Sensing change subsea

With future subsea applications expected to employ a wider range of optoelectronic technologies, FMC has established a dedicated R&D group that focuses on building and expanding a wide range of integrated sensing solutions. Daniel McStay explains why.

The move toward deepwater installations, long distance tiebacks and subsea processing, combined with the need for increased oil recovery and extended equipment life presents significant technical challenges for the offshore oil and gas industry. There is also a growing awareness within the subsea industry of the need for enhanced reliability and integrity monitoring of subsea installations as well as the environmental sensitivity associated with installations in regions such as the Arctic.

One of the emerging technological innovations that will allow the industry to successfully address these challenges is the development of intelligent energy systems for hydrocarbon production and integrity management. The goal of such systems is to achieve total asset awareness to maximize performance by allowing a company to operate its assets at the technical limit of efficiency, cost and safety with minimal environmental impact. Intelligent energy systems include a broad spectrum of technologies capable of collecting, transmitting and analyzing completion, production and reservoir data and enabling control that optimizes the production process.

Various monitoring applications are possible for platforms, FPSOs, moorings and risers.

Optical fiber systems

The last decade has witnessed a rapid expansion in the utilization of optoelectronic technologies to generate and transmit the data required for intelligent energy systems. Optical fibres have long formed the backbone of high data rate communications systems and are used to connect the various components of a subsea infrastructure to broadcast monitoring and control data. Optical fiber systems can provide the offshore oil industry with gigabit communication bandwidths and can support 200km unrepeated step out distances.

The industry has also witnessed a major growth in the use of and investment in optical fiber sensors, primarily for downhole monitoring. These sensors have a number of advantages over electronic systems that make them particularly suited for subsea oil and gas applications. They are made of electrically insulating materials that are chemical resistant and immune to electromagnetic interference. Optical fibers are also capable of recording multiple measurements on a single cable. The fully passive components and sensors have multiplexing capabilities with a wide operational temperature range. The technology is reliable, lightweight and robust.

There are various types of optical fiber sensors and sensing formats which are highly suited to subsea applications. In general, they can be divided into two basic types: intrinsic and extrinsic. Optical fiber sensors use two sensor formats: point and distributed sensing. Intrinsic sensors use properties of the optical fiber itself, such as light scattering, as the basis for the measurement. Extrinsic sensors use properties of a separate component, such as a fluorescent material or flexible membrane, which is coupled to the optical fiber as the basis for the measurement. It is possible to have multiple sensing components distributed along a single optical fibre, which may be individually addressed. Alternatively, distributed sensing, of temperature and strain for example, along the entire length of an optical fibre is also possible.

Commercial downhole optical fiber sensor systems, for example those used to measure the temperature distribution in a well, are now well established in surface fields. These systems are able to capture a far greater number of downhole measurements, resulting in more accurate analysis during either the well drilling or production phase. To realize the benefits of downhole optical sensing in subsea wells, however, it is essential to have an optical fiber feed-through on a subsea tree. It must not only act as an efficient optical connection at the high temperatures and pressures found on the downhole side of the interface, but must also provide the primary and secondary pressure barriers. The system must be capable of multiple-mate and de-mates during an operational lifetime of 25 years. FMC Technologies has collaborated in a joint industry project (JIP) that included an operator, connector manufacturer and downhole sensor provider to develop an optical feed-through for use on FMC’s subsea trees. The first optical fiber feed- through was successfully installed in a horizontal subsea tree and deployed in late 2009.

Seabed optoelectronics

When developing intelligent subsea systems, enhanced monitoring and control technologies for seabed installations, structures and production systems are essential. Despite the benefits of optical fiber sensors outlined above, very little use is currently made of optical fiber and other optoelectronic technologies for permanent monitoring of seabed production assets. The current and likely future usage of optoelectronic technologies to facilitate intelligent energy systems subsea include sensors deployed downhole, within subsea processing systems, and on subsea trees. Various monitoring applications are also possible for platforms, FPSOs, moorings and risers.

