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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 + 50°C)• diameter is about 10mm (varies with manufacturer)• low angular divergence (diameter increases very sl

owly 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

100inverts manholebeteen disatance horizontal

inverts manholebetween difference (%)

heightgrade

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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

• owner’s 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.