Suction Caisson Response Under Sustained Loop Current Loads

Abstract

Abstract The capacity of suction caissons to withstand uplift loads relies on suctionsinduced beneath the caisson base and within the internal soil plug. Thesesuctions can be relied upon for short-term loading such as due to wave action, but are more questionable for the sustained loading from loop currents, whereloading periods of several days or even weeks are possible. This paper combines results from centrifuge model tests and finite elementanalysis to elucidate relationships for the proportion of the short termcapacity that can be relied upon for various durations of loading on suctioncaissons. The paper addresses issued related to the reduction in holdingcapacity due to pore pressure dissipation effects. The combined results from finite element analysis and two independent sourcesof centrifuge testing present a consistent picture of long term suction caissonload holding capacity. Loads approaching the undrained capacity can besustained for a few months. Substantially lower loads can be held for extremelylong durations. Intermediate loads can be sustained for determinate lengths oftime dependent upon caisson dimensions and soil properties. For typical caisson geometries and consolidation properties relevant to Gulf ofMexico (GOM) conditions, it is found that there is negligible reduction incapacity for holding times of up to 100 days, which is well in excess of timesrelevant for loop currents, and that loads in the region of 85% of theshortterm capacity may be held for periods of up to 2 years with only smalldisplacements. The paper presents these and additional results and provides comments on theimplications for suction caisson foundation design under long term loading, such as would be encountered in mooring systems for floating productionstructures exposed to loop current loading like those commonly occurring in theGOM. Introduction Loop current considerations in the GOM are an important aspect of suctioncaisson design for facilities located in deepwater. Vukovich et al. (1979) havedescribed the characteristics of these loop currents. The circulation in the GOM is dominated by the loop current (LC), an energeticcurrent of warm water that enters the Gulf through the Yucatan Straits. As itenters the Gulf, it extends slightly to the north, loops back around to thesouth, and exits the Gulf through the Florida Straits. But the loop current isdynamic in nature and its position and strength change with time. Once or twicea year, it extends well to the north, forming an intense clockwise flow thatmay reach as far north as 29 deg. In an attempt to return to the more normalsoutherly extent, it slowly pinches off the extension, resulting in a closedcirculation and the generation of a warm-core, anticyclonic ring, or eddy. Thisloop current eddy (LCE), no longer constrained, migrates to the west at a rateof 2-4 km/day, bringing deep, strong currents to deepwater exploration andproduction operations along the way. loop current eddies can be 200-400 km indiameter, with swirl velocities of up to 5 knots and rotation rates of 5-10deg/day.