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Conventional ship mooring in ports has many shortcomings such as a high safety risk, low efficiency and high labor intensity. In order to explore and develop the theory and key technologies of intelligent automatic mooring systems, this paper takes an intelligent mooring system based on a parallel anti-rolling mechanism as the research and development object. A new mooring method integrating ship hydrodynamics, mechanism kinematics and intelligent algorithms is proposed. Through numerical simulation and comparative analysis of the model, the motion inhibition effect of mooring ships under different working conditions is obtained. The results show that the control strategy and intelligent algorithm of the system can realize the active control of the wharf mooring ships and achieve the goal of improving wharf stability conditions through an intelligent mooring system.

The paper studies the parametric stochastic roll motion in the random waves. The differential equation of the ship parametric roll under random wave is established with considering the nonlinear damping and ship speed. Random sea surface is treated as a narrow-band stochastic process, and the stochastic parametric excitation is studied based on the effective wave theory. The nonlinear restored arm function obtained from the numerical simulation is expressed as the approximate analytic function. By using the stochastic averaging method, the differential equation of motion is transformed into Ito’s stochastic differential equation. The steady-state probability density function of roll motion is obtained, and the results are validated with the numerical simulation and model test.

The motions of the floating vertical axis wind turbine (VAWT) with consideration of the coupling between the aerodynamics and the hydrodynamics are calculated in this paper. The surge-heave-pitch coupling nonlinear equations of the floating VAWT are established. The aerodynamic loads are obtained by applying the double-multiple-stream tube theory with consideration of the dynamic stall and the floating foundation motion. The motion performances of a 5 MW Φ - Darrieus type floating VAWT are studied with its foundation of the Spar type with heave plates. It is shown that the amplitude of the heave motion of the floating VAWT is small when the heave plates are equipped. The effects of the wind and wave loads on the floating VAWT motions are assessed. The results show that the mean values of the surge and the pitch of the floating VAWT are mainly related to the wind loads; the standard deviations of the surge, the heave, and the pitch are mainly related to the wave loads. In the regular wave cases, the frequencies of the surge, the heave and the pitch are dominated by the wave frequencies, and the components of the 2P responses (caused by the aerodynamic loads) of the pitch are small. The 2P responses of the pitch are more significant in the irregular wave cases as compared with those in the regular wave cases.

The prediction and control of vortex-induced vibration (VIV) is one of the key problems for riser design. The effect of platform heave motion on VIV of deep sea top-tensioned riser (TTR) is presented by means of numerical simulation and experiment in this research. First, the heave motion was modeled as a parametric excitation, and the governing equation of VIV of riser considering the parametric excitation was established. Then, the dynamic response of TTR was calculated numerically by the finite difference method based on the Van der Pol wake-oscillator model. Finally, a validation experiment was carried out at the towing tank of Tianjin university. The results show that the VIV response at the bottom of riser is significantly increased due to the platform heave motion, especially in the situation of low current velocity. The larger amplitude and the higher frequency of the platform heave motion with the greater influence are generated on VIV of TTR. In particular, the value of 0.5 times, 1 time, or other multiples of the platform heave frequency will be included in the vibration frequency component of TTR when the platform heave amplitude is large and the frequency is high.

Focusing on the 75 m depth offshore area, an articulated foundation is proposed for supporting National Renewable Energy Laboratory (NREL) 5 MW offshore wind turbine (OWT). Through the overall sensitivity check on hydrostatic performance, the main parameters are set to meet the requirements of stability and economy. An in-house code was programmed to simulate the dynamic response of the articulated offshore wind turbine (AOWT). The aerodynamic load on rotating blades and the wind pressure load on tower are calculated based on the blade element momentum theory and the empirical formula, respectively. The hydrodynamic load is simulated by the three-dimensional potential flow theory. The motions of foundation, the aerodynamic performance of the wind turbine, and the loads on the articulated joint are documented in different cases. According to the simulations, the articulated offshore wind turbine shows feasibility to work in the particular area.

