Effect of Spoilers on Submarine Pipeline Stability

ABSTRACT In previous OTC papers the emphasis of the, spoiler effect was on the improved self-burial process itself, as a cost-saving alternative to mechanical trenching. The present paper presents important new aspects which yield attractive additional cost reductions, even if no complete burial is required. These aspects were studied and quantified using advanced numerical models as well as small scale and large scale laboratory experiments. Important spoiler effects are:Much quicker tunnel formation and pipeline subsidence immediately after pipelaying.Significant, favorable change in hydrodynamic forces.Through the combined spoiler effects, pipeline stability increases to such an extent that remarkable reductions are allowed on concrete weight coating, with an additional gain in pipeline safety, since spanning problems and vortex shedding are much reduced and the pipeline is well embedded after a very short time. Results of a field test are presented, based on observations and measurements concerning the effect of a spoiler on the self burial performance of an actual laid pipeline. It is concluded that the spoiler significantly enhances the self burial result, even in circumstances where otherwise less optimum conditions for natural self burial occur. INTRODUCTION Four years after the first publication [1], the patented, scour-stimulating submarine pipeline spoiler has turned from a mere promising idea into a full-scale tested, cost-saving product, which has been applied for the first time in the North Sea in June, 1988. During an extended period of research and development, frequent communications were held with operators, fishery representatives, pipeline engineering companies, and the Dutch State Supervision of Mines (as the main source of pipeline burial regulations for the Dutch North Sea sector). In this way many valuable comments and suggestions have been incorporated both with respect to the design of the final product and with respect to the type of tests and analyses which have been conducted in order to prove its function and its cost-effectiveness. In the course of this evolutionary process, some interesting but somewhat impractical research items of the spoiler have been truncated, in favour of a much more practical approach towards a no-nonsense product which combines an ingenious exploitation of naturally occurring near-bed flow phenomena with relatively simple hardware and straightforward application. In the crucial phase of pipeline stability analysis under currents and waves, full use has been made of the remarkable ways in which flow kinematics, sediment transport, and pressure distribution in the immediate vicinity of a submarine pipeline are affected by the spoiler. Practical implementation of innovations in the offshore branch is generally subject to hesitation. Operators want to deal with proven concepts. They are eager to cut costs, but not at the expense of operational safety. The practical application of spoilers on submarine pipelines forms no exception to this rule. It is a matter of step by step strengthening the proof, thereby reducing the doubts. After all, even full-scale laboratory tests do not completely cover all aspects of real conditions.