Principle of Laser Pipelaying Control

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Principle of Laser Pipelaying Control
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Alignment Lasers

• Main Purpose
• define a visible laser line which can be
  interrupted anywhere
• the centre of the spot taken as a reference point
• Incorporate
• own built-in self-levelling system OR
• designed to fit into conventional surveying
  instrument tribrachs to enable levelling to be
  achieved

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Alignment Lasers

• used in conjunction with special targets on which
  the beam is viewed in order to check if there is
  any deviation from the designated direction
• Beam
• remains at a constant small diameter (10-20 mm)
  at any point along its working range OR
• others have a lens system and need to be focused
• enables a smaller well-defined dot to be
  established at a particular location
• if intercepted at other locations, beam diameter
  may not be usable and refocusing will be necessary

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Alignment Lasers

• the latter system is slightly more expensive to
  produce because of additional lens system
  required
• used in any application requiring the setting out
  of line, level, slope or verticality
• In construction
• controlling pipelaying, pipejacking, tunnelling,
  rail alignment and dredging
• most of the them are purpose-built laser
  alignment equipment and have been developed to
  perform the particular task

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Pipe Lasers

• most contractor tend to use purpose-built
  instruments which come under the general title of
  pipelasers
• used with targets fitted inside the pipe
• extremely robust and totally waterproof
• able to cope with adverse ground conditions and
  handling that they inevitably receive on
  construction site

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Pipe Lasers

• Waterproof
• not affected by flooding and will continue to
  function even if totally immersed
• withstand pressure exerted by 10m head of water
• can be used in a wide range of temperatures, (-25
  to 50C)
• diameter is about 10mm (varies with manufacturer)
• low angular divergence (diameter increases very
  slowly with distance)
• no focusing of the beam is required

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Pipe Lasers

• enables very long ranges to be achieved before
  divergence has too great an effect on the size of
  the laser spot (max. range 300m)
• Safety point of view
• most conform to Class 3A safety category
• necessary when laying pipes in bright sunlight in
  order that the beam can be seen clearly on the
  target
• so greater safety precautions must be taken

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Pipe Lasers

• Self Levelling
• use electronic level-vial system
• manually operated level bubble which has to be
  centred initially to level the instrument
  roughly, then the automatic levelling system
  takes over
• instrument accurately levels itself in both the
  grade-axis and the cross-axis

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Pipe Lasers

• no danger of using the instrument when it is not
  level, since each incorporates some type of
  warning system which is activated while the
  self-levelling procedure is being carried out
• serves to notify the operator that the instrument
  is not yet ready to be use
• usually the beam flashes and, in some machine, a
  light also flashes on the display panel
• warning system also operates when the instrument
  is knocked-out of level during use

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Pipe Lasers

• if possible, the instrument will re-level itself,
  but one must remember that it will not necessary
  re-level at the same height as before since it
  may have been moved in a vertical direction
• in addition, it will not necessary re-align
  itself on the target
• as a result, setting up on unstable ground must
  be checked regularly
• accuracy of the self-levelling system should also
  be checked regularly

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Pipe Lasers

• incorporates motors which are activated by
  electronic circuitry within the instrument
• used to control the both the grade and line

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Pipe Lasers

• Grade
• dialled in by turning a control knob
• amount introduced is indicated on the instrument
  control panel, usually a LCD display
• can be altered in increments of 0.002 over
  ranges from 30 to -10

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Pipe Lasers

• for some machine, separate display panels are
  used to avoid confusion in indicating positive
  and negative grades
• required grade is calculated from
• a greater positive grade range is provided since
  pipes are usually laid uphill

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Pipe Lasers

• Line
• controlled by simply turning a dial on the
  control panel
• exact figures are not displayed since pipes are
  always laid with the aid of targets
• the line is controlled by turning the dial until
  the beam hits the centre of the target

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Pipe Lasers

• besides manual mode, most lasers have a
  remote-control facility either by cable (12m) or
  wireless (80m)

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Pipe Lasers

• Supports
• available to deal with any requirement, e.g. used
  inside or outside the pipe, above ground or in a
  manhole
• always used in conjunction with translucent
  targets
• beam is generated either exactly down or some
  known distance below its centreline, e.g. 15mm
  below the centreline of a 150mm diameter pipe

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Pipe Lasers

• For larger-diameter pipes
• adjustable-length legs can be fitted to enable
  the beam to be projected either along the
  centreline or at some known height above the
  invert level
• For manhole locations
• adjustable cross-bracing is available to which
  the laser can be attached
• For above-ground work
• tripod mountings can be used

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Pipe Lasers

• Targets
• available in a variety of sizes and to fit into
  every size of pipe down to 150mm diameter
• ruggedly made and easy to read
• some are adjustable for centering in a range of
  pipe diameters and are fitted with 2 level vials
  to enable them to be levelled either
  right-side-up or upside-down
• others are specially designed either self-centre
  or to fit exactly into pipes of a particular
  diameter

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Problem

• Refraction
• during pipelaying, laser beam may be refracted as
  it travels through the air in the pipe
• Occur
• when placing a warm pipe into a cold trench (or
  vice versa)
• when a long stretch of pipe is left exposed in
  the trench such that it is heated by the sun on
  top and cooled by earth and possibly water on the
  bottom

