Linear position sensors show ability to monitor in many applications

An electro-mechanical sensor, the LVDT produces an electrical output that is linear and directly proportional to the position of a physically separate movable nickel-iron alloy core. Typically, the LVDT body is held in place and the ferrous core is attached to the moving element on the piece of equipment requiring position feedback. Its operating principle is the same as any transformer where a primary coil induces voltage in secondary coils that are 180 degrees out of phase. Coupled only by magnetic flux, there is no need for any physical contact between the core and coils. The primary coil is typically excited with a sinusoidal 3VAC RMS, between 2.5 and 10 kHz.

Microprocessors, along with new materials and manufacturing processes, enhance the performance capabilities of LVDT linear position sensors, making them more suitable for a wider range of applications. For example, new corrosion-resistant/high-temperature materials have enabled linear position sensors to withstand harsh conditions found in many offshore and subsea applications. LVDTs can be hermetically sealed or vented, and constructed from a variety of materials such as stainless steel, Monel, or Inconel. Units can withstand conditions such as:

• Pressures from vacuum to 30,000 psi (207 MPa)
• Temperatures from -203 ˚C to + 220 ˚C (-333 ºF to 428 ºF)
• High vibration 2-2 kHz +100 g (DC)
• Shock tolerance 1,000 g.

When suitably housed and manufactured, linear position sensors can withstand years of use in seawater and corrosive acids, and very high pressures and temperatures in conjunction with the chemical abuse.

Drilling platforms

In offshore drilling, linear position sensors monitor the structural integrity of platforms and pipelines. Attached to pipes or tubes – including production risers, catenary risers, tendons, and platform legs and braces, like large strain gauges – LVDTs monitor structural movement to ensure stability. As the pipe/tube/cable is strained, LVDTs report position change to a surface control system that can take steps to counter the motion. The sensors provide measurements with resolution to a fraction of a microstrain. For the huge loads on these structures, the total movement being measured is typically less than 2 mm (1/10 in.).

As measurements must be obtained in seawater depths to 7,500 ft (2,286 m) and with external pressure of 3,800 psi (26 MPa), LVDTs for these applications are designed with stainless steel and other exotic material such as Inconel, titanium, and Hastelloy for pressure and corrosion resistance. These materials enhance the reliability of this instrument so it can operate for at least 10 years, even if the device is fully exposed to seawater.

Example of an offshore LVDT, hermetically sealed with a welded subsea connector that is gold plated and rated up to 7,500 psi. Dependent upon ocean temperature and depth levels, the LVDT casing is typically composed of special alloy that supports long-term operation in different elements.

To get a useful signal to the surface, a 4-20 mA two-wire, loop-powered I/O is used, along with the LVDT, to minimize any noise over long transmission lines. Offsets can be made easily in the data acquisition system on the platform. A major advantage of an LVDT for this task is the sensor reliability regardless of offsets due to pressure and/or temperature. Also, high-density microelectronics allow signal conditioning and processing functions to go inside the LVDT housing rather than in an external box.

Choke valve position

LVDTs provide position feedback of subsea tree choke valves to ensure proper gas or oil flow through a subsea tree. They are remotely controlled by either hydraulic or electric actuators.

Position feedback is required to monitor and control choke status. Single or redundant LVDTs on valve actuators can ensure that when the chokes are nearly closed, flow is shut off entirely. Failure to completely close a choke could result in an environmental disaster. There are several sizes of sea chokes, but typical full strokes range from 2 to 12 in. (5 to 30 cm).

The LVDT for this application is either hermetically sealed or contained in an enclosure filled with oil. When an oil bath protects components, an LVDT does not need to be hermetically sealed as do sensors submerged directly into sea water. There, exotic materials are needed to protect coils and electronics.

In either case, typically a 4-20 mA signal from the sensor’s built-in electronics is scaled to the full range of the gate valve. A current signal is used over a voltage because the 4-20 mA signal can run over long cables with no voltage drop, and only two wires are required rather than four wires as for voltage signals. When the 4-20 mA signal is scaled to the full range of the gate valve, 4 mA equates to the valve closed position and 20 mA equates to the fully open position. For ball or butterfly valves, either a Rotary Variable Differential Transformer (RVDT) is used or the rotation motion is converted into a linear motion.

Transformer tank monitoring

In deep sea applications, it is impossible to run long power cables to drive electric motors and instrumentation from a top drive system without significant line losses. To reduce these losses, a power transformer on the sea bed gets high voltage from the topside. High-voltage transformers require non-conducting oil for both cooling and isolation. On the sea bed, the oil tanks must be protected against sea water contamination.

Bellows are used to pressure compensate subsea power transformers. Pressure compensators keep the insulating oil pressure close to that of the water pressure outside the transformer to avoid damage from pressure differential. The bellows are constructed to keep sea water out. There may be several bellows at one subsea installation. The movement of the bellows must be recorded to avoid power transformer failure.

Hermetically sealed LVDTs are designed to be submerged into sea water and mounted onto bellows to measure their position. The LVDT provides position feedback to a surface-based monitoring control. The LVDT serves as the eyes and ears of the compensator’s underwater operation.

Other subsea applications

LVDTs also are used in subsea towers to monitor extension of safety cables, providing critical information in case of severe weather or earthquake. This is a result of safety and environmental policies mandated by oil and insurance companies.