analysis of a spool-riser system
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Analysis of a Spool-Riser System
Renata Hermano Almeida da Silveira
Projeto de Graduacao apresentado ao
Curso de Engenharia Naval e Oceanica da
Escola Politecnica, Universidade Federal do
Rio de Janeiro, como parte dos requisitos
necessarios a obtencao do tıtulo de Engen-
heiro.
Orientadores: Murilo Augusto Vaz
Benjamin Dubois
Rio de Janeiro
Junho de 2021
Analysis of a Spool-Riser System
Renata Hermano Almeida da Silveira
PROJETO DE GRADUAÇÃO SUBMETIDO AO CORPO DOCENTE DO CURSO DE
ENGENHARIA NAVAL E OCEÂNICA DA ESCOLA POLITÉCNICA DA
UNIVERSIDADE FEDERAL DO RIO DE JANEIRO COMO PARTE DOS REQUISITOS
NECESSÁRIOS PARA A OBTENÇÃO DO GRAU DE ENGENHEIRA NAVAL E
OCEÂNICA
.
Examinada por:
________________________________________________
Prof. Murilo Augusto Vaz, Ph. D.
.
________________________________________________
Marcelo Caire, D.Sc.
________________________________________________
Rafael Familiar Solano, D. Sc.
RIO DE JANEIRO, RJ – BRASIL
Junho de 2021
Silveira, Renata Hermano Almeida da
Analysis of a Spool-Riser System/ Renata Hermano Almeida da Silveira
– Rio de Janeiro: UFRJ/ Escola Politecnica, 2021.
XIV, p.74 : il.; 29,7 cm.
Orientador: Murilo Augusto Vaz, Benjamin Dubois
Projeto de Graduacao – UFRJ / Escola Politecnica / Curso de
Engenharia Naval e Oceanica, 2021.
Referencias Bibliograficas: p.63-64.
1. Pipeline. 2. Riser. 3. Spool. 4. FEA. I. Vaz, Murilo Augusto. II. Uni-
versidade Federal do Rio de Janeiro, UFRJ, Engenharia Naval e Oceanica.
III. Analysis of a Spool-Riser System.
iii
Acknowledgements
Even though this report is signed by one person, several others participated,
helped, guided and supported me to some extent during the activities presented in this
piece of work or even before it ever began. If it were not for the following people, this
report would never find its way to the reader.
I will be acknowledging the orientation and supervision of Benjamin Dubois and
Murilo Vaz during the duration of this project. Thank you for keeping the topic as
interesting as possible and for being patient and helpful whenever I needed.
I would also like to thank UFRJ institution and its employees, specially Professor
Marta Cecilia Tapia Reyes who made possible for me to participate in the ENSTA-
Bretagne double degree exchange program, which has changed me as a professional and
as a person.
It is as important to thank all my new friends made during this exchange, they
became a family across the globe and will always have a special place in my life. My
acknowledgements extend to my friends from Brazil who have always supported me,
being only one phone call away.
Most of all, I want to thank the unconditional encouragement from my parents,
Tania and Renato, who have worked so hard to provide me with the best and that
often gave up their own dreams so that their children could reach theirs. To my sisters,
Marina, Maria Clara and Nathalia, who always stood by my side, believed in me more
than myself and are my forever best friends.
Last but not least, thanks to Daniel who besides of filling my days with laughter,
love and joy, stayed by my side rooting and encouraging for me, no matter what.
iv
Abstract of Undergraduate Project presented to POLI/UFRJ as a partial fulfillment of
the requirements for the degree of Engineer.
Analysis of a Spool-Riser System
Renata Hermano Almeida da Silveira
Adivisors: Murilo Augusto Vaz and Benjamin Dubois
Course: Naval and Ocean Engineering
A spool is a pipe section which is used to connect a pipeline to another subsea struc-
ture or riser, it ensures the continuity of fluid transport. This element can have various
shapes, and its main function is to absorb efforts coming from the flowline and prevent
them to arise to the riser/subsea equipment. Mechanical analyses of this element be-
haviour under environmental and operation loads are often important subject during a
subsea field project.
This work, through a comparative analysis between AutoPIPE and Abaqus software,
aims to develop a methodology for this structure analyses to be made routinely with
Abaqus. It is focused on model definition approaches to best represent the study case
and the applicability of this methodology in the routine of Pipeline Engineers.
The study was motivated by the lack of accurate information on how to analyse subsea
pipeline systems with AutoPIPE, since this software is primarily intended for topside
analysis. In addition, it was considered that the engineers working with pipelines usually
have greater knowledge in the use of Abaqus instead of AutoPIPE.
Key-words: Pipeline, Riser, Spool, FEA.
v
Resumo do Projeto de Graduacao apresentado a Escola Politecnica/UFRJ como parte
dos requisitos necesserios para obtencao do grau de Engenheiro Naval e Oceanico
Analise de um Sistema Spool-Riser
Renata Hermano Almeida da Silveira
Orientadores: Murilo Augusto Vaz e Benjamin Dubois
Curso: Engenharia Naval e Oceanica
Um spool e uma secao de tubo que e usada para conectar um duto o a outra es-
trutura submarina ou riser, garantindo a continuidade do transporte de fluido. Esse
elemento pode ter varias formas, mas, seja qual for a sua forma, sua principal funcao
e absorver os esforcos provenientes do duto e impedir que eles cheguem intensamente
ao riser/estruturas subsea. Analises mecanicas do comportamento desse elemento sob
cargas ambientais e operacionais sao frequentemente assuntos importantes durante pro-
jetos novos e de extensao de campo de petroleo.
Este trabalho, atraves de um estudo comparativo entre os softwares AutoPIPE e Abaqus,
visa desenvolver uma metodologia para que essas analises de estrutura sejam feitas
rotineiramente com o Abaqus. Ele se concentra nos parametros de definicao de modelo
para melhor representar o caso de estudo e a aplicabilidade dessa metodologia na rotina
dos engenheiros de dutos.
O estudo foi motivado pela falta de informacoes precisas sobre como analisar sistemas
de dutos submersos com o AutoPIPE, uma vez que esse software e primordialmente des-
tinado a analises topside. Alem disso, foi considerado que os engenheiros que trabalham
com dutos normalmente possuem maior conhecimento na utilizacao do Abaqus que do
AutoPIPE.
Palavras-chave: Pipeline, Riser, Spool, FEA.
vi
Acronyms
BC Boundary Condition(s)
CD Chart Datum
CF Concentrated force
DF Distributed force
DOF Degrees of freedom
FEA Finite Element Analysis
HOC Hang Off Clamp
HY Hydrotest
MSL Mean Sea Level
N/A Not Applicable
NLGeom Geometric non-linearity
OP Operation
PIP Pipe in pipe
RFA Reaction force in A direction
RMA Reaction moment around A direction
SIF Stress intensification factor
UA Displacement in A direction
URA Rotation around the A axis
ULC Unitary load case
VBA Visual Basic for Applications
Y Year(s)
vii
Contents
1 Introduction 1
1.1 Context and Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Problem Statement and Objectives . . . . . . . . . . . . . . . . . . . . 2
1.3 Structure of the report . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Theoretical Background 5
2.1 Types of Subsea Piping Structures . . . . . . . . . . . . . . . . . . . . 5
2.1.1 Riser and Pipeline . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.2 Spool . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2 General Coss Section of Rigid Subsea Pipes . . . . . . . . . . . . . . . . 7
2.3 Loads on Subsea Piping Structures . . . . . . . . . . . . . . . . . . . . 8
2.3.1 Loads on Risers . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.3.2 Loads on Spools and Pipelines . . . . . . . . . . . . . . . . . . . 8
3 Software Overview 13
3.1 AutoPIPE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.2 Abaqus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.3 FEMAP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4 Correlation between Abaqus and AutoPIPE 16
4.1 Straight pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4.1.1 Case A: Weight loads . . . . . . . . . . . . . . . . . . . . . . . 17
4.1.2 Case B: Weight, content and buoyancy loads . . . . . . . . . . . 17
4.1.3 Case C: Pressure load . . . . . . . . . . . . . . . . . . . . . . . 18
4.1.4 Case D: Temperature load . . . . . . . . . . . . . . . . . . . . . 21
4.1.5 Case E: Waves and current load . . . . . . . . . . . . . . . . . . 21
viii
4.2 Geometry with a bend . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.2.1 Case A: Weight load . . . . . . . . . . . . . . . . . . . . . . . . 26
