This paper deals with a comparison between the computed and measured motions and tether forces for a deep water TLP. The computations are compared with model experiments which have been conducted in both regular and irregular waves in the Seakeeping Laboratory of the Netherlands Ship Model Basin.
Attention is paid to the prediction of the second harmonic components of the tether forces in regular waves and to the computation in the time domain of the combined wave frequency and low frequency motions and tether forces in irregular seas. The time domain computations have been performed using as input the wave record measured in the basin.
Comparison of results of computations and model tests allow identification of the major cause of high frequency tether force fluctuations and demonstrate the applicability of the convolution integral method in time domain analysis of low frequency TLP motions.
In designing a tension leg platform knowledge of the motion behaviour and tether forces is important. In an earlier OTC-paper by Tan and De Boom  a computation method to predict the first order platform motions and tether forces in the frequency and time domain and the mean and low frequency excursions in the horizontal plane in the frequency domain has been presented. The computations which were based on linear potential theory were carried out for a TLP operating in 450 m water depth. Model tests were performed in regular and irregular waves in the Seakeeping Laboratory of the Netherlands Ship Model Basin to validate the computation method.
This correlation study presented in ref.  demonstrated the general applicability of the computation method for providing the above mentioned data in the preliminary design stage, even though some non-linearities in the platform behaviour were observed. It was found that second harmonic components in the motions in the vertical plane and in the related tether loads cannot be neglected.
In the present paper a comparison between the computed and measured second harmonic components in the tether forces in regular waves is presented. Moreover, the computer program was extended so as to generate the time traces of the motions and tether loads including the low frequency behaviour induced by low frequency second order wave drift forces. The applicability of this numerical method was shown by Pinkster and Huijsmans  for a semi-submersible. A similar correlation is made in the present TLP study.
For this correlation study a TLP was designed having a relatively simple shape. Details - such as bracings - of which no significant effect was expected on the hydrodynamic behaviour were left out. The resulting platform consisted of a square deck supported by four major circular columns, which were interconnected by submerged rectangular pontoons. Vertical mooring tethers were attached at the centre of each of the corner columns. Figure 1 shows the main dimensions of the TLP, whereas in Table 1 the particulars of the loading condition and mooring tethers are listed.