By: John Flynn
Optical fiber has enabled greater bandwidth, more safety, and more configurability in subsea oil and gas systems. Wet-mate connector evolution launched widespread fiber use in the early 1990s, while the development of high-pressure fiber penetrations increased the reach of optics into subsea wells and pressure vessels in the early 2000s. Advancements in optical connector design, such as angled endfaces, have recently enabled a new era of optical sensing.
The future of subsea fiber optics lies in a systems-level design approach. Unique technologies, synergizing fiber optics with integrated electronics, are positioned to increase the efficiency of entire fields, reduce future project CAPEX, enhance reliability, and ensure the continued use of optical fiber, even in challenging market conditions.
In response to the need for higher bandwidth and E&M-resistant communication on oceanographic and defense products, optical wet-mate connectors were developed in the late 20th century. Designers were, at the time, unable to rely on the spring/piston that worked so well for electrical connector sealing as the piston obstructed the light path. Some early designs leveraged expanded beam lenses, but these designs exhibited very high optical losses.
The lowest loss optical connectors, such as Teledyne's Rolling Seal connector released in 1995, utilized ITU-T G. 652-compliant optical fiber and standard telecommunications- grade 2.5 mm optical ferrules, polished to a physical contact (PC) or ultra-physical contact (UPC) grade. Typical optical specifications were .0.5 dB insertion loss and .30 dB return loss. The innovative moving seals at the front of the connector trapped debris and rolled it out of the way before clean, oil-filled volumes were joined and the optical terminations made contact.
Low loss subsea optical connections greatly increased the usage of wet-mate optical connectors, particularly in the oil and gas industry. Now, virtually all subsea oil and gas umbilicals contain one or more gel-filled optical fiber tubes, typically containing 12 optical fibers for various uses subsea. Optical connectors are also present on the majority of subsea oil and gas structures, including tree-mounted subsea control modules (SCMs), standalone subsea distribution units (SDUs), and a variety of other equipment, including router modules or subsea manifolds. The wet-mate optical connector now acts as a critical component in linking the communication network of a subsea field. Most early projects such as ConocoPhillips’ “Little Dotty” in 1997 or Norsk Hydro’s (now Statoil) “Troll Pilot” in 1998 utilized simple point-to-point jumpers, consisting of a wet-mate optical connector on each end, a pressure-balanced, oil-filled hose between them, and the mating connector halves mounted to a subsea bulkhead.
The development of hybrid connectors containing optical and electrical circuits in the late 1990s represented a significant step forward in connector design, joining two enabling technologies into a convenient single package. Hybrid connectors enabled the delivery of electrical power along with high-bandwidth optical communication. Powered temperature, pressure, flow, or erosion sensors on pipelines and trees could now be read topside in virtual real time. Teledyne’s Nautilus Rolling Seal Hybrid (NRH) joined the Rolling Seal optical technology with the already widely used Nautilus electrical pins in order to provide optical connectivity and electrical up to 30A and 1kV. Though used more frequently for oceanographic/communication programs such as Japan’s Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) system or Ocean Networks Canada’s Neptune nodal science project located off the west coast of Vancouver Island, British Columbia, NRH connectors have also enabled cost savings on oil and gas projects, such as Total’s 2013 ultradeep flagship field Egina off the coast of Nigeria.
Cable Terminations & Penetrations
Fiber optic subsea connectors have always required ancillary products, such as the aforementioned pressure- balanced oil-filled hose that constitutes the subsea jumper. One such ancillary product is the cable termination. In order for the topside or shore-side team to communicate with the field, an umbilical cable, typically many kilometers long, must be terminated to the first in a series of wet-mate connectors; this occurs at cable termination assembly.
Early designs for cable terminations incorporated fully pressure-compensated housings. However, these designs have pressure, and therefore, depth limitations, due to the risk of pressure driving the compensation fluid up the length of the umbilical cable. Teledyne took a leap forward with the release the field assembled cable termination (FACT), which leverages a robust optical penetration to allow the separation of the lowrisk, factory-tested, pressure-balanced portion of the assembly from the field-assembled cable-breakout portion. The volumes exposed to the cable are kept at 1 ATM, and no fluid is driven into the cable. This volume isolation methodology is also viable for hybrid cable terminations, employing both optical and electrical breakout regions in order to transform the hybrid umbilical into a hybrid wet-mate connector.
Penetrations, like those used within the FACT design, have also been required more frequently as optical and electrical communication equipment is marinized. Many active and some passive optical components cannot tolerate extreme subsea pressures. Thick-walled pressure vessels are used to contain these components in a 1 ATM volume. However, optical and electrical signals must be able to traverse the pressure vessel wall. Electrical penetrations were easily developed in the mid-20th century by sealing a mechanically robust insulation material over a conductive element. Optical penetrations were a greater challenge due to the fragility of optical fiber in bending and the threat of bend losses during operation. Early optical penetrations, primarily developed before 2000, were often epoxy-based, providing pressure integrity only up to approximately 5,000 psi. Advancements in bonding and sealing to optical fiber have enabled truly hermetic, reliable optical penetrations up to 10,000 psi as a standard. As a result, not only can 1 ATM housings and cable terminations survive at deeper depths, the increased reliability of the sealing methods means less operational downtime and fewer overall system interventions.
