Four stub column tests were conducted for the purpose of making a direct comparison of the effects of pinholes and stress relieving on axial load-deformation behavior. Theoretical analysis of a stress relieved tubular column with pinholes was performed using the finite element method, and the results compared satisfactorily to those of the stub column test. It is shown that the axial load deformation relationship implicit in the present design method is somewhat conservative, and a more rational expression is proposed.
A typical jack-up offshore mobile drill rig, shown in Fig. 1 consists of a buoyant platform, a large mat, three tubular columns and a hydraulic jacking system. The length of the columns is determined by the desired operating depth and environmental characteristics, and wall thickness is varied in an effort to optimize material usage. Platform loads are transmitted to the columns through pins which pass through rectangular pinholes in the columns. Typical drill rig columns have four or six pinholes spaced equally around the circumference at levels spaced from four to eight feet apart. A typical pinhole is shown in Fig. 2. Pinholes are flame cut with the corners shaped to minimize the resulting stress concentration, and are reinforced in order to prevent local failure from the large bearing loads. In order to eliminate residual stresses, and thus minimize fatigue cracking, the columns are stress relieved after the pinholes are made. In the United States, the design of fixed offshore structures has customarily been based on the allowable stress method of the AISC Building Code [5, 6]. The net cross section of the column, assumed to be prismatic, is used in computing the axial rigidity, moment of inertia and slenderness ratio [8], and therefore, the allowable load. The present design procedure tends to be quite conservative. Designers of the subject drill rigs require a rational column design procedure that takes into account the effects of pinholes, stress relieving and varying wall thickness. Currently, Bethlehem Steel Corporation is developing a computer-aided procedure utilizing the Newmark numerical method [9] to determine the strength of these drill rig columns under combined axial and lateral loads. In order to determine the appropriate moment-thrustcurvature relationship for each segment along the length of the column knowledge of the axial load-deformation behavior is required. The objective of this research was to analyze the effects of pinholes and stress relieving on stub column behavior and to formulate an axial load-deformation relationship for a stress relieved tubular column with pinholes.
Four specimens were tested. Table 1 gives specimen data and the distinguishing features of the four specimens. Specimen 1 had no pinholes and was not stress relieved, as is typical of commonly-used tubular columns. Specimen 2 had pinholes but was not stress relieved, and Specimen 3 had no pinholes but was stress relieved. Specimen 4 had pinholes and was stress relieved, as is typical of jack-up offshore mobile drill rig columns. By testing these four specimens, the stub column behavior of each can be directly compared to determine the individual and combined effects of pinholes and stress relieving.
