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The behavior of subsea equipment has been widely studied by companies to minimize risks during subsea installation, which brings huge costs to the operation. The prediction of equipment motions typically is accomplished by utilizing either commercial software or recommended practices from Classification Societies to comply with the minimum requirements for launching operations. This article analyzes subsea installation in time and frequency domains using experimental, numerical, and analytical approaches. Experiments have been carried out at Instituto de Pesquisas Tecnológicas (IPT) to investigate the dependency of drag and added mass coefficients on Keulegan-Carpenter (KC) number and at Laboratory of Waves and Currents (LOC-COPPE/UFRJ) to study equipment and cable responses on the installation. A numerical model is made in orcaflex® for determining the transmissibility of displacement and force in function of frequency ratio, and the time series of manifold responses. Linearized models from DNV (DNV-GL, 2017, "Recommended Practice (DNVGL-RP-N103) - Modelling and Analysis of Marine Operations.") and the so-called natural KC (proposed in the present article) are taken in the investigation and compared to previous methodologies. The natural KC approach arises as an innovative way of investigating subsea equipment as the literature only provides parametric analysis considering an arbitrary KC value (i.e., Pestana et al., 2021, "Subsea Manifold Installation: Operational Windows Estimation Based on Hydrodynamic Model Testing," Ocean. Eng., 219, p. 108364). The analyses notice a good agreement among experiments, orcaflex® model, and natural KC model, whereas the curves found by the DNV method usually overpredict the response of subsea equipment, especially around resonance region. [ABSTRACT FROM AUTHOR]

Numerical modeling of the floating offshore wind turbine (FOWT) dynamics plays a critical role at the design stage of a floating wind project. Still, there exist challenges for verification of efficient engineering models against experimental results. Recently, an experimental campaign was carried out for a 1:96 downscaled model of the OC4-DeepCWind semi-submersible platform with mooring lines made of fiber ropes and chains. Leveraging the results of this campaign, this paper focuses on the development and calibration of a numerical model for the semi-submersible platform with a focus on the dynamic responses under bichromatic waves. In the numerical model, the hydrodynamic loads are modeled based on the potential flow theory with Morison drag. The lumped mass method is applied to model the mooring system. Both free decay tests and bichromatic wave conditions are considered in the model calibration process, and key uncertain parameters (e.g., mooring line length) that affect the response have been identified and discussed. Using the proposed calibration procedure, we establish a reasonably good numerical model for prediction of the platform motion and mooring dynamics. The low-frequency responses of the platform under bichromatic waves are well-captured. These outcomes contribute to the development of efficient numerical FOWT models under experimental uncertainty. [ABSTRACT FROM AUTHOR]

Marine icing due to freezing sea spray has been attributed to many safety incidences. Prediction of sea spray icing is necessary for operational safety, design optimization, and structural health. In general, lack of detailed full-scale measurements due to the complexity and costs make validation difficult. The next best option is that of controlled laboratory experiments. The current study is the first study in the field of sea spray icing that investigates the use of new data science technologies like machine learning and feature engineering for the prediction of sea spray icing based on data collected from controlled laboratory experiments. A new prediction model dubbed "Spice" is proposed. Spice is designed "bottom-up" from experimentally collected data, and thus, if the input variables are accurately known, it could be said to be highly accurate within the tested range compared to existing theoretical models. Results from the current study show promising trends; however, more experiments are suggested for increasing the range of confident predictions and reducing the skewness of the training data. Results from spice are compared with five existing models and give icing rates in various conditions in the middle of the spectrum of the other models. It is discussed how validation from two existing full-scale icing measurements from literature proves to be challenging, and more detailed measurements are suggested for the purpose of validation. [ABSTRACT FROM AUTHOR]

A flexibly connected double-module floating structure can be used as the primary component unit of a large multi-module floating structure, revealing some basic principles regarding the interaction of multiple modules and connectors to the same extent. This study investigated the hydrodynamic characteristics of a double-module floating structure with cable-fender connectors and connector loads through an experimental model test. Due to the weak coupling strength of the connector, the motion response of each unit of the double-module floating structure mainly responds to the wave frequency, and the resonance frequency of the floating unit is close to that of the single floating module. The magnitude of the connector load is related to the relative pitch motion of the two modules. The increased stiffness of the connector can limit the pitch motion of the modules but can also lead to higher forces on the connector. The load acting on the fender in the cable-fender connector is strongly nonlinear, and the shear force on the fender is large; this may be the control load causing fender damage. [ABSTRACT FROM AUTHOR]

This paper proposes a shore-based constant tension mooring system, which improves the cable tension distribution by adjusting the length of the cable to maintain the constant tension of the cable between the ship and the mooring pile in order to solve the problem of poor safety and reliability of the traditional mooring system in the mooring process. First, based on the three-dimensional potential flow theory, this paper uses the hydrodynamic software AQWA to numerically simulate the dynamic response of the traditional mooring system under the coupling of wind, wave, and current in different sea states. Subsequently, a shore-based constant tension mooring system using the principle of volume-varying hydraulic control was studied. On the basis of a comprehensive analysis of the working principle of the constant tension hydraulic control mooring system, a mathematical model of the main working circuit is established. The system was numerically simulated by relying on matlab/Simulink simulation software. Finally, by comparing with traditional mooring systems, the results show that the maximum cable tension of the shore-based constant tension mooring system is significantly reduced so that the tension is controlled within a fixed range, and the safety factor of the mooring cable is significantly improved, thus reducing the risk of mooring system failure and improving the ship's survivability. [ABSTRACT FROM AUTHOR]

The deformation caused by waves and current in an aquatic environment significantly impacts the remaining volume of a net cage. Its fluctuations in magnitude could pose a potential threat to the well-being of cultivated species. There is a critical need for a simple, cost-effective solution to monitor cage deformation in real-time. This study proposes a depth-based method, utilizing just two depth meters attached to the cages floating collar and tube sinker, to estimate the remaining volume. Compared to the experimental measurements from previous studies, this method demonstrates satisfactory accuracies. Under current-only and waves-current conditions, the estimation differences are around 11.6% and 23%, respectively. In contrast, when compared to commonly used volume-based methods, the differences are approximately 1.8% and 9.5%. Despite a tendency for overestimation when the mean cross-sectional area deviates from the original top or bottom area of the cage, this method remains a feasible alternative. Its practicality highlights its potential as an efficient means of monitoring cage deformation. [ABSTRACT FROM AUTHOR]

