overview of design codes for offshore fixed structures · 2017. 11. 20. · outline part 1 – api...

Overview of Design Codes for Offshore Fixed Structures

Presentation at 離岸風機研討會

Albert Ku, Dec/5th/2017, Taipei, Taiwan

OutlinePart 1 – API RP2A

–Background of API RP 2A, Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms

–Historical work in the development of API RP 2A-LRFD 1st

edition (1993)–Development of API RP 2A-LRFD 2nd edition

• Progress/status of API RP 2A-LRFD 2nd edition• API Task Group 19 latest work/AKADEME project• Technical Alignment of API/ISO

Part 2 – API ductility requirement and seismic design provisions

History of API RP 2AOTC 20831 (2010)

Status of RP 2 Series and ISO Alignment

LRFDtask-group

Younan (2014)

Useful Background Papers1. A Series of Late 1980’s Papers on API 2A-LRFD 1st Edition2. API Offshore Structure Standards: Changing Times, OTC 2008, D. Wisch, A.

Mangiavacchi3. RP 2GEO: The New API Recommended Practice for Geotechnical Engineering,

OTC 2010, P. Jeanjean4. API Offshore Standards – Underlying Risk Values and Correlation with ISO,

OTC 2012, D. Wisch, H. Banon, D. Knoll, S. Verret

5. Development of API RP-2A LRFD 2nd Edition, Offshore Structural Reliability Conference 2014, A. Ku, F. Zwerneman

6. Background to New API Fatigue Provisions, OTC 2010, P. Marshall, J. Bucknell

7. New API RP2A Tubular Joint Strength Design Provisions, OTC 2010, D. Pecknold

8. API RP 2EQ – Seismic Design Procedure & Criteria for Offshore Structures, OTC 2010, A. Younan, F. Puskar

9. ISO 19902 Tubular Members Including Damaged and Grouted Members, OMAE 2011, P. Frieze

10. LRFD Calibration of Load Factors for Extreme Storm Loading in Malaysian Waters, Journal of Marine Engineering & Technology, 2014, N. Nicols, R. Khan

Background between API RP-2A LRFD and ISO 19902

API RP 2A-LRFD 1st edition (1993) Used worldwide but not in the US ISO 19902:2007 largely based on API RP 2A-LRFD

1st edition No maintenance of API RP 2A-LRFD between

1993 and 2012 API RP 2A-LRFD 1st edition retracted in 2012

due to lack of maintenance Current effort focuses on adopting ISO 19902

back to US practice (API 2A-LRFD 2nd edition)

Mapping of RP 2 Series and ISO

RP 2A 2nd

LRFD ISO 19902RP 2A-LRFDISO 19902

OTC 20831 (2010), except red-texted boxes

ISO 19901-9

Co-existing WSD 22nd and LRFD 2nd Edition

OTC 20831 (2010)

Co-existing of WSD and LRFDIn the foreseeable future

Correspondence between and Pf

)( fP Is Gaussian probability distribution function

API RP 2A-LRFD 1st Edition Calibration Methodology

Every designed structural member (beam, column, brace, etc.) has a probability of failure (pf). This pfcan also be expressed as reliability index ()

By carefully selecting load and resistance factors it is possible to achieve

Averaged to be similar as implied in WSD

Minimized spread of

By applying simple procedure, can be calculated for various failure modes

Observe from figure to the right, the “averaged” for each of the 2 curves are similar, but spread of LRFD curve is smaller

OTC 5699 (1988)

Comparison of of Different Failure Modes and between WSD/LRFD

Notes:1) Averages taken over important range of W/G

of 2 to 40 except for piles, where W/G is 0.6 to 2. G = L + D and L = 3D

2) by advanced FOSM, Ref: Moses, 81-22

3) Design FormulasWSD: Rn > SF (L + D)

4/3 Rn > SF (L + D + W)where SF and Rn from API RP2A, 12th edition

LRFD: Rn > 1.3D + 1.5LRn > 1.1D + 1.1L + 1. 35W

where Rn is formula in (81-22)OTC 5699 (1988)

Load Factors in API LRFD & ISO 19902 vs. WSD

WSD LRFD

Operating Condition

0.6R = D + L + WR = 1.67D + 1.67L + 1.67W (normalized)

0.95R = 1.3D + 1.5L + 1.22WR = 1.37D + 1.58L + 1.28W (normalized)

Storm Condition

0.8R = D + L + WR = 1.25D + 1.25L + 1.25W (normalized)

0.95R = 1.1D + 1.1L + 1.35WR = 1.16D + 1.16L + 1.42W (normalized)

Note:-R denotes resistance or structural capacity.-D denotes dead loads, L denotes live loads, and W denotes environmental loads due to wind, wave and currents.-Assumes nominal resistance factor of 0.95 for LRFD.