Simulations are conducted in time domain to investigate the dynamic response of a spar-type floating offshore wind turbine (FOWT) under the freak wave scenarios. Toward this end, a coupled aero-hydro-mooring in-house numerical code is adopted to perform the simulations. The methodology includes a blade-element-momentum (BEM) model for simulating the aerodynamic loads, a nonlinear model for simulating the hydrodynamic loads, a nonlinear restoring model of Spar buoy, and a nonlinear algorithm for simulating the mooring cables. The OC3 Hywind spar-type FOWT is adopted as an example to study the dynamic response under the freak wave conditions, meanwhile the time series of freak waves are generated using the random frequency components selection phase modulation method. The motion of platform, the tension applied on the mooring lines, and the power generation performance are documented in several cases. According to the simulations, it is indicated that when a freak wave acts on the FOWT, the transient motion of the FOWT is induced in all degrees-of-freedom, as well as the produced power decreases rapidly. Furthermore, the impact of freak wave parameters on the motion of FOWT is discussed.

With the increase of water depth, the design and use of the top-tensioned risers (TTR) are facing more and more challenges. This research presents the effect of top tension on dynamic behavior of deep-sea risers by means of numerical simulations and experiments. First, the governing equation of vortex-induced vibration (VIV) of TTR based on Euler-Bernoulli theory and Van der Pol wake-oscillator model was established, and the effect of top tension on natural vibration of TTR was discussed. Then, the dynamic response of TTR in shear current was calculated numerically by finite difference method. The displacement, bending stress and vibration frequency of TTR with the variation of top tension were investigated. Finally, a VIV experiment of a 5 m long flexible top-tensioned model was carried out at the towing tank of Tianjin University. The results show that the vibration displacement of TTR increases and the bending stress decreases as the top tension increases. The dominant frequency of VIV of TTR is controlled by the current velocity and is barely influenced by the top tension. With the increase of top tension, the natural frequency of TTR increases, the lower order modes are excited in the same current.

In this work, two different blade structural models are used to estimate the blade deformations and the global structural responses of a 10MW floating offshore wind turbine (FOWT). One model is based on the Euler-Bernoulli beam theory and it is solved by the linear normal mode superposition method. The other model is based on the geometry exact beam theory (GEBT) which can consider the full geometric nonlinearity and large deformation. The control equations of GEBT are discretized by Legendre spectral finite elements. The aero-hydro-servo-elastic fully coupled numerical simulations are conducted in the open-source analysis tool OpenFAST to explore the feasibility of the two different structural models for modeling large scale wind turbine blades. Both the steady-state and dynamic results show that power generation and thrust on rotor are similar for the different blade models. There is a small difference in the results of the blade pitch angle and flapwise and edgewise blade root bending moment at high wind speeds due to the lack of torsion degree of freedom in the mode-based method. The difference between the two models is mainly reflected in the prediction of blade tip deformations. The one-hour short-term extreme blade root bending moments and the damage equivalent fatigue loads at blade root are both compared based on the two models. For edgewise bending moment, the extreme value of GEBT model is found at cut-out wind speed, whereas the linear beam model predicts the extreme value around rated wind speed. For the flapwise bending moment, the extreme value is captured around the rated wind speed for both of the two models, but GEBT model presents a larger value. As for fatigue loads, the short-term 1 Hz damage equivalent loads calculated based on the linear beam model are smaller than GEBT model at almost all load cases for both edgewise and flapwise root bending moment, which implies that the linear beam model may underestimate the life time fatigue damage at blade root.

Unburied subsea pipelines operating under high-temperature and high-pressure conditions tend to relieve their axial compressive force by forming lateral buckles. In order to manage lateral buckling, a sleeper is often employed as a buckle-initiation technique to ensure pipeline integrity. In this study, analytical solutions of third-mode lateral buckling for unburied subsea pipelines with sleeper are derived. The analytical solution is compared with experimental data in the literature and shows good agreement. The stability of the buckled pipeline is investigated by means of an energy analysis and it is found that third-mode lateral buckling has lower energy than first-mode buckling, which means that third-mode buckling is more likely to happen in practice. The influence of sleeper height and sleeper friction on lateral post-buckling behaviour is illustrated and analysed, with particular attention paid to the minimum critical temperature difference, lateral displacement amplitude and maximum stress. Our results show that increasing the height of the sleeper or decreasing the friction between pipeline and sleeper can both be used to decrease the minimum critical temperature difference, but their influence on the maximum stress is opposite.

