Schiehallion FPSO Mooring Integrity Management Top Chain Crack Inspection and Change-out

摘要

As part of BP’s Schiehallion mooring integrity management campaign and risk management programme, BP elected to replace the top chains of each chain leg in the Schiehallion FPSO mooring system in 2011. A critical factor in ensuring successful change-out was assurance of reserve strength in the top chains ‘weakest link’. BP commissioned TWI to develop and validate a diver deployed UT system to detect and size fatigue cracks in the top mooring chain link. BP also commissioned finite element stress analysis and full scale pull tests to confirm the failure stress for chain links containing simulated fatigue cracks and thus set critical crack sizes. The location of cracking, confirmed by analyses of a failed link from a chain failure that occurred in 2006 and subsequent retrieval of top chains, was uninspectable by available NDT tools and procedures. Link 2 (first loadbearing link in the top chain) was identified to be the critical link in terms of fatigue. A new UT system was designed to detect cracks 100mm long and 20mm through-thickness with 90% probability of detection (POD). These values were conservatively determined. The equipment and procedure were established for diver deployment from the chain table, for simplicity in application by the diver, and minimal communications through the dive supervisor required between the diver and the NDT specialist based in the topside dive control room. A robust validated procedure was prepared to ensure maximum confidence in the NDT results and efficient execution to minimise the duration of the inspections and allow the planned change-out of the top chain segments to proceed.