The level of soil stiffness under spudcan footings is an area of intense interest and debate, with many practitioners believing that current jack-up assessment guidelines are overly conservative. In order to evaluate appropriate stiffness parameters, back-analysis of case records of jack-up platforms in the North Sea has been performed. The records relate to three different rigs at a total of eight locations, which include a variety of soil conditions, water depths and sea-state severity. For each site the horizontal deck displacements and the sea-state conditions under storm loading are available. Numerical simulation of the platforms under storm loading was undertaken with varying levels of foundation stiffness. For each set of stiffness one-hour of numerical simulation was performed, with the most severe recorded environmental loading conditions for that site used. The horizontal deck displacements of the measured data and the numerical simulation results have been compared in both the frequency domain and by the magnitude of response. On the basis of the analyses, recommendations can be made for higher stiffness factors then are currently suggested in the SNAME, 1997 Technical & Research Bulletin 5-5A, Site Specific Assessment of Mobile Jack-up Units (SNAME, 1997).
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AbstractSmith et al. [1] established that a thorough inclusion of both 3D and nonlinear effects of waves can reduce the extreme loads computed for a jack-up. A systematic study of a number of jack-ups in different water depths and locations has been performed to extend and generalise the applicability of the results and to provide recommendations for enhancing the current practice as set out in SNAME T&R Bulletin 5-5A. Particular attention has been paid to the extreme dynamic amplification of the response which is difficult to define in an irregular sea.The work builds upon an initial study which considered the load reductions that could be obtained for a LeTourneau 116C jack-up operating in 60m of water in the North Sea and 100m of water in the Gulf of Mexico. To provide a more general coverage the present study provides a more comprehensive assessment considering a wider range of environment and operating conditions and further rig classes. These results have been used to derive simplified methods for inclusion in T&R 5-5A which can robustly represent the effects of 3D and nonlinear waves in an assessment context, without having to perform the more complex and time consuming direct evaluation.The reduction in loads is quantified using a 2 nd order directional wave theory for the irregular extreme wave kinematics coupled with an existing analysis model which simulates jack-up quasi-statics and dynamics. The results show that significant reductions may be gained for TRS areas, whilst more modest reductions are obtained for non-TRS areas. The investigation of the existing recipes for evaluating dynamics revealed inadequacies in the "SIPM" approach in T&R 5-5A. For robust results a modification of one of the alternative methods in T&R 5-5A is proposed.The inclusion of irregularity, nonlinearity and directionality in the description of the kinematics associated with extreme ocean waves allows for a more accurate assessment of jack-up loads. Existing analysis methods which ignore the characteristics of real ocean waves may significantly overestimate the hydrodynamic loads on offshore structures. The explicit consideration of short-crestedness will provide benefits for the design and assessment of jack-ups, extending their versatility for operations in a wider range of locations and water depths.
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