60 research outputs found

    Parametric pitch instability investigation of Deep Draft Semi-submersible platform in irregular waves

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    Parametric pitch instability of a Deep Draft Semi-submersible platform (DDS) is investigated in irregular waves. Parametric pitch is a form of parametric instability, which occurs when parameters of a system vary with time and the variation satisfies a certain condition. In previous studies, analyzing of parametric instability is mainly limited to regular waves, whereas the realistic sea conditions are irregular waves. Besides, parametric instability also occurs in irregular waves in some experiments. This study predicts parametric pitch of a Deep Draft Semi-submersible platform in irregular waves. Heave motion of DDS is simulated by wave spectrum and response amplitude operator (RAO). Then Hill equation for DDS pitch motion in irregular waves is derived based on linear-wave theory. By using Bubnov-Galerkin approach to solve Hill equation, the corresponding stability chart is obtained. The differences between regular-waves stability chart and irregular-waves stability chart are compared. Then the sensitivity of wave parameters on DDS parametric pitch in irregular waves is discussed. Based on the discussion, some suggestions for the DDS design are proposed to avoid parametric pitch by choosing appropriate parameters. The results indicate that it's important and necessary to predict DDS parametric pitch in irregular waves during design process

    Evaluation of Student and Staff Perceptions on L&T Models Across Multiple Disciplines

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    Moving towards Education 4.0, there has been a gradual shift in learning and teaching (L&T) practices worldwide towards active and deep learning (Gardiner, 2015). With technological advancements, different models of learning and teaching utilising digital mediums have evolved, alongside with frameworks to support transitions into enhanced blended learning (Adekola, Dale, & Gardiner, 2017). It was proposed that the students’ learning needs and expectations must be considered in the L&T pedagogy. In Ithaca S+R and the Univer¬sity System of Maryland, parallel comparisons of traditional versus blended courses were conducted (Griffiths, Chingos, Mulhern, & Spies, 2014). In this study, students on the blended courses performed slightly better or as well as those on the traditional courses but enjoyed the course less. At the University of Glasgow Singapore, L&T with different modes of blended instruction was explored. Four courses in Computing Science, Nursing, Mechatronics and Civil Engineering, which were hosted on different learning management systems, FutureLearn, Moodle and xSiTe, were considered. Across these courses, varying lesson plans and proportion of digital versus Face-to-face (F2F) interactions were provided. Lesson plans ranged from supplementary learning with videos to active and blended learning. Two surveys were developed to evaluate the staffs’ and students’ experiences. These included MCQs with a Likert-scale, as well as open ended questions. In this study, quantitative data was imported into Excel for visualisation, while qualitative data was subjected to categorisation and analysis (Braun & Clarke, 2006). Results were collated from at least fifty respondents in each course. The evaluation study for the students was developed on the following areas: (1)Accessibility; (2)Acceptance Levels; (3)Learner’s Gain; (4)Learner’s Experience; (5)Learner’s Perception; (6)Viewing Duration; (7)Repeated Viewing; (8)Useful to Learning; (9)Higher Level Learning; and (10)Acceptance levels on proportion of Videos versus F2F interactions. Similar questions were posed to lecturers. Some of the key findings are as follows: (i) All four lecturers believe that the videos helped to raise the level of classroom discussion and channelled F2F consultation time to enhance the L&T gain for students. (ii) Most learners used a laptop for video viewing. This is closely followed by the smartphone, especially for Nursing. (iii) More than 93% of the learners believe that videos are helpful in their learning. (iv) Concept reinforcement was ranked to be most important approach for successful learning outcomes. Students also appreciate foundational materials and content to evoke active learning and critical thinking. (v) Over 78% of the students felt that they had to repeat the viewing of videos to grasp the concepts. (vi) Across all disciplines, more than 88% of the students felt that videos are useful to learning. Above 79% felt that they are learning at a higher level. (vii) Above 81% of the students are comfortable to engage in blended learning and felt that the optimal proportion of F2F consultation versus video time would be between 40% to 60%. In conclusion, it is evident that students are generally comfortable to engage in blended learning, if a good balance of digital and F2F interaction is provided. Students enjoy learning at their own pace and time. Many of the students felt that the digital content enabled them to review their learning and reinforce their understanding. Improvement in summative assessment scores is also demonstrated, where blended learning is offered to students. This project has provided the necessary guidance needed to develop successful courses for active and blended learning and demonstrates L&T examples with different pedagogical approaches. The results will be studied for future course development and lesson planning across all joint SIT-Glasgow degree programmes

