3D Nonlinear Wave Spreading on Jackup Loading and Response and Its Impact on Current Assessment Practice

Abstract

TX 75083-3836, U.S.A., fax 01-972-952-9435. 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.

Offshore foundation systems for resource recovery: Assessing the three-dimensional response of jack-up platforms

“…This directional spreading formulation is as recommended in ISO (2005) and typical values of spreading factor φ include 0.867 for tropical cyclones at latitude of less than 40 degree, 0.88 for Monsoons in latitudes less than 15 degree and ~0.9 for the North Sea, as discussed in Smith et al (2006) and Hoyle et al (2009). A value of 0.88 is assumed for the numerical examples in this paper.…”

Performance of an example jack-up platform under directional random ocean waves

“…Smith et al [13] also quantified the level of load reduction in jack-ups using a second-order directional NewWave theory and consequently the effect of randomness again was not explicitly included. This study was extended by Smith et al [19] and Hoyle et al [20] by introducing a formula to calculate a kinematic reduction factor based on jack-up dimensions, wave characteristic and water depth and was intended to be applied in conjunction with a regular wave analysis. The formula was proposed by a parametric study conducted using a number of independent variables for five different rig classes.…”