In case that a topside is transported to an installation site by using a deck transportation vessel, a topside could be fixed by using seafastening structures to be installed to a transportation vessel safely. Topside and seafastening structures behave in connection with the transportation vessel. Thus, to check the safety of the structure for the transportation period, the fatigue analysis of the seafastening structures in the coupled behavior of the transportation vessel and the topside is conducted in this work. Rainflow count algorithm is used to analyze a fatigue damage and the life of the seafastening structure. In this study, the number of environmental cases for the analysis is 14 and each case is analyzed to estimate each fatigue damage. And these damages of seafastening structures in each case are summed and fatigue life is obtained from the total fatigue damage. Because many calculation time is needed for all analysis cases, the average sea-state method is proposed. Expectation average values of significant wave height and peak period could be solved to use representative one for analysis cases. This average sea-state case is analyzed only once in the method. The fatigue damage and life of seafastening structure are estimated by using the result of the average sea-state method and compared with the results in common method.
The transportation method for an offshore structure becomes more important as offshore structures are getting larger and an installation depth becomes deeper. In case that a large offshore structure itself cannot be transported once, the structure is transported by splitting it into two parts that are lower support structure and topside. The lower support structure can float on the water surface by using its own buoyancy and thus, is transported by using tugboats. But the topside has not a buoyancy and also consists of equipment unfamiliar with water such as electrical, chemical, mechanical systems and so on. Thus the topside can be transported by using a deck transportation vessel (DTV) to avoid a direct contact with water. To be transported safely to an installation site, topside must be fixed to the transportation vessel by using the seafastening structures.