Structural damping of tensioned pipes with reference to cables

An expression for modal damping (loss factor or logarithmic decrement) is obtained analytically for tensioned pipes where the material damping is the only source of damping. The dependence of modal damping on axial tension, which is well known from general experience with cables, is explained. The expression is extended to give an equivalent modal loss factor when there are other forms of damping such as dry friction, as in the cases of cables and marine risers. Test results on vertical tensioned pipes with pivoted ends and fixed ends are presented. When pivoted joints were used, Coulomb damping in these joints dominated the structural damping behaviour. When the pivoted ends were replaced by fixed ends, the damping values were reduced greatly, and were of a similar level to those measured when the pivots stopped moving in the tests with pivoted ends. It is shown that these low damping values were in part contributed by the dry friction force between the pipe specimen and metal shafts in the push-fit fixings used in the tests. The material damping was predicted by the expression developed to be only a small part of the structural damping in all the tests. These lessons need to be applied to previous results. (C) 1996 Academic Press Limited