The prediction of air-gap and deck impact on column-based platforms in steep and high waves is addressed. Numerical models based on linear and second-order diffraction-radiation analysis are validated against model test data, for various cases including fixed as well as floating structures. Significant higher-order effects are identified from systematic variations in the wave steepness. Wave-in-deck impact loads are modelled by a simple formulation similar to Kaplan's approach, but taking into account amplification effects from the large-volume hull. Predicted load time series compare quite well with model test data. Introductory CFD studies using a commercial Volume-of-Fluid type tool are presented, from which promising results are obtained, and challenges for future improvements are addressed.
The calm-water air-gap under offshore platform decks is an important design factor, and is determined by the expected minimum air-gap in extreme design conditions. For columnbased platforms such as semisubmersibles (Semi's), tensionleg platforms (TLP's) and gravity based structures (GBS's), the prediction of minimum air-gap in harsh environments, and of probabilities of deck impact events, is a challenging task. If linear theory could be assumed, the amplification of waves could be easily predicted by a standard numerical diffractionradiation approach, or, for a circular fixed cylinder, by the classical analytical solution by McCamy & Fuchs /1/. However, experience shows that in steep waves there are significant nonlinear contributions /2,3,4,5/. This includes interaction effects as well as effects in the incident wave themselves. For design in extreme waves it may lead to a significant difference in height levels. In addition to the level prediction itself, nonlinear tools are also needed in the accurate prediction of resulting wave-in-deck loads in case of negative air-gap.
Most standard engineering tools of today are not capable of accurate or robust modelling of these contributions by theoretical approaches alone, and design load values are therefore most often based on model test experience. Still, second-order diffraction-radiation codes for prediction of the free-surface elevation have been developed within the last 10 – 20 years, see e.g. /3,6/. Experiences from use of such modelling and comparison to measurements show that it may represent a significant improvement relative to linear theory /3,7/. However, these and other works also show discrepancies which are not fully explained. Therefore there is a need for more testing and validation of such models. It can also be questioned whether or not higher-order or fully nonlinear models are needed for a satisfactory modelling, due to the strongly nonlinear effects observed. Such models are also in development, see e.g. /8,9/, although the use in engineering applications still seems to be in the future.
Various simplified wave-in-deck load models being used for jacket types of platforms have been reviewed in /10/. One such model was proposed by Kaplan /11/, based on the principle of conservation of fluid momentum. A twodimensional fully nonlinear boundary element method was presented in /12/. None of the simple methods seem to take into account wave amplification due to a large-volume hull.