OTC 5882

Member UC Comparison - Platform A Member UC Comparison - Platform B

Member UC Comparison - Platform C Joint UC Comparison - Platform C

Curve on top means more conservative

WSDLRFD

1980’s Case Studies

Adoption of ISO 19902 for API RP 2A-LRFD 2nd

Edition with Following Modifications

Specific target reliability numbers in the Informative annex removed

Design criteria tied to API RP 2MET and robustness level ultimate strength analysis criteria for Gulf of Mexico (GoM)

Tubular Joint Checks Joint check equations aligned with WSD 22nd edition

Replaced ISO minimum joint requirement with API 50% capacity requirement

Foundation section tied in with API RP 2GEO and resistance factors provided for both pile and shallow foundations

Tied in with API RP 2SIM and API RP 2MOP

Other modifications related to API/ISO technical alignments

AKADEME Project

Key Contractors: API Keystone Atkins Digre Energo McDermott (OFD Engineering) Experia

Funded by API to prepare the 2nd edition of RP 2A-LRFD and to assess the consistency of member and joint unity checks (UCs) based on API RP 2A-WSD 21st edition and ISO 19902 using case study platforms

Undertaken by a group of contractors with API Task Group 19 supervision

Voluntary works performed by McDermott and DNV (BSEE project) also included

Case Study PlatformsNo. Description Water

DepthLocation Exposure

CategoryAnalysis Company

1 4-Leg 274 ft Confidential - McDermott/OFD(volunteer work)

2 8-Leg 360 ft Confidential - DNV(BSEE project)

3 Caisson 45 ft GoM L-3 Energo

4 Tripod 360 ft WesternGoM

L-2 Atkins

5 4-Leg 300 ft Central GoM L-1 McDermott/OFD

6 8-Leg 275 ft WesternGoM

L-2 Keystone

Case Study Platform 3

Platform 3 Member and Joint UC Comparisons

Effect of Hydrostatic Pressure on Member UCs -Platform 4

(a) With Hydrostatic pressure (b) Without Hydrostatic Pressure

Effect of Hydrostatic Pressure on Member UCs - Platform 6

(a) With Hydrostatic pressure (b) Without Hydrostatic Pressure

Discussions of Case Study Results Current case study results show higher consistency between WSD and

LRFD member UCs than 1980’s case studies

Overall, member and joint UCs from ISO 19902 and API RP 2A-WSD are consistent. Minimum joint strength requirements are not included in this comparison

Member and joint UCs from ISO 19902 are slightly higher than those from API RP 2A-WSD for platforms dominated by environmental loading (vs. gravity loading)

Member UC comparisons between ISO and API show more scatter with hydrostatic pressure than excluding hydrostatic pressure

New ISO 19902 Proposed Changes

Paul Frieze (PAFA Engineering) investigated Fred Moses’ earlier work and proposed: Gravity load factor reduced from 1.1 to 1.0 when combined with

environmental loads

Partial resistance factor for compression reduced from 1.18 to 1.10

AKADEME Study Platform 4 L-2 structure

3 leg , 3 pile platform

Western GoM

Pile penetration = 220 ft (B & C), 265 ft (A)

Pile diameter = 48 inches

2 conductors (1 inside pile A)

Normally consolidated to slightly overconsolidatedmarine clays

Metocean Parameters

Wave Height = 63 ft

Wave Period = 12.4 seconds

Surface Current Speed = 1.8 knots

Bottom of Current Profile = 200 ft

Wind Speed (1 hr@10m) = 70 knots

Tide and Surge = 2.5 ftAPI AKADEME Proposed ISO Load and Resistance Factors 27

Platform 4 – All Jacket MembersUC Comparison (Storm, 0 Degree)

API AKADEME Proposed ISO Load and Resistance Factors 28

Original Partial FactorsRevised Gravity Load Factor Only

R = ISO/APIμR = 1.015σR = 0.149COVR = 15%N = 239



Revised Compression Resistance Factor Only

Revised Load & Resistance Factors


Please note:μR = Mean ValueσR = Standard DeviationCOVR = Coefficient of VariationN = Sample Size

ISO/API Technical Alignments

Hydrostatic Checks

Conical Transition

Pile Sleeve Grout Equation

Energo Engineering, “AKADEME Project – API RP 2A vs. ISO 19902 Member and Joint UC Comparisons”, API AKADEME Project Report, 2016.

