|
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.
|
|
