Pressure Tolerant Power Electronics for Subsea Oil and Gas Exploitation

The principal objective for the present research project was to provide fundamental knowledge on material and component packaging for supporting the realization of reliable pressure tolerant power electronic components and circuits. Deep-water subsea oil production installations require complex electrical power systems including large local power converters located at various depths, possibly in excess of 5000 metres. Current designs offered by the industry for subsea converter operations are based on concepts in which the power circuits are assembled in their entirety in steel vessels at atmospheric pressure. As operational water depths and converter power rating increase, pressure vessels become increasingly heavy and unwieldy due to the need to increase wall thickness. Consequently, heat transfer from the power electronics components to the seawater becomes problematic. For this reason, the oil companies are looking for less cumbersome subsea power electronics systems. The development of pressure-tolerant power circuits is a significant step in the right direction, opening the door to reducing device weight and volume. Furthermore, systems such as this will provide opportunities for improved power component cooling systems that will increase reliability and reduce costs. The availability of reliable pressure tolerant converter systems will enable the introduction of new concepts for subsea transmission and electrical power distribution, including the use of DC and frequencies other than the 50/60Hz range. Interesting results have been achieved from the experimental works on materials that were assumed candidates for pressure tolerant packaging of components such as power semiconductors. Properties of insulating materials and chemical compatibility between different insulating and packaging materials have been investigated. It has been demonstrated that the termination area of power semiconductor chips is vulnerable for fibers and other particles when the insulation liquid is in direct contact with the chip surface. This implies that the chip and substrate interfaces need to be protected from direct contact with liquids by one or more materials with the sufficient mechanical, chemical and electrical properties for maintaining the long-term insulation quality. In the project, several candidates for such additional protection has been investigated by experimental research. Live endurance experiments with power components and complete high voltage, high power converter modules in liquid pressurized environments have concluded that all electrical characteristics for the most critical components such as the power semiconductors, IGBT driver electronics and power capacitors are very well maintained under pressure, at least up to 500 bar (5000m sea depth). Indeed, voltage and current waveforms are close to unaffected by pressure, indicating high stability for the electric functionality and operability. The high voltage, high power converter modules that have been demonstrated in the project represent building blocks for multi-megawatt medium and high voltage converters for applications such as motor drives and high voltage power conversions systems (HVDC) Guidelines have been proposed for test and qualification of materials, components and circuits aimed for reliable operation in liquid, pressurized environment. The basis for the proposed guidelines has been prevailing standards such as IEC, with special attention to the relevant matters for pressure tolerant power electronics, and with input from the gained experience and methodologies from the project. The focus has been on materials and components assumed to be essential for the converter power circuits, and that in the start-up phase of the project were assumed to be the most critical regarding liquid and pressurized environment such as the IGBT modules and DC-link capacitors. The detailed project results has been reported to the project partners as SINTEF technical reports and as workshop presentations. The most generic results are published as conference and journal papers. The dissemination includes about 50 SINTEF technical reports, 5 student reports, 6 scientific conference papers, 3 journal papers, 55 workshop presentations and 2 popular science disseminations. In addition valuable contact network has been established with possible component and material manufacturers of future custom pressure tolerant power electronics products. Significant new competence has been provided for the SINTEF staff and students involved in the project. SINTEF is well prepared to utilize the achieved competence and experience from the project by contributing to the realization of reliable components and power distribution systems for the future subsea oil and gas exploiting projects, as well as for other sectors that are demanding new technical solutions, like subsea power grids for the renewable energy sector.

Subsea oil exploitation requires complex electric power systems adapted to a high pressure environment. On a short term there is a need for local supply systems around wellheads with gas boosters, oil pumps, separators and other equipment. At a later stag e one expects to develop energy supply systems for long step-outs where high voltage DC is a viable solution. This all requires local large power converters at the seabed at various depths exceeding 3000 meters. Presently, designs offered by the industry aimed for subsea converter operation are based on concepts where the power circuits are completely assembled in steel vessels at atmospheric pressure. As the sea depth and the converter power rating increase, the pressure vessels gradually become heavy a nd unwieldy devices due to need for increasing the wall thickness. Consequently, the heat conduction from the power electronics components to seawater also becomes problematic. Therefore the oil companies are looking for more feasible solutions for subsea power electronics. Enabling pressure tolerant power circuits is a significant step in that direction, eliminating the above mentioned problems. In an accomplished research project at SINTEF on feasibility of Pressure Tolerant Power Electronics (PTPE) m ajor steps toward realization of PTPE has been taken, and reliable operation of pressurized (300 bar or 3000m sea depth) converter modules has been demonstrated. Encouraged by these results, the industry has now decided to take a further step towards real ization of PTPE for real applications. However, it is also recognized that to achieve PTPE products with sufficient long-term reliability, some remaining fundamental problems need to be solved. The problems and uncertainties are mainly related to material s for electric insulation, and how to apply the materials for appropriate packaging of the most vulnerable components. The applied project will address these very important remaining challenges.