Subsea Well Intervention

Work progresses on advanced Riserless Intervention System

A collaborative effort to develop a riserless, deepwater/ ultra-deepwater subsea well intervention system is getting closer to commerciality, while research continues on a companion shallow-water version that is hydraulically operated directly from the surface.

Robert Corkren and Larry Klentz, Saipem America; and David Larimore, Halliburton Co.

In developing new technologies and methods in the ever-changing E&P industry, companies need to have a strong understanding of harsh deepwater environments and their rigorous challenges. Saipem America and Halliburton understand these challenges and the new interest in deepwater riserless technology. Both firms have many years of combined experience in providing quality deepwater services.

Since signing a cooperation agreement in 2005, Saipem America (formerly Sonsub) and Halliburton have come a long way in developing a true Riserless Intervention System (RIS) for deepwater and ultra deepwater well interventions. Continual communication among operators and a cooperative, integrated project development team, has allowed both Saipem America and Halliburton to take an educated, well-informed path in developing an advanced system.

Originally, the RIS was designed as a means for a reliable well intervention system based on existing technology for use in relatively shallow-water operations in the North Sea market. The idea of riserless intervention systems in the North Sea for shallow-water interventions has come and gone several times in the last five years or so. Recently, however, there has been a huge push for deeper-water and ultra-deep water (water depths greater than 4,000 ft) riserless interventions. The primary drivers are: 1) a need to reduce overall expense of riser-based well interventions; 2) the diminishing availability of deepwater platform rigs; and 3) an economic incentive to increase recoverable reserves on subsea completions.

To address these challenges, we now see a two-fold need in the industry for two completely independent, but cross-compatible, riserless intervention systems. One system is for a fully hydraulic, topside-controlled, shallow-water riserless intervention system. The other would be an innovative, advanced MUX/ ROV, subsea-controlled, deepwater/ ultra-deepwater riserless intervention system. The RIS is made up from major components of both Halliburton's wireline systems and Saipem America's completion workover riser system, Fig. 1. The completion work over riser system was first used in 1992 on the Popeye project in the GOM. It has been continuously improved and upgraded for use to a depth of 7,000 ft using Saipem's patented, concentric pipe, subsea production riser.

Fig. 1. The RIS system is comprised of components from one firm's wireline systems and the other company's completion workover riser system.

SHALLOW-WATER RIS

In conjunction with current developments of the deepwater/ ultra-deepwater RIS, Fig. 2, the companies have taken a step back and are re-examining the shallow-water market, where this all began. Still working in cooperation, the firms have initiated a review of existing technologies that both companies currently have, so that they can develop a more economical, shallow-water RIS.

Fig. 2. Given its free-standing design, the subsea stack will eliminate the risk associated with down lines.

Of course, the trick is to stay within the idea of safe, reliable operations, as well as keep with one of the main goals – to reduce the cost of well interventions to the operator. With negligible modifications to this existing equipment, at a fraction of the cost, the firms have already identified a nine-month decrease in product R&D time to bring the shallow-water system to market. In water depths less than 2,000 ft, there is a significant benefit to running an existing wireline system and electro/ hydraulic umbilical system from a vessel. Conversely, the deepwater/ ultra-deepwater RIS will be controlled through existing Innovator ROV umbilicals, and the necessary fluid control will be accomplished completely via existing subsea pumping technology.

DEEPWATER/ ULTRA-DEEPWATER RIS

As this review continues, research, development and engineering are still underway for the deepwater/ ultra deepwater RIS. Targeting the GOM and West Africa markets, Saipem and Halliburton's deepwater/ ultra deepwater RIS has been designed for a large number of deepwater subsea wells in the GOM and offshore West Africa, in water depths up to 8,000 ft.

The deepwater/ ultra-deepwater RIS will be a remotely operated subsea wireline system with the fewest down lines of any proposed or operational RIS on the market. The deepwater/ ultra deepwater RIS shall be deployable on relatively short notice from any dynamically positioned Class 2 (DP2) vessel that the supplier and the client deem fit for duty. While the deepwater/ ultra deepwater RIS will be controlled remotely from the surface, one or more Heavy Work Class ROVs will be required to assist with tree preparation, deployment, tool landing, latching, unlatching and recovery, backup operations, general observations and retrieval.

