Reducing risk and cost through an optimised inspection approach. February 2017

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Dwain Hodge
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1 Reducing risk and cost through an optimised inspection approach February 2017

2016. Platform was shut down for several months to monitor

Ninian Southern Multiple fatigue cracks found and brace

2 Structural fatigue Maari platform 1.4m long crack found in a horizontal bracing in November Platform was shut down for several months to monitor and repair. Ninian Southern Multiple fatigue cracks found and brace failures recorded since 1980s Alexander Kielland Catastrophic brace failure due to fatigue causing collapse in people died. 2

3 Managing fatigue risk ISO & API-RP-2SIM Evaluation Data Inspection Strategy Risk Category Higher Inspection Interval Ranges 3 years to 5 years Medium 6 years to 10 years Lower 11 years to 15 years Inspection Programme 3

4 Consequence Managing fatigue risk Long fatigue life Short fatigue life Critical structural component Inspection Interval (Years) Likelihood A 5 5 B What about very short fatigue lives? Risky conclusions: Platform is still standing not a problem. My fatigue results are a risk ranking only. C D Performed NDT on one joint and found no cracks so the others should be fine Less critical structural component E F NDT Inspected once before no damage 4

Managing high fatigue risk North Sea Jacket: 37 joints

Fatigue lives Option 1: Inspect Option 2: Do Nothing

Better understand the risk and Inspect only what you need

Unfocussed scope Often cannot be completed Costly Accept

results Advanced analysis can potentially mitigate the

understanding of relative risks Can be perceived as

risk Minimised inspection scope Inspection results can

information readily available to make cost effective

5 Managing high fatigue risk North Sea Jacket: 37 joints with Life < 60 years 18 joints with Life < 10 years Short Fatigue lives Option 1: Inspect Option 2: Do Nothing Option 3: Mitigate via Analysis Option 4. Better understand the risk and Inspect only what you need to Expected action / commonly done. Unfocussed scope Often cannot be completed Costly Accept the perceived risk Qualitatively dismiss the risk / results Advanced analysis can potentially mitigate the fatigue risks to an acceptable level More definitive understanding of relative risks Can be perceived as costly May not fully solve the problem Well understood risk Minimised inspection scope Inspection results can further optimise the future inspection scope Analysis information readily available to make cost effective decisions Lack of cooperation & communication between analysts / inspectors / operators to understand the benefits and implement 5

6 Systematic fatigue risk assessment Option 4. Better understand the risk and Inspect only what you need to 5 STEPS: Step 1: Improve fatigue analysis Step 2: Quantify fatigue risk Step 3: Quantify inspection results (if any) in terms of its effect on risk Step 4: Better understand platform redundancy and re-set risk thresholds Step 5: Use database of inspection effectiveness and costs to plan cost optimised targeted inspections RISK COST 6

Step 1 Improve fatigue analysis Conventional

splash zone assumptions Refining wave steepness (if

7 Step 1 Improve fatigue analysis Conventional Fatigue analysis is conservative. We can improve by: Reviewing the model (joint flexibility) Reviewing SN curves and SCFs Refining splash zone assumptions Refining wave steepness (if spectral analysis) Embedding shell / solid FE models No. of locations:

8 Annual Failure Rate Step 2 Quantify fatigue risk Established methods (DNVGL-RP-C210) to determine probability of fatigue failure based on fatigue analysis results 1E+0 1E-1 1E-2 1E-3 1E-4 1E-5 1E-6 1E-7 ACFM Inspection at 4 years 1E Time (Yrs) Step 3 Incorporate inspection findings 8

9 Step 4 Determine criticality via redundancy Refining the Fatigue Analysis can reduce the likelihood of failure Likelihood Level Demonstrating redundancy reduces the consequence of failure 9

10 Annual Failure Rate Step 5 Develop cost effective inspection plan Applying Quantitative RBI means we calculate the probability of fatigue failure as a function of time, and inspect each time the probability gets too high. 1E+0 1E-1 1E-2 1E-3 Diver inspection at 4 years Diver inspection at 12 years Final Diver inspection at 22 years 1E-4 1E-5 1E-6 1E-7 Reduction in risk is based on Probability of Detection (PoD) 1E-8 Hard Suit Diver NDT Inspection campaign (5 inspections required) ROV NDT Inspection campaign (6 inspections required) Time (Yrs) 10

11 Step 5 Develop cost effective inspection plan RISK COST 11

12 12

13 Thank you If you d like to find out more visit: or contact: Eric.Hoo@atkinsglobal.com Colin.Brice@atkinsglobal.com Matt.Keys@atkinsglobal.com David.Louer@atkinsglobal.com Acknowledgements: Atkins except where stated otherwise 13

Technical Standards and Legislation: Risk Based Inspection. Presenter: Pierre Swart

Technical Standards and Legislation: Risk Based Inspection Presenter: Pierre Swart Agenda Change in Legislation. Where RBI fits in. Application to implement RBI. RBI concepts. Elements of an RBI analysis.