Leak detection and monitoring of subsea structures are becoming increasingly important because of the need to manage production and control fluids. One of the recently introduced approaches to improve monitoring is a new class of optoelectronic subsea sensors based on fluorescence spectroscopy that provides permanent, real-time monitoring for hydrocarbon production systems.

The system, developed by FMC Technologies, is capable of detecting small production fluid leaks over distances of 10 to 12 ft (3 to 4 m) in a subsea environment and can detect many hydraulic fluids at greater distances. The system uses advanced single-emitter LED technology to meet the challenges of lifetime powerconsumption, spatial coverage, and delivery of a cost-effective solution. It is designed for permanent deployment on subsea trees, subsea processing systems, and related equipment to provide enhanced leak detection capability, and it can work at water depths to 9,842 ft (3,000 m).

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The instrument is housed in a pressure enclosure rated for a maximum working depth of 9,842 ft and can be fitted with a camera that provides a visual record of the leak location and volume. (Images courtesy of FMC Technologies)

Today, most leaks are discovered either in the course of routine ROV inspection or when personnel observe surface oil slicks. Relying on visual surveillance is inadequate, especially in light of the fact that exploration and development are being carried out in environmentally sensitive areas.

The focus of leak detection should be to identify the small and chronic leaks that are not detected by conventional leak detection systems based on pressure or mass balance and leaks that are difficult to detect using acoustic or capacitive based systems.

Fluorescence-based leak detection
The new permanent subsea leak detector has been designed employing fluorescence as the basis of the measurement. Fluorescence measurements offer a number of advantages over existing technologies for subsea leak detection. Because hydrocarbons and some hydraulic fluids have specific fluorescence signatures, they can be targeted by the detector. This makes fluorescence systems more selective than conventional systems and less prone to false alarms. Additionally, fluorescence measurements are very sensitive, allowing small leaks to be detected.

Detecting and monitoring chronic leaks can allow operators to schedule planned cost-effective intervention and maintenance, which reduces the chance that a small leak will become catastrophic. The intensity of fluorescence detected correlates with the volume of leaking material in the water column. A more intense reading indicates a larger leak. Not only does this approach provide an immediate indication of the magnitude of the leak, it allows a marginal leak to be monitored over time. This information can be vital in planning effective maintenance or repair programs.

This artist’s impression shows the leak detection and monitoring system in operation.

The basic layout and concept for the new instrument developed for effective permanent subsea leak detection has been adopted for pipeline and riser inspections carried out using ROVs. The leak detection process begins with abeam of light projected from the leak detector unit. The light is at a wavelength suitable to excite the fluorescence of the target material (oil, hydraulic fluid, etc.). A portion of the fluorescence is collected by the leak detector system’s photo-detection unit. The detected fluorescence signal is fed to a controller and processing system that converts the signal into a numerical representation that shows the concentration of oil or hydraulic fluids present in the water. Threshold levels for oil and hydraulic fluids can be set so that an automatic signal sounds when leak levels exceed those identified.

The leak detector can be deployed to monitor a fixed location on a subsea structure or can be fitted to a pan-and-tilt system that allows the light beam to sweep across the structure. When combined with the pan-and-tilt system, the sensor is effective over ranges to approximately 10 ft and provides operators the ability to monitor key areas of subsea equipment such as subsea trees and manifolds.

Developing the system
A detection instrument that is going to be used in a permanent subsea deployment must have low power requirements, but it also must be highly sensitive. Ideally, a system should require a maximum power of less than 10 watts and run over a low-bandwidth communications channel. Typically, the communications are MODBUS or preferably CAN Open for compatibility with the Subsea Instrumentation Interface Standard.

Hydrocarbons and some hydraulic fluids have specific fluorescence signatures that can be targeted by the detector. This sample fluorescence spectrum shows a North Sea crude oil sample with 365 and 405 nm LED excitation. The visible spectrum is shown for reference.

To achieve these requirements, the new FMC system uses recent developments in high-power LED technology that provide high output power and efficiency at wavelengths that are suitable for effective excitation of hydrocarbon fluorescence at 365 and 405 nm. These new LED devices can emit more than 200 milliwatts with electrical to optical efficiencies approaching 15%. When combined with efficient electronics for signal processing systems, a practical leak detection instrument can be realized with a total power consumption of less than 10 watts. The instrumentis housed in a pressure enclosure rated for a maximum working depth of 9,842 ft. The system also can be fitted with a camera that can be turned on following an alarm signal from the leak detector. The camera provides a visual record of the leak location and can eliminate the need to mobilize an ROV to confirm the presence of a leak.

The system has been designed for compatibility with existing subsea control modules and is suitable for retrofit applications. Because it enables early detection of small leaks and the ability to monitor the evolution of leak rate with time, this leak detection system is an important tool that allows operators to comply with environmental requirements, enabling early intervention when necessary and in the end, optimizing subsea production.

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