ABSTRACT

Quasi-static mudline loads for circular pile are calculated for regular and irregular seas. Regarding regular waves, emphasis is given to 5th order Stokes profiles. Consequences of various ways to define the Stokes 5th profile are included. The most conservative choice is to adopt the design wave profile by the crest height and associated mean wave period. Regarding irregular sea states, simulated surface waves and associated kinematics are presented for linear wave theory and second order theory.

INTRODUCTION

The hydrodynamic load on a slender pile structure is reasonably well described by the Morrison load formula. This load consists of two terms, an inertia term which is proportional to the acceleration of the fluid particles normal to the pile, (t), and a drag term that is proportional to u(t)|u(t)|, where u(t) is the particle speed normal to the pile. The kinematics used in Morrison equation is the undisturbed kinematics. This means that diffraction effects must be negligible. For a harmonic wave this is in practise assumed to be fulfilled if (equation), where λ is the wave length and D is pile diameter, Faltinsen (1990). If also (equation), the drag term of the Morrison equation is dominating. The latter will often be the case for structural members of jackets and jack-ups when estimating characteristic loads for design in harsh weather areas. In such an area the ULS wave height may in the order of 28m, while the ALS wave height can be 25%larger.

AIM OF PAPER

In this paper we will focus on drag dominated cases. Since horizontal particle speed is maximum under the wave crest, one has to integrate Morrison equation to the exact surface in order to obtain reasonable design loads. This suggest that the extreme crest heights should be properly estimated in order to obtain accurate global loads on the pile structure. In order to make study a little more complete we will also touch upon cases where the inertia forces also is important.

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