Pipeline Route And Depth-Of-Cover Survey Considerations

Geophysical and geotechnical survey techniques used for shallow water pipeline route investigations can also be used to determine the pipeline depth of burial in shallow water regimes. Multidiscipline surveys enable: 1) mitigation of risks from natural or man-made seafloor hazards; 2) regulatory compliance (archaeological, integrity management, shallow hazards); and 3) establishment of design criteria for pipeline, cable, template, etc.

By operating several data acquisition systems simultaneously, a wide range of survey objectives can be accommodated quickly, minimizing survey costs to client companies.

Multidiscipline survey techniques will provide data not only for depth of cover but can identify the conditions of the pipeline trench, backfill material, surrounding surficial soils, seafloor scour along the pipeline, debris and proximity to other pipelines. Accurate position information is recorded along with the systems data.

By simultaneously operating several systems, a quick determination of conditions that may have an effect on an existing or planned pipeline installation can be realized. A typical system suite includes: 1) positioning – DGPS, 2) subbottom profiler – Chirp, 3) echo sounder – dual frequency, 4) sidescan sonar – dual frequency, and 5) magnetometer – low noise.

Depth Of Cover
Let us consider pipeline depth-of-cover survey techniques – shallow water operations. Four primary approaches are typically used to measure depth of soil cover over an existing pipeline: a) physical probing from the water surface or by diver, b) gradiometer array, c) pulse induction system, and d) chirp subbottom profiler.

The following discussion details the subbottom profiler utilization as it is considered the most effective and least costly survey method of the three. In addition, when other complementary survey equipment is used, additional important data are available to the operator

Figure 1 illustrates typical data provided by a sidescan sonar, subbottom profiler and echo sounder when operated over a pipeline. Additionally, a magnetometer is used to assist in determining the exact location of the line.

Typically, a dual-frequency echo sounder (30 kHz – 200 kHz range) and a subbottom profiler are used to determine depth of cover. The higher frequency system inherently is the most accurate due to a narrow beam width, shorter pulse length and wavelength provided. Specifications for the A typical hydrographic profiler operating at 200 kHz provides accuracy as .01 m 1% water depth. In the case of depth of cover, we are concerned with the window between the seafloor surface and the top of the pipe which will normally be less than 10 feet. In this case, the accuracy would be better than .01 m (.4 inch) + 1% of 10 feet (1.2 inch) or 1.6 inch maximum error.

The sediment within the depth of interest is water-saturated and acoustic velocities are considered identical to that of the water column.

With reference to Figure 1, we can observe the depth of the pipeline from the seafloor to be nine feet. However, due to scour or lack of backfill we also can see the pipeline is six feet from the bottom of the trench. It would be difficult, at best, for a diver to determine where the actual seafloor surface is in respect to the trench bottom and the pipeline.