Frequency-Domain Analysis Of Otec Cw Pipe And Platform Dynamics

Abstract

ABSTRACT A quasi-linear frequency domain analysis of the coupled OTEC CW pipe and platform system has been developed. A finite element model of pipe structural dynamics is employed and the platform is modelled by contemporary ship or stable platform motion theory. Nonlinear fluid forces are included by means of the method of equivalent linearization. The procedure has proven to be extremely versatile in its ability to represent various platform configurations and realistic directional random sea states. The F.E. representation of the CW pipe has the capability of dealing accurately with pipes having variable properties along the length and discrete irregularities such as flexible joints. The procedures have been applied successfully to a wide range of platform and pipe configurations. INTRODUCTION Most of the open sea Ocean Thermal Energy Conversion Plants which have been studied to date employ a cold water pipe approximately 2000 to 3000 feet in length and from five to 100 feet in diameter, depending upon plant capacity. The pipe is suspended vertically below a floating platform and is subject to dynamic excitation from the motion of the platform as well as the wave-induced forces which act on the near-surface portion of the pipe itself. The length to diameter ratio of the pipe is such that it may be expected to be quite flexible in its bending response to these forces, and the resulting deflections of the pipe about its mean position, consequently, will be sufficiently large as to require that the motions of the pipe must be taken into consideration when computing the fluid forces acting on it. In the larger capacity plants especially, the overall length and weight of the cold water pipe are such that there is little possibility of easily retracting it into the floating platform in the event of a severe storm. The pipe must, therefore, be designed structurally to withstand the most severe sea conditions to be expected in its region of operation with an acceptably low level of risk of failure. In carrying out this design process, the dynamic aspects of the pine response to waves, in combination with the dynamics of the platform, must be included in the design analysis procedure. In some of the proposed OTEC systems, the CW pipe is constructed as an essentially continuous tubular structural member. There may be a small number of flexible joints which allow some nearly unrestricted rotational flexibility at the pipe-platform connection and, perhaps, at several locations along the pipe length, and these points divide the pipe into two or three tubular beam-like segments. In other cases, it has been proposed to construct the pipe of a series of short, essentially rigid, tubular segments, with a relatively flexible joint at the junction of each pair of segments. The principal source of flexibility, in this case, comes from these joints, and the segments themselves may be assumed to be completely inflexible.