Bonga Mooring Top Chain Replacement Project – Lessons Learnt from Execution

Abstract Mooring systems are typically designed to secure floaters in position (within prescribed limits). For floating production platforms considering the FPSO type, the prescribed limits are usually the safe excursion limits for adjoining structures rigidly connected to the FPSO such as risers and umbilicals. Mooring systems design for deepwater floating production platforms have evolved over the years to using a combination of chain and either wire or fiber ropes to provide weight and tension optimization on the overall system design. The Bonga FPSO mooring system is of such design having chains in the upper segment connecting to the fairlead, with wire or fiber ropes at the middle segment while the bottom segment consists of chain links towards the anchor on the seabed. With this arrangement, the top chain is susceptible to corrosion due to the closeness to the sea surface and on many occasions leads to premature replacement (before end of design life). Inspection results in a regular asset integrity management survey confirmed that the observed and calculated diameter in some instances exceeded the design corrosion allowance hence leading to a deterioration in service life of the chain. Based on these inspection findings, studies and analysis, the operator of the Bonga field decided to replace the top-chain segments of the Bonga FPSO mooring system with new chain lengths of similar design. This paper presents the execution story of the Bonga Mooring Top Chain Replacement Project and the lessons learnt associated in the top quartile delivery. Given the offshore installation activities for this project involved high risk activities (including heavy lifting, diving, line cutting under tension etc.), this paper details the overarching safety measures put in place to ensure safe execution. It also highlights the various technical challenges faced and the solutions proffered given the complex uncertainties associated with brownfield activity execution. With quite a number of industry key players and stakeholders involved in the project execution, this paper also emphasizes how team integration among various stakeholders ensured timely and efficient execution.