Recommendations for a Fatigue Design Analysis Calibrated Using Structural Reliability Analysis
Authors: Hørte Torfinn , Okkenhaug Siril 
Published: October 2022

DNV is currently running a Joint Industry Project, NorMoor JIP, which is now approaching the end of phase 3. In this phase the fatigue limit state (FLS) of mooring lines with studless chains for permanent units has got the main attention. Phases 1 and 2 was covering the ultimate limit state (ULS) and accidental limit state (ALS) respectively.

The NorMoor JIP has participants from energy companies, engineering companies, rig-owners, manufacturers and marine authorities. It is a global study covering Gulf of Mexico, Northern Europe and Brazil waters. Thus, areas with hurricanes, winter storms and combinations of wind sea and swell are covered. The results from the study will be applicable to most locations around the world, except for areas with squall.

Our motivation for initiating a study on mooring line reliability was that all the global standards (API, ISO, DNV, others) are mostly based on work from late 1990s, when frequency domain analysis was prevalent, and the reliability level implied by the codes was not known. With this backdrop NorMoor Phase 1 and 2 was initiated, and recommendations on more recent and advanced analysis methodology and accompanying safety factors for ULS and ALS was developed.

For FLS DNV calibrated safety factors for permanently moored units using frequency domain calculations in 1998. Now two decades later the age of the fleet of the world’s production units is growing, and FLS is the governing limit state for life extension. The NorMoor JIP Phase 3 was initiated to ensure that the FLS code provides consistent and accurate safety level, and to see if improvements could be made to the analysis methodology. Phase 1 and 2 of the NorMoor JIP has shown that more cost-effective mooring systems can be obtained, without jeopardizing the safety level. Further potential for cost optimization of mooring designs has been investigated in Phase 3 by improving the FLS code.

The focus in the present paper is the methodology and application of structural reliability analysis for mooring line fatigue where the probability of fatigue failure is calculated as function of safety factor. The paper also includes the subsequent calibration of a fatigue safety factor and a recommendation for a revised fatigue design procedure.

A series of 3 papers is published at the OMAE2022 documenting the work done in the NorMoor Phase 3 on the fatigue limit state of mooring lines. The deterministic fatigue analysis method is supported by an accompanying paper at this conference, OMAE2022-81441, in which an importance sampling technique developed in the NorMoor JIP is documented to provide the most accurate fatigue estimates for the minimum of sea states analysed. Furthermore, another accompanying paper, OMAE2022-81465, provides statistical analysis results of a comprehensive database of fatigue test data, including both new and used studless chains. With this input, it has been possible implement an improved fatigue analysis methodology that also accounts for the mean loads as well as degradation based on extent of corrosion.

The present work provides a unique and comprehensive set of results, where advanced reliability methods are used to calibrate a mooring design code where the mooring line tensions are calculated in the time domain. The results provide a basis for regulators, such as ISO, to update their rules. The paper concludes with a recommendation for a revised fatigue design procedure, and the plan is to implement this in the next revision of DNV-OS-E301.