NorMoor JIP, Structural Reliability Analysis for Mooring Lines in Ultimate and Accidental Limit State

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. Our motivation for initiating a study on mooring line reliability started out with questions raised with regards to the applicability of the mooring standards. All the global standards (API, ISO, DNV, others) are mostly based on work from late 1990s, when frequency domain analysis was prevalent while time domain analysis is often applied today. The reliability level implied by these regulations is not known, and we also see that the mooring standards are interpreted and applied differently. The objective of the NorMoor JIP has therefore been to provide a more consistent analysis methodology, and calibrated safety factors for time domain in line with this methodology. This is achieved through reliability-based calibration for a range of different units, mooring systems, water depths and geographical locations. Thus, the NorMoor JIP is intended for global application, and the aim is to get the results implemented into global standards like ISO. 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). The reliability analysis is based on a detailed probabilistic model of the metocean environment, state-of-the-art response analyses and strength data from break load tests. The probability of failure is calculated as a function of the magnitude of the safety factor. It is shown that present regulations result in a significant scatter in reliability level between the cases. An alternative design format with 2 partial safety factors may potentially reduce the scatter. Differences in results for hurricane conditions in the Gulf of Mexico and storm conditions on the Norwegian Continental Shelf are obtained, indicating the need for a different design philosophy for the two locations. The present work provides a unique and comprehensive set of results, where advanced reliability methods are used in combination with detailed response calculations in the time domain. The results provide a basis for calibration of mooring design for ULS and ALS and subsequently for regulators to update their rules. Calibration of safety factors and selection of target reliability level are presented in a companion paper at OTC 2017.