In this paper, we discuss the characteristics of structural response of semi-submersible V.L.F.S. in waves based on rigorous numerical analysis. In this analysis, hydoroelasticity and hydro-dynamic interactions are fully considered. A guide-line for structural design of semi-submersible V.L.F.S. is proposed after relations between the structural response characteristics and structural configuration are discussed.
There are two types of semi-submersible V.L.F.S. proposed. One is column-supported type seen in Kansai Intl Airport 1st Phase, floating structure plan. V.L.F.S. of this type is supported on columns of large diameter with small depth. It is shown that the response characteristics can be dominated by quasi-static response pointed out by a part of the present author. The other type is column and lower-hull supported model represented by MOB, whose shape is obtained by extending the present semi-submersible rigs of two lower-hulls. It is shown that some of the most important response characteristics with the structure of this type are horizontal bending in low frequencies and structural resonance.

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The 4000 m class deepsea drilling vessel will be realized in Japan in near future. The drilling vessel is equipped with deepsea riser, and this paper discusses longitudinal dynamic response of the deepsea riser in hung-off condition. This paper shows that there exist fixed points in the frequency response of strain of hung-off deepsea risers. A fixed point is a point in a frequency response which does not change irrespective of the change of some governing parameters of the dynamic system. The fixed point is originally found in the discreet mass-spring system, and this paper shows that the fixed point exist also for a continuum system, riser & BOP system. The fixed points are shown to exist in the frequency response of axial strain with respect to the damping of riser and also the damping of BOP. Considering the position of fixed points in the frequency response, it can be concluded that the increase of damping does not always improve the dynamic response of hung-off riser. When the external exciting frequency, namely heave motion of vessel, is lower than the fixed point, strain at riser top is increased with increase of damping. On the other hand, if the riser is excited at the frequency larger than the fixed point, response is significantly improved.

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The marine structure using under severe oceanic environment should be designed with careful consideration of keeping the exposed working place to be comfort. In case of severe conditions beyond the criterion of thermal environment, some countermeasures for thermal compensation, such as a working space with windbreakers or sunshades, need to be taken adequately. The designer of an offshore structure should understand the dependence of the thermal stress of human body on the thermal environmental factors such as operative temperature, metabolic heat and clothes.
In order to grasp the relation between the heat stress of human body and the thermal environmental factors, a formula for heat stress is established with a view of calculating the quantity of heat storage using the heat balance model of human body. This formula that is consisted of temperature, wind velocity, metabolic heat and clothes heat resistance, can be defined to be the critical working hours to lay down the limit time when a variation of body foundation temperature exceeds the fixed threshold value. The necessity of several kinds of compensated equipment for the protection of workers against severe exposure environment is evaluated by use of present judgement method.

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