assessment of pipeline free span integrity on mobile · pdf filetransient freespans freespans...
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Assessment of Pipeline Free Span Integrity on Mobile Seabeds
CHRIS MADELEYSUBSEA ENGINEERING ASSOCIATES
PERTH, AUSTRALIA
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Outline
Background Approach Outcomes Direction &Summary
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Background
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Background
Scour Creates Transient Freespans
Freespans Assumed Fixed During Analysis
Rectifications RequiredSpans Move,
Disappear, Self-bury
Overconservative Analysis
Excessive OPEX
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Known Freespan Failures
Cook InletAlaska, 1960s & 70s
Large, reversing daily tidal currents on sandy seabed
Ping Hu PipelineEast China Sea, 2000
Four typhoons exposing buried, near-shore pipeline
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Scour Evolution
Step 0: Pipeline resting on flat sandy seabed
As designed! ☺
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Scour Evolution
Step 1: Scour Initiation
Leckie et al. 2015
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Scour Evolution
Step 2: Scour Growth
Negligible Fatigue Damage
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Scour Evolution
Step 3: Critical Length
Waves and current cause:
1. VIV and Wave Fatigue2. More scour!
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Scour Evolution
Step 4: Touchdown
Scour holes are shallow, hence touchdown will occur before overstressing
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Scour Evolution
Step 4: Backfill
Scour and span re-formation complete before significant fatigue damage is accumulated.
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Scour Evolution
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MOBILEspan Project
Clear Outcome
Build on Existing Knowledge
Combination of Different FieldsVIVProbabilistic
MethodsFatigue
Scour Lab Testing
Field Observations
Tool for Span Assessments
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Approach
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MOBILEspan Approach
Pipeline
Soil FluidMOBILESPAN
MONTE CARLO
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Assessment Framework
Inputs
Monte Carlo Simulation
Failure Probability
Time-domain Analyses
Results
Determine Intervention Requirements
Pipeline Segmentation Split into homogeneous segments and check applicability
Deterministic & statistical, from design, surveys, experimental data, etc.
10 million iterations; compliant with DNV-OS-F101
Full design life including scour and pipeline response
Limit state functions and key indicators
Calculate nominal failure probability
Check simulation results with observations for reasonability, compare results against reliability targets
Time-domain Analyses
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Assessment Framework
Iteration Start
Wave Loads
Timestep Results
Fatigue Damage Ultimate Limit State
VIV Response
Geometry
Structure
Hydrodynamics
Loop
Initialise Iteration
Timestep Inputs
Iteration Results
Complete
ScourScourHydrodynamics
VIV Response Wave Loads
Ultimate Limit StateFatigue Damage
Structure
GeometryCalculated using DNV-RP-F105
& DNV-OS-F101
NEW
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Scour Model
Scour Model
Spacing Onset
DepthGrowthBackfill
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Outcomes
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Approach Dual Benefits
Probabilistic Analysis
Scour Modelling
MO
BIL
Esp
an
Captures Uncertainties in:
Fatigue Curve
Response Model
Soil Properties
Metocean
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Illustrative Results
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Benchmarks Against Survey Data
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Efficient Probabilistic Analysis
Distributed Computing
More Power
Bayesian Statistics
+20×
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Computing in the Cloud
LocalComputing
Single Workstation
Familiar
Upfront cost
Inflexible
CloudComputing
Remote Cluster
Modern
On-demand
Scalable
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Required Iterations
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Direction & Summary
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Phase 2
Project Track
Phase 1
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Conclusions
MOBILEspan approach for scouring spans→ fewer interventions→ significant OPEX savings
Step-change in probabilistic capabilities→ less conservative designs→ significant CAPEX savings
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