Abstract
Fatigue life assessment currently recommended by offshore standards is associated with a large number of uncertainties mainly related to the environmental loads and the numerical model. Recently, for economic reasons, the need for extending the lifetime of existing offshore structures led to the necessity of developing more accurate and realistic predicting models so that damage detection and maintenance can be optimized. This paper proposes the implementation of Structural Health Monitoring Systems in order to extract modal properties—such as mode shapes, natural frequencies, and damping ratios—throughout Operational Modal Analysis (OMA), which is the engineering field that studies the modal properties of systems under ambient vibrations or normal operating conditions. The identified modal properties of the structural system are the fundamental information to update a finite element model by means of an expansion technique. Then, the virtual sensing technique—modal expansion—is used to estimate the stress in the entire structure. Though existing models depend on the load estimation, the model based on OMA-assisted virtual sensing depends on the measured responses and assumes that the loads act as random vibrations. A case study using data from a real offshore structure is presented based on measurements recorded during normal operation conditions of an offshore tripod jacket. From strains estimated using OMA and virtual sensing, fatigue stresses are predicted and verified by applying the concept of equivalent stress range. Both estimated and measured strains are given as input data to evaluate the equivalent stress range and compared with each other. Based on this study, structural health monitoring estimates the fatigue stresses with high precision. As conclusion, this study describes how the fatigue can be assessed based on a more accurate value of stress and less uncertainties, which may allow extending the fatigue life of offshore platforms.
Highlights
Fatigue life is stated in terms of stress ranges that are produced by the variable loads imposed on a structure. e most common variable loads affecting offshore structures are the waves
Because uncertainties are associated with the environmental loads and to the numerical model, it yields to the use of safety factors underestimating the operational fatigue life of the offshore structure. ese uncertainties could be reduced by monitoring the structure with the use of strain gauges
Quality measurements have been applied to the stress estimation findings to quantify the correlation between estimated and measured responses. e Time Response Assurance Criterion (TRAC) values are between 92.5% and 94.5%, which is considered to be high especially for the case of measurements from a real offshore structure and confirms that the strains of a structure can be estimated with good precision by performing structural health monitoring
Summary
Fatigue life is stated in terms of stress ranges that are produced by the variable loads imposed on a structure. e most common variable loads affecting offshore structures are the waves. Current existing models can predict the evolution of fatigue damage over time by estimating the loads based on wave statistics and applying them into a numerical model Several approaches follow these models, as recommended by offshore design codes [1] and more recent studies, for instance, the fatigue methodology proposed by Mourão et al [2] using local damage parameters. In the literature on stress estimation, the modal expansion is one of the most popular process models, which is a linear transformation that expands the system response based on the identified mode shapes [8]. The modal expansion has been applied to nonlinear systems In this way, Nabuco et al [12] used modal expansion based on parameters determined from a linear case and successfully estimated the strain responses of two scaled offshore platforms connected with a friction structure. To the best of the authors’ knowledge, the application of stress estimation on structures in operation is limited to offshore wind turbines [17,18,19,20,21,22], stadium [23], and lattice structure [9, 24, 25]. us, this paper adds important information to stress estimation by applying this technique on an operating offshore platform
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