4.2.2 Case B: Weight, content and buoyancy loads . . . . . . . . . . . 29
4.2.3 Case C: Weight, content, buoyancy, temperature and pressure loads 29
4.2.4 Case D: Weight, content, buoyancy, temperature, pressure, waves
and current loads . . . . . . . . . . . . . . . . . . . . . . . . . 30
4.3 Summary of results and conclusion . . . . . . . . . . . . . . . . . . . . 31
4.3.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
4.3.2 Findings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
5 Methodology for recreating AutoPIPE simulation into Abaqus 33
5.1 Geometry and Mesh - Femap . . . . . . . . . . . . . . . . . . . . . . . 33
5.2 Pipe design data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
5.2.1 Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
5.2.2 Pipe and coating data . . . . . . . . . . . . . . . . . . . . . . . 35
5.2.3 Pressure and temperature data . . . . . . . . . . . . . . . . . . 36
5.2.4 Fitting data . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
5.3 Environmental data . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
5.3.1 Water depth and seawater density . . . . . . . . . . . . . . . . . 37
5.3.2 Waves and currents . . . . . . . . . . . . . . . . . . . . . . . . 37
5.3.3 Hydrodynamic coefficients . . . . . . . . . . . . . . . . . . . . . 38
5.3.4 Marine Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
5.3.5 Soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
5.4 Interface Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
5.4.1 Pipeline Expansion . . . . . . . . . . . . . . . . . . . . . . . . . 39
5.4.2 Platform Movements . . . . . . . . . . . . . . . . . . . . . . . . 39
5.5 Load cases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
5.6 Modeling choices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
5.6.1 Soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
5.6.2 Riser guides/supports . . . . . . . . . . . . . . . . . . . . . . . 43
5.6.3 Valve and flanges . . . . . . . . . . . . . . . . . . . . . . . . . 44
5.6.4 Concrete, anti-corrosion and marine growth . . . . . . . . . . . . 44
5.6.5 Load sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
ix
5.7 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
5.7.1 Displacement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
5.7.2 Reaction force and reaction moment . . . . . . . . . . . . . . . 51
5.7.3 Section force and section moment . . . . . . . . . . . . . . . . . 52
5.7.4 Stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
5.8 Methodology summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
5.9 Software comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
5.10 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
6 Conclusion 62
Bibliografia 63
A Wave theories limits 65
B ASME B31.8 Stress 66
B.1 Hoop Stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
B.2 Longitudinal Stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
B.3 Combined Stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
C Pre-processing sheet 69
D Python script for post-processing 71
x
List of Figures
1.1 Jacket platform and its piping system. . . . . . . . . . . . . . . . . . . 3
2.1 Subsea schematic layout [Ref. [1]]. . . . . . . . . . . . . . . . . . . . . 6
2.2 Most common types of spools: vertical (left) and horizontal (right) [Ref.
[2] and [3]]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3 Pipe Cross Section. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.4 Force diagram - Spools and Pipelines. . . . . . . . . . . . . . . . . . . . 9
2.5 Pipeline end expansion. . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.6 Strain distribution along the flowline [Ref. [1]]. . . . . . . . . . . . . . . 10
4.1 Wave phase orientation for Abaqus and AutoPIPE both with 0-degree
phase angle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4.2 Schematic Case E.1 (left) and E.2 (right). . . . . . . . . . . . . . . . . 23
4.3 Pipe with a 90-degree bend. . . . . . . . . . . . . . . . . . . . . . . . . 25
5.1 FEMAP mesh overview. . . . . . . . . . . . . . . . . . . . . . . . . . . 34
5.2 Abaqus modeled geometry: supports, soil and pipe structure. . . . . . . 42
5.3 Clearance definition for tube-to-tube contact elements. . . . . . . . . . . 43
5.4 Support geometry made for Abaqus model with tube-to-tube contact
elements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
5.5 AutoPIPE loads and BC choices for the main case of study. . . . . . . . 45
5.6 X displacement results - Case GP1T1U20-HY. . . . . . . . . . . . . . . 48
5.7 Y displacement results - Case GP1T1U20-HY. . . . . . . . . . . . . . . 48
5.8 AutoPIPE top view deformed shape, 25 scale factor - Case GP1T1U20-HY. 49
5.9 Abaqus Pipe top view deformed shape, 25 scale factor - Case GP1T1U20-
HY. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
xi
5.10 Abaqus Elbow-Pipe top view deformed shape, 25 scale factor - Case
GP1T1U20-HY. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
5.11 AutoPIPE local axis definition. . . . . . . . . . . . . . . . . . . . . . . 53
5.12 Effective axial force results in local axis - Case GP1T1U20-HY. . . . . . 53
5.13 Bending moment about the local 1-axis - Case GP1T1U20-HY. . . . . . 54
5.14 Von Mises stress - Case GP1T1U20-HY. . . . . . . . . . . . . . . . . . 55
5.15 Axial stress - Case GP1T1U20-HY. . . . . . . . . . . . . . . . . . . . . 56
5.16 Hoop stress - Case GP1T1U20-HY. . . . . . . . . . . . . . . . . . . . . 56
5.17 Example of .bat file to run Abaqus simulations in a row. . . . . . . . . . 59
A.1 Wave theories limits [Ref. [8]]. . . . . . . . . . . . . . . . . . . . . . . 65
B.1 ASME B31.8 SIF definition [Ref. [4]] . . . . . . . . . . . . . . . . . . . 67
C.1 VBA pre-processing sheet. . . . . . . . . . . . . . . . . . . . . . . . . . 70
D.1 Python preliminary script for post-processing. . . . . . . . . . . . . . . . 74
xii
List of Tables
4.1 Results for a straight pipe - Load case A. . . . . . . . . . . . . . . . . . 17
4.2 Results for a straight pipe - Load case B. . . . . . . . . . . . . . . . . . 18
4.3 Abaqus results for a straight pipe - Load case B where buoyancy was
modeled as CF. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.4 Results for a straight pipe - Load case C without friction. . . . . . . . . 20
4.5 Results for a straight pipe - Load case C with friction. . . . . . . . . . . 20
4.6 Results for a straight pipe - Load case D. . . . . . . . . . . . . . . . . . 21
4.7 Results for a vertical pipe submitted to wave load. . . . . . . . . . . . . 23
4.8 Current data - Load case E.2. . . . . . . . . . . . . . . . . . . . . . . . 24
4.9 Results for a vertical pipe submitted to wave and current load. . . . . . . 24
4.10 AutoPIPE results for the geometry with a bend - Load case A. . . . . . . 26
4.11 Mesh influence on Abaqus results for the geometry with a bend modeled
with Pipe31 - Load case A . . . . . . . . . . . . . . . . . . . . . . . . . 26
4.12 Mesh influence on Abaqus results for the geometry with a bend modeled
with Elbow31 - Load case A. . . . . . . . . . . . . . . . . . . . . . . . 28
4.13 Results for the geometry with a bend modelled with 25 elements - Load
case B. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
4.14 Results for the geometry with a bend modeled with 25 elements - Load
case C. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
4.15 Current data- Load case D . . . . . . . . . . . . . . . . . . . . . . . . . 30
4.16 Results for the geometry with a bend modeled with 25 elements - Load
case D. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
5.1 Material Data - Steel. . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
5.2 Pipe Data - Spool and Pipeline. . . . . . . . . . . . . . . . . . . . . . . 35
5.3 Pipe Data - Riser and Topside. . . . . . . . . . . . . . . . . . . . . . . 35
xiii
5.4 Density of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
5.5 Pipe Data - Riser and Topside. . . . . . . . . . . . . . . . . . . . . . . 36
5.6 Fitting weight data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
5.7 Current data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
5.8 Wave data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
5.9 Hydrodynamic coefficients. . . . . . . . . . . . . . . . . . . . . . . . . 38
5.10 Marine growth profile. . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
5.11 Soil properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
5.12 Pipeline expansion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
5.13 Platform displacements. . . . . . . . . . . . . . . . . . . . . . . . . . . 39
5.14 Unitary Load Cases. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
5.15 Combined Load Cases. . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
5.16 Abaqus step sequence. . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
5.17 AutoPIPE reaction force on pipe supports - Case GP1T1U20-HY. . . . . 51
5.18 Abaqus reaction force on pipe supports - Case GP1T1U20-HY. . . . . . 52
5.19 Reaction moment on the hang off clamp - Case GP1T1U20-HY. . . . . . 52
5.20 Abaqus step sequence. . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
xiv
Chapter 1
Introduction
Due to energy demands that increase each day, the exploitation of fossil resources
is still necessary and indispensable. In order to meet these needs, many gas and oil
production facilities have been built in the last few years. Consequently the engineering
behind those facilities was, and still is, forced to evolve quickly into finding new solutions
for offshore challenges. It is in this context of oil extraction and offshore structures that
this project is inserted.
When it comes to offshore exploitation fields, the subsea layout is always a com-
plex schema of flowlines, jumpers, spools, manifolds, Christmas Trees etc. which allow
the oil to flow from the well up to the platform. In order to define a layout, a series
of individual analyses of each component and their interaction with each other must be
assessed. This project studies the interaction of Spool, Riser and associated Flowline.
1.1 Context and Motivation
In the offshore production industry, rigid spools are special shaped pipes that
joins a flowline and a production structure. In the studied case, the spool makes the
connection between an underwater pipeline and a riser. On its turn, the riser is guided
along the platform, supported at its higher level and connected to the platform topside.
The main reasons why risers are not directly connected to the flowline are the
thermal expansion and pressure efforts that income from the flowline and would most
certainly affect riser structure. For this reason the spool is used to make their link and to
absorb those incoming efforts from the flowline, not allowing them to arise so intensely
1
on the riser.
In addition to the pressure and the temperature of the transported fluid, those
pipes are submitted to loading specific to the method of installation, to movements of the
platform (settlements, inclinations, etc ...) and they are also subject to hydrodynamic
forces generated by waves and sea currents. Therefore, the system made by riser, spool
and pipeline needs to stand all the efforts in various combinations during its lifetime
without compromising the fluid flow, the environment and the safety of the platform
staff. Therefore, it must not fail under those circumstances.
Since the system is under complex conditions and the structure is requested in
so many ways, usually finite element simulations are made to design and predict the
structure response to such environment. Most often, these studies are carried out with
AutoPIPE or CAESAR II software that are originally dedicated to the analysis of topside
piping. Some companies use AutoPIPE software to analyse the riser and spool system.
Although the creation of the model is fairly easy, the interpretation of results is not
straightforward.
Therefore, this project wants to be able to provide another possibility for riser
and spool structural studies. It aims in a routinely use of FEA software to carry out
these studies, this one having better modelling capacities and making it possible for
simulations to get even closer to the real case of study.
1.2 Problem Statement and Objectives
The development of a finite element methodology to simulate the structural
analysis of Spool-Riser system is the main purpose of this study. In order to do so, an
existing old project will be this study base case. This study was previously carried out
using AutoPIPE software and it refers to the pre-sizing of a Spool-Riser system that is
placed under a jacket platform, Figure 1.1 illustrates this platform type and its piping
system.
The given pipe geometry will have to resist all efforts from waves, currents, im-
posed displacement due to the slight movements of the platform, internal pressure and
thermal expansion. The system is held in place by a hang off clamp and other supports.
Those support elements can be seen in Figure 1.1 as green arrows and will be better
2
detailed later on.
Figure 1.1: Jacket platform and its piping system.
Therefore, the main objective of this project was to recreate the simulation with
Abaqus and to validate its results with the ones given by AutoPIPE. With the purpose
of accomplishing it, a series of simulations with simplified geometry were made to test
and comprehend the series of events that occurs around the Spool-Riser geometry. The
studies of the influence of mesh refinement and element’s types were also made to allow
the choice of the best modeling design.
1.3 Structure of the report
This project is structured in 6 chapters which are described below:
3
• Chapter 2: discusses the theoretical background around the subsea piping system.
The main characteristics of each component will be presented and the principal
loads and responses will be discussed.
• Chapter 3: is destined to a brief presentation of the two software that are concerned
by this study: Abaqus and AutoPIPE.
• Chapter 4: aims to define modelling correlations between Abaqus and AutoPIPE.
This is made through a series of simple load case studies.
• Chapter 5: the real Spool-Riser simulation is modelled in both Abaqus and Au-
toPIPE. All modelling parameters are presented in this section, including geometry,
load sequence, mesh and boundary conditions. Finally, a final comparison between
Abaqus and AutoPIPE outputs is made.
• Chapter 6: conclusion and way forward.
4
Chapter 2
Theoretical Background
This section is dedicated to present basic information which helps elucidate the
comprehension of the following research work. Therefore it is focused on three major
subsea structures: spool, risers and pipelines.
2.1 Types of Subsea Piping Structures
2.1.1 Riser and Pipeline
Risers and pipelines are structures that transport the well fluid to the platform.
When risers are mainly vertical and connect floating units to the seabed, pipelines are
horizontal and lay on the seabed. Both structures can be either rigid (made of steel),
flexible (made using overlapping layers of steel profiles and layers of polymeric materials),
or hybrids, (a system composed by section(s) of rigid riser and section(s) of flexible
riser). There are many types of subsea ducts which could be used for: drilling, export,
production, completion and injection.
Those structures suffer great influence from external means, such as underwater
pressure, waves and currents, as well as from the movements of the platform (for risers
only). This brings an impact on the efforts suffered by the element, consequently the
structures must be capable of standing such efforts.
The following figure shows an arrangement of subsea ducts.
5
Figure 2.1: Subsea schematic layout [Ref. [1]].
2.1.2 Spool
A spool is a special shaped pipe section which connects two subsea structures
and allow the fluid transportation between them. Spools have two main functions [Ref.
[1]]:
• Allow the connection between a flowline and another subsea structure, compen-
sating for possible installation misalignments;
• As flowlines may suffer displacements (axial expansion and/or walking) during its
lifetime, spools are used to avoid the displacement from propagating and high
loads to propagate to adjacent structures.
