5. Expansion of Mode Shapes and Responses on the Offshore Platform Valdemar

There is a need in the future for maintaining and increasing oil and gas production in the Danish North Sea. Related to this are studies for exploring the potential for extending the lifetime of offshore platforms by implementation of Structural Monitoring Systems (SMS). The project, which this paper is based on, uses an expansion technique as a first step in the sequence of assessing the actual lifetime of a platform. Mode shapes and natural frequencies are estimated using operational modal analysis. The mode shapes are then expanded by expressing each experimental mode shape as an optimal linear combination of selected modes from a finite element model. The offshore platform, Valdemar, which is fully instrumented with accelerometers, GPS, strain gauges and wave radars, is chosen as a case study. Results show that the measured response can be expanded with high precision, which provides valuable information when assessing the actual lifetime of the platform. Also it is shown that the expansion technique can be used for assessment of measurement uncertainties.