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These valves are usually mounted in pairs on both the choke and kill lines. They are opened hydraulically from the surface (0.6galls of fluid is typically required) but once the opening pressure is released, spring force automatically forces the gate valve closed.

In deep water operations, the hydrostatic head of fluid in the opening line tends to open the valve. Some designs counter this be incorporating a system which transmits seawater hydrostatic pressure to an oil chamber on the spring side of the piston to compensate for this effect. Other designs have separate pressure-assist closing lines, figure 5.d shows a Cameron type AF fail-safe valve.

Due to space limitation, the innermost valve on the stack is usually a 90^o type with a flow target to avoid fluid or sand cutting. The outer valve is normal straight through and must be bi-directional, i.e. able to hold pressure from on top as well as below for testing the choke and kill lines.

5.2.1. BOP Control System

The simplest form of BOP control is to assign a hydraulic line direct to each individual function. This presents little problem on land rigs where the large number of control lines required can be easily handled and the distance the control fluid has to travel is not great.

On a subsea stack, this direct control is impractical, too many individual lines would be needed and the pressure drop inside them would be too great for the reaction time to be acceptable.

For this reason, other systems have been developed based on the idea of using one main hydraulic line through which power fluid is sent to the stack and for pilot valves located on the stack to direct it to the various functions on command from the surface. These commands can be easily transmitted to the pilot valves either hydraulically, electrically or acoustically.

a) Hydraulic Control Systems

The main components of a hydraulic control system are shown in figure 5.e. A master hydraulic power unit supplies fluid to both pilot and hydraulic lines via accumulator bottles.

The stack can be controlled from this unit or from a remote control panel on the rig floor or an electric mini panel usually located in the rig office.

Pilot and operating fluid is provided to stack via one of two hose bundles each of which terminates in a Pressure Operating Device (conventionally termed yellow or blue pod) mounted on the lower marine riser package. The pods are identical, one providing complete backup for the other, either one being selected for use from the control panels. A typical hose bundle is made up of a 1" supply hose for the power fluid and up to 64 x ³/₁₆" hoses for the pilot fluid. Inside each pod the pilot lines terminate at pilot valves, each of which is connected to the common power fluid supply. When a particular stack function command is selected, pilot fluid pressure is directed down a pilot line to the corresponding pilot valve.

This valve opens to allow the operating fluid to pass through it and then via a shuttle valve to the operating cylinder. The shuttle valves, which are mounted on the stack, allow the fluid to flow to the operating cylinder from the one selected pod only.

The operating fluid is stored in the accumulator bottles at 3,000psi. This pressure is too high for normal operation of the annulars or rams and so the control pods contains regulators in order that closing pressure can be controlled as required (usually from 0 to 1,500psi), though higher if the situation demands it. The subsea regulator is controlled from surface via a pilot line and another line returns to a panel gauge and gives the ‘readback’ operating pressure downstream of the regulator.

Each control pod is mounted in a receptacle on the lower riser package and can usually be retrieved independently if repairs become necessary. Whilst the stack is being run, the hose bundle is fed out from a power driven reel which is equipped with a manifold so that control of 5 or 6 stack functions can still be maintained during running. Once the stack has been landed and sufficient hose run out, a special junction box on the reel enables a quick connection to be made between the pod and the hydraulic unit.

Some of the hydraulic power fluid is stored in accumulators located on the stack in order to reduce closing times and also to provide a surge chamber effect for the annular preventers.

All the operating fluid on the low pressure side of a function is eventually vented to the sea via the pilot valves.

This, therefore, necessitates the use of environmentally friendly fluid which must also inhibit corrosion and bacterial growth as well as being compatible with anti-freeze additives. Large volume of fluid are prepared and stored near the hydraulic unit and are transferred automatically to the accumulators by electrically driven triplex pumps whenever accumulator pressure falls below a preset level. The pilot fluid circuit is closed.

A turbine type flow meter mounted on the hydraulic unit measures the volume of hydraulic fluid used every time a function is operated. This can indicate for example whether or not a ram is closing fully or if there is a leak somewhere in the system.

Apart from the ‘close’ and ‘open’ positions, it is also possible to place a function in ‘block’

position. In this position, the lines carrying pilot pressure to the pilot valves have a vented spring action in the pilot valves which shuts off the power fluid supply and vents both sides of the operating piston.

Figure 5.D - Fail Safe Valve

Figure 5.E - Hydraulic Control System

b) Electro-Hydraulic Control Systems

The object of the BOP control system is to move sufficient power fluid, at the required pressure, to the operating cylinder in the minimum time possibly. For very long lengths of hose bundles (over 2,000ft or 600m), friction losses inside the small pilot lines result in unacceptably long reaction times. If the diameter of these lines is increased, the hose bundle would be too bulky to handle so an alternative to a purely hydraulic control system is needed for deep water operations.

This is satisfied by electro-hydraulic systems in which the hydraulic pilot valves are operated by electrical solenoid valves in the control pods through lines from surface. High pressure is taken from the main power line in the pod under control of the solenoid valve and is used as pilot pressure to open the pilot valve and thus allow regulated power fluid through to the operating cylinder.

A further refinement to this system reduces all the separate electrical lines in the hose bundle to only two, down which coded multiplexed signals are transmitted. A multiplex package in the control pod decodes these signals and activates the corresponding solenoid valve.

c) Acoustic Control System

Although in both the control systems described above, redundancy is assured through the use of two identical control pods, a further fully independent system is sometimes desired for complete back-up for contingency.

To suit this requirement, acoustic control systems have been designed which can operate certain selected vital stack functions even if the rig is forced off location and, therefore, is not physically attached to the wellhead.

This system basically uses a portable battery powered surface control unit connected to either a hull mounted or portable acoustic transducer to transmit an acoustic signal to a receiver on the stack. The receiver and the battery powered subsea control unit respond to the signal and transmit a reply back to the surface. A subsea valve package on the stack interfaces the acoustic and primary hydraulic systems via shuttle valves. It contain solenoid valves powered by the subsea battery pack (rechargeable only on surface) and pilot valves.

Pilot fluid, provided from a separate pilot fluid accumulator with power fluid, is stored in a separate bank of stack mounted accumulator bottles. These store fluid at 3,000psi and can be recharged via the primary control system. The valve package contains no subsea regulator, hence, the 3,000psi is applied directly to the operating piston.

A secure coded signalling system and noise rejection circuit eliminate the possibility of a function being executed by accident. To improve signal reception on the stack, two subsea transducer are mounted on long horizontal arms which swing down automatically on opposite sides of the BOP stack when it is lowered.

The transmission range for such a system is in the order of one mile or 2km.

5.2.2. Subsea Pods

As already described, the pods contain the regulators and pilot valves required to direct the hydraulic fluid to the various stack functions.

The retrievable type is the most commonly used by the industry. The retrievable male portion of the pod contains all the pod valves, regulators and the hose bundle junction box. Should a pod valve, regulator or hose bundle malfunction, it is quicker and, hence, less costly to retrieve the pod than to retrieve the riser and the lower marine riser package.

5.2.3. Accumulators

Accumulators are used to store hydraulic fluid under pressure. As much accumulator volume as possible is located on the subsea stack in order to reduce operating time and also to enable them to act as a surge chamber for the annular preventers.

Surface accumulators are pre-charged with nitrogen to 1,000psi (70kg/cm²). Subsea accumulators should be precharged with nitrogen to 1,000psi (70kg/cm²) + 45psi per 100ft (10.3kg/cm² per 100m) of water depth to compensate for the hydrostatic head of sea water.

For total accumulator volume refer to the Eni-Agip Well Control Policy.