Spools can have a series of different shapes such as: Z-shaped, M-shaped, L-
shaped etc. The shape choice is highly influenced by the field layout that the structure
is installed in. It is possible to divided spool types into two main categories: vertical and
horizontal. Figure 2.2 illustrates vertical and horizontal Z-shaped spools.
This study is focused on a Z-shaped horizontal spool.
6
Figure 2.2: Most common types of spools: vertical (left) and horizontal (right) [Ref.
[2] and [3]].
2.2 General Coss Section of Rigid Subsea Pipes
Rigid subsea pipe structures are made of high performance steel and, as they
operate under challenging conditions, a number of protection coats are applied inside and
outside of the pipe for protection. Coating mainly protect from corrosive environments,
damage caused by abrasion, falling objects and an insulation coating may be used to
conserve high temperature. Figure 2.3 illustrates a general subsea pipe cross section.
Figure 2.3: Pipe Cross Section.
7
2.3 Loads on Subsea Piping Structures
2.3.1 Loads on Risers
During its lifetime risers are submitted to a number of variant load combinations.
Some loads can be mentioned such as:
• Self weight.
• Waves: the effect depends mostly on the wave period, height and incidental di-
rection.
• Currents: the effect depends mostly on the velocity and incidental direction.
• External and internal pressures.
• Thermal expansion.
• Platform/Vessel displacements.
2.3.2 Loads on Spools and Pipelines
A subsea pipe needs to be stable on the seabed and its stability is directly related
to is weight. However, a structure that is too heavy leads to higher production and
installation costs, on the other hand a structure that is too light will find it hard to be
stable under the effects of waves and currents, being more susceptible to vertical and
horizontal movements.
There are numerous ways to stabilize a flowline on the seabed, varying from
higher wall thickness to concrete coating and anchors, however, the decision making
process usually relies on the global solution cost.
Figure 2.4, originally extracted from Ref. [5] and later adapted for the study
case, illustrates a simplified force diagram acting on a subsea pipe laying on the seabed.
2.3.2.1 Pipeline Expansion
When the pipeline is submitted to pressure and temperature loads it is logically
inclined to expand. However, it may be blocked by the soil interaction with the structure.
8
Figure 2.4: Force diagram - Spools and Pipelines.
The soil frictional behaviour generates a compressive force in the pipeline which works
to restrain the pipe from any movement, expansion included. This friction resistance is
more significant at the middle of the pipe, deceasing toward its end.
As above mentioned, the frictional resistance decreases from the middle towards
the extremities. At a give point along the pipe, the compressive force equals the expan-
sion force and the pipe is restrained from further expansion. This point is considered as
the virtual anchor point (VAP) of the flowline [Ref. [6]].
Figure 2.5 exemplifies the effective axial force acting on a given pipe and the
pipeline end displacement. It is possible to identify the restrained and unrestrained
sections of the pipe.
Figure 2.5: Pipeline end expansion.
9
2.3.2.2 Pipeline Strains
The effects of pressure, temperature and soil-pipe interaction implies to the
pipeline stresses and strains. For length located on the unrestrained section of the struc-
ture, the pipe is free to move and expansion may be noticed. On the other hand, sections
of the restrained zone will face high stresses due to the effects of friction resistance.
In order to get the total pipeline expansion, strains must be integrated along the
flowline from the free end to the anchor point. According to Bai and Bai [Ref. [1]],
a pipeline with zero initial strain and constant cross section, the pressure and thermal
strains are constant along the pipe length and the frictional strain varies linearly. Figure
2.6 shows a graphic representation of the strains along the flowline.
Figure 2.6: Strain distribution along the flowline [Ref. [1]].
The whole expansion is then given by:
∫ Lanchor
o
(εTemperature + εPressure + εfriction) dL (2.1)
10
The following points describe of the possibles strain nature and their mathemat-
ical formulation.
• Temperature Strain
When a high temperature is applied to a pipeline, stresses and strains show up
in the structure. As said before, if the pipeline is found to be unrestrained the thermal
strain will built up on the pipe. This strain is given by:
εTemperature = α∆T (2.2)
where:
α the material thermal expansion coefficient;
∆T thermal variation between initial and final states.
• Pressure Strain - End Cap Effect
Pressure strain can be divided into two natures: End Cap Effect and Poisson
Effect (see next bullet point).
The end cap effect is the contribution of the pressure in the axial direction and
it is related to the pressure effect on closed pipes and bend walls. The end cap strain is
given by:
εPEndCap=
(AiPi − AoPo)
EAs
(2.3)
where:
Ai internal area of the pipe (considering the internal diameter);
Ao external area of the pipe (considering the external diameter);
Pi is the internal pressure;
Po is the external pressure;
As is the steel cross section area.
• Pressure Strain - Poisson Effect
The Poisson effect plays a role in the pipe radial expansion due to the inter-
nal pressure. Once the pipe expands radially it suffers little axial compression. The
longitudinal strain related to the Poisson effect is given by:
11
εPEndCap= (−2ν)
(AiPi − AoPo)
EAs
(2.4)
where:
ν is the Poisson’s ratio.
• Frictional Strain - Soil Interaction
As above discussed, when the pipe starts to expand, frictional resistance builds
up trying to block the imminent movement. To do so, fictional resistance generates
negative strains which oppose the effects of pressure and temperature. For unburied
pipes (which is the study case), the frictional strain is given by:
εfriction =−µwxEAs
(2.5)
where:
µ is the axial friction coefficient;
w is the pipeline line weight;
x is the distance from the free end of the pipe.
12
Chapter 3
Software Overview
3.1 AutoPIPE
Bentley AutoPIPE is a finite element program for calculation of piping stresses,
flange analysis, pipe support design, and equipment nozzle loading analysis under static
and dynamic loading conditions. This software mainly uses the Bernoulli beam theory.
However, in the particular case of a pressure type loading, the theory of thick wall is
considered in order to take into account thickness influence.
The specificity of this software is the integration of different international stan-
dards for the design of pipes. It incorporates ASME, British Standard, API, NEMA,
ANSI, ASCE, AISC, UBC, and WRC guidelines and design limits to provide a compre-
hensive analysis of the entire system. Thus, the stresses as well as bend influence are
calculated according to these codes.
For this software results are given on nodes. For non-nodal data, which is the
case of stresses, two values are provided per node: the value immediately before and the
value immediately after the node. Besides that, stresses are calculated on the external
wall, these are consequently the maximum stresses.
A last subtlety of this software is its way of combining load cases. By default,
each time gap, friction and/or soil elements are used, the analysis is defined as non-
linear. This means that load sequence influences unitary load case results and that this
load sequence is always defined by:
• Gravity case called GR;
13
• Thermal case (T1) applied over the deformed shape of gravitational case.
• Pressure case (P1) applied over the deformed shape of case T1.
• Any user defined case (U) applied over the deformed shape of case P1. This may
comport waves, current, wind, imposed displacement or other load types.
Finally, their combination is made by simple sum of results from each unitary
load case.
3.2 Abaqus
In its turn, Abaqus is a more vast software and is able to analyse any geometry
conceived by the user. It is a multiphysic finite element computation software but
rather adapted to thermomechanical calculations. Abaqus can be assessed by a graphical
interface, called Abaqus CAE, or by input file codes. However, some modelling options
are only available for the last one which is the case of Aqua module used for modelling
all hydro efforts.
Abaqus Aqua module is used to apply steady current, wave, and wind loading
to submerged or partially submerged structures in problems such as the modelling of
offshore piping installations or the analysis of marine risers. This module can calculate
drag, buoyancy, and inertia loading only for beam, pipe, elbow, truss, and certain rigid
elements.
When it comes to element type, Abaqus element library proposes a large variety
capable of analysing models of 2D or 3D beam, linear or quadratic type. As regards
the calculation of pipes, it has two families of recommended elements: Pipe and Elbow.
Unlike others, those two families accept loads of the internal pressure type and are the
ones assessed for this study.
Pipe elements are defined by Bryan [Ref.[7]] as two or three dimensional beams
which mathematical notation includes Timoshenko beam theory, taking into account
shear deformation, axial and bending behaviours. It also includes the response of material
non-linearity and radial expansion of the cross-section caused by internal pressure.
On the other hand, although Elbow elements may also look like beam elements
for the user it has a much more complex formulation. Specially, these elements account
14
for flexibility factors in order to correct results from single beam theory, considering
ovalization and warping of the pipeline cross-section. They are recommended for thin-
walled straight pipes which might suffer collapse or for pipe sections which may already
be curved in its initial configuration (bends).
Just like pipe element, elbow elements use polynomial interpolation along their
length to solve the pipe final condition. However, in order to take into account tube
ovalization, elements from family type elbow use a Fourier-type interpolation on the
circumference. The user has the possibility to choose the desired Fourier number in a
range from 0 to 6, but the higher Fourier number the longer the simulation will take and
more precise results are expected.
Stresses and other outputs in general are calculated per element at its integration
points. The location of those integration points depends on the type of section assigned
to an element, this subject will be later detailed.
Finally, Abaqus load sequence relies on step implementation, being user free to
create any sequence combination according to simulations objectives.
3.3 FEMAP
Just like Abaqus, FEMAP is an engineering analysis program used to model finite
element models to analyse real life engineering problems. Although FEMAP is less used
than Abaqus on engineering problem solving this days, it has an very large scope of
use. Allowing the analysis ranging from basic strength analysis, dynamic simulation,
system-level performance evaluation fluid flow to multi-physics engineering studies.
FEMAP stands out when compared to Abaqus on the subject of modelling and
mesh capacities. it integrates the possibility of CAD import, modelling and meshing
tools that are more user-friendly.
For this study, in chapter 5, FEMAP will be used to define mesh control according
to the study needs.
15
Chapter 4
Correlation between Abaqus and
AutoPIPE
This section describes simple cases that were used to develop a modelling cor-
relation between Abaqus and AutoPIPE. This study was done aiming to understand
how AutoPIPE applies all sorts of loads and, consequently, how to transpose them into
Abaqus. It was also studied with the objective of understanding how both software
interpret pipe geometry, mainly elbows sections.
Those cases were not only studied with Abaqus and AutoPIPE, some times a
third software was used to help guide the comparison. For thermal and pressure cases
a theoretical MathCad sheet gave the expected results for axial expansion and friction
force, on the other hand, cases that included wave and current were also modelled with a
company internal software. More details concerning those software and their application
will be given progressively in the following subsections.
It is known through AutoPIPE documentation that this software is not capable
of computing geometric non linearity and this is to be taken as basis case for all FEA
analysis.
4.1 Straight pipe
For the first tests, a horizontal and a vertical steel pipe, with a 610 mm outside
diameter and 15.9 mm wall thickness have been studied. Those geometries were submit-
ted to several different load cases, results and conclusion can be verified in the following
16
subsections.
4.1.1 Case A: Weight loads
This first case was analysed aiming to allow the author to familiarize with Au-
toPIPE and Abaqus input file method. Besides that, it had also the objective of helping
develop a modelling correlation between software. Thus, a 9km vertical pipe fixed on its
upper end submitted only to weight load was modelled both in AutoPIPE and Abaqus.
In order to make a comparison between software, the following outputs were used for
comparison:
• vertical reaction force on the fixed end;
• vertical displacement on the free end.
