IWCF WELL CONTROL REFRESHER (SUBSEA EQUIPMENT supplement)

CONNECTORS

FAIL SAFE VALVES Valves are usually placed in pairs on the choke and kill lines. They are opened hydraulically from the surface (0.6 gals typically reqd). Spring action automatically forces the gate closed once opening pressure released. To compensate for the tendency in deep water for the hydrostatic head of fluid in the line to cause the valve to open by itself. Some designs have a system which transmit seawater hydrostatic pressure to an oil chamber on the spring side of the piston to compensate for this effect Other designs have a separate pressure-assist closing line.CAMERON TYPE AF

FAIL SAFE VALVES Due to space limitations, the first valve out from the stack (the inner failsafe) is usually a 90O type with a target to avoid fluid or sand cutting. The outer valve is straight through and must be bi- directional i.e. able to hold pressure from on top as well as below for when the choke and kill lines are tested.

Liquid lock between the two valves in each line is eliminated by porting the fluid, exhausted from the pressure chamber when opening the valve, away from the neighbouring valve. Line pressure acting against the lower end of the balancing stem assists in closing the valve.CAMERON TYPE AF

BOP CONTROL SYSTEMSThe simplest form of BOP control is to assign a hydraulic line direct to each individual function. Surface stack systems This is the usual practice The large number of control lines can easily be handled. The distances the control fluid has to travel is not great (reaction times ok). Subsea Stack systems This direct control system is impractical. Too many individual lines would be needed. Unacceptable reaction times due to excessive pressure drop in the lines. For Subsea systems, the standard practice;

One main hydraulic line through which power fluid is sent to the stack Pilot valves located on the stack direct the power fluid to the various functions on command from surface.

The commands can be easily transmitted to the pilot valves in various ways; Either Hydraulically, Electrically, or Acoustically.

BOP CONTROL SYSTEMS Hydraulic System A master hydraulic power unit supplied 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 sent to the stack via one of two hose bundles each of which terminates in a pod (conventionally termed yellow or blue) 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, 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 then 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 3000 psi. This pressure is too high for normal operation of the annular or rams and so the control pods contain regulators in order that closing pressures can be controlled as required usually from 0 1500psi. The subsea regulator is controlled from surface via a pilot line and another line is used to return to a panel gauge 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. Once the stack has been landed and sufficient hose run out, then a special junction box on the reel enables a quick connection to be made between the pod and the hydraulic unit.Hydraulic control System Description of Operations

Hydraulic control System - Shuttle valve operation (Principle of redundancy)Shuttle valveOperatingcylinder

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 vented to the sea via the pilot valves. Hence the need to use environmentally safe fluids which must also inhibit corrosion and bacterial growth as well as being compatible with anti freeze additives. Large volumes of the 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 pre-set level. The pilot fluid circuit is a closed one. 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 whether or not a ram is closing all the way, for example, or if there is a leak somewhere in the system. Apart from the obvious close and open positions, it is possible to place a function in block. In the block 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.Hydraulic control System Description of Operations

Hydraulic System OperationSPM: sub plate mounted

BOP CONTROL SYSTEMS Electro Hydraulic System 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. For very long lengths of hose bundles, (over 2,000 ft) friction losses inside the small pilot lines result in unacceptably long reaction times. If the diameter of these lines was increased, the hose bundle would be too bulky to handle so an alternative to the purely hydraulic control system is needed for deep water operations. The alternative is the electro-hydraulic system in which the 3/16 hydraulic pilot lines are replaced by electrical lines which operate solenoid valves in the control pods. 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 replaces all the separate electrical lines in the hose bundle with just two, down which coded multiplexed signals are transmitted. A multiplex package in the control pod decodes the signals and activates the corresponding solenoid valve.

HYDRAULIC CONTROLSYSTEM WITH ELECTRICCONTROL PANELSSubsea Control Pod

BOP CONTROL SYSTEMS Acoustic System (Surface components)

ACOUSTIC BACKUP SYSTEMSPV: solenoid valve PV: pilot valve

ACOUSTIC CONTROL SYSTEM In addition to the assured redundancy through the use of two identical control pods, a further fully independent system is sometimes desired for complete backup. To suit this requirement, acoustic control systems have been designed which can operate certain vital stack functions even if the rig is forced off location and is not physically attached to the wellhead. This system 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 battery powered subsea control unit respond to the signal and transmit a reply back to surface. A subsea valve package on the stack interfaces the acoustic and primary hydraulic systems via shuttle valves. It contains solenoid valves powered by the subsea battery pack (rechargeable only on surface) and pilot valves. Pilot fluid comes from a separate pilot fluid accumulator and power fluid is stored in a separate bank of stack mounted accumulator bottles. These store fluid at 3000 psi and can be recharged via the primary control systems. The valve package contains no subsea regulator, hence the 3000 psi is applied directly to the operating piston. If necessary, the accumulators can be dumped under control of the primary system. 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, the two subsea transducers are mounted on long horizontal arms which swing down automatically on opposite sides of the BOP stack when it is lowered. Manufacturers claim the transmission range for such a system to be in the order of one mile.

