Numerous offshore wave energy converter (WEC) designs have been invented; however, none has achieved full commercialization so far. The primary obstacle impeding WEC commercialization is the elevated ...levelized cost of energy (LCOE). Consequently, there exists a pressing need to innovate and swiftly diminish the LCOE. A critical challenge faced by WECs is their susceptibility to extreme wave loads during storms. Promising concepts must demonstrate robust design features to ensure resilience in adverse conditions, while maintaining efficiency in harnessing power under normal sea states. It is anticipated that the initial commercial endeavors will concentrate on near-shore WEC technologies due to the cost advantages associated with proximity to the coastline, facilitating more affordable power transmission and maintenance. In response, this manuscript proposes a pioneering near-shore WEC concept designed with a survivability mode that is engineered to mitigate wave loads during severe sea conditions. Moreover, prior investigations have highlighted favorable resonance properties of this novel concept, enhancing wave power extraction during recurrent energetic sea states. This study employs numerical and physical modelling techniques to evaluate wave loads on the proposed WEC. The results indicate a remarkable 65% reduction in wave loads on the moving floater of the WEC during a range of sea states under the implemented survivability mode.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Through-the-thickness stress distribution characterized by the degree of bending (DoB) has a major effect on the fatigue behavior of tubular joints commonly found in steel offshore structures. The ...determination of DoB values is essential for improving the accuracy of fatigue life estimation using the stress-life (S–N) method and particularly for predicting the fatigue crack growth based on the fracture mechanics (FM) approach. In the present research, data extracted from 648 finite element (FE) analyses carried out on 81 tubular KT-joint models was used to study the effects of geometrical parameters on the DoB values in KT-joints subjected to eight different types of loading including two types of axial loading, three types of in-plane bending (IPB) moment loading, and three types of out-of-plane bending (OPB) moment loading. Generated FE models were validated using experimental data, previous FE results, and available parametric equations. Geometrically parametric investigation was followed by a set of nonlinear regression analyses to develop 21 parametric design formulas for the calculation of the DoB in tubular KT-joints under the axial, IPB, and OPB loadings.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The majority of tubular joints commonly found in offshore practice are multi-planar. Investigating the effect of loaded out-of-plane braces on the values of the stress concentration factor (SCF) in ...offshore tubular joints has been the objective of numerous research works. However, due to the diversity of joint types and loading conditions, a number of quite important cases still exist that have not been studied thoroughly. Among them are multi-planar KT-joints subjected to bending loads. In the present research, results extracted from the stress analysis of finite element models, verified against available experimental data, were used to study the chord-side SCFs in two- and three-planar tubular KT-joints subjected to in-plane and out-of-plane bending loads. A geometrically parametric investigation was conducted and then followed by a set of nonlinear regression analyses to derive a SCF parametric equation for the fatigue analysis and design.
•The SCFs in two- and three-planar tubular KT-joints under IPB and OPB loads were investigated.•For the numerical study, a FE model was developed and verified against experimental data.•The intensities of multi-planarity effect in considered load cases were compared.•A large number of FE models were generated to study the geometrical effects on the SCFs.•A new SCF parametric equation was developed for OPB-loaded three-planar KT-joints.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
An assessment is made of the stress distribution and the hydrodynamic response of the preliminary structural design of the tension leg platform of a 10 MW wind turbine. The platform supporting a 10 ...MW turbine is modelled and analysed by the finite element method. The stress distribution of the platform is determined in still water with the turbine at above-rated conditions, and the response of the tension leg platform is estimated in the time domain. The results of the time domain analysis show reasonable agreement between the present results and the available data. To check the design stiffener dimensions, span, and spacing against stress distribution, classification societies’ recommendations are used. The results of the stress distribution analysis indicate that the critical locations of the platform are the interaction of the lower columns with the upper columns and the connection of the tower of the turbine.
