 |
aerial pipeline surveys
Surveying pipelines carrying water and petroleum products with ultra-violet, visible, near and long wave infra-red for leak detection is not a new technique and has been tested with varying degrees of success. Each part of the electromagnetic wavelength has advantages and disadvantages with regard to the vast range of environmental conditions. Modern (Mil-spec) thermal imaging equipment is particularly of new interest since recent technology provides much higher spatial and thermal resolution. Modern computer technology also contributes to more sophisticated post processing and analysing the data.
Water, for example, that has saturated the surrounding earth around a leaking pipe will in some cases provide a significant temperature difference to surrounding dry earth. On a warm day, the temperature contrast or differential will be greater and under these ideal conditions a high specification thermal imager will record the data that can in these circumstances be up to 2°C above or below ambient. Conversely, when dry ground temperature is near equilibrium with standing water and moist earth, new water ingress discrimination can be subtle especially from a long-range aerial perspective.
Thermal data is usually recorded in monochrome
since it is necessarily of higher resolution
than pseudo colour. The human eye will only
discriminate up to 20 grey tones (temperatures)
whereas a computer can discriminate 256. The
images can be colourised in real time if
desirable for a specific project.
The colourised thermal image below, Figure 1. shows a leak (blue/green) from a trunk main water pipe.
Figure 1:
Figure 2:
Example of combined Thermal Image and Visible.
Blue is high density water centred around leak. Note there is no visible evidence. All data is annotated with Lat/Long and time/date.
Temperatures (colours) indicate a density variation associated with the colder water seepage from the ruptured pipe. The ‘density slicing’ and characteristic shape of water leakage helps identify real leaks from natural standing water.
Dry earth will change with ambient temperature variations more quickly than moist and wet areas. Evaporation at the surface lowers the surface temperature. The purpose of this image is to illustrate how saturated earth can vary in temperature differently to surrounding conditions.
Aerial thermographic surveys of both surface reservoir banks and sub-surface reservoirs can show anomalies associated with leakage. An aerial perspective will provide an overall view of the extent and path of a leak. The chart below is used to give a quick indication of moisture density at the surface.
Figure 3:
Example Thermal Image of underground reservoir with high density moisture to one side.
Figure 4:
Thermal Image - Surface reservoir showing water path and collection that was not visible to the naked eye.
Reservoirs can be scanned with infrared equipment very quickly providing a visualisation of temperatures associated with leaks from banks and dams. Water collection down hill from a reservoir can be from a leak and not always associated from a ground perspective. With many years of experience in various types of pipeline surveying local areas are also scanned as part of the whole investigation.
Equipment Used (Horton Levi Ltd.)
Military Specification Thermal Imager
Specifications:
Operating wavelength*: 8-13m-6
- Produces real-time video output in
colour or monochrome CCIR video output, luminance
dependant upon energy radiated by observed surfaces.
- Thermal resolution to equivalent of 0.17°C @ 0.289 hz/m radian (this is
equivalent to an emissivity variation of 0.002).
*This wavelength band is absolutely critical for aerial
surveys representing a window of transmission relatively unaffected by
absorption due to moisture, carbon dioxide or other atmospheric components,
even for path lengths of thousands of metres. This wavelength also gives
greater signal strengths at temperatures close to ambient.
Practical Considerations for Thermal Pipeline Surveys
A high resolution thermal imager will detect difference in radiant energy
emission from the surface observed down to the equivalent of 0.17°C at constant
high emissivity or with an unchanged temperature to the equivalent of 0.002
emissivity variation however these differences are caused.
In practice, this means that pipeline trenches can be detected to less than
1 metre as randomly “warmer” or “colder” than the surroundings – even if the
pipe has been entrenched for many years. In surveys of rural (green top) water
pipelines about 65% of trenches can be identified. This service, in itself,
has corrected clients’ inaccurate maps.
Such trenches are visible for any or all of the following reasons:
- Surface discontinuities cause differential solar gain/radiant loss.
- The properties of the material used for backfill differs from the
surrounding material. This could, for example, simply relate to water
absorption/retention properties.
- Transmission by conduction of cold/warm pipe conditions to the surface.
- Vegetation differences caused by the above.
Pipeline Discontinuities
Pipeline discontinuities, like inspection hatches, washout chambers,
cattle troughs, etc, will be seen if, they are manifest at the surface at or
above the minimum detectable difference (in °C or emissivity).
Leaks, overflows and other fluid loss may be apparent for the same reason. These
are more likely to be detected if:
- the pipeline fluid has an emissivity or a temperature difference to the
ambient surface material (including water in either case).
- The temperature difference between the ambient surface temperature and the
pipeline fluid is as high as possible. This relates to the solar or radiative
history of the surface (hence the dusk/dawn criterion for flying) and to the
thermal history of the pipeline fluid. (e.g. Flow rate, initial temperature,
any trace heating, pressure changes with gases etc,) Pipelines or leaks
under ‘blacktop’ (i.e. Concrete & tarmac) are only likely to be
detected if
the pipe is carrying hot fluid or if the leak is sufficient to saturate earth
at either side of the road.
- If the pipeline is volatile. A petrol leak will appear as a cold area.
For more information on this subject, to learn more about the company or to enquire about a quote, please visit our
corporate website at www.hortonlevi.co.uk
|
|
|
|
|
