Tracerco at cutting edge of flow assurance techniques

One of the UK’s leading process and measurement diagnostic specialists, Tracerco, has put its technology to the test during groundbreaking oil and gas projects.

The company’s flagship level measurement instrument, the Tracerco Profiler™ (the Profiler) has an established reputation for regularly being used in many upstream three-phase oil and gas separation processes. More recently, the company’s team of experts were called in to modify the unit to be used in a unique subsea processing application as part of the Tordis Improved Oil Recovery (IOR) system and to help an oil and gas major eliminate hits on its crude oil tower at a refinery though novel design allowing use in a desalter vessel.

The Tordis Improved Oil Recovery (IOR) system, which was commissioned by Statoil, is the world’s first full field subsea separation, boosting and injection system. It is at the cutting edge of design and technology and owes much of its success to the strong working partnership between FMC Technologies and Tracerco, part of the Johnson Matthey Group.

The collaboration has resulted in an impressive subsea separation monitoring system that combines Tracerco’s separator measurement experience with FMC Technologies’ knowledge of subsea process design and project execution.

In 2005 Statoil contracted FMC Technologies to deliver a full-field separation facility to enable reinjection of bulk water into a non-hydrocarbon reservoir and send hydrocarbons through a multi-phase pump back to the Gullfaks C platform. By installing a full-field subsea separation facility, Statoil expects to improve the Tordis Field’s recovery factor from 49% to 55%. Along with other upgrades to the field, FMC’s separation system will allow Statoil to extract roughly 35 MMbbl extra from the Tordis Field.

Six of Tracerco’s state-of-the-art separator level measurement instruments are central to the project and are being used in the separator and desander vessels. The company has more than 40 years’ experience in developing innovative specialist measurement solutions and has redesigned its topside Profiler unit to cope with the subsea environment.

Andy Hurst, managing director at Tracerco said: “This is a hugely exciting project and we are delighted to be involved. During the next decade, subsea-separating techniques will be used more and more as operators try to make the most of hydrocarbon reserves. The Tordis project will not only make this activity more viable but also sets the industry standard for this type of activity. Through subsea separating technology, energy firms will have the opportunity to maximise well returns and ensure the longevity of field life.”

The Profiler works by giving a density profile of the cross section of the vessel and then gives a visual interpretation of the data much in the same way an MRI scanner would provide a slice of a human body, essentially giving the operator “insight onsite”. The scan of the vessel provides a range of data that can be interpreted to identify the oil / water interface and any emulsion / foaming layers that may be forming.

The unit is based on the gamma radiation absorption principle, using long-term stable Geiger-Muller detectors and consists of a number of density measurement detectors, installed vertically inside the separator vessel. The measured change in density — the process profile — is presented graphically to the operator. The measured interface levels from Digital Control System (DCS) input for separator control and shutdown, include foam, liquid, emulsion, water and sand levels. The information provided by the subsea profiler gives engineers the most accurate vision of the density patterns in the vessel and helps maximise efficiency.

Tracerco was also called in to help a major oil and gas operator prevent unplanned shutdowns during the refining of crude oil. The company was able to develop its existing Profiler unit to address the unique requirements of the operator’s refinery. Tracerco invested more than a year in the development of the technology, which is now being used as an integral part of a desalter unit and it has produced unprecedented results.

Since being installed, the Profiler has eliminated all unplanned desalter shutdowns at the refinery, enabling the company to dramatically increase the number of barrels it can refine per day and significantly reduce chemical usage.

Andy Hurst, managing director of Tracerco said: “This really is a unique partnership and we are delighted that the Profiler has produced such fantastic results. We were approached by the operator to provide a unit to help reduce their crude tower water hits. The temperature and pressure requirements meant that we couldn’t use existing technology but had to use our wealth of experience to develop a new, specialized unit”.

Tracerco has proven experience of using the Profiler at temperatures of approximately 150°C but it had never before been faced with adapting the technology to cope with temperatures of upwards of 250°C.

“The high temperatures and pressures involved were a real challenge for us as it was above the allowable intrinsically safe certification temperature of 125°C,” added Hurst. “We were able to overcome the temperature issues by incorporating a specialist closed loop cooling system to the Profiler. The cooling system was a particular challenge as it had to meet with Tracerco’s design ethos of minimal maintenance requirements. But, with a lot of time spent in research and development, we have come up with a proven reliable and robust solution”

The function of the desalter vessel is to remove suspended salt crystals and water in the crude feed. Salt is naturally present in the oil when it is produced in an oil field. When oil is distilled at a refinery, it is heated to over 426^oC, causing the salt to become very corrosive. To remove the salt, wash water is added to the incoming crude feed, the emulsion this creates must then be quickly separated inside the desalter. Salty water is then discharged out of the bottom and the desalted/dehydrated crude is discharged out of the top of the vessel.

