Introduction

Regular inspection of underwater communications (pipelines and cables) is actual problem of modern oil and gas industry. Specially equipped vessels, towed underwater devices and remote operated vehicles /ROV/ are applied for these purposes as usually, but quality of acquired data does not always allow revealing emergencies at the proper time. "Spot" inspections by ROVs give difficultly comparable data (Baker, 1991; Murray, 1991). The perspective solution of the problem is autonomous underwater vehicles / AUV/ application as "the intellectual carrier" of research equipment (Evans et al., 2003; Kojima et al., 1997). According (Ageev, 2005) the main goals of pipeline and cables inspection are:

1. more accurate position determination (searching and tracking);

2. pipe sagging and freespan detection and measurement;

3. terrain survey on each side of communication by means of high frequency side scan sonar /HF SSS/ and detection of extraneous objects;

4. detection of damages;

5. leakage detection of transported substances (for pipelines).

The pipeline and cable inspection by means of AUV includes two stages: preliminary (communication search and detection) and the main (motion along the communication with carrying out of necessary measurements, i.e. tracking). Exact mutual orientation of AUV and inspected object is required in real time during the tracking stage.

To solve inspection tasks AUV should be equipped with reliable detection systems for inspected object recognition. Video, electromagnetic and echo-sounder data can be used for these purposes. Each of these devices demonstrates optimal results for certain classes of objects in appropriate conditions. For example, metal pipelines have the significant sizes and can be detected by all listed above devices. While underwater cables have a small diameter, because of this applicability of acoustic methods is limited (Petillot et al., 2002). Process of communications search and detection is complicated, as a rule, with a poor visibility of the given objects (strewed with a ground, silted or covered by underwater flora and fauna).

Experiments with the use of AUV for inspection of underwater communications have been carried out for a long time. Usually only one instrument, which AUV is equipped with, is used for object detection.

The first experiments with detection and inspection of metal cables were carried out with the use of AUV "Aqua Explorer 2" (Asai at al., 2000). The AUV was equipped with two external magnetometers which allowed to find out metal cables. However the devices allowed to find out only active cables (i.e. cables with electric current). Devices did not find out cables where there was not electric current for any reasons (for example, because of breakage). Moreover AUV had significant transverse dimensions because of magnetometers installation on pylons.

The basic possibility of video camera use for automatic cables detection and tracking was shown in (Matsumoto & Ito, 1995; Ortiz at al., 2000). And the firsts practical results were obtained with the use of semi-AUV TSL (Scherbatyuk at al., 2000) and ROVs Ventana and Tiburon (Kogan at al., 2006).

The experiments of pipelines tracking with the use of multi-beam echo sounder were carried out on the base of AUV AUTOTRACKER (Petillot et al., 2002) and revealed good enough results. However the used facilities allowed to detect the inspected object only at strict mutual orientation of AUV and the pipeline.

Detection reliability of lengthy metallic objects (cables and pipelines) can be considerably increased by means of synchronous data processing of the all recognition devices. In other words, it is necessary to equip AUV with a plenty of detection systems which work on the basis of different physical principles. For the sake of reliability the information from the different sources is combined into "environment model" and processed jointly. "Environment model" is updated on the basis of new data and can be used for subsequent AUV motion planning. The questions of object recognition, "environment model" structure and AUV behavior during inspection are considered in this paper.

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