Lineamientos referidos a la reducción de brechas

• Run 30 inch conductor string into open hole with 30 inch suspension joint attached to the guidance cone.

• Once landed and set to the correct vertical elevation, cement 30 inch conductor in place according to operator procedures.

• Rotate the drill pipe and pull to release running tool. Pull back to surface.

• Drill the next hole to TD and run the 20-inch casing.

• Attach the 18 ¾ inch wellhead body to the 20-inch casing. Install the bore protector in the wellhead (if not installed at the factory). Run cement stinger into wellhead housing sitting on rotary table and make up the wellhead body to the running tool. Make up running tool to wellhead.

• Run the wellhead body assembly into the suspension joint. Cement.

• Release the running string from the wellhead by rotation and pull back to surface.

• Place the drilling BOP across the spider beams over the moon pool. Make up the hydraulic umbilicals and check all the functions.

• Run the BOP on marine riser. Lock BOP connector onto 18 ¾ inch wellhead. Rig up diverter with choke and kill lines.

• Make up the isolation test tool onto drill pipe string. Run into the wellhead. Test the BOP stack then retrieve the test tool.

• Drill the hole for the 13-3/8 inch casing. Pull back the string and make it up to the bore protector retrieval tool. Run in and retrieve the bore protector.

• Run in the 13-3/8 inch casing string with attached cementing equipment.

• Make up the 13-3/8 inch casing hanger and the pack off to the single trip tool and make this assembly up to the casing string, run in the hole with the drill string and casing.

• Land the hanger into the 18 ¾ inch wellhead. Slack off the weight and cement the string into place. Activate the pack off setting mode of the tool

• Slack off the string weight and close the BOP pipe rams.

• Build up pressure above the tool to set and test the pack off. Open the pipe rams, release the tool from the pack off and then pull it back to the surface.

• Run in the 13-3/8 inch bore protector on the bore protector running tool. Land and lock into the wellhead. Release the tool and pull back to the surface.

• Repeat the above steps to run the next casing strings.

• Start next functions for well – (e.g. Temporary abandonment, permanent abandonment, completion, etc.).

Rev. 0 SUBSEA PRODUCTION SYSTEMS Page 32 2.3 Subsea Christmas Trees

2.3.1 Functions of Subsea Trees

A subsea Christmas tree is basically a stack of valves installed on a subsea wellhead to provide a controllable interface between the well and the production facilities. Some specific functions of a subsea Christmas tree include the following:

• Sealing the wellhead from the environment by means of the tree connector.

• Sealing the production bore and annulus from the environment.

• Providing a controlled flow path from the production tubing, through the tree to the production flow line. Well flow control can be provided by means of tree valves and/or a tree-mounted choke.

• Providing access to the well bore via tree caps and/or swab valves.

• Providing access to the annulus for well control, pressure monitoring, gas lift, etc.

• Providing a hydraulic interface for the down hole safety valve.

• Providing an electrical interface for down hole instrumentation, electric submersible pumps, etc.

• Providing structural support for flow line and control umbilical interface.

Figure 2.17 - Schematic Representations of Different Tree Types

Tree Tubing Head Spool Tubing Hanger Subsea Wellhead

2.3.2 Types of Subsea Trees

2.3.2.1 Dual Bore Tree or Conventional Tree

Until recently, most subsea trees were so-called “dual bore” type trees. A typical dual bore tree is illustrated in Figure 2-18. These trees have a production and annulus bore passing vertically through the tree body with production and annulus master valves and swab valves oriented vertically in the main block of the tree. They are designed to allow vertical access to the main production bore and to the annulus bore during installation and workover operations.

When a dual bore subsea Christmas tree is connected to a subsea wellhead it must interface with the tubing hanger previously installed in the wellhead. The tubing hanger and tree must

be correctly orientated so they mate properly with one another and the production and annulus bores are properly aligned and sealed. Alignment of the tubing hanger in the wellhead is generally accomplished by interaction of a pin and helix between the tubing hanger running tool and the BOP or a pre-machined vertical orientation slot in the BOP connector upper body. The reaction between the pin and the helix causes the tubing hanger assembly to rotate into the correct position. Alternatively, the tubing hanger is rotated until the alignment slot lines up with a spring-loaded alignment key on the running tool. The tree is subsequently aligned by the permanent guidebase.

