Damage Detection To Offshore Platforms Based On Modal Strain Energy Method Considering The Temperature Influence

It is valuable in engineering application to develop global damage detection technology for offshore platforms. For such a huge structure as an offshore platform, the ambient vibration measurement is mainly used for modal identification and damage detection. One of the key problems of this technology is how to eliminate the interference of environmental factors. In ocean environment, the influence of ambient temperature cannot be ignored. Seawater exists temperature difference in depth direction, which is called temperature gradient phenomenon; seawater in different season also exists temperature difference, which appears to be uniform rising. These two kinds of temperature variation are sometimes very significant in offshore area where most of the jacket platforms are located in. In this dissertation, the influence of seawater temperature on modal strain energy method, which is one of the methods for structural global damage detection, were studied. The main work in this paper is described as follows:Firstly, an analysis method of temperature gradient influence on structural global damage detection for offshore jacket platforms is established. Taking a jacket-type offshore platform as a model, the finite element model under the simulated seawater temperature gradient was built regarding the influences of temperature on steel elastic modulus. The accuracy and the feasibility for damage detection using this model was demonstrated as well.Secondly, by detecting and analyzing the damage simulated on this model using mode strain energy method, the influence rules of temperature gradient on damage detection to offshore platform based on modal strain energy method were summarized. It is demonstrated that the temperature gradient weakens the ability of damage location using the method, especially for the members with greater structure redundancy, and it also reduces the precision of damage extent detection, the reduction will be greater for the members with greater structure redundancy and lower damage extent. The modal parameters, which were used in the modal strain energy method, were obtained by two methods. First, the modal parameters based on eigenvalue decomposition of finite element model were used in the analysis, then the modal parameters identified from vibration responses were used in the further analysis. The results obtained form the two methods are coincident, but for the later method, the influence of temperature gradient is a little bigger than the fomer method.Thirdly, on the basis of above research, the damage detection under the condition of seawater temperature uniform rising were studied. The results showed that it had bigger influence on damage location and damage extent identification when bigger temperature difference appeard between the damage model and the baseline model, and the rules of the influence were the same as the influence of temperature gradient. Through comparing the influence of seawater temperature gradient and seawater temperature uniform rising, the fundamental cause of the influence on damage detection caused by seawater temperature is found out, that is the temperature difference between the damage model and the baseline model, and the structural elastic modulus difference caused by it. The bigger the temperature difference is, the bigger the influence is.