Centrifuge Modelling of 3-Leg Jackups with Non-Skirted and Skirted Spuds on Partially Drained Sand

ABSTRACT This paper presents and compares results from twocentrifuge experiments modelling the cyclic load-displacement and pore-pressure responses for a small 3-leg prototype jackup on a medium-dense quartz sand layer. Flat spuds were used in the first test. Flat spuds fitted with short vertical skirts were used in the second test. The data showed extremely high foundation fixities for both types of spud. In the context of these very high values, and in the absence of scour, overall dynamic fixity was slightlyreduced by the presence of skirts. Generally the skirted spuds settled less than the non-skirted spuds. In extreme events the greater pullout capacity of the skirted foundations was confirmed. Pore pressure responses were highly complex. Double-frequency as well as single frequency components were present. Peaks and troughs were not in general in phase with spud loads. Excess pore pressuremagnitudes depended on load magnitudes, cyclic loading frequency, and location in the soil, Positive and negative pore pressure excursions were not symmetric. High suctionsoccurred in parts of cycles. Pore pressure dissipation times also depended on load magnitude, indicating different patterns of pore pressure generation for different loads. There was no evidence of cumulative pore pressure buildup. Considerable work remains to validate and generalise these results. Care is needed in extrapolating the data to larger rigs, to rigs with different relative height-to-leg spacing ratios or different prototype spud diameters, or to rigs on sands with References and figures at end of paper different fundamental properties (such as calcareous sands), or sands with different densities, non-uniformity, or layering. Nevertheless the data support the proposition of benefits available from the use of skirts for jackup footings. INTRODUCTION The response of independent-leg offshore jackups to cyclic environmental wind, wave, and current loading, the dynamic characteristics of the jackup, the internal stresses particularlyat the hull-leg and spud-leg connections, fatigue life, and the ultimate environmental load envelope of the jackup, depend critically on the horizontal, vertical, and rotational stiff nesses and limiting load envelope of the foundations, These in turn depend inter alia on the pore pressure responses in the soils, which depend on the nature of the soil horizons, the characteristics of the soils, their stiffnesses, plasticity, and horizontal and vertical permeabilities, the rate of loading, and the drainage path lengths. In this paper, we use the word "fixity" to refer to stiffness rather than ultimate capacity. We discuss the rotational stiffness of a foundation, that is, the relation between spud moment and spud rotation. We also discuss its horizontal and vertical stiff nesses, and we discuss how these interact with the structural stiffness to create the soil-structure-interaction concept of "fixity". We distinguish the concept of "fixity" as "stiffness" from the different, though related, concept of ultimate capacity. EXPERIMENTAL APPARATUS In centrifuge testing, a l/Nth scale model of a full-size prototype is tested in a centrifuge operated to apply a gravity field that is N times larger than earth's gravity g [1-4]. Underthese conditions, and with appropriate scaling of loads.