The occurrence of free spans on subsea pipeline is, In many situations, inevitable as a result of current action and other influences. If the length of a free span becomes excessive, then rectification is required to ensure that the pipeline is not vulnerable to damage. In the past, the length of pipeline free spans has been estimated visually from ROV video records It is widely acknowledged that this is often a difficult and unreliable estimate.
The purpose of this paper is to outline the capabilities of a quantitative pan assessment technique, based on measuring the vibration signature of the free span The technique has evolved from a concept first proposed by BP personnel The initial development work was funded by BP and subsequent trials were supported by a consortium including BP, Statoil, Norsk Hydro, Total and Texaco. Following these trials the system has been used successfully In the field throughout the summer of 1988 and 1989
In the following, there is a brief discussion of span assessment criteria. This is followed by a description of the information which can be gamed from the pipespan vibration signature. Finally, the instrumentation requires to obtain a free span vibration signature during a pipeline survey is described.
There are at least two reasons why the length of a pipeline freespan may be critical First, the sag of the pipeline under its own weight may lead to a failure of the pipeline as a result of excessive bending stresses Second, if the natural frequency of the span is sufficiently low, it is well known that transverse vibration can occur as a result of the action of vortex shedding induced by current flowing across the pipe.
These two failure mechanisms are considered briefly in the following
Static Stresses- The bending stresses m a span can in principal be calculated from a knowledge of the geometry of the pipeline, the span length etc However, there remain two difficulties. First, the nature of the end conditions of the span are unknown Maximum stress calculations could be based on anything from simply supported to fully fixed conditions In addition, the touchdown point is often difficult to locate precisely, resulting in a degree of uncertainty in the span length.As a result of these uncertainties, it is quite possible for estimates of the stress in a pipeline free span to be in error by a factor of 2 or more in the absence of better information, conservative assumptions must be made. Similar considerations apply to vortex shedding analysis. When a current flows perpendicular to a pipeline, vortices are shed alternately from the upper and lower side of the pipe, giving rise to a dynamic transverse force.
Vortex Shedding- Vortex shedding becomes of Importance if the natural frequency of the pipeline free span is matched by the vortex shedding frequency. In this event, large amplitude vibration can occur, potentially leading to pipeline failure as a result of accumulated fatigue damage. In principle, the natural frequency of a span can be calculated from a pipeline survey based on the estimated span length. As before, however, the difficulties in assessing span length and end conditions introduce large uncertainties. To summarise, there are two separate calculations required to determine whether a particular free span is critical Both rely on input data which in the past was either difficult to obtain or unavailable As a result, conservative assumptions must be made and it appears that resources are frequently wasted in rectifying spans unnecessarily.
