Unsteady aerodynamics of offshore floating wind turbines using free vortex wake model

Among the offshore floating wind turbine software packs, the blade element momentum theory (BEM) and generalized dynamic wake (GDW) model are widely used. A free vortex wake model has been coupled to FAST v7 to do a comparative dynamic analysis between using the BEM theory and GDW method on offshore floating wind turbine. The verification test on the free-wake model has been performed according to the NREL VI experiment in steady and yaw conditions. To analyze the unsteady aerodynamics of floating wind turbine, the OC3 spar type wind turbine has been used to do simulations. The global performances on both the rotor and the platform and their interactions are shown and discussed

How to determine the principal dimensions of FPSO vessel

The evaluation of the principal dimensions of the Floating Production, Storage and Offloading (FPSO) system is one of the most critical tasks at the initial design stage of the vessel. It is therefore important to get this right from the onset. This paper presents a simple method of determining the optimal principal dimensions of FPSO vessels of any specified oil storage capacity. An interactive programme, the Principal Dimensions Programme (PDP) has therefore been designed to accurately evaluate them based on the required cubic number (L× B × D) and the needed oil storage capacity (as the modern segregated vessels are volume-limited). The prediction of these dimensions has been given to ensure a safe operation and optimal performance of the vessels with regards to their motion responses in deep sea waves

Fragility reduction of offshore wind turbines using tuned liquid column dampers

High flexibility of offshore wind turbines (OWTs) makes them vulnerable to excessive vibrations. This paper studies vibration control of offshore wind turbines induced by multi-hazard excitations. A model consisting of entire offshore wind turbine foundation and tower controlled by tuned liquid column dampers(TLCD) considering nonlinear soil pile interaction is established. The model is subjected to wave, wind, and seismic loading. The effect of severity of earthquake on the performance of the structural control device is investigated. A fragility analysis based on acceleration capacity thresholds is performed to estimate reliability improvement using the structural control devices. The fitted fragility functions based on multiple stripes analysis are constructed and compared with the empirical cumulative distribution curves. The results suggest that the use of an optimal TLCD with a mass ratio of 2.5% reduces the fragility of the system by as much as 6% and 12% for operational and parked conditions, respectively

Updating the distributions of uncertain parameters involved in fatigue analysis

Fatigue is an important failure mode in offshore jacket platforms. To evaluate the fatigue damage, these platforms are periodically inspected during their lifetime. Regarding fatigue damage, the information from inspection consists of crack measurement. A Bayesian framework can be used to update the probability distribution of the crack size. The main purpose of this study is to develop a framework to update the probability distributions of all uncertain parameters involved in the fatigue crack growth analysis. This methodology maximizes the benefit of the inspection results by updating several uncertain parameters involved in the fracture mechanics approach. Two sets of cracks are used to obtain the updated distributions for uncertain parameters; prior cracks and simulated reality cracks. By comparing these cracks, the updated distributions for uncertain parameters are obtained. The updated crack size distribution can be used to update the estimation of the probability of failure. To demonstrate the developed framework, a tubular joint in a specific jacket platform is considered and the framework is applied for that joint. The results of the developed methodology indicate that the updated distributions of uncertain parameters shift towards the simulated reality distributions

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

Seismic analysis and design assisted by numerical simulation of slender steel portal frame structures

Lightweight (thin-walled or cold-formed) steel portal frame structure could be a popular and effective alternative to the traditional hot rolled structure and, with care to avoid buckling, could be used in earthquake areas owing to its economy and ease of fabrication and transportation, but no recommendations for seismic design of these structures is provided in the design codes. Accordingly, there is need for a lightweight design that is suitable for earthquake areas, which could be transported using lighter vehicles and erected quickly using smaller plant than is required for conventional hot rolled sections following an earthquake. The present paper shows some stages in the development of an earthquake-resistant frame, designed for by combining numerical finite element investigations with analytical check calculations based on EN 1993-1-3 for cold-formed steel members and EN 1993-1-1 for design of steel structures to estimate the loads on the frame structure within the use of EN 1998-1seismic design requirements. Although the initial buckling modes have been avoided, the frame still needs further modification to improve its ductility. It is planned to use this work to assist with the development of performance-based design recommendations for future structures that cover both thin-walled steel and cold-formed steel portal frame structures

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

Review and application of Rainflow residue processing techniques for accurate fatigue damage estimation

Most fatigue loaded structural components are subjected to variable amplitude loads which must be processed into a form that is compatible with design life calculations. Rainflow counting allows individual stress cycles to be identified where they form a closed stress-strain hysteresis loop within a random signal, but inevitably leaves a residue of open data points which must be post-processed. Comparison is made between conventional methods of processing the residue data points, which may be non-conservative, and a more versatile method, presented by Amzallag et al. (1994), which allows transition cycles to be processed accurately. This paper presents an analytical proof of the method presented by Amzallag et al. The impact of residue processing on fatigue calculations is demonstrated through the application and comparison of the different techniques in two case studies using long term, high resolution data sets. The most significance is found when the load process results in a slowly varying mean stress which is not fully accounted for by traditional Rainflow counting methods

Bow impact loading on FPSOs 2 - theoretical investigation

Steep wave impact pressures and the structural dynamic response of FPSO bows are studied using 1:80 scale instrumented models and time domain simulation. Comparisons are made between experimental and calculated impacts and associated pressures. A simple time history simulation method for bow loading in (non-linear) random seas is developed. Simplified design rules for curved bows are proposed. The rules account for the effect of the size of the loaded area on the average pressure, the rise and decay times of the pressure and the dynamic amplification of the response