Subsea well control enhances horizontal production tree safety

Modular design for flexibility

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In an emergency shutdown situation, the subsea completion tree system's lower ball valve and flapper valve close, the retainer valve closes, and the volume between the flapper valve and retainer valve is vented. Disconnect occurs in 15 seconds.
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Well control systems are vital during completion, flowback, and intervention operations on a well. New well control and safety equipment is required at the seafloor as a result of the horizontal production tree configuration. Proven subsea well control systems are particularly important today for the growing ultra deepwater market.

With horizontal production trees, wellbore fluids are directed from vertical to horizontal by the tubing hanger. The trees utilize many of the same components as conventional trees. Use of a horizontal production tree allows access to the well without removing the tree or disturbing flowlines or control umbilicals.

The use of subsea completions to develop marginal and deepwater fields is growing in popularity as a cost effective development tool. Horizontal production trees are gaining acceptance by the industry as another way to reduce capital costs and operating expenses.

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According to figures from Quest Offshore Resources, approximately 1,800 subsea wells have been installed worldwide since the first subsea well was completed in 1960, representing a significant opportunity for well maintenance interventions. Going forward, Quest estimates that as many as 250 new subsea well completions could be installed annually from now until the end of 2002, and then increasing to as high as 400 during 2003. These figures are based on pending, probable and possible subsea completions as indicated in an accompanying table.

Deepwater discoveries

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The subsea control module of the deep water control system contains 26 solenoid-operated control valves and 42 pressure transmitters inside a pressure-compensated oil-filled enclosure.
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The number of deepwater discoveries worldwide has doubled during the past two years, from 33 announced discoveries in 1,500 ft of water and greater in 1998 to 65 in 1999, according to figures from Offshore Data Services. The trend toward deeper waters is evident in these figures.

The number of wells in water depths of 3,000-5,000 ft has more than tripled from 12 in 1998, to 38 during 1999. There were nine discoveries in greater than 5,000 ft of water in 1999, the first announced discoveries in those water depths since 1996. These include several discoveries in 6,000 and 7,000 ft of water and greater.

As seen from the number of trees already installed plus the number of trees planned or considered possible for installation during the next four years, a very strong well completion market exists. Since virtually every major production tree manufacturer promotes their own horizontal production tree technology as a cost effective option for marginal and/or deepwater fields compared with conventional production trees, it is likely that more of the subsea completions will utilize horizontal trees.

Horizontal tree efficiency

Operators have seen a need for additional well control systems during completion installation, flowback, and intervention operations in order to ensure the safety and environmental capability of horizontal production trees. Operators want to effectively shut in a well and also retain fluids in the riser if it becomes necessary to disconnect. They also want quick and easy reconnection of the riser and landing string after the emergency situation has been effectively managed.

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Schlumberger's solution to these needs is the SenTREE 7, the industry's first modular integrated subsea completion tree system (SCS). The equipment is used in conjunction with a horizontal christmas tree and tubing hanger system, and serves as the primary subsea safety device during completion, flowback, and intervention operations.

It is designed to operate in 10,000 ft of water, and handle pressures up to 10,000 psi with temperatures as high as 325°F. It provides the fast response to isolate the well and disconnect the landing string from the completion string without releasing well bore fluids into the environment.

The SCS results from combining experience in downhole testing with the electronic telemetry used in the company's logging business. Electronics from the company's logging technology have been operated in downhole conditions with temperatures up to 450°F. Those same electronics are utilized in the tree's deepwater control system (DWCS).

SCS components

Components in the system include:

  • Control system: Supplies electrical and hydraulic control functions to operate the valves and latch connector
  • Bleed-off valve: Used to bleed off pressure between the flapper valve and retainer valve prior to disconnect
  • Retainer valve: When closed, the valve retains the contents of the landing string, prohibiting the discharge of the contents into the environment
  • Latch connector: Provides for emergency disconnect and also reconnect capability
  • Flapper valve: Provides the primary well control seal
  • Ball valve: The first valve to close in the emergency disconnect sequence of events.

