Determination of Rational Robustness Criteria for FPSO Design
Authors: Carra C. , Potts A. E. , Johnstone D. , Kilner A. A. 
Published: April 2017

This paper outlines a methodology for developing consistent robustness criteria; accounting for multiple hazards that may occur during long Return Period (RP) events, and accounting for other degradation mechanisms unrelated to long RP events but, based on experience, were root causes of many failures. The "if in doubt, make it stout," philosophy is critically examined against a higher fidelity approach, utilizing new knowledge of reported causes of failure to achieve better design.

With uncertainty when designing FPSO mooring systems, regulatory responses focus on increasing line numbers and capacity (e.g. increasing the Factor of Safety (FOS) from 1.67 to 2.20, or consideration of longer RPs for design.) This design basis response presumes failures primarily occur due to overloading events. Instead of arbitrarily increasing RPs or the FOS, a more robust methodology is developed that utilizes new knowledge of root causes in higher fidelity time domain numerical modelling to better understand true mechanisms of failure, and design accordingly. These higher fidelity methods take account of system response complexities not usually accounted for in conventional accepted design analysis approaches.

An alternate risk based methodology is described which is aligned with System Safety Engineering philosophies, whereby a probabilistic approach is used to ensure overall probability of failure is less than 1/10,000. The approach considers multiple mechanism of failure, arising from both high RP storm events and operational motions, in conjunction with corrosion wear rates as observed from recent studies (e.g. SCORCH JIP). Observations are drawn from mooring failure investigations and on-going in-field mooring monitoring campaigns. The results of recent studies are discussed, revealing that half of the failures investigated were due to time dependent degradation from corrosion and/or fatigue. Where multi-line failures have occurred, the root cause is almost exclusively due to common mode degradation resulting in multiple lines experiencing significant loss of capacity, whereby un-zipping is not due to overload, but stochastic undetected or unremedied loss of capacity. Thus, arbitrarily increasing design storm severity and FOS may not be an informed way of addressing actual failure modes, but a blunt instrument that may introduce unintended consequences and design spirals with new failure mechanisms introduced by pushing the limits of component sizing and manufacturing capabilities. A more rational robustness criteria is developed, that aims to ensure that designers understand their system to a higher level of fidelity. There is no presumption that a tension mechanism of failure during a high RP storm event is governing, but rather, robustness is evaluated using a reliability based approach. This accounts for corrosion, wear, bird caging, out-of-plane bending, and other phenomena discovered in recent studies of actual failures, ultimately delivering a much higher level of design robustness without unnecessarily increasing strength of the system's resistance to an irrelevant failure mode.