BP3381 14-17 Fibre optics by pad58035


									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
                                                                                                                  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
                                                                                                                  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
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
in the long control umbilicals                                                                     Sea, which transmits data at the speed of light. The 300km-

used to connect systems                                                     Ula
                                                                                        NORTH      long cable, 50mm in diameter, is operated by BP’s Central

on offshore platforms with                                                                         North Sea Fibre Telecomms Company (CNSFTC) with

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

                                                                                                     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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