This paper presents results from an experimental investigation on the loading on a rigid slender vertical circular cylinder in unidirectional regular and irregular waves. The ambient flow was measured directly, so that the derived results would not be subject to additional uncertainties associated with the use of wave theories. Reynolds numbers were in the range 104 to 7 × 104 and Keulegan-Carpenter numbers 1 to 20. Morison drag, inertia and lift coefficients are computed from forces measured at one force sleeve and compared with data obtained at the same Reynolds and Keulegan Carpenter numbers, but under uniform flow conditions in a U-tube. In regular waves, the present results show the importance of the non-uniformity of wave-induced flow, and in irregular waves it is clear that the loading is much influenced by the history of the flow.


This paper describes some initial analysis of measurements of local forces on a vertical: cylinder in unidirectional waves. The experiments were carried out as part of a programme (described fully by Irani, Chaplin and Subbiah, 1992) which was aimed at the more complex (and much less well-researched) problem of Morison loading in multidirectional waves. For reference purposes however, a comprehensive series of measurements was made first in regular and irregular unidirectional waves; it is some of these that form the basis of the present paper. These measurements are of interest in their own right. First, because particular emphasis was placed throughout the test programme on obtaining good quality measurements of the particle kinematics at the cylinder's location, thus avoiding the need to rely on wave theories in the derivation of force coefficients. Second, measurements of local forces in waves are to be found in only a few references (e.g. Bullock, 1983; Chakrabarti, 1980), and then usually only in regular waves.

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