Roy E. Olson
Innovative Foundations in Deepwater
Sherif El-Gharbawy (PhD student), Mathew Sileston (MS student)
Since their inception, a little over a decade ago, suction caisson foundations (SCFs) have been built with length-to-diameter (L/D) ratios of 0.5 to 4. Model caissons with L/D ratios as such have been tested with success in previous phases of this project. Currently, we are introducing model caissons with L/D ratios of 2 to 8 to study the effect of increasing caisson length on penetration resistance, pullout capacity, and the feasibility of using suction as the method of installation. The simulated TLP loading will be in the form of static vertical tension, cyclic vertical tension, static inclined tension, cyclic inclined tension, and combined vertical and horizontal cyclic tension.
SCFs have been employed with success as foundations for both offshore large gravity platforms and as anchors for tension leg platforms (TLPs) in the North Sea. World events and the discovery of major petroleum prospects in the deep GoM within the United States Exclusive economic Zone lend the need to examine and exploit the new technology for application within this zone. The thrust of this effort is in line with the stated objectives of the OTRC project and our industry partners; specifically the study of foundations for TLPs in water depths of 6,000 to 10,000 ft in the GoM.
The project entails five major tasks: (1) define variables and technical terminology; (2) survey literature and gather information and database from actual construction and laboratory model testing and numerical analyses; (3) identify applied loads; (4) conduct a comprehensive experimental program to study the behavior of SCFs under accurately simulated pullout loading from environmental conditions; (5) present the offshore petroleum industry with an understanding of the behavior of SCFs in deep waters and a recommendation for design.
Major work on suction caissons has been carried out at the Norwegian Geotechnical Institute. They have concentrated on caissons with the appearance of large footings. Professor Whittle has been heavily involved in modeling suction caissons in a JIP at MIT.
Major uncertainties still exist with respect to the response and capacity of open-ended driven piles to cyclic tensile loading, in spite of periodic efforts to study the problem over many years. The experience base for suction caissons is even narrower than for driven piles. It is believed that introducing a range of L/D ratios for suction caissons approaching that for driven piles will significantly increase their tensile capacity and the applicability of already existing database for driven piles. The ongoing program also effects a more accurate simulation of actual TLP loading than has been done elsewhere.
October 1992 to September 1997
The project final report will introduce a recommended design method that is supported by laboratory and limited field data, and will enable the offshore petroleum industry in the United States to take advantage of suction caissons as a novel foundation system for use in the GoM.
We have presented results of our OTRC-sponsored studies at the ASCE Civil Engineering in the Oceans V conference at Texas A&M University in 1992, an ASCE meeting in Dallas in 1993, in the BOSS conference at MIT in 1994, and in an ASCE meeting in Bahrain in 1994. We have met with personnel from NGI in the United States. We participated in a workshop on suction caissons in Houston. Two papers have been submitted for possible presentation at the Offshore Technology Conference to be held in Houston in May 1997.