Pressure Tolerant Power Electronics for Deep and Ultra-Deep Water
Authors: Pittini Riccardo , Hernes Magnar , Petterteig Astrid 
Published: May 2011

This paper presents results from an on-going research project [1] on pressure tolerant power electronics. The main goal for the research is to provide and demonstrate solutions that enable power electronic converters to operate in pressurized environment. Oil companies have plans for subsea processing of oil and gas. Today concept considers power electronic converters in one bar vessels. As the depth and the converter power rating increase, these vessels become increasingly bulky and heavy. Pressure balanced converters would allow lower vessel wall thickness, thereby giving lower weight, and simpler cooling due to improved heat conduction through the vessel walls. The new concept considers the power electronic converters placed in vessels completely filled with appropriate dielectric liquid able to operate at pressures from 1 bar up to several hundred bars. Dielectric fluids are required to prove their properties especially related to insulation, incompressibility and heat transportation. The methodology presented here considers a mechanical adaptation of components followed by various laboratory experiments for verifying or correcting the proposed solutions [2-4].

Standard off-the-shelf components and special pressure adapted components have been subject to various provocative pressurisation tests up to 300 bar. Tests clarified the need for special adaptation of some components, while others could be used without any modification. Subsequently a full converter phase-leg has been built, submerged in dielectric liquid and tested in full operation up to 300 bar. This phase-leg is based on a press-pack IGBT modified for operation at high pressure. Measurements performed at different pressures demonstrate that there is no relevant difference in terms of electric parameters between this modified IGBT and a standard IGBT. Long-term tests proved the concepts goodness. The next step will test bonded IGBT technology in high-pressure environment, and finally will realise a test demo to be located at a suitable subsea site.

Introduction

Several new oil and gas wells worldwide are located offshore where the sea depth can reach 3000 m. Offshore platforms for oil and gas processing are expensive and often exposed to extreme conditions, for example in the North Sea outside the coast of Norway. For reducing the processing costs and for increasing the recovery factor, many companies have plans for subsea processing of oil and gas that require power electronic converters for various applications. The converter power range is from 0.1-100 kW (valve actuators, UPS etc.) up to tens of MW for variable speed drives for oil pumps and gas boosting. The today's installations are based on concepts where power electronic converters are assembled in one bar vessels. One example of such installation is the Ormen Lange Field in the North Sea which has a design depth of 1100 m and a converters rating of 12.5 MW [5]. As the converter power rating and the design depth increase the vessels become increasingly bulky and heavy due to the high wall thickness required for coping with the pressure difference. Pressure balanced vessels will give significantly reduction of weight. Moreover, the cooling system for removing converter heat-losses may become more reliable and less complex since the heat can be more easily conducted through the thinner vessel walls, instead of by separate heat exchangers Fig. 1. Therefore the oil and gas industry has shown an increasing interest for pressure balanced solutions.