Nonlinear Simulation of Jackup Platform Models
Authors: Liu P. , Massie W.W. , Wolters J.G. , Blaauwendraad J. 
Published: October 2008

This paper summarizes the laboratory studies and associated computer simulation; of three principle jack-up platform models tested in a wave tank. The primary objective of this work is to investigate the influence of hydrodynamic and P-? non-linearities on such rigs. The computations were carried out in the time domain using a nonlinear, dynamic, multiple degree of freedom simulation which includes various hydrodynamic interaction options. Comparison of the computed results with the measured data shows that such simulations are reliable. Further results demonstrate that: Hydrodynamic interaction must include relative rather than absolute velocities, the P-? effect is essential, damping and stiffness at the deck-leg connection can (within certain limits) be 'traded off' and a realistic model should be calibrated using not only the recorded response data but also the recorded internal forces as well. Hydrodynamic parameter studies such as the influences of linearization, free surface treatment, different wave theories, hydrodynamic force cancellation, etc., are also presented.

INTRODUCTION

The primary objective of this work is to investigate the influence of hydrodynamic and P-? nonlinearities on elevated jack-up rigs. This helps 'unravel' the dilemma of the various (conflicting) existing bodies of knowledge on performance of elevated jack-up platforms. Three principle models were tested in a wave tank. The associated simulations were carried out in the time domain using a nonlinear, dynamic, multiple degree of freedom software which includes various hydrodynamic interaction options.

The present work forms a part of a Delft University of Technology project with objective to increase the detailed knowledge of the behavior of such platform components as well as the prediction of the overall structure's elevated behavior and (remaining) lifetime.

THE MODELS

Hydrodynamic model testing on three principle jack-up models was carried out in the wave tank of Ship Hydromechanics Laboratory, TU Delft, during summer of 1988.

Necessary simplifications were made in the model design to concentrate attention on the physical processes to be studied. While some discussion of model scales is relevant, no attempt has been made to reproduce actual field conditions in the models. Instead, the physical models should be seen as full scale structures.

A convenient tank water depth was 2.0 m. Possible wave frequencies in the basin range from about 0.6 to 1.3 Hz with wave heights up to 0.080 m. (Higher frequencies were reached for lower wave heights). These were lower than used for the model design due to wave breaking.

The natural frequency of the structures tested was chosen to be around 1 Hz. The model leg spacing was chosen to include a reasonable hydrodynamic force cancellation effect. The design approach, further, was to choose the leg stiffness such that the model platform has a quasistatic deflection of 2% of the water depth at deck level if the peak force resulting from a design wave was applied to all 3 legs simultaneously. By choosing different leg materials and adjusting deck masses, it proved possible to essentially retain the natural frequency and quasi-static deflection (as outlined above) while using two quite different types of legs. Fig. 1 illustrates the model geometry.