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Fatigue in Jacket Structures With Impaired Integrity
Norwegian University of Science and Technology, Faculty of Engineering Science and Technology, Department of Marine Technology.
2012 (English)MasteroppgaveStudent thesis
Abstract [en]

Present regulations for offshore structures on the Norwegian continental shelf have a requirement for Accidental Limit State redundancy and Fatigue Limit State redundancy in a damaged state. However, the requirement is far more defined for the Accidental Limit State than for the Fatigue Limit State. An increased understanding of factors governing fatigue redundancy would create a basis to form a comprehensive definition. In literature, the term redundancy is defined in several ways. The different methods can be divided into two major categories, namely deterministic and probabilistic approaches. In general, redundancy may be defined as the absence of members whose failure would lead to global collapse. Within both the deterministic and the probabilistic framework, several redundancy factors are usually defined, and there are resemblance between some of them. In probabilistic methods, the reliability method is commonly applied through the First Order Reliability Method. Fatigue damage is a primarily concern regarding the integrity for offshore structures. A near constant subjection to cyclic loadings from wind, current and waves initiates a cumulative damage process which leads to a certain fatigue life for the members in the structure. The lifetime may be calculated using either a fracture mechanics approach or a SN-curve approach. There exists several approaches to calculate the stress levels to be used in the fatigue analyses, and the choice of method is mainly based on whether or not the structure under consideration is dynamically behaving or may be regarded as quasi-static. Also, there may be nonlinearities that needs to be accounted for and naturally this will affect the choice of analysis method. However, large uncertainties are associated with fatigue calculations regardless of analysis approach. Thus, a probabilistic framework is highly relevant in order to estimate the risk of failure due to fatigue. A study on how impaired integrity affects the fatigue life has been performed for two jackets; one highly redundant four-legged jacket and one less redundant threelegged jacket. The main goal has been to investigate the fatigue redundancy of the structures, in order to link up the risk of accelerated fatigue due to damage with the risk of failure due to extreme environmental actions, which may eventually lead to structural collapse. A stochastic fatigue analysis approach was chosen, and the analyses was in agreement with the standards governing the Norwegian continental shelf at the time of this thesis. Pushover analyses has been performed on the jackets to give an insight in their redundancy, and a calculation of changes in the natural periods under impaired integrity has also been done. The four-legged jacket was proven to be highly redundant, and had small changes in the natural period under impaired integrity. The three-legged jacket on the other hand, had some damage cases with a rather large increase. Also, the redundancy factor R4 was significantly lower for the three-legged jacket, thus confirming it to be less redundant than the four-legged jacket. A large change in the natural period will alter the dynamic response, thus the fatigue life is vastly connected to changes in global stiffness. Trying to explain the changes in fatigue life for the two jackets under impaired integrity without using a deterministic approach, i.e. calculate the fatigue life for the specific damage case, was proven to be very difficult. There seems to be no easy way to isolate the severity of the fatigue life reduction since large changes are occurring throughout almost the entire structure for several damage cases, as well as large spread in the values them self. However, there was also found some trends in the results. One of them, was that the closer a member is to the damaged element, the larger is the expected reduction in fatigue life. Another trend, is that a large fatigue accelerator factor is expected in almost every damage case, thus one may expect large changes for most of the damage scenarios. Another vastly occurring phenomenon were the location of the damaged members who gave the lowest fatigue life in the structures. For the four-legged jacket, this involved damage in the caisson supports. The three-legged jacket, however, had the lowest fatigue lives occurring for damage cases in the top frame where there is a lack of deterministic redundancy. There seems to be no fatigue redundancy for the jackets, as there are large fatigue accelerator factors occurring. There is also the very low calculated fatigue life in the most extreme cases. However, there has been found a slight correlation between a large reduction in fatigue life and a large initial fatigue life. What should also be taken into account though, is both the risk related to the fatigue lives found, and the accuracy of the values due to the linearised analysis.

Place, publisher, year, edition, pages
Institutt for marin teknikk , 2012. , 170 p.
Keyword [no]
ntnudaim:7156, MTMART Marin teknikk, Marin konstruksjonsteknikk
URN: urn:nbn:no:ntnu:diva-18530Local ID: ntnudaim:7156OAI: diva2:566015
Available from: 2012-11-08 Created: 2012-11-08

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