Results are shown in table 4.1 for AutoPIPE, Abaqus with Pipe31 elements and
analytical calculations. Since both software were reasonably compatible with the analyt-
ical calculations, no changes were needed to adjust any of the models.
Table 4.1: Results for a straight pipe - Load case A.
Analytical AutoPIPE Abaqus
RFZ [kN ] UZ [m] RFZ [kN ] UZ [m] RFZ [kN ] UZ [m]
-20557 -15.29 -20560 -15.29 -20557 -15.30
4.1.2 Case B: Weight, content and buoyancy loads
This case is essentially the same from section 4.1.1 with an additional content
weight (internal fluid) and buoyancy load that were applied in Abaqus by using the Aqua
module with a distributed load of buoyancy type. This is the first time Aqua module
was implemented in this project, therefore this case aims to understand the way it works
and how to use it properly.
This time result comparison was made once again by the following outputs:
• vertical reaction force on the fixed end;
17
• vertical displacement on the free end.
Results are shown in table 4.2, but in this case they are not a match when it
comes to the vertical displacement on the lower end of the pipe. AutoPIPE calculates
a case where the lower end is in compression, and Abaqus still shows stretching of the
whole pipe. Logically one would expect a result as the one incoming from Abaqus.
It is also noted that a considerable difference between analytical and FEA results
are seen for the axial displacement, this could be realated to the mesh details and will
be further discussed on section 4.2.
Table 4.2: Results for a straight pipe - Load case B.
Analytical AutoPIPE Abaqus
RFZ [kN ] UZ [m] RFZ [kN ] UZ [m] RFZ [kN ] UZ [m]
-17859 -13.26 -17864 6.46 -17839 -12.60
Several tests were made, it was found that for straight vertical pipes AutoPIPE
applies buoyancy as a concentrated force on the lower end of the pipe, this leads to the
different results for axial deformation. Once the same concentrated force was applied on
Abaqus model, it gave almost the same results as AutoPIPE as shown in table 4.3.
Table 4.3: Abaqus results for a straight pipe - Load case B where buoyancy was
modeled as CF.
Abaqus
RFZ [kN ] UZ [m]
-17839 6.49
It is then identified a possible problem when it comes to modelling a Riser on
AutoPIPE: the buoyancy formulation. Other tests were made in section 4.2 to assess the
viability of AutoPIPE modelling for the study case, a Spool-Riser system, as it comprises
a long vertical pipe section.
4.1.3 Case C: Pressure load
This case intends to verify how AutoPIPE calculates section forces related to
pressure load cases and to select an output from Abaqus that corresponds to the same
18
calculation method. To help guide this study, analytical calculations based on section
2.3.2.1 were also carried out using a MathCad sheet which provides the axial displacement
and the effective axial force acting on the pipe. This sheet will be repetitively mentioned
because it was also used for further cases.
The present case consists of a horizontal element that is submitted only to internal
pressure load and is fixed on one of its extremities. The pipe is made of steel, measures
9 km, has an outside diameter of 610 mm and wall thickness of 15.9 mm. A pressure
difference of 100 bar is noticed between external and internal surfaces.
In order to make the correct assumptions on Abaqus, AutoPIPE documentation
was verified and it was discovered that, when it comes to pressure load, this software
takes all pressures as if they were applied in the external diameter of the pipe, having a
conservative approach.
To analyse this situation, three FEA simulations were put in place: AutoPIPE,
Abaqus with Pipe31 elements and Abaqus with Elbow31 elements. This time the fol-
lowing outputs were subject of comparison:
• effective axial section force measured on the fixed end;
• displacement measured on free end .
The effective axial force is a concept commonly used in the offshore industry, it
allows calculation of the global behaviour without having to integrate the internal and
external pressure over the duct wall, according to [Ref. [6]]. The axial section force given
as output by AutoPIPE is the effective axial force, meaning that it does not consider
capped pressure effects.
On the other hand, Abaqus has two different outputs for axial section forces: SF1
and ESF1. The first one is the so-called “true” axial force given by the integral of stresses
over the pipeline cross-section, the other is the effective axial force which simplifies the
influence of the internal and external pressures on the pipeline behaviour. Both outputs
are available for pipe elements, but only SF1 is available for elbow elements. Therefore,
in order to have comparable results between all calculation methods, Abaqus SF1 for
elbow elements was transformed into effective axial force according to the following
equation taken from this software library.
In this equation, pe and pi are respectively external and internal pressures; Ae
19
and Ai are the external and internal pipe section areas as used to define Abaqus pressure
load.
SFaxialAutoPIPE = ESF1Abaqus = SF1Abaqus + peAe − piAi (4.1)
Results from all three FEA simulations are summarized in the table 4.4. This
table provides results obtained with MathCad for analytical calculations, AutoPIPE and
Abaqus for pipe and elbow elements. Results incoming from elbow simulations were
obtained using Fourier ovalization mode varying from 1 to 5 which showed no change in
results for a straight pipe. A perfect compatibility between all methods is noticed. For
this case only, since no friction was applied, the force in the axial direction is zero for all
four calculation methods and therefore is not presented on the results table.
Table 4.4: Results for a straight pipe - Load case C without friction.
MathCad AutoPIPE Abaqus Pipe Abaqus Elbow
Uaxial[m] 1.562 1.562 1.562 1.562
Then, in order to evaluate frictional behaviour of both software, a friction factor
of 0.59 in all directions was imposed in all three calculations methods. Results are
summarized in table 4.5. Displacement results are likely to be considered good enough,
although there is still a slight difference. However, a difference around 4 mm for a 9 km
long pipe is to be considered irrelevant.
Effective axial force this time still has a very good correspondence between meth-
ods, however Abaqus Elbow simulation diverges a little from the others. This is believed
to be related element type modelling approach and the consequent error propagation
when modifying SF1 into ESF1.
Table 4.5: Results for a straight pipe - Load case C with friction.
MathCad AutoPIPE Abaqus Pipe Abaqus Elbow
Faxial[kN ] 1050.2 1050.2 1055.3 1037.1
Uaxial[m] 0.077 0.081 0.074 0.074
20
4.1.4 Case D: Temperature load
For this case, an ambient temperature of 3.7◦C and an internal temperature
of 12.1◦C were taken for a 9 km long pipe, both being constant along the structure
length. The other characteristics of the structure were kept as in case C described above.
Analytical calculations were made using the Mathcad sheet and FEA were carried out
with AutoPIPE and Abaqus. Results are those shown in table 4.6 and were taken in the
same spots as in case C.
Since no friction was applied, the axial force is zero for all four calculation methods
and therefore is not presented on the results table.
Although results may diverge a little, attention is raised to the fact that the
maximum discrepancy is 13mm out of a 9km pipe.
Table 4.6: Results for a straight pipe - Load case D.
MathCad AutoPIPE Abaqus Pipe Abaqus Elbow
Uaxial[m] 0.885 0.884 0.897 0.890
4.1.5 Case E: Waves and current load
In this section waves and current loads will be analysed with Abaqus, AutoPIPE
and a company internal software. This last one allows the analysis of a vertical pipe
that can be submitted to wave and/or currents. It requires as input the wave data, the
pipe geometry, the drag and inertia coefficients and current data. Its outputs are the
force and moment to which the structure is subjected, however, in the internal company
software, no information concerning the application location of this moment is given
and, therefore, only the force output is used for comparison matters.
This case was chosen to be studied to test the use of Abaqus Aqua module for
modelling waves and currents and to define a correspondence between its modelling
methods and AutoPIPE.
One must know that those two FEA software have one major difference when
modelling waves, they have an intrinsic 180◦ phase difference. For AutoPIPE a 0◦ phase
orients the crest of the wave at the origin, whereas 180-degree places its bottom at the
origin. The exact opposite occurs in Abaqus Aqua module as illustrated by figure 4.1.
21
Figure 4.1: Wave phase orientation for Abaqus and AutoPIPE both with 0-degree
phase angle.
This case was modelled as a vertical pipe of 70 m fully submerged, fixed on
its upper end and free on its lowest point. To analyse this situation, two static FEA
simulations were put in place: AutoPIPE and Abaqus with Pipe31 elements. This time
the following outputs were subject of comparison:
• reaction force measured on the fixed end;
• reaction moment measured on the fixed end;
• displacement on free end.
This section first case (case E.1) includes only a wave load. In all three calculation
methods the user must select a wave theory either Airy Wave, Stokes or Stream function.
The region of applicability of each theory can be seen in appendix A.
For the first analysis, a wave height of 15.47 meters and a 12.81 seconds period
was chosen and modelled with Stokes wave theory. This wave has a 90-degree phase
angle for the company internal software and AutoPIPE, therefore a 270◦ for Abaqus.
The considered axis and incoming wave are shown in figure 4.2 below.
Another important point regards the drag, lift and inertia coefficients, they were
set as 1.2 for drag and 3.29 for inertia. This topic is one major difference between soft-
ware, AutoPIPE is able to compute lift efforts, Abaqus Aqua module is not. Therefore,
22
Figure 4.2: Schematic Case E.1 (left) and E.2 (right).
lift was set to zero in AutoPIPE for this section and all that follows, although for this
section it shouldn’t play any role.
Results seem compatible between all three calculations methods as can be seen
in table 4.7.
Table 4.7: Results for a vertical pipe submitted to wave load.
Fy[kN ] Mx[kN.m] Uy[m]
Internal
software
-68.0 -2801.3 (1) -
AutoPIPE -66.6 -1966.0 -8.34
Abaqus -68.1 -1968.5 -8.24
Note:
(1) Result not comparable with the others because its output location is unknown.
The second analysis (case E.2) uses the same wave and coefficients from before
and adds a current defined in table 4.8, see schematic in figure 4.2. Case E.2 results
can be found in table 4.9 and, once again, no major differences were found between all
three software.
23
Table 4.8: Current data - Load case E.2.
Depth [m] Current Velocity, Uc [m/s]
MSL 2.00
-17.5 1.80
-35.0 1.60
-52.5 1.35
-70.0 1.00
Table 4.9: Results for a vertical pipe submitted to wave and current load.
Fy[kN ] Mx[kN.m] Uy[m]
Internal
software
-137.3 -5708.7 (1) -
AutoPIPE -126.1 -3753.4 -15.81
Abaqus -128.9 -3763.4 -15.62
Note:
(1) Result not comparable with the others because its output location is unknown.
24
4.2 Geometry with a bend
The second geometry can be seen in Figure 4.3, it is composed by a horizontal
45 meters long pipe followed by a 90◦ bend with radius of 3.05 meters. The bend is
connected to a vertical section that goes from the seabed to sea free surface and mea-
sures 90 meters. The model rests on a rigid soil with no friction and is tied on its lower
edge (Dx, Dy, Dz, Rx, Ry and Rz = 0) as illustrated by the gray element.
Since the main case of study has several bends along its length this new geometry
was studied to understand how to model bend zones in Abaqus as it is in AutoPIPE.
Therefore two elements families were used in Abaqus: Pipe and Elbow. Their perfor-
mance will be discussed in the following sections. Besides that, this section also means to
define a correct way to combine load cases so Abaqus can represent the same conditions
as AutoPIPE.
Figure 4.3: Pipe with a 90-degree bend.
25
4.2.1 Case A: Weight load
In this case the geometry is only submitted to weight load. The reaction force
on the soil and displacement on the highest point of the pipe were measured, this point
is identified by a green cross in figure 4.3.
