Theoretical and Ohserved Performance of Mobile Rig Footings on Clay

Abstract

ABSTRACT The observed foundation performance of eleven different jack-up mobile drilling rigs at 120 locations in the Gulf of Mexico was compared to their predicted theoretical performance. The foundation elements supporting all rigs included in the study are individual tank-type footings. The near-mudline soils at each rig location were clays; in addition, the soil shear strength and other properties were known (or could be estimated. Reasonable predictions of footing penetration were found to be obtained using the Skempton equation for the bearing capacity coefficient, N1c and strength data from unconfined compression tests on good quality undisturbed samples. Useable estimates of footing penetration were also shown to be possible for the Louisiana continental shelf by extrapolation of soil data on the basis of geologic and other evidence; the potential error tends to increase rapidly with increasing distance from the point where soil properties are known. INTRODUCTION Jack-up mobile drilling rigs with individual tank-type footings and a general configuration as shown in Fig. 1 constitute about 40 percent of the nearly 200 mobile rigs now in service(l). The number of footings ranges from 3 to 14, the footing diameters vary from 16 to 45 ft, and the nominal operating water depths range from 80 to 300 ft. In its operation a jack-up mobile unit floats to location with its legs lifted. At the drilling site, the legs are lowered to the mudline and jacked down into the seafloor soils until the total footing resistance becomes equal to the rig weight. Then the rig begins to lift itself out of the water. The drilling deck continues to climb the legs in order to reach a desired operating height above water level. A rig operator would like to know in advance that the sum of the air gap below the hull (G), the water depth (W), and the footing penetration (D), as shown on Fig. 1, will be less than L1, which is that portion of the total leg length (L) that can safely extend below the hull bottom. Since G and Ware fixed for a specific location and L1 is a constant, the unknown factor needed for advance determination of rig suitability for the location is the probable footing penetration, D. The study reported here was to determine if the penetration of jack-up rig footings into clay could be predicted reliably by conventional or modified soil mechanics procedures. The investigation consisted of a retrospective analysis of actual footing penetration where soil conditions were known or could be inferred. SCOPE OF STUDY Sources of Data. When this study was conceived in 1964, it was re-cognized that the approximate character of performance measurements, coupled with the lack of soil information for many locations, would require the assembly of a large body of data that could be analyzed on a statistical basis.