Water quality measurement

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

As there is a lack of subsea online continuous monitors for produced water quality measurement subsea for reinjection and or discharge purposes, development and qualification of such an instrument become a priority. The alternative is to use a remotely operated vehicle to extract a sample and bring it to the surface for offline 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 reinjection or discharge operations, and therefore water quality.

Measurement technology development

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 reinjection applications.

For oil in water concentration measurement, laser induced fluorescence (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.

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. Likewise, at one stage photo-acoustic based sensors looked promising, but there has been little progress in bringing this technology to the commercial market.

For produced water reinjection, measurements of both solid and oil content, as well as particle size and size distribution, are important. Image analysis, ultrasonic, and a combination of LIF and image analysis based systems are thought to offer good potential for the purpose of produced water quality measurement.

The ultrasonic acoustic based oil in water monitoring system is still a relatively new development, and despite field trials, there are few applications in the oil and gas industry.

It therefore appears that the combination of LIF and image analysis would offer a complete technology for produced water quality measurement. However, this technology is still relatively new to the oil and gas industry.

Ensuring reliability

The reliable operation is paramount subsea equipment is paramount, as once it is deployed it is expensive and time consuming to repair, retrieve or replace. To reduce the risk and to ensure the reliability of these devices, which can then be accepted by the industry, testing and qualification is therefore 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, Rev. 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.

Whilst the aim of all these recommended practices and standards is similar, there is not a completely unified approach in terms of how to achieve increased reliability, risk reduction and ensuring the safe operations of subsea systems.

Qualification testing

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 help to ensure that the subsea equipment will withstand the kinds of environments in which it will be transported, stored and operated, and may include shock, vibration, temperature variations and thermal cycling. 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. Such tests also cover instrument stability, accuracy, repeatability, up time and availability. This helps 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’.

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 suppliers and offshore operators is vital. Subsea separation systems 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. This means that more needs to be done within the oil and gas industry to foster such a close collaboration, in order to help accelerate the development and deployment of subsea water quality measurement technologies.

Mind the gap

Seabed processing strategy helps to maximise oil recovery and minimises 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, which affects the wider up take of subsea separation systems. Clearly there is a strong need to develop subsea water quality measurement devices.

It is thought that with the risk and costs involved in developing these technologies, Joint Industry Projects 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.

Written by Ming Yang, Environmental Consultancy Services Manager at NEL, TÜV SÜD Group, NEL.

Edited by Claira Lloyd

Read the article online at: https://www.hydrocarbonengineering.com/special-reports/11052015/nel-water-article/