Assessment of Pipeline Free Span Integrity on Mobile Seabeds

CHRIS MADELEYSUBSEA ENGINEERING ASSOCIATES

PERTH, AUSTRALIA

Outline

Background Approach Outcomes Direction &Summary

Background

Background

Scour Creates Transient Freespans

Freespans Assumed Fixed During Analysis

Rectifications RequiredSpans Move,

Disappear, Self-bury

Overconservative Analysis

Excessive OPEX

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

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 Fatigue2. 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

Clear Outcome

Build on Existing Knowledge

Combination of Different FieldsVIVProbabilistic

MethodsFatigue

Scour Lab Testing

Field Observations

Tool for Span Assessments

Approach

MOBILEspan Approach

Pipeline

Soil FluidMOBILESPAN

MONTE CARLO

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

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

Scour Model

Scour Model

Spacing Onset

DepthGrowthBackfill

Outcomes

Approach Dual Benefits

Probabilistic Analysis

Scour Modelling

MO

BIL

Esp

an

Captures Uncertainties in:

Fatigue Curve

Response Model

Soil Properties

Metocean

Illustrative Results

Benchmarks Against Survey Data

Efficient Probabilistic Analysis

Distributed Computing

More Power

Bayesian Statistics

+20×

Computing in the Cloud

LocalComputing

Single Workstation

Familiar

Upfront cost

Inflexible

CloudComputing

Remote Cluster

Modern

On-demand

Scalable

Required Iterations

Direction & Summary

Phase 2

Project Track

Phase 1

Conclusions

MOBILEspan approach for scouring spans→ fewer interventions→ significant OPEX savings

Step-change in probabilistic capabilities→ less conservative designs→ significant CAPEX savings