Edward C. Clukey

Effect of Skirt-Tip Geometry on Set-Up Outside Suction Anchors in Soft Clay

An important part of suction anchor design is the determination of the shear strength along the outside skirt wall. Previous work has suggested that when a suction anchor in clay is installed by applying underpressure inside the anchor, the external skin friction may be reduced compared to the skin friction expected for driven piles. The primary reason for this reduction is that the movement of soil at and beneath the caisson tip during installation will be influenced by whether the anchor is penetrated by weight or by underpressure. To further investigate the impact of installation by underpressure, additional finite element analyses have been performed where the skirt installation process has been better followed than in the previous analyses. The movement of soil around the caisson wall was studied for both a flat caisson tip and a tip with a tapered edge of 45° towards the outside of the anchor. The tapering was made to see if it would cause more of the displaced soil to move outside the anchor and thereby increase the mean total stresses and the shear strength along the outside anchor wall. The analyses were made with two separate wall roughness factors for a typical anchor in soft clay.Copyright

Reliability of suction caissons for deep water floating facilities

With the extension of the offshore industry into deeper water the types of offshore structures changed from gravity base structures and fixed platforms supported by driven piles to floating vessels supported with vertical tendons or catenary mooring lines. The anchors for these new systems also changed as increased water depths presented problems for under water pile driving. Suction caissons then became the preferred foundation option for these facilities. A number of significant differences exist between how suction caissons and piles provide resistance and how the foundation system for a floating vessel performs compared to a fixed platform. One of the primary challenges with this new foundation system was the determination of the overall system reliability. An initial study for suction caissons by Clukey et al. (2000) showed that the overall system reliability for suction caissons was not as robust as for a fixed platform. The impact of a single suction caisson failing was found to be more severe and could more easily lead to a system failure. Based on this initial generic study, it was recommended that for extreme loading events the factor of safety (FoS) for Gulf of Mexico (GoM) 100-yr hurricane conditions should be increased from 1.5 (for piles) to 2.0 for suction caissons. This FoS is currently being used in API-RP2SK for vertically dominated loading. However, since that initial study significantly more reliability work has been done at the University of Texas, Det Norske Veritas and the Norwegian Geotechnical Institute. These studies considered a wide variety of cases including different floating vessel types, soil conditions, water depths and loading conditions. The results from this more recent work are compared in this paper. These results show that the current design codes are robust and conservative. Based on the studies opportunities exist to make the design codes less conservative while still maintaining desired levels of reliability. In addition the historical development of suction caissons is presented INTRODUCTON & BACKGROUND Since the early 1990s suction caissons have been used to anchor a variety of deep water facilities. These foundations are cylindrical members but with much larger diameters (4 to 8 m) and smaller depth/diameter ratios than offshore piles. They are installed through deadweight and pumping water from inside the caisson which lowers the pressure inside the caisson (referred to as suction) resulting in an additional downward force. If properly sealed the final uplift resistance is derived from deadweight, external skin friction and reverse end bearing at the bottom of the caisson. A list of installed suction anchors is given in Andersen et al. (2005) showing more than 485 suction installations by 2004 at more than 50 locations in water depths to nearly 2000 m. One early application was the foundation for the Snorre Tension Leg Platform (TLP) installed in the North Sea(NS) in 1991 (Stove et al., 1992). This facility had four foundations, one