    Robust requirements and design optimization for offshore wind turbine structure utilizing approximate model technology

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    We discuss the robust design optimization (RDO) of the support structures of offshore wind turbines to decrease costs and variations considering uncertainties. First, an approximate model derived with design of experiment (DOE) methodology is introduced to replace the time-consuming computational dynamic response analysis of finite element (FE) models for reducing computational costs. Then, a deterministic optimization (DO) without considering the uncertainties is introduced for comparison. The results suggest the feasibility and efficiency of the whole framework of the RDO method, which can be a reference for the design of other offshore support structures

    Multi-objective Optimization Analysis of Ultra-deep Water Drilling Riser under Harsh Environmental Conditions

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    Drilling systems have become an integral part of oil and gas exploration and production particularly in ultra-deep waters. With increasing drilling depth, heavy weight of the riser system and high-top tension requirement become the potential concern. The study mainly focusses on the optimization of buoyancy settings to achieve better operating performance and lighter weight. The multi-objective optimization of operability envelope is a complicated problem due to the presence of discrete design variables and complex analysis process (emergency disconnection, recoil analysis, drift-off) with different FEA models. In this work, an efficient approach for the multi-objective optimization of operability is proposed for the drilling riser in ultra-deep water. The two main contradictory objective functions include minimize the dry weight of drilling riser system and maximize the area of operability envelopes. Each of these conditions or operational scenarios imposes varying design limits for the riser stack-up. The main riser characteristics related to operational and environmental conditions will be considered. According to suitable criteria and requirements, the operational working envelopes will be defined. The economic cost (weight) and drilling riser system performance (operability window) are considered to obtain the better operational performance with Non-dominated sorting genetic algorithm II (NSGA-II). The use of a RBF metamodel approach could solve two critical problems in multi-objective optimization design, including timeconsuming computation and non-convergent problem during iterative loops

    Robust design optimization of supporting structure of offshore wind turbine

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    In this paper, we present a robust design optimization (RDO) framework for the supporting structures of offshore wind turbines that takes uncertainties into consideration. Given the large sizes of turbines and the high complexity of ocean engineering systems, optimizing the supporting structures of offshore wind turbines can significantly save costs while enabling the structures to survive the severe ocean environment. The type of RDO used in our study focuses on reducing the structural weight of a turbine and lowering its variation to achieve stable (or robust) operation and minimal cost. To save computational costs, the design of experiment process was introduced for sensitivity analysis of the design variables and the arrangement of sampling points. The metamodel technology including the Kriging model was used in this study to replace the time-consuming finite element model for dynamic response analysis. A simple test case using a cantilever was introduced first for briefly illustrating the validity of the RDO framework method. Deterministic optimization (DO), which does not consider uncertainties, was conducted simultaneously for comparison. Subsequently, the numerical case of a tripod-type supporting structure of a 5 MW offshore wind turbine was built for formal optimization. The comparison revealed that the reliability of constraints in RDO was much higher than that in DO, whereas the standard deviation in RDO was lower, implying that robust and reliable design results were obtained even under the influence of uncertainties. The results also demonstrated the feasibility of application of the proposed RDO framework method to other offshore supporting structures

    Metamodel approach for reliability-based design optimization of a steel catenary riser

