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

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DHSV Figure A-1 Natural flow well The components mentioned here will be explained more detailed later in this chapter. The build up of the well is as follows. A production casing is perforated along the hydrocarbonbearing formation to allow reservoir fluid to enter the well and flow upward through the tubing. The tubing is the pipe that provides a flow path for the produced fluid from the reservoir. A production packer is located in the annulus to seal off the annular space between the casing and tubing. The tubing string may include accessories like seal assembly, safety valve, packers and pumps. It is suspended from a tubing hanger installed at the wellhead. The tubing hanger provides a sealing between the top of the tubing and the annulus. A series of valves are placed above the tubing hanger, known as the x-mas tree assembly. The x-mas tree production master valve functions as a safety barrier and is capable of cutting of the flow from the reservoir. The production wing valve directs the flow away from the well into a production manifold or storage unit. A swab valve is located in the top of the x-mas tree and used to perform safe vertical re-entries into the tree and well during workover. The control system allows operation of subsea valves during production. A1.2 Well installation Installation of subsea well and downhole maintenance is normally carried out from a drilling rig. It is normal for the well to be drilled, completed, and the tree installed in one continuous operation. On previously drilled wells, the subsea tree would be installed after re-entry, clean up, and completion. As with any offshore operation, the successful installation of a subsea well requires considerable logistics and planning. Additional processing facilities may be required on the drilling vessel to permit initial clean up and testing of the well in order to prevent introduction of sand or debris from the completion into the subsea flowlines. The completion of wells from a floating vessel is somewhat more complicated than from a platform. The motion vessel requires extra care and heave compensation equipment to prevent damaging of completion tubing and seals. A-2
A1.3 Well completion A1.3.1 Tubing The tubing is a string of pipes stretching from the reservoir and up to the wellhead. This is where the produced fluid is led upward to the platform. Different accessories can be mounted on the tubing to control the flow. A1.3.2 Casing program The well consists of a group of concentric pipes, which are tapped together into what is called a casing program. During drilling operation a new casing pipe is added for every drilled diameter. When the hole is drilled, a casing pipe is cemented in place. Usually three casings are used. The outer casing is called a conductor pipe and provides a fundament for the well. When the conductor casing is in place the surface casing, intermediate casing and the production casing is added on. The production casing is also called a linear casing and varies from the other casings by not going all the way up to the wellhead. The main function of the casing is to provide a stabilisation barrier for the well and to protect the production tubing. A1.3.3 Subsea x-mas tree The subsea x-mas tree, an assembly of valves, provides the control of the well during production and is located at the top of the wellhead. Configuration and design of the x-mas tree depends upon several factors. The x-mas tree may be a single block type or several valves joined together; it may be of a horizontal or conventional vertical alignment (dual bore). The vertical tree is designed to connect to a wellhead and interface with the tubing hanger previously installed in the wellhead. A horizontal tree differs from the conventional with the annulus bore branch horizontally out to the side of the tree and the valves are oriented on a horizontal axis. The access to the well bore is gained by removing the internal tree cap. The main purpose of the x-mas tree is to provide an ability to shut in the well at the wellhead. The x-mas tree also fulfils the following functions Direct the production flow from the well Safely stop the flow. Inject protection fluids into the well or the flowline Allow injection of fluid to kill the well. Provide annulus control. Provide access to the annulus during the installation and the production phases. Allow an easy connection of the tree running tool and its associated completion riser. Facilitate all aspects of the maintenance outside of the tree. Operate as a safety barrier. A1.3.4 X-mas tree components The components of a subsea x-mas tree are described in brief with the functions. The production master valve is located on the vertical production bore. The master valve always remains open except for emergencies or during pressure tests of the tree. The production master valve is a fail-safe-gate valve. A-3
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Page 69: APPENDIX A A1 Subsea production sys
Page 73 and 74: A1.5.2 Guidelineless system Several
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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