Measured and predicted dynamic behaviour of an offshore gravity platform

The paper discusses the dynamic characteristics and dynamic behaviour of the Gullfaks C platform as evaluated from full scale measurements and numerical predictions. Two issues are emphasized: identification of a dynamic model for the platform, and a discussion of the dynamic response. In the identification procedure both natural frequencies and mode shapes are used. These are estimated using a multichannel ARMA model. One important parameter is the stiffness of the soil. The identified value for this stiffness is compared with stiffness values estimated from displacement at mudline and calculated base shear. The identified model is furthermore compared with the design model, and the predicted response using the model and a theoretical load model, is compared with measured response. In the discussion of the measured dynamic response the paper features response composition, possible nonlinearities in the soil structure interaction and wave loading. Special attention is given to observed ringing response in the platform.