A Historical Review on Integrity Issues of Permanent Mooring Systems

Abstract

Abstract With several incidents occurring between 2001 and 2011, the relatively high failure rate of permanent mooring systems is raising a concern in the offshore industry. As a response to this, the paper summarizes various incidents and reviews their failure mechanisms and associated integrity issues. In summary, at least eight (8) incidents involved multiple line breaks; some of them leading to vessel drift. Many more with a single-line breakage were accompanied by similar damage in other lines. Many line failures occurred in their early lives, showing a trend of " infant mortality." Chain, connector and wire rope appear to be the top three components that caused most incidents. Several incidents came as a surprise to industry experts. One particularly surprising finding is the novel nature of the failure mechanisms of some incidents, such as out-of-plane bending, chain hockling/twisting, flawed flash welds, low metal toughness, pitting corrosion, etc. Unknown or new failure mechanisms are troubling because, since they are unanticipated, they cannot be easily detected and prevented with existing integrity practices. This paper critically examines the practices, discusses their limitations and identifies possible improvements, while also highlighting future challenges. To raise the bar for the future reliability of mooring systems, some planned improvements on design codes, taking API Standards as a model, are proposed. A key advocate from the authors is to build a healthier industry by more formally sharing lessons learned, so similar failures can be prevented and mooring integrity can be improved. Introduction In the past decade, mooring incidents have been occurring at a high rate. More than twenty (20) incidents have happened to production vessels that are moored on-site for prolonged duration (typically 15–25 years). Among those, at least eight (8) had multiple line damages, or system failure. Some of them led to vessel drifting. Furthermore, the incidents with single-line breakage often have additional lines that sustained damage and could have also failed prematurely if their degradation went undetected. Some of the incidents were of high consequence, causing the vessel to drift a short distance, riser ruptures, production shutdown and small amount of hydrocarbon release. Some were of substantial consequence, requiring effort to repair or replace damaged lines. These incidents are raising concerns for owners and operators. It is important for the industry to better manage mooring integrity and reduce the number of incidents.