The requirement for a dynamic analysis of any offshore riser design is well established. While the mathematical tools are available for such an analysis, the designer must rely upon past experience and judgment to develop the riser design to be analyzed. This de sign task would be simplified if the designer had a "feel" for the effects of the many variables that enter into a riser analysis. The sensitivity of riser behavior to changes in the design parameters would also permit the designer to investigate the importance of variations caused by unexpected weather conditions or changes in operating procedures.
This paper reviews some of the riser analysis techniques which have been presented in the literature. Then, using one of these techniques, the sensitivity of calculated stress in the riser to a number of parameters is presented. While the sensitivity curve s developed cannot be used as a general design guide for all risers because of different riser geometries, the results do indicate trends in calculated stress due to variation in top tension, wave height, drilling vessel motion, etc. These trends are useful to a designer but they also emphasize the need for a complete riser analysis which includes variations in all important parameters.
As drilling is extended into deeper waters, the design of the riser increases in significance. An inadequately designed riser can lead to costly rig downtime and possible catastrophic failure. A delicate balance of riser strength, configuration and supplemental buoyancy must be struck against the environmental effects of current, waves and the operational constraints at the wellhead connection and on the platform.
The design of a riser must include consideration of the many options in pipe size, pipe strength, location and size of the buoyancy modules, the rig positioning and tensioning capabilities and the current profile and wave conditions at the site. These factors which complicate the analysis of the structurally simple riser pipe have different relative effects upon the riser design. A sensitivity analysis which reveals the effect of these parameters is of great interest to those designing deepwater risers. Figure 1 shows a typical deepwater riser.
Riser analytical methods that have been presented in the literature included static as well as dynamic models with varying degrees of sophistication. The dynamic models offer both time and frequency domain and two or three dimensions in space. A brief summary of the available methods is presented in reference 1.
The generally accepted governing equation for the static analysis of risers is given in Figure 2. The equation is derived from a summation of lateral forces on the riser. The bending term is developed from. Bernoulli-Euler beam theory with the higher order curvature terms ignored.
The effective tension is comprised of two terms; the tension in the steel and a pressure effect. The pressure effect is induced when the riser is inclined from the vertical and the net pressure acting generates a lateral force component.