Patent ID: 8448915

Claim:
The method of providing a force for shearing pipe within the bore of a blowout preventer which is located in a subsea location and is subjected to seawater pressure, comprising Providing one or more shear rams for shearing said pipe, Providing one or more pistons in one or more cylinders to provide the force for shearing said pipe when pressure is introduced onto a first side of said pistons, said cylinders having a maximum working pressure Providing a hydraulic supply for said pistons coming at least in part from one or more accumulators, Drawing fluid from said one or more accumulators at a current pressure which does not exceed said maximum working pressure and which declines in pressure to less than said maximum working pressure as said fluid is drawn out, Providing one or more low pressure reservoirs for fluid which are pressured to a pressure lower than said seawater pressure, Providing a valve connected to the second side of said one or more pistons to said one or more low pressure reservoirs which reduces the pressure on said second side of said one or more pistons to increase the pressure differential across said one or more pistons, and further comprising that said pressure differential across said one or more pistons is proximately equal to the maximum working pressure of said one or more cylinders plus an amount to provide a force towards said bore of said blowout preventer stack to offset the force of the pressure in the bore acting on the exposed end of rods of said one or more piston.