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Problem

• if there are pockets of hot and cold air
  present, a temperature gradient occurs
• the beam will be deviated from its intended
  straight path by the different densities

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Overcoming the Problem

• use special blowers to flush the pipe with air of
  a constant temperature while lasers is being used
• standardizes the density of the air within the
  pipe
• air circulation can be further improved if the
  blower nozzle is inserted into the pipe at a skew
  angle to create the corkscrew effect
• mount laser higher up in the pipe
• air temperature may be more constant
• mount laser on top of the pipe
• pipelaying procedure being controlled externally

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Overcoming the Problem

• amount of deflection of the beam increases
  exponentially as the target is moved further away
  the laser
• move laser along the pipe as construction
  progresses and thus minimized the amount of
  deflection
• backfilling the trench as soon as possible after
  the pipe has been laid can help to stabilize the
  air temperature inside the pipe

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Establishing the Line of a Pipe

• owners manuals normally give very detailed and
  easy-to-read descriptions of how the line of a
  pipe can be established
• Main Problem
• initial setting of the laser beam at the required
  direction along which the pipe is to run
• the rest is simply moving the target to the next
  pipe being laid, which is adjusted in position
  and bedded in when the beam hits the centre of
  the target

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Establishing the Line of a Pipe

• initial direction can be set in one of two ways,
  depending on the depth of the work
• Assume
• a ranging rod, peg or batter board has been
  placed on the surface at the next manhole
  location
• a known level has been transferred into the
  bottom of the manhole or trench either by taping,
  vertical taping or some other suitable method

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Establishing the Line of a Pipe

• Shallow manholes and trenches
• with reference to the known level, the laser
  support is adjusted so that the beam is at the
  correct level in the excavation
• the required grade is set and the beam is aimed
  in the approximate direction

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Establishing the Line of a Pipe

• a piece of cord is used as a sight line and is
  attached to a special hook which is usually
  fitted as standard carrying handle of the
  instrument
• the operator lines this in with the ranging rod
  at the next manhole, and adjusts the direction of
  the laser beam using the remote-control device
  until it is on the correct line

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Establishing the Line of a Pipe

• Deep manholes and trenches
• with reference to the known level, the laser
  support is adjusted until the beam is at the
  correct level at the bottom of the excavation or
  in the base of the manhole and at a point on the
  proposed line of the pipe

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Establishing the Line of a Pipe

• a theodolite or special sighting telescope,
  which is usually available as an optional extra,
  si mopunted in such a position that it is
  directly above the laser
• done either by optical plumbing or by mounting
  the sight device on a lone vertical pole attached
  to the laser support

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Establishing the Line of a Pipe

• sighting device is pointed at the ranging rod
  defining the next manhole location, and is used
  to control the line of the excavation
• once a short distance has been excavated, the
  sighting device is tilted downwards to give the
  line on the excavation wall
• the required grade is set on the laser and the
  beam direction is altered using the
  remote-control device until it is on the correct
  line as seen through the sighting device

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Limitation

• restricted to straight pipe runs
• curved section must usually be controlled by
  conventional techniques
• majority of pipelines run mainly in straight
  lines
• application of lasers to their control has many
  advantages over conventional methods

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Advantages of Laser-controlled Pipelaying

• Eliminates the tools for conventional pipelaying
  control techniques
• such as sight rails, profile boards and
  travellers
• awkward to use and have a tendency to be knocked
  over by site equipment
• lasers can be used in areas where surface
  conditions preclude the use of conventional
  techniques, e.g. in marshy and swampy ground
• Speeds up work
• decreases the time required to do the job, up to
  30pipe can be laid in the same time frame

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Advantages of Laser-controlled Pipelaying

• Materials costs are reduced
• as is more accurate than conventional methods
  (better than 10 mm in 100m), trench excavation
  can be to the minimum depth and width required
• estimated that approximately 15 of bed material
  costs can be saved as the laser prevents an
  excavator overdigging
• Less shoring is needed
• the whole pipelaying activity is in a limited
  area
• safety is improved since backfilling can be
  carried out almost immediately

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Advantages of Laser-controlled Pipelaying

• Labour costs are reduced
• elimination of sight rail-traveller technique
  means that fewer personnel are required for
  eyeing-in purposes
• no bankman is needed with an excavator to
  indicate when the correct level has been reached
  (may need for safety purpose)
• Constant check during construction
• any movement or collapse will cause the beam
  either to move on the target or to be blocked
  completely.

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Reference

• Price, W.F. and J Uren (1989) Laser Surveying,
  Van Nostrand Reinhold (International) Co. Ltd.,
  London
• Bannister, A S. Raymond (1984) Surveying, 5th
  ed., ELBS/Pitman, Bath.
• Jackson, J.E. (1977) Plane and Geodetic Surveying
  for Engineers, vol. 1, 6th ed., Constable Co.
  Ltd.
• Schofield, W. (1993) Engineering Surveying, 4th
  ed., Butterworth-Heinemann Ltd., Oxford.