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Design of Offshore Pipelines on Erodible Seabed
Winthrop Professor Liang Cheng
School of Civil, Environmental and Mining Engineering The University of Western Australia
OFFSHORE PIPELINES
• Key links between production wells and storage and processing units (mainly onshore)
• Functions: transport of oil and gas products, power and control fluids
• Costs of pipelines are generally high
• Consequences of failures are high
– Costs associated stoppage of production and repairs
– Environmental and social impacts
TYPES OF OFFSHORE PIPELINES
• Rigid pipe
– Small: 3” to 14”; Medium: 16” to 28”; Large: 30” to 56”
• Flexible pipe
– HP to around 14”ID
– LP to around 20”ID
• Materials: carbon steel, corrosion-resisted Alloy
TYPICAL PIPELINE COSTS
Gas Export Pipeline Cost UKCS
Eng Mat Inst
2nd Trunkline
0
20,000
40,000
60,000
80,000
100,000
120,000
0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000
Pipeline Length*Diameter (km*inch)
Co
st
pe
r k
m*i
nc
h (
US
$)
COSTS FOR PIPELINES
Materials Materials & Fabrication& Fabrication
30%30%
Management Management
& Engineering& Engineering
10%10%
StabilisationStabilisation
30%30%
InstallationInstallation
(excluding Stabilisation)(excluding Stabilisation)
30%30% Materials Materials & Fabrication& Fabrication
30%30%
Management Management
& Engineering& Engineering
10%10%
StabilisationStabilisation
30%30%
InstallationInstallation
(excluding Stabilisation)(excluding Stabilisation)
30%30%
PIPELINE ON-BOTTOM STABILITY
• Trenching or rock berm in shallow waters ( approximately < 30 m)
• Primary stabilization: self weight + concrete coating in medium to deep waters ( > 100 m)
• Primary + Secondary stabilizations: in medium water depth between 30m to 100 m
SECONDARY STABILIZATION METHODS
• Rock Berms
• Gravity Anchors
• Piles, …
PIPELINE DESIGN CHALLENGES
• Corrosion – pipeline integrity
– Anti-corrosion coating and metal cladding
• Flow assurance – steady productions
– Liquid plugging, hydrates, wax, …
• On-bottom stability – present topic
– Resist extreme hydrodynamic forces
– Very expensive
PIPELINE STABILITY DESIGN
• Design requirement: pipeline be stable under extreme environmental conditions
– Absolute stability methods
• Pipeline movements not allowed
• Often conservative, leading to high costs
– Dynamic stability methods
• Pipeline movements are allowed
• Considers seabed resistance changes
• Structure integrity needs to be checked
FLAWS IN CURRENT DESIGN METHODS
• Calculations of wave forces are too conservative – Velocity reduction in wave boundary layers in
force calculations are not considered
• Sediment transport processes ignored – Use static seabed profiles
• Reality: seabed profiles around the pipe significantly modified before storm peaks arrive
– Hydrodynamic loads
– Soil resistance
– Use of a slice of pipeline is not acceptable
Effects of Sediment Transport
• Movement of sandy seabed sediments modifies seabed profiles around pipelines
• Local scour occurs if wave orbital velocity exceeds a critical value
• Local scour can cause pipeline self-burial into the seabed, improving pipeline stability
– Hydrodynamic forces decreases
– Soil resistance increases Dalian University of Technology
SELF-BURIAL MECHANISMS
School of Civil and Resource
Engineering
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B
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A
FIELD EXPERIENCES
• Significant self-burial on sandy seabed
– 80%-100% burial for 60%-80% of the lengths of existing small diameter pipelines
School of Civil and Resource Engineering
Field Measurement Results
• Measurement using high resolution laser surface profilers
• Pipeline is partially buried in the seabed 1 year after installations
• This occurs to entire pipeline
Dalian University of Technology
WAY FORWARD
• Conduct more research
– Quantify the effect of wave boundary layers on hydrodynamic forces on pipelines
– Develop robust and reliable methodology that can take into account the effect of sediment transport on pipeline stability
• Joint Industry Project: StablePIPE JIP
– Aims: develop new design guidelines
– Sponsored by two industry partners
– Involved University and engineering companies
Research Aims and Methods
• Develop a new design method to consider the effect of pipeline self-burial on the stability of pipelines
• Method: Experimental investigation
– Quantify the effects of sediment transport and wave boundary layers on pipeline stability
– Simulate dynamic response of pipelines under extreme environmental conditions
Dalian University of Technology
OUTCOMES
• A new research facility is established
– A large scale recirculating flume
• Able to generate prototype random storm velocity time series up to 2.8m/s with a period of 13s.
• Simulate flow/pipeline/seabed interactions near prototype conditions to overcome scaling difficulties
• Completed a number of research projects
• Developed a new pipeline stability design guideline.
NEW FACILITY
• Functions
– Conduct pipeline stability testing at large scales to reduce scaling effects;
• 1:1 scale for small diameter pipelines (<8 inch)
• 1:5 scale for large diameter pipelines (<40 inch)
– Simulate flow conditions induced by cyclonic storms
• Oscillatory flow up to 2.8 m/s with a peak period of 13 seconds
• Steady currents up to 3.0 m/s
• Combined random storm time series.
O-TUBE CONCEPT
• Flow is generated by controlling the rotation of impeller • One direction only – steady currents • Two directions of equal speeds – oscillatory flows • Two directions of different speeds – oscillatory + steady • Rotations from any spectra – random storms
PIPE CONTROL SYSTEMS
• Model pipe
• Mounting actuators
• Data acquisition system
• Active feedback control operations
MODEL PIPE
• D=0.2 m and L
• Internal DAQ system, communicating via Ethernet
• Up to 1 MHz, 8 channels per box
• Internally pressurised to protect DAQ system.
NUMERICAL MODELS
• SCOUR-2D & SCOUR-3D
– Model scour below pipelines and subsea structures
– Hydrodynamic forces on pipelines and risers
– Vortex-induced vibrations
• WAVEFLUME-3D
– Wave-structure interactions
• Application examples
RESEARCH IMPACTS
SUMMARY
• A new pipeline stability design methodology has been developed and validated against both laboratory tests and field data
• The new design method is being applied to a number of real engineering projects
• More research efforts are needed to improve the design method.