The deepwater/ ultra-deepwater RIS will be comprised of the subsystems and equipment necessary to accomplish a thru-tubing well intervention, including, but not limited to, the intervention unit to be attached to the well; a well control package; all wireline tools necessary to complete the operation; surface control and support equipment; all surface deployment equipment; ROV(s); control pods; and skids, Fig. 3. This method uses a modular approach for installation of the equipment, and the ability to disconnect from the subsea equipment in an emergency situation. A special fail-safe hydraulic umbilical connector allows the surface vessel, in seconds, to disconnect from the well without first retrieving the subsea equipment.

Fig. 3. The deepwater/ ultra-deepwater RIS uses a modular approach for installation of equipment, as well as the ability to disconnect from subsea equipment.

WIRELINE OPERATIONS

For wireline interventions, there are fundamental differences in the way subsea wireline is applied, when compared to conventional wireline operations. In contrast to wireline work on land, offshore platforms and anchored semisubmersible rigs, subsea wireline tool strings are deployed into the well at the seabed level. The subsea well intervention method requires a high level of dependence on communication with the vessel operations, as well as a thorough understanding of a DP vessel's characteristics in open water, Fig. 4.

Fig. 4. Subsea well interventions require considerable communication with vessel operations, plus good understanding of DP vessel behavior.

Subsea well interventions also require strong dependence on the vessel's DP system, which, in turn, relies on a good level of awareness of the vessel's characteristics by the wireline crews. Contingency procedures identify the varying degrees of DP status or possibilities of DP failure, and they provide the emergency protocols to be applied by the various parties directly involved in a subsea well intervention.

Wireline operators require an understanding of the behavior of a DP vessel and the various control systems that help maintain its stability on station. Additionally, it is essential for wireline operators to be familiar with the capabilities of ROVs that provide the functions of subsea survey, monitoring and, on some occasions, physical back-up to the operation of the stack while on the well.

ENGINEERING

To date, Halliburton and Saipem have conducted over six months of engineering analysis on key RIS components. The wireline system is capable of operating in water depths down to 10,000 ft in a 1.5 – 2.0-kt current, with a maximum vessel offset of 2.5%. The intent of the subsea stack is to be completely free-standing, thereby eliminating the risk associated with down lines. The system includes a 90-ft free-standing lubricator capability that can withstand 3.0 knots of current drag force. The BOP has a 5.5-in. bore and is rated for 15,000 psi.

Extensive dynamic modeling and other analysis have been performed to validate the operating characteristics of the wire deployment apparatus and ensure performance, Fig. 5. It is envisioned that a subsea measurement system will be required to account for the effects of wire movement from the surface suspension equipment. A seafloor tool magazine allows work tools (pre-positioned at the wellhead) to be readily changed at the wellhead, utilizing remote ROV techniques. Software developed for this application will enable precise control of the tools by integrating the downhole measurement device signals with the surface instrumentation, and a minimal amount of new automation development.

Fig. 5. The deepwater/ ultra-deepwater RIS is intended to perform wireline interventions in water as deep as 10,000 ft, so extensive dynamic modeling and other tests have been carried out to verify wireline operations in open water.

RISK ASSESSMENT

Deployment of wire from the vessel to the seafloor in open water, of course, introduces additional physical characteristics not consistent with conventional riser operations. Water depth and current profile are known limitations. Here are some additional subsea wireline issues that will not be found on conventional wireline systems:

• The effects of sea water currents will influence the tensions and depth measurements displayed by the wireline Advanced Measurement System (AMS).
• The methods of making up, handling and deploying the lubricator and grease injection head subsea.
• The application of injecting wireline grease, subsea.
• Monitoring grease head-to-cable sealing capability by use of subsea cameras and pressure transducers.
• Maintaining wireline equipment operational control in liaison with the host vessel central control, the DP officers on the bridge and the stack subsea engineer.

