Riser Tensioner Force Variations

ABSTRACT Excessive riser force variation on the upper joint in a riser string can lead to buckling and excessive fatigue. This variation is due to two components of the riser support system. These are the riser tensioning system and the telescopic, or slip, joint. By means of specific examples, the paper reaches two conclusions. The first is that the force variation at the top of the riser string may be much greater than that indicated by monitoring the tensioner system's air tank pressure. The second conclusion is that a major contribution to this variation can be the pressure drop in the air valves. INTRODUCTION The riser tensioners and slip joint, as shown in Fig. 1, form the support system for the riser string used in floating drilling operations. Though the tensioners are the primary support mechanism, their forces are transmitted through the slip joint to the upper joint in the riser string. In many deepwater drilling operations the riser string is isolated in bending from the more massive telescopic joint by an upper ball joint. The upper ball joint interacts directly with the riser string, so the forces evidenced at that joint become the riser string forces due to the tensioner support system. Ideally, the tensioner support system forces at the upper ball joint should provide a net axial load on the riser string and be constant in magnitude as well as direction. However, the non ideal behavior of the riser tensioners as well as the inertia and geometrical effects associated with vessel, slip joint, and riser string motions--result in load variations. In general, then, the upper ball joint force vector is time dependent. No limits as yet have been determined for allowable variations of the riser string forces resulting from the riser support mechanism. Nevertheless, the analytical and quantitative measure of these variations is important in assessing the effectiveness of the support mechanism as well as providing important information regarding boundary conditions necessary for the analysis of the riser string. The purpose of the paper is twofold. The first is to emphasize by numerical examples the strong dependence of the riser tensioner force variations on the character of the assumed losses (pressure drop) in the tensioner system air valves. The second purpose is to present an analytical expression and numerical results for the tensioner system force variations at the upper ball joint, thereby emphasizing the strong effects of vessel motion on the riser string force. TENSIONER ANALYSIS The typical drilling riser tensioner is a hydropneumatic mechanical system as shown in Fig. 2. Its purpose is to provide tension in the cable attached to and supporting the outer barrel of the slip joint. The cable tensioner variation (?T) due to cable motion has been studied by Kozik [1]. A convenient form for Kozik's equation is:(Equation available in full paper)