Overall Dynamic Characteristics of Tension Leg Platforms

Abstract

ABSTRACT In TLP motion analysis, the problem is usually simplified by idealizing the tethers as weightless springs. While this approximation appears to be generally adequate for shallow waters, the relative tether/platform weight ratio increases very rapidly as the water depth increases, thus making it necessary to the at the TLP as a continuous system rater than as a rigid body with only six degrees of freedom. The purpose of this paper is to examine the variation of the overall dynamic response characteristics of a TLP as a function of the water depth, with particular emphasis on such important parameters as the tether/platform weight ratio, the periods of the "principal modes" (i.e., heave, pitch, roll, surge, sway and yaw) and the shapes and periods of the "secondary modes". INTRODUCTION Offshore platform technology for relatively shallow water (approximately less than 1000 feet) is well established. However, as the demand for oil continues to increase, the industry is gradually forced to move into more hostile environments, namely, deeper waters where conventional fixed platforms are no longer feasible and arctic regions where the engineers are faced with a variety of ice-related problems. Two principal concepts have thus far been advanced for oil product ion in deeper waters, namely, the "tension leg platform" which is usually referred to briefly as the TLP and the "guyed tower". A central issue in the design of an offshore platform is the dynamic behavior of the platform under the action of wave, current and wind loads. The wave climate at a specific geographic location is usually represented by a "wave spectrum". A review of wave spectra representing the conditions at major offshore oi1 production areas of the world shows that the most critical wave period range is approximately 5 to 20 seconds. Accordingly, in order to avoid possible resonant behavior, platforms are designed in such a way as not to have a natural period within that particular critical range. In the case of a fixed pl at form, the relevant vibration mode is the so-called "lowest mode", i.e. the mode that corresponds to the lowest frequency and, consequently, the largest period. It is clear that, if the largest natural period of the platform is kept below 5 seconds, the remaining natural periods will be even smaller so that one does not need to worry about them in the process of design. In the case of a tension leg platform, however, the situation is somewhat more complicated as wi11 be apparent in the following section.