Design of Buried Pipelines in Soft Clay: A Case Study

High pressure high temperature offshore pipelines are generally buried for protection. For buried pipelines, upheaval buckling can be triggered in the presence of vertical imperfections when the pipeline compressive state is increased. One of the main uncertainties in the design calculation is the soil resistance to upward and downward movement and the displacement at which the peak resistance is mobilised. For clays the soil response will usually be undrained, certainly for most loading events, however it may also be necessary to consider the drained response (particularly if there is any long term applied uplift). In addition, local flow-round failure mechanism may occur, generally for deeply buried pipelines. If local failure dominates the design, it may not be possible to bury the pipeline. This could have significant cost implications. An experimental programme was carried out to determine the uplift resistance of buried pipelines in soft clay soils. The results of scale model testing and computational analysis (finite element limit analysis) showed that the local behaviour was not applicable. The implications of these results are illustrated in a case study upheaval buckling (UHB) analysis. The overall result was a significant saving on rockdump quantities. The implications for the case study are presented in this paper alongside a description of other backfill requirements and practical considerations for a UHB design.

Natural Element Simulation of Extrusion of Hollow Profiles

Numerical simulation of the extrusion of hollow profiles is a particularly challenging process due to the inherent flow patterns in this type of procedure. Aluminium must flow round the central part of the die and join again prior to exit the die. The quality of this welding line is highly affected by the mechanical conditions within the die, which depend crucially on geometry. In this paper we present a review of our work on the topic, where the Natural Element Method (NEM), a meshless technique, has been applied towards an efficient analysis of the conditions necessary to give a quality welding of the flows giving rise to the final geometry of the extrudate. The use of meshless methods is precisely the novel ingredient in our analysis, since it provides with an accurate description of the position of the flow front and the mechanical conditions (stress, strain, temperature) at the interface. Numerical results are also compared with experimental measures showing the impact of die geometry on the quality of the final design.

Modelling the Installation of Stiffened Caissons in Overconsolidated Clay

Design of suction caissons for installation in overconsolidated clay presents several geotechnical engineering challenges. These include (i) predicting the installation resistance and required ‘suction’ pressure, (ii) ensuring adequate skirt length to account for vertical plug heave, and (iii) accommodating the structural engineering stiffening requirements and their effects on the penetration resistance and plug heave. A suite of centrifuge tests in overconsolidated kaolin clay was carried out to investigate the effects of stiffener geometry on penetration resistance during direct jacking and suction installation. Three caisson geometries were compared: caissons with (i) no stiffeners, (ii) horizontal stiffeners only and (iii) both vertical and horizontal stiffeners. Results show negligible differences in penetration resistance between jacked and suction installation for each caisson type. The magnitude of soil heave within the caisson is seen to be highly dependent on the level of applied suction as well as on the volume of the stiffeners. Observations during and following testing indicated that minimal flow-round of the overconsolidated clay occurred for skirts with horizontal stiffeners. These included (i) linear penetration resistance profiles following penetration of the lowest horizontal stiffener, (ii) a wedge of clay observed only below the lowest horizontal stiffener following extraction, and (iii) unsupported plug heave heights following penetration. A comparison of measured data with back-calculated resistance factors suggests that current design methods adequately predict the measured penetration resistance assuming zero flow-round conditions, implying additional end bearing of the upper horizontal stiffener during penetration was negligible.

CFD Study of the Pressure Distribution on the Back of a 3-D Bluff Body (CUBE) of Air Flow in Different Reynolds Numbers

The study of flow around bluff bodies is important, and has many applications in industry. Up to now, a few numerical studies have been done in this field. In this research a turbulent unsteady flow round a cube is simulated numerically. The LES method is used to simulate the turbulent flow around the cube since this method is more accurate to model time-depended flows than other numerical methods. When the air as an ideal fluid flows over the cube, flow separate from the back of the body and unsteady vortices appears, causing a large wake behind the cube. The Near-Wake (wake close to the body) plays an important role in determining the steady and unsteady forces on the body. In this study, to see the effect of the free stream velocity on the surface pressure behind the body, the Reynolds number is varied from one to four million and the pressure on the back of the cube is calculated numerically. From the results of this study, it can be seen that as the velocity or the Reynolds number increased, the pressure on the surface behind the cube decreased, but the rate of this decrease, increased as the free stream flow velocity increased. For high free stream velocities the base pressure did not change as much and therefore the base drag coefficient stayed constant (around 1.0).