The interaction between surface gravity waves and surface-piercing vertical multi-plate breakwaters in finite water depth is investigated under the assumption of potential flow theory. The time-domain desingularized boundary integral equation method (DBIEM) is adopted to solve the boundary value problem. A decomposition method based on the superposition principle is used to obtain reflected waves. The effect of the vertical plates as a floating breakwater by making full use of Bragg resonance is analyzed. Initially, the waveforms calculated without breakwaters are compared with second-order Stokes regular wave to obtain the appropriate desingularization distance. Then, the interaction of surface gravity waves with the breakwaters with the single plate, double, and triple plates are analyzed. The transmission and reflection coefficients of the breakwater with the single plate are verified with the analytical solutions of previous literature. The transmission and reflection coefficients of the breakwaters with double and triple plates are verified with experimental results. Finally, the transmission and reflection coefficients of surface-piercing vertical multi-plate breakwaters with different plate numbers and drafts are also calculated. The occurring condition of Bragg resonance and its values affected by the plate number and the draft are analyzed. The present results are reference values for the optimal design of vertical plates as a floating breakwater for offshore and coastal engineering applications by fully using Bragg resonance. [ABSTRACT FROM AUTHOR]

The design of a floating wind turbine (FWT) should satisfy the serviceability limit state (SLS) requirement for an efficient and safe operation throughout the entire work life. The SLS requirements are introduced by the owner/developer of the wind turbine facility to achieve serviceability (production of power) or an efficient operation of the facility or a "first step" towards ensuring safety. Currently, there is limited information about SLS requirements in design standards. This study deals with an assessment of current methods, criteria, and procedure for the SLS design check with an emphasis on tilt/pitch and nacelle accelerations in view of power production and its fluctuations. Moreover, other criteria, on the borderline between serviceability and safety criteria, e.g., relating to clearance, are briefly discussed. The criteria relating to power production are illustrated in a case study with a 10-MW semi-submersible FWT considered for an offshore site in the Northern North Sea. Simplified static/dynamic analysis methods for use in the global design phase and high fidelity integrated, dynamic analysis methods for detailed design in terms of serviceability are presented, discussed, and applied in the case study. A good understanding of wind turbine dynamic performance associated with serviceability is essential to facilitate design decision-making. The relative contribution of wind and wave loads to the different SLS criteria is investigated. Finally, the main conclusions are summarized. In lieu of the current state of the art regarding SLS requirements for FWTs, we hope that this study provides a basis for improving design standards and guiding research and engineering practice for the semi-submersible floater design of FWTs. [ABSTRACT FROM AUTHOR]

Froude scaling for Floating Offshore Wind Turbine (FOWT) platforms is typical for understanding and interpreting their behavior and subsequent designs for testing in wave basins. Despite its popularity, the variability and uncertainty of the kinetic responses of such floating structures as a function of scaling require more attention. This work addresses the question of consistency of Froude scaling by comparing the hydrodynamic responses of a range of DeepCwind semisubmersible FOWT scaled models (full model, 1/2, 1/4, 1/9, 1/16, 1/25, 1/36, 1/49, and 1/50). The comparison was made both in the mooring-line tension and bending moment of structural members, which are directly related to their safety limit states. Hydrodynamic forces due to diffraction, radiation, and viscosity along with hydrostatic forces and mooring boundaries are modeled by ansys-Aqwa, which were subsequently converted to bending moment estimates. The variability of kinetic responses like mooring-line tensions and bending moment estimates was investigated for each scaled model, along with the identification of regions of inconsistencies. In the context of offshore renewable energy development through technological readiness levels, the study is especially pertinent for understanding how force variabilities and uncertainties are related to these kinetic responses of semisubmersible platforms. [ABSTRACT FROM AUTHOR]

This paper presents an optimization tool for jacket structures to support Offshore Wind Turbines (OWTs). The tool incorporates several combinations of optimization algorithms and constraint-handling techniques (CHTs): Genetic Algorithm; Differential Evolution (DE); Tournament Selection Method; Multiple Constraint Ranking (MCR); Adaptive Penalty Method, and Helper-and-Equivalent Optimization. The objective function regards the minimization of the jacket weight; the design variables are the diameter and thickness of the tubular members. The constraints are related to natural frequencies and Ultimate Limit State criteria. The candidate solutions are evaluated by full nonlinear time-domain Finite Element coupled analyses. To assess the optimization algorithms and CHTs, a case study is presented for the standardized OWT/jacket structure from the Offshore Code Comparison Collaboration Continuation project. First, a numerical model is built and validated, in terms of masses, natural frequencies, and vibration modes; then, this model is employed to run the optimization tool for all combinations of optimization algorithms and CHTs. The results indicate that, while all methods lead to feasible optimal solutions that comply with the constraints and present considerable weight reductions, the best performer is the combination of the DE algorithm with the MCR constraint-handling technique. [ABSTRACT FROM AUTHOR]

Free-standing hybrid risers (FSHR) are well established in deepwater field developments in the oil and gas industry. The major advantages foreseen for the FSHR are the possibility of safely anticipating production and requiring significantly reduced payloads at the floating production unit. This work addresses the feasibility of a novel proposed optimized concept, the free-standing integrated riser (FSIR), where the pipeline is integrated into the riser by means of a transition section, which is kept in place by a multiple line mooring system, developed to keep the functionality and integrity of the entire riser along its lifetime. The work provides the findings from numerical analysis simulated in orcaflex, a finite element simulation software dedicated to global analysis of risers, considering the design criteria provided in API 2RD Standard. As part of the FSIR evaluation, screening static analysis was performed to identify the most promising configurations for the multiple line mooring system, followed by a series of dynamic analyses to evaluate the behavior of the riser in extreme conditions. In addition, several investigations were performed to assess the functionality and the robustness of the system, including parametric studies to check for installation and dimensional tolerances, assessment of fatigue damage, and checking the impact of pipe size on the design. The results show that the proposed novel FSIR concept can provide a practical and economical solution for deepwater projects. [ABSTRACT FROM AUTHOR]

This paper presents a fully coupled solution in the time-domain, using the finite-differences method to the system of equations that model the dynamic behavior of the riser, blow-out preventer (BOP), and casing strings, when connected for well drilling/completion--the model is suitable to evaluate wellhead fatigue, even when the amplitude of oscillation and accelerations of the BOP are high. Sensibility analysis is used to show the effect of changing the Riser Top Tension to the resulting maximum values of wellhead bending moment and casing stress ranges. For the case where the rig is oscillating around a fixed position and there is no current, using a regular wave, the results show that there are some wave periods for which an increase in the Riser Top Tension reduces the maximum wellhead bending moment and the max casing stress range, therefore increasing fatigue life of the casing and wellhead. The effects of varying the weight of the BOP and soil parameters and the effect of the phase difference between the wave and first-order vessel motion are analyzed. The proposed solution can also be used to perform riser and casing analysis during drift-off/drive-off. [ABSTRACT FROM AUTHOR]