AutoPIPE results are shown in the table 4.10. Results from Abaqus shown in
table 4.11, these were obtained using 4 elements type Pipe31 along the bend area. Since
results were not compatible when it comes to the displacement on the upper end, the
number of elements along the bend was increased in order to study its influence and
choose a mesh that has already converged. Results emerging from this approach can
also be seen in table 4.11.
Table 4.10: AutoPIPE results for the geometry with a bend - Load case A.
RFZ [kN ] UX [m] UY [m] UZ [m]
-312.3 0.0 2.261 -0.064
Table 4.11: Mesh influence on Abaqus results for the geometry with a bend modeled
with Pipe31 - Load case A
Elements along the bend RFZ [kN ] UX [m] UY [m] UZ [m]
4 -312.2 0.0 1.546 -0.050
12 -312.2 0.0 1.543 -0.051
25 -312.3 0.0 1.543 -0.051
50 -312.3 0.0 1.529 -0.052
100 -312.3 0.0 1.529 -0.052
It was noticed that, although the quantity of elements increased, results don’t
seem to be converging to the same values as AutoPIPE simulation. This was taken as an
indication that maybe element type used in Abaqus was not the right one in comparison
to AutoPIPE. The present led to the use of elbow elements for the bend section and some
attempts were made to find the correct Fourier mode, results for different combinations
between mesh and Fourier modes are shown in table 4.12.
Elbow elements were developed to better model bend zone in which ovalization
may occur. Those elements are modelled as beams but their mathematical theory is
26
actually shell type with quite complex deformation patterns allowed. Because elbow
elements use shell formulation, the number of degrees of freedom per element is high.
Those elements also use Fourier modes to model ovalization, all of which leads to more
expensive computational simulations than beam common elements.
From all these results, it is concluded that at least a mode 3 is required in order
to achieve quality results in comparison with AutoPIPE. When it comes to the number
of elements along the bend, although results don’t change enormously from one mesh
to another, a longer analysis of convergence must be done if the required precision is
not yet achieved.
27
Table 4.12: Mesh influence on Abaqus results for the geometry with a bend modeled
with Elbow31 - Load case A.
Elements along the bend Fourier mode RFZ [kN ] UX [m] UY [m] UZ [m]
1 0.0 1.550 -0.051
2 0.0 2.223 -0.065
4 3 -312.3 0.0 2.277 -0.066
4 0.0 2.283 -0.066
5 0.0 2.284 -0.066
1 0.0 1.541 -0.051
2 0.0 2.189 -0.064
12 3 -312.3 0.0 2.229 -0.065
4 0.0 2.236 -0.065
5 0.0 2.236 -0.065
1 0.0 1.539 -0.051
2 0.0 2.186 -0.064
25 3 -312.3 0.0 2.226 -0.065
4 0.0 2.233 -0.066
5 0.0 2.233 -0.066
1 0.0 1.506 -0.050
2 0.0 2.182 -0.064
50 3 -312.3 0.0 2.222 -0.065
4 0.0 2.233 -0.065
5 0.0 2.233 -0.065
28
4.2.2 Case B: Weight, content and buoyancy loads
Since case B in section 4.1.2, page 17, was contradictory when it comes to the
way AutoPIPE applies buoyancy load, this load case aimed to verify that when pipes
with different orientations are put together, the buoyancy is applied as a line load on
AutoPIPE and that results are in line with Abaqus.
The same approach from case A above was used, starting by AutoPIPE followed
by Abaqus with Pipe31 only and then with Elbow31 on the bend area. Results from all
three methods are shown in table 4.13. Based on the analysis made in case A, Abaqus
results were obtained with 25 elements along the bend zone and Fourier mode 3 for
elbow simulations.
Table 4.13: Results for the geometry with a bend modelled with 25 elements - Load
case B.
RFZ [kN ] UX [m] UY [m] UZ [m]
AutoPIPE -271.4 0.0 1.963 -0.056
Abaqus Pipe31 -271.3 0.0 1.252 -0.045
Abaqus Elbow31 -271.4 0.0 1.934 -0.057
Once again, as happened in the previous section, results from Abaqus with el-
ements Pipe31 are not as close to those from AutoPIPE and the implementation of
elbow elements gave better results. It is concluded that elbow elements have a better
correspondence with AutoPIPE and, therefore, should be rather preferred.
4.2.3 Case C: Weight, content, buoyancy, temperature and
pressure loads
This case in based on the previous one and applies temperature and pressure
loads on top of it. The initial state was defined by a temperature of 3.7◦C and the final
by a temperature of 12.1◦C, besides a internal pressure of 100 bar was applied.
Since previous cases suggested that element quantity, for this cases, did not make
a difference in output quality, Abaqus simulations were carried out with 25 elements
around the bend area and Fourier mode 3 for elbows. Results may be seen in table 4.14
for all three simulation methods.
29
Once again they are more consistent between AutoPIPE and Abaqus Elbow el-
ements which restates the need of this type of elements to model bend areas when
resuming AutoPIPE modeling in Abaqus.
Table 4.14: Results for the geometry with a bend modeled with 25 elements - Load
case C.
RFZ [kN ] UX [m] UY [m] UZ [m]
AutoPIPE -271.4 0.0 1.975 -0.062
Abaqus Pipe31 -271.3 0.0 1.337 -0.051
Abaqus Elbow31 -271.4 0.0 1.994 -0.064
4.2.4 Case D: Weight, content, buoyancy, temperature, pres-
sure, waves and current loads
This case applies waves and current loads on the condition of load case C from
above section. A wave height of 15.47 meters and a 12.81 seconds period was chosen
and modelled with Stokes wave theory. This wave has a 90-degree phase angle for
AutoPIPE, therefore a 270◦ for Abaqus as explained in section 4.1.5, page 21.
Besides, a current defined in table 4.15 was applied. Results seem compatible
between all three calculations methods as can be seen in table 4.16, however Abaqus
with pipe elements have a disadvantage because they can’t calculate precisely the dis-
placement. Abaqus simulations were carried out with 25 elements around the bend and
Fourier mode 3 for Elbow elements.
Table 4.15: Current data- Load case D
Depth [m] Current Velocity, Uc [m/s]
MSL 1.81
-22.5 1.66
-45.1 1.63
-67.7 1.45
Seabed 1.37
30
Table 4.16: Results for the geometry with a bend modeled with 25 elements - Load
case D.
RFZ [kN ] UX [m] UY [m] UZ [m]
AutoPIPE 426.1 0.0 2.640 -0.0025
Abaqus Pipe31 426.0 0.0 2.324 -0.0017
Abaqus Elbow31 426.0 0.0 2.639 -0.0019
4.3 Summary of results and conclusion
This section of this report intended to gather useful information concerning
AutoPIPE-Abaqus modelling correlation. It had three main objectives:
• test Abaqus Aqua module;
• verify the coherence between both software;
• define recommended modelling choices to analyse pipes with Abaqus.
4.3.1 Conclusion
Firstly it is concluded that Abaqus Aqua module is able to model correctly almost
all hydrodynamic efforts, applying buoyancy, drag and inertia efforts and applying waves
and currents. However it is not yet capable of modelling lift effort which is a real
drawback. In order to consider lift efforts it should be modelled using a subroutine.
During this phase, a comparison between two types of elements used by the
Abaqus software, pipe and elbow elements, were also matter of analysis. The calculation
being faster with the pipe elements, there was a strong urge to validate the use of such
elements. However, it was proven that those elements do not quite represent the same
as AutoPIPE simulations and, consequently, Elbow elements with Fourier mode of at
least 3 were found to be a better option.
Thanks to this first modelling phase, it was possible to acquire all the necessary
knowledge to model the Spool-Riser system with Abaqus.
31
4.3.2 Findings
The main findings gathered by all tests are listed hereafter and are recommended
for further analysis:
• Weight: should be modelled as line load or gravity load in Abaqus.
• Buoyancy and content loads: well modelled by Abaqus Aqua module with line load
type.
• Waves: waves with 180◦ phase difference from AutoPIPE referential and modelled
via Abaqus Aqua module.
• Current: well modelled by Abaqus Aqua module.
• Hydrodynamics efforts: Drag and inertia well modelled by Abaqus Aqua module,
although Abaqus is not yet capable of analysing lift forces. Lift forces are important
to analyse the spool and the pipeline that rest on the soil.
• Mesh: at bend area it doesn’t seem to influence results.
• Bends geometry: better represented by elbow elements with high Fourier mode.
• Step type: static general without geometric non-linearity shall be used for compar-
ison with AutoPIPE.
32
Chapter 5
Methodology for recreating
AutoPIPE simulation into Abaqus
In this section the export line geometry, composed by topside, riser, spool and
pipeline, was analysed using Abaqus software in both operating and hydrotest condi-
tions. At first, the geometry and mesh were modelled using FEMAP software. Later an
initial Abaqus model case was put in place to adjust simulation parameters, to assure
convergence and to allow initial comparison between AutoPIPE and Abaqus results.
Once the global model was working properly, VBA macros were developed to
generate all Abaqus input files, each of them representing one of AutoPIPE load cases.
AutoPIPE simulations analyses 53 load combinations for hydrotest conditions and 53 for
operations, those cases combine waves and displacements in different direction besides
cases GR, T1 and P1 previously mentioned.
The next step was the comparison of results in terms of displacement, reaction
force and moment, section forces and moments and stresses.
5.1 Geometry and Mesh - Femap
As mentioned before, geometry and mesh parameters were defined with FEMAP.
This choice was made because this software is more user-friendly than Abaqus CAE when
it comes to geometry definition and mesh control. Moreover, FEMAP exports geometry
directly into Abaqus input file format and allows direct manipulation of nodes, elements
and sections.
33
A view of FEMAP geometry and mesh model is shown in figure 5.1, it is composed
of 756 nodes and, consequently, 755 elements. The element size along elbow areas
is around 0.1 meters. For straight sections, elements length has been progressively
increased towards the next bend and reduced close to the bend (bias on both ends). This
strategy was used to reduce the global number of elements and to make de simulation
faster.
Figure 5.1: FEMAP mesh overview.
5.2 Pipe design data
This section presents all pipe parameters that were used in AutoPIPE original
simulations and in Abaqus.
5.2.1 Material
Material properties for all pipes are provided in table 5.1.
34
Table 5.1: Material Data - Steel.
Steel Parameters Value
Density [kg/m3] 7850
Thermal expansion coefficient [C−1] 1.17 x 10−5
Young's modulus [MPa] 207 000
Poisson's ratio 0.30
5.2.2 Pipe and coating data
Pipeline and coating data are presented in tables 5.2, 5.3 and 5.4 .
Table 5.2: Pipe Data - Spool and Pipeline.
Steel Grade Value [mm]
Nominal outer diameter 610.0
Corrosion allowance 3.0
Nominal wall thickness 15.9 / 19.05 (1)
Anticorrosive coating thickness 2.2 / 3.5 (2)
Concrete coating thickness 70.0
Notes:
(1) Thickness of 19.05 mm has been considered only for spool bends.
(2) 3.5 mm of coating is considered for line pipes without concrete.
Table 5.3: Pipe Data - Riser and Topside.
