Subsea-Fiber Wet-Mate Connectors Require Careful Design To Balance Cost, Performance
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As the hunger for data grows, long stepouts become more common, and fiber communication becomes standard, the use of fiber in subsea oil and gas fields is set to increase. While optical loss along fiber itself is low and data-collection possibilities are high, these applications will only be fully realized if wet-mate fiber connectors can achieve high optical performance consistently. Excellent results are achieved with compact design, a modular approach, and an emphasis on cleanliness.
Wet-mate fiber connectors have been on the market for years and are used increasingly in the subsea oil and gas industry. While they have been moderately successful, the technical challenges should not be underestimated. Industry specifications demand high levels of optical performance and extensive qualification programs. The balance between performance and cost of the finished product has always been difficult to achieve.
Subsea fields are seeing longer stepouts, which are suited to using fiber as the key communication network. Furthermore, the subsea industry is seeing an ongoing drive to use fiber sensors in downhole systems (e.g., distributed temperature sensing), allowing greater amounts of data to be transmitted topside. Other applications such as direct-current and fiber-optic distribution and pipe-in-pipe heating are also emerging. For reasons of flexibility and practicality, the wet-mate fiber connector plays an important role in all of these.
The key technical challenges are a mixture of mechanical design, operation and maintenance, and operational environment.
Mechanical Design. The core of a standard single-mode fiber is 9 µm, and it must be completely aligned in every dimension for optical performance to be achieved.
Manufacturing, Operation, and Maintenance. Cleanliness of the optical ferrule faces on every single mate is crucial. Without it, performance is degraded or lost. In the worst case, permanent damage is transferred by a dirty ferrule face mating with a clean one.
Operational Environment. A wet-mate connector is a sealed, oil-filled, pressure-balanced mechanical device that will be handled in harsh topside conditions in extreme temperatures before being deployed to sea depths of up to 4,000 m.
Operation and Maintenance. High optical performance is required on all lines for up to 1,000 mates. Subsea connectors typically are mated only a few times in deployment and lifetime operation, but the same connectors are used for testing topside where they see many more mates.
Compact Size. Space on subsea equipment structures is always at a premium, and designs always need to be as efficient with space as possible.
A new wet-mate fiber-connector development was considered with a strategy combining performance requirements, product robustness, competitive product cost, manufacturability, and usability. Two key decisions determined the initial design approach.
- The resulting connector should complement existing products, including familiarity of design for end users and use of the same materials.
- An off-the-shelf optical ferrule should be chosen that has been used in many other industries, is already manufactured in high volumes, and is relatively inexpensive. From the beginning, the decision was made to use an angled-physical-contact version, a type of optical connection that provides better return-loss performance than that of a straight physical contact.
The high-level connector design—overall size, shape, and exterior features—was based on a well-established electrical-controls connector range. The optical ferrule then was chosen through a concept-design phase. Comprehensive component qualification during this phase ensured that the chosen ferrule was fit for subsea operation.
With the optical components and the general approach to the connector design established, the next criteria to be considered were the connector size and cost.
Initially, the objective was to use exactly the same dimensions as the well-established electrical-controls connector, which would have reduced component costs through economies of scale, but the mating mechanism required for precise optical-ferrule alignment would not fit into the equivalent connector bodies. The resulting design, however, did use some equivalent dimensions that provided good commonality.
Designing to Cost
Some cost-saving decisions can be made at the beginning of a development on the basis of previous experience. Examples here included ensuring that no machined components required welding, thereby eliminating expensive control of suppliers and procedures, and building in an anticross-mating feature to ensure that the existing controls connector could never be mated with the fiber connector regardless of how similar they might be. Designing to cost is an ongoing process that starts with product strategy. By keeping a few key aspects as active parts of the development process, however, the groundwork is done and reaching a target cost is more likely.
For a subsea connector system, the connector pair is only the beginning. In addition to the cost, complexity, and size tradeoff, the design approach also must consider a modular system. Oil and gas subsea applications are never exactly the same, so a connector system needs to provide key elements in such a way that clients can use them as effectively as possible.
The challenge, then, for the connector supplier is how to find the balance between standard configurations, which can bring economies of scale, and custom configurations so that clients can have flexibility with system configuration.
A 12-way system—connectors handling 12 optical lines simultaneously—lends itself to good modularity and system flexibility.
The qualification program encompassed several key specifications. The program took the most onerous criteria from each specification and created a matrix of testing that satisfied all specifications simultaneously.
The program was planned in detail simultaneously with the development schedule. The forward planning enabled flexibility during the qualification testing phase.
For subsea connectors, the tests themselves are familiar and include pressure tests, temperature cycling, and a wide range of mechanical tests. For a fiber connector, however, the method of checking performance nearly always is through optical performance—insertion loss and return loss at 1310-, 1550-, and 162-nm wavelengths on all 12 lines.
The connector build program before the qualification was treated as a preproduction run in every respect by building enough connectors to refine assembly processes, tooling, and documentation.
Part of the preproduction run proved the final crucial aspect for subsea fiber connectors: cleanliness. A particle larger than 4 µm—invisible to the naked eye—has the capacity to stop optical transmission completely if it is anywhere near the surface of the optical ferrule face. Particles also have the ability to cause permanent damage if they have been ground into the ferrule face. At that point, even thorough cleaning of the ferrule face will not bring back its optical performance.
Because cleanliness at this level cannot be controlled through visible means, necessary precautions must be taken to eliminate as much contamination as possible. A high-level clean room (Fig. 1 above) was planned early in the development and was production-ready for the preproduction run before qualification. The main connector mechanism is built and filled with oil in the clean room.
In an oil-filled and pressure-balanced subsea connector, the optical ferrule faces are sealed inside and, once built, cannot be accessed. The assembly starts by twice cleaning every individual component with ultrasonic cleaning baths. The entire connector assembly then is conducted in a Class 5 clean room.
The faces of the optical ferrules are inspected to the latest possible point in the build process. The connector is filled with oil after the oil has been vacuumed, cleaned, and monitored for contamination levels using a particle monitor. Optical performance is checked again before the connector can leave the clean room.
If the connector is built in this way, data show that the optical performance remains excellent even when it is exposed to long-term and onerous qualification tests that emulate subsea conditions.
Optical performance was the usual measure of test success. Mechanical tests, such as maximum misalignment of connectors, were some exceptions.
Throughout all tests, optical performance was maintained within specification on all 12 lines and across the number of connectors and jumpers built.
The durability test is one of the harsher tests of the qualification program. It requires 750 mates at ambient pressure and temperature, followed by 250 mates at 450 bar in a pressure vessel filled with sand and silt. The intention is to simulate repeated connector use topside before it might be deployed to a maximum of 4000 m subsea and expected to continue performing for its design life of 30 years.
The off-the-shelf multifiber ferrule chosen was small, high-performing, and relatively inexpensive. It provided the basis for a fiber connector that meets oil and gas industry specifications and requirements. Further, its standard nature and high degree of usage in other industries provides the advantage of commonality.
A thorough and well-managed design process ensured the development of a connector that performs well and passed a thorough qualification program. The design strategy and modular product range ensured that the fiber connector is compact and that the system is adaptable for many different applications. By tracking component costs, using the right materials, and considering production processes, the connector system is cost-effective. A good-quality build combined with thoroughly clean practices will result in a robust product that has excellent performance and can withstand the rigors of a harsh environment.
Subsea-Fiber Wet-Mate Connectors Require Careful Design To Balance Cost, Performance
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