Note: Descriptions are shown in the official language in which they were submitted.
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"All electric subsea boosting system"
The present invention relates to an all electric subsea boosting system for
well
fluid boosting by compressing hydrocarbon gases and/or pumping hydrocarbon
liquids comprising one or more subsea boosting stations and one or more long
step-out power supplies. A boosting station may consist of compressor(s)
and/or single or multiphase pump(s).
An offshore gas field may be developed with seabed installations which are
tied
back to a terminal onshore or an existing platform. The seabed installation
comprises of one or more production templates where each template produces
well fluid through manifold headers which are connected to one or more~
pipelines. Said pipelines transport well fluid to an onshore terminal or; ~anr
existing platform (receiving facility) for further processing. Processed gas
and~
condensate are exported to the market. One or more umbilicals for power,
control and utility supplies are installed from the receiving facility to said
subsea
installations.
For the initial production phase, well fluid may flow to the receiving
facility by
means of the reservoir pressure. Later in the production phase, or at start-up
of
the production, well fluid boosting is required in order to maintain the
production
level and to recover the anticipated gas and condensate volumes. The
conventional solution for such well fluid boosting facility is an offshore
platform.
However, a subsea boosting system may be an alternative to or in combination
to said platform solution.
The present invention seeks to provide an all electric subsea boosting system
to
replace or assist the use of an offshore platform.
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That the system is all electric means that it is controlled and operated with
electrical power, and does not have a hydraulic system for assisting opening
and closing of valves.
In accordance with the present invention, this object is accomplished in an
all
electric subsea boosting system where said system comprises one or more
subsea compression stations and one or more long step-out power supplies.
An all electric subsea boosting system in accordance with the present
invention
has a number of advantages compared to a booster platform solution.
Said system is safe to human injuries and fatalities due to remote operation,
reliable, cost effective, environmental friendly and comprises few parts which
make the system less complicated and easy to operate.
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The present invention will now be ' described and with reference to the
accompanying drawings in which:
Fig. 1 shows a schematic overview of the all electric subsea boosting
system in accordance with the present invention.
Figure 2 shows a subsea main power system single line diagram.
Figs. 3 and 4 show a typical all electric subsea boosting station layout in
accordance with the present invention.
Figure 5 shows a boosting station process flow diagram.
Figure 6 shows a schematic overview of main modules and parts in a
subsea boosting station according to the present invention.
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Figure 7 shows a typical power and control architecture for a subsea
boosting system.
Figure 1 illustrates the all electric subsea boosting system. Said system
comprises one or more subsea boosting stations and one or more long step-out
power supplies.
The long step-out power supply is defined from the connection point at the
receiving facility to and including the main subsea transformer.
Such long step-out power supply comprises the following subsea components:
- Subsea main transformer with pressure compensation system
- High voltage penetrator(s)
- Umbilical termination head
- Combined or separate power and coritrol umbilical, including:
- Main electrical supply
- Utility power (optional)
- Fibre optic lines for control signals
- Barrier lines (optional)
The boosting station is connected directly to at least one subsea production
template and is designed for boosting well fluid from said production
templates.
Well fluid from the production templates is routed via one of the template
manifold headers, via the infield flow lines and to connectors on the suction
side
of the boosting station.
The boosting station is connected to export pipelines with flow lines to each
pipeline. Compressed gas will be transported in said export pipelines to the
receiving facility.
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Figure 2 shows a main power system single line diagram for a subsea boosting
system.
High voltage power, control and utilities are supplied from receiving
facilities
with one or more power and control umbilicals.
The high voltage (HV) power cables will be connected to the subsea main step-
down transformer and the transformer will be installed on the subsea boosting
station with the umbilical attached.
The single line diagram shows the power distribution system for the main
subsea electrical consumers.
Figures 3 and 4 show a typical subsea compression station layout.
