Serpentina FPSO Mooring Integrity Issues and System Replacement: Unique Fast Track Approach

Abstract

Abstract This paper provides an overview of the first documented industry intervention involving replacement of a permanent mooring system due to seabed trenching near the suction piles. Another unique aspect of this intervention was the use of readily available hardware for installing a temporary mooring system to minimize operational risks. Brown-field interventions are often required to address mooring integrity issues or life extensions. These are more challenging than new (green-field) designs and installations due to existing infrastructure. This includes the Floating Production Storage and offloading System (FPSO) hull, risers and flowlines. Also, the production shutdown has to be minimized while maintaining operational safety. The present paper summarizes mooring integrity issues and the complete mooring system replacement of the Serpentina FPSO offshore Equatorial Guinea (EG) within ten years of service. A partial mooring replacement campaign was initially planned to address wire rope degradation and chain chafing damage on selected legs. However, seabed trenches near the suction piles, discovered at a late stage, required a complete change in the project plans. The revised scope included installation of a new set of anchors and off the shelf mooring hardware for all nine anchor legs, to address the risk of potential mooring failure in an expedient manner. Installation of new anchors and mooring lines for this project was executed in roughly 6 months from the discovery of the trenches. The planning and execution of this fast track project is discussed, along with the multiple challenges that needed to be addressed. Industry has limited experience in seabed-trenching related mooring issues and associated mitigation. The importance of seabed trench inspection for taut-mooring systems is discussed. Finally, lessons learned in planning and execution of the project in safe and expedient manner is summarized.