Buried subsea pipelines under high temperature conditions tend to relieve their axial compressive force by forming localised upheaval buckles. This phenomenon is traditionally studied as a kind of imperfect column buckling problem. We study upheaval buckling as a genuinely localised buckling phenomenon without making any ad hoc assumptions on the shape of the buckled pipeline. We combine this buckling analysis with a detailed state-of-the-art nonlinear pipe-soil interaction model that accounts for the effect of uplift peak soil resistance for buried pipelines. This allows us to investigate the effect of cover depth of subsea pipelines on their load-deflection behaviour. Furthermore, the influence of axial and uplift peak soil resistance on the localised upheaval behaviour is investigated and the maximum axial compressive stress during the buckling process is discussed. Parameter studies reveal a limit to the temperature difference for safe operation of the pipeline. Localised upheaval buckling may then occur if the pipe is sufficiently imperfect or sufficiently dynamically perturbed.

Simulations are conducted in time domain to investigate the transient response of a SPAR-type floating offshore wind turbine in scenarios with fractured mooring lines. Towards this end, a coupled aero-hydro-elastic numerical model is developed. The methodology includes a blade-element-momentum model for aerodynamics, a nonlinear model for hydrodynamics, a nonlinear restoring model of SPAR buoy, and a fully nonlinear dynamic algorithm for intact and fractured mooring cables. The OC3 Hywind SPAR-type FOWT is chosen as an example to study the dynamic response after one of its mooring lines is suddenly broken. The motions of platform, the tensions in the mooring lines and the power generation performance are documented in different cases, including different fractured cables and different shutdown strategies. An interesting finding is that in terms of drift distance, it might be more dangerous to shut down the turbine in certain scenarios. Furthermore, the potential impacts of mooring failure on adjacent FOWTs are discussed for two designs of the wind farm.

The objective of this paper is to study the nonlinear coupling internal resonance of the heave, roll, and pitch response performance of a spar platform when their frequencies are in the ratio of 2:1:1 under wave and vortex exciting loads. The three degree-of-freedom (DOF) nonlinear coupled equations are established by considering a time-varying wet surface with a first-order wave force in heave and pitch and a vortex-induced force in roll. The first-order steady-state response is solved using the multi-scale method in heave main resonance. The multiple solutions of the motion equations are discussed using the analytic method and a numerical simulation. A sensitivity analysis is conducted to test the influence of the damping and internal detuning parameter. The regions of multiple solutions are found, and the jump phenomenon exists with the changes of the wave excitation. The regions of multiple solutions depend on the values of damping and detuning parameter.

He, X.; Tang, Y.G.; Liu L.Q., and Zhang, R.Y., 2018. Design of the Mooring System for a Floating Wave Energy Converter. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 1351–1355. Coconut Creek (Florida), ISSN 0749-0208.The mooring system is important for a floating wave energy converter since it provides a positioning function and significantly affects the motion characteristics of the wave energy converter. In the present study, a novelty mooring system, which works as a single point mooring assisted with a yaw-restrain system, is proposed. Based on a potential flow theory, a numerical study is carried out. The response amplitudes induced by the wave under different incident angles are presented. The numerical study shows that the incident angle is a key factor of the motion amplitude, and that for each special wavelength, there is an optimal incident angle. A coupled analysis is carried out for the wave energy converter with its mooring system. A controlling method is presented for a given incident angle.

This paper presents a study on the motion response of a tension-leg platform (TLP) under first- and second-order wave forces, including the mean-drift force, difference and sum-frequency forces. The second-order wave force is calculated using the full-field quadratic transfer function (QTF). The coupled effect of the horizontal motions, such as surge, sway and yaw motions, and the set-down motion are taken into consideration by the nonlinear restoring matrix. The time-domain analysis with 50-yr random sea state is performed. A comparison of the results of different case studies is made to assess the influence of second-order wave force on the motions of the platform. The analysis shows that the second-order wave force has a major impact on motions of the TLP. The second-order difference-frequency wave force has an obvious influence on the low-frequency motions of surge and sway, and also will induce a large set-down motion which is an important part of heave motion. Besides, the second-order sum-frequency force will induce a set of high-frequency motions of roll and pitch. However, little influence of second-order wave force is found on the yaw motion.

