Soil movement around a flat or externally bevelled caisson tip was investigated numerically using a large-deformation finite element (FE) approach. Comparing the results, it was found that the bevel was effective in terms of displacing more soil to the outside of the caisson, reducing the inside soil plug heave and giving a slightly higher increase in total stresses along the outside of the caisson wall during suction installation. During both self-weight embedment and suction-assisted installation, the proportion of the caisson wall accommodated by inward soil flow remained about 20% smaller for the bevelled caisson than for the flat-tipped one.
Suction caissons - also referred to as suction anchors or suction piles - are nowadays the most popular type of offshore anchor throughout the deepwater hydrocarbon-producing areas. They are so called because of the assistance of under-pressure or suction to install them, following penetration under self-weight. Compared with traditional anchor solutions, such as driven piles or drag anchors, they have significantly higher reliability and economic efficiency.
Modern caissons are usually cylindrical units made of steel, open at the base and equipped with a valve at their top lid, like inverted buckets. Mooring loads are applied by anchor chains usually attached to pad-eyes on the sides of the caissons. Compared with open-end piles, suction caissons are generally larger in diameter, shorter in length and much thinner walled. Typical diameters range from 3 to 8m, with aspect ratios (embedded length divided by diameter) of 2 to 6, and ratios of diameter to wall thickness of 100 to 2001. It should be noted that the focus here is on relatively slender caissons, installed in soft clay for anchoring floating structures, rather than short suction-installed caissons (or skirted foundations), such as might be used in dense sands as foundations for fixed structures.
For caisson design dominated by axial capacity, it is critical to assess the external shaft friction accurately, since that provides some 40 to 50% of the transient uplift capacity and an even greater proportion of sustained uplift capacity, where the reversed end-bearing cannot be relied upon.
During caisson installation, positive excess pore pressures are generated in the surrounding clay due partly to reduction in effective stress as the clay is sheared and remoulded, and partly to the increase in total stress as the penetrating wall displaces the soil outwards. After installation, consolidation, or ‘setup’, occurs and the shear strength of the clay and local effective stresses increase. The amount of strength regain, the final radial effective stress acting on the wall and the timescale for consolidation all depend on the pore pressures generated during installation, which in turn are affected by the magnitude of outward soil movement.
Accordingly, in practice some caissons have been fabricated with an external taper or bevel at the tip in an attempt to push more soil outwards during penetration2, 3, 4, 5. The aim is to negate the effect of suction installation, which may tend to draw a greater proportion of soil into the caissons