5.1 Introduction

Pipeline spanning can occur when the contact between the pipeline and seabed is lost over an appreciable distance on a rough seabed. An evaluation of an allowable free-span length is required in pipeline design. Should actual span lengths exceed the allowable length, correction is then necessary to reduce the span to avoid pipeline damage. The flow of wave and current around a pipeline span can result in the generation of sheet vortices in the wake. These vortices are shed alternately from top to bottom of the pipeline resulting in an oscillatory force exerted on the span. This chapter provides information about the determination of the allowable pipeline span length, based on the avoidance of vortex shedding induced oscillations. Both in-line and cross-flow vortex shedding induced oscillations will be discussed and evaluated. There is also a brief discussion and equations are presented for calculating the fatigue life of a pipeline based on the stresses incurred due to vortex shedding induced oscillations in a free span. This chapter will provide information based only on vortex shedding induced oscillations due to currents, which in most deepwater pipelines is the limiting factor for the allowable span length. It does not address vortex shedding induced oscillations due to wave motions or wave and current combined motions.

5.2 Problem Description