Sea spray icing on ships and marine structures depends on a complex correlation between metocean parameters and vessel characteristics. Sea spray icing rates have mostly been investigated and given as a function of general metocean parameters. The existing models suffer from the lack of experimental data. More experimental data are required for better prediction models and understanding of the icing process. This article presents results from a comprehensive cold laboratory study of the dependence and trends of sea spray icing rates related to eight parameters. Experiments were performed simulating sea spray from a nozzle toward a vertical surface in the freezing environment. This study presents 20 unique tests structured into eight experiments, each of which focuses on change in icing rates due to one independent variable. Results showed that the sea spray rate dependence of the investigated parameters complies with the existing knowledge; however, preliminary analysis points out various unintentional covariates for most experiments that call for further investigations. This is the greatest number of variables tested in one set of experiments to date and serves as valuable sea spray icing data experimental data--a limitation for the evaluation of previous models that pointed out the lack of enough icing measurements in this field of research. [ABSTRACT FROM AUTHOR]

A systematic study of hydrodynamic coefficients for simplified subsea modules has been performed to support the estimation of the coefficients needed for planning of subsea installation operations. The coefficients are assessed for a nearly two-dimensional test setup. The tests are performed as forced oscillations at various amplitudes and periods, representing the forces on the module lowered through the water column. The importance of each of the main components of the subsea modules--mudmat, protection roof, and process equipment of different shapes inside the modules are studied in fully submerged condition. Results for the module elements, generic contents, and different combinations of these elements are presented. For the tested modules, damping is generally the dominating hydrodynamic force. However, the presence of the content inside the modules will generally increase the importance of added mass. Estimation of the hydrodynamic coefficients by summation of the coefficients for the individual structure elements generally overestimates the damping, compared to the coefficients measured for the complete modules. For added mass, estimation based on summation gives generally good results. [ABSTRACT FROM AUTHOR]

The reliable design of offshore aquaculture structures (OAS) for fish farming in the open ocean is vital to the marine aquaculture industry in the future. However, the lack of easy-to-access numerical tools for the dynamic analysis of OAS challenges the sustainable development of marine aquaculture. This article presents a newly developed numerical library under an open-source, finite-element analysis code, Code_Aster, enabling the dynamic analysis of OAS. A numerical model of OAS is first developed using the present numerical library and then validated against experiments. The validation shows a good agreement in terms of structural motions and tensions in mooring lines. After that, the dynamic responses of this model are analyzed subjected to irregular waves and current conditions from field measurements on an offshore fish farm site. The results indicate that a negative mean pitch angle will occur when the current velocity is large. [ABSTRACT FROM AUTHOR]

Increased deployment of offshore wind turbines is seen as an important pathway to increase green renewable energy production. Improved and rapid identification of extreme events and evaluation of hydrodynamic loads due to such events is essential to reduce the cost of energy production. Numerical modeling to pre-screen sea states and to identify the crucial events to prioritize model tests will make a major contribution to reduce design times and costs for such structures. In this effort, a highly efficient and nonlinear numerical model based on the Laplace equations is used to generate undisturbed wave kinematics. Such a simulation is used to identify extreme wave events in a sea state realization, and further, the wave loading due to such events are evaluated using Morison formula. Events screened in this manner can then be transferred to a high-resolution model such as a Navier-Stokes equation-based solver to investigate the hydrodynamics in detail. The implementation and application of such an approach in the open-source hydrodynamic model REEF3D is presented in this work. [ABSTRACT FROM AUTHOR]

The comprehensive usage of an array of natural or artificial semi-circular breakwaters (SCBs) as supporting structures to secure the floating/fixed structures are received increasingly more consideration in recent years. The performance characteristics of bottom-fixed SCB in the presence of a floating dock are investigated under the framework of linear wave theory. The edge conditions such as continuity of velocity and pressure along the SCB and zero-flow condition near rigid surfaces are adopted. The multi-domain boundary element method (MBEM) is used to examine the wave attenuation performance of the whole breakwater system in two dimensions. The correctness of the present study's numerical results is confirmed by performing the comparative study with the readily available experimental and analytical results reported by various researchers. The reflection, transmission, energy loss coefficients, and fluid force experienced by floating dock are examined as a function of incident wave properties and breakwater physical properties. The Bragg resonant reflection and performance comparison with other shapes are also studied. The study results strongly suggest that the damage of floating structures and leeward locales due to incident wave stroke is minimized with the introduction of porosity for SCB located on the seabed by dissipating the wave energy. A pair of bottom-fixed SCBs having 10-20% breakwater porosity placed far away from the dock is identified as the optimal choice to reduce the fluid force experienced by the dock and wave transmission coefficient. [ABSTRACT FROM AUTHOR]

Green water and slamming wave impacts can lead to severe damage or operability issues for marine structures. It is therefore essential to consider their probability and loads in design. This is difficult, as impacts are both hydrodynamically complex and relatively rare. The complexity requires high-fidelity modeling (experiments or CFD), whereas a statistically sound analysis of rare events requires long durations. High-fidelity tools are too demanding to run a Monte-Carlo simulation; low-fidelity tools do not include sufficient physical details. The use of extreme value theory and/or multi-fidelity modeling is therefore required. The present paper reviews the state-of-the-art methods to find wave impact design loads, which include response-conditioning methods, screening methods, and adaptive sampling methods. Their benefits and shortcomings are discussed, as well as challenges for the wave impact problem. One challenge is the role of wave non-linearity. Another is the validation of the different methods; it is hard to obtain long-duration high-fidelity wave impact data. [ABSTRACT FROM AUTHOR]

This work aims to investigate the complex fracture behavior of a large opening box girder (LOBG) using three-dimensional (3D) finite element method (FEM). A numerical model is developed to simulate the fracture of LOBG under individually or jointly applied torsion and bending loads. The crack damage is introduced into the side plate and bottom plate of LOBG. The influencing factors such as crack angles, crack locations, and load types are emphatically discussed. Additionally, the crack growth angles are also predicted during the analysis. The results present that the large opening can generate significant effects on the crack growth of the side plate crack (SPC) and the bottom plate crack (BPC). Under the condition of torsion or hogging, SPC grows more easily than BPC, while BPC is relatively prone to grow under sagging condition. Compared with the torsion condition, the initial crack angle can significantly reduce the stress intensity factor (SIF) under the bending condition. Furthermore, the cracks that gradually approach transverse frames are also more likely to induce mode I fracture under torsion conditions. These findings from the present study can reveal insights for a better understanding of fracture behavior for LOBG. [ABSTRACT FROM AUTHOR]

