September 2019

30 OFFSHORE ENGINEERING EQUIPMENT offshoreequipment.co.uk August 2019 Offshore  Improved interface measurement accuracy Emerson’s Denny Nelson explains how the latest guided wave radar level transmitters can reduce product losses and costs in separation processes by reducing the minimum detectable thickness of the top liquid layer. Optimising the efficiency of separation processes on offshore platforms and floating production storage and offloading vessels (FPSOs), by ensuring that oil and water flow into independent channels with minimum cross- contamination, relies on technology that can accurately and reliably perform interface level measurements. However, these measurements can be challenging, and inaccuracies can lead to undesirable and costly outcomes, such as oil reaching the water tank, or water being sent further along the process. At a multiple well pad facility, unaccounted-for or unauthorised hauling of produced oil in the water tank could cost over $1 million a year in lost revenue. Should there be excess water in the oil tank, this could result in an unexpected loss of capacity, possibly leading to a spill or a well shut-in from a high-level alarm. Technology choices The most basic means of measuring an interface level is through sight glasses on the side of vessels. However, since sight glasses require regular maintenance and rely on operators conducting the measurements, this method is labour- intensive, time-consuming and prone to human error. Also, process conditions can have an impact, such as when condensation prevents operators from making accurate measurements. Alternative interface level measurement technologies include floats and displacers, capacitance transmitters, ultrasonic transmitters, differential pressure meters, and magnetostrictive sensors. However, these technologies also have limitations in terms of their accuracy and reliability in certain process conditions, and require regular maintenance. Guided wave radar The measurement accuracy, repeatability and minimal maintenance requirements of guided wave radar (GWR) level transmitters make this technology a widely-applied solution for interface level measurement applications such as oil above water. GWR devices guide a low- energy pulse of microwaves down a probe, and the energy is reflected back to the transmitter, first at the point of the initial liquid level, and again at the interface level. By measuring the time delay between the transmitted and received echo signals, the transmitter can calculate the distance to the liquid surface and the interface. One restriction when using GWR devices in interface applications is that the top product layer – in this example, the oil – must be of a certain minimum thickness to enable the transmitter to distinguish the echo signals from the two liquids. This minimum detectable thickness has typically been between 50 and 200 millimetres, depending on the transmitter model and probe type. However, the latest GWR transmitters enable this minimum detectable thickness to be reduced to 25 mm. This improvement further prevents unwanted product ingress, helping users to increase the efficiency of their separation processes, and reduce their product losses and costs. This enhancement has been enabled by a software algorithm that allows the transmitter to detect signal peaks that are closer together without having to decrease its signal bandwidth and therefore reduce its high sensitivity and its ability to handle disturbances. Measurement of a thinner top layer can be performed right to the top of the tank when these transmitters are used with a large diameter coaxial probe. These probes provide the strongest return signal, have no upper dead band, and their accuracy is not affected by obstacles on the tank wall. The ability to measure to the top of the vessel enables tank capacity to be optimised and therefore increases throughput and profitability. Emerson will be exhibiting at SPE Offshore Europe, stand 3B80. To learn more about how GWR technology can improve the efficiency of your separation processes, visit Emerson.com/Rosemount5300. Figure 1 Figure 2 Figure 3 Figure 1 - A typical GWR level and interface measurement application. Figure 2 - Emerson’s Rosemount™ 5300 Guided Wave Radar Level Transmitter with large coaxial probe provides functionality that enables the minimum detectable thickness of the upper liquid layer to be halved, to just 25 mm, with measurement right to the top of the tank. Figure 3 - Reducing the minimum detectable thickness of the upper liquid layer improves insight into the separation process.