Modeling experiments of pipeline inspection

Investigation of AUV behavior during inspection of pipeline, which is partly filled up with ground, was carried out by means of modeling. The modeling environment includes:

• 3D-model of bottom surface and artificial objects (pipeline in this case);

• AUV dynamic and kinematics models;

The model of bottom surface (fig. 9) along with model of artificial objects allows to form any terrain shapes, including concave and ambiguous (Pavin, 2005). The model includes three basic levels of detailed elaboration. The micro level corresponds to accuracy of the echo sounder (centimeters). The middle level corresponds to AUV motion during updating the data (decimeters). The macro level describes the general bottom characteristics. The model allows to set characteristics for each of three levels. For more careful test of identification algorithms the terrain model has been added with an opportunity of generation: "visor" and "stone". The macro-relief is used as a skeleton on which middle- and micro-levels are placed according to the certain rules.

The echo sounder model is applicable for calculation as single beam (with the fixed diagram of an orientation), and multi-beam echo-sounders. Thus, the basic characteristics of the echo sounder are taken into account: their layout onboard the AUV and the sensor direction, range and precision of measurements, directional diagram.

The echo sounder was modeled by several acoustic beams (from 5 up to 20) directed fan-shaped downwards in sector of 40°. AUV mission consisted of detection of the pipeline, motion along it, pipeline sagging detection and its contouring. Many variants of terrain, pipeline position and various number of echo sounder beams were modeled. In fig. 9-11 the results of partly buried pipeline detection are demonstrated. The pipeline diameter is 1.6 m. The level of pipeline hiding in sea bottom is 50% (on average). The number of used echo sounder beams - 10. Motion altitude is 3 m. Motion speed is about 0.8 m/ s and depends on echo sounder data update rate.

Scheme of modeling experiment is presented on fig. 9. Terrain "coverage" by echo sounders is represented by blue strip of variable width along the AUV lane (left picture). Right picture illustrates 3D-view of the main inspection phase. It includes freespan detection and survey. AUV has moved along the pipeline, found out sagging, contoured it and renewed motion further along the pipeline. In fig. 10 echo sounder data of AUV portside and starboard are shown (5 sonar beams from each side). The figure presents results of automatic sagging detection on the base of these data. In fig. 11 the probability of pipeline presence in echo sounder field of vision is illustrated. It is supposed that pipeline "exists" if this value is positive. In this case AUV offset is calculated (blue points on the graph) and used in control.

Fig. 9. Pipeline detection (left picture) and 3D-view of pipeline inspection (right picture)

Portside echo-sounder distances

Portside echo-sounder distances

Starboard echo-sounder distances Fig. 10. Echo sounder data and pipeline freespan assessment during sagging inspection
Fig. 11. Pipeline detection probability and mutual offset assessment

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