Steel Grade Value [mm]
Nominal outer diameter 610.0
Nominal wall thickness 19.05
Anticorrosive coating thickness 3.5
Concrete coating thickness N/A
35
Table 5.4: Density of materials
Material density Value [kg/m3]
Hydrotest fluid 1030
Operation fluid 92
Steel 7850
Anticorrosive coating 940
Concrete Weight coating 3050 (High Density)
5.2.3 Pressure and temperature data
Data in table 5.5 was used for AutoPIPE analysis and consequently for this
project.
Table 5.5: Pipe Data - Riser and Topside.
Parameter Value
Design pressure [barg] 96.5
Maximum design temperature [◦C] 71.0
Hydrotest pressure [barg] 101.3
Hydrotest fluid temperature [◦C] 12.2
5.2.4 Fitting data
The geometry of study is composed of a valve and two flanges that represents an
additional weight to the structure, their weight can be found in table 5.6. Their location
will be shown later on.
Table 5.6: Fitting weight data.
Fittings Weight [kg]
Flange 870
Valve 7000
36
5.3 Environmental data
5.3.1 Water depth and seawater density
For the spool and riser stress analysis, a water depth of 70 m/CD is used and a
seawater density is defined as 1026 kg/m3 .
5.3.2 Waves and currents
Extreme omni-directional wave and current data are presented in table 5.7 and
table 5.8, they are both used for stress analyses. This wave is considered to be applied
at MSL = CD + 4.8 m.
Table 5.7: Current data.
Current Velocity, Uc [m/s]
Depth [m] Return Period
1 year 10 years 100 years
MSL 1.44 1.61 1.78
-10.0 1.35 1.50 1.66
-20.0 1.32 1.47 1.63
-30.0 1.30 1.45 1.60
-40.0 1.27 1.42 1.56
-50.0 1.27 1.38 1.52
-60.0 1.18 1.32 1.45
-73.4 0.96 1.07 1.18
Table 5.8: Wave data.
Extreme Hmax [m] Extreme Thmax [s]
Return Period Return Period
1 year 10 years 100 years 1 year 10 years 100 years
11.9 15.47 19.06 11.21 12.81 14.25
Note that CD stands for Chart Datum which is level to which both tidal levels and water depths are
reduced. Generally it matches the lowest astronomical tide.
37
5.3.3 Hydrodynamic coefficients
Hydrodynamic coefficients that have been used in the original AutoPIPE calcula-
tions are shown in table 5.9. However, Abaqus is not yet capable of applying lift efforts
and for comparison objectives this factor was suppressed from AutoPIPE simulations.
Table 5.9: Hydrodynamic coefficients.
Hydrodynamic coefficients Drag Lift Inertia
Riser 1.2 0.0 3.29
Single pipe with concrete 0.9 0.9 3.29
5.3.4 Marine Growth
Marine growth profile used in AutoPIPE modeling approach is shown in table
5.10. Its density is considered as 1435 kg/m3.
Table 5.10: Marine growth profile.
Depth [m/CD] Radial Thickness
-20.0 to +4.8 100 mm
-30.0 to -20.0 Linear variation form 30 mm to 100 mm
-seabed to -30.0 Linear variation form 10 mm to 30 mm
5.3.5 Soil
Table 5.11 summarizes soil properties.
Table 5.11: Soil properties.
Parameters
Type of soil Sand/Gravel
Soil stiffness 1250 N/mm/m
Axial friction coefficient 0.49
Lateral friction coefficient Hydrotest / Operation 1.16 / 1.06
38
5.4 Interface Data
5.4.1 Pipeline Expansion
Table 5.12 presents pipeline expansion considered for AutoPIPE and Abaqus
analysis.
Table 5.12: Pipeline expansion.
Load Case Pipeline Expansion [m]
Hydrotest 0.11
Operation 1.38
5.4.2 Platform Movements
Table 5.13 resumes the imposed displacement at platform levels considered for
this study, considering 100 years return-period meteocean data. Those displacements
were applied on support levels since those structures are the connection between the
pipe and the platform.
Table 5.13: Platform displacements.
Element Elevation Extreme 100-year Displacements [mm]
Topsides + 21.50 m/CD 130.6
HOC + 14.00 m/CD 124.8
Support 1 + 7.30 m/CD 121.9
Support 2 - 5.50 m/CD 107.9
Support 3 - 19.50 m/CD 88.4
Support 4 - 27.60 m/CD 73.6
Support 5 - 35.50 m/CD 43.9
Support 6 - 43.50 m/CD 25.0
Support 7 - 53.50 m/CD 9.9
39
5.5 Load cases
Since AutoPIPE analyses very quickly lots of load case combinations, it was
needed to put in place a code to generate all Abaqus input files and to turn all simulations
one at a time. Therefore, four main input files were written and used to compare results
from AutoPIPE and Abaqus and to evaluate element type influence on result quality.
Those four files were:
• Hydrotest case with pipe elements only
• Hydrotest case with elbow and pipe elements
• Operation case with pipe elements only
• Operation case with elbow and pipe elements
Those files had general information concerning the model, for instance: the ge-
ometry, material, element and section definition, wall thickness, sea level, soil properties
among others.
Later, a VBA code was used to manipulate strings and write all steps and load
definition in accordance with what was presented by AutoPIPE model for both Hy-
drotest and Operation conditions. Input information for each case was transposed from
AutoPIPE into Excel which later fed the VBA code. This code created the input file
for all unitary load cases shown in table 5.14 and for their combination, shown in table
5.15. To allow all cases to turn one at a time, a file .bat was used to fed Abaqus.
40
Table 5.14: Unitary Load Cases.
Load Case Corresponding load
G Weight with marine growth and buoyancy
T1 Pipeline expansion and thermal expansion of the riser and spool
P1 Pressure (Depending on studied case hydrotest pressure or design pressure)
U1 Support displacement following +X (operating displacement) (1)
U2 Support displacement following -X (operating displacement) (1)
U3 Support displacement following +Y (operating displacement) (1)
U4 Support displacement following -Y (operating displacement) (1)
U6 Wave and current following +Y (100 Y in Operating, 10 Y in Hydrotest) (1)
U7 Wave and current following +X (100 Y in Operating, 10 Y in Hydrotest) (1)
U8 Wave and current following -X (100 Y in Operating, 10 Y in Hydrotest) (1)
U9 Wave and current following -Y (100 Y in Operating, 10 Y in Hydrotest) (1)
Notes:
(1) Sign convention as per Figure 5.5.
Table 5.15: Combined Load Cases.
Combination Name Corresponding ULC Combination Name Corresponding ULC
GT1 G+T1 GT1P1U13 G+T1+P1+U4+U9
GT1P1 G+T1+P1 GT1P1U14 G+T1+P1+U1+U9
GT1P1U1 G+T1+P1+U1 GT1P1U15 G+T1+P1+U2+U6
GT1P1U2 G+T1+P1+U2 GT1P1U16 G+T1+P1+U3+U7
GT1P1U3 G+T1+P1+U3 GT1P1U17 G+T1+P1+U4+U8
GT1P1U4 G+T1+P1+U4 GT1P1U18 G+T1+P1+U1+U8
GT1P1U6 G+T1+P1+U6 GT1P1U19 G+T1+P1+U2+U9
GT1P1U7 G+T1+P1+U7 GT1P1U20 G+T1+P1+U3+U6
GT1P1U8 G+T1+P1+U8 GT1P1U21 G+T1+P1+U4+U7
GT1P1U9 G+T1+P1+U9 GT1P1U22 G+T1+P1+U1+U7
GT1P1U10 G+T1+P1+U1+U6 GT1P1U23 G+T1+P1+U2+U8
GT1P1U11 G+T1+P1+U2+U7 GT1P1U24 G+T1+P1+U3+U9
GT1P1U12 G+T1+P1+U3+U8 GT1P1U25 G+T1+P1+U4+U6
41
5.6 Modeling choices
5.6.1 Soil
Soil was chosen to be modeled as a rigid surface. An interaction between this
surface and all pipes nodes was set so the pipe would not be able to pierce the soil.
Friction was defined in Abaqus according to table 5.3.5, page 38, meaning that axial
friction coefficient was determined as 0.49 and lateral as 1.16 for hydrotest and 1.06 for
operation.
However, this represents a difference between software modeling approaches.
AutoPIPE only accepts one friction coefficient per soil element. This means that the
user needs to predict in which direction (axial or lateral) each pipe element is more likely
to move and define consequently its friction coefficient. In this case, Abaqus seems to
have a more realistic approach.
Figure 5.2 bellow illustrates the soil in green and the pipe structure in red, its
possible to verify that the pipe is not capable of passing thought the soil.
Figure 5.2: Abaqus modeled geometry: supports, soil and pipe structure.
42
5.6.2 Riser guides/supports
Riser guides and supports were modeled using tube-to-tube contact elements (
named ITT elements). This type of element is used model the interaction between two
tubes where one tube lies inside the other or between two tubes that lie next to each
other [Ref. [9]]. As stated before in previous sections, along riser length seven supports
were modeled according to AutoPIPE model.
For the study case, those supports where modeled as pipe-in-pipe structure and
one of the parameters to model such interaction between tubes is contact clearance.
Clearance is defined as the distance between the outer pipe inner diameter and the inner
pipe outer diameter, as shown in Figure 5.3 taken from [Ref. [10]].
Figure 5.3: Clearance definition for tube-to-tube contact elements.
On the original model made in AutoPIPE, supports have no clearance however,
to make convergence easier on Abaqus it was set as 2 mm in this software. Figure 5.4
shows a section of Abaqus model and illustrates tube-to-tube contact modeling.
A modeling difference relies on the fact that, for AutoPIPE, those supports are
defined as points that are blocked in their local horizontal plane. On the other hand,
Abaqus modeling has a pipe-in-pipe approach which is more realistic from interaction
point of view, but blocks movement on global horizontal plane.
A last parameter regarding support modeling is its rigidity. On AutoPIPE model
supports are defined as rigid and according to its documentation that means that the
Young’s modulus of this element is a thousand times the Young’s modulus of the defined
material. Therefore, the same modeling choice was made for Abaqus model, although
this software could provide a more realistic analysis of support behavior if it was desired.
Figure 5.2 in the above section shows the pipe geometry in red, its supports in
blue and the soil in green. This image is provided in order to give the reader a visual
reference on the model geometry.
43
Figure 5.4: Support geometry made for Abaqus model with tube-to-tube contact
elements.
5.6.3 Valve and flanges
The original geometry is composed by a valve and two flanges that should be
correctly modeled on Abaqus. Both components are modeled as an additional weight
on AutoPIPE, however valves are considered as line loads and flanges as concentrated
forces. Both structures can be seen and located on the geometry sketch in figure 5.5.
Another detail concerns those elements rigidity. According to [Ref. [11]], al-
though flanges are more rigid than a regular pipe, its real rigidity does not affect analysis
results to any significant extent. Thus, AutoPIPE uses regular pipe rigidity for flange
elements and the same approach is used on Abaqus.
On the other hand, according to [Ref. [11]] , valves rigidity influences results and
are consequently considered on AutoPIPE and, in this software, its Young’s modulus is
a thousand times the Young’s modulus of the regular pipe. Logically the same approach
was used for Abaqus modeling. It has to be noted that this valve is located on the
topside area where loads are limited (no waves).
