Halil Karadeniz

An integrated reliability-based design optimization of offshore towers

After recognizing the uncertainty in the parameters such as material, loading, geometry and so on in contrast with the conventional optimization, the reliability-based design optimization (RBDO) concept has become more meaningful to perform an economical design implementation, which includes a reliability analysis and an optimization algorithm. RBDO procedures include structural analysis, reliability analysis and sensitivity analysis both for optimization and for reliability. The efficiency of the RBDO system depends on the mentioned numerical algorithms. In this work, an integrated algorithms system is proposed to implement the RBDO of the offshore towers, which are subjected to the extreme wave loading. The numerical strategies interacting with each other to fulfill the RBDO of towers are as follows: (a) a structural analysis program, SAPOS, (b) an optimization program, SQP and (c) a reliability analysis program based on FORM. A demonstration of an example tripod tower under the reliability constraints based on limit states of the critical stress, buckling and the natural frequency is presented.

Uncertainty modeling in the fatigue reliability calculation of offshore structures

Abstract This paper presents a procedure of modeling uncertainties in the spectral fatigue analysis of offshore structures with reference to the reliability assessment. Uncertainties of the fatigue damage are generally embedded in response characteristics of the stress process and the damage-model used. Besides commonly accepted uncertainties in offshore structural analysis, which are associated with the modeling of structures and the random wave environment, there are also uncertainties arising from joint flexibilities that occur during the response, the wave–current and water–structure interactions. Uncertainties in joint flexibilities are associated with degradation of member connectivities during a response process. Uncertainties introduced by the wave–current interaction are related to the modeling of a random sea state, applied wave loads and water–structure interaction effects in general. The water–structure interaction, which is an important phenomenon to be considered in the analysis of dynamic-sensitive structures, introduces some added hydrodynamic damping. The associated uncertainties are reflected in the response analysis via the damping term. Therefore, in a quasi-static response analysis, these uncertainties disappear. In the spectral fatigue damage, in addition to the uncertainties of stress statistical characteristics there are some other uncertainties associating with the damage-model used. These uncertainties are related to experimentally determined fatigue data and configurations of selected joints at which damages are likely to occur due to high stress concentrations. This paper presents these uncertainty issues with emphasis on the application of a reliability assessment. However, some other uncertainties arise from approximations inherent in the model. They are assumed to be either comparatively negligible or can be considered within the current uncertainty models so that they are not treated further in this paper. In the calculation of the fatigue damage, a non-narrow banded stress process is used.

Optimization of Steel Monopod-Offshore-Towers Under Probabilistic Constraints

In this work, economical design implementation of a circular steel monopod-offshore-tower, which is subjected to the extreme wave loading, is presented. The mass of the tower is considered as the objective function. The thickness and radius of the cross section of the tower are adopted as design variables of the optimization. Moreover, stress or buckling is specified as probabilistic constraints. The numerical strategy employed for performing the optimization uses the International Mathematics and Statistics Library (IMSL) routine that is based on the sequential quadratic programming. The first-order reliability method (FORM) is used for the reliability calculation from a specified limit state function based on the stress or buckling. A demonstration of an example monopod tower is presented.

An improved fatigue analysis for offshore structures

Abstract This paper presents a spectral fatigue damage calcualation on offshore structures subjected to non-narrow banded Gaussian stress processes. The calculation is based on use of the Palmgren-Miners rule and a multilinear S-N fatigue model. In the calculation, an equivalent stress range, which is defined as dependent on the absolute value of stress maxima and a parameter, is used. This parameter is calculated in terms of the spectral bandwidth measure of the stress process and the slope of the S-N line used. For this purpose, the rainflow cycle counting algorithm and the Monte-Carlo procedure are used to determine stress ranges and cycles from time simulations of a number of stress spectra of a monopod structure. A relation between the equivalent stress range parameter and the commonly known damage correction factor is constructed. Cumulative damage arising during a sea state and in a long-term period is presented.

Reliability-based optimisation of offshore jacket-type structures with an integrated-algorithms system

Reliability-based optimisation (RBO) is a powerful tool for including uncertainties in the optimisation process, in which structural and reliability analyses and optimisation algorithms based on mathematical or evolutionary computation concepts have to be combined effectively. This process is rather complicated and difficult to carry out for large structural systems such as steel offshore structures. In this paper, a calculation system of integrated algorithms for the RBO of the offshore towers is presented. The calculation process is composed of a structural analysis package (SAPOS) based on the finite element method, a reliability analysis program based on the first-order reliability method and an optimisation program based on sequential quadratic programming using the International Mathematics and Statistics Library. In the RBO analysis, multiple limit states based on different criteria are used to check a probable failure condition and to identify the limit state criterion. An offshore jacket-type structure is considered as an example to demonstrate the applicability of the implemented algorithm to realistic structural systems.

