Oil in Water (OiW) is a general term for petroleum-based compounds in water. In some countries, e.g. the USA and Brazil, it is also referred to as Total Oil and Grease (TOG) and is one of the most common and highly visible forms of water pollution worldwide. From a treatment and operational standpoint, OiW or TOG can be further divided into dispersed and dissolved. Dispersed oil usually specifies OiW in the form of small droplets, which may range from sub-microns to hundreds of microns in size. Mainly comprised of hydrocarbons, both aliphatic and aromatic, dispersed oil can also contain heteroatom compounds such as organic acids. Dissolved oil usually refers to OiW in a soluble form. Aliphatic hydrocarbons in general have a very low solubility in water. However, the single ring or two ring aromatic hydrocarbons, together with compounds like organic acids and phenols, form the bulk of dissolved oil.
OiW measurement is a specialised subject, relevant to water and wastewater treatment, however, any industry that discharges wastewater has an interest in OiW measurement. Historically, the main areas of application have been for municipal wastewater treatment works, shipping, and the oil and gas industry in relation to wastewater treatment process control and optimisation, discharged water regulatory compliance monitoring, and environmental protection. Unlike many other parameters, OiW is also a method defined parameter. Without a method specified, reported figures of OiW concentration may mean very little.
OiW can be measured in a lab using a bench top method or in the field using an online analyser. Lab bench top methods include reference and non-reference methods. Reference methods are important for defining what OiW is and also for regulatory compliance monitoring. Non-reference methods, including online analysers, are important for process control, optimisation and trending.
Key issues and challenges related to OiW measurement include:
• No single reference method that is universally accepted and adopted
• Lack of understanding of measurement uncertainties associated with OiW results
• Lack of methodologies available in accepting the use of online OiW analysers for discharge reporting
OiW measurement methods
There are many different OiW measurement methods available. Reference methods include Infrared Absorption, Gas Chromatography and Flame Ionisation Detection (GC-FID) and Gravimetric. Non-reference methods include lab bench-top methods and online/inline analysers.
Whilst reference methods are extremely important for OiW definition and for regulatory compliance monitoring, they are not always user-friendly and are often impractical for certain applications.
Field measurement methods are often needed for a variety of reasons, including their ease of use, rapid results, low cost, portability, and no potential requirement for use of solvents and other consumables. In the case of online analysers, they can provide minute-by-minute (if not second-by-second) data, which is extremely useful for process control and optimisation.
Online OiW monitors have been developed, predominantly based on five different techniques: Laser Induced Fluorescence (LIF), light scattering, microscopy image analysis, ultrasonic acoustic and Ultraviolet (UV) fluorescence. UV fluorescence-based technologies are the most used online OiW monitoring technologies of all. However, LIF technology continues to gain acceptance and market share due to the availability of LIF probes which can be inserted directly to the process pipeline. Also, LIF-based monitors from the leading suppliers are equipped with an ultrasonic cleaning capability to mitigate optical window fouling. However, all fluorescence-based monitors (LIF included) are affected by droplet size and the ratio of aromatic to total hydrocarbons. Microscopy image analysis-based monitors offer the advantage of providing both the concentration and size of oil droplets and solid particles, and are therefore popular for produced water reinjection operations. Also, with images available – one can see the particles/droplets on a computer screen – they are increasingly used for process optimisation. Light scattering is quick, robust and is well applied in the shipping industry.
The presence of oil is a risk to the treatment of drinking water; a tiny amount of oil can have a disproportionate impact, tainting the drinking water even at an extremely low concentration. Similarly, for sewage treatment works, the presence of oil in trade effluent can have an impact on the treatment process. As a result, a maximum amount of OiW (or TOG) in a trade effluent is usually specified. In the UK and USA, this is typically around 100 mg/L.
For the shipping industry, the discharge of bilge water, i.e. water collected from the engine room and pumping areas, is strictly regulated by the International Maritime Organisation (IMO). For treated bilge water, the discharge limit for oil content has been set at 15 ppm (parts per million).
In the upstream oil and gas industry, for every barrel of oil produced there are approximately five barrels of water co-produced. For offshore production, roughly 75% of this water is treated and then discharged into the ocean with the rest being reinjected into a reservoir for disposal or for pressure maintenance. For onshore production, roughly 90% of this water is treated and reinjected either for disposal or for reservoir pressure maintenance, with the rest being treated and then reused
or discharged. If produced water is to be discharged, regardless of location, the oil content in the discharged produced water must meet a discharge standard set by the regulators around the world. In the North Sea, a monthly average of 30 mg/L as agreed by the OSPAR (Oslo-Paris) Convention, must be met. For many of the newer installations, however, a reduced figure is often set and agreed between the operator and the corresponding regulatory body/authority of the individual nations. Measurement of OiW concentration plays a very important role both for water treatment operations and for discharge regulatory compliance monitoring.
OiW measurement uncertainty
All measurements have an uncertainty associated and the measurement of OiW is no exception. The uncertainty of a measurement is defined as the size of the margin of doubt related to the measurement. To fully express a measurement result appropriately, it requires three elements:
1. The measured value from a method or a device.
2. The uncertainty of the measurement, which is the margin around the measured value within which one expects the true value lies with a given level of confidence.
3. The level of confidence attached to the uncertainty, which is a measure of the likelihood that the true value of a measurement lies in a defined uncertainty margin.
For OiW measurement, the sources of uncertainty are linked to each of the steps involved in the process of obtaining an OiW result. There are two main components which are respectively related to the sampling and measurement method. However, sampling could potentially contribute more uncertainty than the measurement method itself.
