Separation and produced water re-injection or discharge forms an integral part of subsea processing strategy. Its wider acceptance within the oil and gas industry, however, will depend on the availability of a reliable and continuous measurement of the quality of the water being injected or discharged.

To date there is only one instrument that has been developed and deployed specifically for subsea separation and produced water re-injection applications. Little information is available in the public domain in terms of how this instrument was developed and qualified, and how it has performed since its deployment.

As there is a lack of subsea online continuous monitors for produced water quality measurement for re-injection and/or discharge purposes, development and qualification of such an instrument becomes a priority. The alternative is to use a remotely operated vehicle to extract a sample and bring it to the surface for off-line analysis, which is an extremely expensive process. There is also a long delay in obtaining the measurement. This is detrimental to the effective control of subsea separation and produced water re-injection or discharge operations, and therefore, water quality.

Continuous monitoring of the quality of produced water on the surface is already challenging enough. This is evident from the fact that there is still no approved online oil-in-water monitor for the purpose of reporting in the whole of the North Sea or the Gulf of Mexico.

The development of a continuous on-line water quality measurement device for subsea applications will have many more challenges in comparison to surface applications because:

•  Subsea is a much tougher environment •  Water quality measurements will often need to include both oil and solid in water •  There is 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 •  There is little previous experience to build upon •  The market is relatively small, yet the development costs can be significant

Subsea water quality measurement is required to support re-injection for pressure maintenance or re-injection for disposal or direct discharge into the marine environment.

If the produced water is re-injected for disposal or for pressure maintenance, water quality in terms of both oil concentration and solids will be important. This is because both oil droplets and solid particles can damage the formation and impair the injectivitity of produced water. In the case of re-injection for pressure maintenance, quality in terms of solids concentration, solid particle size and side distribution is perhaps more important from a reservoir management point of view than that related to oil. Produced water re-injection for pressure maintenance will affect oil production rate, and also the total amount of oil that can be ultimately recovered from an oil field in addition to ‘getting rid of’ the water safely.

The lack of water quality measurement instruments for subsea applications is clearly an important reason why discharged separated produced water is unlikely for the foreseeable future. Without a reliable and accurate water quality measurement device that can be used for regulatory compliance monitoring, regulators will unlikely permit such a discharge.

Potential measurement technologies

There are a number of technologies and instruments on the market that have been considered for being developed into measuring water quality for subsea separation and produced water re-injection applications. The measurement techniques used by these technologies include photo-acoustic, erosion, microscopy image analysis, Laser-Induced Fluorescence (LIF), light scattering and ultrasonic acoustic.

For oil-in-water concentration measurement, LIF is now well established and represents a good candidate for subsea applications, as it has enabled manufacturers to construct an analyser with a probe that can be inserted directly into a pipeline. Photo-acoustic based sensors looked promising at one stage, but there has been little progress in bringing this technology to the commercial market. Light scattering is also a well established technique, which has been used for Oil Content Meters (OCMs) that have been fitted into thousands of ships. Light scattering based oil-in-water monitors have been used for produced water oil content measurement, however, their measurement can be affected by the presence of solid, gas bubbles and, like many of the other optical based oil-in-water monitors, fouling can be an issue.

For sand detection and monitoring, both erosion (intrusive) and acoustic (non-invasive) based technologies have been developed to provide useful information for equipment protection and sand production management, but these are not thought to be sensitive enough for produced water applications.

For produced water re-injection, measurements of both solid and oil content are important, as well as particle size and size distribution. There are many different types of particle size analysers available on the market, but for the purpose of produced water quality measurement, image analysis, ultrasonic and a combination of LIF and image analysis based systems are thought to offer good potential.

The ultrasonic acoustic based oil-in-water monitoring system is still a relatively new development, as despite field trials few applications are known in the oil and gas industry. The combination of LIF and image analysis, therefore, seems to offer a complete technology for produced water quality measurement. This technology, however, is still relatively new to the oil and gas industry.

One of the critical issues in having a reliable and robust subsea water quality measurement device is related to fouling of the optical windows that are common to many of the potential technologies mentioned earlier. Fouling may result from oil coating or scale formation. Common mitigation methods currently deployed on the surface include ultrasonic, jetting sprays, and the use of high velocity fluids passing through the optical windows of the sensors that could prevent the fouling occurrence in the first place. Applications of these methods subsea are being investigated.

