Fiber Optic Downhole Sensors for Subsea Wells: Remaining Problems
Authors: Eriksson Klas , Baggs Chris , Theobald Mike 
Published: October 2008

Fibreoptic sensors are being increasingly used in wells with dry wellheads. However, for subsea wells some challenges remain to be overcome. These are e.g.:Low allowed attenuationConnector designs (e.g. tubing hanger connector for horizontal trees does not exist)Interrogation electronics difficult to marinise, which in turn force dedicated fibres all the way from the downhole measurement point to topsideInstallation procedures / processes

This paper discusses potential fibre optic sensing system solutions for a typical subsea field in detail. Technology gaps are identified and the status of critical components presented. Current limitations for various sensor systems are discussed. A control buoy alternative is presented as one possible solution, which may enable the use of fibre optic sensors for subsea wells.

Introduction

Fiber-optic sensors have been deployed in many wells for measurement of pressure and temperature. Optical sensors for other parameters are also emerging, e.g. downhole multiphase flowmeters. In figure 1 we show an installation of the ABB DOGS (Downhole Optical Gauge System) in a test well in Brunei. The DOGS system consists of single or multi point pressure and temperature sensors. Such systems are available from other suppliers also.

Pressure sensors are essentially point sensors, although several can be located on the same fiber (quasi-distributed). Temperature sensors can either be point sensors (with several located on the same fiber) or distributed (measuring the temperature profile along the fiber).

As such sensors are being considered for deep subsea wells, there are some hurdles to overcome, e.g.:The optical fiber must be brought through the tubing hanger connector. Such connectors are uncommon for conventional subsea trees, and non existent for subsea horizontal trees (although some development work is ongoing)The allowable attenuation in the fiber is quite small. In a practical subsea system, multiple fiber optic connectors are necessary in the signal chain, and the attenuation in these connectors soon consumes the available signal budget.A large part of downhole sensing systems do not survive installation and initial operation.

Typical Downhole Fiber-optic Measurement Techniques

In figure 2, we show an example of a typical system for measurement of a temperature profile along a well, called a Raman Distributed Temperature Sensing system (DTS). A Raman scanner works in principle by sending a brief laser light pulse down an optical fiber.

At all points along the fiber, minute portions of this light are scattered and reflected due to various mechanisms, one of which is the Raman effect. The Raman effect will cause the reflected light to have a different wavelength than the incident light.

By measuring the wavelength of the reflected light, and the time at which it was reflected, the temperature at each location along the fiber can be measured with high precision. An alternative technique (although with more complex physics and equipment) is the Brillouin effect, which can also be used for Distributed Temperature and Pressure Sensing.