Rapid direct-mode shape expansion for offshore jacket structures using a hybrid vector

A rapid direct-mode shape expansion methodology is proposed for offshore jacket structures. When sensors are not positioned at an underwater portion of the offshore structure, rapid mode shape expansion techniques should be used for structural damage detection. This work applies a hybrid vector to expand the mode shape. The hybrid vector is constructed by measured values at master degrees of freedom (DOFs) and constant values at slave DOFs based on a series of modification factors. One theoretical development is that the proposed approach is a direct method for unmeasured component estimation, which contrasts with traditional methods. The other development is that the proposed approach is a non-iterative method, which ensures the efficiency of unmeasured mode shape value estimation; a jacket platform is chosen for numerical studies, and the results show that the approach provided a reliable estimation of unmeasured components, even for higher mode shapes, when modelling errors are neglected and the measurement noise is in a certain range. The performance of the proposed method was also investigated using dynamic tests from a steel platform experiment. The results confirm that the proposed method outperforms commonly used existing expansion methods.

► We propose a new method dealing with spatially incomplete experimental mode shapes for three dimensional structures.
► A hybrid vector is defined and values at slave coordinates are estimated without iteration and moelling errors are neglected.
► The proposed approach can be applied to three dimensional structures, such as offshore platforms.
► The approach can expand mode shapes more accurately and reliably for the lower order mode shapes.