Figure 1. Through use of a unique magnetic coupler, the actuation piston and hydraulic operating system connected to the DepthStar tubing-retrievable safety valve is isolated from the tubing well bore. As shown here, the outer and inner magnetic sleeves are separated by a well pressure containing housing, creating a chamber for the actuating piston that is absolutely isolated from the well bore. Use of the magnetic coupler makes it possible to remove all moving seals that are normally found in a subsurface safety valve completely from the wellbore environment.

In the oil field, primary technology drivers focus on safety and economic efficiency. Unfortunately, the deeper, harsher environments into which developments are moving often place these drivers in conflict. While equipment must maintain integrity in deeper, more challenging environmental conditions, it must not sacrifice safety standards and economic efficiency. This is particularly true for subsurface safety valves where conventional solutions are being pushed to the limits of their capabilities.

To answer the challenges of ever-increasing water depths, new freedom in placement and operating characteristics of the subsurface safety valve was needed. Previous deepwater safety valve solutions required either higher control system pressures to operate, often exceeding the pressure capabilities of the subsea umbilical, or additional seals and the long-term retention of gas-charged chambers to provide for reduced operating pressures.

Magnetic coupler

Halliburton’s DepthStar tubing-retrievable subsurface safety valve (TRSV) is a new development in well completion equipment for deep and ultra-deepwater applications that provides step-change improvements in reliability, safety and cost of overall offshore infrastructure. The DepthStar TRSV provides for a virtually unconstrained valve setting depth while operating at a consistently low control line pressure regardless of depth or well pressure, a critical factor in deepwater wellhead applications. Through use of a unique magnetic coupler, the actuation piston and associated hydraulic operating system are removed from the wellbore environment.

With the incorporation of the magnetic coupler into the TRSV, Halliburton brings several industry firsts in safety valve reliability: 1) there are no moving seals within the well bore where potentially harmful fluids and pressures can work against them; 2) the only type of seals that are used in the well bore are metal-to-metal seals, the most reliable in use; and 3) this is the first surface-controlled subsurface safety valve that can permanently isolate well fluids inside the production tubing and keep them from migrating into the TRSV control system.

How it works

The actuating piston of the TRSV is connected to the outer sleeve of the magnetic coupler located in a dedicated chamber of the TRSV outside of the well bore. The inner sleeve of the magnetic coupler is incorporated into the production flow tube within the well bore of the TRSV. Separating the inner and outer sleeves of the magnetic coupler is a pressure-containing housing made of a non-magnetic metallic material. When control pressure is applied to open the TRSV, the piston and outer sleeve of the magnetic coupler is shifted. This motion translates to the production flow tube within the well bore through the magnetic coupling force. The translation of the production flow tube opens the safety valve flapper and places the TRSV into well production mode. Release of the hydraulic pressure on the piston allows a mechanical spring to return the TRSV to the closed position, and the magnetic coupler provides a fail-safe closed condition.

To deliver transparent compatibility with other well tools that could be in the proximity of the DepthStar TRSV, Halliburton paid particular attention to the magnetic field surrounding the coupler. The techniques and methods employed in its design and construction ensure the magnetic field communicates primarily between the two sleeves of the coupler while allowing only a negligible field within or outside the TRSV. Well tools commonly run through (e.g., memory logging tools and perforating guns) or along the outside of (e.g., TEC and fiber optic lines) the TRSV were physically tested and verified that any residual magnetic field, no matter how small, had absolutely no measurable effect on the functions of those items.

Long-term integrity

To provide the long-term integrity of the magnetic coupling force, Halliburton engineers chose to use a rare earth-type magnetic material. The aging properties of this class of magnetic materials are superior to other types available and will retain the required strength for 20 years or more, even at elevated temperatures of 300°F (148°C) and greater.

What does this technology mean to the completion and subsea controls engineers? The TRSV gives the completion engineer the flexibility to place the subsurface safety valve at a depth based on the requirements and goals of the completion and not based on the limitations of the safety valve. For the subsea controls engineer it could mean a paradigm shift in controls architecture. Today’s standard is to install a control system with an umbilical from the host platform to the subsea wellhead, which incorporates a high-pressure circuit (typically 10,000 psi) dedicated solely to operate the safety valve and a low-pressure circuit to operate the other wellhead equipment. Operating at a consistently low control line pressure regardless of depth or well pressure, the TRSV allows for controls architecture that requires only the low-pressure circuit in the umbilical and the high-pressure circuit eliminated.

Aside from the reliability gains that can be realized by operating at a lower pressure, there can be an economic advantage to installing the TRSV as well. The cost savings from eliminating just the high-pressure control line from the subsea umbilical can be significant, often more than the cost of the TRSV itself. When also considering that the topside equipment for the high-pressure circuit is no longer required, the reduction in controls equipment investment can be significant.

DepthStar TRSVs have been installed in the Middle East, West Africa, Gulf of Mexico and Brazil. In a recent installation, a major Gulf of Mexico operator desired to place a subsurface safety valve more than 3,000 ft (915 m) below the mudline in a deepwater subsea completion. (Typical installation depths are 500 to 1,000 ft or 152.5 to 305 m below mudline.) Using the TRSV gave the operator the flexibility to set the valve at this significant depth below the mudline, yet still be able to open the valve with 5,000 psi or less of control line pressure and put the well on production.

Looking ahead to the future, another milestone in subsurface safety valve technology is on the horizon. Using this technology as an enabler, Halliburton’s electrically actuated subsurface safety valve is closer to reality than ever before. A key technical challenge to realizing electric actuation for subsurface safety valves was a reliable, long-term means of isolating the electric actuator and controller from wellbore fluids and pressure. The TRSV, with its dedicated, fully isolated chamber outside the well bore, answers this challenge with the architecture that provides a base platform for active development of an electrically actuated subsurface safety valve technology.