This paper describes an experimental program undertaken to understand better the hydrodynamic loading of multitube production risers. Test results are described, and a generalization is given for engineering applications. Sample design calculations are performed. Introduction In recent years, much attention has been paid to analyzing the response of drilling risers. Most effort has been focused on the development of structural and analytical techniques to predict the stresses in riser systems. The analytical tools are, to a certain extent, calibrated by full-scale measurement. API published a comparison] of nine different, independent calculations of riser response that were in close agreement.Normally, drilling risers have a simple cross section for which the hydrodynamic forces are fairly well known. Extensive studies during many years have established insight into the flow patterns around a single smooth cylinder in constant flow and the corresponding forces. For multitube production risers the hydrodynamic load pattern is far more complex.Research is continuously improving the knowledge of flow-induced forces, particularly the influence of these parameters: high Reynolds number at prototype condition, effect of roughness, proximity effect of closely spaced cylinders, current/wave interaction, and effects of irregular seas.The state of the art regarding hydrodynamic loading on single cylinders in constant and oscillatory flow as a function of varying roughness has been published by Miller and Sarpkaya. Correspondingly, multiple cylinders have been considered by Zdravkovich, Bushnell, and Sarpkaya.To understand better the hydrodynamic loading of multitube production risers, Kongsberg Vapenfabrikk initiated an experimental program early in 1977. This program consisted initially of five riser configurations and one single cylinder to be tested in constant and oscillatory flow. A comparison with published data was made to check the experimental procedure and results against the state of the art. The experimental program later was extended by 18 additional geometries, sponsored by 10 oil companies as a joint industry program.The tests in constant flow were carried out at the Danish Hydraulic Inst. (DHI), while the oscillatory flow tests were performed at the Hydraulic Research Station (HRS). Det norske Veritas was consultant for the project and supervised the experimental work and the data interpretation procedures.This paper describes the experimental program, the test results, and a possible generalization for arriving at a simple engineering application of results.The discussion of sensitivity of important parameters in determining design loads is based on an actual design case. Design calculations performed by Kongsberg Vapenfabrikk for a production riser in deep water are presented. JPT P. 881^