Smith et al.  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 2nd 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.
Ocean waves are irregular, nonlinear and directionally spread (short-crested). However, the analysis methods currently recommended within SNAME T&R 5-5A  do not take all of these characteristics rigourously into account when assessing jack-up loading and response. Deterministic wave force calculations (quasi-static) use periodic (regular), unidirectional wave models such as Stokes 5th. Whilst nonlinearity is included, this approach neglects the irregularity, or random nature of wave excitation. On the other hand, the current recommended approach for the calculation of wave induced load effects on jack-ups (dynamic) using stochastic/random waves, uses the kinematics computed from the superposed linear Airy wave components and, consequently, the nonlinearity of extreme waves is neglected. This analysis method also does not take into account the shortcrested nature of real seas.