If a project includes a gas compression, the concept design becomes far more complex. It becomes necessary to examine compressor designs and driver selection for a number of concept options in considerable detail. Compressors consume a vast amount of power and the choice of drivers has a big impact on overall project cost, technical complexity and schedule.

For gas compressors driven by electric motors, the choice of motor driver, whether a fixed speed motor or a variable speed motor, is a major consideration. The fixed speed motors are very low in cost but difficult to start online and require special consideration in the design of the electrical system. To achieve direct online start, it becomes necessary to install more turbine power and a robust electrical system.

Variable speed motors are significantly more expensive, are of a bespoke design and their speed control system is still undergoing technical development and requires detailed investigation into the availability of proven equipment.

Regardless of the driver choice, whether fixed speed or variable speed electric motor, the electric power supply must be adequate to drive the compressor motor. This would normally require a power station with a number of gas turbine generators including spare units to meet the compressor power demand and would add considerably to the project cost. For this reason gas compressors are very often driven by gas turbines especially in remote locations and offshore platforms.

Compressors used in the oil and gas fields are a bespoke design and are tailor made for each project. However, gas turbines are available in a limited number of models with a fixed power output. Selecting one specific gas turbine at the concept design stage is not a preferred option as it limits the number of manufacturers who would be willing to bid for the project; compressor selection must also take into account gas turbines from different vendors to provide competitive advantage. This requires detailed knowledge of gas turbine designs available from various manufacturers and their performance characteristics such as speed, power turbine ratings and effect of ambient temperature on available power.

When developing a Concept Study for an oil and gas production facility, MSE uses its knowledge of turbo-machinery and past experience to select and optimise the design. For gas fields, however, reservoir performance impacts on compressor selection and its operation over time. Experience has shown that to design a compressor, without taking into account the changing reservoir pressures and flows over time, results in poor performance and failure to meet the expected production forecast.

For this reason MSE always develops an integrated model of the gas field which combines reservoir, wells, gathering system and central processing facility using their proprietary model GASMAN with gas compression systems. This model is first validated against the reservoir performance obtained from the data supplied by the client. The GASMAN model is calibrated and used to perform History matching. Once validated, the model is then used to determine future performance of the field with and without gas compression.

MSE's compressor specification takes into account the effect of declining reservoir pressures and flows over time. The compressor specifications are then used to develop the design of gas compressors and their aerodynamic performance which is verified using the CENTRIF program. Alternatively if compressor performance data is available from the manufacturer or from field tests, the MSE GASMAN model can use this data directly.

The future performance of reservoir and wells carries the usual risks associated with forecasting sub-surface performance. The integration of compression system with the reservoir has been used to mitigate these risks by designing a flexible compression system that can compensate if the reservoir and well performance turned out to differ from the original predictions.

However this option is only available during the Concept Design stage where it is possible to build a certain amount of flexibility and redundancy in compressor selection. The extent to which this redundancy is built in depends upon the perceived risk that reservoir performance may deviate from the prediction.

The cost of providing the redundancy in the design at the concept stage is usually low. However, changing compressor design once it has been installed would be prohibitive in most cases.

Using the GASMAN model, MSE can optimise compressor selections and driver power which would compensate for reservoir uncertainties.
 

CENTRIF & GASMAN