In recent years development in the offshore industry has moved key processes from surface facilities to subsea, with the major focus being on subsea boosting and gas compression. This has led equipment manufacturers to develop high performance subsea pumps and compressors, driven by increasingly powerful, efficient and compact electric motors. These motors, together with the bearings and seals in the pumps and compressors, need to be lubricated and cooled efficiently by a lubricant with unique properties suited for operation in the subsea environment. This paper will focus on the challenges that were overcome in the development of a lubricant to meet both the performance and environmental requirements.
The lubricant is normally supplied to the subsea unit via an umbilical from the host surface facility, and is one of the vital keys to the reliability of this (rotating) equipment. The performance characteristics of the lubricant must cope with industry trends such as increasing process stream temperatures, higher pressures and higher shaft rotational speeds. In combination with these demands tightening environmental legislation governing the use and discharge of chemicals into the oceans requires that the industry adopts more environmentally responsible lubricant technologies.
The development process began by understanding in detail the conditions inside the various rotating elements within the pump/compressor. Critical parameters which will be reviewed include long term thermal stability, lubricant viscosity, lubricity, load carrying properties and water contamination management. With shaft speeds up to 6000rpm, and high simulated process temperatures, the product would be thermally stressed beyond limits typically seen in these applications. The screening tests on bespoke test rigs, selection of environmentally responsible lubricant components will be discussed, together with an overview of the global offshore environmental trends.
This approach demonstrated the superior performance of the lubricant by testing it beyond the expected conditions to provide enhanced reliability assurance and reduce operational risk. The final lubricant solution will increase the operating envelope of subsea pumps and compressors, offer improved reliability, and being fully compliant with the tightest offshore environmental legislation.
