Assessment of Pipeline Free Span Integrity on Mobile Seabeds
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
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