Axial Compression with Hydrostatic Pressure

API WSD (4) = ISO (1)




API vs. ISO Hydrostatic Code Calibration: Axial Compression and Hydrostatic Pressure

Axial Tension with Hydrostatic Pressure

API vs ISO Hydrostatic Code Calibration: Elastic Local Buckling under Hydrostatic Pressure

API vs ISO Hydrostatic Code Calibration: Inelastic Local Buckling under Hydrostatic Pressure

ISO 19902 Combined Axial and Capped-end Stress Expressions

Marine vs. Rational BuoyancyMarine Buoyancy Rational Buoyancy

Fbuoy

Fbuoy

conservative estimate of local hydrostatic stresses ( = 0.5, fixed end restraints)

more accurate, direct inclusion of local hydrostatic stresses

Conclusions – Hydrostatic Pressure

API RP 2A-WSD 21st edition and ISO 19902:2007 1st edition have identical code check equations when members are in axial compression with hydrostatic pressure.

For members in axial tension with hydrostatic pressure, the code check equations are different. API RP 2A-WSD 21st edition code check equations provide a reasonably conservative fit to existing test data.

It is recommended to adopt API RP 2A-WSD 21st edition equations for hydrostatic code checks.

Study on 2) Conical TransitionAKADEME Case Study Platform 3

Helideck EL (+) 71.1 ft

Main Deck EL (+) 50.8 ft

EL (+) 22.3 ft

Water Surface EL (+) 10 ft

EL (+) 0 ft

Mudline EL (-) 35 ft

WT=2 in

WT=2 in

• L-3 structure• Free standing caisson• Pile penetration = 110 ft• Pile diameter = 72 inches• Water depth = 45 ft• Clays overlying dense sands

Comparison of Conical Transition Code Check Results

Note: Governing UC ratios are highlighted in red.

• API RP 2A-WSD computes UCs based on tensile strength instead of yield strength for the total stress check.

• ISO 19902:2007 1st edition produces the most conservative result.

• API RP 2A-WSD 21st edition and NORSOK standard produce comparable results.

Maersk FEM Results Compared to Codes- Axial Compression

Maersk report: “Plastic Hinge Model for Tubulars and Conical Transitions. Results of detailed FEM analyses condensed into a beam Model” Rambøll Ref. 862601/340_0001(1)

Conclusions – Conical Transition The UC value for the conical transition member of Platform

3 based on ISO 19902:2007 1st edition is much higher than the UC value based on API RP 2A-WSD 21st edition (nearly twice as high), which indicates that ISO 19902:2007 1st

edition code check equations are more conservative.

API RP 2A-WSD computes UCs based on tensile strength instead of yield strength for the total stress check. Thus, it is recommended to consider adopting the NORSOK conical transition code check equations for the future revisions of API RP 2A-LRFD and ISO 19902.

API RP 2A-WSD 21st edition and API RP 2A-LRFD 1st edition have identical code check equations for conical transitions.

Study on 3) Grouted ConnectionDemand/Capacity API RP 2A-WSD ISO 19902:2007

Transfer Stress only Axial Force

Axial Force&

Torsion

Allo

wab

le T

rans

fer

Stre

ss

Ope

rati

ng C

ondi

tion

Extr

eme

Load

Co

ndit

ion

(including 1/3 allowable stress increase)

without shear keys

with shear keys fg is the lesser of fg,sliding and fg,shear

kred is reduction factor for effects of movement during grout setting;Assume no movement kred = 1

