Assessment of Pipeline Free Span
Integrity on Mobile Seabeds
CHRIS MADELEY
SUBSEA ENGINEERING ASSOCIATES
PERTH, AUSTRALIA
Outline
Background
Approach
Outcomes
Direction &
Summary
Background
Background
Scour Creates
Transient Freespans
Freespans Assumed
Fixed During Analysis
Rectifications Required
Overconservative
Analysis
Excessive OPEX
Spans Move,
Disappear, Self-bury
Known Freespan Failures
Cook Inlet
Alaska, 1960s & 70s
Large, reversing daily tidal currents on sandy seabed
Ping Hu Pipeline
East China Sea, 2000
Four typhoons exposing buried, near-shore pipeline
Scour Evolution
Step 0: Pipeline resting on flat sandy seabed
As designed! ☺
Scour Evolution
Step 1: Scour Initiation
Leckie et al. 2015
Scour Evolution
Step 2: Scour Growth
Negligible Fatigue Damage
Scour Evolution
Step 3: Critical Length
Waves and current cause:
1. VIV and Wave Fatigue
2. More scour!
Scour Evolution
Step 4: Touchdown
Scour holes are shallow, hence
touchdown will occur before overstressing
Scour Evolution
Step 4: Backfill
Scour and span re-formation complete before
significant fatigue damage is accumulated.
Scour Evolution
MOBILEspan Project
Tool for Span
Clear
Outcome
Assessments
Field
Scour Lab
Build
on ExistingObservations
Knowledge
Testing
Probabilistic
VIVCombination of Different Fields
Fatigue
Methods
Approach
MOBILEspan Approach
Pipeline
Soil
MOBILESPAN
Fluid
MONTE CARLO
Assessment Framework
Pipeline Segmentation
Inputs
Monte Carlo
Simulation
Time-domain Analyses
Results
Failure Probability
Determine Intervention
Requirements
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
Assessment Framework
Iteration
Start
Initialise
Iteration
Timestep Inputs
Scour
Hydrodynamics
Calculated using
DNV-RP-F105
& DNV-OS-F101
Iteration
Results
NEW
Geometry
Structure
VIV Response
Wave Loads
Fatigue Damage
Ultimate Limit State
Complete
Timestep Results
Loop
Scour Model
Scour
Model
Backfill
Spacing
Growth
Onset
Depth
Outcomes
Probabilistic
Analysis
MOBILEspan
Approach Dual Benefits
Captures Uncertainties in:
Fatigue Curve
Scour
Response Model
Modelling
Soil Properties
Metocean
Illustrative Results
Benchmarks Against Survey Data
Efficient Probabilistic Analysis
More
Power
20×
Distributed
Computing
+
Bayesian
Statistics
Computing in the Cloud
Local
Computing
Cloud
Computing
Single Workstation
Remote Cluster
Familiar
Modern
Upfront cost
On-demand
Inflexible
Scalable
Required Iterations
Direction & Summary
Project Track
Phase 1
Phase 2
Conclusions
MOBILEspan approach for scouring spans
→ fewer interventions
→ significant OPEX savings
Step-change in probabilistic capabilities
→ less conservative designs
→ significant CAPEX savings