Autonomous underwater vehicles for pipeline and cable inspections 21 Overview

The current need of energy transport such as electricity, petroleum and gas have provoked an increasing amount of underwater infrastructure such as cables and pipelines. In order to maintain these infrastructures with a suitable degree of safety and reliability, periodic inspections for preventive maintenance are necessary.

Damages in the submarine pipelines due to suspended (free-span) sections, craft anchorages or fishing activities can cause a strong environmental impact and to cut off certain critical supplies or communication lines. Although leakages of a submarine pipeline are not frequent, the consequences of an eventual spillage to the environment may be severe and irreversible. On the other hand, a greater demand of inspections and preventive maintenance are necessary due to pipeline ageing. In addition, many parts of the geographical areas with submarine infrastructure are located in deep waters (500-3500m.), constituting another challenge for the current technology.

A similar situation is observed in the case of preventive maintenance of submarine cables, because there is not an international standard to carry out it. A great reason to introduce these periodic surveys for the preventive maintenance is to reduce the repair time and, therefore, the profit losses due to the impossibility of information transmission through the cable. In addition, the spillages of pollutants of the damaged electrical submarine cables have also a dramatic impact in the fragile marine environment. To minimize it, it is necessary to urgently locate any possible damage, in order to take the necessary precautions for avoiding the pollution. Therefore, the above-mentioned maintenance also includes the recognition (and the corresponding decision about navigation behaviour of the submarine robot) of wastes located in the proximity of the inspected object. Thus, the shape recognition of fishing nets, rocks, mines, anchors, and other debris, should be also considered. There are then two main motivations for preventive maintenance: to avoid infrastructure damage and for ecosystem preservation, which are closely related. Based on the previous observations, it is clear that one of the most outstanding applications for AUVs is pipeline and cable tracking for maintenance purposes. This explains the increasing interest on commercial exploitation of periodical underwater inspections.

Currently these inspections are done with ROV or TUD as mentioned, but these approaches have two basic drawbacks when compared to an AUV without a physical link to the surface: a) the lower quality of acquired data due to umbilical perturbation over position, and b) the higher cost to be invested in the ship and its crew each time that an inspection has to be undertaken. These two unwanted characteristics become enhanced, as the surveys depths are greater. For instance, offshore petroleum exploitation is being shifted to deeper waters as the resource is becoming scant. In contrast, AUVs allow a smother and faster navigation (over the typical three quarter knots of ROVs), and then a more reliable data acquisition is obtained. In fact, an AUV can reach positions in global coordinates and navigate in autonomous way with low position error, and is able to follow certain sensors readings considering them for planning the desired and possible trajectory to be tracked the vehicle's control systems. However, there are other limitations like the pressure that the submarine can stand when the depth increases and the endurance in terms of battery power. These are still open problems from standpoint of new materials.

During the last years successful trials have been done with AUVs applied to cable and pipeline tracking. Among them, the paradigmatic Twin-burger 2, guided by cam images (Balasuriya & Ura, 1998), although for deep and opaque waters it is preferable to use sonar or a fusion of many sensors like in RAIS (Antonelli et al., 2001). Also the EU funded AUTOTRACKER Project, in which the authors participated, was thought to show that the current technology is mature enough to face this autonomous underwater pipelines and cables inspections in deep water up to three thousand meters. Some reports on preliminary successful results may be found in (Evans et al., 2003) and (Acosta et al., 2005). They were the antecedents for the AUVI prototype, also supported by the EU and the University of Balearic Islands, (Acosta et al., 2006), and (Acosta et al., 2007). The AUVI was constructed mainly to test computational intelligence algorithms for planning and replannig of vehicle's trajectories and tasks, and is the ancestor of the current prototype ICTIOBOT.

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