| Communications in Public and Private Industry |
| Optical Fibre Communication for Onshore Oil
and Gas Production Field |
| By Kwok-Hong Mak, Newcastle-upon-Tyne, England,
44-191-226-2253, makk@pbeurope.co |
| Optical fibre continues to be incorporated into
more and more communication designs as an alternative to traditional
communications systems, such as radio. Electrical utilities in particular
are taking advantage of using their overhead power line systems either
as communication bearers or as a means of mounting the optical fibre
cable system. |
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In November 1998, we were appointed by a client to carry out a
conceptual study of a supervisory control and data acquisition (SCADA)
system for an onshore oil and gas production field. As part of the
conceptual study, we performed detailed technical and cost evaluations
of two principal communication options—radio and optical fibre.
The optical fibre option involved using existing power transmission
and distribution infrastructures to support optical fibre cables.
This approach provides secure and reliable communication links and
can reduce installation and maintenance costs significantly.
Although the study and our findings were specific for our client’s
onshore oil field, the principles used are equally applicable to any
electrical utility that is considering using its overhead power lines
as either communication bearers or as a means of mounting the optical
fibre cable system.
Overview of an Oil Field Operations
Each oil field generally comprises a number of oil wells, water supply
wells, remote degassing stations and central processing facilities.
Oil from wells is collected within each field by pipeline at the remote
degassing stations. At each station, a test separator tests samples
from individual wells for oil, gas and water content. After testing,
the sampled oil, gas and water are aggregated with the main flow.
The main flow is then aggregated and passed by bulk pipelines to a
central degassing station where oil, gas and water are separated by
a three-stage separation process.
Oil is collected under pressure at each oil well in the field. As
oil is extracted, the pressure in the underground “reservoir”
lowers, so water is injected into the rock strata below the oil at
the edges of the oil field in order to maintain reservoir pressure.
This water comes from water supply wells at the field, each of which
is associated with a “cluster” of water injection wells.
At each water supply well, water is extracted from rock strata at
approximately 1500 m (4,900 feet) by a submersible pump. A surface
pump is then used to boost the water pressure in the supply main to
the injection wells.
The majority of the water supply wells and remote degassing stations
are located around the perimeter of the oilfield. Power is supplied
to these locations via the 33 kV overhead power line system.
We were commissioned to look at a SCADA system that had been installed
at the oil field in 1986 to monitor the surface pump status and some
other operational parameters associated with the water supply wells
and remote degassing stations. A radio-based communication system
operating in the 450 MHz frequency band was used to transmit SCADA
data. Frequently, the SCADA system, now out of service, had exhibited
poor performance. The problems were thought to be due to radio transmission
problems.
A new state-of-the-art water injection SCADA system to be installed
at this oil field will meet the client’s present-day operational
requirements. It will consist of a master station at the Central Facilities
and remote terminal units (RTUs) at the water supply wells and remote
degassing stations.
Evaluation of Radio Option
Radio waves in the UHF (ultra high frequency) frequency range (300
- 3,000 MHz) are used widely for speech and data communications. The
two primary merits of radio systems are that no physical connection
(such as wire) is required and capital costs are generally lower than
those of equivalent cable systems if large numbers of very tall radio
antenna supporting structures are not involved.
There are fundamental problems in the initial planning of radio systems,
however, including the following:
- Difficulty obtaining suitable radio frequency assignments
- Possible interference from other nearby
radio systems
- Problems with propagation, including path
loss and possible fading effects
- “Line-of-sight” route limitations, which could
mean costly tower or mast structures for transmission over hilly
terrain
- Electromagnetic interference
- Possible impact on performance by atmospheric conditions.
Nevertheless, in suitable circumstances, a point-to-multi-point polling
radio system can provide an economic solution for the transmission
of telecontrol and telemetering signals. But, was this option suitable
for the oil field concerned?
At this particular oil field, all the water supply well clusters and
remote degassing stations lie within a 20-km (12-mile) radius of the
radio base station site. The intervening terrain is undulating with
sand dunes exceeding 50 m (165 feet) in height, however, so we had
to calculate the radio path profiles between the base station antenna
and each of the remote sites. The purpose was to determine the minimum
antenna heights needed to achieve an unobstructed line of sight, based
on a Fresnel zone clearance of 0.6. A fade margin of 40 dB was also
used for obtaining an availability figure of 99.99 percent for the
radio links.
