Water Hammer Induced Vibration of Steel Pipelines Repaired with FRP Composites

Abstract

Since the 1990s, there has been an increasing trend to repair corroded or eroded pipelines, conveying oil, gas or water, by using fibre-reinforced polymer (FRP) composites. This relatively new repair technique involves wrapping the corroded part of the pipeline with a so-called FRP overwrap. FRP materials are lightweight, have high relative strength and do not corrode, making them an effective repair solution.

The viability of this repair technique has been proved by the numerous research programs which have been performed. However, most of the literature regarding the design of FRP overwraps does only consider a static internal pressure. In this thesis, the behaviour of steel pipelines, repaired with FRP overwraps, subjected to water hammer conditions, has been investigated. Water hammer is the occurrence of pressure waves in the conveyed liquid, due to abrupt changes to steady flow conditions, for example the rapid closure of a valve.

An approximate dynamic model, describing the radial vibration of steel pipes with a FRP overwrap, due to water hammer conditions, has been derived. The model is based on the theory of thin-walled cylinders, and the laminate stiffness matrix for a FRP laminate. In order to take the steel pipe into account, the laminate stiffness matrix was modified. Basic water hammer theory was used to find the magnitude and velocity of the water hammer-induced pressure wave. These properties defined the exciting load in the dynamic analysis of the repaired pipe wall. The derived governing equation was solved analytically by applying boundary conditions and utilising the properties of Fourier series. This resulted in series expressions for the radial deflection and the pipe wall stresses, as functions of the distance from the valve, and time since valve closure.

The model was also implemented on representative examples, with two different FRP materials; E-Glass/Epoxy and T300/5208 Carbon/Epoxy, and the influence of the thickness of the FRP overwrap was investigated. For both these materials, the model predicted the maximum radial deflection, due to the water hammer, to decrease if the overwrap was thin, and increase if the overwrap was thick. The reason was found to be that the natural frequency of the pipe is significantly altered when a thick FRP overwrap is applied, because of its low density, compared to the steel pipe. As the FRP overwrap gets thicker, the water hammer-induced vibration approaches a state of resonance, increasing the amplitude of the vibration. The rate, at which the amplitude increases, with respect to the thickness of the overwrap, will depend on the density and stiffness of the FRP material. The increased amplitude nevertheless causes an increased maximum radial deflection, and thereby increased stresses. During the design of a FRP overwrap for a pipe which is susceptible to water hammer conditions, it will therefore be important to ensure that the changed natural frequency does not lead to unacceptable stresses.