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14 Frontiers April 2002
Let there be light
Engineers in BP are lit up about using optical fibres for continuous monitoring
of wells and faster communications offshore. Nina Morgan finds out how fibre
optics are proving their worth downhole
ike Theobald first ‘saw the light’ fibres, with BP now regarded as the industry optical sensors because permanently
M in the mid-1990s when working
on BP’s ETAP project in the North
Sea, one of the first offshore developments
leader when it comes to championing the use
of fibre optic sensors in wells. The company’s
interest in fibre optics took off in the late
installed conventional electronic sensors
can be unreliable, particularly as
temperatures rise above 120°C.
to take advantage of fibre optic sensors. 1990s in the wake of several important ‘For permanent sensing, fibre optics
‘A lot of people couldn’t believe that the light deepwater discoveries. One of these is the offers many advantages for measuring
could travel all the way from the platform – Thunder Horse field in the Gulf of Mexico – important well parameters such as
a distance of 10 to 12km – and then make a field with huge hydrocarbon reserves, temperature and pressure,’ Williams notes.
measurements downhole in the wells,’ but situated in 1800m of water – with ‘These simple passive devices are very
he recalls. ‘That was a real breakthrough downhole conditions characterised by very robust and reliable because they don’t have
in downhole data acquisition. Fibre optics high pressure and temperature (Frontiers any active components as do conventional
is a technology whose time has come in September 2001). In the face of such a gauges. There are other advantages too:
BP. It’s a fantastically exciting area.’ challenging development scenario, BP’s the optical devices are very light and small;
As a member of BP’s upstream engineers recognised that continuous they don’t require subsea power supplies
technology group in the UK, Theobald is just surveillance – permanent sensing and or electronic interrogation units; and the
one of many people around the world in BP monitoring of downhole temperature and fibres themselves provide a ready means
enthusing over new applications for fibre pressure in wells – would be crucial. to transmit the sensor data to the surface.’
optics in their part of the business. Although ‘The more information we have, the (See panel, top of page 16.)
the use of optical fibres was taken up in a big better we can manage the reservoirs,’
way by the telecommunications industry in explains Brock Williams, project manager Testing times
the 1980s, their application in offshore for the intelligent wells team, based in To ensure that appropriate technologies
communications systems (see panel, bottom Houston. ‘Any place where intervention – would be available and tailored to its needs,
of page 16), and for downhole monitoring of that is, re-entering a well to carry out an BP began working with optical fibre specialist
key parameters in development and operation – is complex and expensive, you contractors CiDRA (now Weatherford
production wells, is much more recent. are very reluctant to do it. This is where Sensing Systems) of the US and Sensa
BP and Shell were among the first oil permanent sensing comes into its own.’ of the UK two years ago to encourage the
companies to show serious interest in optical BP’s attention became directed towards development of a variety of suitable fibre >>
Frontiers April 2002 15
close as one metre over the entire length
Fibre optics in the well of the well bore – typically several thousand
metres long – to provide distributed
temperature sensing (DTS), establishing
a temperature profile throughout the well.
‘DTS is a new type of sensing option
made possible by the use of optics,’ explains
Wellhead Instruments Williams. ‘It is opening up new insights into
equipment
what is happening downhole and offers
unparalleled opportunities for monitoring the
health of a well.’ DTS is already being used
by BP onshore at Wytch Farm in the UK and
the Marlin field in the Gulf of Mexico. A DTS
Well
installation is also planned for the Mahogany
field offshore Trinidad during 2002.
The first fibre optic gauge tested by BP
Fibre optic cable was a pressure gauge developed by CiDRA,
running inside well
which was trialled initially in BP’s Pompano
deepwater development in the Gulf of
Above: An optical fibre with integrated Mexico in April 2000. Then in 2001 CiDRA’s
sensors is threaded into a well on BP’s Bragg grating sensor optical pressure gauges were subjected
Pompano platform in the Gulf of Mexico integrated into fibre to gruelling real-life stress tests in several BP
wells. An optical pressure gauge installed in
Right: Key components of a downhole a well in BP’s Valhall field in the Norwegian
optical sensing system sector of the North Sea survived unscathed
when the well was repeatedly perforated
during routine completion of the well. The
>> optic sensors. Wolfgang Schollnberger, the line, providing an early warning of gauge continued to function flawlessly in
then BP vice president, reservoirs, possible blockages caused by the formation spite of the fact that eight reservoir fracture
describes the impetus behind the move: of hydrates or wax. It could also make it jobs – involving rapid heating and cooling of
‘We were looking for continuous well feasible to monitor actual pipeline cool-down the well bore and associated rapid changes
monitoring, but at commodity prices.’ profiles when production is stopped. in pressure – were later carried out.
The ensuing development work has The advent of fibre optics has also led Optical pressure gauges also survived
resulted in BP’s engineers now being able to the development of new ways to monitor similar tough treatment in three wells in BP
to take advantage of optical sensors for wells. The fibre itself can be used to take Alaska’s North Star Field. After putting the
measuring downhole pressure, temperature, temperature measurements at intervals as systems through their paces, Williams notes:
seismic activity and fluid
flow in some of the field
developments in the Gulf of
Mexico, the North Sea and
Bringing bandwidth offshore
Alaska. Further deployments These days, personnel working on BP’s Forties platforms
Key
of optical sensors are Fibre connected point of presence (PoP) in the North Sea jokingly complain that they no longer have
BP’s CNS fibre
planned in Trinidad. NSC-1 fibre
time to nip out for a cup of coffee while waiting for a software
Optical fibres are also Sleipner application to download. The reason? A new fibre optics
proving useful for Cruden Bay to Stavanger cable running from Cruden Bay, north of Aberdeen, to the
monitoring stress and strain Aberdeen Forties BP-operated Ula field in the Norwegian sector of the North
Everest
in the long control umbilicals Sea, which transmits data at the speed of light. The 300km-
SCO
used to connect systems Ula
NORTH long cable, 50mm in diameter, is operated by BP’s Central
TLA
SEA
on offshore platforms with North Sea Fibre Telecomms Company (CNSFTC) with
ND
subsea equipment on the Valhall operational support from Data Marine Systems Ltd, and
seabed, and in different was installed in October 2001. The system provides a reliable
to Lowestoft
types of riser systems – broadband telecommunications link to platforms which are
the pipelines that transport located on the cable or within microwave radio range – currently around 16 installations.