5.6.4 Concrete, anti-corrosion and marine growth
To take into account those three terms on Abaqus model, their weight was incor-
porated as equivalent density applied to the simple pipe itself. Besides, this parameter
also influenced hydrodynamic efforts, the total outer diameter related to the additional
external coating was used on the Aqua module for buoyancy, drag and inertia forces.
44
Figure 5.5: AutoPIPE loads and BC choices for the main case of study.
45
5.6.5 Load sequence
AutoPIPE has a specific load sequence that was previously mentioned. It is
recalled that the software starts by applying gravity case, composed by weight, buoyancy
and content loads. Then from this case deformed shape of case it applies thermal
case and, on top of that, pressure case. Finally, it applies imposed displacement and
hydrodynamic forces together. Creating combined cases as defined on section 5.5.
On the other hand, Abaqus load sequence is defined by steps. Consequently,
for this model steps were set in the same order as AutoPIPE, but due to convergence
difficulty imposed displacement and waves were separated in two different steps. Step
sequence, loads and time can be seen in table 5.16 below. This is not truly the same
modeling approach from AutoPIPE, but the following sections will prove it to be a good
one.
To model a complete wave in Abaqus Aqua module the total simulation time
should be equal to or a multiple of the wave period, because in Abaqus waves starts to
propagate from the beginning of the simulation. Therefore, in this study, the step time
of all steps that come before wave loading was defined as very small in comparison with
the last one. This last step is used for wave propagation, adding drag and inertia efforts
into the simulation.
Table 5.16: Abaqus step sequence.
Case
condition
Step
number
Total
timeLoads
HY/OP 1 0.01 Self weight, content weight and buoyancy
HY/OP 2 0.01Loads Step 1 + thermal expansion and
internal pressure
OP 3 0.01 Loads Step 2 + change of temperature
HY/OP 3/4 0.01 Loads Step 2/3 + platform displacement
HY/OP 4/512.78/
14.21Loads Step 3/4 + wave and current loading
46
5.7 Results
To allow the post processing of all load cases for both Hydrotest and Operation
conditions the best approach would be the implementation of python codes that interact
with Abaqus .odb/.dat files and extracts the desired information. However, due to this
report deadline the mentioned code has not yet been implemented and this section results
were extracted using only recorded macros on Abaqus.
Due to the enormous amount of results, only case GP1T1U20-HY will be pre-
sented but all others should have given results with around the same quality and will
likely be correctly validated by comparison with AutoPIPE. This case was randomly cho-
sen among those that combine all types efforts and as described in section 5.5, table
5.15, case GP1T1U20 is the junction of cases G, T1, P1, U3 and U6.
5.7.1 Displacement
Displacement results for case GP1T1U20 in hydrotest conditions can be seen in
figures 5.6 and 5.7 that show the displacement along the pipe length in X and Y direction
respectively. In those figures the blue area corresponds to the topside part, the green
area stands for the riser length, yellow part for the spool section and the amber for the
pipeline. Besides, the dotted lines show the location of the hang off clamp and the other
seven guides. Continuous lines correspond to results from AutoPIPE, Abaqus with pipe
elements and with both elbow and pipe elements.
The most significant difference seen in figure 5.6 between Abaqus and AutoPIPE
is located at the end of the spool and along the pipeline. This difference is believed
to be related to the friction assessment on AutoPIPE. This software can only take one
friction factor per element, this means that a pipeline would have either its true axial
friction or its lateral friction, on the other hand Abaqus can set two different frictions,
one in each direction. AutoPIPE friction was chosen in accordance with the expected
movement of the element, therefore it was defined an axial friction coefficient of 0.49
for the pipeline.
47
Figure 5.6: X displacement results - Case GP1T1U20-HY.
Figure 5.7: Y displacement results - Case GP1T1U20-HY.
48
In figure 5.7 there is also a divergence along the pipeline. It is believed that
Abaqus represents a closer situation to the reality since in this area this graphic stands
for the lateral movement of the pipeline and the friction coefficient should be 1.16 and
not 0.49 as it was defined in AutoPIPE. It is believed that, if pipeline lateral movement
was restricted in AutoPIPE, axial displacement would increase and be compatible with
Abaqus.
On the other hand, the start of the topside also shows incompatible results
between Abaqus and AutoPIPE. This time, although the difference is minor, results
coming from Abaqus seem more reliable. This because for this specific load case the
start end of the topside has an imposed displacement on +Y as described in section
5.5, page 40. Therefore, the displacement on this node should be equal to the imposed
one, this result is as expected for both Abaqus simulations, but AutoPIPE presents a
displacement that is inferior to the one applied.
Spool deformed shapes from all three simulation methods may be seen in figures
5.8, 5.9 and 5.10. In figure 5.8, AutoPIPE deformed shape is represented in red and
the unformed geometry in black. For the other two images, extracted from Abaqus,
deformed shape is rainbow colored and unformed is, once again, represented in black.
Figure 5.8: AutoPIPE top view deformed shape, 25 scale factor - Case GP1T1U20-
HY.
49
Figure 5.9: Abaqus Pipe top view deformed shape, 25 scale factor - Case GP1T1U20-
HY.
Figure 5.10: Abaqus Elbow-Pipe top view deformed shape, 25 scale factor - Case
GP1T1U20-HY.
Those images show deformed shapes that are very close to each other in shape
and magnitude as also indicated by figures 5.6 and 5.7. Although little divergences
show that Abaqus simulations using elbow-pipe elements may give closer results to the
50
ones from AutoPIPE, this divergence is so tiny, as endorsed by figures 5.8 to 5.10, that
simulations using only pipe elements may be a better choice from displacement point of
view, since it takes around 1/10 of the total time of an elbow-pipe simulation.
5.7.2 Reaction force and reaction moment
The following tables, table 5.17 and 5.18, provide the reaction force on the
supports in the global axis for all three models. Results seem a little more compatible
for AutoPIPE and Abaqus Elbow-Pipe simulation. This was expected since simple cases
from section 4.1, page 16, had already given good results for reaction forces when
Abaqus Aqua is used.
Table 5.19 provides the reaction moment on the hang off clamp in the global
axis for all three models. In section 4.1 indicated that reaction moment wouldn’t have
a great divergence between FEA methods, however it is not the reality. Results provide
around the same magnitude, but components seem a little off and further investigations
are recommended.
Table 5.17: AutoPIPE reaction force on pipe supports - Case GP1T1U20-HY.
RFX [kN ] RFY [kN ] RFZ [kN ]
HOC 8.9 31.8 262.4
+7.3m/CD 8.0 132.4 0.8
-5.5m/CD 5.1 122.1 0.5
-19.5m/CD 2.8 54.7 0.2
-27.6m/CD 3.1 50.1 0.3
-43.5m/CD 5.9 52.1 0.5
-53.5m/CD 3.1 0.9 0.3
-63.5m/CD 11.5 33.6 34.0
51
Table 5.18: Abaqus reaction force on pipe supports - Case GP1T1U20-HY.
Abaqus Elbow/Pipe Abaqus Pipe
RFX [kN ] RFY [kN ] RFZ [kN ] RFX [kN ] RFY [kN ] RFZ [kN ]
HOC 8.8 31.3 260.4 8.8 31.8 262.4
+7.3m/CD 7.1 129.0 0.7 7.1 128.9 0.7
-5.5m/CD 4.5 123.3 0.4 4.5 123.4 0.4
-19.5m/CD 2.6 54.7 0.2 2.6 54.6 0.2
-27.6m/CD 3.3 50.5 0.3 3.3 50.6 0.3
-43.5m/CD 5.2 52.2 0.5 4.5 51.4 0.4
-53.5m/CD 3.7 3.3 0.3 2.1 5.8 0.2
-63.5m/CD 17.1 32.0 33.4 19.9 29.0 30.7
Table 5.19: Reaction moment on the hang off clamp - Case GP1T1U20-HY.
MX [kN.m] MY [kN.m] MZ [kN.m] MR[kN.m]
AutoPIPE 3.877 8.027 22.955 24.625
Abaqus Pipe31 0.443 18.110 26.480 32.658
Abaqus Elbow31 10.605 7.002 31.962 34.406
5.7.3 Section force and section moment
Section forces and moments were also subject of comparison between software.
In order to compare those variables, their value in local axis was rather used. Therefore
it was important to define a beam direction on Abaqus that corresponds to the one from
the base software. For AutoPIPE, pipe main axis is defined by the points order. As an
example, if there is a point called A00 that is linked to one A01, the element main axis
is defined from A00 to A01, as illustrated by figure 5.11.
AutoPIPE gives 6 outputs for section forces, three in global axis: x, y, z; and other
three in local axis: axial, shear in local direction 1 and 2. But one may be aware that,
for axial section force this software does not consider capped pressure loads, meaning
that it actually presents the effective axial section force.
52
Figure 5.11: AutoPIPE local axis definition.
As stated in section 4.1.3, page 18, Abaqus has an output called SF (section
force) and another called SM (section moment), both being measured in local axis.
By default, Abaqus axial section force, SF1, takes pressure load into consideration, but
fortunately the software has also an output called ESF1 (effective axial section force) that
simply recalculates axial force without capped pressure effect. Therefore, comparison
between section force in axial direction should be made between AutoPIPE axial force
and Abaqus EFS1 output, as per section 4.1.3.
Other section forces and section moments are calculated in the same way in both
software and, consequently, Abaqus output SF and SM can be used. Magnitude results
from both, outputs, may be seen in images 5.12 and 5.13 bellow.
Figure 5.12: Effective axial force results in local axis - Case GP1T1U20-HY.
53
Figure 5.13: Bending moment about the local 1-axis - Case GP1T1U20-HY.
From those graphics it is possible to conclude:
• Topside and riser:
– results are in very good agreement.
• Spool and pipeline:
– Effective axial force shows some disagreement specially on the pipeline area
which could be related to friction assessment. This was not the case in the
simple cases from section 4.1, page 16.
– Section moment is overestimated with Pipe31 which can be explained by the
stiffness of those elements. However, for elbow model a better result was
expected since it takes it account bend flexibility.
54
5.7.4 Stress
As for the section force, both software have different methods for calculating
stress and consequently some adjustments must be done in order to compare the correct
outputs.
As previously mentioned, AutoPIPE integrates different international standards
for pipe design and, thus, the stresses and bend influence are calculated according to
the user chosen code. For the study case ASME B31-8-2003 [Ref.[4]] code was used.
This standard provides stress intensification and flexibility factors to analyse elbows and
its methods are described in appendix B.
When it comes to Abaqus, correct outputs choices must be taken in order to
have comparable results with AutoPIPE. Abaqus has integrated outputs for the von
Mises Stress which is calculated according to analytical formula given below:
σvm =
√1
2[(σ11 − σ22)2 + (σ22 − σ33)2 + (σ33 − σ11)2 + 6(σ2
12 + σ223 + σ2
31) (5.1)
However, results from AutoPIPE are based on standards and in order to compare
them, Abaqus results were reassessed based on ASME standard.
The following three graphics present the von Mises stress, axial stress and hoop
stress for all three simulations.
Figure 5.14: Von Mises stress - Case GP1T1U20-HY.
