Model Updating of a Jacket Structure by Mass Reallocation Method and Design Optimization Considering Unity Check

Abstract

This dissertation proposes a mass reallocation method for structural model updating of a finite element (FE) model on an offshore jacket structure and proposes an optimal design of jacket structures considering unity check criterion. Several offshore jacket structures have been operated as ocean research stations (ORSs) in Korea. In 2011, the Gageocho ORS was attacked by Typhoon Muifa, and its structural members and several observation devices were severely damaged. After this event, the Gageocho ORS was rehabilitated with a 5-m height to account for the 100-yr extreme wave height, and with concrete grouting in the four legs of the jacket. The vibration measurement system was equipped to monitor the structural vibrational characteristics including natural frequencies and modal damping ratios.

As the first part of this thesis, the mass reallocation method is presented for structural model updating of the Gageocho ORS based on the experimentally identified natural frequencies. A preliminary FE model is constructed based on design drawings, and several of the candidate baseline FE models are manually built, taking into account the different structural conditions such as corroded thickness. Among these candidate baseline FE models, the most reasonable baseline FE model is selected by comparing the differences between the identified and calculated natural frequencies. The baseline FE model is updated based on the identified modal properties, and by using the pattern search method, which is one of direct search optimization methods. The mass reallocation method is newly proposed as a means to determine the equivalent mass quantities along with the height and on a floor.

It is found that the natural frequencies calculated based on the updated FE model are very close to the identified natural frequencies. It is expected that these results, which are obtained by updating a baseline FE model, can be useful for establishing the reference database for jacket-type offshore structures, and assessing the structural integrity of the Gageocho ORS.

As the second part, this thesis proposes an optimal design of the jacket structure. Generally, offshore jacket structures are made up of steel members, and therefore, the safety standards for the structures usually concern the steel components. This study hence aims to provide a safe and lightweight design of jacket structures only composed of steel materials without a large amount of concrete in the jacket legs.

The structural elements are grouped according to their roles. Based on the actual design, the rate of change in the inner diameter of sections within a group is defined as a design variable. The optimization is carried out by a genetic algorithm to minimize the total weight of the parts to be modified. To meet the conservative safety standards in the offshore field, both the maximum stress and the unity criteria are applied as constraints in the optimization. For greater safety confidence, extreme environments are assumed. The maximum of marine attachment thickness and section erosion in the splash zone is applied. The design load is defined as forces induced by extreme waves, winds, and currents in the same direction. Each of the eight load directions surrounding the structure is considered to design the structure in a balanced and safe manner.

As an outcome, a design much lighter that satisfies the strict safety criteria is found. This proves that the structural optimization method could help save material costs and thus is beneficial for designing offshore structures. There is a significance in that this approach is broadly applicable to other offshore structures or offshore wind turbines in the future.