High Temperature, High Pressure Applications
Fiber optic sensing has grown over the past 20 to 30 years to include a full suite of core technologies, Distributed Temperature Sensing (DTS), Distributed Acoustic Sensing (DAS), and discrete Fiber Bragg Grating (FBG) sensors to name a few. If used within a subsea well, not only must the optical fiber cable and/or sensor housing survive the pressure and temperatures, often greater than 15,000 psi and 120°C), but a reliable penetration and connection method is required at the X-tree/Tubing Hanger (XT/TH) interface.
The release of the 2007 Intelligent Well Interface Standardization (IWIS) joint industry project (JIP) marked a milestone in alignment between major oil and gas operators on a vision for subsea wells. Key market drivers at the time were an increase in recoverable reserves, reduced intervention, and improved viability for brown-field projects—many of which are still relevant. Chief among the technologies required to bring IWIS to life, was an Optical Feed-through System (OFS), combining an optical wet-mate connector with a penetrator and possibly dry-mate connector in the subsea well. Despite great leaps in technology, a successful OFS deployment has not yet occurred. Suitable 30-year HTHP material choices, non-epoxy fiber termination, and high temperature pressure-compensation are a few of the remaining technical hurdles to overcome before a truly viable OFS comes to market.
IWIS eventually made way for the Subsea Fiber Optic Monitoring (SEAFOM) group, which released the detailed OFS qualification and acceptance testing standard TQP-01 in 2011, providing a new level of standardization for potential OFS manufacturers. Ongoing JIPs continually provide a forum for operators and all levels of equipment manufacturers to collaborate, draft industry specifications, review recent deployments, share lessons learned, or advertise. Because of these efforts, work still actively continues on OFS technology, though the current cost environment has noticeably slowed the market growth.
In recent years, wet-mate optical connector improvements have continued to push optical technology boundaries, integrating some technology already common in the terrestrial telecom industry. Angled physical contacts (APC), a standard of terrestrial connectors (e.g., FC/APC or SC/APC), are a requirement for wider adoption of distributed sensing technologies and low-reflectivity FBG-based systems. The 8° angle on the endface directs reflected light out of the fiber waveguide, thereby improving the connector return loss (RL). For wet-mates, this improved the RL from ≥30 dB to ≥45 dB, a reflection noise floor suitable for reflective sensing as described above. Teledyne's APC Rolling Seal supported the all-optical 4D Life of Field Seismic monitoring in Petrobras' Jubarte field in 2010. The system consisted of more than 35 km of sensing cables arranged over an area of 9 sq. km. In total, 96 wet-mate optical contacts supported over 700 4D sensing stations.
Additionally, optical connector manufacturers are increasing channel counts to support more wells per single optical connector. In one scenario, a single 24-channel optical connector could be used to terminate all available optical fibers from an umbilical cable. That connector could then support six X-trees, each with subsea electronics modules (SEMs) requiring two fibers each. While such a solution seems to offer a substantial cost savings, it also presents a single point failure condition, a consideration that persists as a concern as channel count and contact density continue to increase.
When evolutionary changes, such as increasing channel count, are made to qualified product lines, overall reliability must remain at the forefront of the product development process. The epoxies, thermoplastics, and metals used in the system must be rated for at least 30 years in the host environments. Standard Arrhenius acceleration models offer an initial guide to material compatibility testing of new designs. However, building on API 17N the reliability and risk management volume of the API 17-series, Teledyne has begun to leverage more advanced analysis techniques, such as Weibull analysis, Chi-squared analysis, and stepstress Accelerated Life Testing models, in order to provide a more complete picture of reliability and give operators true confidence in material, product, and system lifetime.
Finally, as the oil and gas industry struggles to cope with depressed prices, progressive architecture changes offer a new, deeper level of cost savings. Teledyne has recently qualified the Electrical Optical Flying Lead (EOFL) to be initially deployed in 2018. In a traditional system, communication signal and power came into the Umbilical Termination Assembly (UTA) from the surface and it was then distributed to the router module for electrical to optical conversion; the system encompassing two redundant router modules for every six X-trees. The router modules were then connected to each X-tree. With the introduction of the EOFL, it is now possible to remove both subsea router modules from the field, replacing each with a line-retrievable flying lead assembly containing the media converter, saving close to an estimated $2,000,000 per six-well distribution system.
The unique EOFL product combines a Nautilus 12-pin electrical wet-mate connector on one end with a Nautilus Rolling Seal Hybrid on the opposite end and a 1-ATM canister containing ISO 13628-6-tested electrical- to-optical conversion boards. Fully qualified optical and electrical penetrations ensure 30-year integrity of the 1-ATM housing.
Though subsea connection technology continues to march forward, systems-level approaches and re-considerations of connection philosophy, along with more traditional product cost-cutting techniques, will ensure the continued use of fiber optics in the world of subsea oil and gas.