Calibration of seabed fixity and system damping from GSF Magellan full-scale measurements

Full-scale measurements and analyses of environmental conditions and dynamic response of the GSF Magellan jack-up for calibration of seabed fixity have now been completed for five winter seasons. During this time, the unit was operating at the Elgin and Franklin platform sites in the North Sea in about 90 m of water depth. The instrumentation system installed on the jack-up acquired continuous data for five winters, and storms with up to 8 m significant wave height were recorded. For selected storms, time domain dynamic response analyses were carried out with adjusted soil spring stiffnesses that result in the same measured and computed natural frequencies. Using spectral analysis method, the modal damping was evaluated by trial and error such that the measured response peak matched the simulated response peak at the natural frequency. The system damping was computed using the half-power method from both measured and simulated response. This paper discusses the analysis methodology and results in an effort to enhance the industry's understanding of jack-up foundation behaviour, and it includes comments on the conservatism in SNAME T&R 5-5A (2002), as well as in the updates for SNAME proposed in a Noble Denton/Oxford University study commissioned by IADC. The comments and conclusions of this paper are case-specific, and care should be exercised in extrapolating them to other jack-ups or other sites.