Fatigue Stress Estimation of an Offshore Jacket Structure Based on Operational Modal Analysis

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.

A Comprehensive Review on Design, Monitoring, and Failure in Fixed Offshore Platforms

“…It can be said that this stage is of significant importance, and the accuracy of the results is tied to the precision of the initial measured information. Therefore, the method selection and accuracy of sensors is very important in evaluating fatigue life [76]. Failures that occur under dynamic loading conditions are generally called fatigue failures.…”

“…It can be said that this stage is of significant importance, and the accuracy of the results is tied to the precision of the initial measured information. Therefore, the method selection and accuracy of sensors is very important in evaluating fatigue life [76]. Figure 15 presents the comparison of optimization results of critical zones which is related to an SPD19A platform located in the South Pars field in the Persian Gulf.…”

Data‐driven virtual sensing and dynamic strain estimation for fatigue analysis of offshore wind turbine using principal component analysis

“…Since the proposed technique is based on regression, the reader should note that it is similar to the modal expansion technique, [8][9][10][12][13][14][16][17][18] which utilises mode shapes instead of singular vectors. The transformation matrix works as a subspace reduction that removes any noise or response perpendicular to the new subspace of the singular vectors.…”

“…In 2020, Tarpø et al 16 studied the application of expanded experimental mode shapes for stress estimation, concluding that the expansion is equivalent to updating and calibrating the system model. Nabuco et al 17 applied dynamic strain estimation on an offshore tripod structure during operation with high accuracy. There, the uncertainty of the estimated strain response was lower than it is typically assumed in a design process; thus, the strain estimation could reduce the number of planned inspections.…”

Virtual Sensing and Strain Estimation on an Offshore Wind Turbine Using Supervised Learning

“…Tarpø et al [11] studied expansions of experimental mode shapes used for stress estimation and they concluded that expansions of mode shapes is equivalent to updating the system model. Nabuco et al [12] estimated the dynamic full-field strain of an offshore tripod structure during operation with high accuracy. Tarpø et al [13] proved that the modal expansion technique applies to a subsystem within a nonlinear and time-varying system when the nonlinear and time-varying effects act externally on the subsystem.…”