Subsea Safety Equipment

During tests, in addition to the downhole valve, other safety systems can be run sub-surface. These systems give an additional safety feature if the downhole valve fails to close and on offshore floating rigs can provide unlatching facilities if the rig moves off location.

These safety tools are divided into two categories :

- for use on floating rigs- for use on fixed rigs (both on and offshore)

Floating rigs

During drilling from a floating vessel (a semi-submersible or a drill ship), the subsea blowout preventer (BOP) stack normally consists of four or five BOP's :

- An annular preventer on top

- Blind/shear rams

- Two or three BOP's to close on drillpipe or test strings

The riser, which brings the mud back to surface, is landed on the top of the BOP stack with a hydraulic connector that allows disconnection when necessary. The BOP's and the hydraulic connector are controlled from the surface by hydraulic hoses running in bundles outside the riser.

During testing, the drillstring running through the BOP stack is full of oil or gas. Should it become necessary to pull the drilling vessel off location, the well would be held in control only by the blind/shear rams, which might have been damaged while cutting the pipe. Re-entry would be impossible because of pressure below the blind/shear rams. To overcome these problems a subsea valve is inserted in the test string.

The purpose of a subsea valve is to provide a failsafe seabed master valve to close the drillstring and allow disconnection during testing from a floating drilling vessel. This disconnection could be necessary due to rough weather, dragging or loss of anchor, or failure of a dynamic positioning system. The sequence of disconnection would be as follows :

- The valves in the subsea valves are closed

- The hydraulic assembly is unlatched from the valve assembly

- The hydraulic assembly and the string above it are pulled clear of the BOP stack

- The BOP blind rams are closed

- The riser is disconnected

The vessel can move off location, leaving the well under control.

Three tools are part of the system :

- The EZ tree valve which provides the well control and the unlatching capabilities

- The Retainer valve (now part of the latest design of EZ tree) which keeps the landing string contents safely contained during the unlatch

- The Lubricator valve which is designed to be used when wireline operations are scheduled to reduce the length of wireline lubricator above the flowhead.

E-Z Tree Subsea Safety Valve

The E-Z Tree subsea safety valve is made up on the test string and landed in the subsea BOP stack to provide a safety valve and unlatch system during a drill stem test. After the E-Z tree subsea safety valve has been landed, pipe rams are closed around the slick joint below the valve assembly to seal the annulus. (See following diagram).

Experience has proved that a flapper valve is the most reliable type of fully opening subsurface safety valve. The E-Z tree valve assembly combines a flapper valve with a ball valve that can cut up to 15/32 inch wireline or 11/2 inch coiled tubing (with a special ball). This valve combination provides the simplest, most reliable system, holds pressure from below and can be pumped through to kill the well in an emergency.

The E-Z tree valve is connected by a three or four line hose bundle to a surface control console on the rig floor.

The flapper and ball valves are opened by applying hydraulic pressure on one line and closed by a spring force after bleeding off this pressure. Valve unlatching is performed by applying pressure from the console on the second line. The third line is to give an extra, assisting force when shearing wireline or cable. Should hydraulic pressure be lost or the hose cut, unlatching can be performed mechanically by shearing a pin and rotating 14 turns to the right. The complete

hydraulic operating mechanism is contained in the latch assembly, which can thus be serviced after unlatching - a major advantage.

Chemical injection is possible directly at the flapper valve through the fourth hose line.

The Retainer valve, which closes in conjunction with the E-Z tree valve, isolates the landing string from the riser after disconnection to ensure that no hydrocarbons are dumped, thus reducing the risk of pollution

The E-Z tree assembly can be tailored to fit most BOP stacks by adjusting the fluted hanger on the hanger mandrel.

When the valve and latch assemblies are connected, their combined length is less than 7 feet. In some BOP stacks, that makes it possible to close the pipe rams on the slick joint and still have room to close the blind rams above the latch assembly.

Lubricator Valve

The lubricator valve is a surface operated hydraulic valve run one or two joints below the flowhead during well testing operations. It enables the top of the test string to be used as a lubricator for the introduction of wireline tools, and reduces the amount of surface equipment required.

The lubricator valve is connected by a two or three line bundle to a surface operating console. One line opens the valve, one line closes, and the third may be used for chemical injection if required.

The valve can be pressure tested from above (with hydraulic pressure maintained in a closure line), or below.

The lubricator valve is balanced and, in case of hydraulic failure, remains in its last position, either open or closed. If the valve is closed, however, application from above of an 1800 psi differential pressure allows pump-through to kill the well.

The hydraulic closure system enables the ball to cut up to 15/32 inch wireline or 11/4 inch coiled tubing (with a special ball) in an emergency.

