Application of Multiple Spuds (Dowels) to Development of Sliding Resistance for Gravity-Based Platforms
Authors: Gerwick B.C. , Potter R. , Matlock H. , Mast R. , Bea R.G. 
Published: December 2007

Gerwick Jr., B.C. U. of California-Berkeley Potter, R. Potter, R. Sohio Petroleum Corp. Matlock, H. ERTEC Western Inc. Mast, R. SPE ABAM Engineers Inc. Bea, R.G. SPE PMB Systems Engineering Inc. PMB Systems Engineering Inc. October 1984

The critical design parameter for many concrete and steel gravity-based platforms in shallow and moderate depth waters is their resistance to sliding under high lateral forces such as those imposed by ice or earthquakes. Conventional methods of increasing sliding resistance include adding ballast, enlarging base size, and the provision of skirts. All of these have limitations because of draft, cost, or overloading of the foundation as well as problems in seating and removal. Multiple large-diameter steel spuds or dowels, individually penetrated after seating of the structure, offer a viable penetrated after seating of the structure, offer a viable alternative for the transfer of lateral shear forces into the foundation. They are especially suitable to many arctic locations where water depths are shallow and weak seafloor soils overlie more competent soils. Typical spud sizes range from 60 to 96 in. [150 to 240 cm] in diameter and may penetrate 20 to 40 ft [6 to 12 m] below the seafloor. They are installed individually by vibratory or impact hammers augmented by jetting or drilling. Assessment of ultimate shear resistance must consider several potential modes of failure. These can be balanced in each specific case to fail under overload in a desirable sequence, first in sliding and then in bending, before damage to the structure itself. The spuds are free to move vertically relative to the structure, hence the structure will maintain contact with the soil during any consolidation settlements and can rotate slightly (tilt) as necessary to develop bearing resistance to overturning moments. Under impact loads such as those from a multiyear ice floe, the compliance of the spuds in bending and the soils in strain absorbs significant energy, thus reducing the maximum loads. In seismic zones, spuds can be employed to develop high shear resistance without significant increase in the attracted inertial force. Multiple spuds appear to be a practicable solution to provision of sliding resistance to high lateral forces for provision of sliding resistance to high lateral forces for structures in shallow water, weak soils, or where relocation is anticipated as with exploratory structures. Because the number and penetration depth of the spuds can be increased or decreased penetration depth of the spuds can be increased or decreased readily, the concept is very flexible in meeting varying soil, ice, and seismic criteria.

Introduction

The structures for current and planned developments in the arctic are faced with the principal problem of resisting high lateral forces from sea ice. In the shallow waters of the Beaufort Sea, and presumably in the Chukchi Sea and Norton Sound as well, the critical failure mode is that of sliding. The lateral forces from the ice must be transferred through the structure and into the foundation soil. This same problem occurs in North Sea gravity structures under storm waves. There the probable failure plane is usually on or close to the interface between the base of the structure and the seafloor. Enlarged bases and steel skirts have been the adopted solution, with the skirts penetrating several meters through the weaker soils to mobilize increased shear strength below. In the water depths typical of the North Sea, there is enough net weight of structure and ballast available during installation to "dove" the skirts into the seafloor the required distance. Gravity-based caissons recently have been proposed for shallow water sites in other parts of the world where the foundation soils consist of partially consolidated sands. Some of these sites are subject to strong earthquakes. The resistance to displacement by sliding is a function of the net submerged weight of the structure and the friction factor at or just below the structure/soil interface. Although adding solid ballast to the structure can increase this resistance, it also increases the inertial forces developed-especially if more structure and more volumetric displacement are required to contain the ballast. While a true description of the seismic response of such a structure/water column/soil system is very complex and the matter of potential liquefaction of the sands must be addressed also, it appears that the addition of an enlarged base and solid ballast may not be the most effective solution because of the increased seismic forces generated. In the southern Beaufort Sea, which is the area of principal interest in this paper, there are a number of areas (including Harrison Bay) where 10 to 30 ft [3 to 9 m] of recent silts and clays overlie the relatively strong Pleistocene sands. The upper portion of these Pleistocene soils is relict permafrost, ice portion of these Pleistocene soils is relict permafrost, ice bonded to varying degrees, in temperature balance (30.2 to 28.4F [-1 to -2C]) with the recent silts above it.

JPT

P. 1719