Subsea Pipeline Free Span Assessment

01 Wave and current modelling 6 topics

• Sea states and how they are characterised
• Short-term modelling of waves
• Long-term modelling of waves and currents
• Wave scatter diagrams
• Wave and current histograms
• Building wave and current histograms from return-period values

02 Wave and current loads 3 topics

• Wave theories and how to select them
• Wave kinematics
• Drag and inertia forces

03 Introduction to free spans 4 topics

• Where free spans occur
• Free span characterisation
• Parameters affecting free span response
• Interacting spans

04 Free-span-induced failure modes 3 topics

• Fatigue from in-line and cross-flow vortex-induced vibration (VIV)
• Fatigue from direct wave and current cyclic loading
• Strength (yield) failure under self-weight and extreme environmental loading

05 Free span assessment approaches 4 topics

• Screening
• Detailed fatigue assessment with a simplified structural response model
• Detailed fatigue assessment with finite element modelling
• Applicability and limitations of each approach

06 Safety factors 3 topics

• Effect of the pipeline safety class
• Effect of certainty in free span parameters
• Safety factors for screening criteria

07 Structural response of a free-spanning pipeline 5 topics

• Pipe–soil interaction: stiffness and damping
• Effective span length
• Natural frequencies and the effect of pressure and temperature
• Stress amplitudes
• Static bending and deflection

08 Screening-level assessment 6 topics

• Required wave and current information
• Reduced velocity
• Check against in-line vibration
• Check against cross-flow vibration
• Check against direct wave and current cyclic loading
• Limitations of screening-level assessment

09 S-N curves 7 topics

• What is an S-N curve?
• Palmgren–Miner rule
• Classifying the welds in a pipeline
• Stress concentration factors (SCF)
• Internal and external fatigue
• S-N curves to DNV-RP-C203
• S-N curves to BS 7910

10 In-line VIV 6 topics

• Key in-line parameters
• In-line response model
• In-line VIV amplitude
• In-line stress range
• In-line vibration frequency
• In-line fatigue damage

11 Cross-flow VIV 7 topics

• Key cross-flow parameters
• Cross-flow response model
• Cross-flow VIV amplitude
• Cross-flow stress range
• Cross-flow vibration frequency
• Cross-flow fatigue damage
• Cross-flow-induced in-line motion

12 Direct wave and current cyclic loading 3 topics

• Vibration frequencies
• Force model
• Fatigue damage

13 Fatigue capacity and total fatigue life 5 topics

• Comparing in-line VIV damage and force model damage
• Fatigue capacity in each sea state from in-line VIV and cross-flow-induced motion, integrated over the long-term current distribution
• Fatigue capacity in each sea state from cross-flow VIV, integrated over the long-term current distribution
• Fatigue capacity in each sea state from direct wave and current loading
• Integrating fatigue damage over all sea states

14 Ultimate limit state (ULS) check 3 topics

• Maximum in-line and cross-flow loads
• Local buckling check to DNV-OS-F101
• Code check to PD 8010-2

15 Multi-mode response and finite element assessment 4 topics

• Finite element modelling
• Extracting mode shapes and natural frequencies
• Fatigue damage at each point along the pipeline
• Extracting the maximum damage

16 Free span assessment of in-service pipelines 4 topics

• Survey information
• Splitting the pipeline into sections
• Free span assessment
• Intervention