2.3.2.2 Mono Bore Tree

A typical mono bore tree is similar to a conventional dual bore tree but differs in that it utilizes a simpler riser system to install the tree and tubing hanger. Additionally simpler styles of mono bore tree exist which are generally used on mud line completions in shallow water.

When producing a well, the annulus between the production tubing and the well casing must be accessible to relieve thermally induced pressure build up. In order to accomplish this, tubing hanger and tree systems must enable access to the annulus under the tubing hanger.

Both conventional and mono-bore trees (except the basic mudline style trees) utilize a port Figure 2.18 - Example of a Compact Dual Bore

Tree

Rev. 0 SUBSEA PRODUCTION SYSTEMS Page 34 through the tubing hanger. This port, as well as the production bore, must be closed before removing the BOP or the subsea tree.

On a conventional style tree, the annulus port is typically sealed with a wire line plug run and retrieved through a multi-bore completion riser or a riser with a diverter mechanism. This riser is generally expensive and dedicated to the tree system. Refer to descriptions of riser systems elsewhere in this document for detailed descriptions.

In the mono bore tree system the tubing hanger is run on drill pipe or tubing and the annulus is accessed through a hose bundle. Opening and closing of the annulus is accomplished by means of a “shiftable” plug or valve in the annulus bore. The disadvantage to this, as compared to the dual bore system, is the requirement for moving parts within the tubing hanger that must be left subsea for the life of the completion.

Some designs incorporate a second plug or valve, ported in series with the primary plug, which can be actuated as a backup to close the annulus if more redundancy is desired.

The mono bore tree obviates the need for a true vertical annulus bore through the tree.

2.3.2.3 Horizontal Tree

Another type of subsea Christmas tree that has gained popularity since its introduction in 1992 is the “horizontal” tree. A typical horizontal subsea trees are illustrated in Figures 2-19 and 2-21. Its most obvious distinction from the dual bore tree is that the production and annulus bores branch horizontally out of the side of the tree body and the valves are oriented on a horizontal axis. The horizontal tree has no production or annulus swab valves. Access to the well bore is gained by removing the internal tree cap, or a wireline plug within the internal tree cap, and a wireline plug in the tubing hanger. The horizontal subsea Christmas tree is sometimes referred to as a “side valve tree” or SpoolTree™. Other distinguishing features of the horizontal tree, in addition to the valve arrangement from which it gets its

Figure 2.19 - Deepwater Guidelineless Horizontal Tree

Figure 2.20 - Monobore Tree

name, are: 1) the tubing hanger is installed in the tree itself, rather than in the wellhead and 2) the top of the tree is designed so the BOP may be landed onto the tree. This arrangement allows the tubing string to be recovered without first retrieving the tree.

Horizontal Tree technology was conceived and developed to run and retrieve well bore tubing through an installed tree providing a simple and efficient work-over capability. Originally, this type of technology seemed ideally suited for Electric Submersible Pump (ESP) applications, where frequent pump maintenance or replacement may be required. Well interventions were most commonly caused by the need to repair downhole problems as opposed to subsea tree equipment problems.

The concept was extended to include standard production and injection wells in the belief that horizontal technology offered much greater benefit over conventional technology, at least in some applications.

The benefits and drawbacks of both horizontal and conventional tree technologies have been the subject of many debates for several years. The newer horizontal tree technology has been shown to have significant merit in order to have acquired at least 50 % of the market in less than six years. It is probable that both completion technologies will have a vital part to play in future oil and gas developments and the possibility of a winner for all applications is unlikely.

Figure 2.21 - Horizontal Trees

Rev. 0 SUBSEA PRODUCTION SYSTEMS Page 36 2.3.2.4 ADVANTAGES of Horizontal Trees

• Tubing recovery is simplified. The ability to perform tubing work-over and some drill-through operations without the need to recover the subsea tree and disturb the associated production flowline/controls connection is beneficial. This is particularly attractive for wells with planned or scheduled tubing work-over intervention or complex down-hole completions with the higher probabilities for down-hole failures requiring rig intervention.