wellhead fluids through the ‘Effectively what we have done is to give the offshore community the same fast communications
water column to the host and connectivity we have onshore,’ explains BP’s Catherine Robertson, fibre delivery manager, based
facility. They offer potential in Aberdeen. ‘The challenge now is to use that bandwidth to change the way people work and to improve
in other areas too. For safety and production efficiency.’
example, a fibre laid along- Currently the bandwidth is used for telephone calls, video conferencing and data transfer between onshore
side a subsea pipeline could and offshore. But other applications, ranging from direct transfer of data from downhole drilling, well logging
make it possible to monitor and seismic surveys, to monitoring of plant and equipment from onshore, are also being investigated.
temperature changes along
16 Frontiers April 2002
Making light work
How an optical fibre works Optical sensing using a Bragg grating
Light striking the inside surface Light striking the surface of the core at greater A Bragg grating is photoimprinted onto a section of optical fibre.
of the fibre optic core at than the critical angle experiences repeated total The grating acts as a wavelength-specific reflector of light. When the
less than the critical angle internal reflection. It cannot escape the core and grating is subject to strain, the reflected wavelength shifts in a linear
is refracted into the cladding is ‘bounced’ along the entire length of the fibre manner. Thus the grating can be viewed as an optical strain gauge
Critical angle Fibre core
Photoimprinted Bragg grating
Incoming
light
Reflected Transmitted
Light wavelength
source Glass core wavelengths
Optical cladding Strain
Plastic buffer coating
Since the mid-1980s, optical fibres – flexible strands of pure silicon glass wavelength of light but allows all other light to pass through. The fibre
just twice the thickness of a human hair – have revolutionised optic cable carries incoming light down to the Bragg grating and also
telecommunications. By bouncing pulses of light off the internal sides returns the reflected light back to instruments on the surface.
of the fibres, a process known as total internal reflection, optical fibres If a strain is applied to the fibre containing the grating, for example
are able to transmit vast amounts of digital data (including audio, video by changing temperature or pressure, a different wavelength is
and other information) over a single line. Now optical fibres are helping reflected. By precisely measuring this shift in reflected wavelength,
to revolutionise downhole sensing. the temperature or pressure at the grating location can be determined.
The latest fibre optic sensors use fibre Bragg gratings. These are Bragg grating-based sensors are already available to measure
created by using lasers to alter the internal structure of the fibres at a properties such as temperature, pressure, flow rate, liquid and gas
particular location on the fibre such that it reflects back a very specific phase fractions, strain and seismic waves.
‘We feel very confident that the technology Installing sensors, such as those used for intensity the further they have to travel –
is adequately robust to withstand all DTS and flowmeters which are ideally placed and subsea wells are sometimes located
field applications.’ over the reservoir section of the well, is even 30km or more from the host platform.
more testing, particularly when the well However, there are ways around this.
Making the connections completion is carried out in two stages. In If a continuous optical link is not possible,
Despite this confident outlook, there are this situation an additional optical connection the optical information from the downhole
still some challenges to overcome in the must be made down in the well where sensors can be processed electronically
use of fibre optic sensors. One of the access space is very limited, temperatures in a subsea pod on the seafloor, and then
biggest is developing better ways to install are high and the fluids can be harsh. ‘By routed through a modem in the subsea
the sensors in wells, particularly in subsea working with companies that have developed control module. The modem converts the
field developments where wellheads are subsea optical connectors for transatlantic data into an electrical signal which can be
located on the seafloor. cables and defence transmitted conventionally to the platform
Success here depends
on achieving good ‘wet’ Fibre optics are applications, we are
optimistic that suitable
through copper wires built into the subsea
control system umbilical.
optical connections
underwater, in contrast
opening up new optical wet connectors for
use on subsea trees and in
Those working with fibre optic sensors
believe their benefits outweigh the effort
to the ‘dry’ connections
that can be made on a
insights into what downhole applications will be
developed,’ says Williams.
needed to overcome installation and
connection problems of this kind.
platform. Although dry is happening ‘One of the most exciting things about
connections are now Linking up fibre optics is that their potential use as
routine in the optics downhole Once the downhole sensing devices is vast,’ observes Theobald.
industry, trying to bring connections are made, the ‘The majority of conventional data acquisition
together fibres just 0.1mm thick, perfectly task of bringing the data up to the platform techniques that have been used so far have
aligned in an underwater setting, presents still remains. The ideal is to find ways to the potential for being converted to a fibre
a significant challenge. Nevertheless, this establish a direct fibre optic link to the optic system.’
feat has already been conducted successfully surface from the subsea tree. Although this And, adds Williams: ‘Thanks to the use of
several times in subsea ‘trees’ – the direct fibre optic solution requires very little fibre optics, the industry vision of “intelligent
equipment on the seafloor that acts to control hardware, no additional electronics, and is wells” which will provide us with reliable,
the pressure and flow of liquids and gases easy to configure, it is not possible to adopt continuous and real time information about
coming out of the wells. in all cases. Optical pulses lose some of their their behaviour, is fast becoming a reality.’ ■
Frontiers April 2002 17
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