Unburied subsea pipelines operating under high temperature and high pressure (HT/HP) conditions tend to relieve their axial compressive force by forming lateral buckles in an uncontrolled manner. In order to control lateral buckling, a distributed buoyancy section is often employed. In this study, analytical solutions are deduced for lateral buckling of unburied subsea pipelines with a distributed buoyancy section. An energy analysis is employed to investigate the stability of the buckled pipeline. The influence of the length and weight of the distributed buoyancy section on pipeline buckled configurations, typical lateral buckling behaviour and the minimum critical temperature difference is illustrated and analysed. The results are shown to be in good agreement with experimental data in the literature. The effect of imperfections is also discussed and an error analysis is conducted for one of the main assumptions of the proposed analytical method. The results show that increasing the length or decreasing the weight of the distributed buoyancy section can both be used to decrease the minimum critical temperature difference. The maximum compressive stress will decrease with decreasing weight of the distributed buoyancy section. However, the influence of the length of the distributed buoyancy section on the maximum compressive stress is complicated.

The PDFs (probability density functions) and probability of a ship rolling under the random parametric and forced excitations were studied by a semi-analytical method. The rolling motion equation of the ship in random oblique waves was established. The righting arm obtained by the numerical simulation was approximately fitted by an analytical function. The irregular waves were decomposed into two Gauss stationary random processes, and the CARMA (2, 1) model was used to fit the spectral density function of parametric and forced excitations. The stochastic energy envelope averaging method was used to solve the PDFs and the probability. The validity of the semi-analytical method was verified by the Monte Carlo method. The C11 ship was taken as an example, and the influences of the system parameters on the PDFs and probability were analyzed. The results show that the probability of ship rolling is affected by the characteristic wave height, wave length, and the heading angle. In order to provide proper advice for the ship's manoeuvring, the parametric excitations should be considered appropriately when the ship navigates in the oblique seas.

Unburied subsea pipelines operating under high-temperature and high-pressure conditions tend to relieve their axial compressive force by forming lateral buckles. Uncontrolled lateral buckling may lead to pipeline failure. In order to control lateral buckling, a sleeper is often employed as a buckle-initiation technique. In this study, analytical solutions of lateral buckling for unburied subsea pipelines with sleeper are derived. An energy analysis is employed to investigate the stability of the buckled pipeline. The influence of sleeper height and sleeper friction on pipeline buckled configurations and typical lateral buckling behaviour is illustrated and analysed. The results are shown to be in very good agreement with experimental data in the literature. We also discuss the effect of imperfections and conduct an error analysis of one of the main assumptions of the proposed analytical method. Our results show that increasing the height of the sleeper or decreasing the friction between pipeline and sleeper can all be used to decrease the minimum critical temperature difference. However, only the sleeper height is effective in substantially reducing the maximum compressive stress.

Unburied subsea pipelines operating under high temperature and high pressure (HT/HP) conditions tend to relieve their axial compressive force by forming lateral buckles. Uncontrolled lateral buckling can lead to pipeline failure. In order to control lateral buckling phenomenon, distributed buoyancy method is employed as buckle initiation technique. In this study, analytical solutions for lateral buckling of unburied subsea pipelines with distributed buoyancy section are derived. Fitting method is used to get the relationship between the equivalent axial compressive force and the half-length of the buckled section. The influence of the length and the weight of the distributed buoyancy section over typical behaviors of lateral buckling is illustrated and analyzed. The results show that increasing the length of distributed buoyancy section or decreasing the weight of distributed buoyancy section can all be used to decrease the minimum critical temperature difference and reduce the maximum axial compressive stress. However, the corresponding lateral displacement amplitude increases. The best selection of the weight of distributed buoyancy section is about half reduction of the original weight of pipeline.

Unburied subsea pipelines designed to operate under high temperature and high pressure (HT/HP) conditions tend to relieve their axial compressive force by forming lateral buckles. Uncontrolled lateral buckling can lead to pipeline failure, such as collapse. Thus, controlled lateral buckling is an effective and inexpensive method to accommodate axial thermal expansion and to control pipeline lateral buckling. In this study, analytical solutions for uncontrolled and controlled lateral buckling of unburied subsea pipelines are derived. And the interaction between two adjacent buckles for controlled lateral buckling is considered. The typical behaviors of uncontrolled and controlled lateral buckling with different lengths of feed-in zone are illustrated and analyzed. The effects of length of feed-in zone and lateral soil resistance on controlled lateral buckling are also presented. The results show that shorter length of feed-in zone and smaller lateral soil resistance will lead to smaller lateral buckling amplitude, maximum bending moment and maximum axial compressive stress. Therefore, reducing the length of feed-in zone or lateral soil resistance can be an appropriate method to be used to control lateral buckling. Finally, the analytical solutions are validated by use of the finite element modelling. The analytical results show good agreement with finite element analysis results.

