The semi-submersible vessels, as special transport ships, have unique characteristics in order to achieve their transport capacity by comparison with the typical transport ships, such as oil tankers, bulk carriers and container ships. The conventional transport ships have specific target cargo and have been given clear design combinations of working conditions (loading in alternate hold, etc.) by classification societies. There are no definite requirements of the calculated the working conditions for submersible vessels from the classification societies because of the complexity and variety of the cargos transported by the large-scale submersible vessels. The design loads of the deck and the combinations of the working conditions are studied for the large-scale semi-submersible vessels. The basic design idea of the deck loads and working conditions are also summarized for the large-scale semi-submersible vessels. The issues to be considered under the typical design working conditions are presented to ensure that the design of working conditions not only covers the typical loading requirements of the ship, but also is not too conservative, which will in turn affect the member design and the cargo loading capacity of the semi-submersible vessels.

The requirements of the loading capacity of semi-submersible vessels are increasing with the diversified and large-scale marine equipment. The 4-island semi-submersible vessel has a fully-integrated main deck that is especially suitable for the transportation of the ultra large vessels and the marine structures. It introduces the structural design and strength evaluation of the conventional 3-island semi-submersible vessels, and discusses the deck load, allowable bending moment shear force and mid-ship section design that affect the cargo capacity of the semi-submersible vessel. These design points of the 3-island semi-submersible vessel are also applicable for the 4-island semi-submersible vessel. Compared to the conventional 3-island semi-submersible vessel, the 4-island semi-submersible vessel has the unique arrangement characteristics of the narrow and long side island that is overhanging the main hull. The structural design should consider comprehensively about the influence of the pipes and equipment for the ventilation and exhaust on the hull structure for the optimization design, and meet the requirement of heavy cargo transportation. It introduces the general idea of the structural evaluation of the 4-island semi-submersible vessel and the analysis of the relevant structural strength and the fatigue of key nodes. The vibration of the overall 4-island semi-submersible vessel and that of its side island are also evaluated to ensure the performance of the ship.