New tools extend deepwater well test, intervention capabilities

Thomas J. Leeson
Expro Group
Aberdeen

Recent advances in control system and tool design address the challenges faced in testing and cleaning up deepwater wells.

These advances reduce capital equipment costs, simplify operational management, and extend operating envelopes.

Carrying out well testing or clean-up operations in deep water presents a number of challenges, particularly in regard to:

• Managing the hydrostatic head experienced at the seabed
• Requiring swift emergency disconnection when working from a dynamically positioned vessel
• Preventing and managing hydrates.

To maintain operational flexibility, subsea tools must provide the same full range of functionality expected during conventional subsea operations. In addition, tool reliability becomes even more important because of more-costly downtime in deepwater activities.

Finally, appropriate preparation and execution of operational programs is a key element in delivering exceptional service. It should be recognized that the pool of competent engineers and operations personnel is significantly smaller for deep water than for conventional operations.

Deep water

Even a brief exposure to the limited well testing and clean-up operations that have been conducted in deep water (greater than 1,000 m) confirms that these operations present a difficult set of challenges.

Well testing and clean-up, after completion or intervention, can be classed together because of the common issues raised by producing hydrocarbons to surface via a landing string. Of course, differing operational objectives will modify the program and thus apply particular issues and constraints on the equipment and service providers.

Deepwater costs, and thus the costs associated with downtime, mean that reliability is a key attribute to be incorporated in designing equipment for this environment, even more so than for conventional operations. This is exacerbated by the general remoteness of locations and the time required to recover and rerun deployed equipment from the seabed and the well.

In addition, technical barriers thrown up by the deepwater environment require innovative approaches to tool design.

For well testing/clean-up, the technical challenges can be grouped as follows:

• Understanding the impact of operating from a dynamically positioned vessel
• Providing fast, effective control systems
• Managing the effects of high hydrostatic pressures, both internal and external, on all deployed equipment
• Suppressing and managing hydrates.

Landing string design

The key safety considerations for landing string design, as always, are to maintain provision for emergency well isolation for retaining the riser contents and for disconnecting both landing string and riser in case of planned and unplanned events.

In addition, one wishes to have sufficient flexibility to:

• Deploy large tool strings for long periods of time on both wire and coiled tubing while the well is live
• Maximize the operational envelop with respect to weather and vessel motion/excursion
• Respond effectively to unplanned events and recover from them with a minimum of disruption.

A thorough Hazop program and the equipment proposed will highlight the potential unplanned events to be considered, and of course clever equipment design can eliminate or simplify the required response to many of these.

Fig. 1 [135,844 bytes] shows a typical well test string configuration.

Subsea test tree

The subsea test tree (SSTT) is the main well control device in the landing string, and as such many of its functions are safety critical. In addition, any failure in reliability is likely to severely limit operational flexibility and may prevent collection of the reservoir appraisal data that was the original objective of the operation.

The primary functions of a conventional SSTT must be maintained, and these are principally:

• Fail-safe close, full-bore valves, complete with independent actuation and multiple cycle capability
• Ability to cut wire and coiled tubing and subsequently seal
• Secondary control method for all safety-critical functions
• Disconnection facility with subsequent closure of the blowout preventer (BOP) blind/shear rams
• Relatch capability with complete restoration of both hydraulic and electronic control and monitoring functions
• In-built control logic to prevent disconnection prior to valve closure
• Relatch, pressure test capability
• Pump-through capability
• Through porting of electrical and hydraulic communication for data acquisition, smart well control, chemical injection, and sub-wellhead tool control.

However, for deep water the following three needs must be added:

1. Minimal actuation volumes to optimize response times
2. Disconnection under high tensile and bending loads
3. Reliable, fast, emergency and contingency disconnection methods.

Providing these features in a cost-effective manner requires clever, innovative, specialist engineering. The failure to include any of these attributes will, at best, limit the operational program, and thus the value of the operation.

