Note: Descriptions are shown in the official language in which they were submitted.
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Subsea pipeline power transmission
This invention relates to subsea pipeline power transmission systems, methods
and apparatus.
The term subsea is used in this application as this is conventional
terminology,
however, it will be understood that this covers any underwater situation.
In many circumstances where subsea pipeline systems are used, there is a
desire to operate equipment at locations which, in the general sense, are
remote. That is to say, although the equipment is situated adjacent to the
pipeline itself it is not near any other facility or infrastructure. Such
pieces of
equipment might,' for example, be sensors which monitor the integrity or
operation of the pipeline system.
One of the problems with such remote pieces of equipment is providing a
suitable power source. Whilst batteries can be used these are unattractive for
various reasons including their limited life, their expense and environmental
concerns.
It is an object of the present invention to provide methods, systems and
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apparatus which allow the supply of power to remote equipment in subsea
pipeline systems.
According to a first aspect of the present invention there is provided a
subsea
pipeline power transmission system comprising a pipeline, an electrical power
supply connected to the pipeline at a first location, and connection means
provided on the pipeline at a second location for connection of a load to the
pipeline to allow the load to receive electrical power from the power supply
via
the pipeline wherein the pipeline has a plurality of cathodic protection
anodes,
each of which is electrically connected via respective impedance means to the
pipeline.
According to a second aspect of the present invention there is provided a
method of subsea pipeline power transmission along a pipeline having a
plurality of cathodic protection anodes comprising the steps of:
applying electrical power to the pipeline at a first location; and
electrically connecting a load to be supplied to the pipeline at a second
location;
wherein each anode is electrically connected via respective impedance
means to the pipeline.
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According to a third aspect of the present invention there is provided
apparatus
for use in a subsea pipeline power transmission system or method comprising:
an anode arrangement comprising, a sacrificial anode arranged for
mounting on a pipeline and impedance means having one terminal connected to
the anode and another terminal arranged for connection to said pipeline; and
an electrical power supply arranged for electrical connection to a
pipeline.
According to a fourth aspect of the present invention there is provided an
anode arrangement for use in a subsea pipeline power transmission system, the
arrangement comprising, a sacrificial anode arranged for mounting on a
pipeline and impedance means having one terminal connected to the anode and
another terminal arranged for connection to said pipeline.
The anode arrangement may include further terminals allowing the connection
of a load across the impedance means.
The impedance means may comprise inductance means. Preferably the
impedance means comprises filter means. The impedance means, especially
when comprising filter means, may be arranged to give a high impedance to
time varying signals within one or more selected ranges of frequencies and a
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low impedance to signals outside the selected range or ranges. The impedance
means can be arranged so that the real part of the impedance is substantially
zero. This means that there is little or no attenuation of the do components
of
signals passing through the impedance means.
The use of inductance means and particularly filter means has advantages when
the metallic structure is used to carry power currents because these means can
be chosen to offer high impedance to the time varying signals used for power
supply thereby reducing losses, whilst offering low impedance to the currents
used for cathodic protection. Minimising losses is particularly important when
transmitting power rather than merely trying to detect a signal. Limiting loss
to
a realistic level is necessary to give a practical system.
An embodiment of the present invention will now be described by way of
example only with reference to the accompanying drawing which schematically
shows a pipeline system embodying the invention.
The drawing shows a subsea pipeline system which comprises a pipeline 1
provided with a plurality of anodes 2 which are electrically connected to the
pipeline 1 via respective filter means 3.
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A power supply 4 is electrically connected to the pipeline 1 towards one end.
This location will typically be at a main facility or some other place
provided
with good infrastructure such that the provision of a power supply 4 is not
problematic.
5
Although not shown in detail, as is common practice in this field, the
pipeline
system is provided with a cathodic protection system of which the anodes 2
form an essential part. Cathodic protection currents flowing in the pipeline 1
to improve corrosion resistance will be do currents. Thus, the filter means 3
provided at each anode are arranged to have substantially zero impedance to do
currents.
On the other hand, the filter means 3 are arranged to have a very high
impedance to the power supply currents delivered by the power supply means
4. In this system the power supply means applies a current typically having a
frequency in the order of 30 to 100 Hz. The filter means 3 are arranged to
have
a high impedance to signals having the appropriate frequencies in this range.
The filter means 3 may be designed so that at the transmission frequency it
gives an impedance of at least two orders of magnitude greater than the
characteristic impedance of the pipeline (with anodes removed) when acting as
a transmission system. This means that whilst the cathodic protection currents
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can flow to the anode substantially unimpeded, the losses from the pipeline 1
as far as the power supply current is concerned are greatly reduced.
The frequency of current used to transmit power is chosen with regard to two
main factors. Lower frequencies call for more bulky and expensive components
in the filter means whereas as frequency is increased, skin effect in the
pipeline
becomes problematic. The frequency at which skin effect begins to compromise
performance may be determined empirically on a test length of pipe but can be
expected to be in the range of 50 to 100 Hz for most typical pipes.
The above arrangement means that loads 5, i.e., pieces of equipment which
need electrical power, can be connected to the pipeline 1 at locations which
are
remote from the power supply 4. As shown in the drawing, a load 5 may, for
example, be connected directly to the pipeline 1 and provided with a separate
earth terminal E, or may be connected directly across one of the filter means
3
associated with a particular anode 2 where the equipment to be driven is
located at or near an anode 2.
The provision of suitable impedance means, preferably as in this embodiment
filter means 3, between the pipeline 1 and the anode 2 makes a power supply
system of this type feasible. For example, if no impedance means 3 are
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provided, then power supply in this manner might be possible in a subsea
pipeline over a distance of say only 300 to 400 metres. However, with the
filter
means included, it can be possible to transmit power over a distance of say 10
kilometres. In the present system the loss of power might typically be in the
order of 0.5 to 1dB per kilometre and as such, if the power supply 4 applies
150 watts to the pipeline 1 then a load at a 10 kilometre distance from the
power supply 4 should be able to draw a power in the order of 50 to l5watts.
It has been determined that effectively stopping leakage from the anodes gives
a 10ø improvement in power transmission capabilities over 10 kilometre subsea
pipelines.
It will be appreciated that although an ac current is applied to the pipeline
1 for
transmission, this signal may be locally converted into a do signal using
known
techniques if this is required.
