ABSTRACT This paper presents some significant results of analytical investigations of the effect of various problem parameters on the nondeterministic response of a marine riser to random wave forces and deterministic steady current forces. It is shown that the response to random wave forces is highly dependent upon the spectral distribution of energy density as well as the total average energy density. Several modes of vibration, not necessarily including the fundamental mode, are found to contribute significantly to the random response. INTRODUCTION A marine riser is a long, slender pipe used in, offshore drilling operations. Extending from a fixed or floating platform at the sea surface to a wellhead connection at the sea floor, the riser contains the drill string, which it guides and supports, and the drilling mud, which it returns to the surface for reuse. Fig. 1 depicts a typical riser installation. Risers with outside diameters of less than 2 ft are commonly used in water several hundred feet deep.1–3 Such a long, slender structure tends to buckle under its own weight unless it is supported in some manner. To prevent buckling and reduce deflections, a tension force that is usually somewhat greater than the total submerged weight of' the riser system is applied at the surface end of the riser. The parameter, top tension ratio, is defined as the ratio of this tension force to riser submerged weight. Near the upper end of the riser, there is normally a slip joint that permits lengthening and shortening of the riser to compensate for changes in water depth caused by tides and waves. At the bottom end, where the riser connects to the blowout preventer, there is a flexible ball joint that permits some angular rotation without excessive bending when misalignment occurs. In addition to its own weight and the supporting tension, the riser is also subjected to the hydrodynamic forces of currents and water waves and to stresses resulting from the relative lateral displacement of the riser ends, which occurs when the surface platform drifts off station. In riser installations, it is necessary to limit the bottom rotation to a few degrees. The maximum rotation must be less than that permitted by the design of the ball joint, which is usually about 100 It is seldom possible, however, to attain such a rotation without producing excessive stresses in the riser. Moreover, in most installations, if the rotation is more than a few degrees, the drill pipe rubs against the riser, the ball joint, the blowout preventer, and the well casing, resulting in wear that is great enough to constitute failure. Bottom rotation is controlled, in part, by the positioning system used to keep the surface support platform on station. Whether a conventional mooring system or a-dynamic positioning system is used, accurate information describing the behavior of the riser in its operating environment is needed for proper design of the system.