Non-linear finite element (FE) analysis is a powerful tool for determination of ultimate capacities of steel components. To produce reliable predictions of structural strength, it is of fundamental importance that non-linear finite element analyses are based on a validated methodology. This paper focuses on physical test campaigns, which are designed with the purpose of providing an experimental foundation for calibrated and validated non-linear finite element assessments of plated components, which form part of typical offshore structures. The experimental work discussed in the current paper is a continuation of the experiments of tubular members and joints, which were carried out by Maersk Oil in the late nineties. The objective is that the two-test campaigns together shall allow for development of a non-linear finite element methodology with calibrated failure criteria providing either characteristic or mean level of capacity/response for the main details in typical offshore structures. This paper particularly focuses on the testing campaign completed in 2017 which forms the basis for the calibration of the failure criteria covering plated sections. [ABSTRACT FROM AUTHOR]

For structural design engineers, there is an apparent gap in how the hydrostatic pressure is treated between the American and European systems. In API RP-2A, the beam-column equations treat the axial and bending capacities the same as there is no hydrostatic pressure. This is physically not correct, as member utilization is a combination of hydrostatic, axial, and bending actions. In contrast, the ISO and NORSOK beam-column equations include reductions of axial and moment capacities due to hydrostatic effect. In this paper, available actual test data are compared with the API and ISO capacity equations. A third set of capacity equations provided by Chen et al. is also considered. Unity check (UC) results show that, although API equations lack the proper hydrostatic reduction in axial/bending capacities, it is compensated by the separate checks of hoop buckling and ultimate strength. For engineering applications, similar member designs will be obtained by either the European or the American systems. [ABSTRACT FROM AUTHOR]

Flow-induce motions (FIM) small-scale model tests were performed for the Jappaku floating offshore wind turbines (JPK), a FOWT developed to operate in Brazilian waters. This paper aims to investigate the presence of FIM on the JPK to show the importance of heave plate (HP) design and to show how HP mitigates FIM. Three different HP dimensions were tested and compared with the condition without HP. In addition, two different incidence angles of the current were tested, namely, 0 deg and 180 deg. The results showed amplitudes in the transverse direction similar to the diameter of the external platform column for the case without HP. These amplitudes are higher than the ones observed for previous deep-draft semi-submersibles found in the literature. Conversely, the largest HP dimensions implied in mitigation of the FIM amplitudes, i.e., very low amplitudes, were observed. The presence of the central column played an essential role in FIM and significantly modified the amplitudes in different current incidences. Due to the different diameters of the external and central columns, the FIM presented two different branches of response related to the vortex-shedding frequency around the columns. The results showed that significant FIM could occur for this specific JPK investigation even with HP. The HP design has a positive effect on reducing dynamic behaviors due to the wave and current incidences. Therefore, its design must be included in the preliminary stages of FOWT developments. [ABSTRACT FROM AUTHOR]

Oceans are harsh environments and can impose significant loads on deployed structures. A wave energy converter (WEC) should be designed to maximize the energy absorbed while ensuring the operating wave condition does not exceed the failure limits of the device itself. Therefore, the loads endured by the support structure are a design constraint for the system. Furthermore, the WEC should be adaptable to different sea states. This work uses a WEC-Sim model of a variable-geometry oscillating wave energy converter (VGOSWEC) mounted on a support structure simulated under different wave scenarios. A VGOSWEC resembles a paddle pitching about a fixed hinge perpendicular to the incoming wave fronts. The geometry of the VGOSWEC is varied by opening a series of controllable flaps on the pitching paddle when the structure experiences threshold loads. It is hypothesized that opening the flaps should result in load shedding at the base of the support structure by reducing the moments about the hinge axis. This work compares the hydrodynamic coefficients, natural periods, and response amplitude operators from completely closed to completely open configurations of the controllable flaps. This work shows that the completely open configuration can reduce the pitch and surge loads on the base of the support structure by as much as 80% °. Increased loads at the structure's natural period can be mitigated by an axial power take-off damping acting as an additional design parameter to control the loads at the WEC's support structure. [ABSTRACT FROM AUTHOR]

Estimation of the hydrodynamic loads based on strip theory using the Morrison equation provides an inexpensive method for load estimation for the offshore industry. The advantage of this approach is that it requires only the undisturbed wave kinematics along with inertia and viscous force coefficients. Over the recent years, the development in numerical wave tank simulations makes it possible to simulate nonlinear 3-h sea states, with computational times in the order of real time. This presents the possibility to calculate loads using wave spectrum input in numerical simulations with reasonable computational time and effort. In the current paper, the open-source fully nonlinear potential flow model REEF3D::FNPF is employed for calculating the nonlinear wave kinematics. Here, the Laplace equation for the velocity potential is solved on a s-coordinate mesh with the nonlinear free surface boundary conditions to close the system. A technique to calculate the total acceleration on the s-coordinate grid is introduced which makes it possible to apply strip theory in a moving grid framework. With the combination of strip theory and 3-h wave simulations, a unique possibility to estimate the hydrodynamic loads in real time for all discrete positions in space within the domain of the numerical wave tank is presented in this paper. The numerical results for inline forces on an offshore wind mono-pile substructure are compared with measurements, and the new approach shows good agreement. [ABSTRACT FROM AUTHOR]

An analytical solution of the dynamic response of offshore wind turbines under wave load with nonlinear Stokes's wave theory and wave-structure and soil-foundation interactions is developed. Natural frequencies and the corresponding modes are obtained. The effect of the wave-structure interaction, the added mass, the foundation stiffness, and the nacelle translational and rotational inertia on the motion of the structure is investigated. The nonlinear loading provided by the drag term of Morison's equation is successfully handled. A parametric study to examine the effect of the structural parameters on the dynamic response is conducted, and the results of the proposed analytical solution are compared to numerical ones. The proposed method has the following advantages: (a) it is accurate and straightforward because of its analytical nature, (b) it does not ignore the drag term in the wave loading by keeping its nonlinearity nature, (c) the structure of the wind turbine is modeled as a continuous system, (d) it takes into account the effect of the rotational and translational inertia of the nacelle on the dynamic response, and (e) it provides an interpretation of the effect of the sea level variation in changing the natural frequencies. [ABSTRACT FROM AUTHOR]

