Lubrication evolution

Castrol oil blend, 220C, at 4 weeks, with anti-corrosion additives. Its color clarity indicates that it is stable after exposure to high temperatures.

A new environmental lubricant supports keysubsea boosting and compression technology. Castrol Offshore’s Chris Morrissey reports.

In recent years, the offshore industry has seen key processes move from surface facilities to subsea, along an increased usage of increased oil recovery techniques. This has led equipment manufacturers to develop high performance subsea boosting pumps and gas compressors, driven by increasingly powerful, efficient and compact electric motors. These motors, together with the shaft bearings and seals, need to be lubricated and cooled efficiently by a lubricant with specialist properties suited for operation in the subsea environment.

The lubricant is normally supplied to the subsea unit via an umbilical from the host surface facility and is vital to the reliability of the 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. Lubricating fluids are important in helping to ensure reliability and reducing operational risk.

Functional requirements of the lubricant

Typical mineral oil, 150 C, at 4 weeks. The darkening indicates degrading after thermal aging.

The subsea equipment plays a vital role in extracting oil and gas reserves, and ultra-high reliability is a necessity. The key internal components typically comprise of radial bearings, and thrust bearings and mechanical seals. A fluid is needed to lubricate, cool and clean away wear particles. In addition, the lubricant is needed to remove heat from the powerful electric motor; a good electrical insulator is necessary, too. Advances in machine technology mean that lubricating oils are being thermally stressed beyond limits previously seen and, with shaft speeds up to 6000rpm and high process temperatures, there are a number of key performance challenges that must be overcome, together with meeting tight environmental legislation.

The process must start by gaining a detailed understanding of the conditions inside the various rotating elements within a specific pump and compressor. Critical parameters that should be reviewed include long-term thermal stability, lubricant viscosity, lubricity, load carrying properties and water contamination management.

The lubricant will interact with many different components in the system so material compatibility is vital. The mechanical seals act as the barrier element for the internal lubrication system and are designed to leak a small amount of barrier oil to the process stream. The frictional losses in mechanical seals are related to rotational speed and pressure and result in local temperature rises. As shaft sizes and speeds increase, the heat generation at the sealing interface is significant, and may lead to high peak temperatures. The fluid is required to reduce friction, remove heat and retain its integrity. Formation of deposits will upset the seal gap causing increased leakage and potential failure.

Together with ever-increasing performance demands, the environmental legislation relating to chemicals discharged by the offshore industry continues to become tighter and more widespread. The legislation was first developed for operations in the North Sea, but now extends to other regions including the Gulf of Mexico, Canada, West Africa and Australia. However, the Oslo and Paris (OSPAR) Convention requirements remain the most stringent. The OSPAR legislation covers the North East Atlantic and states that all fluids that have the potential to be discharged to sea by operational discharge or by leaks that cannot be prevented should be assessed and registered. OSPAR currently requires that every individual component in a product is tested for marine toxicity, biodegradability and bioaccumulation. To meet evolving environmental legislation, it is essential that environmentally-responsible components are used in the development of a lubricating fluid.

Lubricating fluid development process

Conventional, low viscosity lubricating oil formed the baseline for the product development program, with the first step being the selection of a suitable base fluid. The oil needs to be of a low viscosity to reduce shear in rotating elements, provide low levels of leakage across the mechanical seals, effectively remove heat from the electric motor and be environmentally compliant. Performance enhancing additive components are then chosen that will deliver the key functional requirements, and levels are optimized to deliver maximum performance. For example, if the additive level is too high, it might have a detrimental effect on the electrical insulation properties or form deposits on mechanical seal faces. By developing an oil with increased thermal stability, the fluid is more likely to meet the current and future needs of more powerful pumps and compressors. In addition, the lubricant may need to cope with upset conditions such as seawater contamination and high breakdown voltage requirements. The lubricant must also successfully pass all original equipment manufacturer (OEM) functional testing.

The importance of validation testing

Thermal stability is a key product attribute, and once the initial thermal stability tests are complete the formulation must then undergo physical and chemical testing at higher temperatures and for longer durations. This involves testing at up to 220°C, with samples monitored during the test period to ensure the fluids remained stable with no visible deposits. The aged samples are then subjected to more in-depth testing, including infrared (IR) analysis which uses a fingerprint technique to identify any changes in the chemistry compared to an unexposed sample. If the IR traces remain virtually identical, this indicates that the fluid has remained stable throughout the test.

It also is essential that the formulation performs well against testing for insulation properties (breakdown voltage). This is determined by running an IEC156 test, in which two electrodes are immersed in the oil, and the voltage is increase until an electrical short occurs. This value must be high enough to prevent shorting under high voltage operation within the motor.

The final lubricant will be exposed to internal pump and compressor parts such as elastomer and thermoplastic seals and motor internal parts and there are methods in place to analyze the compatibility of materials. The seals need to be tested for compatibility with the oil by immersion testing according to NORSOK M710 procedure. They are aged at higher temperatures to accelerate the testing. For the motor internals that contains epoxy resins and cable insulations, additional electrical measurements are carried out on sub-assemblies throughout the test period.

Conclusion

Functional testing to ensure performance assurance is increasingly critical, and Castrol Offshore has designed a method to demonstrate the superior performance of a lubricant. The approach involves testing the fluid beyond the expected conditions to provide enhanced reliability assurance. The screening tests that are carried out take place on bespoke test rigs to ensure that the performance of the lubricant is monitored in an environment that is as close to operational as possible. Due to the comprehensive testing program, Castrol Offshore has recently been involved in the development of a lubricating/ barrier oil that meets the machine performance and environmental needs of the current and the next generation of subsea boosting pumps and wet gas compressors. The lubricant is fully compliant with the tightest OSPAR environmental legislation and outperforms conventional mineral oil solutions on thermal stability, whiledelivering good electrical insulation and drying capability. The final lubricant solution will increase the operating envelope of subsea pumps and compressors.

Chris Morrissey joined Castrol Offshore in 2004 as product advisor for the subsea business. He moved on to manage the technical service team, and then to specialize in product qualification, where he worked closely with many subsea equipment vendors. He is currently team leader for offshore product development within Castrol’s central technology function.

Before joining Castrol, Chris has held several design and development positions within Parker Hannifin UK, and John Crane Ltd. These positions allowed him to gain a wide experience in the design and application of hydraulic equipment and high performance mechanical sealing. This work spanned many different business sectors such as chemical processing, paper making, automotive manufacture and oil & gas.

Chris has a B Eng. Degree in Mechanical Engineering from the University of Hertfordshire