Abstract
Oil and gas production in the Norwegian Sea has been ongoing for decades. The Åsgard B platform produces gas and condensate from the Mikkel and Midgard reservoirs from subsea wellheads with 50 to 80 km long pipeline tie-backs. Because the production started to decline, several innovative technologies were investigated to extend the production and increase the total recovery from the field. Artificial lift, by applying compressors installed on the seabed close to the wellheads, was the most favorable solution. An all-electric hermetically sealed integrated motor-compressor was selected to fulfil this duty and an extensive technology qualification program over 10 years was initiated.
Machine qualification activities included both component and full scale testing levels with the aim to arrive at a low risk subsea compressor solution with utmost robustness and reliability. The component testing level placed major focus on screening available materials to withstand exposure to the raw wellstream fluid from the Åsgard field. Full scale motor compressor testing was performed in a dedicated test facility with hydrocarbon fluids to investigate overall machine performance and enhancement via the introduction of new machine components from the material screening testing.
Valuable operational experience from the qualification efforts were collected and used to establish the design and material selection boundary conditions for the final dedicated subsea compression solution. A particular negative operational experience was fouling induced machine thermodynamic and rotordynamic performance degradation and the need to introduce integrated online washing and machine wet gas compression compatibility to reduce operational risks. To preserve machine robustness and mitigate potential corrosion and erosion problems, a machine concept with unconventional low operating speed and enhanced materials was selected. The world’s first subsea compressor with the above concept attributes was successfully manufactured, tested, installed, and operated subsea at a depth of 260 meters.
The qualified machine has been designed and tested for presence of free liquids such as water, hydrocarbon condensate, and glycol in the gas stream and demonstrated full capability to operate within the complete compressor performance map with liquid content exceeding levels found in normal gas-condensate fields. An available high performance wellstream compressor can reduce the complexity and cost of future subsea compression systems.