On the basis of the linearized theory of water waves, the present study has demonstrated a semi-analytical method to assess the hydrodynamic performance of a pair of partially immersed barriers just above a thick bottom-standing barrier. By means of the eigenfunction expansion method, a system of the first kind Fredholm-type integral equation involving a horizontal component of velocity as unknown functions is developed for the interaction of water waves with both types of barriers. The multiterm Galerkin approximation is adopted to determine these unknown functions having square root singularities at the submerged edge of the thin barriers and one-third singularities at the corners of the thick barrier. In order to overcome such types of singularities, Chebychev polynomials for half-singularities and ultra-spherical Gegenbauer polynomials for one-third singularities with suitable weight functions have been taken into consideration. The numerical examples of both reflection and transmission coefficients are presented to examine the hydrodynamic performance of breakwater. Some fascinating results like resonant frequencies are obtained for practical engineering. At the same time, reflection coefficients for the present breakwater agree reasonably for the limiting cases with the previously available results. [ABSTRACT FROM AUTHOR]

This work proposes a taut-leg deep-water mooring configuration with buoys, designed specifically to allow a quick and easy visual identification of the rupture of any mooring line of the system. This addresses one of the main concerns of the offshore oil and gas industry: the significant number of events of mooring line failure that has been recently observed in actual operations in deep and ultra-deep water scenarios; field experience indicates that, in current spread-mooring systems, several weeks may pass until a failure is detected. The proposed configuration also presents the additional advantages of reducing the mooring radius and the tensions on the top of the lines. The configuration is evaluated and compared with a standard taut-leg system, considering the same base case study of a typical deep-water FPSO, and using up-to-date numerical methods implemented into an in-house fully coupled nonlinear time-domain dynamic analysis tool. The results indicated that the proposed configuration provided significant improvements in the cost and performance of the mooring system, in terms of line lengths, mooring radii, tensions, and offsets; and confirmed that the buoy emerges and reaches the surface in the event of a line rupture, irrespective of the point where the line has broken. This indicates that the resulting configuration is a promising technical alternative to traditional standard taut-leg systems in deep and ultra-deep water scenarios. [ABSTRACT FROM AUTHOR]

The erosion wear due to raindrop plays a vital role in the service life of wind turbine components, especially blades. Furthermore, the rain impact angle and the environmental conditions have a major impact on erosion wear. In the present work, the rain erosion performance of glass fiber reinforced polyester (GFRP) blade material in offshore and onshore environmental conditions has been studied. A whirling arm test rig was created and used to conduct the erosion tests on both offshore and onshore conditions. To simulate the offshore and onshore environmental conditions, experiments were carried out using both fresh and saltwater. The trail runs were carried out for time duration (30-90 min) at different impact angles (0-90 deg) by running the whirling arm tester at different impact velocities (30-70 m/s). During the experimentation, the impact velocity varied from 30 to 70 m/s. The impact angle was taken as 0 deg, 30 deg, 45 deg, 60 deg, and 90 deg, respectively. The results showed that erosion wear in the offshore conditions was more in all the cases as compared to onshore conditions. Furthermore, the maximum mass loss was at 45-deg impact angles for both conditions, which was further endorsed by Scanning Electron Microscope (SEM) analysis. Very little work has been reported on the optimization of erosion wear response of Offshore and Onshore wind turbines by using different design of experiment techniques. The erosion testing reveals that the GFRP blades exhibit a ductile erosion mechanism, which was further explained by using three-dimensional surface plots with the help of Image J software. [ABSTRACT FROM AUTHOR]

This article presents an investigation of a newly discovered motion instability phenomenon for floating wind turbines, first observed in numerical aero-hydro-servo-elastic simulations. The source of instability is identified as antisymmetric aerodynamic stiffness coupling terms in roll and yaw caused by the turbine thrust force. Analytical expressions for the roll-yaw and yaw-roll stiffness coupling terms are derived, using the actuator disk analogy of the rotor plane. It is demonstrated that a two degrees-of-freedom linear equation of motion without explicit external forcing qualitatively captures the instability. Based on this model, an analytical stability criteria is derived. An intuitive physical explanation of the phenomena is provided, considering the energy flow of the system. An important finding is that dissipative forces, in the form of aerodynamic and hydrodynamic damping, are needed to fully explain the observed instability. It is demonstrated, counterintuitively, that increased damping in yaw reduces the stability margin. This is explained by the fact that dissipative forces result in a phase difference between the harmonic roll and yaw motion in the coupled roll-yaw modes. [ABSTRACT FROM AUTHOR]

New types of fish farms are often larger and structurally more complex than conventional fish farming structures, and associated challenges concerning safety and costs increase correspondingly. Thus, increased precision in structural design is required, with estimation of hydrodynamic loads on nets as an important topic. Today, both load coefficients for nets and measured netting dimensions are given with relatively high uncertainties. New knowledge for netting materials with high solidities as well as scaled netting commonly applied in model tests is included in the presented study. Results from towing tests and the development of a new mathematical expression for local drag coefficients (for netting twines) indicate that drag coefficients are not only dependent on solidity and Reynolds number, but may also be affected by the velocity reduction and the local velocity at the twines. [ABSTRACT FROM AUTHOR]

A series of physical tests and finite element (FE) analyses are conducted to evaluate the failure of smooth (conventional) and textured (proposed concept) pipes. To do so, hydrostatic pressure tests are performed on aluminum beverage cans (ductile failure) and additively manufactured Ti6Al4V-0406 titanium pipes (brittle failure). Mechanical material properties are obtained from tensile tests of coupon samples. In the absence of physical burst pressure tests, FE models are validated against experimental results of external pressure tests and are used to predict the buckle initiation (Pi) and burst pressure (Pb) capacity of the textured pipes with different number of circumferential triangles, N, and base angles, α. Results show that buckle initiation pressures of the textured concept is 2.34 and 1.80 times greater than those of the smooth aluminum cans and titanium pipes, respectively. However, the burst pressure of the textured pipe can only get 3% greater than the smooth pipe. Based on the current results, a textured pipe with N = 6 and α = 30 deg is the optimum textured design. [ABSTRACT FROM AUTHOR]

Design life of offshore structures is in general in the 20-to 30-year range, with some cases going up to 50 years. Fatigue is one of the major design criteria for such structures. Climate change may affect the fatigue life of offshore structures, it would be necessary to update the design procedures to take into account climate change effects on structural performance. This paper aims to investigate the impact of climate change in the long-term fatigue life of offshore structures due to wave loading. For this purpose, available environmental conditions for two locations (South East Brazilian Coast and North Atlantic Ocean) generated by the HadGEM-2S global climate model, considering representative concentration pathway (RCP) 4.5 and RCP 8.5 future scenarios and the historical (past) scenarios are considered. The assessment in both locations is performed for two structural models: an idealized stress spectrum for a generic fatigue hot-spot and a steel lazy wave riser (SLWR) connected to a floating production storage and offloading (FPSO). Fatigue life is estimated using the S-N curve approach. Results show that the impact on the fatigue life depends on the structure dynamic characteristics, on the geographic location and mainly on the greenhouse emission scenario. In general, for the Brazilian location, when compared to the historical scenario, most of the future scenarios lead to slightly higher fatigue damages (lower fatigue lives). On the other hand, for the North Atlantic location, there is not a clear trend for future climate change scenarios. [ABSTRACT FROM AUTHOR]