In the present paper, data extracted from the finite element analysis of 118 models, which were validated against the test results obtained from an experimental study, were used to investigate the ...geometrical effect on the weld-toe stress concentration factors (SCFs) in internally ring-stiffened tubular KT-joints subjected to four different types of out-of-plane bending (OPB) loads. Although the tubular KT-joints are commonly found in offshore jacket structures and despite the crucial role of SCFs in the evaluation of the tubular joint׳s fatigue performance, the SCFs in internally ring-stiffened KT-joints subjected to OPB loads have not been investigated so far and no design equation is available to determine the SCFs for OPB-loaded joints of this type. In the present research, geometrically parametric study was followed by the nonlinear regression analysis to develop a new set of SCF parametric formulas for the fatigue design of ring-stiffened KT-joints under the OPB loadings.
•The SCF distribution in internally ring-stiffened tubular KT-joints was experimentally investigated.•118 FE models were generated and analyzed for the parametric study of OPB-loading-induced SCFs.•Geometrical effects on the weld-toe SCFs at the saddle position under OPB loadings were studied.•A set of fatigue design formulas was proposed to determine the SCFs in OPB-loaded joints.•Significant difference exists between SCFs in axially loaded and OPB-loaded stiffened KT-joints.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
•SCFs in FRP-strengthened tubular KT-joints were studied under three types of brace axial loading.•An optimization study was conducted to find the most suitable fibers orientation in the FRP ...layup.•Totally 1458 FE models were generated and analyzed to study the effect of joint geometry and FRP parameters.•A new set of parametric fatigue design formulas was developed for the SCF calculation in FRP-strengthened tubular KT-joints.
This paper presents a brand new application of the fiber-reinforced polymer (FRP) materials as a strengthening tool for reinforcing offshore steel tubular KT-joints to reduce the stress concentration factors (SCFs). The finite element (FE) approach was validated against the experimental data on the FRP-strengthened joints as well as the well-known standards on the analysis and design of SCFs in tubular KT-joints. Having been validated, the numerical modeling methodology was used for an optimization study, which was conducted to find the most practical and effective fibers’ orientation in the FRP layup. The study continued with 1458 FE models under three different axial brace loading conditions. Three different types of FRP materials, including glass/epoxy, glass/vinyl ester, and carbon/epoxy, were considered as reinforcing materials. Results of the parametric studies showed that apart from the loading scheme, by using CFRP as the strengthening material, more than 50 percent reduction in SCF values was achieved at the saddle position of the central brace. Based on the parametric study results, a parabolic trend was observed for relative SCFs (ψ) against β, in which the minimum values of ψ occurs at β=0.5. Increasing γ and τ decreased the FRP effectiveness while increasing θ enhanced the FRP efficacy in reducing the SCFs. Finally, practical parametric equations were presented for fatigue design of unstrengthened and FRP-strengthened steel tubular KT-joints. The performance of the proposed formulae was examined not only by the statistical indices and the recommendations of Fatigue Guidance Review Panel but also by an external validation analysis the performance of the new equations in predicting the SCF values in their range of applicability was investigated.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
In the present research, results from the finite element (FE) stress analysis of 108 models, validated using test data obtained from the experimental investigation, was used to propose probability ...distribution models for stress concentration factors (SCFs) in internally ring-stiffened tubular KT-joints under four types of in-plane bending (IPB) loads. The SCF is one of the crucial parameters in the fatigue reliability analysis of tubular joints. It exhibits considerable scatter which calls for greater emphasis in accurate derivation of its governing probability function. According to the knowledge of the authors, no comprehensive research has been carried out on the probability distribution of SCFs especially in stiffened tubular joints commonly found in offshore structures. What has been used so far as the probability distribution of SCFs in the reliability analysis of this type of joints is mainly based on assumptions and limited observations, especially in terms of distribution parameters. Based on a FE parametric study, a set of sample databases was prepared for the maximum central- and outer-brace SCFs; and density histograms were generated for these samples. Nine different probability density functions (PDFs) were fitted to these histograms. The maximum likelihood method was used to compute the parameters of fitted distributions. In each case, using a developed computer code, Kolmogorov–Smirnov and chi-squared tests were applied to evaluate the goodness of fit. Finally, the best-fitted distributions were selected and after substituting the values of estimated parameters for each distribution, six fully defined PDFs were developed for the maximum weld-toe SCFs of central and outer braces in internally ring-stiffened tubular KT-joints subjected to four different types of IPB loading.
