Monitoring of riser motion and loads is increasingly being used to assess metal fatigue and to assist rigs in operating safely. Monitoring systems provide valuable information on unexpected incidents. This paper shows how sensor data have been used in combination with structural analysis to assess consequences of a loss of tension for a top-tensioned CWO riser. In order to avoid that a single failure in a tensioning system causes total loss of tension and riser buckling risers should be operated using both rucker tensioners and top drive. However, prior to the retrieval of the riser only top tension was applied. Due to a power out the top drive tension was lost, and the resulting set down caused the riser to buckle. The consequence could in the worst case have been permanent damage to key riser components such as the Lower Tapered Stress Joint and Weak Link. However, as the riser was equipped with a monitoring system that provided accurate data on deformation at key locations, it was possible to show that the system had not been exposed to permanent deformations and could be safely used further. It is advisable to perform continuous monitoring of tension and put in place mitigation to avoid that such incidents lead to permanent damage in the future.

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