An analytical study is presented here to investigate the scattering of oblique flexural gravity waves by a pair of totally submerged vertically placed porous barriers, located at some distance from each other, for a homogenous fluid flowing over a porous sea-bed. A thin ice-sheet, replacing the usual free surface, is considered as the upper surface where it is treated as a thin elastic plate by following Euler-Bernoulli beam equation. The complete analytical solution, under the assumption of small-amplitude theory and structural response, is acquired by employing eigenfunction expansion and least square method for the problem of flexural gravity waves interacting with the submerged porous barriers. Subsequently, computation for the reflection and transmission coefficients, energy loss and wave forces are carried out and discussed for different parameter values corresponding to the ice-sheet, porous sea-bed, and porous barriers. This study establishes that the oscillatory behavior exhibited by the reflection of the waves. It further shows that when the inertial effect of the porous-effect parameter of the barriers is increased, the minima in wave reflection occur. The vertical porous barriers are found to dissipate a significant portion of the wave energy when an increase in the inertial effect of the porous barriers is affected. The hydrodynamic force on the barriers also follows an oscillatory pattern, and it increases when the length of the barrier is increased. Furthermore, wave transmission decreases significantly due to the energy dissipation by the porous sea-bed. It is demonstrated that corresponding to various structural parameters, almost no reflection and full transmission take place for an impermeable sea-bed and also when only real porosity parameter of the porous sea-bed is considered. The effect of the ice-sheet on the propagation of the flexural waves is also examined by obtaining a number of results for variation of various parameters. Variation in the elastic parameter of the floating ice-sheet is observed to command a considerable influence when the wave impinges upon the submerged vertical porous barriers. [ABSTRACT FROM AUTHOR]

This article presents an experimental test program and numerical analyses conducted on aluminum alloy drill pipes with two different geometries. Small-scale characterization tests were conducted to determine both the material mechanical properties and the fatigue S-N curves. Full-scale fatigue tests of the components are also presented. A finite element model of the drill pipes, including the tool-joint region, was developed. The model simulates, through different load steps, the tool-joint hot-assembly and the experimental loads to obtain the actual stress distribution during the full-scale tests. Maximum stress amplitude in the aluminum pipes was found to be coincident with the edge of the connector, at the same location where failure was observed in full-scale tests. The study revealed that such pipes present a complex stress state near their connection to the steel tool-joints due to their geometry and the residual stresses induced during the assembly of the steel connectors onto the aluminum pipes. Finally, multi-axial fatigue models were calibrated with the results of the small-scale tests and applied to the stress-strain state obtained numerically. Theoretical predictions were correlated with full-scale fatigue test results. [ABSTRACT FROM AUTHOR]

Floating offshore wind turbines are subjected to higher tower fatigue loads than their fixed-to-seabed counterparts, which could lead to reductions in turbine life. The worst increases are generally seen in the tower axial fatigue, associated with the tower fore-aft bending moment. For a spar type platform, this has been shown to increase by up to 2.5 times and for a semi-submersible platform, by up to 1.8 times. Reducing these loads would be beneficial, as the alternative of strengthening the towers leads to increase in cost. Here, two offshore floating wind turbine systems, of the spar type, are analyzed and selected responses and tower fatigue are compared: one incorporates a variable-speed, variable-pitch-to-stall blade control system and a back twisted blade, and the other a conventional pitch-to-feather control. The results are then compared with those obtained in an earlier study, where the same turbine configurations were coupled to a semi-submersible platform. A weighted wind frequency analysis at three mean turbulent wind speeds of 8, 13, and 18 m/s highlights that the impact of the back twist angle magnitude and initiation point on tower axial fatigue life extension was the same for both platform types. Compared with their respective feather base models, an increase in the tower axial fatigue life of 18.8% was seen with a spar platform and 10.2% with a semi-submersible platform, when a back twist angle to the tip of -6 deg was imposed along with the variable-speed, variable-pitch-to-stall control. [ABSTRACT FROM AUTHOR]

A new type of umbilical cable named "high-current composite umbilical cable" is composed of electronic cables, optical cables, steel tubes, and structural strengthening components. It can be regarded as a key piece of industrial equipment in subsea production systems that provide control functions, high electric current, and hydraulic remote transmission. When it is oriented at a power supply with a relatively high rated current, power transmission will produce a lot of heat. Then, the cross-sectional temperature increases, which affects the performances of its material and mechanical responses. Therefore, electro-thermal-mechanical coupled analysis is critical for the cross-sectional design of the high-current composite umbilical cable. Accordingly, a multi-physics coupled analysis was performed based on two typical umbilical cable cross sections. Finite element models were established and subjected to electro-thermal analysis to obtain a temperature distribution of the two sections at different current capacities. Based on results of temperature field analysis, the section models were subjected to thermo-mechanical analysis. The results of the two types of analyses are compared and differences are discussed, which illustrate the multi-physics coupled effect cannot be neglected. The armored layers will relatively reduce the heat dissipation performance, but compared with the umbilical cable model without the armored layers, the model with double-armored layers is less affected by temperature, so its capacity of resistance external pressure is relatively better. The proposed coupled analysis methodology provides a new guidance for the design of the high-current composite umbilical cables. [ABSTRACT FROM AUTHOR]

The present work proposes a novel structural configuration for floating wind farms after an investigation into the structural configurations of current floating wind farms. The uniqueness of the structural configuration lies in the ability to adjust its heading to wind inflow directions, i.e., the self-adaptive property. To verify the existence of the self-adaptive property, a structural design is performed, and a quasi-static framework to analyze the responses is established. To simplify the loads on wind turbines under yawed inflow conditions, empirical expressions for wind loads are proposed, based on which the quasi-static analysis is performed under the yawed inflow direction of 90 deg from the structural heading. The analytical result indicates that under yawed inflow conditions, the structural heading will eventually turn to align with the wind inflow direction on account of the structural configuration and the wind turbine control. Then, the effects of inflow wind velocity, initial offset, number of wind turbines, complex environmental conditions, and hydroelastic properties on the responses of the structure are investigated. [ABSTRACT FROM AUTHOR]