Initial seabed applications of optoelectronic sensor technologies include monitoring of subsea pipelines, tiebacks and flow lines. The use of optical fiber distributed temperature sensors can give an indication of leaks and flow disruption, such as hydrate and wax formation in pipelines via induced temperature changes. This sensing capability may be combined with distributed strain sensing, either in the same optical fiber or a dedicated strain sensing fiber.

One application under investigation is the use of optical fiber Bragg grating temperature sensors for monitoring of the insulation and thermal condition of subsea trees, manifolds and processing equipment. This provides a more accurate means of determining the allowable shut- down times of the equipment and also indicates areas where the insulation is not performing as desired.

Optoelectronic sensor technology offers the potential to radically change the amount, quality and nature of information generated from subsea equipment. Future subsea trees and processing systems will incorporate a multitude of optoelectronic sensors including sensors to measure the pressure, temperature and flow of the production fluid, as well as characterizing the composition of the fluid, erosion/ corrosion, electric current and voltage sensors. Displacement sensors may also be used to confirm the activation and position of valves and chokes. These sensors will be used to provide enhanced information both on the production fluid and the functioning of the tree or processing system. The ability to multiplex optical fiber sensors allows duplicate sensors to be incorporated into the tree or processing systems to provide back-up in the event of a sensor failure. This improves the reliability of the overall monitoring system. The ability to operate at elevated temperatures also suits the technology to addressing the challenges of high temperature high pressure applications.

Bragg sensors mounted on the internal wall of insulation placed around the choke module of a subsea tree.

Production, integrity monitoring

It is important that intelligent energy systems provide users with useful information and not merely sensor data. To optimize production, solutions for intelligent energy are based on a common vision for system enhancement. Rather than simply developing standalone sensors and equipment, a holistic approach to system enhancement is possible when using expertise from production equipment design to software solutions for analysis and modeling. In such an approach, the sensors are integrated with advanced communications, signal processing and modeling systems to generate the information required by users.

FlowManager is a model-based software system that calculates and predicts information relevant to flow assurance from real-time measured production data. It is used for production allocation, monitoring, optimization and reservoir management. The system optimizes the field architecture during the early phases of planning and design. It also increases reservoir recovery rates and improves production planning during the operational lifetime of the reservoir, thus maximizing the operator’s return on investment.

Environmental monitoring

There is an increased awareness and industry focus on effective subsea environmental monitoring. This is another area in which optoelectronic technologies are able to offer new monitoring solutions. Conventional permanently deployed leak detectors on subsea equipment are only capable of measuring at a small fixed location and are therefore reliant on the leak occurring in or close to this location by mechanically collecting the hydrocarbons. Alternatively, they may rely on secondary indicators of leaks such as noise, which are often difficult to detect due to background interference.,/p>

Daniel McStay has been working with FMC Technologies as an optoelectronics consultant since 2005. Having developed an appropriate business strategy, he was tasked with establishing a subsea optoelectronics research capability within the company.

FMC is developing a new optoelectronic permanent subsea leak detector which sweeps a beam of light over large areas of subsea equipment to monitor for leaks. The system is capable of performing spectroscopic analysis of the light and can detect and quantify the presence of oils and hydraulic fluids from leaks. The system is highly sensitive, capable of detecting concentrations of a few parts per thousand or better for oils and hydraulic fluids in the water column. It is suitable for monitoring on trees, templates, manifolds and subsea processing systems. The ability to continually monitor and assess leaks will allow operators to plan intervention and maintenance in a cost effective manner and potentially reduce the chance that a small leak becomes catastrophic.

Great potential

Optoelectronic technology offers the potential to greatly enhance the monitoring of subsea production. Although the last 10 years have seen the widespread adoption of optical fibre systems, it is important to remember that the spectrum of optoelectronics technologies covers far more than just optical fibers.

Fluorescence-based oil-in-water monitors, laser ranging systems and LIDAR (Light Detection and Ranging) are among the many examples of optoelectronic technologies that have been employed in surface oil and gas applications. Photocatalytic-based polishing of production fluids to remove hydrocarbons from discharge water has also been developed. The use of imaging systems and lasers for subsea inspection is well established. More recently the potential for high data rate subsea wireless communications has attracted interest.OE

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