Fifteen million sq miles of sedimentary basins lie between 1,000 ft and 10,000 ft. Drilling and production in increasing water depth will demand an equally increasing reliability of the subsea equipment; components must be as autonomous and separate as possible so that the failure of one of them does not endanger the whole assembly; link between the surface and the subsea equipment must be reduced as much as it can be due to their vulnerability.
Previous subsea blowout presenter systems, whether fully hydraulic or electro hydraulic, are using multisided and require large-diameter expensive and troublesome umbilical?s. Now the industry has available a remote control system for BOP's stack in deep-sea drilling operations that only requires a pair of wires coupled with two hard line multiplex electronic units.
A backup acoustic control system is also attached to the main system, in order to undertake the main function in case of accident on the multiplexed cable. As for the hydraulic energy, it can be pumped from the surface as usual, or directly produced on the stab assembly by means of a subsea hydraulic power unit in closed circuit.
The MATRA Bops? remote control system is designed according to 10,000-ft water-depth specifications. Two hard line multiplex electronic systems (TESUMA) give full redundancy to the control, and an acoustic system (TELTAC) is used as a backup, in case of emergency. Two independent electro hydraulic units, directly connected to each hard line multiplex, deliver the necessary power to the presenters.
A typical BOP stack consists of a certain number of organs hydraulically operated such as: connectors, rams, wedge locks, annular presenter, choke and line valve operators, and so on. The operation of most of these organs should be made in limited time (4s to 20s) at a pressure of 1,500 psi above the ambient pressure. Other organs, such as bag-type BOP and ball joint, should have a pressure regulated from 1,500 to 3,000 psi.
The operation of the stack is insured by two hydraulic units, coded yellow and blue, located on the stab assembly. Each unit consists of a bank of accumulators and an electro hydraulic block. The two sets of accumulators normally isolated can be remotely coupled. The total useful capacity of hydraulic fluid allows one full cycle of all stack function operations, plus an additional availability of 25 percent of the operating volume with a pressure decrease from 3,000 to 1,200 psi over ambient.
The subsea accumulators are of the bladder type in cylindrical shape.
There is no possible absorption of the nitrogen precharged by the fluid. Recharge is currently 1,000 psi above ambient hydrostatic exhaust pressure, the minimum pressure being then 1.2 times the recharged pressure and the maximum pressure: 3, 000 psi over.
The capacity of each accumulator ranges from 12.5 to 50 gal and provides approximately between 0.25 for 10,000 ft and half volume for 1,000 ft of useful fluid.