without shear keysℎ� = 0

without shear keys

with shear keys

API RP 2A-WSD vs. 2A-LRFD 1st (w/ Shear Keys)2A-WSD 2A-LRFD 1st

Operating condition

Extreme loading condition

x 1.8

API RP 2A-WSD vs. 2A-LRFD 1st (w/o Shear Keys)2A-WSD 2A-LRFD 1st

Operating condition

Extreme loading condition

0

0

0x 1.8

Case Study 1 – Platform X• Omnidirectional metocean conditions were

applied. operating condition extreme loading condition

0 degree

Ds = 93 ints = 2.5 inDp = 84 intp = 2.5 inL = 44 ft

Assume:h = 0.5 ins = 20 infcu = 5000 psi

Case Study 2 – AKADEME Platform 5• Jacket legs and through piles were assumed to be

grouted in this study.• Omnidirectional metocean conditions were

applied. operating condition extreme loading condition

0 degree

Platform North

Ds = 58.5 ints = 1.25 inDp = 54 intp = 1.75 in

Assume:h = 0.5 ins = 20 infcu = 5000 psiL = 100 ft

Grouted Connection Unity ChecksAPI RP 2A-LRFD 1st vs. API RP 2A-WSD 21st

Base Equation:

API WSD API LRFD 1st

Operating

Condition� + � + � ≤ ��

1.3� + 1.5� + 1.215� ≤ (0.9 × 1.8)��

� . � �� + � . � �� + � . � �� ≤ �� (normalized)

ExtremeCondition

� + � + � ≤ 1.33��

� . � �� + � . � �� + � . � �� ≤ �� (normalized)

1.1� + 1.1� + 1.35� ≤ (0.9 × 1.8)��

� . � �� + � . � �� + � . � �� ≤ �� (normalized)

Grouted Connection Unity ChecksAPI RP 2A-LRFD 1st vs. ISO 19902:2007 1st

Conclusions – Grouted Connection The grouted connection code check equations in API RP 2A-LRFD

1st edition yield consistent results with those from API RP 2A-WSD 21st edition and ISO 19902:2007 1st edition.

The API RP 2A-LRFD 1st edition has similar formation of interface transfer strength but 1.8 times higher than the allowable transfer stress in API RP 2A-WSD 21st edition.

It is recommended to adopt the API RP 2A-LRFD 1st edition grouted connection code check equations for API RP 2A-LRFD 2nd edition.

Conclusions for ISO/API Jacket CodeWork performed to date supports the approach of Modified

Adoption of ISO 19902 for API RP 2A-LRFD 2nd Edition.Overall, member and joint UCs from ISO 19902 and API

RP 2A-WSD are consistent.Key technical issues studied:

–Hydrostatic pressure–Conical transition–Pile sleeve grout equation

ISO 19902 is currently being updated. API/ISO committees will work closely to harmonize the next editions of API RP 2A-LRFD and ISO 19902.

Presentation Part 2API RP2A Ductility Requirement and Seismic Design Process

Ductile Framing

(source: API RP2A)

Non-Ductile Framing

(source: API RP2A)

API Ductile Design Features - Jackets

Legs and enclosed piles remain elastic for 2 x strength level

Vertical diagonal bracing configured so tension and compression braces get equal shear

No K bracing allowed Braces remain elastic for 2 x strength level in lieu of

above 2 requirements Horizontals between legs at all framing levels with

capacity for load redistribution Vertical diagonals: kL/r<80 D/t<1900/Fy Non-tubulars in vertical frames are AISC

compact sections or meet 2 x strength level

Vertical Bracing Schemes

K Diamond X Diagonal

Comparative Ductility of Vertical Schemes

Effect of Strong Horizontal Planes

API/ISO Earthquake Code Recommendations

API 2EQ is a modified adoption of ISO 19901-2:2004

Two level approach. Extreme Level Earthquake (ELE) – strength design.

Abnormal Level Earthquake (ALE) – ductility check.

Seismic maps for USA offshore in API 2EQ.

Seismic maps for offshore locations worldwide in ISO 19901-2:2004.

Extreme Level Earthquake (ELE)

Platform performs in an elastic manner.

Response spectrum analysis (or time-history).

Code-based or site specific spectra as input.

300 to 500 year Return Period typical.

70% allowable stress increase for API code check.

Do members/joints fail?

Site-specific response spectra based on PSHA and site response analysis may be used instead of the code-based spectra.

Abnormal Level Earthquake (ALE)

Platform typically performs in an inelastic manner.

Time-history analysis (or pushover) to check the “ductility” of the platform.

