The motions of floating moored structures are affected by first order wave loads which are proportional to the wave amplitude and associated with the wave frequency. On the other hand, second order wave loads are proportional to the square of the wave amplitude and related to the sum or difference of a pair of frequencies of the irregular sea. Although the second order loads are usually smaller compared to the first order loads, these loads can excite resonance motions in frequencies that the system has very low damping. Therefore, second order wave loads have particular importance in the design of mooring systems.
The multi-body system composed by Tension Leg Platform (TLP) and Tender Assisted Drilling (TAD) is particularly susceptible to the second order effects, due to the very low natural frequencies of their horizontal modes and the very high natural frequencies of the vertical modes of the TLP.
This work presents a numerical study of second order wave loads on the TLP-TAD multi-body system. An extensive hydrodynamic analysis focus on the hydrodynamic interactions between the floaters and how these effects modify the second order loads on the platforms was performed. The second order quadratic transform functions (QTFs) were evaluated using the indirect and the direct method. Moreover, the importance of the free surface integral was checked. Finally, the accuracy of Newman approximation for the low-frequency QTF was evaluated.