Simplicity is likely to deliver the reliability necessary to be truly cost-effective.

Retainer valve

A retainer valve has the principle function of containing the landing string contents prior to any disconnection from the SSTT. It, therefore, has to hold pressure from above only. In addition, its control must be integrated with the unlatch mechanism to ensure the correct sequence of events cannot be lost.

It is also imperative that the trapped volume between the retainer valve and SSTT can be vented prior to unlatching the landing string. To minimize lost time, the integrity of the vent system should be restorable without the need for redressing at surface.

To permit well kill if valve control is lost, conventional design incorporates "fail as-is." However, it is possible to consider a fail-close design as long as there is a method for accessing bypass ports to allow killing operations.

Again access to a range of design is essential for providing the fit-for-purpose solution required for particular circumstances.

Control system

All the previously discussed tools need an integrated control system for responding with enough speed to manage operations from a dynamically positioned vessel. The disaster scenario in this case is a drive-off (or even drift-off), requiring an emergency disconnection as the landing string angle and associated bending forces increase.

Clearly, response times for direct hydraulic control deteriorate in direct relationship to water depth and thus umbilical length.

Assisted hydraulics, using downhole ports to vent open line control fluids have been used successfully, for some time, to depths of about 1,300 m with response times, from initiation to disconnect, of about 20 sec.

However, at depths beyond this where dynamically positioned rigs are required, 15 sec or less will be needed for such an operation. This time allows for a reasonable vessel excursion zone at surface, compatible with the constraints of bending moments and rig floor stick-up.

Thus, electro-hydraulic systems are necessary. Electrical signals to a control pod run as part of the landing string above the retainer valve permit the energy stored in an accompanying accumulator to drive tool functions.

These systems offer the following principal advantages:

• Fast response times independent of water depth
• Smaller umbilical diameters
• Flexible reconfiguration of control logic
• Increased number of hydraulic output options
• Remote emergency shutdown (ESD) initiation.

The next step is to use multiplexed, or addressed, signals to minimize the complexity of the umbilical bundle and provide the opportunity for feedback signals from condition monitoring points. In addition, the control logic can be easily integrated with the rig BOP controls.

As an example, Expro's latest electro-hydraulic system (Fig. 2 [141,434 bytes]) incorporates a number of features such as:

• System feedback of internal and external pressures and temperatures and integrity of barrier fluids
• Inferred valve position from feedback of hydraulic volumes and pressures
• Event logging for future sequence confirmation and analysis
• Dual redundancy of surface, umbilical, and downhole components
• Subsea processor system that provides continuous fault monitoring and minimizes data transfer and umbilical complexity.

The advantages of multiplexed systems can be a simplified umbilical bundle minimizing cost, real time and memory feedback of key operating parameters, increased reliability, and reduced operator intervention.

In addition, these tools must be designed to withstand the hydrostatic conditions found in deep water. This has been a major recent concern and has required the development of multiple electrical barriers in the confined space available within the BOP stack and marine riser.

The use of high-spec miniaturized components, originally developed for other industries, the development of new fabrication techniques, and a philosophy of employing nonconductive control fluids have increased confidence in system reliability. However, it is recommended that rigorous qualification testing at expected conditions confirm reliability prior to field trials and operations.

Hydrate management

Hydrate formation is well known as a potential hazard of deepwater operations due to the combination of relatively high hydrostatic pressures and low temperatures, at the seabed. Unluckily for us, this is just about the worst possible location for solids formation, as there is a significant risk of valve cavities being blocked and valve function impaired or lost.

Clearly the best strategy for hydrate management would be prevention. A number of procedures already exist of course, mainly based around chemical injection, and in the short term we must ensure that whatever well control tools are deployed, the tools are compatible with the chemical injection technique.

In addition, the use of heating has been proposed.

But both of these tactics appear to be expensive, particularly when prediction techniques carry a wide margin of error. However, the alternative of losing well control because of valve malfunction caused by solids formation, and the subsequent risk to life, environment, and property certainly justifies the additional costs.