• The spool tree is suitable for tubing up to 7” OD whereas the dual bore tree is limited to 5-1/2” OD. The larger bore can also accommodate a larger number of down-hole hydraulic control lines, chemical lines and electrical transducer penetrations with the capability to provide full bore annulus circulation or injection.

• The large bores possible with this system are consistent with the usual objective to reduce the number of wells. However, reliability may be compromised by a more complex completion.

• The ability to use standard, drilling BOP stacks for installation and work-over. All the completion operations except for running the subsea tree and debris cap are performed through the drilling BOP stack. This eliminates the need for a dedicated open water completion riser system.

• All completion work is carried out through or within the protection of a BOP stack.

• The ability to use single string tubing or casing as an installation and completion riser allows a cheaper riser to be configured than a conventional dual bore riser. The BOP stack’s choke and kill lines are used to circulate the annulus or riser fluids prior to disconnection and recovery of the riser system. The production tubing annulus access bypasses the tubing hanger and uses metal sealing valves for annulus isolation. This provides maximum space through the tubing hanger body for big bore completions.

• Subsea tree installation or recovery is greatly simplified by using drill pipe instead of a dedicated riser system.

• The Subsea Tree provides an integral and precise, passive tubing hanger orientation system with no requirement for BOP modifications, interaction or datum’s.

• Subsea tree provides new, exact and retrievable tubing hanger landing, locking, orientation and sealing profiles, not dependent on the condition of wellhead internal profiles. A damaged hanger sealing profile in the wellhead, is not significant to a Horizontal Tree. The same benefit with a conventional tree system requires expensive additional tubing hanger adapter or tubing spool equipment.

• The tubing hanger-to-subsea tree interface is tested and verified at the time of landing the tubing hanger in the tree while the BOP stack is still in place. Should problems arise, this offers the possibility for recovering the tubing hanger and taking immediate remedial action without tripping the stack. A conventional tree-to-wellhead/tubing hanger interface cannot be verified until after the BOP stack has been recovered and the tree installed. A failure to interface properly can have serious time/cost implications, especially if the tubing hanger is damaged or not in the correct orientation when the tree lands.

• Subsea tree single-piece spool body construction provides the maximum tree spool strength characteristics and reliability with minimum failure modes. These are considered to be stronger than conventional trees.

• Successful subsea tree installation is not dependent on the full integrity of the wellhead internal sealing profiles. There are greater probabilities for successful installation on existing and perhaps old exploratory wellheads of uncertain integrity.

The tree readily adapts to different wellheads from different vendors.

• Horizontal trees are compact, have a low profile and an excellent strength-to-weight ratio.

• Subsea component ‘building blocks’ can be arranged into many different tree layouts.

This has given considerable flexibility to horizontal tree configuration and improved the opportunity of mass produced tree equipment by allowing the flexibility to manufacturers. Tree internals can be standardized while external characteristics can be varied or moved to suit the application.

• A Horizontal subsea tree design, using guidelines, can be readily converted to a guideline-less and funnel-down, wellhead re-entry system. This is achieved by adding a bolt-on funnel to the bottom of the tree. A funnel-down, BOP stack, wellhead re-entry system can be used for guideline-less re-re-entry to a Horizontal Tree with little or no change to the standard guideline subsea tree. This will provide the lightest possible guideline-less subsea tree weight.

Figure 2.22 - Dual Bore Tree Stacked on Top of Tubing Adapter on Shop Floor

Rev. 0 SUBSEA PRODUCTION SYSTEMS Page 38 2.3.2.5 DISADVANTAGES of Horizontal Trees

• The tubing must be pulled first before retrieving the tree. Horizontal Tree recovery requires that the down-hole completion is recovered first, with the associated well killing operations through the BOP stack. Rationalization of this disadvantage is based on intervention data, that suggests that subsea tree failures, requiring the tree to be recovered, are a low percentage of all major failures requiring intervention. By far, the

greatest percentage of failures, relate to the failure of down-hole equipment, such as safety valves, gravel packs, etc. This suggests that intervention savings are actually likely to be accrued due to the use of Horizontal Tree technology, as down-hole work-over frequency is much greater than the probability of tree recwork-overy.

• A drill-and-complete scenario for Horizontal Trees currently requires two BOP trips.