The subsea boosting station, comprises the following modules and parts:
- One or more compressor trains and/or single or multiphase pump(s)
- One or more circuit breaker modules
- Inlet and outlet manifolds
- Inlet coolers
- Inlet sand trap
- Parking location for main transformer and power umbilical termination
head
- Required installation tools
- High voltage electrical system
- Process system
- Utility power system
- Control system
- Barrier system
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The compressor train is the main equipment required for compressing the well
stream. The compressor train comprises the following modules and parts:
- Compressor module
5 - Compressor Variable Speed Drive (VSD)
- Anti-surge valve and actuator
- Anti-surge cooler
- Separator/scrubber module
- Pump module
- Pump VSD
- Remote and manually operated valves
- Interconnection piping
- Control system including control modules
Common to the compressor trains is a power and control umbilical connection
system and a valve manifold fitted with flow line connection systems.
The station power distribution system consisting of removable circuit breaker
modules and variable speed drive modules are arranged together at one end of
the station structure adjacent to the subsea main transformer. The actual
mating mechanism for the high voltage wet mate connectors will be dependent
upon the chosen power connection system.
The piping manifold is formed to provide a balanced symmetrical routing
through each of the compressor trains. Emphasis is given to avoid high stress
levels, ensuring flexibility for connection operations.
The modules are provided with local guiding/docking and are locked into
position by dedicated mechanisms.
Intervention for ROV is designed for minimum top and one side access.
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Access to modules for vertical removal/installation is provided from the top
and
sides of the protective structure.
Smaller removable modules such as control pods, control valves and certain
instrumentation units are provided as individual units and/or included within
one
of the main modules as removable items, these modules/items are run on
dedicated intervention running tools.
The compressor is directly driven by a high-speed motor. The electrical motor
is
cooled with hydrocarbon gas with a pressure regulated to be equal to or as
close to the suction pressure as possible. Said gas source can either be
conditioned gas supplied to the subsea compression station from an external
source, discharge gas from the, compressor module or suction gas to the
compressor module. Said hydrocarbon gas for electrical motor cooling might be
conditioned prior to entering into the electrical motor and said hydro-carbon
gas
might also be replaced by other suitable gases. Alternatively the motor may be
fully canned with main cooling from the gas flow.
The compressor is able to meet the design operational conditions over the
production period with declining production wellhead pressure. Re-bundling of
the compressor can be performed as part of a maintenance program.
A magnetic bearing system is used for each of the subsea compressor
modules.
The system includes magnetic radial and axial bearings as well as run-down
bearings.
Material properties of the compressor unit is suitable for operation with
relevant
content of H2S and CO2.
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The compressor and material, properties are designed for the liquid fractions
and solids content coming with the gas stream from the upstream separator.
The size and distribution of the liquid droplets and solids particles is
dependent
on the separator design.
The boosting station manifold is equipped with a remote operated isolation
valve facilitating by-pass of the compression trains.
The boosting system is designed to handle the continuous fines/sand
production. The rotating equipment is protected against wear and degradation
from solids. This will ensure high efficiency, long life and reliability.
The compressor(s) have anti-surge control recycle line designed for full
recycle
flow at maximum continuous speed (105%). The anti-surge control valve is
' electrical actuated, axial stroke and is , located close to the compressor
discharge at high point. An anti-su'rge re-cycle cooler is included downstream
of
the anti-surge valve in the re-cycling pipe loop.
The compressors have a discharge pipe equipped with a remote operated
isolation valve. A non-return valve is fitted in the compressor discharge pipe
upstream of the isolation valve.
The boosting station is able to handle liquid backfiow from the downstream
export pipeline. The boosting station is isolated and pressurised to avoid
liquid
ingress due to back-flow from multiphase export pipelines.
The separator separates liquid/solids from the gas which in turn is ingested
into
the pump and compressor, respectively.
The separator is designed to separate liquids and solids from the gas flow to
avoid excessive erosion of the compressor. Right separator design is chosen to
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secure that solids are not clogged or fixed anywhere in the separator or its
internals.
The condensate pumps are able to handle the liquid production and boost it up
to the required discharge pressure. The pumps are variable or fixed speed
driven.
The pumps are able to handle the continuous and intermittent sand production
in the liquid stream from the separators.
The boosting station has tie-in connection for well fluid discharge. Each of
these
are equipped with ROV (remotely operated vehicle) operated valves for routing
of the well fluid to the different pipelines.