Site specific ground motion records as input.

3000 year Return Period (or more) typical.

Does the platform collapse?

Global X Component

-1-0.8-0.6-0.4-0.2

00.20.40.60.8

1

0 5 10 15 20 25

Time (sec)

Acc

eler

atio

n (g

)

Global Y Component

-1-0.8-0.6-0.4-0.2

00.20.40.60.8

1

0 5 10 15 20 25

Time (sec)

Acc

eler

atio

n (g

)

Global Z Component

-0.60

-0.40

-0.20

0.00

0.20

0.40

0.60

0 5 10 15 20 25

Time (sec)

Acc

eler

atio

n (g

)

Overall API/ISO Process

Determine Seismic Risk Category (SRC) - higher risk (e.g., manned platforms) = safer design

Determine need for simplified or detailed procedure

Establish Return Period for ALE Establish Return Period for ELE based on the

ductility expected of the platform design Design the platform for ELE (strength design)

and check its ductility/performance for ALE

API/ISO Processfor Ground Motion

(source: ISO 19901-2:2004)

Site Seismic Zones

Seismic Zone Acceleration (g) - 1 sec0 <0.031 0.03-0.102 0.11-0.253 0.26-0.454 >0.45

Example Seismic Map

ISO Map for Central America

(source: ISO 19901-2:2004)

Seismic Risk Category Based on exposure level and seismic zone at the platform site

Note that L2 was removed in API 2EQ (can’t “evacuate” personnel for earthquakes)

(source: ISO 19901-2:2004)

Seismic Design Requirements

(source: ISO 19901-2:2004)

Target Probability of Failure

•Use of Return Periods as a basis for design worldwide can lead to inconsistencies•A 1,000 year RP earthquake in California is different than a 1,000 year RP earthquake in SE Asia•API/ISO uses an alternative approach of a consistent target annual probability of failure, Pf

(source: ISO 19901-2:2004)

Standard API/ISO Spectrum(1,000-yr Return Period)

This shows a “generic” earthquake response spectrum taken from API 2EQ (Nov. 2014). Notice how the “acceleration” that a platform will experience depends on the platform’s dynamic characteristics (periods).

(source: API 2EQ, November 2014)

Scale Factor for ALE Spectrum(Simplified Seismic Action Procedure)

(source: ISO 19901-2:2004)

Correction Factor for ALE Spectrum(Detailed Seismic Action Procedure)

(source: ISO 19901-2:2004)

Seismic Reserve Capacity Factor (Cr) for ELE Spectrum

rC = Variable up to 2.80as demonstrated by analysis.

The recommendations for ductile design in 5.3.6.4.3 are followed and a non-linear static pushover analysis according to API RP 2EQ is performed to verify the global performance of the structure under ALE conditions.

rC = Variable up to 2.00as demonstrated by analysis.

The recommendations for ductile design in 5.3.6.4.3 are followed, but a non-linear static pushoveranalysis to verify ALE performance is not performed.

rC = 1.40

The structure has a minimum of three legs and a bracing pattern consisting of leg-to-leg diagonals with horizontals, or X-braces without horizontals. The slenderness ratio (KL/r) of diagonal bracing in vertical frames is limited to no more than 80 and FyD/Et ≤ 0.069. For X-bracing in vertical frames the same restrictions apply, where the length L to be used depends on the loading pattern of the X-bracing.A non-linear analysis to verify the ductility level performance is not performed.

rC = 1.10 If none of the above characterizations apply.

Minimum ELE Return PeriodsConsequence Category Minimum ELE Return Period

Low Consequence 50Medium Consequence 100

High Consequence 200

•Cr is a function of the expected ductility of the platform design•The intent is to determine a RP that can be used to design the platform elastically, such that it will meet the ALE performance criteria•The Cr factor of 2.0 or higher should be demonstrated by detailed analysis

Summary

Latest API RP2A-LRFD, and ISO 19902 developments were discussed

Some of the succinct features in API/ISO seismic design provisions were also discussed

Technologies in API RP-2A and/or ISO 19902 for designing/building a fixed platform are quite mature

Design code expertise can be leveraged through industry professionals who participated in API/ISO committees

End of PresentationQ & A

overview of design codes for offshore fixed structures · 2017. 11. 20. · outline part 1 – api...

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