The tension leg platform is at present the most promising concept for the exploitation of offshore fields in very deep water. One of the major tasks in the design of such structures is a correct evaluation of the wave-induced motions they represent the major contribution to the forces acting on the tethers, which are the most critical structural components of a TLP.

The TLP dynamic behaviour is characterized by low frequency, large amplitude in plane motions (surge, sway and yaw) induced by the slowly varying drift forces and by the first order responses to the direct wave excitation in all six degrees of freedom.

Although such dynamic behaviour is common to a large variety of moored vessels, TLP differs in that it may experience also a resonant condition in heave, roll and pitch motions. Despite the fact that the tether stiffness is chosen to keep the natural periods of these motions out of the wave frequency range, an excitation is still present, due to the second order, high frequency, wave forces. Consequently, tether slack may be experienced or tether fatigue life may be drastically reduced by high frequency cyclic loads.

The importance of a correct evaluation of the fluid-structure interactions, together with an accurate mathematical modelling of the rigid body dynamics is now evident.

Nowadays a good level of accuracy has been reached in the estimate of the first order wave forces either using programs based on 3-D potential theory or, in some cases, a Morrison type formulation (refs 1–2). Many efforts have been made to evaluate the wave forces up to the second order in both the low and high frequency range (refs 3–41, but several uncertainties are still present. In particular, no reliable tools are available for the estimate of the low and high frequency damping, which is, of course, of paramount importance when dealing with resonant motions. Furthermore all the computer procedures which have been developed are very expensive and cannot be used for parametric studies or in an early stage of a project.

For this reason Tecnomare has developed a computer program, specifically devoted to the TLP dynamic analysis, which can tackle the problem at different levels of accuracy and make possible a substantial saving of computer resources A comparison has been performed with experimental results relevant to a TLP in 800 m water depth, which shows how even a simplified approach may be sufficiently accurate.

TLP CONFIGURATION

The TLP analysed in this study has been designed to operate in 800 m water depth A square deck is supported by four circular columns with bottom footings (Fig. I), which are Interconnected by submerged circular pontoons The tether system comprises four tubes of 50 cm external diameter placed at the centre of each column.

The pre-tension (bouyancy-weight) and the tether axial stiffness were chosen to give the following natural periods at the rest position:

(Table available in full paper)

Fig. 1 Front and top view of the TLP (units m)

METHOD OF ANALYSIS

This content is only available via PDF.
You can access this article if you purchase or spend a download.