Subsea blowout prevention (BOP) equipment

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The subsea BOP equipment may comprise either a single stack or a two-stack system. The single stack system is preferred. The single stack system consists of one large bore BOP stack (generally 18 3/4") with a working pressure classification of 10,000 psi or 15,000 psi.

The two-stack system consists of a 2000 psi WP large bore BOP stack (generally 21 1/4") and a smaller bore stack (generally 13 5/8") with a working pressure classification sufficient to meet the maximum anticipated surface pressure.

A number of variations in the position of pipe rams, shear rams, and kill- and choke line connections are in use in subsea BOP stacks throughout the industry, as many have been designed by other Companies with different well control philosophies and cannot be changed without major expense. However, if a particular stack configuration is accepted by a Company, the drilling manual/well programme should highlight the differences, limitations and possibilities of the particular layout.

The principal items of the stacks are listed below. These items are listed in order from the top to the bottom of the stack.

1 Single stack system

A single stack system built into a guide frame with lifting attachment shall, regardless of the rated working pressure, contain the following:

·Riser adaptor.

·Flexible joint.

·One annular preventer, but preferably two with a hydraulic connector located above the (lower) annular preventer. It is acceptable to have a 5000 psi WP annular preventer on a 10,000 psi WP BOP stack, and a 10,000 psi WP annular preventer on a 15,000 psi WP BOP stack.

·Four ram type preventers all equipped with ram locks. One of the preventers to be equipped with blind/shear rams and the other three with the correct size pipe rams or Variable Bore Rams (VBRs). At least one ram type preventer below the blind/shear rams shall be equipped with fixed pipe rams. Each preventer shall have two 77.8 mm (3 1/16") bore side outlets rated to the same working pressure as the ram preventer. One side outlet of each preventer shall be fitted with two fail safe valves each for connection with the kill- and choke lines. The valves shall be 'fail-safe' in the closed position. The remainder of the side outlets shall be blanked off.

·The female part of the lower hydraulic connector.

·Two hydraulic or electro-hydraulic control pods each with 100% redundancy and provided with remote control pressure regulation to meet the waterdepth requirements.

2 Two-stack system

A two-stack system contains a large bore and a small bore stack.

2.1 Large bore stack

The 2000 psi WP stack shall contain:

·The male part of the upper hydraulic connector.

·One annular preventer.

·Two ram type preventers both equipped with ram locks; one preventer to be equipped with blind/shear rams and the other preventer with proper size pipe rams. The ram type preventers shall each have two 3 1/16" bore side outlets fitted with two fail safe valves each for connection with the kill- and choke lines.

·The female part of the lower hydraulic connector.

·Two hydraulic or electro-hydraulic control pods (100% redundancy). These pods shall be provided with remote control pressure regulation to meet the water depth requirements.

The BOP stack shall be built into a guide frame, provided with a lifting attachment.

2.2 Small bore stack

The small bore stack shall, regardless of the working pressure, contain the following:

·The male part of the upper hydraulic connector, or a riser adaptor.

·One annular preventer with the same pressure rating as the ram type preventers: a 5000 psi WP annular preventer on a 10,000 WP BOP stack or a 10,000 psi WP on a 15,000 psi WP BOP stack is acceptable.

·Four ram type preventers with ram locks. One preventer to be equipped with blind/shear rams and three preventers with proper size pipe rams or Variable Bore Rams (VBRs). At least one ram type preventer below the blind/shear rams shall be equipped with fixed pipe rams. Each ram type preventer below the blind/shear rams shall have two 3 1/16" bore side outlets rated to the same working pressure as the ram preventer. One side outlet of each preventer shall be fitted with two fail safe valves each for connection with the kill- choke lines. The valves should be "fail-safe" in the closed position. The remainder of the side outlets shall be blanked off.

·The female part of the lower hydraulic connector.

·Two hydraulic or electro-hydraulic control pods (100% redundancy). These pods shall be provided with remote pressure regulation to meet the waterdepth requirements.

The BOP stack shall be built into a guide frame, provided with a pick-up attachment.

3 BOP operating and control equipment

In subsea operations, BOP operating and control equipment shall include:

·An independent automatic accumulator unit rated for 20,700 kPa (3000 psi) WP for use with the subsea BOP control system complete with a soluble oil/water reservoir, automatic proportioning equipment for soluble oil, and a control manifold.

Without recharging, the accumulator capacity shall be adequate for closing and opening all ram type preventers and one annular preventer around the drillpipe, and for closing again one ram type preventer and one annular preventer around the drillpipe and holding them closed against the rated working pressure of the preventers. The unit shall include one electrically driven triplex charging pump and two air-driven pumps for charging the accumulators. The unit shall also be fitted with a regulator, similar to the Koomey type TR-5, which will not "fail open", causing loss of operating pressure. The unit shall be located in a safe area away from the drilling floor and the spiderdeck.

Part of the accumulators may be installed on the BOP stack for quicker response of the functions, and for operation via an acoustic control.

·Two graphic remote control panels, each one clearly showing 'open' and 'closed' positions for all subsea functions. When a two-stack system is used, the panels should each contain an overlay, clearly showing all functions and provided with controls for the 13,800 kPa (2000 psi) WP BOP stack. One panel must be located near the driller's position. The other panel to be located at a safe distance from the substructure and adjacent to the escape route from the drilling unit, or in the toolpusher's office. A meter for indicating control fluid flow should be located on each remote control panel.

The panels should be connected to the control manifold in such a way that all functions can be operated independently from each panel.

·A dual hydraulic or electro-hydraulic cable and/or hose system providing 100% redundancy of control for all functions of the BOP stack.

The nature of the interconnections between the surface control equipment and the subsea BOP stack depends on the type of control system. For the direct hydraulic and pilot-operated hydraulic systems, integrated multiple hose bundles are commonly used. For electro-hydraulic systems the electrical interconnections may be combined into integrated 'umbilical' cable bundles. Alternatively the hydraulic hose can be handled separately. As a general rule, the original total lengths of the flexible control cables and hoses should be 90 m (300 feet) greater than the maximum waterdepth for which the system is designed.

·An emergency control system sometimes referred to as an acoustic system to close at least one ram type preventer, the shear rams, and open one hydraulic connector, in the event that the BOP functions are inoperable due to a failure of the control system.

The accumulator bottles on the BOP stack should be fitted with non-return valves to prevent accidental dumping and should be of sufficient capacity for one activation of each of the emergency control functions plus 50%.

·Electrical equipment shall be suitable for the zone in which it is installed (Zone 1 or 2).

BOP control fluid shall be protected against freezing.

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