Seabed shear stress amplification around subsea pipelines: Part 1, 2D parametric study with currents

2014 ◽  
pp. 317-324
Author(s):  
T Griffiths ◽  
F Zhao ◽  
M Kalkhoven ◽  
W Shen ◽  
M Xu ◽  
...  
Keyword(s):  
Shear Stress ◽  
Parametric Study ◽  
Stress Amplification ◽  
Subsea Pipelines

Sediment Attractors: Seabed Shear Stress Shadows Around Subsea Pipelines Cause Net Sediment Accretion

2015 ◽  
Author(s):  
Fuyu Zhao ◽  
Terry Griffiths ◽  
Wenwen Shen ◽  
Scott Draper ◽  
Hongwei An ◽  
...  
Keyword(s):  
Shear Stress ◽  
Bedload Transport ◽  
Shear Stresses ◽  
Sediment Accretion ◽  
Cfd Modelling ◽  
Ansys Fluent ◽  
Steady Current ◽  
Stress Amplification ◽  
Subsea Pipelines ◽  
Transport Potential

This paper presents interpretation of the results of 2D CFD modelling using ANSYS Fluent, which has been undertaken for a parametric range of over 200 cases, including over 60 different seabed geometries, pipe diameters and seabed roughnesses as well as a range of steady current, wave and combined wave / current cases. Through analysis of the results including evaluation of seabed shear stress amplification factors compared to far-field ambient values, integration across the seabed of seabed shear stresses and bedload transport potential, the conditions under which sedimentation can be expected are predicted. The results have relevance to improving our understanding of sedimentation (backfilling) around subsea pipelines under live bed conditions, since the presence of shear-stress deficits or shadows leads to enhanced accretion of sediment in the region of a pipeline, even where there is localised amplification of shear stress right next to the pipe. The results are expected to enable better approaches design of subsea pipeline stability on erodible seabeds, or on impermeable rocky beds where veneers of mobile sediment are present.

Gravity Anchors Astride Subsea Pipelines Subject to Oscillatory and Combined Steady and Oscillatory Flows

2012 ◽  
Author(s):  
Xu Zhao ◽  
Liang Cheng ◽  
Ming Zhao ◽  
Hongwei An ◽  
Wei He
Keyword(s):  
Shear Stress ◽  
Oscillatory Flow ◽  
Stokes Equations ◽  
Numerical Study ◽  
Shear Stresses ◽  
Bed Shear Stress ◽  
Oscillatory Flows ◽  
Steady Current ◽  
Stress Amplification ◽  
Subsea Pipelines

This writing presents results of simulating oscillatory and combined steady and oscillatory flows past gravity anchors astride subsea pipelines. It can be considered a companion to a previous numerical study on steady currents past gravity anchors. The gravity anchor system comprises large arch-shaped concrete blocks positioned at intervals astride offshore pipelines, and it is engineered to provide innovative and cost-effective secondary stabilisation for high-capacity gas-transporting pipelines serving in severe metocean conditions, e.g. cyclone-prone offshore areas. A free-settling marine object bottom-seated on the seabed, however, the gravity anchor may subside into scour pits around its base due to locally disturbed flow regimes, imposing integrity risks on the pipe. Also, the effect of gravity anchors on hydrodynamic loading on nearby pipeline lengths is of interest. The present study employed a Petrov-Galerkin finite element method to solve the three-dimensional Navier-Stokes equations in direct numerical simulation. Firstly sinusoidal flow oscillating perpendicularly to the pipe beneath gravity anchors on an immobile bed was simulated at a Keulegan-Carpenter number of 10 and a pipe Reynolds number of 1000. Then, a steady current co-directionally superimposed on the aforementioned oscillatory flow was modelled at a ratio of steady current velocity to oscillatory flow velocity amplitude of 1. With sediment transport capacity related to bed shear stresses, the time-averaged bed shear stress amplification around gravity anchors in oscillatory flow was revealed first, and found to be consistent with laboratory observations of scour patterns. The effect of superimposing steady flow onto oscillatory flow on bed shear stress amplification was then explored. Lastly, hydrodynamic forces on pipelines in the vicinity of gravity anchors were gauged. The present work intends to shed light on the initial seabed responses with regard to the scour process around gravity anchors immersed in the oceanic wave boundary layer, as well as the effect of gravity anchors on hydrodynamic loadings on pipelines.

Analytical Simulation of the Composite Ice Wall

1989 ◽  
Vol 111 (3) ◽  
pp. 174-180 ◽  
Author(s):  
P. Corder ◽  
T. Kozik
Keyword(s):  
Shear Stress ◽  
Closed Form ◽  
Normal Stress ◽  
Transverse Shear ◽  
Parametric Study ◽  
Beam Theory ◽  
Transverse Shear Stress ◽  
Transverse Normal Stress ◽  
Analytical Simulation ◽  
Sandwich Configuration

A system of linear, closed-form stress equations for a steel-concrete-steel sandwich configuration, i.e., the “Composite Ice Wall,” was derived incorporating a formulation of classical beam theory. The stress terms include the longitudinal normal stress, the transverse shear stress and the transverse normal stress. These equations were programmed using Pascal and a parametric study was conducted. Some of the results are included herein. The analytical model produces principal stress contours and centerline deflections very similar to those in the classical beam for comparable pressure loadings.

