ABSTRACT

A model is described for the calculation of wave loads on pile template offshore platforms in random directional waves. The model incorporates drag and inertia coefficients measured in laboratory and field. The force coefficients vary with amplitude to diameter ratio and surface condition of each member. A method is described for estimating the appropriate amplitude for a member in random waves and current, so that time varying coefficients are not required. Random directional waves are simulated using a fast Fourier transform simulator and linear wave theory. The linear wave theory kinematics are modified in order to produce reasonable velocities near the free surface. Conditioned simulation of forces on the Exxon Ocean Test Structure and the Shell COGNAC platform are compared to measurements on these platforms in severe storms. The conditioning helps eliminate much of the random variability and permits the comparison of wave force time series rather than probability distributions.

INTRODUCTION

This paper describes a random directional wave force (RDWF) model and presents comparisons between RDWF calculations and measured platform forces. Computation of forces on offshore platforms consists of two nearly distinct parts. The first part is the determination of water kinematics and dynamics. This describes the motion of the water due to waves and currents and the pressure gradients in the water. The second part is the determination of the forces on the platform assuming known water motions. These two aspects of the problem are separable because it is assumed that the presence of the platform has a negligible effect on the water motions. The random directional wave (RDW) theory provides a more realistic description of the ocean surface and more accurate water motions (1, 2) during storms than do regular unidirectional theories commonly used in design. Both the increased accuracy and the need to include random process theory in the dynamic analyses of platforms have spurred the development of the RDWF model. The RDWF model uses force coefficients based on the best available experiments. In the discussion below no attempt is made to relate the force coefficient model to the wealth of sinusoidal and/or unidirectional data available as those flows do not accurately represent actual offshore conditions. This paper presents the force coefficient model of RDWF. Comparisons of the force coefficient model with laboratory and field data are made. Force simulations are used to demonstrate the limitations of available data for calibrating calculations in extreme conditions. The complete RDWF model is verified by comparison with force measurements made on the Ocean Test Structure (OTS) (3) and on the COGNAC platform. The bias and covariance of the model is shown. This paper describes the force calculation model. The parameterization of the force coefficients with the amplitude to diameter ratio is discussed. The method of selecting the amplitude of motion in a random wave environment in the presence of a current follows. Comparisons are made with the two best data sets available: a laboratory experiment referred to as 2D-Forces (4) and the well known offshore experiment OTS(3).

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