Lessons Learned From Norwegian Mooring Line Failures 2010–2013

Abstract

15 failures of offshore mooring lines have occurred in the period 2010–2013 on the Norwegian Continental Shelf. The failures are cause by a mixture of overload, fatigue and mechanical damages. Failure statistics from the period 2010–2013 are presented and compared it with earlier statistics. In the period 1996–2005 a high number of cases were observed. The industry reacted reasonably, and the number of incidents was clearly reduced for several years. In 2010 the number of cases started to increase again, rising questions on how to improve again. Some of the old failure modes have disappeared, as dragging of anchors and failures of chains above 20 years of age. But several other failure modes have appeared or reappeared. Differences between different types of units are found. The failure frequencies are highly uncertain, but the failure rate from 2010–2013 have been in the order of magnitude • single line failures: 92*10−4 per line year and • double line failures: 12*10−4 per line year. No triple line failures have occurred in 2010–2013, but using data from 2000–2013, the failure rate have been in the order of magnitude 2*10−4 per line year. The paper discusses possible changes in practice, regulations and standards, on quality of materials, maintenance, ALS-, FLS- and ULS-design.