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Pipe laying operations

In many marine activities, ranging from navigation to fishing, and from offshore platform construction and pipeline laying, a thorough knowledge of the sea-floor topography (bathymetry) is vital.

The figure shows an admiralty chart that details the bottom topography of the North Sea. This charts are developed from echosounding data.

Mapping the seabed with ship-based echosounders is expensive. However, if this traditional method is used in conjuction with space techniques, such as satellite radar imaging, the costs for gathering such bathymetric information may be reduced substantially. In fact, data from each of these techniques complements one another.

Echosounding provides very accurate depth measurements along a line, whilst satellite radar images offer synoptic, two dimensional overviews.

Combining the data provides detailed information too costly to obtain from echosounding alone.

Radar images, such as those derived from the ERS-1 satellite, show the bottom topography in swallow waters, when currents are present.

This figure shows the effect caused by the underlying topography on the sea surface, and so on the intensity of the returning radar signal.

Three factors permit mapping of the ocean bottom using this technique:

  • Interactions between tidal flow or currents and the bottom topography cause changes in the speed of surface water flow.
  • Variations in the surface flows speed cause variation in the surface waves.
  • Variations in the surface waves create changes in the surface response to radar impulses.

    Traditionally, pipeline laying operations follow these steps:

  • Selection and survey of the most suitable track, based on available charts.
  • Survey of additional tracks on both sides of the main one, up to several hundred metres in width.
  • Definition in the final pipe route corridor.

  • Execution of a single survey after pipe laying to confirm that the pipeline has been laid according to design criteria.

    Today, tradition merges with modern techniques. One example is the Bathymetric Assessment System, developed in Holland, which combines echosounding and ESR-1 SAR observations.

    Starting from radar imagery, a first-guess of depth is made using a relatively simple imaging model which describes how the radar can read the bathymetry. Based on this depth estimate, a detailed conventional survey is planned. The imaging model is used to reconstruct the final depths from radar imagery and using the conventional depth observations as standards.

    The system has been used succesfully in surveying an area northwest of Zeebrugge, off Hollands North Sea coast. This ERS-1 image has been aquired over the area, on the 1st of September 1993. The yellow box denotes the mapped area.
    The site includes a corridor 32 Km in length and 200 m in width. Pre-route contour (depth) maps were prepared, which were similar and comparable to admiralty charts.

    An absolute accuracy in the order of 30 cm has been achieved, which is comparable to that obtained by most standars echosounders. Similar systems can be used in other swallow water regions of the world.

    (Wensink, ARGOSS, Am Emmeloord/The Netherland)

    Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry

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