The problem with existing proposed methods is encountered during a drive-off condition. A drive-off condition occurs when, by accident or design, the surface vessel is forced to move away from its position over the well without first recovering the equipment attached to the well. With other systems, in the event of a drive-off, the operator must shut-in the well and release the lubricator and emergency disconnect package, so that the intervention equipment can be lifted free. To make this emergency recovery, the lubricator and emergency disconnect package must be kept continuously attached to heavy surface winches. Unless the crew can react very quickly, the subsea equipment will remain dangling below the vessel for some period of time while the vessel operator tries to regain control.

The deepwater/ ultra-deepwater RIS avoids this problem by simply disconnecting the surface-deployed umbilical from the subsea equipment, leaving the lubricator and blowout preventer safely attached to the well by means of patented hydraulic quick disconnects. This method eliminates the need to keep surface winches attached to the subsea equipment, which further enhances the system's safety and operability, thereby allowing disconnection in a matter of seconds.

The umbilical connector, used to connect the control umbilical to the RIS equipment, is fail-safe, in that it is hydraulically powered to connect and remains connected until hydraulically powered to release. Normal operation of the connector is controlled through the umbilical. However, a secondary release system, powered by the ROV, is provided as a contingency. The multiple-hose passages of the umbilical are sealed by check valves that are opened as the connector is powered to the connect condition. They automatically close as the connector is powered to release. The connection and disconnection is accomplished with a patented, single hydraulic cylinder that grabs the female half of the connector and pulls the connector halves together to engage the individual hose connector. Disconnection is achieved by extending the hydraulic cylinder with pressure from the hydraulic hose or by intervention of the ROV.

The blowout preventer is equipped with three sets of rams, two in the production bore and one in the annulus. It also has a bypass loop for each bore to equalize pressure across the rams after they have been closed.

CONTINGENCIES SEPARATING RIS FROM OTHER SYSTEMS

Saipem America and Halliburton have reviewed several possible scenarios as part of the initial development program. Below are just a few of the situations analyzed during our development reviews with major operators in the Gulf of Mexico. In the event of any one of the following situations, the RIS controls have been designed as a completely redundant system – from the control systems topside down to the seabed.

Control system failure. This situation involves an ROV override to close the BOP, and disconnecting the lubricator package and the subsea control unit. If necessary, at such a time, there would be a recovery to the surface for repairs. As a contingency, it will be possible to flush the lubricator prior to shutting in the BOP. In either situation, there will be three additional barriers above the BOP.

Vessel DP failure. In seconds after a vessel DP failure, an Emergency Shutdown (ESD) sequence will be initiated. Different stages of ESD can be programmed into the software to shut-in valves in any sequence, including delays and flying lead, quick disconnects. In the event of a control system failure, the BOP is equipped with a dead man circuit to shut-in valves.

ROV failure. All ROVs present on-board will be fully serviced and commissioned prior to their deployment. But in the event that an ROV fails during service, it will be recovered back to the vessel's deck and repaired. The backup ROV system will be used for the continuation of intervention while the failed ROV is being repaired.

Onset of bad weather. In the event that weather conditions worsen to the point where intervention cannot be completed as planned, the decision may be taken by the vessel superintendent, in conjunction with the client representative, to shut-in and temporarily abandon the RIS without disconnecting from the subsea tree. Alternatively, a complete recovery of the system to the installation vessel can be initiated, if time permits.

Wireline tools stuck across BOP/ tree. Integrated, shearable tool string sections will be spaced out across BOP cutting rams. Additional barriers have been designed, and are located, at both the top and bottom of the lubricator.

Abandonment and recovery. The BOP is shut-in and left on site. The control system and lubricator will be retrieved. A last-resort contingency would be to return to the location with a full workover riser system to resume operations.

CONCLUSIONS

The multi-service company approach in developing the RIS technology is based on the firms' collective experience, existing technology, and the necessary collaboration between the vessel and well services to deliver high-quality subsea intervention capabilities.

Oil and gas operators have estimated that wireline will account for as much as 70% of all well intervention requirements during initial development of the deepwater subsea market.

The RIS developed by Saipem America and Halliburton provides greater flexibility and improved safety. Its modular design allows the use of a smaller, DP vessel equipped with a Heavy Work Class ROV to safely deploy and operate the system. The fail-safe umbilical connector provides safer, quick disconnection and emergency drive-off capability. A proven blowout preventer design and highly experienced crews give great confidence that workover operations can be conducted in a safe, efficient manner.