1 Proceedings of OMAE 2010 29 th International Conference on Offshore Mechanics and Arctic Engineering June 6-11, 2010, Shanghai, China OMAE2010 - 20094 ON THE MECHANICS OF PIPE WALKING: CASE OF A BURIED PIPELINE Vincent O. S. Olunloyo Department of Systems Engineering Faculty of Engineering University of Lagos Charles A. Osheku Department of Systems Engineering Faculty of Engineering University of Lagos Adekunle O. Adelaja Department of Mechanical Engineering Faculty of Engineering University of Lagos ABSTRACT The influence of soil sedimentation on the pipe walking phenomenon is investigated analytically via a set of coupled nonlinear partial differential equations where the effects of operating variables such as internal fluid temperature variation, prestress and internal pressurization, Coriolis and axial accelerations of the internal fluid and cross sectional area change are fully captured. For this problem, a segment of an offshore pipeline resting on the ocean floor is idealized as elastic beam on an elastic foundation using recently refined Euler-Bernoulli beam theory. By invoking integral transforms, closed form analytical expressions for displacement of the fluid-pipe-soil interaction system associated with pipe walking is computed. Simulated results showed that pipe walking phenomenon is strongly dependent on sedimentation level, friction at the interface of pipe- ocean sub soil layer, temperature variation, fluctuations in internal fluid pressurisation and oscillatory strain of the pipe in both transverse and longitudinal modes. Keywords: transverse and longitudinal vibrations; fluid conveying beam, pipe walking phenomenon 1.0 Introduction In practice, pipelines are usually subjected to high temperature, high pressure (HT/HP) conditions especially in deep and ultra deep offshore locations. However, the tendency for slight variation particularly with repeated heat up and cool down cycles can set up flow variables fluctuation along the flowline. In recent studies, part of failure modes in subsea pipelines are attributed to pipe walking phenomena associated with fluid conveyance in field operations resulting from pipe elongation along the axial direction. It has been observed that where temperature gradients arising from significant temperature differentials from one end of pipe to the other there is net overall displacement towards the cold end. A cumulative axial displacement of 7m (Carr et al 2003) and walk per cycle of between 15mm and 350mm (Carr et al 2006) were reported for a 2 km long pipeline. Transient temperature profiles were also presented and postulated to be responsible for this phenomenon. For the case of pipelines under the North Sea, hot fluids typically enter from one end to be discharged at the other while heat is lost to the surrounding seawater along the pipeline. As the process continues, the pipeline is progressively heated up along its entire length and this process gives rise to non-linear strain along the pipeline. However these pipelines are susceptible to sediment coverage associated with debris flow process, at the bottom of the ocean. On the other hand, movement of sediment at the bottom of the ocean can be initiated from construction and maintenance exercises such as excavation and backfilling operation on trenched or buried offshore pipelines. Within the context of soil sedimentation modelling some interesting results have been reported, such as: duration and rate of sedimentation (Long et al, 1998); sediment concentration (Julien, 1995); nature of construction activities (Ritter, 1981; TERA, 1996); nature and sizes of particles in relation to their tendency to precipitation (Bandaloo, 1978 and Anderson, 1998) and the rise in bed material resulting from increase in sediment deposit (Conner et al, 1993). However, these investigations have not taken into consideration effects of sedimentation on pipeline vibration at the bottom of the ocean. Recently when Olunloyo et al (2004, 2007a) investigated the effect of sediment cover on the level of vibration of subsea pipelines in the transverse direction they concluded that irrespective of the seabed geology, increased sedimentation cover helps to control the level of vibration.