Erosion-Based Sand Sensor: Subsea System Developed For The Tordis Field
Authors: Skavang Kristian , Braaten N.A. , Sirnes Geirfinn , Johnsen Roy 
Published: October 2008

A new type of subsea sand monitoring system is presented. The sand probe is based on measuring the sand induced wear on elements placed in the oil flow. The probe requires no on site calibration and can measure in all flow regimes. This is the first erosion based subsea sand detection system that can give continuous on line, quantitative information on sand production.

INTRODUCTION

The CorrOcean Sand Monitoring System was originally designed for topside use, in a joint development project with Norsk Hydro. This paper describes the development that were done in order to meet the requirements for a subsea system as specified by Saga Petroleum, the operator of the Tordis field in the North Sea.

This sand monitoring system utilizes the well known ER (Electrical Resistance) technique for measuring erosion on thin corrosion resistant sensing elements. The CorrOcean system was developed as a result of the need for sensors that can give real quantification of the sand content in two or three phase flow without any on site calibration. The sand probe has gone through an extensive test program in a dedicated test rig at the Norsk Hydro Research Center at Herøya, Porsgrunn - Norway. Several hundred tests were conducted over a two year period, and these tests have successfully demonstrated that the erosion based sand monitoring system can offer quantification of the sand content in any type of multiphase flow (l). The tests should that the probe can be used equally well in all flow regimes, including the intermittent regime (slug flow), and that it is not affected by varying turbulence levels. Since the accumulated metal loss on a sensing element is measured, the system has really no lower limit on the smallest detectable sand concentration given a long enough measurement interval. The sand probe has been through a complete calibration in the test program, ...and hence no further on site calibration (injection of known quantities sand) is needed. The system has a very rugged design, with high reliability, which is particularly beneficial for subsea applications. This simple measurement technique requires only a minimum of electronics subsea, which reduces the chance of equipment failure.

THE TORDIS FIELD DEVELOPMENT

The Tordis Field Development is a subsea production system located in the southern part of Block 34/7 on the Norwegian continental shelf. Operator of the Tordis field is Saga Petroleum as.

The installation will be operated from the Gullfaks C platform 12 kilometers away. The water depth at the subsea installation is approximately 200 meters, and the reservoir is located at approximately 2100 meters below the seafloor, and is mainly an oil producing reservoir. The Tordis development consists of a central manifold with production and water injections trees installed in a circle around the manifold, as shown in Fig. 1. The installation is connected to the Gullfaks C platform through two 10 inch production flowlines, one 10 inch water injection flowline and two control umbilicals, one of which is laid with a strapped-on 2 inch methanol line.