ABSTRACT

Leak detection and monitoring on subsea structures, e.g. subsea tree and subsea processing systems (SPS), is an area of increasing interest regarding production and control fluids in the oil and gas industries. Current techniques, such as capacitive (dielectric) based measurement or passive acoustic systems, have limitations. This paper reports an optoelectronic solution based upon fluorescence spectroscopy to provide a permanent monitoring solution.

This paper presents a new class of optoelectronic subsea sensor for permanent, real-time monitoring of hydrocarbon production systems. The system is capable of detecting small leaks of production or hydraulic fluid (ppm levels) over distances of 4–5m in a subsea environment. Ideally, systems designed for such applications should be capable of working at depths of up to 3000m unattended for periods of 20-plus years.

The system uses advanced single-emitter LED technology to meet the challenges of lifetime, power consumption, spatial coverage and delivery of a cost-effective solution. It is designed for permanent deployment on subsea tree, SPS and associated equipment to provide enhanced leak detection capability.

Introduction

There is an increased awareness from operators, legislators and authorities about effective leak detection of production fluids on subsea oil production equipment (Carlsen and Mjaaland, 2006; Det Norske Veritas, 2009). The need for subsea leak detection is a combination of safety, economic and environmental factors.

Although subsea leak detectors are routinely specified for subsea installations with a number in use, in reality the majority of leaks are still discovered either through routine remotely operated vehicle (ROV) inspection or observation of surface oil slicks. With the increase in subsea field development, there is a reduction in surface facilities and so less chance for visual observation of leaks. In any scenario the reliance on visual observation of a leak is unacceptable, and especially so when potential exploration and development in environmentally sensitive areas, such as the Arctic (Protection of the Arctic Marine Environment, 2002) and Barents Sea, are considered. This has put a focus on the detection of small and chronic leaks that are not detectable by conventional leak detection systems, which are based upon pressure or mass balance and are difficult to achieve using acoustic or capacitive based systems.

The detection of small and chronic leaks is desirable from a number of perspectives. In highly environmentally sensitive areas, even small leaks can be regarded as significant. Detection and monitoring of chronic leaks can allow intervention and maintenance to be planned in a cost-effective manner, as well as potentially reducing the chance of a small leak becoming catastrophic.

There is thus considerable interest with in the subsea oil and gas industry for new sensor technologies that provide more effective leak detection and are suitable for permanent deployment on subsea equipment. Attractive properties for a permanent subsea leak detection system include: spatial coverage; compatibility with the subsea control system (low power and communications); detection of all leaks; selectivity and reduced false alarms; rated for 3000m operation; and cost effective and durable.

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