An integrated framework including distinct algorithms for optimization of offshore towers under uncertainties

A reliability analysis is usually required to carry out design optimization of large structural systems to incorporate the uncertainties on the parameters such as material properties, external loads, manufacturing condition, etc. This procedure is called Reliability Based Design Optimization (RBDO), and requires a structural analysis program, a reliability analysis and optimization tools to couple effectively. In this paper, an integrated framework is proposed to implement the RBDO of the offshore towers. It has two distinct approaches to evaluate the probabilistic constraints; namely Reliability-Index based Approach (RIA) and Performance Measure Approach (PMA). The proposed framework also suggests Sequential Quadratic Programming (SQP) and Differential Evolution (DE) as optimization methods. Examples of monopod, tripod, and jacket towers under the reliability constraints based on limit states of the critical stress, buckling, and the natural frequency are presented to demonstrate the applicability of the implemented algorithm to realistic structural systems.

Introduction to Random Vibration and Stochastic Analysis

This chapter is included to provide basic information and essential formulation of random vibration and stochastic analysis that needed in the offshore structural analysis. It contains seven sections. The first section describes briefly random vibration and its occurrence in practice. The second section explains some definitions in the probability theory and presents their formulations that to be used in the probabilistic analysis. The third section explains calculation of probability moments of random variables and random functions. Then, commonly used probability distribution models are presented. In the fourth section, random processes, ensemble averages and expected values, stationary and ergodic processes are explained, and then differentiation of stochastic processes are summarized. In the fifth section, spectral analysis is explained with emphasis on band-limited, narrow-banded and broad-banded processes, crossing analysis and probability distribution of maxima. The sixth section presents input–output relations of stochastic processes and transfer functions. In the seventh section, some illustrative examples are provided.

Optimization of Steel Monopod Offshore-Towers Under Probabilistic Constraints

In this work, economical design implementation of a circular steel monopod-offshore-tower, which is subjected to the extreme wave loading, is presented. The mass of the tower is considered as the objective function. The thickness and radius of the cross-section of the tower are adopted as design variables of the optimization. Moreover, stress or buckling is specified as probabilistic constraints. The numerical strategy employed for performing the optimization uses the IMSLLibraries routine that is based on the Sequential Quadratic Programming (SQP). The FORM is used for the reliability calculation from a specified limit state function based on the stress or buckling. A demonstration of an example monopod tower is presented.

Reliability Analysis of Offshore Structures

This chapter is devoted to the reliability analysis of offshore structures. It contains seven sections. The first section describes uncertainties in general and gives information about the reliability methods. The second section presents basic definitions and structural reliability methods in more detail. Calculation of the reliability index β by the FORM and SORM methods are explained for nonlinear failure functions of non-Normal correlated design variables in general. The calculation algorithms and flow diagrams are presented. Then, the numerical integration (NI) and Monte Carlo simulation (MCS) techniques of the Level-III (exact) reliability methods are summarized. The third section presents the inverse reliability method and its calculation algorithm. The fourth section describes uncertainties in the spectral stresses and fatigue damages of offshore structures, which arise from different origins. The fifth section formulates stress spectrum and spectral moments in a reduced uncertainty space. The sixth section explains the fatigue reliability calculation of offshore structures and provides calculation algorithms. The seventh section demonstrates the reliability calculations.

Reliability Based Optimization of Steel Monopod Offshore-Towers

In this work, the implementation of reliability-based optimization (RBO) of a circular steel monopod-offshore-tower with constant and variable diameters (represented by segmentations) and thicknesses is presented. The tower is subjected to the extreme wave loading. For this purpose, the deterministic optimization of the tower is performed with constraints including stress, buckling, and the lowest natural frequency firstly. Then, a reliability-based optimization of the tower is performed. The reliability index is calculated from FORM using a limit state function based on the lowest natural frequency. The mass of the tower is considered as being the objective function; the thickness and diameter of the cross-section of the tower are taken as being design variables of the optimization. The numerical strategy employed for performing the optimization uses the IMSL-Libraries routine that is based on the Sequential Quadratic Programming (SQP). In addition, to check the results obtained from aforementioned procedure, the RBO of the tower is also performed using the genetic algorithms (GA) tool of the MATLAB. Finally, a demonstration of an example monopod tower is presented.Copyright