Testing and qualification

Once it is deployed, subsea equipment will become expensive and time consuming to repair, retrieve or replace. Consequently, its reliable operation is paramount. To reduce the risk and ensure the reliability of these devices, which can then be accepted by the industry, testing and qualification is critical.

As testing and qualification of subsea production control equipment is a relatively new discipline, there is no established protocol specifically for the testing and qualification of produced water quality measurement devices. ISO 13628 Petroleum and natural gas industries – design and operation of subsea production systems – Part 6 Subsea production control systems, 2004; DNV RP A203 Qualification of new technology, December 2012; and API 17 Q Subsea Equipment Qualification, Revised 1 January 2010 provide some good guidance in terms of what is required. Of the three documents, the ISO 13628 provides the most detailed information regarding the types of tests that may be required on a subsea water quality measurement device.

While the aim of all these recommended practices and standards is similar, in terms of increasing reliability, reducing the risk and ensuring the safe operations of subsea equipment and systems, there is not a completely unified approach in terms of how to achieve it. Also, every company will have a different perception of risk, so when tests are carried out, the criteria of what can be accepted will differ from organisation to organisation.

Qualification tests serve three main purposes:

•  To demonstrate functional requirement •  To screen-out faults and manufacturing/assembly defects •  To improve robustness and reliability

Generally there are two main types of qualification test – environment and duty.

Environment tests may include shock, vibration, temperature variations and thermal cycling. These tests help to ensure that the subsea equipment will withstand the kinds of environments in which they will be transported, stored and operated. Here the DNV RP A203, API 17Q and in particular the ISO 13628-6, should be closely followed.

Duty tests may include those related to function and performance, e.g. responses to a change in process conditions such as temperature, pressure, salinity and chemicals, also instrument stability, accuracy, repeatability, up-time and availability. These tests help to ensure that the equipment is fit for the specific application. Here the ISO 15839 should be used as a key reference.

For all types of test it is important that instrument developers communicate with the testing organisations and discuss the testing requirements in detail. This is because some of the testing organisations, in particular those associated with environment tests, may not be as familiar with the standards and recommended practices associated with subsea equipment testing as it is such a new ‘science’.

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 are also carried out to ensure that they can work alongside other instruments and equipment.

This means that close collaboration between instrument suppliers, subsea separation systems providers and offshore operators is vital. Subsea separation systems providers and operators have the experience of 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, therefore, to foster such a close collaboration to help accelerate the development and deployment of subsea water quality measurement technologies. This is particularly important under the current climate where the oil price stays relatively low.

Filling the technology gap

Seabed processing strategy helps to maximise oil recovery and minimise flow assurance issues and subsea separation, and produced water re-injection or discharge forms an important part of this. Yet water quality measurement remains a technology gap that affects the wider up-take of subsea separation systems. Clearly there is a strong need to develop subsea water quality measurement devices, which has been recognised by NEL, operators and the likes of RPSEA (Research Partnership to Secure Energy for America).

In the past five years, NEL has conducted two Joint Industry Projects (JIPs), aimed at accelerating the development of these devices. While the first JIP focused on reviewing potential technologies and establishing functional specifications, the second concentrated on performance testing of a number of selected technologies and identifying technology gaps. With the success of these two projects, a third JIP has been initiated and is underway. The project supported by three operators and one subsea separation systems provider is aimed at developing subsea water quality measurement technology to Technology Readiness Level (TRL) 5.

In the USA, a government supported project, entitled Subsea Produced Water Sensor Development Project, was initiated in 2014. The primary goal of the project is to develop sensors that can be used for regulatory compliance monitoring purposes for the discharge of produced water separated subsea. The project, which aims to bring potential sensors to a TRL 3 or 4 as defined in API RP17N, consists of two phases.

Phase 1 (nine months) is about concept proof of a new and emerging oil-in-water measurement technique based on using Confocal Laser Fluorescence Microscopy (CLFM), developing technical requirements for subsea produced water discharge sensors and conducting a gap analysis on some of the existing oil-in-water measurement sensors.

Phase 2 (15 months) is about design, development and performance testing of a number of sensors that will have been selected from Phase 1. The 24 month project is expected to be completed by September 2016.

It is thought that with the risk and costs involved in developing these subsea sensing technologies, JIPs that involve operators, subsea separation system providers, independent testing organisations as well as technology suppliers, offer the best route to successfully develop subsea water quality measurement technology and to fill the technology gap. With the research and development efforts on-going at present, good progress in developing an accurate, reliable and robust subsea water quality measurement device can be anticipated in the next four to five years.

Published: 16th Sep 2015 in AWE International