Under high temperature and high pressure (HT/HP) conditions, unburied subsea pipelines may buckle horizontally on the seabed in an uncontrolled manner, which may lead to the failure of the pipeline. So buckle initiators, such as sleepers, will be employed to trigger buckles at planned locations. In this study, mathematical models are proposed to simulate lateral buckling of subsea pipelines triggered by dual sleepers with a gap between them. Analytical solutions of the vertical and lateral deformation for the pipeline with dual sleepers are derived first. Comparisons between the results obtained in this study and in the literature are presented and discussed. Then, the influence of the dual sleepers on the buckled configuration and the post-buckling behaviour is analysed. A parametric analysis about the minimum critical temperature difference, the displacement amplitude and the maximum stress is discussed in detail. Finally, the influence of nonlinear lateral pipe-soil interaction on the post-buckling behaviour is discussed. The results show that the formula derived in this study can simulate both single sleeper and dual sleepers to trigger lateral buckles. When dual sleepers are employed, the minimum critical temperature difference and the maximum stress are reduced compared to single sleeper, which will decrease further through increasing the spacing between the dual sleepers, increasing the sleeper height or decreasing the sleeper friction coefficient. The sleeper friction coefficient must be controlled strictly.

The VAWT (vertical axis wind turbine) has advantages in the development of large-scale offshore wind power. This paper presents a motion study of a 5 MW floating VAWT composed of the Φ type Darrieus wind turbine and the spar type floating foundation with heave plates. A computing code of aerodynamic loads was developed considering the dynamic stall and the floating foundation motions. The motion equations of the floating VAWT were established and solved numerically. Model tests were conducted, and the results of heave and pitch motion in test were compared with that of numerical calculation. The PRAOs (pseudo response amplitude operators) of numerical calculation are found to agree well with the experimental data with wave only conditions, and have some little discrepancies with wave and wind conditions. The surge-heave-pitch motions of the floating VAWT were analyzed. The results show that the aerodynamic forces have minimal influence on the heave motions of the floating VAWT, while they obviously increase the mean values of surge and pitch motions. The surge, heave and pitch frequencies of the floating VAWT are dominated by the wave frequencies, and the 2P (twice-per-revolution) response of pitch motions is not significant.

We present a study of the nonlinear coupling internal resonance for the heave roll and pitch performance of a spar platform under the wave and vortex-induced loads when the ratio of the frequencies of heave, roll and pitch are approximately 2:1:1. In consideration of varying wet surface, the three DOFs nonlinear coupled equations are established for the spar platform under the effect of the first-order wave loads in the heave and pitch, and vortexinduced loads in the roll. By utilizing the method of multi-scales when the vortex-induced frequency is close to the natural roll frequency, the first-order perturbation solution is obtained analytically and further validated by the numerical integration. Sensitivity analysis is performed to understand the influence of the damping and the internal detuning parameter. Two cases with internal resonance are shown. The first case is that no saturation phenomenon exists under small vortex-induced loads. The first order perturbation solution illustrates that only the vortex-induced frequency motion in roll and the super-harmonic frequency motion in heave are excited. The second case is that the vortex-induced loads are large enough to excite the pitch and a saturation phenomenon in the heave mode follows. The results show that there is no steady response occurrence for some cases. For these cases chaos occurs and large amplitudes response can be induced by the vortex-induced excitation.

The dynamic analysis of a Tension Leg Platform (TLP) in random wave is investigated by considering the set-down of a floating body. The nonlinear restoring stiffness is derived with the set-down motion of a floating body and the coupled motion of the tension leg and platform and the differential equations of the motion are established. The study focuses on the influence of the set-down motion on the nonlinear response of the platform. By considering different significant wave heights and currents, motion responses of the platform are calculated and compared. The analysis shows that the set-down motion significantly increases the heave motion with low frequency and the equilibrium position of the heave motion with the set-down motion is much lower than that without set-down motion. The results in this paper indicate that the set-down motion has a major impact on the safety of the platform inproduction operation, and it is also a threat to the strength of tension legs and risers.

