Reliability of Marine Structures in the Context of Risk Based Design

Teixeira, Angelo Manuel Palos (1997) Reliability of Marine Structures in the Context of Risk Based Design. MSc(R) thesis, University of Glasgow.

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The aim of this thesis is to establish the suitability of the theory of structural reliability as the appropriate tool to establish the notional probability of failure for different structural designs with respect to the same adverse state in the context of risk based design. In chapter 1 the problem is introduced by a brief presentation of some accident statistical data on total losses of the world merchant fleet. The analysis of this data shows that only an overall risk model can explain the difference between the real and the notional safety levels, which are estimated from the structural reliability theory. Although a safety index can make no claim of representing an absolute measure of the probability of failure, it can be included in an overall risk model providing an efficient mean of comparing the safety levels of different structures. In this context, two examples of integration between risk and reliability analysis are then presented. Chapter 2 reviews the development of the methods suggested by a number of writers in recent years for the calculation of an "index of reliability" and the associated estimates of failure probability. In chapter 3 the methods of structural reliability are used to assess the reliability of the primary ship structure of four tankers with respect to the ultimate collapse moment. The stochastic model of still water bending moment is defined for one individual voyage based on available data from general ship statistics. The evaluation of the wave induced load effects that occur during long-term operation of the ship in the seaway is carried out for the North Atlantic. A more rigorous formulation of the reliability problem is defined by requiring that the structure is safe under the combined maximum of still water and wave induced bending moment that occurs in a reference period. The reliability results using these two formulations are compared showing that these formulations can be related to each other and the choice of one or the other is a matter of standardisation in order to allow the ship structures to be compared. The results of the reliability analyses are used to assess the partial safety factors that can be applied in a probabilistic based design rule for a defined target safety level. As an example, the design formula is used to redesign the midship section of one of the sample ships in order to meet the target failure probability considered in the rule development process. The reliability formulation is also applied to different ship types with the objective of achieving indications in the safety levels of the different designs. The reliability results of one containership and two different designs of a bulk carrier are compared with the ones obtained for tankers. Additionally, the variability in notional reliability levels that result from the ships being subjected to different wave environments in European coastal waters is quantified. In chapter 4 a reliability formulation is proposed for thermally insulated plates subjected to pool fires. The basic features of the fire model, of the heat transfer through the passive protection and of the collapse temperature of plates are described. A systematic study on plate collapse under heat loads with uniform distribution in the plate was performed using a non-linear finite element code that accounts for the elasto-plastic behaviour and for the changes in the material properties. The load-shortening behaviour of plates with different aspect ratios, slenderness and initial imperfections are presented. Since plate elements are part of a structure, its boundary conditions are far from being fixed. This effect is studied using elastic supports as well as localised heat loads. The basic mechanisms that influence the shape of the flame of a pool fire are described and a first order second moment approach is used to quantify the uncertainty of the heat loads and to describe the importance of the governing variables in the limit state function. The limit state function is defined in terms of steel temperature and the reliability index is determined by a time independent first order method. An example of the reliability analysis of a fire wall protected with insulation is provided. A different reliability formulation is defined when the heat load is not applied to the whole plate surface, but instead is localised in area. In this case the plates are able to sustain additional in-plane compressive loads before collapse. Therefore, it is appropriate to formulate the reliability problem in terms of stresses because this is the condition that will govern collapse. Calculations are presented concerning the effect of the different parameters on the reliability of plates, and in particular the effect of the size of the heated area is quantified.

Item Type: Thesis (MSc(R))
Qualification Level: Masters
Additional Information: Adviser: C Guedes Soares
Keywords: Ocean engineering
Date of Award: 1997
Depositing User: Enlighten Team
Unique ID: glathesis:1997-75884
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 19 Dec 2019 09:15
Last Modified: 19 Dec 2019 09:15

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