Optimising Compressor Re-wheel Design for Mature Assets

The main challenge of re-wheeling, however, is to optimise the new design and get it right first time. MSE has carried out many re-designs and re-wheels of gas compressors which are in successful operation. With this experience, MSE has developed a range of software systems, design tools and proven methodologies for developing a redesign which is rigorously tested before its implementation.

The redesign is performed with the CENTRIF program, which identifies the new re-wheel configuration for optimum performance. This new design is tested for off-design performance to ensure it has the correct performance envelope to accommodate the new reservoir conditions.

To ensure that the new re-design will operate in the production facility we test the new compressor using the GASMAN model. GASMAN is the integrated model of the production system which includes the gas compression system, the processing facility, the gas turbine drive, the recycle system and the process control set points.

To more accurately optimise the compressor design, the effect of changing reservoir and well performance must also be included in the GASMAN model. This is achieved by the integration of the subsurface system with the compression facilities; this provides a more robust solution and ensures that the reservoir and sub-surface performance risks are included in the new compressor design.

The GASMAN model includes a sophisticated material balance method for reservoir performance and for determining well hydraulics. The model includes the gas influx and water influx terms, which allows for water aquifer support. The model has been extensively tested and history matched with many large and small gas fields using the reliable data.
This is achieved by downloading historical data from the DCS systems of the existing facility.

For reservoir and well performance it is possible to perform the history matching for the past 5 - 10 years or longer if the data is available in a suitable format. Recently the GASMAN model was used to history match a gas field which has been in operation for the past 40 years. However, for compressor performance only recent data is required for validating the model.

After the GASMAN model has been verified against field data with the current compressor design, the alternative compressor designs are installed. The model is run in the dynamic mode for the next 5 - 10 years to verify the compressor performance. After a few iterations it is possible to identify the most appropriate compressor designs which would provide the best performance with the reservoir and well combination. It is possible to design compressors which would last for longer periods but MSE's experience has been that this compromises the compressor design and marginalizes its performance.

The revised compressor design after its test is then passed on to compressor vendor for final confirmation. The vendor selection is then re-run in GASMAN to verify the performance. It is normal practice to degrade the performance of compressor from new to allow for in service performance variations. MSE has developed and maintained a large database of machinery performance; the database provides a good indication of expected performance variations in the field.

Re-design through Re-wheel

The alternative to a re-design and replacement of equipment is "re-wheeling". A re-wheeling consists of changes to the compressor internals and can be implemented in a few days, without requiring extended shut down. Re-wheeling is a very expedient and cost effective option for production optimisation of mature assets. The re-wheeling does not require piping changes or removal of casings; it is limited to the removal of the compressor internals and their replacement with new optimised set of internals.

If the re-design of a production facility is like a major "surgical operation", then re-wheeling can be viewed as "Key-hole surgery" which can be performed quickly; the production facility regains the lost capacity and production gets restored.
Re-wheeling is a low cost, low risk and high reward operation. The original compressor internals are retained as a backup to further limit risks.