Lee Hudson at Macro Sensors considers how submersible LVDT linear position sensors offer long-term performance in subsea measurement control systems
New materials and construction techniques have extended the use of LVDT linear position sensors in subsea applications. Housed in materials such as Inconel, LVDTs have become more chemical resistance to seawater and other corrosive acids to provide long-term reliable operations for many years. As a result, LVDTs are becoming more popular alternatives to less reliable linear position technologies such as pots and magnetorestrictive sensors for position measurement in subsea monitoring systems.
One of the most difficult environments for any sensor to survive is under water. In subsea applications, the environment poses several performance criteria for the feedback sensors including operation in high pressures and resistance to chemically hostile seawater at depths as much as 12,000ft. As seawater attacks common metals, sensors constructed of stainless steel often become corroded and pitted, which ultimately leads to sensor failure. Location, temperature, salt level and micro-organisms in seawater can accelerate corrosion rates.
To withstand deep sea environments, Macro Sensors has constructed a new series of submersible LVDT Linear Position Sensors constructed of Inconel. The Inconel offers excellent protection against corrosion due to higher content of nickel, chromium and molybdeumn. These superalloys enhance the already high-reliability of the LVDT assembly, ensuring that it can meet service life requirements of at least 20 years, even if the device is fully exposed to seawater. Reliability is of critical importance due to the cost of replacing subsea hardware.
When encased in special alloys, the company’s SSIR 937 Series Submersible LVDT Position Sensors can provide the same reliable performance known for on land in seawater depths of 15,000ft with external pressures of approximately 7500psi in excess of 1 million hours Mean Time Between Failures. Another major advantage of using LVDTs for this sub-sea application is its extraordinary repeatability (error less than 0.01 percent of full range), regardless of offsets due to pressure and/or temperature.
LVDT accuracy and long-term operation are particularly needed for Finite Element Analysis (FEA) in subsea monitoring systems. Some typical applications for these Sensors include monitoring structural movement for long-term FEA (Finite Element Analysis) of pipelines, derricks, moorings, choke valves, extensometers and other critical high stress members on offshore oil platforms.
As an example, in order to ensure oil platform stability, these devices can be used to measure the extension of the structural members of oil platforms to a fraction of a microstrain. To ensure oil platforms don’t shift, movement is measured to less than 2mm. Because required measurement must be obtained in seawater depths down to 7500ft., with external pressure of approximately 3800psi and where service life requirements are a minimum of 20 years, these sensors offer resistance to pressure and corrosion while offering the high-reliability and long-term operation required of this application, even when fully exposed to sea water.
These devices are also ideal for providing position feedback required for monitoring and controlling status of chokes or valves to ensure proper gas or oil flow through ‘Subsea Christmas trees’. A Subsea Christmas tree refers to an assembly of valves, spools and fittings for an oil well, which, in formation, resemble a decorated tree. Its function is to prevent the oil or gas release from an oil well, while directing the flow of fluids from the well. Valves or chokes on the Christmas tree are used to open and close oil pipes as they bring oil from the sea bed. They are remotely controlled by either hydraulic or electric actuators.
Position feedback is required for monitoring and controlling choke status. Single or redundant LVDTs mounted on valve actuators are used to ensure that the chokes are closed. Failure to completely close a choke could result in an environmental disaster as recently experienced in the Gulf. There are several sizes of sea chokes, but typical full strokes range from 2 to 12 inches.
In these and other subsea applications, these LVDTs resolve the problem of getting a signal back to the surface, even in great water depths. Macro Sensor has developed 4-20ma two-wire, loop-powered electronics, with comparably high-reliability, to meet the 20-year lifetime requirement. A 4-20mA I/O also minimises any noise over long transmission lines. Offsets can be easily made in the data acquisition system on the platform above.
A major advantage of using an LVDT for this task is that the extraordinary reliability of the sensor, regardless of offsets due to pressure and/or temperature, permits those offsets to be easily corrected in the data acquisition system on the platform above.
As the search for oil and gas goes into deeper water, LVDT Linear Positions have become more dominantly used, replacing other technologies such as load cells in marine and offshore applications. Depending upon the temperature, salinity, oxygen levels and depths of the oil and gas fields, the LVDT, with its hermetic seal and non-contacting operation, is often the only technology that can deliver accurate and reliable performance in subsea conditioners.
Lee Hudson is Application Engineer at Macro Sensors
Macro Sensors
