Scour-induced Free Spans

Abstract

ABSTRACT The paper reviews the origin and effects of scour induced free spans as part of the self lowering process of submarine pipelines. Aspects of single free span development like initial embedment, the onset of scour, the rate of scour development, relevant stopping criteria, multiple span formation and critical free span development are described and discussed. 1. INTRODUCTION Self lowering of submarine pipelines is known to happen in several parts of the North Sea. Regular surveys of self lowering pipelines gave good insight in the overall result of self lowering and self burial. However, the actual interaction between seabed and pipeline is more complex and is not yet completely understood. This paper reviews the origin and the effects of scour induced free spans as part of the self lowering process of submarine pipelines in erodible seabeds. 2. SINGLE FRRE SPAN DEVELOPMENT Immediately after laying of the pipe, it will settle somewhat in the seabed. Because of:Pipeline installation Increased contact pressure at the touchdown point due to the suspended span, and dynamic effects of the laying operation will contribute to initial embedment in the order of 2-3% of the pipediameter (in case of medium sand).Small scale hydrodynamic action Even moderate wave and current actions will induce hydrodynamic forces which lead to, albeit small, lateral motions of the pipeline which will also contribute to initial embedment. Hydrodynamic forces representing 10-20% of the submerged weight have induced cyclic lateral motions and eventually settlement of 5-15% of the pipeline diameter. See Figure 1 and 2.Seabottom unevenness The presence of small scale bottom irregularities at the moment of installation will result in non-uniform contact pressures and thus contribute to non-uniform initial embedment. It may be enough to avoid gaps under the pipeline in the "troughs" and embedment in the seabed corresponding to the average ripple height may be obtained. Scour induced free spans need a start. Sometime, somewhere along the pipeline there must be an onset of scour. Due to irregularities in the seabed and variations in initial embedment, there will most probably, be locations along the pipeline where the scour will be initiated.In case no gaps are present, the onset of scour will occur at locations where the embedment is minimal, or at locations where the soil conditions are such that the wave generated pressure differences between either sides of the pipeline cause the intergranular flow in the sand, under the pipe, to initiate scouring.If small gaps are present, those gaps will cause the onset of scour. Model investigations showed that even in situations with moderate tidal velocities scour under a pipeline will occur due to increased bed shear stresses in almost any small gap below the pipe, also in cases with no sediment transport on the flat seabed. After onset of scour, a scour induced free span will develop:–the gap under the pipe will deepen till equilibrium depth;