Figure 1: Proposed UHF Radio Telemetry System for SCADA Data |
We found that the required antenna height at a significant number
of remote sites is dependent on the height at which the antenna is
mounted at the central site. The higher the antenna is mounted at
the central site, however, the more prone it will be to external interference
from nearby radio systems. With a new, 90-m (295-foot) -high radio
tower at the central site, only nine remote sites would require antenna
heights of between 30 m (98 feet) and 53 m (174 feet). The absolute
minimum height for remote site antennae is considered to be 20 m (66
feet), however. This height is required to ensure an unobstructed
line of sight operation at the oil field, where there is a regular
presence of ground fog.
Figure 1 shows the proposed radio telemetry system, comprising a radio
base station at the oil field central site and a remote radio terminal
at each water supply well and remote degassing station. The radio
base station at the central site will be common to all remote radio
sites, so duplicate transceivers with automatic changeover facilities
are proposed to improve the resilience of the system.
Evaluation of Optical Fibre Option
Optical fibre technology is now regarded as the most powerful and
versatile communications medium available. It presented a potentially
attractive alternative to radio, provided we could use the 33 kV overhead
lines feeding the oil field water supply wells and remote degassing
stations system as either communication bearers or as a means of mounting
the optical fibre cable systems. Optical fibre systems have been in
service for many years and are very reliable as long as due care is
exercised during installation of the cable and its jointing.
The merits of fibre-optic cable links are:
- Long distance transmission without repeater,
typically 120 km (72 miles)
- Immunity to electromagnetic interference
- Extremely large potential bandwidth, typically
100 GHz/km (0.6 miles)
- No approval required from the radio regulatory authority (unlike
radio systems).
There are three basic types of aerial optical fibre cable for use
on overhead power lines:
- Composite optical ground wire (OPGW),
which is basically optical fibres integrated within the static
ground wire (i.e., earthwire). It can be used in place of a conventional
earthwire.
- Optical fibre cable attached to
the earthwire by either wrapping or lashing.
- All Dielectric Self-Supporting (ADSS) optical
fibre cable suspended below the phase conductors between the power
line supports.
Optical Ground Wire. It is generally accepted that
the OPGW is the most effective type of optical fibre cable although,
for an existing overhead power line, its use means the existing earthwire
must be scrapped.
OPGWs are manufactured with mechanical and electrical characteristics
similar to conventional earthwires. In many cases, the OPGW can be
manufactured such that its diameter and weight is equivalent to that
of the earthwire being replaced, so it will not affect the applied
loading on the overhead line supports. The optical fibres, typically
between 4 and 24 of them, are contained either in a metal tube or
in a slotted metal spacer, depending on the manufacturer’s design,
set within the metallic strands that make up the earthwire. The overall
construction is designed to ensure total immunity of the communication
circuits from the effects of short circuits on the power system and
lightning strikes.
Installation of OPGW is basically similar to that of conventional
earthwire. Figure 2 shows the mounting position of OPGW on a typical
single pole structure at the oil field.
Of the three basic types of aerial optical fibre cable, OPGW has the
longest service history. So far, more than 11,000 km (6,600 miles)
of OPGW cables have been installed worldwide by one manufacturer alone.
OPGW provides the best environment for the optical fibres because
it protects them from man, nature and the elements. Reported OPGW
problems/failures are rare.
Costwise, OPGW is the most expensive. Also, at lower voltages the
power circuits must be disconnected during installation, which sometimes
may be highly undesirable for operational reasons.
Figure 2: Installation Method for Optical Ground Wire (OPGW)
Cable |
Figure 3: Installation Method for Lashed Cable |
Lashed/Wrapped Cable. Lower cost alternatives to
OPGW are the so called “wrap” and “lash” cable
systems. In both systems the basic cable is formed out of a plastic
tube that contains the optical fibres. The cable is attached to the
earthwire either by wrapping it spirally or by lashing it to the earthwire.
In the latter case, the cable and the earthwire remain parallel and
either a continuous spirally wound tape is applied to keep the two
in the required position or separate metallic spirals are applied
to clip the optical fibre cable to the earthwire. The lashed type
of installation is illustrated in Figure 3.
Lashed or wrapped optical fibre cable is a commonly adopted solution
for retrofitting existing lines as long as the earthwire and tower
structure can withstand the additional mechanical loading. Such a
cable is necessarily of a relatively light construction and requires
very careful installation to avoid subjecting the optical fibres to
excessive mechanical stress. There is also a greater risk of damage
from short circuits or lightning, as compared to OPGW.
With lashed cables, the area that is subjected to wind forces is considerably
larger than that of the original earthwire. This lashed arrangement
also creates an aerofoil section that causes lifting of the amalgamated
cable/earthwire under high wind speed conditions. This lifting can
give rise to galloping of the optical fibre cable and the earthwire,
which can cause damage to either or both components.
