Non-linear Wave Load Model to Predict the High Frequency Deep Wave Impact Forces on Offshore Structures

Abstract

A simplified wave impact load model to predict the additional impact (Ringing) load due to a high frequency steep wave at near breaking condition is presented in this paper. Model tests and field experience of offshore structures have shown that loads on large surface piercing structures, such as columns of concrete gravity structures, in steep wave condition are considerably larger than those assumed in conventional design calculations. In extreme, near breaking wave conditions, offshore platforms exhibit significant resonant response called ringing. The ringing effect occurs like irregularly spaced bursts of response at the natural frequency of the structure. The bursts must be superimposed on the response predicted by traditional linear theory. The wave load model is developed based on Morison?s equation supplemented with a wave slap term. Water particle kinematics used in the present model is predicted using the Stokes fifth order wave theory. The simplified wave impact load model is validated with experimental results by Zou and Kim (1995).The validation of load model shows good agreement with the experimental and analytical values obtained by Zou and Kim (1995). A computer program has been formulated based on the model to obtain the wave loads at all discretised nodes, along the length of column from instantaneous free water surface to the bottom of the column at mud level for the given depth of water, wave height, period of time and size of the column and to calculate the non-dimensional load coefficient for all nodes. The predicted wave loads vary hyperbolically along the length of the column. The simplified wave impact load model enables practical applicability without involving tedious computational efforts and saves computational time and space.