(Run the BOP stack; drill well; recover BOP stack; run tree; re-run BOP stack; finish complete; recover BOP stack).

• A Horizontal Tree does not include master and/or swab valves in the vertical bore of the tree to provide first-line barrier protection to the environment. It relies on a wireline plug to provide the first line barrier protection. Care must be taken to ensure that the critical wireline plug sealing surfaces in the tubing hanger and tree cap are not damaged during wireline operations.

• The subsea tree must be designed to withstand the loadings associated with a deepwater BOP stack and riser system.

• The bore of the subsea tree may be exposed to a very harsh drilling riser environment requiring special provisions for bore protection and bore cleaning in order to ensure successful tubing hanger installation and valve reliability.

Figure 2.23 - Dual Bore Split (Upper and Lower) Body Tree

• Tubing hanger installation requires the use of a sophisticated BOP subsea intervention tree and landing string system in order to provide for safe flow testing, wireline and coil tubing intervention and emergency disconnect scenarios. This adds complexity and time to the tubing hanger and down-hole completion, installation process. The Tubing hanger installation requires simultaneous control of the tubing hanger running tool, Subsea Intervention Tree and landing string system, BOP and subsea tree’s work-over functions. This involves up to four umbilicals and their control panels.

• The tubing hanger hydraulic and electrical penetrations exit through the side of the subsea tree’s spool body. Control of hydraulic functions and monitoring of electrical functions is typically not provided although available, during installation of the tubing hanger system.

• The side outlet penetrations for control and electrical functions are additional leak paths in the primary tree bore during drilling and completion operations

• ROV’s must be used to connect/disconnect work-over controls between the BOP and Subsea Tree.

• A landing string leak or failure during well test or well clean up can divert hydrocarbons to the rig floor, burst the marine riser, or evacuate the marine riser allowing it to collapse under hydrostatic pressure.

Figure 2.24 - Dual Bore Tree Being Deployed

Rev. 0 SUBSEA PRODUCTION SYSTEMS Page 40 2.3.2.6 ADVANTAGES of Conventional Dual And Mono Bore Trees

• Only one BOP trip is required in a drill-and-complete scenario. In addition, no temporary well abandonment plug is required between the BOP stack recovery and the tree installation as the tubing hanger serves that purpose.

• The subsea tree can be recovered without having to recover the tubing hanger and down-hole completion because the tubing hanger lands in the wellhead and not in the subsea tree.

• The subsea tree is not required to withstand high loads associated with a Drilling BOP stack.

• Work-Over control connections are normally made between stab rings mounted on the tree mandrel and the LRP connector. No ROV is required.

2.3.2.7 DISADVANTAGES of Conventional Dual And Mono Bore Trees

• The wellhead bore sets the tubing hanger outside diameter, leaving only a limited area for downhole access. This restricts the largest possible production bore size when including all the other down-hole penetrations required. Particularly the annulus bore that provides a circulation path that can also be sealed with a wireline plug. The 2” annulus bore is selected for the minimum reliable wireline plug size and exceeds the flow requirements. The available space is even more severely limited when considering a concentric tubing hanger design or for the need for annulus injection or gas lift capabilities.

• If deepwater wells tend toward intelligent completions and/or simultaneous production from different reservoirs, conventional tree technology is inherently limited by the restricted space inside a wellhead. An alternative would be to use the hybrid tree, which lands a conventional tree on top of a horizontal tree, for these applications.

Figure 2.25 - Dual Bore Guidelineless Tree on Test Stand

• The subsea tree must be recovered in order to perform a tubing work-over. This disturbs the production flowline and umbilical connections. This creates new opportunities for damage to other hardware that is not easily recovered.

• A Monobore riser with a selector crossover mechanism at its base, in order to provide wireline access to the annulus can be unreliable.

• The subsea tree is typically installed on the dedicated work-over riser and wireline BOP intervention system in order to provide for flow testing, wireline and coil tubing operations, and emergency disconnect. This adds complexity and time to the

• The subsea tree is typically installed on the dedicated work-over riser and wireline BOP intervention system in order to provide for flow testing, wireline and coil tubing operations, and emergency disconnect. This adds complexity and time to the