Spoilers For Stimulated Self-Burial Of Submarine Pipelines

ABSTRACT Full-scale laboratory tests on a 20" O.D. pipeline with a pair of 5" spoilers show a tremendous increase in local scour as compared to a plain pipeline, indicating a strong self-burialpotential. INTRODUCTION Spoilers on pipelines cause an increase of local turbulence and thus stimulate local sediment pick-up. As a result, the pipeline settles in its self-made trench and may be completely covered afterwards by natural backfill. In previous papers [I, 2] the concept of spoilers for stimulated self-burial of submarine pipelines has been introduced (patents pending), and an explanation of the pipeline burial process was given in terms of alternating "tunnel-erosion" and "leeside erosion".The concept was validated by small-scale laboratory tests. Stimulated self-burial may be applied where the sediment transportability under the given hydraulic conditions meets certain minimum requirements. In these conditions, the new method is much cheaper than artificial trenching. As compared to self-burial of plain pipelines on the other hand, stimulated self-burial significantly increases the speed, the ultimate burial depth, and the area of applicability in terms of sediment characteristics (silty and shelly sand) and hydraulic conditions. As a major step towards a proven, commercially applicable product, an extensive test program was recently started in order to demonstrate the effect of spoilers in full-scale conditions. In this paper, the first results are presented which deal with tunnel-erosion underneath fixed pipelines in uniform flow. TEST SET-UP AND CONDITIONS The aim of the tests was to check whether the previous small-scale test results [1] would be corroborated if carried out at full scale. Therefore the pipeline was conditioned in the same way as before, viz. fixed in between the side panels of a channel with a very small initial gap between the pipe and the sand bed. The tests were carried out in a 100 m long, 3 m wide and 3 m deep channel. Unidirectional flow with a depth-averaged velocity V = 0.60 m/s was applied. There were no waves. Over a length of 75 m a sand bed was present with D50 = 0.22 mm and with a thickness of 1 m. The waterdepth over the sand bed was 1.70 m. The pipeline was installed half-way the sand bed length, perpendicular to the flow direction. First a plain 20" O.D. (0.50 m) pipeline was mounted. The steady current caused tunnel erosion. During the development of the scouring process the bed configuration was monitored by echo sounding in three ranges along the channel, until approximately equilibrium depth was attained. Then,5" (0.125 m) high spoilers were mounted both on top and underneath the pipeline, and the test was continued with the same hydraulic conditions. In this way a good impression can be obtained of the additional scouring potential of spoilers, as compared to the scour caused by a plain pipe.