For a typical oil in produced water result of 15 mg/L, obtained by manual sampling and analysis using the OSPAR GC-FID method, previous estimation carried out at TÜV SÜD National Engineering Laboratory showed that the measurement uncertainty could be as much as ±49% with a 95% confidence. Such a high level of uncertainty could have serious implications to regulatory compliance monitoring, performance assessment of online OiW analysers, and the development of acceptance criteria for using non-reference OiW measurement methods for discharge reporting.
Filling the knowledge gaps
Online continuous OiW analysers have been used by the oil and gas industry for produced water management for many years. Until recently, the use of these online analysers has been mainly limited to process trending for surface manned installations, with very few examples of their use for produced water discharge reporting.
Guidelines for the use of online continuous OiW analysers for discharge reporting purposes have been available. They were developed with manned installations in mind. However, these guidelines had not been put into practice, checked, or verified in the field before being issued. A revisit was considered necessary to see if they can be simplified and/or improved.
Also, with an increasing emphasis on maximising oil and gas recovery and cost-effective production, there is increasing interest in developing and deploying unmanned platforms and subsea separation systems. Discharge of significant amounts of produced water from these unmanned installations will not be possible without the availability and use of reliable online OiW analysers. As yet, no guidelines currently exist for accepting an online OiW analyser for reporting the discharge of produced water from these installations.
With this in mind, we initiated a Joint Industry Project (JIP) entitled “Making Online (OiW) Analysers for Reporting (MOAR) – Manned, Unmanned and Subsea Installations”. The JIP was aimed at filling knowledge gaps, making recommendations to update existing guidelines, and ultimately making the use of online OiW analysers for produced water discharge reporting a common practice.
The JIP had the following objectives:
• To provide a status report on online OiW monitoring technologies and on the use of online analysers for reporting purposes
• To understand and establish uncertainties associated with OiW sampling and measurement
• To refine acceptance criteria for using online OiW analysers for reporting purposes
• To develop guidance for accepting online OiW analysers for unmanned and subsea applications
• To propose a set of changes to be made to the existing guidelines in relation to using online OiW analysers
The JIP was conducted and completed in 2020 with the support of six international oil and gas operators and the UK Government’s Department for Business, Energy & Industrial Strategy (BEIS).
Following the completion of the project, BEIS forwarded an input paper based on the outcome of the JIP to the OSPAR Offshore Industry Committee (OIC) meeting in March 2021, subsequently OSPAR agreed to further review the JIP final report, to discuss and agree possible changes to the existing OSPAR guidelines in relation to the use of online OiW analysers for discharge reporting.
Following on from the success of the last JIP, we have now initiated a new JIP aimed at conducting field trials to ensure what will be recommended for changes to the existing OSPAR guidelines will be practical and implementable. The new JIP will have following objectives:
• To demonstrate the feasibility of using the 95% prediction interval for online OiW analysers’ correlation validation in a field
• To check the practicality of the newly proposed methodology for accepting the use of online OiW analysers for reporting for both manned and unmanned installations
• To develop a methodology for calculating real time uncertainties associated with OiW data derived from online analysers for reporting
• To write up a draft updating the relevant sections of the existing guidelines taking into consideration the findings from the latest and previous JIPs
• To investigate alternative methods of accepting online OiW analysers for discharge reporting
It is anticipated that up to three field trials will be conducted covering typical manned and unmanned production and produced water discharge scenarios.
The outcome of the new JIP will provide industry, regulators and authorities with the confidence needed to implement an updated OSPAR guideline. Ultimately, it will allow for the industry to use online OiW analysers for produced water discharge reporting for both manned and unmanned installations, which brings many benefits including safer operations, improved efficiency, more accurate discharge information and the possibility of automation/unmanned production.
For those who will participate the latest project, it will provide them with an opportunity to shape future industry standards and practices related to the use of online OiW analysers for produced water management.
Measurement of OiW is an important subject for various industry sectors in relation to water intake protection, wastewater treatment process control and optimisation and regulatory compliance monitoring. There are many methods available to measure OiW concentration, both lab and field based. Lab based reference methods remain extremely important for the definition of OiW and compliance monitoring, whilst field methods are needed for process operations, optimisation and tending purposes.
OiW is a method defined parameter, therefore, it is always important to mention the measurement method used when quoting OiW concentration results. All measurements have uncertainty associated. However, due to the nature of OiW measurement – mostly wastewater related – few have paid much attention to its uncertainty until recently. Yet, it is vitally important as it impacts on discharge compliance monitoring and the assessment and acceptance of alternative measurement methods. Recent work at TÜV SÜD National Engineering Laboratory indicates that there is a substantial amount of uncertainty associated with OiW results obtained by manual sampling and subsequent analysis using a reference method such as the OSPAR GC-FID method.
Use of online OiW analysers has increased in recent years, for operations and for discharge reporting purposes. Online analysers offer many benefits, not only providing minute-by-minute information and reducing the need for manual sampling and analyses, but also potentially offering more accurate OiW discharge information. As regulators demand ever improved environmental performance and industry seeks to continually reduce operating costs, these benefits mean that online analysers are likely to become increasingly popular. To make the use of online OiW analysers for produced water discharge reporting a common practice for the offshore oil and gas industry, collaborative research projects have been initiated by us to close the knowledge gaps. As a result, the subject of uncertainty associated with OiW results is now better understood. In addition, sound criteria have been developed for accepting the use of online OiW analysers for oil in produced water discharge reporting purposes and a new JIP has been launched. The new project will aim at conducting up to three field trials covering typical manned and unmanned produced water handling and discharge scenarios, to ensure that any proposed changes to the existing official guidelines on using online OiW analysers will be practical and implementable.