55
Figure 5.15: Axial stress - Case GP1T1U20-HY.
Figure 5.16: Hoop stress - Case GP1T1U20-HY.
It can be seen that:
• Von Mises:
– AutoPIPE stress profile generally matches the one from Abaqus Pipe sim-
ulation. However, at bend locations Abaqus Pipe elements tend to super
56
estimate stresses, which could be seen as conservative behaviour.
– Abaqus Elbow-Pipe simulation show stresses that have less significant differ-
ence when compared to AutoPIPE. However, considerable discrepancies are
found at the end of the spool which could be better understood with further
spool studies.
• Axial Stress:
– All three curves have similar profiles although offset off 40 MPa can be seen
in some specific zones.
– Those peaks follow the tendency of section moment results in section 5.7.3,
page 52 and were expected to be seen at axial stresses.
• Hoop Stress:
– The two valleys seen in the spool zone (yellow) correspond to the spool bend
which are modelled with a higher thickness than the rest of this element.
Therefore a lower hoop stress was expected.
– All three similations show a hoop stress profile that generally matches each
other.
In conclusion, stress profiles are pretty much compatible between all three simu-
lations, however, Elbow elements seem to be more suitable to bend modelling based on
the better superposition of von Mises stress it has shown.
5.8 Methodology summary
Based on the previous sections a preliminary methodology for the analysis of
spool-riser system using Abaqus is drafted below.
• Geometry:
– Created using FEMAP based on isometric drawings.
– Mesh generated in FEMAP.
57
– Careful element and node numbering shall be done in FEMAP to avoid
Abaqus post-processing difficulties.
– Geometry is then exported from FEMAP as .inp file to be used in Abaqus.
• Abaqus model:
– Sections should be defined either as Thick pipe, Pipe or eventually Elbow.
– Material shall be defined and assigned to sections.
– Riser guides shall be modelled with ITT elements in accordance with its
isometric drawings dimensions and emplacement.
– Soil is defined as rigid surface with axial and lateral frictions.
– Valve is ideally modelled as line loads with increased rigidity.
– Flanges are to be defined as concentrated load with no extra rigidity.
– Buoyancy is modelled as distributed load by the use of Abaqus Aqua module.
– Waves and currents are modelled with Aqua module. Note that waves with
0◦ phase have its bottom at the origin.
– Drag and Inertia are modelled as distributed loads by the use of Aqua module.
• Abaqus step sequence:
– All steps together must lead to a total simulation time equal to a wave period
and initial steps must be shorter that the one destined to waves, because in
Abaqus waves start to propagate from the beginning of the simulation. The
following table provides the recommended step sequence based on the waves
analysed during this study.
• Load case definition - VBA:
– Complete the input sheet for each load case for HY an OP conditions (see
appendix C );
– Click on the blue button and all the .inp files will be automatically generated.
– This pre-processing toll also generates a .bat to run all .inp files one at a
time.
58
Table 5.20: Abaqus step sequence.
Case
condition
Step
number
Total
timeLoads
HY/OP 1 0.01 Self weight, content weight and buoyancy
HY/OP 2 0.01Loads Step 1 + thermal expansion and
internal pressure
OP 3 0.01 Loads Step 2 + change of temperature
HY/OP 3/4 0.01 Loads Step 2/3 + platform displacement
HY/OP 4/512.78/
14.21Loads Step 3/4 + wave and current loading
• Run Abaqus .inp files:
– Use of the .bat generated by VBA macro.
– This file can be schedule to run in specific time and looks like the one bellow.
Figure 5.17: Example of .bat file to run Abaqus simulations in a row.
• Post-processing VBA and Python:
– This code is being developed. Ideally the user will fill a form made in Ex-
cel/VBA or python thinker and would get a file with all the worse cases results
of all required outputs.
59
– An example of Abaqus python scripting that was used in this report can be
seen in appendix D. Post-processing automatic scripts shall be inspired in
this one.
5.9 Software comparison
Based on the different studies performed, the advantages and disadvantages of
Abaqus compared to AutoPIPE are provided.
• Advantages:
– Better mesh and element control;
– Possible to use NLGeom if desired;
– Possible to consider plastic behaviour if desired;
– Possible to model internal pressure/temperature on the internal wall;
– Better control of load cases and load sequence;
– More realistic friction assessment;
– Better assessment of imposed displacement;
– Support modelled via PIP which it more realistic.
• Disadvantages:
– Not possible to directly model lift efforts. To do so a subroutine should be
implemented/programmed;
– Takes more time to run;
– Input file requires careful manipulation because once node quantity is elevated
it can get complex;
– Takes more time for model creation;
– Knowledge of integration point location and quantity is required for right and
quick post-processing;
– When post-processing several different load cases, it is recommended and
automatized tool to make this task easier.
60
5.10 Conclusion
The goal of this second phase was to compare results incoming from Abaqus
and AutoPIPE when considering a Spool-Riser geometry. To do so, although a first
comparison between elements type had already taken place and indicated that elbow
elements were a better choice, this factor was once again studied when modelling the
spool-riser system and two models were compared:
• with only pipe elements
• with pipe elements on the straight parts and the elbow elements on the bent parts
After result post-processing for displacement, reaction force and moments, they
were all validated as sufficiently good matches between both software approaches. How-
ever, section force, section moment and stress results were less compatible between
software. It is believed that is has mostly to do with the soil modelling that is not the
exact same in Abaqus and AutoPIPE. To better evaluate it, new simpler cases like the
ones from chapter 4 should be done focusing on soil behaviour.
Although Pipe simulations had shown a good stress profile, it had sometimes an
offset around 40MPa that was not irrelevant for the study case. On the other hand,
elbow-pipe simulations better compatibility to AutoPIPE simulations. For now it seems
reasonable to affirm that Spools could be modelled using pipe elements for the straight
parts and Elbow elements with high Fourier number for bend zones.
To continue such analysis it should be first verified that other load cases men-
tioned in section 5.5 also lead to the same conclusions as case GP1T1U20-HY.
However, it is recommended a longer comparison between Abaqus and AutoPIPE
with simple cases focusing on soil behaviour, lift analysis, and bend stresses.
61
Chapter 6
Conclusion
The objective of this project was to define the methods to model and analyse a
spool-riser system with Abaqus software and to prove its feasibility. A first step was the
discovery of each of the studied offshore structures, followed up by a phase of simple
cases study under AutoPIPE and Abaqus to calibrate all modelling choices. This first step
led to the assumption that both Pipe and Elbow element families would be sufficiently
good to re-model AutoPIPE simulation.
Once the main modelling choices had been made based on the simple cases,
the geometry was defined with FEMAP software, exported into .inp format and, finally,
manipulated with a VBA macro to generate all load cases. Abaqus cases were ran to
allow a comparison with AutoPIPE and pyhton scripts were written for result extraction.
It was confirmed that both Pipe and Elbow had quite good correspondence with
AutoPIPE simulation. Finally, although the model behaviour is globally the same, the
comparison highlighted significant discrepancies that shall be later investigated, partic-
ularly for the parts in contact with the soil. This can be done by performing additional
tests on simple cases (e.g. Z-shape spool in contact with the soil).
Therefore, it is necessary to perform additional tests on the models prior to use
Abaqus on a complete project. Furthermore, given the relative complexity of the model
definition and elevated computational time it is recommended to use Abaqus only for
spool optimization, unless pre- and post-processing tools are developed and validated in
the future.
62
Bibliography
[1] BAI, Q. B. E. Y., Subsea Pipeline Design, Analysis, and Installation. Gulf Profes-
sional Publishing, 2014.
[2] LYSSAND, T., “Design of Subsea Spools: Investigating the Effect of Spool Shape”,
University of Stavanger/Subsea7 Norway, , 2015.
[3] NOGUEIRA, G., “Avaliacao da fadiga em jumpers rıgidos devido a vibracoes in line
produzidas pelo fenomeno do VIV”, Escola Politecnica, UFRJ, , 2015.
[4] ASME, Gas Transmission and Distribution piping systems, Report ASME B31.8-
2003, The American Society of Mechanical Engineers, 2003.
[5] QUEIROZ, J. O., “Analise de estabilidade de dutos rıgidos submarinos sujeitos a
acao de ondas e correntes marinhas”, Escola Politecnica, UFRJ, , 2011.
[6] SPARKS, C., “Comportement mecanique des tuyaux: influence de la traction, de la
pression et du poids lineique”, Oil & Gas Science and Technology, v. 38, pp. 475–
490, 1983.
[7] BRYAN, B. J., “STATIC ANALYSIS OF A PIPING SYSTEM WITH ELBOWS”,
ASME PVP Conference, pp. 02–03, 1994.
[8] MEHAUTE, B. L., An Introduction to Hydrodynamics & Water Waves. Springer,
1976.
[9] SIMULIA, “Abaqus 6.14 Analysis User’s Guide”, http://ivt-
abaqusdoc.ivt.ntnu.no:2080/v6.14/books/usb/default.htm, 2014, (Acesso em
23 Maio 2020).
[10] UNKNOWN, “Abaqus Tube-to-Tube modeling”, http://www.lhe.no/.
63
[11] BENTLEY, “Bentley Technical Support Group”,
https://communities.bentley.com/products, 2020, (Acesso em 23 Maio 2020).
64
Appendix A
Wave theories limits
Figure A.1: Wave theories limits [Ref. [8]].
65
Appendix B
ASME B31.8 Stress
B.1 Hoop Stress
Hoop stress is calculated as shown in equation below:
Sh =(Pi − Pe).D
2t≤ Sy.F1.T (B.1)
In which:
Pi = Internal design pressure
Pe = External pressure
D = Nominal outside diameter of pipe
F1 = Hoop stress design factor (=0.5 in operation, 0.9 in hydrotest as per ASME B31.8,
[Ref. [4]]
Sy = Specified minimum yield strength (SMYS)
T = Temperature derating factor
t = Pipe wall thickness
B.2 Longitudinal Stress
The Longitudinal stress (SL) is calculated as below:
| SL |≤ Sy.F2.T (B.2)
In which:
66
SL = max(σa + σb;σa − σb)
σb = Resultant bending stress =
√(iiMi)2+ioMo)2
Z
Mi = In-plane bending moment
Mo = Out-of-plane bending moment
ii = In-plane stress intensification factor for bends, see bellow image B.1, shall not be
less than unity
io = Out-of-plane stress intensification factor for bends, see bellow image B.1, shall not
be less than unity
Z = Pipe Section Modulus
σa = Axial stress(positive tensile or negative compressive) = Faxl
A
Faxl = Axial force
A = Pipe section area
F2 = Longitudinal stress design factor (=0.8)
Figure B.1: ASME B31.8 SIF definition [Ref. [4]]
B.3 Combined Stress
To comply with B31.8 requirement, the combined stress formula shall be checked
as follows:
√S2h − SLSh + S2
L − 3S2t ≤ Sy.F3 (B.3)
Sh = Maximum hoop stress
St = Torsion stress
F3 = Combined stress design factor (=0.9)
67
Sy = SMYS (including temperature de-rating as per DNV OS F101)
68
Appendix C
Pre-processing sheet
69
Figure C.1: VBA pre-processing sheet.
70
Appendix D
Python script for post-processing
71
72
73
Figure D.1: Python preliminary script for post-processing.
74
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