Summary and Recommendations for Safe Mooring System Design in ULS and ALS
Authors: Okkenhaug Siril , Hørte Torfinn , Paulshus Øivind 
Published: October 2017

DNV GL is currently running a Joint Industry Project, “NorMoor JIP”, with participants from oil companies, engineering companies, rig-owners, manufacturers and marine authorities. It is a global study covering Gulf of Mexico, Northern Europe and Brazil waters.

Our motivation for initiating a study on mooring line reliability was that all the global standards (API, ISO, DNV GL, others) are mostly based on work from late 1990s, when frequency domain analysis was prevalent. The reliability level implied by these regulations is not known, and we also see that the mooring standards are interpreted and applied differently.

Thus, there is a need for a mooring design code with a consistent analysis methodology and with safety factors that are in line with this methodology and calibrated at an appropriate target reliability level. This is achieved through reliability-based calibration for a range of different units, mooring systems, water depths and geographical locations. The focus in the present paper is the calibration of safety factors and selection of target reliability level. The underlying probabilistic analysis results used for the calibration are reported in two accompanying papers at OMAE 2017, [1] and [2], dealing with structural reliability analyses for the ULS and ALS respectively.

For mobile units frequency domain analyses are common, and although the main attention in the JIP is towards time domain analyses, it is part of the JIP to calibrate safety factors for frequency domain analyses as well. The annual extreme value distribution of line tension for all cases is calculated in time domain and is applied both in the calibration of safety factors for time domain and frequency domain analyses.

It is seen that characteristic tensions from time domain analyses are likely to be higher than those from frequency domain analyses. The dilemma of not being penalized when using more refined time domain analyses is discussed, and different safety factors have been suggested for use with time domain and frequency domain analyses.

A discussion about target reliability level is included, and the target levels are proposed with basis in the existing mooring design practice for mobile units, where frequency domain analysis is prevalent. Different targets are proposed depending on consequences of failure.

Calibration for different design formats are carried out. The current format using a single safety factor is challenged with a format with two safety factors. The objective is to arrive as close as possible to the target reliability for all cases analyzed.

A different design philosophy is needed in the Gulf of Mexico in order to achieve acceptable risk, and options are discussed.

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. ULS and ALS are covered here, and a potential phase 3 of the JIP will cover the fatigue limit state.

When the NorMoor JIP is completed the plan is to implement the results into DNVGL-OS-E301, [5].