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(Mil Underwater) Watertight Prototype Feature

Advances in electronics are enabling subsea equipment to deliver enhanced capabilities, but the increased system complexity imposes tough integration challenges

Advances in electronics are enabling subsea equipment to deliver enhanced capabilities, but the increased system complexity imposes tough integration challenges. Specialist skills such as harness design, cabling layout and installation, custom interconnects and multi-layer sealing are needed to ensure reliability in deep offshore waters.

Ocean technology – a market whose total global value is estimated in terms of trillions of dollars - is taking advantage of improvements in electronics and computing to increase performance and take on new roles in offshore activities. These include maintaining equipment such as drilling stations, pipelines and communication cables, data gathering for various monitoring and research activities, security surveillance, defence and homeland security.

In proven applications such as data loggers and Remotely Operated Vehicles (ROVs), innovations such as signal processing and high-speed communications allow enhancements such as increasing the numbers of data channels supported, raising digital video quality or improving remote control allowing more complex operations to be performed.

In addition, access to high-performance real-time electronics provides an important enabler for subsea Autonomous Unmanned Vehicles (AUVs). These types of vehicles represent a rapidly growing market sector, and could replace expensive naval equipment and crews in activities such as cable laying, harbour patrols, offshore inspection and maintenance, subsea surveying and exploration.

However, designing with the latest components to process ever-larger numbers of data-collection channels, or to execute complex robotic movements under water, is only a part of the challenge. Building the complete system as an undersea-worthy unit capable of operating reliably at depth and when subjected to hazards such as impacts, shocks or vibration calls for specialist skills in building systems for extreme environments.

Prototyping Extreme Solutions

The deployable end product is typically not achieved directly “by design” but is usually completed more quickly and cost effectively by developing a working prototype to meet performance and packaging requirements. This was the approach taken by a recent project, in which Tekdata Interconnections technology provided specialist post-design services to help Systems Engineering & Assessment Ltd (SEA) complete delivery of an undersea data-gathering pod for use at depths up to 100 metres.

When Tekdata joined the project, a working prototype had been through basic tests, including immersion in water at depths up to two metres. Tekdata was asked to assess and improve the performance of the interconnections and harnessing throughout the entire pod, helping the project to progress from the prototype stage to full production readiness.

The system comprises a large number of data gathering channels, with extensive on-board signal processing and data storage plus supporting subsystems such as power supplies. The wiring and functional modules are closely spaced within the enclosure. When deployed operationally, a hinged external array of transducers is connected to the electronics within the pod. Only basic waterproofing had been provided for the sensor interconnecting cables, using a 3-metre length of hose potted into the array.

Mark Howitt of Tekdata takes up the story. “We first analysed the transducer assembly using X-ray inspection, which revealed the need for design improvements to provide waterproofing capable of performing at the specified operating depth. We redesigned the assembly to incorporate five distinct layers of waterproofing to ensure that no single failure mode would break the seal.”

At the points where the cable entered the pod, Tekdata also designed water-tight feed-throughs – again, protected by five layers of waterproofing - thereby ensuring consistent performance throughout the entire cabling assembly. This multi-level waterproofing strategy proved its strength when a 250kg metal assembly was accidentally placed on the backshell of the outer seal at one of the corners; the worst possible place. Only two of the five waterproofing layers were damaged, and the integrity of the seal was not compromised. The damaged layers were quite easily repaired.

Innovative Performance Interconnects

Tekdata has specialist expertise in the design of feed-through wiring for harsh environments. The feed-through avoids the need for a set of connectors to pass an electrical cable through a panel or bulkhead, thereby saving cost, bulk and weight, avoids a number of assembly challenges, and also eliminates sources of significant electrical insertion losses. Without connectors, several potential points of failure are also eliminated. The pod placed a high demand on the number and performance of the feed-throughs; many individual points of entry were required, each one protected by multiple levels of sealing capable of withstanding high external pressures at the specified operating depth of 100 metres. Figure 2 shows the feed-through developed for the SEA data-pod project.

