AUV motion during the pipeline inspection has the features connected with freespan investigation. In the literature the task of main stage is not formalized and usually reduced to the list of the AUV desirable actions. In the paper the following sequence of actions is offered for the implementation of main stage (fig. 8):
1. AUV moves directly above the pipeline, thus:
• leakage detection is fulfilled (the methane sensor is used usually for these purposes);
• the video filming of the pipe surface is carried out;
• SSS imaging is carried out on both sides of the pipeline for detection of extraneous objects;
• pipe saggings and freespans are found out (on the basis of echo sounder data), and their lengths are estimated;
2. in the case of pipe sagging detection AUV moves away from the pipeline with backward motion for performance of SSS imaging;
3. AUV fulfills SSS imaging of sagging from both sides;
4. AUV comes back to the pipeline for continuation of inspection;
5. on completion of communication line inspection, the AUV fulfills backward motion for execution of SSS survey at the offset of 20-25 meters from inspected object.
AUV motion during cable tracking (Inzartsev & Pavin, 2008) is the same as for pipeline one (excluding item 2). The main goals of inspection can be achieved at AUV motion above the pipeline at height of 1-3 m. Thus, the inspection task is realized as the following algorithm (fig. 8).
The control heading ®tCTRL during pipeline or cable tracking can be presented as the sum of the following values: direction of the cable/pipeline ®tCAB, the crossing angle crossQ of the inspection object (for "zigzag" trajectory) with corresponding sign Sidet (determining a left-right-side AUV direction movement) and the trajectory stabilization function stab() above the inspection object (at movement lengthways the inspection object):
0CtrL =0CAB + Sidgt,sia6(8tCTRL) (19)
Where: öf0™- - calculated distance from cable/pipeline to "AUV stabilization point"; border() - size of the inspection border zone. The "AUV stabilization point" is located on the vehicle shape with coordinates: AXCTRL, AyCTRL - in the connected coordinate system and XfCTRL, YfCTRL - in absolute coordinate system.
AUV search trajectory is characterized by the crossing angle function cross() and the trajectory stabilization function border(). Examples of the possible dependences used during
AUV field tests are given below:
cross(pEE ) = COT + COT • exp(- Kcross • pEE) (25)
stob(sCTRL ) = qfë • tanh(Kstob • SCTRL ) (26)
Where: ôminborder u ômaxborder - minimal and maximal border size of the search zone; Kborder -coefficient of border reduction; tymincross u ^maxcross - minimal and maximal cross angle; Kcross -coefficient of the cross angle reduction; tymaxstab - maximal angle of mutual AUV-to-object offset compensation; Kstab - AUV trajectory stabilization coefficient at movement lengthways the inspection object.
AUV control system calculates object survey zone and control heading. Thus, the trajectory of underwater vehicle will represent oscillatory movement along the inspection object (fig.
6, 13). The amplitude of fluctuations is inversely to the object existence probability near the AUV. The maximal estimation of the probability changes movement to a direct line. When estimation is reduced (loss of the object) the oscillatory movements appear again.
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