Hjelmeland_subbed.qxp 26/3/09 05:01 Page 52
Successfully Boosting the Mutineer and Exeter Fields
Offshore Western Australia
a report by
Mads Hjelmeland
1
and Andrew Eaglesham
2
1. Framo Engineering AS; 2. Santos Ltd
The Mutineer and Exeter fields are located in the Santos-operated common pressure-containing housing with design pressure of
Engineering & Construction
permit WA-191-P, approximately 150km north of Dampier on the 3,000psig. The pump and motor cartridge contains all parts
north-west coast of Australia. Sanction was given in the second susceptible to wear and tear, including all pump instrumentation. It
quarter of 2003, with first oil expected in mid-2005. The project is retrievable by a light intervention vessel supported by a remotely
development plan was based on subsea production from the two operated vehicle (ROV).
fields, tied back to a floating production, storage and off-loading
vessel (FPSO), as illustrated in Figure 1, with water depths in the area The pump is installed in a free-standing subsea module, which also
in the range of 140–160m. contains the manifolding, pump bypass line, isolation valves and
individual multiphase flow meters for the well connections. Each
Total recoverables at Mutineer and Exeter were estimated at some well is tied directly into this multiphase pump module without an
100 million barrels (bbl) of oil, and the production capacity of the additional separate manifold structure being required (see Figure 2).
facility is 100,000 barrels of oil per day (bopd). The crude oil density
was 43ºAPI, with a gas–oil ratio (GOR) as low as 10 standard cubic The PhaseWatcher Vx multiphase flow meters provide continuous
feet (scf)/bbl, with a corresponding bubble point pressure of some measurement of the production from each well. Of particular
80psia. Based on the characteristics of the fluids, it was therefore interest is oil production and WC development and monitoring of
expected to produce the fields at fairly low pressures, with moderate the individual flow rates for ESP optimisation and control purposes
to low gas volume fractions (GVF). The reservoir is pressure- (see Figure 3).
supported by an aquifer, and water production is expected to
increase with time, ultimately beyond 90% water cut (WC). Although the well production had been predicted to contain low
amounts of free gas even at wellhead pressures below 100psia, a
The Solution multiphase pump based on the helicoaxial pumping principle was
The artificial lift solution selected for the Mutineer and Exeter fields selected for the two seabed pumping stations. The helicoaxial
was a combined subsea boost system. Each well was designed to technology offers flexibility to boost production even with
produce by a moderately rated dual redundant electric submersible significantly increased gas content. In comparison, a conventional
pump (ESP), operating in series with a single multiphase pump on single-phase pump will gas-lock with 5–10% of free gas at inlet. The
the seabed at each of the two fields. During the initial production seabed pumps installed at the Mutineer and Exeter fields in 2005
phase, the wells were expected to flow naturally, with the seabed have a motor rating of 1,100kW and a rated maximum speed of
pumps started to maintain target production with reducing reservoir some 3,800rpm. The maximum total equivalent flow rate that can be
pressure. At some later stage, the reservoir pressure would no longer handled per pump is 100,000bopd. Similarly, the multiphase flow
enable flow to the seabed, and the ESPs would be used to feed the meters and associated control pod can be individually retrieved by
seabed pump. diver assistance, while the control pods for hydraulic valve operation
can be replaced by ROV support.
While seabed pumps are installed and retrieved by light intervention
vessels, ESPs normally require rigs for pump replacement. Initially, this A self-contained power and control module on the FPSO contains
was a major concern for the operator due to the high leasing cost and variable speed drives for each of the two seabed pumps, in addition to
low availability of such rigs in the area. These two issues were lube oil supply and control systems. Power is supplied to the pumps
identified as likely to cause significant economic impact through through a power and control umbilical, allowing the operators on the
deferred production. The worst-case low-reservoir-pressure scenarios FPSO to remotely control the seabed pumps. The control system
indicated that the wells could not flow naturally to the seabed and automatically detects operation outside the operating envelope and
that downhole pressure support was therefore required to cover for performs corrective actions to maintain target conditions.
this case. Unfortunately, at the time the associated power rating for
the ESP system to maintain the target production was high and The Benefits
beyond the current references for installed ESPs, if used without Several advantages are associated with the combined subsea boost
another boosting system. Also, based on the fact that the reliability system for Mutineer and Exeter fields. Primarily, the system has the
of ESPs normally decreases with higher power ratings, a more robust capability to provide the required lift, even in the worst-case low-
solution with increased redundancy and less risk was sought. reservoir-pressure scenario. This is achieved with moderately rated
ESPs, which are only required to boost the production to the seabed
The multiphase pump is the conventional electric drive helicoaxial, when combined with a subsea multiphase pump. This strategy could
and the electric motor is close-coupled and encapsulated in a allow for the downhole pump to be designed and operated within the
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© TOUCH BRIEFINGS 2009
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