Fixed structure motion measurement and structural damping calculation

Many fixed offshore platforms are approaching their original design lives, but if their lives can be extended they remain the most costeffective solution to ongoing production of the field. Operators and designers now have access to relatively low-cost, high-accuracy instrumentation tools that can be used to monitor the loading and dynamic response of fixed offshore platforms. This paper details a measurement campaign to monitor real-time loading of two fixed offshore platforms, a gravity-based Well Head Platform and a braced monotower, and to calculate their damping as an input into structural analysis and fatigue modelling. The effective structural damping levels were found to be similar, and substantially greater than those currently recommended in codes and in the literature for offshore platforms. The use of these more realistic, higher damping values has the effect of decreasing calculated fatigue damage, thereby allowing production to be extended. Further, instrumentation of platforms provides operators real-time proxy information on risky loading conditions and structural changes on the platform. This can complement a risk-based inspection framework, whereby relatively riskier locations are targeted for priority inspections and, if necessary, repair or modification. The illustration of the techniques used in this study of offshore platforms near their original design life will be of significant benefit to offshore operators, platform designers and contractors, and classification societies.