Analysis of the Dynamics and Stability of Visco- Elastic Pipes Conveying a Non-Newtonian fluid.
A Thesis Submitted to the School of Postgraduate Studies, University of Lagos.
Flow induced vibration of pipeline and riser systems are strongly dependent on internal fluid flow parameters as well as the mechanical properties of the conveyance vessel. The steady growing demand on the transport of primary energy sources such as oil and gas necessitated larger diameters and higher operating pressures. In order to reduce the amount of steel in the pipes there was a search for higher strength materials that could be used to reduce the wall thickness, as permitted by advances in manufacturing processes. Some studies on the mechanics of sandwich elastic systems as effective vibration and noise reduction mechanisms have stimulated the possibility of replacing stainless steel pipes with sandwich pipes especially in deepwater environment. Recent analysis has indicated that crude oil is a Newtonian fluid at high temperature, but exhibits non-Newtonian behaviour due to its complex mixture of hydrocarbons. This thesis presents our attempt to find an alternative material to steel as offshore fluid conveyance medium, by analytically investigating the dynamics of viscoelastic pipes in offshore environment using Euler-Bernoullis beam theory. By idealising the viscoelastic pipeline resting on the sea bed as a viscoelastic beam that is resting on an elastic continuum, a non-linear, boundary value partial differential equation governing the fluid- structure- soil interaction mechanics is formulated. The material property of the beam-model pipe is described by the Kelvin-Voigt type viscoelastic constitutive relation. By linearizing the governing partial differential equation matching the problem physics, under slight perturbation of the internal fluid velocity and other flow variables closed form analytical results for the system dual natural frequencies and consequently stability under external excitation are computed for field designs. We were able to find out that stability for both single and sandwich viscoelastic pipes is low in offshore environment compared with that of steel pipe. However, when sandwich pipes are arranged in such a way that thicker viscoelastic material is on the inside and thinner steel material on the outside, natural frequency is tremendously improved which demonstrates good stability. Results also show that the more the flow behaviour index of the conveyed fluid the more the stability, which makes dilatant fluid to be more stable than Newtonian fluid which in turn is more stable than pseudoplastic fluid. In the case of sandwiched pipes, the results obtained in this work emphasized the importance of arrangement of the pipes for offshore applications.