Subsea sensors are at multiple locations on the trees, manifold, and flowlines. The subsea transducers/sensors’ locations on a subsea tree. Tree-mounted pressure sensors and temperature sensors measure upstream and downstream of the chokes. Software and electronics in the SCMs compile sensor data and system status information with unique addresses and time-stamp validations to transmit to the topside MCS as requested. Various transducers and sensors are used in a subsea production system:
- Pressure transducer;
- Temperature transducer;
- Combined pressure and temperature transducer;
- Choke position indicator;
- Downhole pressure temperature transducer;
- Sand detector;
- Erosion detection;
- Pig detector.
Pressure Transducer (PT)
Pressure transducers are typical subsea sensors located on a subsea tree, manifold, etc.. A subsea PT typically operates using the force-balance technique, in which the current required by a coil to resist the movement of the detecting diaphragm gives a measure of the applied pressure. The diaphragm therefore does not actually deform; hence, devices can be built to withstand high pressures. Such devices can achieve an accuracy of 0.15%, although the output current circuitry may reduce this to an overall error band of around 0.5% of full scale. Because pressure transducers are flange mounted, they cannot be removed subsea if they fail. It is possible, however, to specify a transducer with dual gauges in one housing, providing a dual redundant sensor.
Temperature Transducer (TT)
Some TTs operate by measuring the output of a thermocouple, which is a simple device whose output is proportional to the difference in temperature between a hot and a cold junction. The hot junction is the one measuring the process, and the cold junction is at the head itself. In theory, the sensing element should be as close to the process fluid as possible. However, if the sensor were simply fitted into the production bore, there would be no other physical barrier between the process and the environment, other than the sensor itself. The usual approach therefore is to install a thermowell or use a pocket drilled into the tree block.
Pressure/Temperature Transducer (PTT)
A sensor design is available in which pressure and temperature elements are combined into one package. In this design, the temperature sensor is located in a probe, which is designed to be flush mounted into the process pipework. This also helps reduce errors due to hydrate formation. For sensors upstream of the production master valve, regulations may require that there be one or two block valves before the sensor, so, although the pressure measurement is unaffected, the temperature measurement may be subject to thermal inertia depending on its location.
Detecting sand in the produced fluids can be an important part of a proactive strategy for:
- Managing short-term damage of vulnerable equipment (e.g., chokes);
- Managing long-term damage of pipe/flowline;
- Warning of reservoir collapse;
- Preventing separator level control problems.
The detector works either by monitoring the noise generated by sand impacts on a solid surface or by measuring the erosion damage of a target inserted into the flow. This leads to two types: acoustic and electrical detectors. Acoustic signals generated by the particles impinging on the walls of piping are monitored by the detector, which is usually installed immediately past a bend of the piping. However, the accuracy is affected by the noise of flowlines or pumps. The electrical detector is based on measuring the change of electrical resistance of thin sensing elements, which can be eroded by sand. The system reads the erosive effect directly with high accuracy. Software tools can be used to predict the worst case erosion on critical pipe elements such as chokes.
 Society for Underwater Technology, Subsea Production Control, SUT, Subsea Awareness Course, 2008.
 B. Laurent, P.S. Jean, L. Robert, First Application of the All-Electric Subsea Production System Implementation of a New Technology, OTC 18819, Offshore Technology Conference, Houston, 2006.
 C.P. William, Subsea Control Module, U.S. Patent 6,161,618, 2000.
 International Electro-technical Commission, Functional safety of electrical/electronic/ programmable electronic safety-related systems, IEC 61508, 2010.
 J. Davalath, H.B. Skeels, S. Corneliussen, Current State of the Art in the Design of Subsea HIPPS Systems, OTC 14183, Offshore Technology Conference, Houston, 2002.
 International Organization of Standards, Petroleum and natural gas industries - Design and operation of subsea production Systems - Part 6: Subsea production control systems, ISO 13628, 2000.