Establishing Platform Foundation Capacity Using Topside Vibration Measurements

This paper describes how a structural vibration monitoring procedure was used in combination with structural analysis modeling to determine the foundation capacity of an existing offshore platform. The project was initiated due to concern that the foundation capacity was marginal for a needed expansion of the topsides process system. The field work consisted of conducting a sequence of vibration measurement for a range of increasing deck loads. Since the platform’s resonant frequencies depend on the structural stiffness and mass, each deck loading (mass) produced a slightly different resonance condition. The set of resonant frequencies obtained for the different deck loads allows one to deduce the stiffness of the structure-foundation system. The stiffness of the steel structure (jacket and deck) was established with structural analysis. The remaining unknown is the axial and lateral stiffness of the pile foundation. Using the torsional resonant frequencies, the lateral stiffness of the pile array and individual piles was established. Then using the translational resonance, the axial pile stiffness was found. Based on these analyses, the platform’s foundation capacity was estimated to be significantly greater than that predicted by the standard pile capacity design recipe. The structure was deemed adequate for the planned topside modification.