Session: 02-04-01 Fatigue and Fracture Reliability 1

Paper Number: 81441

81441 - Mooring Fatigue Damage Prediction Based on Importance Sampling 

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 of mooring line studless chains for permanent units has got the main attention. Phases 1 and 2 was covering the ultimate and accidental limit states respectively. The 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.  

Our motivation for initiating a study on mooring line reliability was that all the global standards (API, ISO, DNV and 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 not interpreted and applied in a consistent manner. 

The environmental model applied in mooring fatigue analyses is developed from an environmental database relying on measurements and/or hindcast models. Traditionally, mooring fatigue analyses have been based on scatter diagrams, extracted from this database. More recently an alternative method is to apply an entire calendar year in the calculations, and the selected year is the one which is assumed to produce the average yearly damage corresponding to the complete environmental database. Both options imply a set of assumptions which are challenged in this paper. 

As part of NorMoor JIP, DNV has also developed a new fatigue analysis methodology based on importance sampling of the environmental database. This new technique is both accurate when benchmarked against the analysis of the complete database and effective in terms of computational effort. In addition, an estimation of the statistical error of the calculation is obtained. 

Presenting Author: Daniel Merino Hoyos COWI AS

Authors:

Siril Okkenhaug DNV
Daniel Merino Hoyos COWI AS
Eivind Tørset Magnussen DNV
Michael Macke DNV