The DWCS is a multiplex (MUX), consisting of a subsea accumulator module, subsea control module, and a subsea spanner joint. The three modules of the DWCS total 78 ft in length. The DWCS sits atop the subsea completion tree and supplies the controls for the tree.

The subsea accumulator module (SAM) consists of eight accumulators arrayed around a central mandrel and stores hydraulic fluid at 5,000 psi and 10,000 psi. It is the termination point for the electro-hydraulic umbilical from the surface and also interfaces with the landing string.

The diameter of the single umbilical from the rig or surface vessel is only 1.5-in., and contains a seven-conductor armored cable, a high pressure hydraulic fluid line and a chemical injection line inside a polyurethane outer sheath. A single, 10,000 ft umbilical saves rig time and space by serving the subsea completion tree system, surface flowhead, and the running tools for the tubing hanger and tree cap.

The subsea control module (SCM) contains 26 solenoid operated control valves and 42 pressure transmitters. The SCM also contains two flowmeters, two temperature transmitters, six hydraulic filters, and two subsea electronic modules. The entire SCM is contained inside a pressure compensated oil-filled enclosure.

The subsea electronics module (SEM) is the electronic heart of the DWCS and an integral part of the subsea control module. Three pin connectors provide the electrical interface between the surface control unit (via the SEM) and all valves and pressure transducers of the SCM.

A subsea spanner joint extends the SCM and SAM above the BOP flex joint. The spanner joint contains the 9 5/8-in. annular BOP sealing area and is the interface to the subsea completion tree.

A master control station at the surface utilizes a Windows NT based system to communicate with the subsea equipment through Schlumberger's "CTS" high-speed telemetry link. The station controls and monitors the topside functions via the flowhead control panel and also monitors the status of the three remote shutdown panels.

System options

Three types of control systems are available depending upon the type of vessel used, whether moored or dynamically positioned, and in shallow or deepwater operations. Direct hydraulic control is utilized for shallow water and anchored vessels while enhanced direct hydraulic (E-D-H) is a fast response communication method for deepwater anchored vessels. E-D-H is used in water depths from 1,500 to 5,000 ft. The system uses accumulators at the surface with a hydraulic pod attached to the landing string. To close the well and disconnect is achieved in approximately two minutes at 4,500 ft of water.

Communication in applications with a dynam ically positioned vessel is provided by the DWCS whereby a signal is sent to and received by the subsea electronics module. The control module is actuated from the surface utilizing established telemetry and equipment adapted from Schlum berger's wireline logging tools. In an emergency shut down situation, the lower ball valve and flapper valve close, the retainer valve closes, the volume between the flapper valve and retainer is vented, and disconnect occurs in 15 seconds.

The philosophy regarding redundant safety systems is not to merely duplicate the safety system but to install two different systems. The flapper valve permits higher fluid pumping rates if well conditions deteriorate into a kill situation. With a two ball valve safety system, it is possible that not enough volume of fluid would pass through to effectively kill the well. With a flapper valve there is no resistance encountered, and there is only one ball valve to pump fluid by, resulting in a better chance of killing the well.

Tree operations

Several major international operators have awarded multi-year contracts to the company for use of the subsea completion tree and associated services. These contracts include two separate three-year agreements by Texaco for use in the Gulf of Mexico and the North Sea; a three-year contract with BP Amoco in the Gulf of Mexico; and a two-year agreement with a third major Gulf of Mexico operator.

The first of three subsea completions in Texaco's Gemini field during summer 1999 set a world water depth record for subsea completion tree systems at 3,400 ft. For these particular wells, service was performed from an anchored drilling, using an enhanced direct hydraulic control system. The subsea completion tree served as part of the landing string system to install the internal tree cap.

Subsequent to the world water depth record on the Texaco well, another late 1999 installation for another major operator set a new water depth record for a subsea completion tree system in the Gulf of Mexico at a depth of 4,650 ft. An enhanced direct hydraulic control system also was utilized on this multi-well development.

The SenTREE 7 system is being introduced into the North Sea Captain field as a large bore intervention alternative. Fifteen completions will be installed for Texaco during the three-year contract.

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