Marine Pipeline Inspection Using A Tracking Technique

Abstract

Abstract The pipeline tracking technique offers a considerable gain in efficiency in comparison with the currently used pipeline surveying technique. The new technique enables the survey vessel to move more or less parallel to the pipeline, instead of crossing time and again at right angles. The tracking technique employes a standard subbottom profiler together with two detectors, suspended from paravanes on each side of the ship. A colour-television system provides the helmsman with readily intelligible reflection information, which enables him to keep track of the pipeline. The tracking technique was extensively tested in cooperation with the Netherlands Ministry of Transport and Public Works on their survey vessel 'Volans'. The test included surveys in the Dutch, German and Danish sectors of the North Sea over the Pennzoil, Placid and Emden-Ekofisk pipelines, the last-mentioned of which is partly covered with rocks. A smaller diameter interfield line was also tracked successfully. 1. Introduction Marine pipelines carrying oil and gas from offshore fields to onshore storage facilities require periodic inspection. In areas where the pipelines rest on the seabed they are exposed to strong currents, to impacts induced by anchors or fishing gear. Their condition and lateral position must be checked at regular intervals. In other areas such as the Dutch sector of the North Sea, they are frequently buried in the seabed and here it is also necessary to check the depth of burial. In the course of time several systems have been devised to aid in pipeline detection and inspection1,2. Detection is possible, e.g. with video equipment, side scan sonar, and forward or sector scanning sonar if the pipe is exposed. The subbottom profiling system and the (electro) magnetic methods can be used if the pipeline is buried. If pipelines are leaking, hydrocarbon detectors can be used and in the case of electrical leakage from the cathodic protection system, this can also be sensed3. For the determination of burial depth both pipeline and seabottom must be detected. The tool most suitable for this purpose is the subbottom profiler. A subbottom profiler is a device generating acoustic pulses which are transmitted through the water and reflected by the seabed and by any interface that separates layers with different acoustic impedances. The system detects, ampliries and processes the returned signals. These signals are recorded and displayed as a profile on a recorder. The subbottom profiler uses acoustic signals with frequencies in the range from 1 to 10 kHz. The acoustic source is capable of producing a signal with sufficient bandwidth to achieve the required resolution and at the same time sufficient amplitude to achieve the required penetration (up to 50 m). An advantage of the subbottom profiler over the electromagnetic method is that it can be simply operated from a small surface ship, whereas with the electromagnetic method the sensing equipment must be kept in the immediate vicinity of the pipeline (closer than 10 m) owing to the high conductivity of the sea- water. Submarines are used for this purpose.