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    A reliability-based design optimization (RBDO) methodology is presented for the design of a steel catenary riser (SCR) under dynamic environmental loads. The purpose of this work is to optimize the cost of products subjected to probabilistic constraints. Searching for the optimal design of the riser in a wide range of design variables is computationally very expensive if time-consuming codes for dynamic analysis are necessary in the iteration process. In this study, the effectiveness of the proposed RBDO using a metamodel is firstly studied and validated through a beam test, then applied to the industrial dynamic optimization problem. The design variables of structures are assumed to be uncertain, and some other parameters such as loading and material properties are considered random. The performance function is approximated using metamodels to avoid time-consuming finite-element analysis during the optimization iteration. A single-loop method is used to decouple the double-loop RBDO problem. The reliability is finally confirmed through Monte Carlo simulations. According to the analysis, the presented methodology is more rational and realistic compared with deterministic optimization

    Dynamic and Fatigue Analyses of Stress Joint for Deepwater Steel Catenary Riser

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    As the offshore industry continues to progress developments in deepwater, Steel Catenary Riser (SCR) offers great advantages over other risers. In order to provide the hang-off with sufficient stiffness, stress joint is used to connect the riser with platform. When stress joints are located at the top of a deep marine riser, it is greatly affected by both axial and bending stress due to great cyclic loading. So it is necessary to do some research on dynamic and local fatigue analyses for stress joint. In this work, global dynamic analysis for a SCR is performed firstly, then local boundary condition obtained from the previous analysis are applied to the stress joint FE model for time domain dynamic and multiaxial fatigue analysis. Results indicate that the stress level is far lower than yield limit of material and damage induced by fatigue needs more attention. Besides, the damage character of the two types of stress joints differs: for TSJ, the place where stress joint connects with riser is easy to occur fatigue damage; for SSJ, the most probable position is at the place where the end of inner sleeve pipe contacts with the riser body. Compared with SSJ, TSJ shows a higher stress level but better fatigue performance, and it will have a higher material cost. Considering various factors, designers should choose the most suitable type and also geometric parameters

    Parametric Resonance of the Free Hanging Marine Risers in Ultra-Deep Water Depths

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    The study is focused in the parametric instability of the deep-sea risers due to the platform heave motions. As offshore hydrocarbon resources exploration and exploitation moving to much deeper waters, risers play more important roles than before, and face with many technological challenges. The riser resonance can produce disastrous results, such as environment pollution and economical loss. In this work, firstly, the governing motion equation of the marine riser is formulated. Then the stability behavior of the risers with and without nonlinear damping is investigated by employing the Floquet theory. During the numerical solution of the governing equation, the coupling between the modes was considered. Finally, special attention has been paid to the effect of damping for the parametric unstable region changes. The results show that damping can effectively reduce unstable regions. Several useful suggestions are proposed for the design of deep-sea riser structures

    Fatigue reliability based design optimization of bending stiffener

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    A methodology for fatigue reliability based design optimization is proposed for the design of bending stiffener. Bending stiffener is employed to protect the upper connection of umbilical/flexible riser against damage from overbending. It is prone to cause fatigue failure due to the wave induced vessel motions. Therefore, its fatigue character has a great impact on the safety of oil and gas production and we should pay more attention to it. In addition, the fatigue analysis involves material, geometric, and loading uncertainties, hence the reliability analysis is performed for considering the influence of uncertain factors. In this work, the fatigue reliability based optimization involves the fatigue analysis and complex optimization algorithms the metamodel is used to reduce the computational cost. Three metamodels are constructed by the optimum Latin hypercube method. Then, the optimum metamodel is selected for the optimization through the accuracy evaluation. The feasibility of the methodology is verified by a test case of beam. Finally, it is applied to the fatigue reliability based design optimization of bending stiffener. The results demonstrate that this methodology is rational and improves the fatigue reliability of bending stiffener
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