Patent ID: 8251148

Claim:
A system for active heave compensation of a device in an offshore arrangement, particularly on board a floating structure, comprising at least one double-acting, single piston rod hydraulic cylinder which is connected to the device which is to be heave compensated, the piston rod side of the cylinder displacing less volume than the opposite, piston side of the cylinder, a hydraulic power unit for providing hydraulic high-pressure fluid to the currently active side of the hydraulic cylinder, a control unit which regulates the supply conditions of the high-pressure fluid to the currently active side of the hydraulic cylinder, low-pressure hydraulic fluid concurrently being permitted to leave the currently passive side of the hydraulic cylinder, wherein the hydraulic power unit comprises a pump unit which via respective conduits are connected to the two sides of the hydraulic cylinder for forming therewith a substantially closed hydraulic system, wherein hydraulic fluid delivered by the pump unit to the conduit to the currently active high-pressure cylinder side is taken at least partly from the conduit to the currently low-pressure passive cylinder side, the control unit regulating the output of the pump, and wherein the hydraulic system further comprises means which compensate for the displacement volume difference between the two sides of the hydraulic cylinder, said means comprising an accumulator for low-pressure hydraulic fluid and associated piping constructed such that the two conduits from the pump unit are connected through pressure or electric actuated valves to the accumulator for low-pressure hydraulic fluid in order for the accumulator to receive fluid from, or deliver fluid to, the currently low-pressure passive cylinder side as necessary in order to maintain fluid balance between the suction side and discharge side of the pump unit, said valves being arranged to close the accumulator against the currently active cylinder side and open the accumulator towards the currently passive cylinder side.