Written by Macro Sensors
www.macrosensors.com
positionsensors@macrosensors.com
Subsea environments are one of the most difficult areas for any sensor to perform, especially when performance life of as much as 20 years is expected in an underwater application.
In subsea environments, seawater depths of 15,000 ft can exert external pressures of approximately 7500 psi on a sensor. Many sensors fail over time in high pressure deep sea environments, causing undue expense related to sensor replacement. Often, the maintenance service cost in replacing the sensor is more than the sensor itself. That’s why many applications require extended sensor life as part of operating specifications.
Seawater also attacks sensor metals at varying water depths, resulting in corrosion accelerated by the different levels of oxygen, temperature, pH, chlorinity, biological activity, electrical conductivity and velocity flow rates present in seawater at various depths. Corrosion can occur in the form of pitting, crevice or intergranular that leads to sensor failure.
Stagnant or polluted waters are additional triggers that often promote sulfate-reducing bacteria that can affect sensor materials’ performance. The high electrical conductivity of seawater promotes macro cell corrosion and increases galvanic corrosion, which accelerates temperature rise as well, further promoting corrosion.
Microbially-induced corrosion also is a very serious problem that affects sensor operation based on different service conditions and materials used in sensor construction, especially low-grade austenitic stainless steels. It is a corrosion process involving material degradation that normally occurs on welded joints and leads to weld failure if not checked and treated in time.
As a result of the effects of pressure and seawater on sensors, subsea applications pose special challenges for reliable operations. Depending upon the temperature, salinity, oxygen levels and seawater depths, the LVDT (linear variable differential transformer), when hermetically sealed and constructed of special alloys, is often the only technology that can deliver accurate and reliable performance in subsea conditions.
What is an LVDT?
An LVDT is an electromechanical sensor that converts the rectilinear motion of an object – onto which it is mechanically coupled – into a corresponding electrical signal. Available in a variety of measurement ranges, an LVDT linear position sensor can measure movements as small as a few millionths of an inch to up to ±20 inches.
Figure A shows the components of a typical LVDT. The transformer's internal structure consists of a primary winding centered between a pair of identically wound secondary windings, symmetrically spaced about the primary. The coils are wound on a one-piece hollow form of thermally stable glass reinforced polymer, encapsulated against moisture, wrapped in a high permeability magnetic shield, and then secured in cylindrical stainless steel housing. This coil assembly is usually the stationary element of the position sensor.
In operation, the LVDT's primary winding is energized by an alternating current of appropriate amplitude and frequency, known as the primary excitation. The LVDT's electrical output signal is the differential AC voltage between two secondary windings, which varies with the axial position of the core within the LVDT coil. Usually this AC output voltage is converted by suitable electronic circuitry to high level DC voltage or current that is more convenient to use.
How LVDTs Withstand Deep Sea Environments
Hermetically sealed LVDTs
In applications where sensors are surrounded by corrosive or pressurized media, a hermetically-sealed LVDT is recommended to ensure outside media does not enter the windings, making units impervious to water and chemicals that can shorten sensor life or reliability.
These types of LVDTs feature a hermetically-sealed assembly with a heavy-wall, metal housing and integral metal bore liner either made from 316 Stainless Steel or Super Nickel Alloys. This construction allows free movement of the core while sealing out the surrounding media from the windings.
In addition to its heavy-duty housing, bore liner and end washers are welded together to form a hermetic seal free from oxidation-producing faults that may cause leakage. Sensor coils are wound on a one-piece hollow form of thermally stable, glass reinforced polymer, encapsulated against moisture, wrapped in a highly permeable magnetic shield, and, then, secured in a cylindrical metal housing. For severe applications, the coil windings may be further protected with special potting compounds or encapsulating resins.
Leads can be sealed with a glass-sealed header or compression bushing, sheathed in stainless steel sheathing or Teflon coated so connections don’t fail in extreme environments. Typical hermetically-sealed LVDTs can withstand operating pressures up to 3000 psig. The hermetically sealed construction also lets the core to withstand temperatures up to 400°F.
Special Housing Materials
Many common stainless steels such as 304 and 316 should not be used for sensors that will operate in direct contact with seawater. To survive subsea environments, the LVDT casing must be composed of special alloys (see Chart 1) that provides extended chemical resistance to seawater. These superalloys enhance the already high-reliability of the LVDT assembly, ensuring that it can meet extended service life requirements, even if the device is fully exposed to seawater at depths up to 15,000 feet with external pressures of approximately 7500 psi. Reliability is of critically important due to the cost of replacing sub sea hardware.
Water Type 17-4PH stainless 316L
stainless steel Nickel Alloy 625
Fresh water Yes Yes Yes
De-Ionized Water No Yes Yes
Ground water No Yes Yes
Brackish water No Yes Yes
Sea water No No Yes
In shallow warm waters, Monel is ideal as its metal composition resists sea life forming on it. Housing and core carrier made from stainless steel will not survive well in shallow warm waters. Monel 400 is a special nickel-based alloy that provides excellent resistance against pitting and attack by micro organism, enabling sensors to perform in shallow and warm waters with high levels of oxygen. At depths of 2000 feet or more where the temperature is around 5°C, Stainless Steel is acceptable.
In subsea applications with depths of 7500 ft. and external pressures surpassing 3500 psi, Inconel offers excellent protection against corrosion due to higher content of nickel, chromium and molybdeumn.
When designed with Inconel 718 for pressure and corrosion resistance, an LVDT assembly can provide reliable operations for many years, even if the device is fully exposed to seawater.
While these alloys cost more than stainless steel, they offer total immunity against localized corrosion as well as oxidizing and reducing elements.
Where Subsea LVDTs Operate
LVDT Linear Position Sensors are widely used for condition monitoring systems as a part of control and safety functions for applications in offshore platforms, desalination systems, mooring cables, seafloor wellheads, and oil and gas gathering systems. During operation, sensors can be submerged in seawater at varying depths, based on tides, up to 1,000 feet.
LVDT accuracy and long-term operation is particularly needed for 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. For example, subsea LVDTs 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.
Subsea LVDTs 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 as part of a subsea control module. Single or redundant LVDTs mounted on valve actuators are used to ensure that when the chokes are nearly closed, flow is entirely shut off. 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 form 2 to 12 inches.
LVDTs are also used in subsea towers to monitor extension of safety cables, providing critical information in case of severe weather or earthquake, leading to the evacuation of the drilling platform and closing of the oil well. This application came about as a result of safety and environmental policies mandated by oil and insurance companies.
SUMMARY
As demand for sensors to be in contact with seawater increases for applications such as loading systems, subsea mooring cables, control valves, chokes, desalination plants and platform stability, it is important to consider the seawater environment, location and conditions when specifying a sensor. Materials must be carefully selected to withstand high pressures and variable corrosion properties to ensure trouble-free operation. The long-term cost of ownership should be taken into consideration as some locations, particularly on the seabed, are not easy to access to replace failing sensors without spending a lot of money and downtime.
As Subsea LVDTs provide the same reliable performance as standard units but with more chemical resistance to seawater, they are becoming more popular alternatives to less reliable linear position technologies such as pots and magnetorestrictive sensors for position measurement in subsea monitoring systems.
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