This paper presents a multitude of lessons learned during the 5 years spent on the design, manufacturing and delivery of the on- and off-vessel mooring components for the Appomattox semi-submersible.
Appomattox is Shell's newest and largest deepwater oil & gas production asset in the Gulf of Mexico (GoM). It was successfully moored about 260 kilometers Southeast of New Orleans, in June 2018 using a 16-point chain-polyester-chain mooring system. It was the culmination of the work done by a dedicated team of Shell civil-marine engineers, mooring equipment suppliers and an offshore installation contractor. Shortly after completion of mooring lines hook-up operations, the polyester ropes were pre-stretched, and the FPS was moved into position for the start of the Steel Catenary Riser (SCR) system installation. Appomattox started production in May 2019. The mooring equipment has so far been performing as expected.
The delivery of Appomattox mooring components was particularly challenging due to an unusually large number of stringent constraints including: 40-year design life, first application of fairleads with underwater chain stoppers in the GoM to mitigate out-of-plane chain link bending fatigue, polyester rope properties fine-tuned to ensure that installation and in-service requirements are met, stringent inspection oversight to enforce project-specific requirements, Thermal Spray Aluminum (TSA) on portions of the platform chains and novel chain handling equipment and method for mooring lines hook-up operations, just to mention a few. The design of the overall system and management of the mooring equipment vendors was performed using mostly in-house resources.
The paper starts with some general information about the Appomattox development. The three subsequent sections present the mooring system, the delivery team and the overall timeline. The lessons are then organized in sections by mooring component types (on-vessel, polyester ropes, chains, and various connectors). Some reflections on how stakeholders influenced certain characteristics of the mooring components are disseminated throughout the paper. The paper also includes some observations about how the equipment performed in the field even though the lessons learned about the commissioning and offshore installation are left for a separate future paper.