The oil and gas industry faces ever-increasing demand for improvements in the efficiency of production to increase the amount of oil recovered from subsea fields. Along with those improvements comes an increased need to enhance the reliability and integrity monitoring of subsea installations.

One of the technological innovations that will allow the industry to successfully address these challenges is the intelligent energy system or digital oil field. Though there is a broad variety of such systems, their common features include the reliance on the flow of data that follows that of the hydrocarbons coupled with advanced data management and processing.

Optoelectronic sensors

Optoelectronic technologies are playing an increasing role in the sensing and transmission of the data needed by intelligent systems. The initial oil and gas industry interest in optoelectronic sensing technology centered largely on the use of optical fiber sensors for in-well pressure and temperature monitoring and, later, on distributed temperature sensing (DTS) and seismic monitoring. However, wider opportunities for the application of optoelectronic technologies exist. The development of advanced monitoring solutions for subsea infrastructure and production systems is a crucial area in which the technology already has made inroads and is set to be a major component of future subsea intelligent systems. It is important to adopt a holistic approach to get the right sensor producing the right data.

Tech transfer

There is a wide range of optical fiber sensor technologies that have been proven in other industries and that are now being adapted for subsea applications. These sensors monitor things such as strain, pressure, flexing, and vibration. One example of this is the use of optical fiber Bragg grating temperature sensors for monitoring under the insulation of subsea trees. This provides accurate data during cool-down tests to minimize the likelihood of hydrate formation. The increasing use of electrical systems and rotating machinery subsea makes it important to have reliable data on electrical and mechanical parameters. Because they are immune to electromagnetic interference, optical fiber sensors are well suited to electrical system monitoring using, for example, DTS to detect faults in electrical power lines and all-optical electric current sensors based on the Faraday effect.

As illustrated by the use of a spectroscopic remote sensing system as a permanently installed subsea leak detector, optoelectronics encompasses more than just optical fiber sensors. The leak detector works by sweeping a beam of light across subsea equipment and detecting the resultant fluorescence, offering ultra-high sensitivity with a real-time part-per-million detection capability. Spectroscopic sensing techniques also are capable of characterizing production fluids and therefore offer the possibility of developing in-line analyzers that will facilitate the optimization of subsea processing systems. Optical powering is another optoelectronic technology area that offers advantages for subsea applications, particularly for remote locations where powering and data transmission via electrical cables can be problematic. In a recent example, optical power was transmitted through an optical fiber and converted into electrical power that was used to operate a video camera and optical communications module. The video images were transmitted to the operator along an optical fiber using the module.

Subsea optical communications

In addition to developing new optoelectronic sensors, it is essential to implement an appropriate and cost-effective subsea communication infrastructure. The principal communications technology for subsea field developments today is optical fiber, which has good noise immunity and is capable of long-distance transmission at high data rates. The cost of a 24-fiber bundle inside a subsea control umbilical is one-fourth that of an electrical cable quad. Implementing a fiber bundle in an umbilical from the outset is a low-cost pre-investment in future infrastructure – even if high data transmission requirements are not there initially. It also is worth mentioning the emerging area of subsea optical wireless technology, which offers some of the advantages of surface wireless. There is much interest in the possibilities of this technology for subsea sensor networks, but current systems have restricted ranges and powering challenges that limit their usefulness.

For networks with many sensors the cost of connectors, penetrators, and cabling is a significant challenge. To address this, alternative connection and cabling options that adapt proven optical networking techniques and components for use in subsea sensor networks are being pursued. The new devices are designed to be used in conjunction with existing subsea optical connectors and pressure-barrier penetration connectors. These include passive and active optical components housed in purpose-built subsea cabling and encapsulation systems. The passive multiplexing devices allow ready connection to multiple locations via low-cost breakout units, while the active multiplexing devices provide access to multiple optical fibers through a single fiber pressure-barrier penetration. Critically, each of the optical fibers can incorporate numerous optical sensors, thus dramatically reducing the connection cost per sensor.

Condition performance monitoring

The greatest opportunities and challenges for optoelectronic sensing lie in providing the data for processing systems that will enable them to generate more accurate and useful information for users to optimize the field production. For this it is essential to synchronize the nature and amount of the sensor outputs with the requirements of data processing such as condition performance monitoring (CPM) systems. FMC Technologies established an optoelectronics group specializing in sensors that works with the in-house CPM team to develop sensor solutions. Data from the optoelectronic sensors then can be processed by the CPM systems to provide information relating to the status and predicted future states of the equipment. In producing this information, the CPM system uses a range of process and device models, databases, and technical condition indices (TCIs). The TCIs are mathematical models describing the behavior of a particular component and are useful tools to assess the status and remaining life of the component. In its simplest form the TCI for a component is structured to allow the ready identification of when the component is operating optimally, when it may soon require attention, or when it requires a shutdown. In the CPM system the TCI for an individual component is linked into a hierarchy that includes information on backup and redundancy within the overall system. The system is able to understand that even if the data from sensors on a component merit a shutdown, the TCI may indicate only a warning-level alarm because of the built-in backup or redundancy. The CPM is designed to be a proactive subsea equipment surveillance system linked with a 24/7 collaborative expert environment for diagnosis, advice, and problem-solving.

The use of optoelectronics now spans the entire subsea hydrocarbon production process. With an optical communications backbone in place and optoelectronic sensing devices generating the right data, it is possible to optimize production. With these data CPM systems will enable a move from reactive to proactive service by generating and aggregating information from individual components to ensure an optimized maintenance strategy.