Figure 5 shows a subsea boosting station process flow diagram.
The process in the subsea boosting station is envisaged in the following
paragraphs.
The well fluid from a tied-in production template is distributed to a
separator
equipped with an electric actuated isolation valve in the inlet pipe. The well
stream is further routed via the compressor by-pass line before compressor
start-up and the by-pass valve is closed when the compressor(s) are brought
into operation.
The need for inlet/input coolers will depend on required compressor inlet
temperature and the physical location of the compressor station in relation to
the production template(s) and the heat transfer from the connecting flow
lines
to the seawater. The cooling required is dependent on the well stream inlet
temperature, the required inlet temperature to the boosting system and the
hydrate formation temperature. Additional cooling in the in-field flow lines
from
the production templates is possible.
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The compressor allows recirculation for anti-surge protection and start-
up/shut-
down operations. The recycle cooler and recycle loop is designed for full
recycle
flow at compressor maximum continuous speed (105%).
Most of the solids are removed in the separators. Sand/fines/solids entering
the
boosting station will be separated out in the separator and transported via
the
liquid pump to the discharge pipeline. However, a sand trap for accidental
sand
production may be used to remove sand from the inlet well fluid.
Gas demisting and gas-liquid separation is performed by use of scrubbers.
Tolerance to sand/solids/fines in the well stream is made acceptable with
regard
to entrainment, clogging in demisting equipment and drainage system and also
accumulation in vessel bottom. Continuous production of fines is handled in
the
boosting station, without jeopardizing operation and performance.
Terrain induced slugging and transient slugging may be expected. The
separation vessel is designed to have safe and efficient handling of liquid
slugs.
The slug handling philosophy is to accumulate the specified slug volumes in
the
separator units. The liquid slugs entering the boosting station will
accumulate in
the separator before being pumped to the station discharge by the liquid
pumps.
The design also ensures stable operation for moderate slugging with minimum
use of liquid level control devices and minimum impact on compressor
operation due to inlet pressure transients. The internals are designed for the
thrust and vibration caused by the expected slugging.
The liquid boosting system consists of single or multiphase condensate pumps
with fixed or variable speed drives. The pump discharge pipes are equipped
with a non-return valve upstream of the discharge isolation valve.
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Anti-surge control is made possible by monitoring the compressor suction flow
rate, temperature, pressure together with compressor discharge pressure and
temperature.
5 The well stream is inhibited by MEG injection at the wellheads to prevent
hydrate formation.
The MEG, condensate and water is separated out in the separator in the
boosting station and pumped to the station discharge header by the condensate
10 pumps. Sufficient MEG content will ensure hydrate prevention of these parts
of
the system.
The gas separated out in the separator will have none or only small quantities
of
MEG.
A schematic overview of main modules and parts in a subsea boosting station
pilot set-up used for tests in the intended environment is shown in Figure 6:
The subsea facilities comprise remotely actuated valves to control the flow of
produced gas and the injection of chemicals. The remotely operated valves are
electrically actuated
Local instruments (transmitters) is provided to measure pressure, temperature,
gas flow rate and record the anti-surge valve position.
The different types of valves, the condition monitoring system and the
transmitters are interfaced via the subsea control modules.
Interfaces with subsea variable speed drives and circuit breakers, distributed
control system and emergency shut down systems are foreseen.
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A typical power and control supply architecture use for the boosting system
architecture is shown in Figure 7.
Interface and closing of control loops between the variable speed drives
circuit
breakers and compressors control system may be via the receiving facilities
control system main bus. All information, alarms and interlocks between the
two
systems should be handled by the distributed control system.
The receiving facilities distributed control system controls all control loops
defined "slow". This is typically opening and closing of manifold valves and
condition monitoring systems. The subsea control system has inter-connection
links to handle potential subsea shutdown requirements.
Dynamic control loops, which requires quick response, are the anti-surge
controller and the magnetic bearing controller. These loops shall be closed
subsea if required.
Anti-surge algorithms are identically implemented for all compressor stages.
The control algorithms include features for suction and discharge pressure
override, i.e. limiting the discharge pressure or increasing the suction
pressure.