The oil discharged must be less than 0.5% water, otherwise when heated it could over pressurize the distillation column. To ensure oil dehydration, electrostatic grids are added to maximise coalescence. But, if too much water reaches the grids it could cause them to electrically overload and shutdown. To prevent this, emulsion breaking chemicals are added to the incoming crude to assist in oil/water separation. The continuous change of oil/water concentrations make it very difficult to optimize the process and if salt crystals reach a distillation column, the damage could be extensive.

With the view inside the vessel, the Profiler gave the operators confidence in the operation of the vessel and allowed the operations team to raise the oil/water interface higher in the vessel near to the electrostatic grid. Once this was achieved a much smaller emulsion layer with a clean water outlet and no process upsets was quickly seen.

The Profiler is just one of a suite of products and services that Tracerco provides As part of its Precision Diagnostics portfolio of services there is a range of other monitoring techniques that can be used to ensure the effective flow of oil and gas through an oil and gas processing facility or pipeline. The techniques are non-intrusive and can be used to accurately diagnose processing problems.

For instance, as remaining reserves of oil and gas become more difficult to access and fields that were thought previously uneconomic to develop become viable there is an increasing requirement to attain longer subsea tiebacks to production facilities. The flow lines involved in longer tiebacks will become more prone to blockage from deposition such as hydrates or waxes. It is therefore important that if these deposits and blockages do form that they can be examined and quantified so that the appropriate intervention action can be taken.

Tracerco uses three key techniques: gamma-ray transmission scanning for accurate point measurements, tomography measurements for detailed density mapping of pipeline contents and radioisotope tracer injections to determine deposit profiles and inventory over long distances.

Using basic scanning techniques used in topside processing, the amount of deposit can be determined from outside of the pipeline, the problem arises though when the pipeline is buried as in many subsea applications. Therefore, tracer techniques, similar to a barium meal used in medical technology can be used to determine deposit location.

Should further, more detailed investigations be required, then a technique similar to that of a CAT scan, tomography, can be used to give a picture of the inside of the pipe.

The gamma-ray transmission technique, Tracerco Diagnostics™ Scan, uses a small sealed source and a sensitive radiation detector positioned on adjacent sides of the pipe. The measured radiation intensity is directly related to the amount of deposit in the pipeline.

The tomography scanning technique, Tracerco Diagnostics™ ThruVision, is an advanced method of gamma-ray scanning. Multiple scans around a pipeline’s circumference using a single point source and a phased array of sensitive radiation detectors enables a very detailed map of pipeline contents to be produced. By doing numerous scans, many intersection points are created like a child’s spirograph, each of these intersections has a relative density which is equated by a highly sophisticated computer programme and are used to create a density map. These on-line, non-intrusive testing methods and use of small levels of material, mean that there is no disruption to normal operations.

The tracer injection technique requires the injection of a small sharp pulse of low activity radioactive tracer into the line, enabling the fluid velocity between externally mounted radiation detector loggers to be measured to better than 0.1% accuracy. Comparing this to the pipeline flow rate for the duration of the test enable the amounts of deposit between successive detectors to be calculated. Unsealed radioisotope tracing techniques are regularly used for checking or calibration of installed flowmeters, measurement of flow in systems where no flowmeters are installed and flow distribution studies in multiflow systems. The tracer used is designed to follow a particular material through a system. Sensitive radiation detectors are placed on the outside surface of a pipe and detect the unsealed tracer presence upon its flow past specific positions. These measurements can be used to directly measure fluid velocity, flow rate, phase distribution and deposit inventory.

Last year, Tracerco was requested to help a Floating Production Storage and Offloading (FPSO) vessel in the North Sea to locate a restriction in a subsea production system that was affecting production. The deposit location was known to be somewhere between the subsea manifold and the FPSO but how much of a restriction was unknown. The platform had tried pouring glycol into the system to remedy the situation with no effect. As a result, further information was required so that correct intervention action could be chosen.

Basic gamma scanning was first used on the flexible riser system at half meter intervals with scans being taken in the 3, 6, 9 and 12 o’clock positions.

Within six hours of being onboard the platform, the location of the deposit had been found and the extent of the problem quantified. A large amount of deposit was found at the bottom of the loop with some moderate build-up of deposit in the vertical section. From the information supplied, the platform was able to determine that retro jetting was applicable to the scenario and subsequently a retro jetting skid was mobilised and the deposit cleared.

Radioisotope technology offers powerful and well-proven inspection techniques for accurately measuring the amount and location of pipeline contents such as waxes or hydrate deposits, in instances where pipeline conditions are uncertain. Until recently these direct, non-intrusive measurements of pipeline contents were thought impossible, but the technology is now being used by many operators and its use is rapidly growing around the world.

Tracerco has developed an in-depth experience of flow assurance techniques during the last 40 years. It remains at the cutting edge of this specialist area, providing technical assistance to a broad spectrum of oil and gas related clients.