Model Tests To Establish a Design Method for TLP-Tether Systems

Abstract

ABSTRACT Model tests were conducted in regular and random waves with a view to establishing a design method for tension leg platform (TLP)-tether systems. Linearized and nonlinear analytical methods were used, and the calculated values were compared with measured values. The nonlinear method was used in analyzing a TLP tether system under such critical sea conditions as would cause the tethers to snatch a situation that should be avoided in the design of TLP tethers - and under unstable conditions with all tethers at one corner of the TLP broken off. The results of these investigate are reported in this paper. INTRODUCTION The tether system is one of the important elements in ensuring the safety and reliability of a tension leg platform (TLP). These factors of safety and reliability are influenced to a great extent by the variation in tether tension due to the wave induced motion of the TLP. To determine the dynamic characteristics of the TLP, the authors used a dynamic response analysis system (part of the Offshore Floating-Structures Design System, OFDS, which was developed to evaluate the stability and dynamic motion characteristics of floating structures) and a method of analysis that considers nonlinearity due to the dynamic motions of the structure. Model tests were also conducted to prove the validity of these methods of analysis. TLP ANALYSIS Linearized Analysis OFDS performs the following principal functions:Stability analysis.Analysis of static loads (wind, current and drift forces).Analysis of static equilibrium (offset value and mooring force).Analysis of the dynamic response of structures in waves.Structural analysis. To evaluate the dynamic characteristics of the TLP, the fourth function above was used. A TLP was assumed to be a rigid body with six degrees of freedom. Of the fluid force acting on the floating body, diffraction and radiation forces acting on its large-mass sections such as the hull and columns were analyzed by three-dimensional potential theory. The drag force on the TLP due to the relative velocity of water particles was obtained through linearization and iterative calculation.1,2 Forces that act on individual elements of a TLP can be expressed as:(mathematical equation available in full paper) The right side of Eq. (1) contains the terms for the inertial force, damping, drag force, righting force and wave force. Mi and Mai are the mass and added-mass matrices of an element i, respectively. Ni is linear damping, Ki is the restoring force coefficient matrix, and U is the velocity matrix of the fluid. Di is nonlinear damping and Fi is structural weight and wind load. The wave force is calculated at a mean offset position due to wave drift, current and wind. The restoring force is a nonlinear force owing to a drawdown effect. For these reasons, a linearized motion equation is solved by iterative calculation.