It is generally possible to install the optical fibre cable with the
power circuit remaining energised although, for safety reasons, the
electrical clearance necessary for spirally wrapping either the optical
fibre cable or the support wrap usually precludes this on voltage
lines below 66 kV. Other arrangements for installing earthwire supported
cable using manually applied pre-formed spiral clips or other suitable
devices should permit live-line working. Live-line installation will
be considerably more costly because of the extra safety precautions
that must be taken.
Once installed, the cable is maintenance free generally, although
support taping and clips have failed on various of the lashed systems;
the wrapping technique has also had its failures. The installed cost,
however, is generally less than OPGW, even allowing for the additional
cost of live-line installation.
Figure 4: Installation Method for All Dielectric Self Supporting
(ADSS) Cable |
All Dielectric Self-Supporting Cable. The ADSS was
developed without any metal so that it could be installed parallel
to the phase conductors. Normally, the ADSS is mounted on the overhead
line support structure and strung below the bottom phase conductor
of the overhead line, as illustrated in Figure 4.
The ADSS has to be self-supporting, so tensile members are needed
to strengthen the cable. As a result, the overall diameter of ADSS
cable is generally larger than that for the wrapped/lashed type for
an equivalent optical fibre count. Wind effects are noticeable on
the relatively large-diameter ADSS cable, so careful checks must be
made on the capability of the supporting structures.
ADSS cable is set at a lower height than the phase conductors, thus
reducing the additional overturning moment from the ADSS. On the other
hand, installation below the phase conductors may give lower than
desirable ground clearance which, due to prevailing site conditions,
can be reduced even further by drifting sand.
The ADSS format is generally considered to be the next best after
the OPGW as an environment for optical fibres. Experience on extra
high voltage power lines has shown, however, that deterioration of
the sheath material caused by a phenomenon known as “dry band
arcing” can result from the cable being subjected to high electric
field strength (due to its physical location in relation to the phase
conductors). This concern makes ADSS cable more suitable for use at
voltages of 132 kV and less.
ADSS is suitable for live-line installation provided there is sufficient
electrical clearance for safe installation. Overall, ADSS costs about
the same as lashed-type cable—the cable itself tends to be more
costly but installation is generally simpler and less costly.
Telecommunications Network Design
The cost of the optical fibre cable will represent the major proportion
of the cost of an optical fibre communication system, so we optimised
the cable route to provide communications for all sites with the minimum
length of cable. An optimised route also offers the highest resilience
of the communication system.
The logical route would follow the perimeter power lines; with the
optical fibre being looped into each of the remote sites fed from
the main power ring. A complete fibre ring permits communication in
either direction, so there is inherent redundancy, and communication
to all points on the ring can be maintained in the event of a failure
at any point.
Figure 5: Proposed optical fiber telecommunications network |
We considered a number of optical fibre network configurations, each
having its advantages and disadvantages. The preferred optical fibre
telecommunications network (Figure 5) requires the least amount of
optical fibre cable, 170 km (102 miles). It consists of two separate
optical fibre rings, each of which has almost half the total number
of RTUs. Although a single cable failure on one of the optical fibre
rings can affect two remote RTUs, we do not consider this to be a
major drawback.
At each remote site, an optical line terminal equipped with the necessary
interfaces is required. Each remote terminal acts as a node on the
optical fibre ring. At the central site, master optical fibre node
equipment is required for each optical fibre ring.
Conclusion
In view of the significant initial capital cost advantage that optical
fibre options have over radio, and the very substantial technical
superiority of a fibre solution, we concluded that an optical fibre
based communications system should be used for the new water injection
SCADA system at the oil field. We further considered that the technical
advantages of OPGW more than offset any cost premium over the other
types of lashed or ADSS cables installation, providing that operational
constraints permit the necessary power line outages could be arranged.
A technical presentation was given to the client in March 1999. The
client concurs with the findings and recommendations detailed in our
conceptual study report and is currently making arrangements to proceed
with the implementation of the new water injection SCADA system. |
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Kwok-Hong Mak is a Chartered Engineer and a member of the Institution
of Electrical Engineers, UK. His areas of specialization are in
communications and control systems including fibre-optic communications,
microwave radio links, VHF/UHF mobile radios, SCADA systems, digital
transmission and telephony equipment for applications in electricity
supply and transportation industries. Recently, he prepared a technical
specification for a fibre-optic communications system for speech
and data facilities based on the synchronous digital hierarchy (SDH)
technology for the Syria 400 kV power transmission network.
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