•The SCF distribution in internally ring-stiffened tubular KT-joints was experimentally investigated.•108 FE models were generated for parametric SCF analysis of KT-joints under four types of IPB loads.•Extracted data was used to propose six fully defined PDFs for the maximum SCFs.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
This research investigates the effect of stress concentration factors (SCFs) in KT-connections retrofitted with concrete under axial loadings. 972 finite element models of KT-connections with various ...types of concrete and geometrical parameters have been modeled and analyzed using ANSYS APDL. SOLID 186 was used to mesh all parts of the model and to design the weld profile AWS recommendations were followed. The interaction between concrete and the chord was defined using ANSYS contact capability. Available experimental data was utilized to validate those models. Afterwards, the validated models were used to study the SCFs in different kinds of KT-connections filled with concrete. The results indicate a reduction of the maximum SCFs by around 50%–60% at the saddle points, and 17%–43% at the crown points compared to empty KT-joints. The reduction effect increases significantly when the concrete nominal compressive strength grades from C2, C40, and C50. In addition, in all types of loading conditions at the saddle points, the SCFs values increase. Among the nondimensional parameters, θ significantly affects SCFs crown points at the outer braces and saddle points at the central brace. Despite the clearly identified advantage of using concrete filling to reduce SCFs in the KT-joints, there are no studies or equations on these joints filled with concrete. Therefore, to calculate SCFs in KT-connections filled with concrete, some new formulae are developed and validated against the UK DoE acceptance standard.
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•Stress concentration factors were investigated for concrete-filled tubular KT joints under axial loadings.•Effects of dimensionless geometric parameters and concrete grads were investigated thorough FE analysis.•The FE modeling was validated using experimental data on tubular joints.•Parametric equations were proposed to calculate maximum SCF in concrete-filled tubular KT joints.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Stress concentration factors (SCF) in steel tubular joints have gained considerable attention since it is the crucial parameter for fatigue design of steel tubular structures. Meanwhile, among ...various strengthening methods, Fiber Reinforced Polymer (FRP) wrapping has shown to be an effective scheme for reinforcing steel structures. This study is dedicated to the numerical investigation of SCF values in FRP-strengthened tubular KT-joints under the action of In-Plane Bending (IPB) and Out-of-Plane Bending (OPB) moments on the central as well as outer braces with three various practical loading patterns per each moment. The Finite Element (FE) modeling methodology for joint geometry and FRP generation was validated against the available data in the literature and well-known codes. The effects of different geometrical parameters along with the FRP parameters on the SCF values were studied through post-analyzing a total of 2920 FE models. The numerical parametric studies indicated that FRP strengthening could lower the SCF values in KT-connections by about 30 and 55 percent under IPB and OPB moments, respectively. Using the driven data, practical fatigue design formulas were developed for predicting SCF in FRP-strengthened KT-joints under bending moments, and reliability of the proposed equations was examined using Fatigue Guidance Review Panel recommendations.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
In the paper, the most suitable probability distributions for SCFmax in DKT-Joints stiffened with FRP under axial loading conditions are studied. To this end, 1296 models were generated and then ...analyzed in the ANSYS to provide five reliable samples. Created FE models were validated using previous laboratory and FE data. Afterward, to predict the appropriate probability distribution models, 12 different kinds of distributions were utilized to check the fit distribution curve to the samples. Next, the accuracy and reliability of the distribution curves were assessed with the help of goodness-of-fit tests, such as the Kolmogorov-Smirnov (KS) and the Chi-squared (CS). The results of the analysis reveal that the Birnbaum-Saunders distribution is the best option. Furthermore, based on the probability graphs, it was concluded that the developed distribution is suitable enough for the given data sets.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