Many studies have been done on the heave-pitch unstable coupling response for a spar platform by a 2-DOF model. In fact, in addition to the heave and pitch which are in one plane, the nonlinear unstable motion will also occur in roll. From the results of the experiments, the unstable roll motion plays a dominant role in the motion of a spar platform which is much stronger than that of pitch. The objective of this paper is to study 3-DOF coupling response performance of spar platform under wave and vortex-induced force. The nonlinear coupled equations in heave, roll and pitch are established by considering time-varying wet surface and coupling. The first order steady-state response is solved by multi-scales method when the incident wave frequency approaches the heave natural frequency. Numerical integration of the motion equations has been performed to verify the first-order perturbation solution. The results are confirmed by model test. There is a saturation phenomenon associated with heave mode in 3-DOF systems and all extra energy is transferred to roll and pitch. It is observed that sub-harmonic response occurs in roll and pitch when the wave force exceeds a certain value. The energy distribution in roll and pitch is determined by the initial value and damping characteristics of roll and pitch. The energy transfers from heave to pitch and then transfers from pitch to roll. Due to the influence of the low-frequency vortex-excited force, the response of roll is more complicated than that of pitch.

In this paper, the effects of second-order difference-frequency wave forces on the global motion of an offshore wind turbine system with a large displacement under the survival condition are studied. In this case, the hydrodynamic force is the main force because the blades are feathered to reduce the lifting force. The first-order hydrodynamic forces are calculated by WADAM, while the second-order wave forces are calculated by a customized MATLAB module. Then the hydrodynamic coefficients are transferred to the wind turbine analytical code FAST. Through the comparisons of dynamic responses between the first- and second-order numerical models, it is found that the second-order wave forces significantly influence the motion of floating wind turbine under the survival condition. Moreover, neglecting the second-order force significantly underestimates the tension forces in the mooring lines.

The three-dimensional natural vibration characteristics of water inside a moon pool of an ocean structures are studied. The governing equations are derived based on the linear potential flow theory, and the boundary condition of the total opening bottom suggested by Molin is adopted. A semi-analytical method is used to solve the governing equations, and the natural frequencies and the motion modes are obtained. Two types of motions are studied: (1) the piston motion in the vertical direction, and (2) the sloshing motion of the free surface. The influences of moon pool's structural parameters on the natural frequencies, and the modal shapes are analyzed.

Simulations are conducted in time domain to investigate the dynamic response of a SPAR-type floating offshore wind turbine under the scenarios with freak wave. Towards this end, a coupled aero-hydro numerical model is developed. The methodology includes a blade-element-momentum model for aerodynamics, a nonlinear model for hydrodynamics, a nonlinear restoring model of SPAR buoy, and a nonlinear algorithm for mooring cables. The OC3 Hywind SPAR-type FOWT is chosen as an example to study the dynamic response under the freak conditions, while the time series of freak wave is generated by the Random Frequency Components Selection Phase Modulation Method. The motions of platform, the tensions in the mooring lines and the power generation performance are documented in different cases. According to the simulations, it shows that the power coefficient of wind turbine decreased rapidly at the moment when freak wave acted on the floating structure.

Distributed buoyancy method is one of the buckle initiation techniques used to trigger controlled lateral buckling at planned locations for subsea pipelines operating under high temperature and high pressure (HT/HP) conditions. Deviations from a straight profile for pipelines may be introduced by the pipe-laying vessel’s sway motion during the installation process. In this study, analytical solutions of lateral buckling are deduced for imperfect unburied subsea pipelines with a distributed buoyancy section. The effect of initial imperfections on buckled configurations and typical post-buckling behaviours is illustrated and analysed. The results show that, compared to the case without initial imperfection, lateral displacement amplitude becomes larger when initial imperfection exists. Maximum compressive stress increases when wavelength of initial imperfection is smaller than buckled length of pipeline. However, maximum compressive stress decreases when wavelength of initial imperfection is larger than buckled length of pipeline. So it’s better to introduce longer wavelength of initial imperfection.

Comparing to Φ-type and H-type VAWT (Vertical Axis Wind Turbine), the amplitude changes of the aerodynamics acting on Helical-type VAWT are much smaller, so Helical-type VAWT has advantages in steady output power and avoiding fatigue of structure. Considering the characteristic of helical-type VAWT, this paper modifies the semi empirical method of calculating aerodynamic loads and compares with CFD results. A comparison is presented between CFD results and experiment results to confirm the model used in CFD. Single parameter analysis and muti-parameters analysis are carried out to study the influence of structural parameters on the dynamic torque. Based on an objective output power as 5MW, the parameters of wind turbine are adjusted, and optimal values of these parameters are determined.

