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Nowadays, natural load excitation cannot detect all modal problems when testing vibration responses of offshore platforms. To overcome this issue, the present paper proposes a novel method to detect structural damages of offshore platforms based on grouping modal strain energy. This method divides the unit modal strain energy into axial tension-compression and bending. It may not only expand known modals and overcome the incomplete modal parameters under natural load excitation, but also could estimate the damage locations based on low-order modal parameters. The proposed method was verified by numerical simulations. Based on simulations on different damage conditions of a jacket offshore platform, damage locations could be determined accurately through comprehensive analysis of grouping modal strain energy distribution. [ABSTRACT FROM AUTHOR]

A new damage assessment method is proposed for aging offshore platforms based on dynamic tests, which can provide information on whether damages occurred between the times of two adjacent measurements. A new damage indicator is defined to reduce the influences of existing damages accumulated before the first measurement. One theoretical improvement is that the requirement for using the stiffness of the finite element model to replace that of the measured model is unnecessary in the calculation of the modal strain energy. The other is that damages that occurred in periodic measurements of aging platforms can be assessed separately, even for those platforms that have not been tested during their previous service life. A numerical jacket platform is chosen, and the results show that the proposed method can properly identify the damages that occurred between the two measurements and can estimate the damage severity, even when 5% corrupted mode shapes are used. In addition, the method also can greatly reduce the influence of the damages accumulated before the first measurement, which means that the platform׳s integrity can be assessed by implementing periodic dynamic tests without any other test data. [ABSTRACT FROM AUTHOR]

Damage may occur in the pile foundations of high-pile wharfs due to the complex and diverse working conditions. To identify the damage in pile foundations in early stage is very important for the operation and maintenance on wharfs. Research on pile damage identification based on modal strain energy is one of the hot research topics. This study presents the concept of the modal strain energy residual variability using the characteristics of sensor placement and pile mode shapes of high-pile wharfs, which provide a simple, accurate and practical method to identify the damage in pile foundations of high-pile wharfs in early stage. The qualitative and quantitative relationships between the modal strain energy residual variability and element damage extent are deduced. This concept not only enhances the sensitivity of the modal strain energy to damage but also realizes the decoupling of damage element identification, which is easy to calculate in practical engineering. A damage identification method is constructed base on the concept, and the accuracy of the method is verified by a finite element numerical simulation and model test. A sensor placement method for engineering detection is studied by comparing the damage identification effect in different sampling intervals. The results show that this method can accurately locate a damage area and accurately identify the damage extent in numerical simulation and model test, which provides a new method for the practice of pile foundation damage identification of high-pile wharfs. • Location damage identification based on modal strain energy residual variability. • Quantitative damage identification based on the modal strain energy residual variability. • S implification of the dynamic analysis of a high-pile wharf. • Effect of the sampling interval on damage identification. [ABSTRACT FROM AUTHOR]

Abstract: The development of robust techniques for early damage detection for offshore structures is crucial to avoid the possible catastrophe caused by structural failures. This article applies the cross-model cross-mode (CMCM) method for damage detection that is capable of identifying the damage to individual members of offshore jacket platforms, when limited, spatially incomplete modal data is available. Basically, the CMCM method is classified as a direct, physical property adjustment model updating method. Implementing this method requires only a few modes measured from the damaged structure. In dealing with spatial incompleteness, this paper investigates both model reduction and modal expansion techniques. Specifically, either Guyan (static condensation) or SEREP (system equivalent reduction expansion process) transformation matrix, between the master and slave degrees-of-freedom, is employed in the model reduction or modal expansion process. One theoretical development is an iterative procedure to compute the transformation matrix associated with the (unknown) damaged structure. Numerical studies are conducted for a jacket platform with multiple damaged members based on synthetic data generated from finite-element models. The results suggest that (i) Guyan scheme always outperforms SEREP, (ii) model reduction is always better than modal expansion, and (iii) the CMCM method in conjunction with iterative Guyan reduction approach yields the best damage location and severity estimate. [Copyright &y& Elsevier]