Optimising non-destructive examination of newbuilding ship hull structures by developing a data-centric risk and reliability framework based on fracture mechanics

This thesis was previously held under moratorium from 18/11/19 to 18/11/21Ship structures are made of steel members that are joined with welds. Welded connections may contain various imperfections. These imperfections are inherent to this joining technology. Design rules and standards are based on the assumption that welds are made to good a workmanship level. Hence, a ship is inspected during construction to make sure it is reasonably defect-free. However, since 100% inspection coverage is not feasible, only partial inspection has been required by classification societies. Classification societies have developed rules, standards, and guidelines specifying the extent to which inspection should be performed. In this research, a review of rules and standards from classification bodies showed some limitations in current practices. One key limitation is that the rules favour a “one-size-fits-all” approach. In addition to that, a significant discrepancy exists between rules of different classification societies. In this thesis, an innovative framework is proposed, which combines a risk and reliability approach with a statistical sampling scheme achieving targeted and cost-effective inspections. The developed reliability model predicts the failure probability of the structure based on probabilistic fracture mechanics. Various uncertain variables influencing the predictive reliability model are identified, and their effects are considered. The data for two key variables, namely, defect statistics and material toughness are gathered and analysed using appropriate statistical analysis methods. A reliability code is developed based Convolution Integral (CI), which estimates the predictive reliability using the analysed data. Statistical sampling principles are then used to specify the number required NDT checkpoints to achieve a certain statistical confidence about the reliability of structure and the limits set by statistical process control (SPC). The framework allows for updating the predictive reliability estimation of the structure using the inspection findings by employing a Bayesian updating method. The applicability of the framework is clearly demonstrated in a case study structure.Ship structures are made of steel members that are joined with welds. Welded connections may contain various imperfections. These imperfections are inherent to this joining technology. Design rules and standards are based on the assumption that welds are made to good a workmanship level. Hence, a ship is inspected during construction to make sure it is reasonably defect-free. However, since 100% inspection coverage is not feasible, only partial inspection has been required by classification societies. Classification societies have developed rules, standards, and guidelines specifying the extent to which inspection should be performed. In this research, a review of rules and standards from classification bodies showed some limitations in current practices. One key limitation is that the rules favour a “one-size-fits-all” approach. In addition to that, a significant discrepancy exists between rules of different classification societies. In this thesis, an innovative framework is proposed, which combines a risk and reliability approach with a statistical sampling scheme achieving targeted and cost-effective inspections. The developed reliability model predicts the failure probability of the structure based on probabilistic fracture mechanics. Various uncertain variables influencing the predictive reliability model are identified, and their effects are considered. The data for two key variables, namely, defect statistics and material toughness are gathered and analysed using appropriate statistical analysis methods. A reliability code is developed based Convolution Integral (CI), which estimates the predictive reliability using the analysed data. Statistical sampling principles are then used to specify the number required NDT checkpoints to achieve a certain statistical confidence about the reliability of structure and the limits set by statistical process control (SPC). The framework allows for updating the predictive reliability estimation of the structure using the inspection findings by employing a Bayesian updating method. The applicability of the framework is clearly demonstrated in a case study structure

Review of available probabilistic models of the crack growth parameters in the Paris equation

Crack growth rate parameters of the Paris equation are crucial inputs in the engineering critical assessment (ECA) of structures containing flaws. In fracture mechanics based reliability analysis, probabilistic models of these parameters are often used. Despite the considerable body of research in this area, there is significant variability among available models. This paper reviews the current available models in the literature and addresses areas requiring further research with a view to assisting probabilistic flaw assessment. The effect of the existing variability in crack growth model parameters is investigated by fracture mechanics analysis of a case study crack

Estimation of weld defects size distributions, rates and probability of detections in fabrication yards using a Bayesian theorem approach

Estimation of probability detection curves for non-destructive evaluation (NDE) typically involves the manufacturing of a high number of defect specimens followed by trial NDE and statistical analysis of the data based on the hit/miss approach. This is a time-consuming and costly procedure. Besides, probability of detection (POD) depends on a number of variables, such as human factors (operator), and the testing environment, resulting in a significant mismatch between those POD curves generated in the lab and those in practice. One application of POD curves is in the quality control of welded joints [1]. Weld quality is often characterised by the number of defects found and their size which is, inevitably, dependent on the POD of the employed NDE. Therefore, a predefined generic POD curve has certain limitations. In this paper, a method of estimating POD curves based on the Bayesian theorem of conditional probability is presented and its applicability is validated by studying an existing database under both Bayesian and the hit/miss methods. Overall, the POD predicted by the Bayesian theorem is found to be consistent with the commonly used hit/miss model. Finally, the Bayesian model is used to estimate the POD, and the true weld defect size and frequency in two ship manufacturing yards. The estimated weld defect size and frequency models provide valuable information to estimate the fatigue and fracture reliability of ship and offshore structures. It is shown that one of the yards has both better weld quality production and superior NDE detection. This will have a valuable benefit for weld quality control (QC) programmes through saving the testing resources

A review of nondestructive examination methods for new-building ships undergoing classification society survey

