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This paper presents a k- turbulence model for simulation of steady current and itsinduced vortex shedding caused by the presence of an offshore pipeline. Performance of the modelaround a circular cylinder above a wall with gap to diameter ratios of 0.1, 0.35 and 0.5 underdifferent flow regimes with Reynolds numbers of 1500, 2500 and 7000 is studied. The flow field iscomputed with solving the Reynolds Averaged Navier-Stokes equations (RANS) the seabed underpipeline is treated as a plane boundary with no-slip boundary condition on pipe surface. Thegoverning equations are solved using Finite Volume Method in a Cartesian coordinate system.Based on the numerical solutions, the flow field, vortex shedding and distribution of shear stressdue to the presence of the pipeline near seabed are studied. In addition the mechanism of vortexshedding with different gap to diameter ratios is examined with focusing on the effect of vortexshedding on bed shear stress. It is found that the k- turbulence model can well predict the flowfield and its induced vortex shedding around a pipeline hence it can be easily applied forsimulation of scour below an offshore pipeline.

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Determination of allowable free span length plays a crucial role in design of offshore pipelines. The seabed intervention cost and safety of an offshore pipelines project are largely influenced by pipelines free spanning during the project life time. Different criteria are proposed by both the current designing guidelines and researchers there is however lack of comprehensive assessment of independent parameters affects the design length of free span. In this note, it is intended to investigate the effects of seabed formation along with axial force on Natural Frequency of offshore pipelines. Based on this assessment a new simple formula is proposed. Finally, to evaluate the result of this study, the allowable free span length of Qeshem Island pipelines is calculated as a case study and compared with those of the DNV (1998) and ABS (2001) guidelines and the modal analysis.

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In this paper, a two-dimensional Reynolds Averaged Navier-Stokes (RANS) model is developed to simulate the shoaling, breaking and overtopping of a solitary wave over a vertical breakwater. Turbulence intensity is described by using a k turbulence closure model and the free surface configuration is tracked by Volume Of Fluid (VOF) technique. To validate the numerical model the simulation results is compared with the Xie (1981) experimental data and a very good agreement between them is observed. The results revealed that wave height and wave energy decrease considerably during the reflection from vertical wall, which illustrates a considerable energy lost during the impaction and wave overtopping process. The turbulence production during the broken wave interaction with vertical breakwater is very significant consequently the vertical breakwater undergoes sever turbulent and dynamic drag force.