Japan Marine Science and Technology Center plans the TLP sea test using a prototype floating offshore structure which has been designed by Ship Research Institute, Japan Ministry of Transport, for the joint at-sea experiments. The TLP sea tests will be excuted in this and next years. The test will be performed to evaluate the performance and reliability of the tension mooring system in the real ocean environment. The site is 40m deep and 3km offshore facing the Japan Sea. The environmental condition is favorable in summer, which allows us very easy installation work. It becomes, however, severe in winter. The prototype floating ocean structure is not originally designed for TLP. Moreover this test structure is made as small as the motions of the structure and the mooring line tensions can be measured in relatively severe sea conditions upon keeping enough stability. The air draft is only 5.5m. Consequently the great waves will crash the upper structure causing shock loads in the tension mooring line. By the 1/40 scale model test the critical wave heigh is proved to be 7.2m. Therefore, the test period during which the sea test can be excuted safely is decided from May till October. The stress check for yielding and buckling is proved that the working stresses under the critical environmental conditions are much less than the allowable stresses.