Abstract

The need for quantifiable inspection methods to evaluate the potential for life extension has become increasingly important for a large number of floating production facilities nearing the end of their design life. This paper discusses the use of a risk-based progressive inspection methodology to identify, quantify and analyze the condition of mooring systems and ultimately determine fitness for purpose.

Due to an increase in mooring system failures and the growing knowledge of failure mechanisms, many operators are moving towards more comprehensive inspection plans in addition to the minimum requirements as set by Class. Using a risk-based inspection plan and focusing on known areas of vulnerability allows for a higher probability that issues are identified before they become catastrophic.

Welaptega's experience with in-service mooring inspection has exposed numerous cases worldwide where mooring systems have been severely degraded due to accelerated and unexpected deteriorative mechanisms. In each case a decision to either replace the moorings or keep them in service had to be made.

Inspection programs using a number of Welaptega's measurement and modelling tools has allowed operators to identify the most severely degraded sections in a mooring system. Data collected makes it possible to produce 3D models of chain links which are used to evaluate the extent of material loss. The application of finite element analysis enables the client to quantify the residual strength of the mooring chain and estimate remaining fatigue life; from here a decision to extend the life of the mooring system can comfortably be made. In the past, finite element analysis combined with break testing has proven that even severely degraded mooring chain may still be fit for purpose in some circumstances.

The ability to provide high quality and quantitative subsea intelligence will continue to allow operators to make risk-based decisions, increasing the economic viability of field life extension without compromising asset integrity.

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