The sensitivity of the response of typical marine risers to choices of hydrodynamic coefficients is assessed. A time and frequency domain study of typical drilling and production risers for water depths from 500 to 3000 feet investigates the influence of the technique used to include the hydrodynamic coefficients in the riser program as well as the value of the coefficients for a range of environmental conditions. Data are presented to guide the designer and analyst in determining how to model the riser hydrodynamic coefficient and how accurately the coefficients must be known.
The results of the study show that for most cases, constant coefficients based on the maximum values of Reynolds and Keulegan-Carpenter numbers occurring anywhere along the riser are sufficient for determining operating parameters such as top tension and base angle. For those designs where fatigue life or clearance with adjacent structures is critical, more sophisticated techniques using coefficients based on local Reynolds and Keulegan-Carpenter numbers may be necessary. Overall, the study shows that riser responses are generally insensitive to the values of the hydrodynamic coefficients used to predict the motions. Fatigue life and clearance with adjacent structures can be affected if the uncertainty in the coefficients is sufficiently large.
The engineer is faced with three basic problems when analyzing any of the marine risers incorporated into many of today I s offshore drilling and production systems. First, an accurate description of the riser is physical properties and its operating environment must be developed. Second, a computerized riser analysis program of demonstrated accuracy must be used to predict the riser's responses. Third, the results of the computer analysis must be interpreted in a manner consistent with the actual field operation of the riser. Each of these problems can be solved to a certain degree.
The riser's physical properties can be obtained from the designer or the equipment manufacturer. Estimates of the local waves and current, and their probability of occurrence can be obtained from the oceanographer. Computerized riser analysis programs are available which have been verified by model test data and comparison with simple closed form solutions or other riser programs. Discussions with the operating people can help in understanding how the riser is actually used and the important operational problems. Much work continues to be done to improve our understanding of each of these problem areas.
The purpose of this work was to aid in developing an understanding of the importance of accurately specifying the flow conditions surrounding the riser; specifically the hydrodynamic drag and mass coefficients, CD and em. Most riser programs use a form of the Morrison equation to describe the relationship between the relative water velocities and accelerations, and the hydrodynamic forces on the riser. This formulation requires the input of drag and mass coefficients appropriate to the local flow conditions around the riser. Many model and full-scale test programs have been conducted to determine these hydrodynamic coefficients.
