This paper presents a practical ballast system for a twin hull design semisubmersible as an answer to the problems which could result from faults both interior and exterior to the system.
The design presented is then examined through a fault analysis technique common to other industries and applicable to the life-sustaining ballast system. This examination confirms the design philosophy that a single fault or reasonable multiple faults should not lead to destabilization of the vessel.
The ballast system of a semisubmersible drilling vessel enables the crew to keep the vessel at the prescribed draft and trim as changes in variable load occur. While drilling, the draft is kept where the motion characteristics of the rig are most stable. The primary reason for this stability is the minimum reaction of the vessel to wave motion. Using the example of a typical twin-hulled vessel with six columns of 30-foot diameter (Figure 1), this vessel would only experience a change in buoyance forces of 2,700 short tons while encountering 20-foot waves. The vessel reacts minimally to such forces because of its large mass and the damping forces created by Newtonian fluid flow around the lower hulls. For the same vessel, though, a change of 135 tons in variable load would alter the draft one foot. Thus, the ballast system would have to take on board or discharge 32,000 gallons of water to stay at the same draft. The location of the tanks where this 32,000 gallons of water is taken or discharged depends on the location of the 135 tons of variable load change.
The system which enables the crew to do this is made up of a number of subsystems, all of which are important to the safety of the crew and vessel. At the same time, these systems overlap and interlock in such complex ways that a fault in one subsystem might have a disastrous result on other subsystems and the safety of the vessel.
The typical ballast system is made up of a number of tanks in the lower hulls usually arranged similarly to these in our hypothetical system (Figure 2). The tanks are made up of sections of the lower hull and are connected to the main piping of the ballast system in the pump room or rooms. Most semisubmersibles have only one pump room in each hull. The pump room can be located anywhere in the lower hull: fore, mid, or aft. Our hypothetical system configuration is a single central pump room; furthermore, the pump room is located on the inboard side of the hull, where it is protected from damage and is accessible, being under the center column (Figures 1 and 2). Each tank is connected to a ballast system piping manifold by a single tank line which serves to both fill and empty that tank.