An Analysis of Marine Risers For Deep Water

Abstract

A model is used to calculate this static and dynamic behavior of a marine riser supported from a floating vessel. Analysis examples demonstrate the consequences of extrapolating an existing riser design into deep water, and show that the dynamic behavior of the riser, caused primarily by vessel response to waves, is a significant design factor in all water depths. Introduction The marine riser is a conductor pipe used in floating drilling operations to convey drilling fluid and to guide tools between the drilling vessel and the wellhead at the ocean floor. The essential features of marine riser design were defined by Fischer and Ludwig who showed, with a static analysis, the importance of tensioning the riser to prevent buckling and to control deflections and stresses. An analysis by Tidwell and Ifrey further illustrated these effects. Fischer and Ludwig recognized the possible effects of dynamic behavior on the riser, but reasoned that these effects would not be a critical design factor for water depths up to 1,000 ft if the riser diameter were held to a minimum. As exploratory drilling operations move into deeper water, the dynamic effects in riser design become of increasing concern. The significance of dynamic behavior for riser design was recognized by National Engineering and Science Co. (NESCO) in their analysis of the 14,000-ft-long drilling riser for Project Mohole. Their study showed that riser Project Mohole. Their study showed that riser dynamic behavior was a significant factor in design, and that the most significant portion of the dynamic behavior was caused by motions of the drilling vessel, rather than by wave forces on the riser. The analysis model described in this paper was formulated to analyze, both statically and dynamically, the types of marine risers presently used in floating drilling operations. The analysis method employs a numerical integration scheme that differs from the series solution method employed by Fischer and Ludwig, and the finite difference methods used by Tidwell and Ifrey and by NESCO. The numerical integration method is particularly suited for an accurate representation of parameters that vary along the length of the riser and for efficient implementation on a digital computer. The purpose of this paper is to describe the mathematical model that was developed, and to illustrate the usefulness of the model with an example that shows the effects of extending a riser into deeper water. The paper is divided into two parts and an appendix. The first part describes the basis for the analysis model; the method of solving the equations in the model is described in the Appendix. The second part of the paper describes the analysis of a 16-in. riser in water depths from 400 to 2,000 ft, and discusses the effects of increasing water depth on riser design and operations. Analysis Model The analysis model of the marine riser is based on the general linear differential equation for a beam column with lateral loads in a vertical plane. A solution to the differential equation is obtained by a numerical integration method for a specified force distribution along the length and boundary conditions at each end. This section describes the equations used in the analysis model; the solution method is outlined in the Appendix. JPT P. 455