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Better level measurement

Published:  27 August, 2015

In today’s economic climate of low oil prices and reduced demand, producers are under pressure to maximise the efficiency of their existing offshore assets by minimising maintenance, reducing costs, and maximising process availability. ODEE reports.

Unplanned shutdowns caused by the failure of equipment or underperforming processes are a significant cause of lost production and inefficiency. Modern industrial automation plays a critical role in understanding the health of equipment and increasing visibility into production process. This helps to identify potential problems at an early stage so that timely action can be taken before production is interrupted.

To support efficiency improvement programmes a greater amount of reliable and accurate information is needed by operators to better control and optimise offshore processes. The latest measurement devices can support this requirement, especially those incorporating diagnostics for monitoring device health, and wireless capability that enables quick and easy installation and removes the need for costly wiring that adds weight and causes disruption during installation.

Many critical processes on offshore platforms and FPSOs rely on accurate level measurement to optimise processes and maintain plant availability. These include separators, scrubbers and storage tanks, where the traditional approach is to use mechanical float switches or displacers to monitor liquid levels. But these mechanical devices are affected by changes in media density and have moving parts that wear or get stuck and require scheduled maintenance. Regular recalibration is required to maintain accuracy. However, the need for more reliable continuous measurements is migrating users to modern, low maintenance technologies such as guided wave radar (GWR), non-contacting radar and vibrating forks.

For many offshore applications, GWR is the technology of choice for level measurement. GWR provides an accurate, reliable and maintenance free way to measure level in a wide range of applications. GWR level transmitters are not density dependent and measurement accuracy is unaffected by high turbulence or vibrations. Since there are no moving parts to stick or wear, maintenance costs are reduced and the problems of false readings, which can result in hazardous situations, are avoided.

In a GWR installation, the device is mounted on top of the tank or chamber with a probe extending to the full depth of the vessel. A low energy pulse of microwaves is sent down the probe and when the pulse reaches the media surface, a reflection is sent back to the transmitter. The transmitter measures the time taken for the pulse to reach the media surface and be reflected back and an on-board microprocessor accurately calculates the distance to the media surface using ‘time-of-flight’ principles.

Diagnostic values such as Signal Quality can be remotely monitored while the device remains in service. By tracking Signal Quality Metrics, plant personnel can automatically monitor for process build up on the probe and for unwanted surface conditions. This awareness lowers maintenance costs by enabling a preventative maintenance programme that reduces or eliminates manual checks.

For installations where there are obstructions in the tank, non- contacting radar is often considered as an alternative. Non-contacting radar doesn’t have a probe and works by sending a signal through the vapour space that bounces off the surface and returns to the gauge. Like GWR, changes in pressure, temperature, and vapour space conditions have no impact on the accuracy. In addition, radar devices have no moving parts, therefore maintenance is minimal.

Vibrating fork level switches are ideal for low level and high level alarm applications supporting applications requiring overfill prevention. The switches operate on the principle of a tuning fork, whose frequency varies depending on whether the fork is immersed or dry. The latest generation vibrating fork level switches feature built in diagnostics and fast update rates that make them suitable for a wide range of critical applications. In addition, the design of the fork is such that they require very minimal maintenance, ideally suited to offshore installations.

The continuous data provided by these intelligent measurement sensors can be integrated into a platform’s existing automation system to increase visibility into the process. Status information and device diagnostics are easily accessible from the control room, reducing maintenance requirements.. With the growing number of unmanned platforms, a fully automated system also provides the opportunity to manage and operate offshore assets remotely from an onshore control room using satellite communications. Onshore operations are able to monitor critical levels and predict, detect and correct conditions that could potentially lead to unplanned downtime.

Wireless makes it easy to

install level instrumentation

In many offshore applications, the lack of an existing cabling infrastructure has held back the installation of additional level instrumentation. The cost of installing new wiring, which can include ducting and cable trays, can be significant and involve third party contractors. This all adds up to increased risk, and weight on the platform. There is also the inconvenience and issues of performing the work whilst the site is operating.

