Effects of Free Spans on Lateral Stability of Offshore Pipelines: A Local Approach

Current industry practice on determination of necessary weight for lateral stability of offshore pipelines makes many important assumptions on the boundary conditions and initial configurations of a subsea pipeline. The pipe is assumed to be on a flat seabed, the distance between local constraints of the pipeline is assumed to be so large that the boundary conditions at the ends have no influence on the maximum lateral displacement, and the contact between seabed and pipeline is assumed to be uniform. Pipelines are in reality placed on rough seabed, and the contact between pipeline and seabed may be very non-uniform. Therefore it is necessary to investigate the effects of free spans and their influence on lateral stability. Pipelines in free spans experience much lower hydrodynamic loads, including lift, drag and inertia, but at the same time a reduced overall resistance. The local influence of the free span is governed by span gap, span length, soil properties, pipe geometry, effective axial force and pipe submerged weight. Local studies of lateral stability on span shoulders have been performed using a simplified approach according to the absolute stability criterion and penetration models according to DNV-RP-F109, and the influence with respect to design according to the absolute static stability criterion is shown. It is demonstrated in this paper that local displacements on free span shoulders may occur even if the pipe satisfies the absolute stability criterion. However, the overall effects of free spans [2] are fairly minor on the global pipeline behaviour, even for pipes with a high ratio of spans to contact points. Therefore, the displacements demonstrated in this paper are likely to be local to smaller areas on span shoulders.