Fixed rigs

E-Z valve

The EZ Valve is a subsurface safety valve for use on land rigs or offshore fixed rigs. This valve is designed for installation in the BOP stack, where the rams can be closed around a slick joint above the valve, protecting the annulus, while the E-Z valve ball can be closed to secure the well at BOP level.

It can also be placed some distance below the BOP stack, for example below the seabed on a jackup rig.

The E-Z valve is normally closed by a spring and nitrogen pressure, and is opened by hydraulic pressure applied from a console on the rig floor.

The E-Z valve is designed primarily to hold pressure from below, but it can be pressure tested from above by holding pressure on the ball closure assist line.

E-Z valve closure can be assisted by hydraulic pressure, enabling the ball to cut up to 15/32 inch wireline or 11/4 inch coiled tubing in an emergency (with a special ball).

Chemical inhibitor can be injected below the valve, if required.

Preparation of EZ Tree

EZ Tree Make-up Procedure / Dummy Run

Although the E-Z Tree system could well be thought to be the concern of the DST crew, traditionally, it is maintained and prepared by the surface testers.The EZ Tree assembly should have been made up with chain tongs on the deck, pressure tested, function tested, shear pins checked and drifted prior to being picked up to the drill floor. For ease of handling it is recommended that a pup joint be installed on top of the EZ Tree. The reason for doing a dummy run with the EZ Tree is to verify that the assembly will pass through and sit correctly in the BOP stack and that the BOP rams will close in the correct position.

Procedures

- Pick up EZ Tree assembly and latch elevators onto pup joint.

- Ensure that there is at least one stand of tubing in the rotary table and stab the bottom x-over onto it. The EZ Tree assembly should be made up to the tubing, care should be taken that none of the joints above back off. (Recommended torque for 4 1/8" and 4 1/2" stub acme joints is 5000 ft. lbs).

- Lower the string in the slips and torque up the bottom x-over, fluted hanger, spacer (if one is used) and slick joint.

- Pull out the slips and run the hanger through the rotary table - the bushings will have to be pulled for this operation. Set 5" drill pipe slips on the slick joint and torque up the slick joint to the valve body.

- Lower the valve assembly as much as possible on top of the slips. Torque up the saver sub to the manifold sub and upper centraliser (if one is used). This operation can be quite difficult as the tongs will be approximately 9 ft. up in the air when the joints are being made up.

- Pull out the slips and run the EZ Tree valve and latch through the rotary table - the bushings will have to be pulled for this operation. Set the slips on the saver sub and torque up the upper centraliser, x-over and pup joint.

- Pull out the slips and run the EZ Tree latch / manifold sub to approximately 4 ft. above the rotary table. The hydraulic hoses can now be attached to the EZ Tree - care should be taken so that no tools fall down the hole and that no hydraulic pressure is applied to the control hoses.

- Pull the EZ Tree assembly out of the hole to the slick joint, replace the bushings and slips and ensure that the driller has slacked off as much weight as possible onto the slips.

- At the console pressure up line B (to fully retract the flapper operator) and pressure up line C to unlatch. Pick up on the elevators and unlatch the EZ Tree, observe console and manifold for any leaks. Torque up the valve body (2000 ft.lbs).

- To relatch the EZ Tree, lower the hydraulic assembly onto the valve and slack off weight. Bleed off the pressure in lines B and C to allow the latch assembly to engage. Get the driller to pick up on the elevators to ensure that the EZ Tree has latched. Slack off all the weight onto the slips again.

- Pressure up line A (to open the flapper and ball valves). If a methanol injection EZ Tree is being used, methanol can now be injected. Observe console and manifold for any leaks, and check that the methanol injection has little or no back pressure.The dummy run can now be done with the hoses still connected, but normally this operation is done with all the hydraulic hoses removed. It is imperative that caps are installed on the manifold to protect the hydraulic assembly. Note that the valve will be in the closed position. By not using the hoses the dummy run can be done relatively quickly. Another method to do a dummy run is to use only a fluted hanger and the appropriate x-overs, it is important that there is a stand of tubing below the hanger to keep it centralised in the BOP stack. The area where the pipe rams are expected to close should be painted white or covered with "denso tape", so that the ram markings will be clearly visable.

- Run the assembly into the hole to the required depth, care should be taken when entering the BOP stack and when landing off in the wear bushing.

- Check spacing and depth with the driller and company representative and then close the relevant pipe rams.

- Open the pipe rams and pull out of the hole with the EZ Tree assembly. When the assembly is at surface set it down on the slips and measure the marks left by the pipe rams on the slick joint / drill pipe, verify the space out calculations.

The dummy run has now been completed and the EZ Tree assembly can now be laid back down on the deck or racked back in the derrick, ready for use.