The Use of Direct Tension Monitoring of Mooring Lines in Reducing Conservatisms in Design and Analysis Models

Abstract

The offshore oil and gas industry is continuing to push intodeeper water and onerous environments, using increasingly complicated vessels and equipment. This, combined with changing global weather patterns has increased uncertainty in offshore production operations. Monitoring mooring lines can help reduce this uncertainty by calculating fatigue damage estimates during major storm events. This greater understanding helps to optimise inspection and maintenance schedules and assess the likelihood of future mooring line failures. Added to this, if the deterioration or failure of a single mooring line is not detected, increasing loads on the remaining lines may result in additional failures. This is regarded as a system failure and could lead to significant consequences for both well control and riser integrity, resulting in huge costs for operators. Unless the operator inspects on a regular basis or monitors the mooring system in real time it is impossible to know for certain whether all mooring lines are in place. A popular technique for monitoring mooring systems is tomeasure mooring line angle (using accelerometer based inclinometers), to infer theoretical mooring line tension. Whilst these systems are effective at alerting operators to a line failure, the fact that tension must be inferred requires a certain amount of conservatism in the estimate. This conservatism can be difficult to quantify. This paper documents the sea trial of a well proven mooring connector combined with new mooring line technology capable of measuring both mooring line angle and direct line tension. This allows for a unique comparison between the two measurements. This paper will detail the results of this comparison, showing the accuracy of inclination based monitoring systems and the level of conservatism factored in when tension is inferred. The data published in this comparison can be used to reduce levels of uncertainty in offshore operations and thus reduce future levels of conservatism in design and analysis models. This can help save costs and increase efficiency for future operations, whilst also helping support safetystrategies.