This paper describes a method to study multi-spanning subsea pipelines undergoing vortex-induced vibrations (VIV). When spans are located close to each other, the behaviour of one is influenced by its neighbouring span. This makes the analysis of fatigue life more complicated as it is not yet fully known how this interaction affects the response of each span.
The numerical model herein proposed is formulated in the time domain and calculates the VIV response of free spans while accounting for pipe-soil interaction. The pipe is modelled as an Euler-Bernoulli beam and solved with finite elements and modal superposition. The soil is modelled as external forces, which depend on pipe position and velocity in time. This allows the pipe to disconnect from the soil during vibration. A non-linear static analysis is performed to find the initial deflection of the pipe (due to weight and drag forces), and thereafter the stiffness of the pipe is linearized around this equilibrium position. The novelty of the model is the consideration of the non-linear pipe-soil interaction (loss of contact) on the prediction of VIV amplitudes for multi-span configurations.
Due to irregular seabed conditions a subsea pipeline can never completely follow the contours of the bed. This leads to free spans, which are parts of the pipe that are suspended above the seabed. These spans are subjected to vibrations due to flow passing around the pipe, the so-called vortex-induced vibrations (VIV). In the design of the pipeline these vibrations have to be taken into account as they induce fatigue damage during the life of the pipe, affect allowable stress design checks and can damage concrete coatings.
Often, free spans are located close to one another. If two or more free spans are said to be so close together that they interact, the spans are defined as a multi-span. Free spans in a multi-span configuration can vibrate significantly different than if the spans were isolated, which is why in multi-span analysis the neighbouring spans have to be taken into account. This paper proposes a time-domain model using a wakeoscillator to predict the vibration of multi-span pipelines in which proper support conditions (between spans) are considered.
The DNV-RP-F105 code is commonly used in the design of pipelines. This code predicts the maximum VIV amplitude based on structural and hydrodynamical properties. It relates fatigue life to the natural frequencies and mode shapes of the pipe, and assumes linear soil behaviour. However, soil is shown to behave nonlinear, which means the pipe may lift up from the bed. This can have severe consequences when investigating multi-spans.
