Recent tension leg platform (TLP) model tests have revealed strong transient resonant responses during extreme wave frequency responses. It is said that these responses, termed ringing, are associated with large-crested, near breaking wave events. In this paper, as a first step of giving light on the occurrence mechanism of such nonlinear phenomenon, first-order, second-order sum frequency and higher order wave force measurement tests are carried out in regular and irregular waves. At the same time, wave height distributions of the wave field inside the TLP are also measured to investigate the correlation between the occurrence condition of higher-order wave force and the wave height deformation. The first and second order wave force measurement tests are used to validate the predictions based on a newly developed first-order and second-order diffraction theory for an arbitrary three dimensional floating bodies. As a result, not only first-order wave forces but also second order sum frequency forces can be predicted from the second-order diffraction theory except for high frequency range ( kL >6, k : wave number, L : distance between columns of TLP). However, a significant third-order surge force occurs when kL becomes larger than 4.4 and has the same characteristics as the first-order force. Furthermore, nonlinear force higher than second order occurs even in irregular waves when a crest pairing of waves occurs inside TLP. Such forces seem to be caused by what the surface elevation inside TLP become more steep and nonlinear, as compared with the incident wave, by a hydrodynamic nonlinear interaction between columns.