ABSTRACT

A technique has been developed for acquiring and analysing the vibration signature of pipeline freespans The information derived can provide a cost effective means of avoldlng unnecessary free span rectification work

When assessing pipeline freespans, two aspects require to be considered separately First, the stresses occurring in the span as a result of its own weight and current drag require to be estimated Second, the susceptibility of the span to vortex Induced vibration requires to be considered In the absence of any quantitative measurements, span assessment requires to be made on the basis of conservative assumptions regarding such unknowns as span length, end fixity, axial tension etc

Careful analysis of the vibration signature gives a quantitative measurement relating to each of the above, in addition to allowing more direct assessment of the vulnerability of the span to vortex shedding

Experience has shown that the application for this system can dramatically reduce the rectification work required

1. INTRODUCTION

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 thls paper is to outline the capabilities of a quantitative span 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, Statoll, Norsk Hydro, Total and Texaco Following these trials the system has been used successfully in the field throughout the summer of 1988, 1989 and 1990

In the following, there is a brief discussion of span assessment criteria This is followed by a description of the information which can be gained from the pipespan vibration signature. Finally, the instrumentation required to obtain a free span vibration signature during a pipeline survey is described

2. Span Assessment

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

2 1 STATIC STRESSES

The bending stresses in 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

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