Subsea processing is an effective technology that contributes toward maximizing recovery of a field’s oil and gas reserves. Two important parts of the subsea processing strategy are subsea separation and produced water reinjection or discharges, and subsea seawater treatment and injection.

Separating water at the seabed offers many additional benefits in terms of minimizing flow assurance issues, easy separation, reduced use of production chemicals and energy savings. However, for these to be realized, the measurement of water quality needs to be addressed since without it operations of subsea separation and produced water reinjection and subsea seawater treatment and injection systems could not be effectively controlled and run.

Subsea water quality measurement is required to support subsea reinjection for pressure maintenance or for disposal or direct discharge into the marine environment. Presently there are no instruments available on the market that can be used for continuously monitoring the quality of produced water separated subsea.

This absence is an important reason why there has been no wider uptake of subsea separation systems—without a reliable and accurate water quality measurement device, regulators cannot permit produced water discharge subsea.

More challenges subsea

The development of a continuous online water quality measurement device for subsea applications will have many more challenges compared to surface applications because:

  • Subsea is a much tougher environment;
  • Water quality measurements will often need to include both oil and solids in water;
  • There are a limited number of potential technologies;
  • There is a lack of qualification testing facilities and standards;
  • There is a lack of regulator involvement for developing procedures and standards; and
  • There is little previous experience to build upon.

If the produced water is reinjected for disposal or for pressure maintenance, water quality in terms of oil concentration and solid concentration, as well as particle size, will be important. This is because both oil droplets and solid particles can damage the formation and impair the injectivity of produced water. In the case of reinjection for pressure maintenance, injectivity impairment can affect the oil production and net oil recovery.

Technology development

In the quest to develop a suitable water quality measurement mechanism for subsea separation and produced water reinjection or discharge applications, a number of technologies have been considered. The measurement techniques previously explored and used include photo-acoustic, erosion, microscopy imaging analysis, laser-induced fluorescence (LIF) and ultrasonic as well as a combination of these.

For oil-in-water concentration measurement, LIF is well established and thought to be a good option for subsea applications. This allows manufacturers to construct an analyzer with a probe that can be inserted directly into a pipeline.

For sand detection and monitoring, both erosion-based (intrusive) and acoustic-based (non-invasive) technologies have been developed to protect equipment and for effective sand production management. However, these are not proving to be sensitive enough to support produced water applications.

For produced water reinjection, measurements of solids and oil content as well as particle size and size distribution are important. There are many different types of particle size analyzers available on the market. For the purpose of produced water quality measurement, image analysis, ultrasonic and a combination of LIF and image analysis-based systems are considered to have potential.

Reduce risk

All subsea equipment will be expensive and time-consuming to repair, retrieve or replace once installed. To reduce the risk of failure and ensure equipment reliability, testing and qualification is critical. However, there is no protocol specifically established for produced water quality measurement devices.

A few existing industry-recommended practices and standards provide some good guidance in terms of what is required. These include ISO 13628:2006: “Petroleum and natural gas industries—design and operation of subsea production systems—Part 6 Subsea production control systems,” which provides the most detailed requirements regarding the types of tests that may be needed for a subsea water quality measurement device. Other useful documents include DNV RP 203 “Qualification procedures for new technology” and API 17 Q “Subsea Equipment Qualification, Rev. 1 January 2010.”

There is, however, not a completely unified approach on achieving increased reliability, reducing risk and ensuring the safe operations of subsea equipment and systems. Also, different companies will have a different perception of risk, so acceptable test criteria may differ from organization to organization.

Qualification tests

Generally there are two main types of qualification test—environment and duty. They serve three main purposes:

  1. To demonstrate functional requirement;
  2. To screen out faults and manufacturing/assembly defects; and
  3. To improve robustness and reliability.

Environment tests may include shock, vibration, temperature variations, thermal cycling and electromagnetic compatibility.

Duty tests may include those related to function and performance. This includes responses to a change in process conditions such as temperature, pressure, salinity and chemicals. It also includes instrument stability, accuracy, repeatability, uptime and availability. These tests help to ensure that the equipment is fit for the specific application.

For all types of tests it is important that instrument developers communicate with testing organizations and discuss testing requirements in detail. This is because some of these organizations, in particular those associated with environmental tests, may not be as familiar with the standards and recommended practices.

Furthermore, subsea water quality measurement devices will ultimately be part of a subsea process control system and will need to be integrated into the overall process control system. It is therefore advised that integration tests also are carried out to ensure that they can work alongside other subsea instruments and equipment.

This means that close collaboration between instrument suppliers, subsea separation equipment providers and offshore operators is vital. Subsea separator providers and operators have the experience in successfully qualifying subsea separation equipment in the past and therefore know not only the qualification process but also the acceptance criteria.

More needs to be done within the oil and gas industry to foster a close collaboration to help accelerate the development and deployment of subsea water quality measurement technologies.

Technology stumbling blocks

Water quality measurement remains a technology stumbling block, which affects the wider uptake of subsea separation systems.

It is thought that with the risk and costs involved in developing these technologies, joint industry projects (JIPs) that bring together operators, subsea separation system providers, independent testing organizations and technology suppliers offer the best route to successfully developing industry-accepted subsea water quality measurement technologies.

Following two earlier JIPs carried out between 2009 and 2013 in which potential technologies were reviewed, tested and a gap analysis performed, NEL has started a third JIP supported by major operators and subsea separation system providers. The two-and-a-half- year project is aimed at helping vendors develop their technologies up to Technology Readiness Level 5.

Clearly there is a strong need to develop subsea water quality measurement devices as produced water reinjection or discharge forms an integral part of seabed processing. However, while water quality measurement remains an issue, with the R&D work currently being undertaken, there is now a high expectation that this technology gap will be substantially closed in the next few years.