Despite a number of efforts, hydrate formation and suppression are not well understood at the molecular level. It is therefore believed that, notwithstanding any recent or future advances in prevention techniques, it is imperative that the industry find a method to address these fundamental issues. Only then can our current approaches be cost-effectively optimized.

Expro is working with operators at addressing hydrate prevention through the use of novel technology; however, the lack of confidence in predictive methods is limiting the applicability of the conceptual solutions proposed. It is therefore more likely that, in the short term, tools to facilitate the removal of blockages will be available before prevention systems.

Horizontal tree intervention

Horizontal, or spool, trees have provided a cost-effective alternative to conventional trees principally through the reduced capital cost of intervention tools and the simplicity with which completion installation, clean-up, and subsequent intervention can be undertaken. Fig. 3 [147,622 bytes] shows a typical string layout for this type of operation.

Any intervention system designed for deep water must maintain these fundamental advantages to deliver the benefits of choosing this particular tree design.

The principle functions of the component systems of the landing string are very similar to that for well testing. However, the interface with the tubing hanger running tool, or tree intervention tool, must be added. This necessitates additional through-porting of control functions and a reduced height within the BOP stack to space-out the intervention tree while still accommodating all the previously listed functions.

In addition, it is necessary to provide full through-bore access, up to 73/8 in. in some cases, to give full access to large bore production completions. Further clever engineering is therefore required to service the full range of operations expected.

Expro has now successfully run over 100 completions using this type of tool, gaining a huge experience in these types of operations. Over 25 sets of these unique tools are now available to a variety of specifications incorporating features such as 73/8-in. through-bore, remateable electrical penetration, dual, independently operated, fail-safe close, ball valves capable of cutting 23/8-in. coiled tubing complete with braided wire line and subsequently sealing, and secondary and tertiary disconnection systems.

These provide exceptional operational flexibility and accommodate a variety of intervention and completion operations without compromising well control integrity.

This experience has now been reapplied to the design phase to improve tool performance and operational flexibility. The new generation of tools incorporates increased actuator forces and leverage to reduce closure times and minimize the risk of frictional "bind-up."

The design has been simplified to reduce the number of components, and the length of the tool shortened to accommodate use in exceptionally short BOP stacks.

The features and benefits incorporated in the Expro horizontal tree intervention tools include:

• Unique cutting profile that provides single cut of coiled tubing and wire line
• Pump-through capability to kill well at high rates
• Short tool length to enable pipe rams to be closed below and shear rams to be closed above the tree in latched conditions
• Independent ball closure to allow lower ball to cut coiled tubing or wire and upper ball to seal
• Upper valve pressure support from above to enable string and latch seals to be pressure tested
• Rapid response ball valves to provide unlatching within 30 sec using simple hydraulic control
• Functional redundancy with secondary cutting and unlatching systems
• Versatile interface that functions with a variety of tubing-hanger running tools or tree-cap running tools.

Conventional tree intervention

Conventional christmas trees and completion installations with dual-bore tubing hangers (production and annulus bores) do not provide the option to kick-off, clean-up, or test the well prior to suspending the completion.

Retrieving the marine riser and installing the tree with a lower riser package (LRP) normally follow this step. This sequence becomes complicated when the christmas tree is not immediately available-a real possibility during fast-track field development.

If a suspension string is left in the well, returning to the well at a later date adds significant rig time to the overall program. However, this can be negated if the completion landing string contains a well control and emergency disconnection device that also provides access to both tubing hanger bores, such as in Expro's dual-bore completion tree (Fig. 4 [133,647 bytes]).

With this tool it is possible to clean up the well and intervene, if necessary, immediately after landing the completion.

The well can then be suspended with wire line plugs in both bores of the tubing hanger, kill fluid above a deep-set plug, and hydrocarbons across the perforations to minimize loss of well productivity caused by kill fluid damage.