Identification of Potential Location and Extent of Localized Interface Debonding in the Wheelpath of Asphalt Pavements

2018 ◽  
Vol 2672 (40) ◽  
pp. 371-381 ◽  
Author(s):  
David Hernando ◽  
Jeremy A. M. Waisome ◽  
Jian Zou ◽  
Reynaldo Roque
Keyword(s):  
Shear Stress ◽  
Parametric Study ◽  
Asphalt Pavements ◽  
Future Research ◽  
Interface Debonding ◽  
Potential Zone ◽  
Future Performance ◽  
Field Evidence ◽  
Performance Predictions ◽  
Stress States

Debonding between asphalt layers is usually modeled as a global or “smeared” phenomenon across the entire lane. However, field evidence indicates debonding may be limited to a portion of the interface. The objective of this study was to determine the potential location and extent of localized interface debonding in asphalt pavements so that more realistic interface conditions and pavement responses can be employed in future performance predictions. A parametric study was conducted to locate stress states potentially conducive to interface debonding. Factors considered included asphalt concrete (AC) layer thickness, AC-to-base stiffness ratio, interface compliance, tire size, and traffic wander. The parametric study showed existence of a zone of high shear stress coupled with low confinement for a broad range of depths (1–3 in. below the surface) and extending to 2 in. from the tire edge. Given the drop in confinement immediately outside the tire edge and that shear stress magnitude in this zone was similar to shear strength values reported in the literature, it was concluded that the repetition of these critical stress state conditions can cause localized debonding of an interface located about 2 in. below the pavement surface. Existence of a potential zone of localized interface debonding around the edge of a tire can promote a debonded strip below the wheelpath, which is consistent with field observations. The width of the debonded strip can extend to 42 in. Future research efforts should examine the stress redistribution associated with the presence of a debonded strip below the wheelpath.

Drained Breakout Resistance of a Pipeline on a Mobile Seabed

2015 ◽  
Author(s):  
Joe G. Tom ◽  
Simon H. F. Leckie ◽  
David J. White ◽  
Scott Draper
Keyword(s):  
Sediment Transport ◽  
Parametric Study ◽  
Numerical Study ◽  
Strong Impact ◽  
Relative Importance ◽  
Subsea Pipelines

This paper describes a numerical study investigating the effect of sediment transport and associated changes in the local seabed profile on the drained breakout resistance of subsea pipelines. Limit analyses were conducted assessing the breakout response of a pipeline placed on a cohesionless Mohr-Coulomb material considering different seabed profiles around the pipeline. These profiles were determined from surveys of a pipeline on an erodible seabed. The parametric study shows the relative importance of various parameters describing the seabed profile geometry, including the local pipe embedment and the adjacent slope of the seabed. Significant changes in drained resistance occur due to changes in local pipeline embedment resulting from scour induced pipeline lowering and/or sedimentation. The seabed slope local to the pipeline also has a strong impact. The assumption of a flat seabed can lead to predicted seabed resistance that differs significantly from the actual value, accounting for a more natural seabed profile.

Bed Shear Stress Distribution Around Offshore Gravity Foundations

2015 ◽  
Author(s):  
Nicholas S. Tavouktsoglou ◽  
John M. Harris ◽  
Richard R. Simons ◽  
Richard J. S. Whitehouse
Keyword(s):  
Shear Stress ◽  
Stress Measurement ◽  
Flow Direction ◽  
Shear Stresses ◽  
Bed Shear Stress ◽  
Stress Amplification ◽  
Bed Roughness ◽  
Scour Protection ◽  
Models Of Gravity ◽  
Maximum Amplification

Offshore gravity foundations are often designed with complex geometries. Such structures interact with the local hydrodynamics and generate enhanced bed shear stresses and flow turbulence capable of scouring the seabed or destabilizing bed armour where deployed. In the present study a novel bed shear stress measurement method has been developed from the camera and laser components of a Particle Image Velocimetry (PIV) system. The bed shear stress amplification was mapped out around six models of gravity foundations with different geometries. Tests were repeated for two bed roughness conditions. The structures tested included uniform cylinders, cylindrical base structures and conical base structures. The flow field around the models was also measured using PIV. The results of this study reveal that the conical base structures generate a different hydrodynamic response compared to the other structures. For uniform cylinders the maximum bed shear stress amplification occurs upstream, at an angle of 45° relative to the flow direction, and measurements were found to agree well with numerical results obtained by Roulund et al. (2005). In the case of the cylindrical base structure the maximum amplification occurs upstream at a similar location to the uniform cylinder case. For the conical base structures the maximum amplification of the bed shear stress occurs on the lee side of the structure, with the magnitude dependent on the side slope of the cone. The bed shear stress results were validated against stresses derived from analysis of the flow fields obtained by the PIV measurements performed under the same test conditions. Conclusions from the study are that the structure with the cylindrical base foundation produces the lowest bed shear stress amplification and that an increase in the bed roughness results in an increase in the amplification of the bed shear stress. These findings have direct implications for design of scour protection. In addition the flow reattachment point behind the foundation is dependent on pile Reynolds number (ReD). This suggests that the results of this study may be extrapolated for higher pile Reynolds using the method described in Roulund et al. (2006).

Sign in / Sign up

Export Citation Format

Share Document