INTERSUB has pioneered the development of instrumented manned submersibles to inspect large production pipelines (see OTC paper n° 3071, 1978). Such vehicles provide detailed information on pipeline condition, but their efficiency is not optimal since dive duration is limited and collected data have to be processed off-line after each dive on the support vessel. The use of unmanned tethered submersibles significantly reduces these drawbacks thanks to their umbilical: unlimited power can be transmitted to the vehicle from the surface and data can be collected and processed in real time permitting 24 hour operations. However, operational and technical problems must be solved such as navigation and tracking of the vehicle, remote control and real time data processing.
The paper describes the technical solutions that have been selected to form a system operated in the North Sea during the 1979 season: an unmanned submersible with a full autopilot capability, pipeline inspection sensors, computer software for real time navigation, data processing and chart plotting, and a specially converted support vessel with dynamic positioning and automatic tracking capabilities. The paper shows how the various components of this system have been combined together into a coherent and operational spread. Emphasis is put on reliability and weather limitations and operational results will be presented. This system constitutes a new and very advanced application for ROVs where compared with their traditional use as remote TV cameras and significant advances have been made, such as: the operation of a large unmanned vehicle, or the application of dynamic positioning to the tracking of a moving target. Cost effectiveness of the new system will be presented and the conclusion will be devoted to a technical, economical and operational comparison with manned submersibles.
Large production pipelines have to be regularly inspected in detail to check status. At the present time, most of the work is achieved by using manned submersibles. These vehicles are extensively instrumented and besides video inspection can perform the different measurements as required by the customer: depth of burial, corrosion and anode status, shape and evolution of the trench, etc ". These measurements combined with accurate positioning are recorded on cassettes inside the sub, then, after the recovery, the are processed by computer on the support vessel. The drawbacks of this technique are indeed the limited dive duration (maximum 8 hours) of the manned subs and the fact that the customer gets the results of the survey only after the dive. Consequently, if some areas of the pipeline appear suspect, a second survey may be necessary for a closer inspection. The use of Remotely Operated Vehicles (R.O.V.'s) can overcome the dive duration drawbacks, having the vehicle powered from the surface by the umbilical. However, existing R.O.Vs. were poorly instrumented and the operation of these vehicles was very delicate due to the fact that the ship could not maintain a fixed relative position over the vehicle, endangering the umbilical and eventually dragging the vehicle.