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Design of a 3 kV Subsea Electrical Junction Box

A client’s project called for a subsea, high voltage electrical junction box that was submersible to 1000 ft, lightweight, and compact. Given the operating conditions, tight space constraints, and multiple connector requirements, a custom design was required. We managed all aspects of the design from component sizing and housing design to fabrication oversight and field testing support.

Innovative Solutions

The electrical components in the system consisted of an array of six custom, high-power 3 kV inductors. Our first step was to establish the ideal shape and size of the inductor array to fit best within the tow body’s geometry, which we were concurrently developing. Through close collaboration with the inductor manufacturer, we were able to tailor the final design of the inductor so that the form factor and wiring access were optimized for our housing design.

Given the large rectangular size of the enclosure and the requirement that the housing wall thicknesses and weight be minimized, it was clear that a pressure-balanced, oil filled junction box was required. A pressure vessel or dry-type subsea enclosure must withstand the full pressure for the rated depth. This leads to a heavy walled enclosure usually of cylindrical or spherical form factor. An enclosure of this form was not an option for our system. Choosing the pressure-balanced enclosure allowed us to meet all the project requirements.

Pressure-Balanced System

Unlike a pressure vessel type subsea housing, a pressure-balanced system consists of an oil-filled water-tight housing in conjunction with a flexible membrane or bladder often called a pressure compensator. The electrical components inside are submerged in an insulating oil. A fluid connection is made between the housing interior and the pressure compensator. Ambient seawater pressure acts on the outer surface of the bladder which then compresses the oil in the bladder and the connected housing. This maintains the internal enclosure pressure at the same level as the outside seawater pressure. As the submerged depth of the housing increases, the oil pressure inside the housing increases to match the seawater pressure. With no net force on the housing, we were able to use lightweight and low-profile materials in the design. Hence, the pressure-balanced system was the ideal subsea housing solution for the application.

We continued to the next step of designing the housing itself. To minimize conductive surfaces and to give the housing body the necessary toughness we selected a specialized plastic as the construction material of the main part of the housing. An acrylic cover was used because its transparency made it easy to verify that all air was purged from the housing and that electrical terminal connections stayed tight and connected.

Housing Mock-up

As the subsea enclosure not only housed the inductor components but also served as a junction box, there was a need to conduct a lot of electrical wiring and splicing inside the housing. Before our client made any investments into the housing fabrication, we created a wooden mock-up to quickly and effectively validate the design. We were concerned about whether the allocated space in the housing would be adequate for wire routing. The mock-up revealed that the space was sufficient enough for wiring and splicing. Mock-ups are an affordable vehicle to gain useful insights and detect costly oversights before fabrication.

After the design was completed and validated, we worked with our partner fabricators to build our design. Working closely with the fabricators, we ensured the proper mechanical fit-up and performed electrical hi-pot testing once all electrical components were installed. Following final checkout we delivered the housing fully assembled to the test site, where we provided field support for testing the system. Testing was conducted at the U.S. Navy’s System Measurement Platform. Over multiple deployments, the system worked as designed.

Subsea Housing Pressure Compensator
Subsea Housing Pressure Compensator
Subsea housing mock up
Subsea housing mock up.
Electrical system field testing
Electrical system field testing