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TX 75083-3836, U.S.A., fax 01-972-952-9435.Abstract FPSO [Floating Production Storage and Offloading] structures have been accepted as a sustainable economic solution for deepwater development projects. Short to medium length (typically 15 to 25m) large diameter driven piles are often used to anchor FPSOs. The loading in such piles during a storm can be resolved into two components: (a) Lateral load, which is one-way cyclic; (b) Tensile (upward) load, which is typically only a few percentage of the lateral load.The greatest uncertainty in the analysis is the load carrying capacity of the pile, since the cyclic storm loading results in progressive degradation of the soil (sand or clay) supporting the pile. Thus understanding the degradation of the supporting soil is critical, for a safe, economic design. This paper thus has two aims: (a) to propose criteria and considerations for design of such piles; (b) to set out simple modifications in the p-y formulation that will provide a safe working envelope for the full range of ground conditions likely to be encountered at different sites.A parallel is also drawn to the approach routinely used by the geotechnical earthquake engineering profession, and reported centrifuge tests have been used to validate the proposed modification.

This paper presents 53 well-documented cases of the short-term set-up of open-ended steel pipe piles driven at two fields in the North Sea. Increases in soil resistance during driving (set-up) were observed when pile driving was re-started after delays typically ranging between 24 h and 100 h. The soil encountered at the sites consisted of overconsolidated sands and clays, where the clays varied from very stiff clay (su of 250 kPa) to very hard clay (su of 800 kPa). The field records were analysed to derive correlations between the length of the delay and the increase in soil resistance, or set-up. The available data have been classified in terms of pile diameter, pile tip penetration and the undrained shear strength of the clay strata. The data indicate significant set-up at such sites, even in the short term, with the rate of increase following a logarithmic curve. The data relate to practical issues associated with installation of piles, and do not address the long-term load-carrying capacity of the piles.

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This paper was selected for presentation by the OTC Program Committee followingreview of informationormtained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Offehore TechnologyConference and are subject to correctionby the author(a).The material, as presented, does not neceessrily reflect any positionof the Offshore Technology Conference or its officers. Permission to copy Is restricted to an abstract of not more than 300 words. Illustrationsmay not be copied. The abstract should contain mnapicuous acknowledgment of where and by whom the paper Is presented. ABSTNACTThis paper reviews the methode generally used by oil companies to determine the conductor eetting depth required to avoid hydrofracture of cohesive soils during drilling for the first casing string. Traditional approaches are compared with an approach developed by the authors, and the results of each method are compared with teat data obtained during geotechnical site investigations offshore.A case history is presented which showe the effect of the authors' deeign method on the required conductor setting depth, and indicates the considerable possible cost savings and safety benefits available from effective conductor design.