Challenges for an Ultrasonic Interface Detection System for Offshore Separators

K-E. Frøysa1, E. Bjørndal2 and I. Klepsvik1

1CMR Instrumentation, Christian Michelsen Research AS, Bergen, Norway, Email: kjellef@cmr.no

2Current affiliation: 3-Phase Measurements AS, Bergen, Norway

ABSTRACT

In offshore separators, gravity is used for the separation of water, oil and gas. In practice, there may in addition be a sand layer in the bottom of the separator, an emulsion layer between the water and the oil layers, and a foam layer above the oil. This means that a separator is a complex dynamic system. Mapping of interfaces (and of thickness of layers) inside such a separator is therefore important in order to drift such a separator in an optimal manner.

Traditionally, interfaces are measured by differential pressure, by buoyancy principles, or by a nuclear method. Such methods are intrusive. This represents a maintenance problem, especially for sub-sea applications. They are also not necessarily able to detect an emulsion layer.

A way of addressing the above mentioned shortcomings of the traditional methods might be to consider ultrasonic interface monitoring methods, where an ultrasonic transducer is clamped to the lowest point of the outside separator wall. The ultrasonic signal generated from the transducer, propagates through the separator wall, and upwards through the interior of the separator. From the reflected signals interface levels are measured.

The present paper addresses the detection of the water-oil or water-emulsion interfaces, in addition to estimation of the emulsion thickness. The challenges faced in order to obtain a robust detection are discussed. These include the effects of a possible sand layer at the separator bottom, the dynamics of the emulsion layer, and non-homogeneities in the water phase. Applications within process monitoring and interface regulation are discussed. In addition, the possibilities of monitoring the sand layer build- up are briefly discussed.

Keywords Acoustic level meter, pulse-echo mode, clamp-on configuration