In the offshore industry, modeling pipe vibrations due to current is important to predict structural fatigue life. In the case of Wake Induced Oscillations (WIO), clashing is also an issue during the design phase to be able to define enough clearance to prevent clashing. If not possible, it is then needed to estimate contact energy between pipes and ensure that clashing is acceptable. Wake induced oscillations are difficult to predict, involving Vortex-Induced Vibrations (VIV) at relatively high frequency and small displacement together with larger motion at lower frequency leading to potential contact. A hydrodynamic model is proposed to predict WIO of a flexible pipe in the wake of an upstream pipe. The structural displacement of the two pipes is computed with a classical finite element model. The pipes are linked thru two dimensional strips where the hydrodynamic loads are computed based on the pipe distance in the strip. Since both pipes are flexible, the upstream pipe is subjected to VIV while the downstream pipe is subjected to the mean wake created by the pipe, to VIV as well as WIO. Each effect is represented by a simplified model. A Blevins model is used to represent the quasi-static drag and lift forces on the downstream riser. The VIV on the upstream riser is computed with a Van der Pol oscillator model, and a similar model is used for the downstream riser with an added term to account for the upstream riser presence. Experimental results on two tandem jumpers are used to validate the approach in steady current and in current plus wave. The database has a large number of model tests with different initial gaps between the two jumpers. Dynamic response of the two pipes is measured thru accelerometers and tension sensors. Some of the configurations exhibit clashing and/or overlapping of the jumpers. Amplitude and spectra of vibrations are compared to the proposed model; the general characteristics of the interaction (contact, overlapping) are also addressed.

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