Abstract

Most of the studies on spudcans are currently based on circular geometry; in this paper, the penetration process in the clay of rectangular spudcans is investigated. Centrifuge experiments and the PIV technique report the soil flow state in the penetration. Subsequently, the experimental results of the bearing capacity are compared with the predicted values of classical methods. It is found that the soil back-flow is complicated by the effect of aspect ratio and cone angle due to the non-axisymmetric characteristic of the rectangular spudcan. Compared with the classical solutions, the experimental bearing capacity results are reasonable.

INTRODUCTION

Jack-up platforms have the advantages of strong mobility, stable operation and high efficiency, and are widely used in offshore drilling and maintenance work. The typical installation process is shown in Fig. 1. After the modern jack-up units are towed to a predetermined position, before raising the hull, the spudcans will penetrate the soil through preloading until the design load is balanced with the penetration resistance and the hull is raised to provide enough air-gap for subsequent operations (Young et al., 1984). Therefore, a clear understanding of the penetration process and an accurate prediction of the penetration resistance profile are required to stabilize the spudcan at a predetermined position for operational requirements or possible storm loads.

Generally, typical strength characteristics of seabed soil, the undrained shear strength will increase linearly with depth, expressed as

(equation)

where sum and k are undrained shear strength at the mudline and the gradient of increase in su along depth, z is the depth below the soil surface.

When the penetration depth is shallow without soil flow-back, the bearing capacity can be expressed as

(equation)

Where Nc is the dimensionless bearing capacity factor and y′ is the effective unit weight of the soil.

When soil back-flow occurs, the bearing capacity can be expressed as

(equation)

Where p0′ is the effective overburden pressure generated by the backflow soil, V is the volume of spudcan and leg embedded in the soil and A is the spudcan cross-sectional area at the maximum section. It can be found that a clear understanding of the soil back-flow state and the selection of suitable Nc and representative su values are vital factors in determining the accuracy of the evaluation of the penetration resistance profile.

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