Deepwater accumulators provide a supply of pressurized working fluid for the control and operation of subsea equipment, such as through hydraulic actuators and motors. Typical subsea equipment may include, but is not limited to, blowout preventers (BOPs) that shut off the well bore to secure an oil or gas well from accidental discharges to the environment, gate valves for the control of flow of oil or gas to the surface or to other subsea locations, or hydraulically actuated connectors and similar devices.
Accumulators are typically divided pressure vessels with a gas section and a hydraulic fluid section that operate on a common principle. The principle is to precharge the gas section with an inert, dry, ideal gas (usually nitrogen or helium), pressurized to a pressure at or slightly below the anticipated minimum pressure required to operate the subsea equipment. Hydraulic fluid will then be added (or “charged”) to the accumulator in the separate hydraulic fluid section, increasing the pressure of the pressurized gas and the hydraulic fluid to the maximum operating pressure of the control system. The precharge pressure determines the pressure of the very last trickle of fluid from the fluid side of the accumulator, and the charge pressure determines the pressure of the very first trickle of fluid from the fluid side of the accumulator. The discharged fluid between the first and last trickle will be at some pressure between the charge and precharge pressure, depending on the speed and volume of the discharge and the ambient temperature during the discharge event. The hydraulic fluid introduced into the accumulator is therefore stored at the maximum control system operating pressure until the accumulator is discharged for the purpose of doing hydraulic work.
Accumulators generally come in three styles—the bladder type having a balloon type bladder to separate the gas from the fluid, the piston type having a piston sliding up and down a seal bore to separate the fluid from the gas, and the float type with a float providing a partial separation of the fluid from the gas and for closing a valve when the float approaches the bottom to prevent the escape of the precharging gas. A fourth type of accumulator is pressure compensated for water depth and adds the precharge pressure plus the ambient seawater pressure to the working fluid.
The precharge gas can be said to act as a spring that is compressed when the gas section is at its lowest volume/greatest pressure and released when the gas section is at its greatest volume/lowest pressure. Accumulators are typically precharged on the surface in the absence of hydrostatic pressure and subsequently charged with hydraulic fluid on the seabed under full hydrostatic pressure. The surface precharge pressure is limited by the pressure containment and structural design limits of the accumulator vessel under surface ambient conditions. Yet, as accumulators are used in deeper water, the efficiency of conventional accumulators decreases as application of hydrostatic pressure causes the gas to compress, leaving a progressively smaller volume of gas to charge the hydraulic fluid. The gas section must consequently be designed such that the gas still provides enough power to operate the subsea equipment under hydrostatic pressure even as the hydraulic fluid approaches discharge and the gas section is at its greatest volume/lowest pressure.
As shown in FIGS. 1 and 2, accumulators may be included, for example, as part of a subsea BOP stack assembly 10 assembled onto a wellhead assembly 11 on the sea floor 12. The BOP stack assembly 10 is connected in line between the wellhead assembly 11 and a floating rig 14 through a subsea riser 16. The BOP stack assembly 10 provides emergency pressure control of drilling/formation fluid in the wellbore 13 should a sudden pressure surge escape the formation into the wellbore 13. The BOP stack assembly thus prevents damage to the floating rig 14 and the subsea riser 16 from fluid pressure exiting the seabed wellhead.
The BOP stack assembly 10 includes a BOP lower marine riser package (LMRP) 18 that connects the riser 16 to a BOP stack package 20. The BOP stack package 20 includes a frame 22, BOPs 23, and accumulators 24 that may be used to provide back up hydraulic fluid pressure for actuating the BOPs 23. The accumulators 24 are nested into the BOP stack package 20 to maximize the available space and leave maintenance routes clear for working on the components of the subsea BOP stack package 20. However, the free space available for all required BOP stack package components such as remote operated vehicle (ROV) panels and mounted control pods, and related equipment has become increasingly difficult due to the increasing number and size of the accumulators 24 for drilling operations in deeper water depths. Depending on the depth of the wellhead assembly 11 and the design of the BOPs 23, numerous accumulators 24 must be included on the frame 22, taking up valuable space on the frame 22 and adding weight to the subsea BOP stack assembly 10.
The use of traditional accumulators at extreme water depths requires large aggregate accumulator volumes that increase the size and weight of the overall subsea equipment assemblies. Yet, offshore rigs continue moving further and further offshore to drill in deeper and deeper water. Because of the ever increasing envelop of operation, traditional accumulators are becoming unmanageable with regards to quantity and location inside existing stack frames. In some instances, it has even been suggested that in order to accommodate the increasing demands of the conventional accumulator system, a separate subsea skid may have to be run in conjunction with the subsea BOP stack in order to provide the required volume necessary at the limits of the water depth capability of the subsea BOP stack. With rig operators increasingly putting a premium on minimizing size and weight of the drilling equipment to reduce drilling costs, the size and weight of all drilling equipment must be optimized.