Case studies have shown this loss of performance can be as much as 30%, depending on reservoir characteristics.

The tool uses a compact ball-valve actuation system that provides coiled tubing cut-and-seal capability while maintaining the tool space-out below the top set of BOP rams. The tool also incorporates all emergency disconnect/reconnect latch functions of the subsea test tree. It is complete with hydraulic and wet-mateable electronic through-porting for downhole tool and tubing hanger function communication.

In addition, the tool acts as the tubing hanger orientation joint, using the BOP pin in the conventional way to orient the hanger prior to landing.

This technique has been employed in a number of fields and has delivered considerable cost-saving benefits, especially through saving rig time by batch installing christmas trees within an accelerated field development program.

Monobore landing string

Another innovation has been to remove the need for a dual-bore landing string, thus cutting capital cost and reducing operational complexity. This was achieved by incorporating a bore selector tool immediately above the completion tree ( Fig. 5 [102,568 bytes]).

This tool selectively directs tool strings to either the production or annulus bore of the tubing hanger by the manipulation of a hinged flapper, while maintaining the ability to suspend the completion with tubing hanger plugs.

Use of this tool requires the modification of the completion tree to include a circulation port in the annulus bore that provides hydraulic access from the BOP choke and kill system.

A third annulus valve is added as a barrier below this port, while the uppermost annulus valve is inverted to hold pressure from above during clean-up operations.

The tool operates on two hydraulic control signals, providing immediate function. This is a major improvement over previous systems that required retrieval to surface.

Configuration is "fail as-is" to facilitate tool string removal, and the option of tubing pressure providing production bore access maximizes flexibility in the event of kill or suspension operations.

This design has now successfully completed a number of wells, and variants are being designed or built for several operators.

Monobore riser tree installation

With access to both bores from a monobore riser proven, the same principle can be applied to tree installation. Such a tool has recently been developed ( Fig. 6 [126,873 bytes]) to interface with a conventional LRP/EDP (lower riser package/emergency disconnect package). The tool is inserted between the EDP and a riser stress joint.

In one particular instance a dual-bore stress joint was available and the redundant annulus bore was employed to provide an exterior telltale providing eyeball remote-operated vehicle (ROV) confirmation of the flapper position.

It is envisaged that this mechanism can be upgraded to provide ROV override, if required. Also if required, a circulation path can be provided via the workover umbilical (or additional flexible hose) and the tree vendor's LRP.

This tool has now successfully run several trees with a simple monobore riser reducing the overall equipment capital cost and simplifying operations.

Operations management

Having suitable equipment is only half the battle. The preparations for operations must be rigorous, as any omission has the potential to be extremely expensive.

To identify the possible failures and their consequences, each design element and the program must be assessed by a Hazop. Then the appropriate steps can be identified in advance to permit effective management of such situations should the need arise.

Even better is to include in the tools the ability to avoid the potential outcome of any failure or provide simple functionality to manage the impact without operational intervention. It is therefore extremely important to ensure that the engineers employed to design equipment address these issues and devise innovative approaches, if necessary, for safety-critical systems and for those systems that have the potential to prevent the meeting of operational objectives.

Using competent operations personnel and service providers is essential. All those in positions of management or supervision must understand the particular hazards posed by deepwater operations and the impact of equipment and operational philosophies selected. As with other operations, the proper training, supervision, and competency assessment of individuals are integral parts of providing appropriate personnel.

Finally, it is imperative that, similar to hardware, operational control systems, including safety management systems, are integrated to avoid anomalies or contradictions. The development of integrated operational and contingency procedures must be considered part of this process.

All these points may be considered as taken for granted, as they equally apply to operations in conventional water depths. Nevertheless, the costs of failing to consider these topics will be greater in deep water, and the pool of personnel available to meet these requirements is considerably smaller.

In addition, many shortcomings or conflicts may only be exposed by the extreme nature of deepwater operations.

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