One way of overcoming these constraints is to use wireless enabled level measurement devices. Wireless vibrating fork and radar transmitters have all the functionality and features of their wired equivalents but with the additional benefits and convenience provided by wireless technology. On existing installations, replacing older technology with the latest level measurement devices reduces maintenance and enables accurate measurement. Where no cabling exists, wireless presents an unparalleled way to add visibility in a wide range of offshore applications. Wireless level transmitters are quick and easy to install. They can be accessed from the gateway – configured and reliably transmitting data to a control system or data historian in just a few hours.

Separator applications

In separator applications, real time performance data enables process control to optimise product quality. For condensate oil and water production tank applications, reliable real time data enables monitoring between times when the tank requires emptying, minimising the risk of spills.

For example, Rosemount 5300 GWR from Emerson provided a solution for level measurements on a Floating, Production, Storage and Off-loading (FPSO) vessel, where there were a number of challenging applications. These included medium pressure, low pressure and high pressure gas compression scrubbers, glycol contactor suction scrubber and glycol contactor. The changing process conditions and the presence of foam and vapour as well as dirty sticky fluids made this a very difficult level application.

The sensitivity of the Rosemount 5300 allowed it to distinguish between a hydrocarbon layer and water. As the hydrocarbon layer builds up, the GWR can track it and report its presence as well as the water level. This increases the accuracy of the level reading and allows the scrubber to operate more efficiently. The ease of obtaining echo curves from the Rosemount unit was a key to understanding the extent of the hydrocarbon layer build-up.

The GWR was proven to be a highly reliable device well suited for the widely varying process conditions of the FPSO. The more accurate and reliable level readings increased the safety of the platform operations. Shutdowns due to level measurements were eliminated, which in turn led to increased production. Reduced shutdowns and increased production have resulted in significant cost savings.

Flare knock-out tanks

Flare knock-out tanks serve as recovery units and allow the offshore platform to flare (burn) vapours. The tanks usually contain water with oily contaminants and there may be rapid changes to the contents. They are usually part of emergency management system and operators face environmental fines for overspill so accurate level measurement is critical.

The traditional mechanical systems used to measure level in flare knock-out tanks can be inaccurate since they are sensitive to changes in density and are costly to maintain. Capacitive systems, which can also be used for these applications, are expensive and time consuming to install as they require calibrating. Their need for frequent maintenance and inherent potential for inaccuracy makes capacitive systems a less attractive option for these applications.

Both non-contacting and guided wave radar measurement devices can be used for continuous level measurement in flare knock-out tanks. They can be installed using either a stilling well or direct connection and commissioning is usually completed with just a few clicks. Radar based level measurement devices are immune to vapour space changes, independent of density changes and require minimal maintenance. and are therefore ideally suited to this application The effective and accurate measurement they provide enhances safety and reduces the risk of spillage and environmental damage.

For example, an offshore oil platform operator in the South China Sea was struggling to meet pollution requirements and had to re-evaluate their control scheme. Natural gas and water was separated from the crude oil and the gas was vented and combusted in a flare stack. However, not all the gas was combusted and its condensate accumulated on top of the seawater in a vessel below the stack.

The operator was using a mechanical float-based system to measure the total overall level in the vessel, with the aim of removing the condensate for treatment rather than dumping it in the sea. Measurement was hampered by the surrounding wave action, readings were unreliable and frequent maintenance was needed.

Installation of a Rosemount 3302 guided wave radar allowed the operator to accurately measure both the overall level and condensate interface irrespective of local conditions or sea state. The operator was able to identify the quantity of condensate, and pump it out and process as fuel. The seawater could be discharged to the ocean without contaminants. These types of measurements could easily be done now with an integral wireless guided wave radar.

Summary

Improved level measurement can support the need to maximise the efficiency of existing offshore assets, whilst reducing operation costs. Replacing traditional level measurement with modern guided wave or non-contacting radar devices improves accuracy and reliability. Being able to continuously monitor a process helps to optimise processes and the increased visibility enables operators to identify problems much earlier that could interrupt production. With the availability of wireless enabled level devices achieving these improvements has become much easier, with many of the cost and installation obstacles removed.

For further information please visit: www2.emersonprocess.com

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