NorMoor JIP, Mooring Design Code Calibration

Abstract

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 Northern Europe, Gulf of Mexico and Brazil waters. 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. 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 for time domain analysis 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 structural reliability analysis results used for the calibration are reported in an accompanying paper at OTC 2017. Structural reliability analysis has been applied to calculate the probability of mooring line failure for intact mooring systems (ULS) and damaged mooring systems where one line is removed (ALS). These results are used as input to the calibration process where the optimum safety factor is calculated as a function of target probability of failure. For ALS, empirical failure data are utilized to assess the probability that a line is missing. 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 calibration is carried out to avoid significant under design and excessive over design. The target reliability level is determined based on evaluations of the current mooring design practice. Final recommendations on safety factors and accompanying analysis methodology are presented. Due to the nature of the hurricanes, it is shown and discussed that a different design philosophy is needed for the Gulf of Mexico. 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.