Fibre Optic Based Sensor Network for Condition Monitoring of Subsea Production Machinery
Authors: High Geoff B. 
Published: October 2008

This paper describes a unique and comprehensive fibre optic sensor technology to provide accurate control and diagnostics for subsea and down-hole pump and motor combinations. Existing comparable technology is restricted to monitoring the variable frequency power supply at the platform, or very low bandwidth applications for ESP's down-hole with limited analysis capabilities. In this system, the performance and condition (particularly if it is approaching failure mode) of an electric motor is monitored by the temperature of bearings and windings, and by spectral analysis of the mechanical vibration signature and the motor driving current. There are major cost advantages in control for optimising performance, increasing life expectancy, and diagnostics for scheduled maintenance in place of catastrophic failure and replacement philosophy. The sensor system is likely to be a relatively low cost item which could also be embedded into motor units during manufacture, but able to provide comprehensive real-time data from remote installations. A technique for metering grid currents using in-fibre Bragg gratings piezoelectrically-driven by a conventional current transformer is demonstrated. We find a resolution of 0. 7 A/ Hz frequencies up to 10kHz.

Introduction

The sub-sea oil industry is on the threshold of a revolution in production technology. The move into deeper waters and the drive for cost savings through lower capital and operating expenditure is forcing the industry to invest in novel production techniques, which make better use of existing technology and infrastructure. This can be seen in the growth of the use of Electric Submersible Pumps (ESP's) and the ongoing interest within the industry in the development of sub-sea and down hole separation, and also the long term investment which has taken place in the development of handling techniques for multiphase fluids, including multiphase pumps and flow meter technology. Much of this technology is based around the installation of large pump-motor combinations both sub-sea and down hole. Currently ESPs placed in the well bore are in the 200–500 kW range, whilst the motors being developed for both water reinjection and multiphase pumping are in the 1–5 MW range. The power supply to these motors is usually a variable frequency drive. The cost effectiveness of these systems is driven by their performance and reliability, both of which require the motors to operate within a narrow optimum performance band. Running the motors outside of this region significantly reduces performance and life expectancy, but as existing performance monitoring consists of monitoring the variable frequency power supply at the platform, and possibly the differential pressure in the produced fluid across the motor/pump, maintaining optimum performance is next to impossible These systems, along with other proposed advances in down hole and sub-sea technology require comprehensive sensor systems to provide accurate control and diagnostics. There are major cost advantages in control for optimising performance, and diagnostics for scheduled maintenance, in place of a catastrophic failure and replacement philosophy. Providing this support down hole with electrical instrumentation is not practicable, principally due to a combination of the hostile environment, and the difficulty in providing reliable communications pathways for a number of sensors.