Classification societies require ship manufacturers to perform nondestructive examination (NDE) of ship weldments to ensure the welding quality of new-building ships. Ships can contain hundreds of kilometers of weld lines and 100% inspection of all welded connections is not feasible. Hence, a limited number of weldments are specified by rules of classification societies to be inspected on a sampling basis. There is a variation between the rules and guidelines used by different classification societies in terms of both philosophy and implementation which results in significant discrepancy in the prescribed checkpoints, numbers, and their locations. In this article, relevant sections of the rules of mainstream International Association of Classification Societies members are studied and potential ways of improving them are discussed. The authors have endeavored to make this study as comprehensive as much as possible. However, given the challenges of covering every single aspect and variable related to NDE in the classification societies’ rules and guidelines reviewed here, the authors can only attempt to cover the key features

A fracture mechanics framework for optimising design and inspection of offshore wind turbine support structures against fatigue failure

Offshore wind turbine (OWT) support structures need to be designed against fatigue failure under cyclic aerodynamic and wave loading. The fatigue failure can be accelerated in a corrosive sea environment. Traditionally, a stress-life approach called the S-N (stress-number of cycles) curve method has been used for the design of structures against fatigue failure. There are a number of limitations in the S-N approach related to welded structures which can be addressed by the fracture mechanics approach. In this paper the limitations of the S-N approach related to OWT support structure are addressed and a fatigue design framework based on fracture mechanics is developed. The application of the framework to a monopile OWT support structure is demonstrated and optimisation of in-service inspection of the structure is studied. It was found that both the design of the weld joint and non-destructive testing (NDT) techniques can be optimised to reduce in-service inspection frequency. Furthermore, probabilistic fracture mechanics as a form of risk-based design is outlined and its application to the monopile support structure is studied. The probabilistic model showed a better capability to account for NDT reliability over a range of possible crack sizes as well as to provide a risk associated with the chosen inspection time which can be used in inspection cost-benefit analysis. There are a number of areas for future research, including a better estimate of fatigue stress with a time-history analysis, the application of the framework to other types of support structures such as jackets and tripods, and integration of risk-based optimisation with a cost-benefit analysis

Weld defect frequency, size statistics and probabilistic models for ship structures

Ships undergo cyclic loading which combined with weld defects can cause fatigue failure. Remaining fatigue life of structures containing defects can be estimated using the defect size. The defect data for ships is non-existent in literature or belong to old offshore structures. In this research, the data collected from two ships are presented. The statistical analysis of the data shows that the Hybrid Laser Welding has lower defect rates than other common arc welding processes indicating that less quality control inspection may be allowed. The defect length values from the studied ships were smaller than those from offshore structures

Application of probabilistic fracture mechanics in risk based non destructive examination of new building ships

Risk based methods can be used to optimise None Destructive Examination (NDE) planning of new building ship hull structures. A key step in this method is estimation of failure probability of fabrication weld defects under fatigue loading. This is achieved through probabilistic fatigue and fracture mechanics assessment. In this paper probabilistic fatigue and fracture mechanics analysis of a butt weld for a ship deck plate is presented

Sensitivity analysis of design parameters for reliability assessment of offshore wind turbine jacket support structures

Offshore Wind Turbine (OWT) support structures are subjected to hostile environments, defined by highly stochastic loads and complex soil-structure interaction, and thus the need for a probabilistic approach towards design. The study carried out herein presents the sensitivity analysis of these inherent stochastic variables imposed on a complex OWT support structure via purpose-developed modular non-intrusive structural reliability assessment formulation. The results from this study reveal that the uncertainties in the wind speed is a structural design driving factor and the hydrodynamic load effects are secondary to this, for the ultimate (ULS) and Fatigue Limit States (FLS) while their relative sensitivities on the Serviceability Limit State (SLS) cannot be clearly distinguished but are seen to have a dominant impact. Also, it was inferred that incorporating correlation between the variables have a significant impact on the reliability of the structure in the ULS design

A reliability-constrained optimisation framework for offshore wind turbine support structures

A new approach that links finite element analysis (FEA) and a genetic algorithm (GA) with Monte Carlo simulation (MCS) for the reliability-constrained optimal design of offshore wind turbine (OWT) support structure is presented. This approach has been applied to optimise the NREL 5MW OWT on OC3 support structure. The objective function minimises the weight of the support structure, constrained to design and performance limits, by attaining the desired reliability level. A response analysis of the reference OWT is performed to estimate design loads. Then, deterministic optimisation (DO) is carried out to find the candidate design solutions. After that, a reliability assessment is conducted to apply the target reliability constraint. This study reveals that the design of a monopile support structure is mainly driven by fatigue limit state. Also, there is no guarantee that DO candidate designs always meet the structural reliability target level in all capacities

A reliability-constrained optimisation framework for offshore wind turbine support structures

A new approach that links finite element analysis (FEA) and a genetic algorithm (GA) with Monte Carlo simulation (MCS) for the reliability-constrained optimal design of offshore wind turbine (OWT) support structure is presented. This approach has been applied to optimise the NREL 5MW OWT on OC3 support structure. The objective function minimises the weight of the support structure, constrained to design and performance limits, by attaining the desired reliability level. A response analysis of the reference OWT is performed to estimate design loads. Then, deterministic optimisation (DO) is carried out to find the candidate design solutions. After that, a reliability assessment is conducted to apply the target reliability constraint. This study reveals that the design of a monopile support structure is mainly driven by fatigue limit state. Also, there is no guarantee that DO candidate designs always meet the structural reliability target level in all capacities