VIV OF FREE SPANNING PIPELINES: COMPARISON OF RESPONSE FROM SEMI-EMPIRICAL CODE TO MODEL TESTS

The purpose of this paper is to compare predictions of vortex-induced vibrations (VIV) from a semi-empirical program to experimental data. The data is taken from a VIV model test program of a free span pipeline using a long elastic pipe model. Both in-line (IL) and cross-flow (CF) vibrations are compared. The Norwegian Ormen Lange field development included pipelines laid on very uneven seafloors, resulting in many free spans. As part of the preparations for this field development, VIV model tests of single- and multi-span pipelines were carried out at MARINTEK for Norsk Hydro, which later became a part of Statoil. The VIVANA program is a semi-empirical frequency domain program based on the finite element method. The program was originally developed by MARINTEK and the Norwegian University of Science and Technology (NTNU) to predict cross-flow response due to VIV. The fluid-structure interaction in VIVANA is described using added mass, excitation and damping coefficients. Default curves are available or the user may input other data. VIVANA originally included only cross-flow excitation but pure in-line excitation was later added. Recently, simultaneous cross-flow and in-line excitation has also been included. At present, the excitation in the cross-flow and in-line directions is not coupled. Coefficients for simultaneous cross-flow and in-line excitation have been proposed and are available in VIVANA. In this paper, response predicted by VIVANA has been compared to the Ormen Lange model tests for selected test series. The analyses with pure IL loading gave good estimates of IL response up to and beyond the start of CF response. The analyses with combined CF and IL loading gave good response estimates for the test series with a long span. The experiments with short spans tended to give CF and IL mode 1 response while the present version of the program gave IL response at higher modes. The present coefficient based approach is, however, promising. Further work should aim at establishing better coefficients and to understanding the interaction between CF and IL response.