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

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Is it necessary to install a downhole safety valve in a subsea oil/gas well? 6 Well intervention 6.1 Intervention types In this subchapter a description of different intervention types and the most important intervention equipment will be explained. The theory is based on ref. [14], [21], [33] and [34]. There are two types of well interventions, light and heavy (also known as workover). An intervention requires a separate vessel or workover rig and is therefore complicated due to the motion of the vessel. The subsea well intervention costs are very high especially when operating in deep water. Equipment has to be rented and brought out to the location. Getting the equipment out to the site requires a lot of planning and logistics. If a failure should occur in between scheduled interventions it might take months before an intervention can be performed. This could result in a production stop and be of great cost to the operating company. 6.1.1 Light intervention Light interventions include operations performed within the flow conduit inside the tubing string and the x-mas tree. There are many types of light interventions, e.g.: • Plugback operations • Coiled tubing operations • Fishing operations • Wireline operations In some cases a failed DHSV can be repaired by a wireline operation if it is arranged for insertion of an insert valve in the DHSV. An insert Wireline Retrievable Surface Controlled SubSurface Safety Valve (WRSCSSV) can be installed as a backup inside the failed DHSV and be activated by wireline. The insertion of an insert WRSCSSV reduces the diameter of the production tubing and thus the production is reduced. When the wireline method is applied a tool string attached to a wire is run by gravity force into the well. Wireline methods are easy to operate and well known to the operating companies. Short time is needed for the rigging and rig down. A problem may occur if the method is applied in deviated and horizontal wells due to the dependability on gravity. 6.1.2 Heavy intervention (workover) A workover on a subsea completed well require the use of a semi submersible drilling rig with heavy lift capabilities. A workover involves a pulling of the production tubing. When a vertical subsea x-mas tree is installed the x-mas tree has to be removed as well. The tubing has to be pulled for various reasons, e.g. to replace or repair the DHSV. During the workover a blowout preventer (BOP) stack and a drilling riser installed. Normally several jobs are done during one workover. A full workover is required for repairs or replacement of the DHSV. 6.1.3 Workover equipment The following section will describe some of the main equipment used when a workover is needed to replace a DHSV. Diploma thesis, NTNU 2002 36
Is it necessary to install a downhole safety valve in a subsea oil/gas well? Workover control system A workover control system connects to the x-mas tree and overrides the installed control system at seabed. It controls the lower riser package and tubing hanger running tool during installation and workover. Workover control systems normally include an emergency shutdown system (ESD). The ESD system secures a safe shut-in and prevents the release of well fluid for all operation modes. Risers Several risers serve different purposes during the different stages of a workover. The completion/workover riser is used for wireline intervention and retrieval of the x-mas tree. Separate bores are included in the riser to provide communication with the tubing and the annulus. The lower riser package is connected to the end of the riser and functions as a barrier during workover. Riser pipes are used when running the blowout preventer (BOP) from the workover rig. An iron roughneck screws new pipes on for every ten meters until the BOP reaches the seabed. Blowout preventer (BOP) The blowout preventer (BOP) system is a set of valves installed on the wellhead to prevent the escape of pressure either in the annular space between the casing and tubing during drilling, completion and workover operations. Most BOP stacks are designed with a triple ram configuration, which provides positive protection against blowouts and secures the well in emergencies [27]. The BOP is normally hydraulically operated from the workover control system. When installed the BOP is run on guidelines and attached to the wellhead. Prior to the operation a remotely operated vehicle (ROV) attaches the guidelines to guideposts of a square wellhead landing base. The BOP is close to the seabed, the guidelines are tightened to find the right position for connection to the wellhead. The BOP is locked on and the operation can begin. An illustration of a BOP is shown in Figure 6-1. Diploma thesis, NTNU 2002 37
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Page 69 and 70: APPENDIX A A1 Subsea production sys
Page 71 and 72: A1.3 Well completion A1.3.1 Tubing
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

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

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