Stimulated Self-Burial of Submarine Pipelines

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Abstract
Under certain conditions submarine pipelines bury themselves down to two diameters below the original sea bed. A field example is presented, and an analysis is given of the possible mechanisms involved. Seeking a wider application of this phenomenon, a stimulated self-burial method is introduced, using fins attached to the pipe. Tentative laboratory tests show promising results. INTRODUCTION The answer to the question whether a specific pipeline on the seabed should be buried or not depends on a variety of arguments, stemming from corporate engineering practice, government regulations, international conventions, etc. All these arguments are somehow used in cost/risk analyses, where factors of widely different nature play a part, each with their own margin of uncertainty. The accumulated uncertainties may create such an amount of maneuvering room that the original question may be answered either way. There are quite some uncertainties involved with a pure technical natural including such items as:pipeline resistance to impactspipeline/bed interaction under currents and wavesdefinition of adequate pipeline stability under hydrodynamic forces and the related design criteria adoptedlast but not least the burial techniques themselves. All items just mentioned are subject to a continuing development of theoretical, empirical and technical know-how, thereby possibly affecting the outcome of the burial question. An interesting development in the area of pipeline burial was the recent discovery of the act that pipelines bury themselves down to three pipe diameters under certain circumstances. A surprising example of this phenomenon in the Dutch North Sea sector will be presented hereafter. Needless to say that this is an important asset in attempting to minimize the capital cost, especially for marginal oil and gas fields. In fact this phenomenon has recently led the Dutch authorities to relax their pipeline burial regulations. Meanwhile, specific research is being conducted in the framework of MATS (Netherlands Marine Technological Research) aiming at a better understanding and subsequent predictability of self-burial for specific pipelines. However, there are natural limits to the self-burial potential of pipelines, depending on pipeline characteristics, hydraulic conditions, and the nature of the sea bed. In order to broaden the applicability of self-burial, and to cope with conditions where plain pipelines would not or not fast enough bury themselves, the Delft Hydraulics Laboratory has investigated the feasibility of a new method (patent pending), called stimulated self-burial. The method essentially seeks to stimulate controlled local erosion by using plastic fins attached to the pipeline. Laboratory tests showed promising results so far, which may lead towards a cheap new pipeline burial method, thereby shifting the answer to the original question in positive direction.

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