Showing papers by "Jørgen Fredsøe published in 1993"

Influence of cross section on wave scour around piles

Abstract: The study extends the investigation of scour around circular vertical piles in waves, reported in 1992 in two works by Sumer et al. In the present paper, the influence of crosssectional shape of pi...

A Review of Wave/Current-Induced Scour Around Pipelines

Abstract: A comprehensive review is presented of scour around pipelines in the case of noncohesive sediment. The review is organized In four main sections, namely the two-dimensional scour, the three-dimensional scour, the eifect of scour on forces on and vibrations of pipelines and the mathematical modelling ofscour process. Over sixty works were included in the review.

Self-Burial Of Pipelines At Span Shoulders

Abstract: The process of self·burial of pipelines at span shoulders has been investigated experimentally in the laboratory in steady currents. Only the noncohesive sediment bed is considered. A rigid cylinder has been used as the pipe model. In all the tests, the experimental conditions were arranged such that the pipe is supported by a sand ridge in the middle. When the length of this supporting ridge is decreased to a critical value, the pipe begins to sink in the sand due to soil failure. The scour, the sinking and, at a later stage, the backfilling processes were monitored by video in plane and side views simultaneously. A simple formula adopted from soil mechanics regarding the bearing capacity of soil was found to give a satisfactory result in relation to the sinking of the pipe at the span shoulder.

Experimental investigation of wave boundary layers with a sudden change in roughness

Abstract: This study deals with turbulent oscillatory boundary-layer flows over a plane bed with a sudden spatial change in roughness. Two kinds of ‘change in the roughness’ were investigated: in one, the roughness changed from a smooth-wall roughness to a roughness equal to 4.8 mm, and in the other, it changed from a roughness equal to 0.35 mm to the same roughness as in the previous experiment (4.8 mm). The free-stream flow was a purely oscillating flow with sinusoidal velocity variation. Mean flow and turbulence properties were measured. The Reynolds number was 6 × 106 for the major part of the experiments, with a maximum velocity of approximately 2 m/s and the stroke of the motion about 6 m. The response of the boundary layer to the sudden change in roughness was found to occur over a transitional length of the flow. The bed shear stress over this transitional length attains a peak value over the bed section with the larger roughness. It was found that the amplification in the bed shear stress due to this peak could be up to 2.5 times its asymptotic value. Also, it was found that the turbulence is quantitatively different in the two half periods; a much stronger turbulence is experienced in the half period where the flow is towards the less-rough section. The present experiments further showed that a constant streaming occurs near the bed in the neighbourhood of the junction between the two bed sections. This streaming is directed towards the section with the larger roughness.

Forces On a Pipeline Oscillating In Transverse Direction In Steady Current

Abstract: This study deals with the forces on a cylinder placed near a plane wall and exposed to a steady current, the cylinder simulating the suspended span of a pipeline and the wall the sea bottom. The cylinder was oscillated (forced oscillations) transversely in a direction perpendicular to the flow. Instantaneous pressure distribution around the cylinder was measured, and the in-line and lift-force components were calculated from the pressure distributions. Reynolds number of the study was about 6 x 104. The range of the reduced velocity was from 3 to 11. Three different values of the gap-to-diameter ratio, e/D, were tested, namely 1, 0.6 and 0.35. The values of amplitude-to-diameter ratio A/D were 0.9, 0.75, 0.5, 0.25 and 0 for e/D = 1: A/D = 0.5, 0.25 and 0 for e/D = 0.6 and A/D = 0.25 and 0 for e/D = 0.3. The results indicate that both the in-line force and the lift force are influenced by the bed proximity. The results demonstrate that the energy is transferred from the flow to the cylinder for small amplitude vibrations (such as A/D = 0.5 and below), for a certain range of the reduced velocity including the lock- in region but excluding large values of the reduced velocity. For large amplitude vibrations (such as A/D = 0.75 and above), however, the energy transfer takes place in the opposite direction, from the cylinder to the flow, and this occurs even in the lock-in region. The latter suggests that the upper limit for transverse vibrations of a flexibly-mounted cylinder would lie somewhere between A/D = 0.5 and 0.75. The existing data confirms this result.