Focus on the 75-meter-depth offshore area, an articulated buoy is proposed for supporting NREL 5 MW offshore wind turbine. Based on the optimization, the main parameters are set for better hydro performance and less environmental loads. According to the quasi-static approach, the intact stability was examined. Then, an in-house code was programmed to simulate the dynamic response of the articulated offshore wind turbine. The aerodynamic load on rotating blades and the wind pressure load on tower are calculated based on the blade element momentum theory and the empirical formula, respectively. The hydrodynamic performance is simulated by the 3-D potential flow theory. The motions of platform, the loads on the articulated hinge and the power generation performance are documented in different cases. According to the simulations, the articulated offshore wind turbine shows good hydrodynamic performance under operation conditions.

Controlled lateral buckling is triggered by distributed buoyancy section at predesigned sites to release the axial force induced by high temperature and high pressure in subsea pipelines. Due to the larger diameter and smaller submerged weight of distributed buoyancy section, compared to the normal pipe section, imperfections are more easily introduced at the location of distributed buoyancy section. In this study, an analytical model is proposed to simulate lateral buckling triggered by a distributed buoyancy section for an imperfect subsea pipeline, which is validated by test data. Semi-analytical solutions are derived. First, snap-through buckling behaviour is discussed. Then the influence of initial imperfections on buckled configurations, post-buckling behaviour, displacement amplitude and maximum stress is discussed in detail. The results show that there is no snap-through phenomenon for large amplitude of initial imperfections, which appears only when the amplitude of imperfection is small enough. The displacement amplitude increases with the amplitude of initial imperfections, and it first increases and then decreases with wavelength of initial imperfection. Compared to a perfect pipeline, the maximum stress amplifies for relative small wavelength of initial imperfections. Therefore, a large enough wavelength of initial imperfection should be introduced.

The nonlinear pipe-soil interaction model and the initial imperfections are predominantly factors that affect the lateral buckling behaviour of subsea pipelines with a high-temperature. In this study, mathematical models, taking these two factors into account, are proposed to simulate lateral buckling of subsea pipelines. Analytical solutions are derived from the assumption of rigid-plastic pipe-soil interaction, while numerical results are obtained when considering nonlinear pipe-soil interaction. The analytical and numerical results have an excellent agreement except for the negligible discrepancy in the pre-buckling state. The discrepancy is induced by the difference of mobilisation distance in rigid-plastic and elastic-plastic pipe-soil interaction models. After the analysis of snap-through phenomenon, the influence of breakout resistance, amplitude and half-wavelength of imperfection on the post-buckling behaviour is studied. Finally, the upper and lower bound critical temperature differences are discussed as well. The results show that the hysteresis cycle between pre-buckling and post-buckling states may appear under cyclic thermal loading. The snap-through phenomenon and the hysteresis cycle can take place more easily for large breakout resistance, small amplitude and large wavelength of imperfection. In the post-buckling state, the maximum stress uplifts with increasing breakout resistance or amplitude of imperfection, while it reduces with escalating wavelength of imperfection.

Subsea pipelines with a free suspended span segment may be subject to upheaval buckling under high temperature, which may lead to a failure mode such as local buckling, fracture and fatigue. Mathematical models are established to study the upheaval buckling behaviour of subsea pipelines with a free span segment. Then, the accurate locations of the maxima of displacement and stress are obtained. The buckled configuration and the typical post-buckling behaviour and their dependence on free span length are analysed. Furthermore, a detailed analysis is presented for critical temperature difference, displacement amplitude and maximum axial compressive stress. Finally, the influence of touchdown on post-buckling behaviour during the process of upheaval buckling is discussed. The results show that the pipeline is prone to upheaval buckling when the free span length is large enough and the critical temperature difference decreases with increasing free span length. The displacement amplitude and the maximum stress within the span increase with the increase of the free span length before touchdown. However, after the pipeline touches the bottom of the free span, the maximum stress within the span decreases significantly compared to the case without touchdown, which becomes further smaller for smaller depth of the free span.