Is it necessary to install a downhole safety valve in a subsea ... - NTNU

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The wing valve is located on the horizontal outlet from the x-mas tree. If it becomes necessary to close the well, the wing valve is the first to be closed. This is where the produced fluid is flowing through. The swab valve is located on the vertical bores of the tree above the wing valve. The swab valve is used to perform safe vertical re-entries into the tree and well during workover. The crossover valve connects the production bore to the annulus bore via a crossover service line. The annulus master valve is located on the vertical annulus bore. It is normally closed and opened only if fluid is to be injected to the annulus bore. The annulus wing valve is normally closed. It closes the side outlet oft the tree block to isolate the service line during production and intervention. Gate valves are the most common type valves in the x-mas tree. The gate valves are normally operated either hydraulically, by mechanical override or remotely operated vehicle (ROV). A1.4 Subsea wellhead The subsea wellhead system is the primary component of a subsea production well. It functions both as a pressure vessel and a structural support at the seabed. During oil and gas production the wellhead support and seal the x-mas tree and holds the casings and the tubing. In the wellhead, the tubing hanger and casing hangers are connecting and carrying the load of the tubing and casings. The tubing head is attached to the uppermost casing head and supports the tubing string. The tubing hanger is an integrated part of the wellhead. It seals and locks the tubing inside the wellhead housing. Seals isolate the production and annulus fluids and prevent leakage. The tubing hanger can either be locked to the wellhead housing or directly via a casing hanger lock down profile, or locked in the last casing hanger suspended in the wellhead. Hydraulic and electrical communication with downhole equipment requires the use of penetrators in the tubing hanger and x-mas tree. The casing head is attached to the wellhead on a casing hanger. A seal is provided to avoid leakage of annulus fluids into the next casing or to the surroundings. A1.5 Guiding system Two different guiding systems have been developed for well completions and workover, guidelines and guidelineless. These systems are used to guide the Blowout preventer (BOP) down to the seabed and place it in the right position. A1.5.1 Guideline system The guideline system is the most conventional and experienced method used for subsea equipment installation. Four guidelines are connected to guideposts of a square wellhead landing base. The BOP is lowered down on the guidelines. When the BOP is closing in on the seabed they are tightened. This helps the BOP to connect to the wellhead at a precise position. The guidelines can be installed and retrieved by ROV or a diver. A-4
A1.5.2 Guidelineless system Several manufactures have provided a guidelineless system for many years. Due to the variation in equipment from the different manufacturers different methods are used for the different manufacturer. One solution is described here. The guidelineless x-mas tree is run on a completion riser. A sonar or TV tool is used to locate the wellhead and connect the x-mas tree to it. Then the x-mas tree is oriented with reference to the tubing hanger. After the tree is oriented in the right position, the pressure on 4 jack landing cylinders is bleed off and the xmas tree is landed. A1.6 Tubing string accessories A1.6.1 Downhole Safety Valve (DHSV) Downhole in the production string a Downhole Safety Valve (DHSV) is located to provide a shut in of the well if dangerous situations occur. The DHSV is normally hydraulically operated from the surface, with control line pressure being supplied through control line strapped to the tubing. If the hydraulic pressure in the control line is cut a spring forces the valve to close. The fail-safe flapper or ball valve DHSV is also activated by an emergency shut down (ESD) system and if any physical damage should occur on the control system. The DHSV in the North Sea are set at up to 50 meters below the seabed. The hydrostatical weight of the hydraulic control fluid set the depth in which the DHSV is set. The hydrostatical weight is balanced by the use of a control line. If the pressure in the control line is increased the DHSV will close. A1.6.2 Seal Assembly The seal assembly is connected as a part of the production tubing and equipped with sealing gaskets on both sides. The seal assembly provides a possibility for the tubing to stretch without rupturing the material during pressure and temperature changes in the well. A1.6.3 Production packer A production packer is constructed to increase the efficiency of the oil and gas production. The packers are functioning as a barrier to isolate the annulus from reservoir fluid and are a necessary part of the well. A tubing-to-casing seal and pressure barrier is provided. The production packer also provides an anchoring of the well. A well can have multiple packers to isolate it into different zones. A1.7 Control system The control system must be configured to meet necessary requirements of the well. The control system includes various surface equipment and umbilical in addition to the subsea control equipment. The control system may be operated by different configurations optical, electric or hydraulic. A direct hydraulic system is the most reliable system today. One hydraulic line is required to operate each device. A pilot hydraulic system stores the hydraulic pressure at the work site with pilot valves to effect the actuation. A signal is sent from the platform by a hydraulic (can also be optic or electric) signal line in the umbilical. The umbilical also carries a main pressure line with hydraulic fluid to the hydraulic pressure system. A pilot valve interprets the signal and actuates the valve, e.g. the DHSV. There are two different hydraulic pressure systems. A high-pressure system at 10.000psi activates the A-5
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Page 69 and 70: APPENDIX A A1 Subsea production sys
Page 71: A1.3 Well completion A1.3.1 Tubing
Page 75 and 76: Appendix B In this appendix a figur
Page 77 and 78: Appendix C Fault trees constructed
Page 79 and 80: Production Wing valve ITL WV Produc
Page 81 and 82: Component: Production Packer Failur
Page 83 and 84: Reliability data dossier Component:
Page 85 and 86: Component: Production wing valve Fa
Page 87 and 88: Component: Crossover line Failure r
Page 89: Appendix F The guide-words used in
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Is it necessary to install a downhole safety valve in a subsea ... - NTNU

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