Aquaculture has been the world's fastest-growing food producing method and grown to become the second-largest export industry in Norway during the past 40 years. Usually, the high-value fish such as Atlantic Salmon (Salmo Salar) is raised in a multi-cage fish farm, where the flow interactions between fish cages exist. In this study, the interactions between fish cages are implemented into the numerical program, FhSim, to investigate its influences on the responses of a multi-cage fish farm. Tensions in anchor lines, drag force, and cultivation volume of each cage in a full-scale 4 × 2 multi-cage fish farms under different flow directions are analyzed numerically. The discrepancies of the responses based on three cases, i.e., (i) without any wake effects, (ii) with only cage-to-cage wake effect, and (iii) with all the wake effects, are compared and discussed. The results indicate that neglecting the wake effects will overestimate the total drag force of the eight cages up to 128% and underestimate the total cultivation volume of the eight cages as much as 42%. This study can provide suggestions on how to consider the wake effects during the design of the multiple-cage system. [ABSTRACT FROM AUTHOR]

As part of the Mechanics of Ice Rubble project, recent experiments have been carried out to study the strength and failure behavior of ice rubble beams and the freeze bonds that form between individual ice blocks. In this study, we present results obtained from a newly developed model for the three-dimensional (3D) discrete element modeling (DEM) open-source code LIGGGHTS. The ice model contains normal and shear springs that operate between neighboring particles which are bonded or that overlap due to compressional stresses. Energy dissipation is accounted for by using a viscous damping model. Using this DEM model, medium-scale freshwater ice rubble punch tests have been simulated. Rubble specimens were generated by "raining" individual DEM ice pieces into a rectangular form and compacting the rubble mass to achieve the target porosity. Before the compacting pressure was removed, bonds between contacting blocks were introduced with parameter values based on representative freeze bond experiments. The rubble beam was then deformed by pushing a platen vertically downward through the center of the beam until failure occurred. Two types of block size and shapes have been simulated: cuboid blocks generated based on the size distribution of the actual rubble, and rubble blocks generated by image processing of actual blocks of broken ice used in the comparison experiments. The mechanism of flexural rubble failure for both cuboid block [s4.2] simulations and empirical block [s4.3] simulations is in line with experimental results; however, the empirical block simulations provide a significantly better estimation of the failure force. [ABSTRACT FROM AUTHOR]

The development is presented of an analytical model for the prediction of the stochastic nonlinear wave loads on the support structure of bottom-mounted and floating offshore wind turbines. Explicit expressions are derived for the time-domain and frequency-domain nonlinear exciting forces in a seastate with a significant wave height comparable to the diameter of the support structure based on the fluid impulse theory. The higher-order moments of the nonlinear load are evaluated from simulated force records, and the derivation of analytical expressions for the nonlinear load statistics for their efficient use in design is addressed. [ABSTRACT FROM AUTHOR]

Welded connections between tubulars and transverse plates are used to build together part structures such as support of flare towers and wind turbines and deck structures on foundation structures. They are used in traditional jacket structures for oil and gas production and in support structures for wind turbines. These welded connections are typically made from the outside resulting in a weld root on the inside and a weld toe on the outside. Different S-N curves apply to these positions; therefore, stresses both on the inside and the outside are needed for fatigue assessment. From the actual design, it is noted that the thicknesses of the tubulars being connected can be different. Also, the diameters of the tubulars can be different. In addition, the fabrication is associated with some fabrication tolerances that provide local eccentricity moments to be transferred through these connections. In this paper, analytical expressions for stress concentration factors for these connections are presented based on classical shell theory. The stress concentration is dependent on the radial restraint from the transverse plate and the eccentricity of the neutral axes in the tubular thickness of one tubular relative to the other tubular. The superposition principle is used to derive resulting stress concentration factors for the inside weld root and the outside weld toe. [ABSTRACT FROM AUTHOR]

The presence of rain-induced leading edge erosion of wind turbine blades (WTBs) necessitates the development of erosion models. One of the essential parameters for erosion modeling is the relative impact velocity between rain droplets and the rotating blade. Based on this parameter, the erosion damage rate of a WTB is calculated to estimate the expected leading edge lifetime. The environmental conditions that govern this parameter have site-specific variations, and thus, rain and wind loading on a turbine differ for onshore and offshore locations. In addition, there are wave loads present in the offshore environment. The present paper tries to provide guidelines for erosion modeling and investigates whether there are differences in erosion of blades due to (1) varying rainfall conditions modeled using different droplet size distributions for onshore and offshore locations in combination with (2) winds of varying turbulence intensities and (3) wave-induced loads. Aero-hydro-servo-elastic simulations are carried out for an onshore wind turbine (WT) and a monopile-supported offshore WT. Furthermore, erosion variables such as the relative impact velocities and the associated erosion damage rate of a blade are analyzed for various blade azimuth angles. The study shows that the rainfall intensity and turbulence intensity minorly influence the impact velocity and pressure but have a substantial effect on the overall erosion damage rate. Additionally, a significantly higher erosion damage rate is found for blades exposed to offshore rainfall conditions than for blades under onshore rainfall conditions. Furthermore, no substantial influence on erosion is found because of wave-induced loads. [ABSTRACT FROM AUTHOR]

It is well known that material properties undergo significant changes with temperature. In order to meet extreme environmental requirements for ships and offshore structures operating in Arctic regions, the effect of temperature on material behavior needs to be considered. In recent studies, significantly higher fatigue strength was observed for base materials and welded joints in comparison to room temperature. Fatigue strength increased even for temperatures far below the allowed service temperature based on fracture toughness results; however, sub-zero temperatures fatigue data are scarce and the effects of steel strength and welding type on fatigue strength changes are unknown. Material selection for ships and offshore structures is typically based on empirical Charpy and fracture toughness relations at the design temperature, minus a safety margin. Thus, this study presents material test results including fatigue tests of butt-welded joints, tensile test, and Charpy impact toughness tests at room and sub-zero temperatures of different structural steel types. Additionally, the effect of welding techniques and steel strength are discussed. The results can be used to extend design approaches for ships and offshore structures subject to sub-zero temperatures and to improve material selection for ships and offshore structures operating in Arctic regions. [ABSTRACT FROM AUTHOR]

This work presents an enhanced hybrid methodology for the analysis and design of floating production systems (FPS). The semi-coupled (S-C) procedure exploits the advantages of coupled and uncoupled models, incorporated into a three-stage sequence of analyses that can be fully automated within a single analysis program, presenting striking reductions of computational costs. The procedure begins by determining, through a full nonlinear static coupled analysis, the mean equilibrium position of the FPS with its mooring lines and risers. Then, it automatically evaluates equivalent six degrees-of-freedom (6DOF) stiffness matrices and force vectors representing the whole array of lines. Finally, these matrices/vectors are transferred to the dynamic analysis, solving the global 6DOF equations of motion restarted from the static equilibrium position. This way, the S-C methodology represents all nonlinear effects associated with the lines and considers their influence on the dynamic behavior of the hull. However, in some situations, it could still overestimate dynamic amplitudes of low-frequency (LF) motions and/or underestimate amplitudes of line tensions. Thus, to improve the overall accuracy, enhanced procedures are incorporated to better represent damping and inertial contribution of the lines. Results of case studies confirm that this methodology provides results adequate for preliminary or intermediary design stages. [ABSTRACT FROM AUTHOR]

