Dynamic interaction of subsea pipeline spans due to vortex-Induced Vibrations

When currents are present around a subsea pipeline, vortices are shed alternately from the upper and lower sides of the pipe. The vortex shedding can induce vibrations in areas where the pipe is suspended above the seabed. These vibrations are called Vortex-Induced Vibrations (VIV). VIV have been and still are a cause of concern for pipeline designers. If a pipeline vibrates during a large part of its design life, then damage due to fatigue is expected. It is therefore of great importance to understand how pipelines respond to the flow-induced vortices and how the vibrations can be mitigated. Recent research has addressed the dynamic interaction of adjacent free spans. Dynamic interaction can occur when two or more parts of a pipeline that are suspended above the seabed are separated by a narrow support. In that case the vibration of a span can be different than when it is considered separately. This area is the focus of this research. The aim is to give more information on how the support properties effect the dynamic interaction of the free spans. The properties under investigation are: the support length and the type of soil model. For a multi-span, nonlinear soil behaviour such as pipe lift off could play a role in the vibration. The implication of this would be: change of dominating response frequency, different vibration shape of the pipe and impact loads on the pipe. In this thesis a model is proposed that calculates VIV of a subsea pipeline; the Pipe-Wake model. This model allows for response of the pipeline in the time-domain, which makes it possible to add nonlinear soil effects. For clay soils a suction model is proposed that allows the pipe to break free from the seabed after the suction force reach a threshold value. For sand two models are possible: no suction or full suction. The former means that in upward motion no soil resistance is noted, while in the latter the soil resistance is equal in both directions. The VIV are modelled using a wake-oscillator. A wake-oscillator is a phenomenological model that captures the behaviour of VIV accurately. The two main aspects of this are the lock-in region and the self-limiting vibration. The pipe itself is modelled as an Euler-Bernoulli beam using the Finite Element Method. A nonlinear static analysis is used to calculate the pipes initial position. For the dynamic calculation the stiffness of the beam is linearized around the static deflection. Modal analysis is used to calculate the response of the pipe. A soil model, which is dependent on soil type is then added to complete the Pipe-Wake model. With this model the case study is replicated. The general conclusion of the research is that simplifying the support by either shortening it, or by assuming that soil behaves the same for both soil penetration and release, is an oversimplification in the case of a multi-span set-up. For high flow velocities this can lead to disconnection of the pipe from the support. The consequence of this is larger vibration amplitudes for lower flow speeds in relation to linear soil behaviour. Modelling the support shorter than it is in reality will lead to a drop in natural frequency and mode shape. It is therefore recommended to verify these conclusions experimentally and see if this would have implications for the design of multi-spanning subsea pipelines.Bottom Founded StructuresOffshore EngineeringMechanical, Maritime and Materials Engineerin