nan Bolton

Centrifuge Model Tests on Anchor Piles for Tension Leg Platforms

Four centrifuge model tests were conducted to study the lateral response of large-diameter piles in clay subject to large lateral displacements. The objectives were to quantify the cyclic response for lateral loading of two closely-spaced piles loaded in line, and to establish the nature and the extent of any gap that may form between the piles and the soil as a result of static or cyclic loading. This paper describes the tests and the results, and shows how the results were used as part of the foundation design considerations for the URSA tension-leg platform. Introduction One method to connect the tendons of Tension Leg Platforms (TLPs) to the foundation piles is to drive the piles with the tendon bottom receptacle attached to the piles. This method was first used on the Mars TLP (Garside et al., 1997). Because it is a free-head pile, this direct-connect method may result in lateral deflections that exceed the criteria on which RP2A (1993, 2000) is based (see Matlock, 1970). The lateral deflections for the lateral tests in soft clay at Sabine, for example, did not exceed about 20% of the pile diameter. In addition, the Mars design was conservatively designed by assuming that large lateral displacements would create a “gap” between the pile and surface soils. The gap was calculated as 4C/γ, where C was the soil strength and γ was the buoyant soil density. In order to study the potential for gap formation and to better understand the group behavior of piles under large lateral displacements, a centrifuge test program was undertaken at Cambridge University. . References, tables and figures at end of paper The URSA TLP in Mississippi Canyon Block 809 (Digre et al, 1999; Gatlin, 1999) and the properties from one of the geotechnical investigation sites conducted as part of the design process served as the prototype for the centrifuge tests. The centrifuge tests simulated the lateral response of a 100-inch diameter pile embedded to 200 ft in a soil that modeled the geotechnical strength properties from one of the Ursa site investigations. For the group piles, a spacing-to-diameter ratio (i.e., s/d ratio) of 3.08 was used. Beam-column analyses on the prototype pile using the Matlock criteria and expected maximum design loads were used to develop the model pile sizes and test parameters. The final prototype pile design used at Ursa, while close to the pile and soil studied in the centrifuge test program, did not exactly match the pile and soil used in the test program. The results of the test program, however, were assumed to be close enough to be applicable to typical Gulf of Mexico TLP direct-connect pile designs such as the Ursa TLP. This paper reports selected results from four centrifuge model tests. The tests provide data of pile-head loaddisplacements, bending moments, inferred shear loads and inferred lateral pressures (p), and displacements (y) under monotonic and cyclic loading conditions. This paper compares results with API RP2A (1993, 2000) design criteria for lateral loading. The paper also includes design recommendations for laterally loaded piles with large displacement. Centrifuge Experimental Setup Centrifuge modelling techniques are widely used for offshore and other civil engineering applications to provide information for design (Schofield, 1980; Craig, 1983; Craig et al, 1984, 1988; Springman, 1993; Taylor, 1994; Murff, 1996; Clukey and Phillips, 2002). These techniques are fundamentally better than “single gravity” tests because stress levels in a centrifuge model can be arranged to be the same as in the corresponding “prototype” or full-size foundation. This is particularly important for model testing of soil, whose behavior depends strongly on stress. Figures 1 and 2 show the modeling arrangement used for the four tests. A clay sample was contained within a 850mm diameter cylindrical tub. The clay was consolidated in three layers, A, B, and C. Prior to each centrifuge test, each of one