The hydrodynamic behaviour if a tension leg platform is analysed using two complementary mathematical models. The objective is to determine the motion of the structure under wave action and calculate the tensions in the mooring lines.
A first analysis is performed using a linear motion response program based on diffraction-radiation theory. Response amplitude operators are obtained for the motions and the mooring line tensions, over a range of wave periods and directions. The importance of coupling terms due to inertia, damping and mooring stiffness components is shown. The limitations of conventional linear analysis are described.
A second analysis is performed to investigate the non-linear effects using a non-linear motion response program based on direct integration in the time domain. Significant increases in motions and mooring line tensions are obtained. The causes are examined in detail, in particular nonlinear couplings due to the mooring system and various effects closely linked to the water structure interaction in the splash zone.
This paper presents. some of the results relative to the study of the hydrodynamic behaviour of a proposed tension leg platform.
Such structures have different motion responses to that of most freely-floating bodies such as semi-submersibles because of their high mooring system stiffness. Since the early work by Paulling and Horton (Ref. I), the basic characteristics of tension leg platforms have been well explained in a number of papers (Ref. 2, 3, 4). In particular it is not evident that the classical assumptions of linear theory can be successfully applied to such structures as non-linear effects are important and have significant influence on their dynamic behaviour.
Experimental investigations on model tests show that classical methods based on linear or linearized analysis are not completely adequate when applied to tension leg platforms and other methods must be used, which offer the possibility of representing essentially non-linear phenomena; these can be time-domain simulations (Ref. 5, 6, 7) or iterative frequency-domain methods (Ref. 8).
It is the aim of this study to assess the importance of non-linear effects by examining the dynamic behaviour of one particular design of tension leg platform. Two mathematical models are used. The first performs a linear diffraction-radiation analysis to establish the hydrodynamic characteristics of the structure and the linear response. The second carries out a non-linear three-dimensional time-domain simulation to assess the importance of non-linear effects with increasing wave steepness.
Both models are well established and have been successfully applied to a wide variety of floating objects including barges, tankers, semi-submersibles floating gravity platforms and articulated towers. They are used here to study the behaviour of a tension leg platform and determine the tensions in the mooring lines.
The structure considered consists of eight columns making up four tension legs linked to vertical tethers. The water depth is 150 m (Fig. 2).