The Marine Riser- A Procedure for Analysis

Abstract

Oil drilling and producing operations have been moving farther and farther offshore to exploit new gas and oil resources. Marine operations pose problems not encountered in land operations such as weather, ocean waves and currents, limited storage space, corrosion, and transportation difficulties. One method of offshore operations places the wellhead on or near the ocean bottom. A drilling rig on the surface provides the working area for performing operations incident to drilling and completing a well. With this arrangement, most operators install a large pipe that extends from the wellhead up to the drilling rig. This pipe is known as the marine riser. The riser forms a continuation of the wellbore which guides tools into the well and provides a conduit for returning drilling mud to the surface. As operations move to deeper water, wind forces that push the drilling rig from the centerline of the well, ocean current forces, and the weight of the riser itself, all contribute to the complex loading of the riser. Presently, tensile forces [usually applied to the riser by constant tension winches and/or buoyant chambers] have resisted some of these loadings, while the lateral supports applied by the drilling platform and wellheads have resisted others. As the length of the riser increases, the loadings become critical and a thorough analysis of the riser is required. In order to analyze the probable behavior of a marine riser for use in drilling to depths in excess of 300 ft, a mathematical model was developed to describe a ballasted, variable cross-section, variable tensioned riser. The resulting fourth order, nonlinear differential equation describing the static case was solved by using finite-difference approximations on an IBM 360, Model 40 computer. Present analysis capabilities allow examination of worst-case conditions in terms of allowable top and bottom angles, bending moments and resulting shear forces. Consider an 18-in O.D. riser [1/2-in. wall] in 600 ft of water with 18 ft of platform offset from the centerline of the hole [3 percent of the depth]. Oceanographic surveys have indicated that there is usually little or no current effect below 300 ft from the surface in open water. The assumption for Fig. 1 is that the current force acts only over the upper 300 ft of the risers. Assume also that the current varies linearly from 3 knots at the surface to zero 300 ft below the surface. [In actual practice the computer program will accept any desired current velocity profile.] The18-in. O.D. riser weighs approximately 201 Ib/ft in water for a total weight of 120,600 lb. Figs. 2 through 5 are the computer plots of the 18-in. O. D. riser. Fig. 2 shows the riser has been deflected by the current beyond the 18 ft of offset [Max. X= 18.42 ft, 528 ft from the bottom]. The numbers along the X axis of each graph are E [exponential] Format, so that 1.842E01 = 1.842 × 10 ft =18.42. Fig. 3 illustrates the slope, measured in degrees from the vertical axis. The angle of the riser at the bottom is 3.80°, which is well within ball joint capabilities, and the top angle is -0.69°.