DRILLPIPE SHUT-OFF VALVES If a kick is taken whilst the Kelly is removed from the drill string, flow up the pipe might be so strong that it is impossible to stab a safety valve. In this case a fast shut-off coupling, designed to be dropped over the drill pipe and automatically latch under the tool joint can be used. The coupling is very heavy and has a Kelly cock either integral with it or screwed on top of it. It should be ensured that any drill pipe that might be used has tool joints that are well within API specifications, otherwise the couplings ability to latch and seal correctly would be in doubt.

STORM VALVE (i) In the event that the string has to be hung-off whilst waiting on weather or pulling the BOP stack for repair, the well can be secured by using a storm valve. This valve is run above a retrievable packer and serves both as a means of sealing off the drillpipe and as a means of releasing the running string. It is a sleeve type valve operated by left hand rotation. As the upper mandrel is backed out of the upper body, it pulls up and closes the valve sleeve. Continued rotation then releases the upper mandrel which is retrieved with the running string. To recover the string, the mandrel is carefully stabbed into the upper body and turned to the right. This will move the valve sleeve down and allow a check to be made for any pressure build-up which might have occurred under the valve.

STORM VALVE (ii)

The coarse threads should always be checked after using the valve since they can easily be damaged if the mandrel is run into the upper body before being made up. The retrieving string should be well centralized to try and reduce this problem. Another difficulty often encountered when retrieving the valve is that solids can drop out of the mud and settle inside the upper body, in which case they have to be carefully washed out.

Some models of storm valve have an expendable plug on the bottom held in place by shear pins. If wireline operations are necessary after the valve has been re-opened, then dropping a sinker bar will knock out the plug to provide an unrestricted opening through the valve. Alternatively, the plug can be pumped out with the valve in the closed position.

Diverters If shallow gas pockets are encountered whilst drilling an offshore well, it is vital that the well is not shut in. Instead the well should be allowed to flow with the produced gas being vented downwind on the rig. For this a diverter system is required. For offshore use, the overboard lines of the diverter system are usually 12 or more in diameter in order to minimise the back pressure on the formation during high flow rates which are usually a feature of shallow blowouts. The control system to close the diverter element and open the overboard line valves are also generally interlocked so that once the element is closed, one of the valves automatically opens and the flowline valve closes. Another feature of a shallow gas blowout is that large amounts of sand and stones can be produced. In this event, seawater should be pumped down the drill string at maximum rate to reduce the risk of sparks igniting the gas. Ideally, the overboard lines would contain no elbows or sharp radius bends.

With the Hydril DS valves, it is possible to select the evacuation direction (starboard or portside) while keeping one line open at all times. On floating rigs, the diverter is incorporated below the rotary table, above the slip joint. As far as pressures are concerned, the weak point is the sealing at the slip joint. Its WP is generally 500 psi.Diverters Hydril DS valves

Diverters Used on Floating Rigs On floating rigs, the diverter is incorporated below the rotary table, above the slip joint. As far as pressures are concerned, the weak point is the sealing at the slip joint. Its WP is generally 500 psi.(1) - Closing Line(2) - Outer packer(3) - Vent Line(4) - Seals(5) - Insert Packer(6) - Diverter Lockdown Dogs(7) - Diverter Housing(8) - Insert Packer lockdownDetails of diverter used on floating rigs

With this system, the fluids of the well are directed to the shakers when the diverter is open. The evacuation line (vent line) being closed. When the diverter is closed, the fluids of the well are evacuated through the vent line, the line to the shaker being closed.DivertersDetails of Hydril FS assembly (diverter open)Details of Hydril FS assembly (diverter closed)

HYDRIL TYPE GL SUBSEA HOOK-UP

Accumulator Calculations Usable Volume Requirements

Accumulator Calculations Usable Volume RequirementsIf for example, the water depth is 305 m (1,000 ft), this corresponds to; 305 x 10.1 = 3,080 kPa (1,000 x 0.445 = 445 psi).The required pressures are found as follows:

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