Vortex Induced Motions of the Horn Mountain Truss Spar

Abstract

Vortex Induced Motion (VIM) of a Spar platform is an important consideration in the design of Spar moorings and risers. During the design phase, the VIM characteristics of a Spar configuration are evaluated by hydrodynamic model testing. The basis for hydrodynamic model testing is that geometric and dynamic similitude between prototype and model parameters is preserved. Reynolds number and Froude number scaling are the two relevant scaling parameters for Spar VIM model testing. However, simultaneously satisfying Reynolds and Froude scaling for the model and prototype conditions is practically impossible. This leads to compromises in model test Reynolds number scaling and inherent uncertainties in extrapolating the model test VIM predictions to full-scale design conditions. Hence measurement of full-scale Spar VIM responses provides valuable data for validation of design assumptions. BP’s integrity management efforts for its Gulf of Mexico deep water production facilities include monitoring of the environmental conditions, and response of the hull, mooring and riser systems. Field measured data for VIM of BP’s Horn Mountain Truss Spar, presented in this paper, shows that the Horn Mountain Truss Spar VIM response is within original design assumptions. The maximum VIM measured in the field (A/D ≈ 0.26) is well below the acceptable design value of A/D = 0.5. The field data is analyzed and results presented in the form of Spar VIM response time traces, Spar VIM response versus Reduced Velocity and variation of period of VIM oscillation with Spar offsets. The field data shows VIM response characteristics consistent with model test observations.