This paper gives a detailed description of the main hydrodynamic characteristics of different types of green water events occurring on a fixed structure in a series of experiments. High-speed video, at 250 fps, was used to capture the flow details from regular waves produced inside a wave flume. The green water events were classified according to their type and the results of the evaluation from the apparent and effective interaction features. The air cavities in the plunging-dam-break (PDB)-type events were almost spherical for longer waves. PDB with large, elliptical cavities, and hammer-fist (HF)-type events occurred with steeper waves. The highest wave front and bow run-up velocities were found in PDB and HF events, respectively. The size of the cavities in PDB events increases and becomes more elliptical for higher bow run-up velocities. The results of these experiments show that not all events can be treated in the same way as the dam-break (DB)-type events when designing structures or implementing models, as not all interactions with the deck happen when the shipping water crosses the bow edge. [ABSTRACT FROM AUTHOR]

The structural design of offshore wind turbines is based on the consideration of coupled dynamic phenomena. Wave loads cause the dynamic oscillation of the monopile, and the dynamic oscillation of the monopile affects the wave loads. The boundary conditions of the gravity-based foundation-monopile-turbine system are mostly affected by the flexural stiffness of the foundation plate, the elastic and creep behavior of the soil, and the inertia (translational and rotational) of the wind turbine mass. The design of the foundation should consider the dynamic response of the soil and the monopile, and the dynamic response of the soil and the monopile is affected by the design parameters of the foundation. The initial conditions of the system yield transient dynamic phenomena. A braking wave at t = 0 causes different dynamic response than the steady-state conditions due to a harmonic wave load. In the present work, an integrated analytical model simulating the above dynamic phenomena is proposed. With the aid of double integral transforms and generalized function properties, a solution of the corresponding differential equations for the monopile-soil-foundation system and the boundary and initial conditions is derived. A parametric study is carried out, and results of the effect of the design parameters and soil properties are presented and discussed. [ABSTRACT FROM AUTHOR]

This paper deals with the design of a compact gearbox for the DTU 10 MW reference offshore wind turbine. An innovative gearbox concept consisting of a fixed planetary stage and a differential compound epicyclic stage is proposed. Power splitting and compound epicyclic transmission technologies are employed, which can effectively reduce the gearbox size. The power transmission principle of the gearbox is described and power distribution ratios in two transfer paths are calculated via the geometrical and mechanical relationships among the gearbox components. The gearbox is designed based on the design loads and fatigue criteria by referring to relevant international standards. A high-fidelity drivetrain numerical model is established by means of a multi-body system (MBS) approach. Then, the power distribution ratios in two transfer paths are compared between the simulation results and the design values with a good agreement. Resonance analysis of the drivetrain model is conducted by means of the Campbell diagram and energy distribution of components, and the results show that no severe resonance phenomena appear in this drivetrain model. Additionally, the load-sharing behavior of the gearbox model is assessed under different environmental conditions, and the results indicate that the compact gearbox has a good load-sharing performance. Such a compact gearbox design, which fulfills all the design requirements for offshore applications, could be a good alternative for offshore wind turbines. [ABSTRACT FROM AUTHOR]

Conical transitions have wide applications in wind turbine foundation as well as oil and gas jacket type of structures. The junctions where tubular and cone meet experience a sharp stress rise from shell edge effects. Like all structures experiencing sharp stress rises, fatigue considerations are critical. In addition to fatigue, the existing offshore structural design standards also require ultimate limit state checks. It is known from the lower bound theorem of plasticity limit analysis that the junction local edge effects do not impact the global capacity. Designing for the local junction ultimate limit state contains wide variations among existing design standards. In this paper, the design practices from API RP-2A, NORSOK N-004, and ISO 19902:2020 draft are assessed. They are compared to the shell plastic yield criteria of Hodge and Ilyushin. In addition, this paper provides a semi-analytical plasticity solution to determine junction plastic deformations. The formulation is based on cylindrical shell equations coupled with deformation plasticity theory. It is found that the growth of the junction plasticity zone is limited, which is consistent with the anticipation from the lower bound limit analysis theorem. The observations made in this paper show that the local junction plasticity is a secondary issue compared to other design considerations. Its ultimate limit state design equation can afford to be more lenient if chooses for future standards' development. [ABSTRACT FROM AUTHOR]

For the design of tubular conical transitions, the axial, bending, and hoop stresses at the junctions are required. Among the offshore design standards, API RP-2A, ISO 19902, and NORSOK N-004, various equations exist for the same stress quantity which may cause confusions. The quality of these existing stress formulae will be examined in this paper. The tubular conical stress equations used in the offshore industry started from Boardman's studies in the 1940s. Recently, Lotsberg re-formulated this problem and applied the results to stress concentration factor (SCF) applications. This paper solves the same set of shell equations but the formulations are cast in a different form. This new format allows for an in-depth examination of existing code equations. In addition, the formulation as presented can be used for modifications to gain higher accuracy. Several recommended new stress formulae are provided. It is observed that the existing code provisions' accuracy quickly deteriorates for cases where plate thickness in tubular and cone differ. The recommended approach is based on theoretical framework of shell mechanics, which better facilitate tubular/cone force balances when compared with existing equations. The sectional relationships among moment, shear, and hoop loads are also treated consistently using shell theory. The resulted improvements make the recommended formulae more accurate than the existing provisions. [ABSTRACT FROM AUTHOR]

To obtain an optimum connection parameter combination for modular floating structures of multiple functions, an integrated methodology for the assessment and optimization is proposed in the present work, out of consideration for the balance between the functional performance and economic effect. To illustrate the whole process, two types of modular floating structures, i.e., a modular wave energy converter (WEC) and a modular mobile offshore base (MOB), are taken as the cases. For the two cases, connection configurations are first specified based on structural functions as well as environment conditions; then, quantified measurements are established and studied, based on which the following process of selection of optimization algorithms is done and the optimum solutions are obtained, making up a whole process of specification of the connection parameter combination for modular floating structures. The proposed methodology possesses the potential of offering guidance for the phase of preliminary design of connection structures of modular floating structures. [ABSTRACT FROM AUTHOR]