The company’s Cryoconnect division has demonstrated the performance potential of feed-through interconnects in another of the most extreme environments known: the technology is employed successfully in space observation projects including the Herschel Space Telescope and the Planck Satellite now orbiting some 1.5 million kilometres above the earth. The feed-throughs connect enclosed sensors operating at temperatures as low as 0.1K to non-cooled electronics via ultra low-mass cables, allowing instruments to observe radiation remaining from the “big bang”, free of thermal noise, using a compact cooling system (cryostat).

As a part of the undersea pod project, Tekdata also re-engineered the transducer-to-pod cable to accommodate movement of the hinged transducer array without imposing excessive stress on the interconnections. This successfully eliminated a significant point of failure, producing a more robust, long-lasting design.

“We were also tasked with integrating an additional external transducer array, which showed signs of corrosion damage after initial immersion testing,” says Tekdata’s Mark Howitt. “To remedy this we replaced the damaged flying connectors and converted the transducer connectors into hermetically sealed feed-throughs.” As a result, all opportunities for water ingress during deployment resulting in corrosion were effectively eliminated. “In addition, Tekdata delivered an extremely low-cost solution,” says Seamus Bergan, SEA Marine Business Director. “This was important since none of the costs for this transducer array had been budgeted as part of the project.”

Bringing it All Together

Development of the prototype then progressed to assembly of the internal equipment and cabling. Since the pod had been designed to exceedingly tight space and weight limitations, the internal units had to be fitted close together, and also close to the structural strengthening pillars placed strategically within the chassis. This left exceedingly small spaces for fitting the harnesses and fibre optics. At this stage, the internal equipment and cabling had never been through so much as a trial fitting; the units had been designed independently using CAD. To further complicate the challenge, all transducer wires from the potted external array had to be of identical length in order to prevent phase shift between transducers.

Working with limited information including schematics, connector/pin-out definitions and mechanical CAD drawings, Tekdata generated suitable wiring schedules and specified components such as backshells and connectors capable of meeting the diverse constraints on dimensions, shapes and sizes. A number of custom backshells were also designed, to minimise the headroom needed for connectors, and other units were modified to incorporate low-profile right angles. Taking advantage of specialist in-house wiring skills, combined with workshop facilities to cut chamfers, add and tap mounting holes, make and modify mounting brackets and disassemble / reassemble arrays of PCBs, Tekdata was able to position the closely spaced sub-units and lay out the cabling neatly with clearly identifiable and traceable connections. Figure 3 shows an internal view of the cabling developed to interconnect the pod’s electronic boards and sub-units.

At this stage of the project the integration team also remedied some defects found in COTS sub-assemblies provided by external suppliers. These were found to have a number of incorrect connections between earth and zero-volt rails. Having documented and verified these defects, Tekdata made the necessary modifications to separate and isolate the circuits.

“To complete this part of the project, we were able to draw on wiring skills such as lacing, which is almost never needed when building consumer or industrial equipment. However, our focus on technically challenging products means we have these skills available,” continues Howitt. “This enables us to produce neat cabling that can be installed quickly and accurately in a production situation, and easily inspected and maintained in the field.”

Following the refinements and modifications to the pod and its cabling, the integration team was able to assemble the pod and fit the water-tight lid at the first attempt. Final updates were applied to the wiring schedule, reflecting the adaptations made during assembly. The result of all this work was that the lid fitted at the first attempt onto the base of the pod. The potted transducer array wires were all of equal length, having passed through the highly waterproofed feed-through. SEA’s test engineers were able to follow the Harnessing scheme easily, and subsequent testing showed the pod to be fully functional and ready for deployment at the design depth of 100 metres.

Conclusion

Ocean technology including monitoring equipment, ROVs and AUVs, has tremendous potential to help nations and corporations improve security, reduce operating costs, manage territory and harness valuable natural resources.

To take full advantage of advanced electronics and computing power to realise future innovations offering ever-more advanced capabilities, equipment vendors need to partner with specialists offering proven prototyping skills and cabling & interconnection expertise. These competencies are needed to ensure that ambitious feature-rich designs are able to stand up to robust usage in the harshest of environments. Few companies, however, can include successful projects for activities in outer space - as well as inner space - among their credentials.

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