The Effect of the Sea-Bottom Proximity on the Fatigue Life of Suspended Spans of Offshore Pipelines Undergoing Vortex-Induced Vibrations
Authors: Tsahalis Demos T. , Jones Warren T. 
Published: October 2008

Abstract
Fatigue life calculations have been made for suspended spans of offshore pipelines exposed to steady flow and undergoing vortex-induced vibrations. The effect of the proximity of the sea bottom was taken into account by utilizing the results of related model tests. It was found that the effect of the sea-bottom proximity is to increase the fatigue life of a suspended span of given length or equivalently to increase the safe length of a suspended span for a given fatigue life. Furthermore, the effect of the pipe-to-wall gap to pipe diameter ratio on the fatigue life of such spans has been determined.

INTRODUCTION
A steady flow about a bluff body, such as a pipeline with a circular cross section, will separate and form a wake. Because of the instability and mutual interaction of the separated shear layers, vortices form and periodically shed from alternate sides of the body. The result is steady and unsteady drag forces in line with the flow and unsteady lift forces? in the transverse, or cross flow direction. If the pipeline is flexible and/or flexibly supported, it begins to vibrate under the action of these periodic forces. The vibrations of the pipeline modify the flow and lead to a nonlinear interaction between the elastic and the fluid systems. The maximum amplitude of inline vortex-induced vibrations is much smaller than the maximum amplitude of transverse vortex induced vibrations, so their contribution to the fatigue life of a pipeline is small. Consequently, attention is focused on the transverse vibrations which will be referred to as "vortex induced vibrations" hereafter. In the last ten years a number of comprehensive overviews of this important class of fluid-structure interaction problems have appeared [1-4].
Experience with unburied offshore pipelines laid in strong current areas has shown that such pipelines may develop unsupported spans due to sea bottom being scoured out from under the pipe due to current action. In turn such unsupported spans exposed to currents may undergo vortex induced vibrations which may affect the fatigue life of the pipeline.
Due to lack of data for the vortex-induced vibrations of a flexible cylinder in close proximity to a plane boundary, fatigue life calculations of suspended pipeline spans that determine the maximum safe length of such spans have been based, out of necessity, on data for the vortex-induced vibrations of an isolated flexible cylinder, i.e., no wall effect.
In a recent experimental study, Tsahalis and Jones [5] determined the effect of the proximity of a plane boundary on the vortex-induced vibrations of a flexible pipe. They found that the proximity of the plane boundary has the following effects on the response of the pipe as compared to the response of the isolated pipe, i.e., no wall effect (see also Figure 1). First, the increase of the response frequency with the flow velocity is less steep. Second, the first perceptible vibrations take place at a higher flow velocity. Fourth, the maximum amplitude is achieved at a higher flow velocity. Fifth, the maximum amplitude is reduced. Sixth, once the maximum amplitude is achieved it appears that it remains constant for higher flow velocities.