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To investigate the coupled effect of a control system between a wind turbine and floating platform, in this paper, a self-designed blade pitch control system is applied for coupled aero-hydrodynamic simulations of a semisubmersible floating wind turbine by using the open-source program OpenFAST. While the motion responses of the floating platform are integrated into the blade pitch control, the generator speed control loop with the active disturbance rejection control algorithm is used to achieve the simultaneous regulation of generator speed and blade pitch angle. The aero-hydrodynamic characteristics of floating wind turbines under different wind fields and different control strategies are discussed. The results show that compared with the traditional baseline control strategy, the proposed integrated individual pitch control (I-IPC) and integrated collective pitch control (I-CPC) strategies not only reduce the load on the blades and tower while increasing the generator power but also improve the surge, roll, and pitch motion responses of the floating platform. The generator speed control loop can be used to effectively regulate generator speed while reducing fluctuations in the aerodynamic torque and output signals. • A generator speed control loop was integrated into the simulation of coupled platform-mooring-nacelle system. • Active disturbance rejection control is applied to regulate blade pitch and reject the external disturbances. • Investigation of the dynamic response by implementing the integrated control strategy. • Study the coupling effect of I-IPC and I-CPC under combined wind-wave excitation. [ABSTRACT FROM AUTHOR]

• A three-dimensional model for VIV predictions of curved flexible cylinder in varied flow angles. • Comprehensive validation of the numerical model is conducted against the publicly available experiments. • Variation of the incident angle significantly affects the three-dimensional VIVs patterns. • The structural power-in regions in three directions are synchronized at oblique flow. Vortex-induced vibrations (VIVs) of steel catenary risers (SCR) considering various flow incident angles are modeled and investigated. The dynamics of the flexible cylinder are first described using the Euler-Bernoulli beam theory with the absolute nodal coordinate formulation (ANCF). To capture the nonlinear shear characteristics of the flow, a discrete point vortex theory-based van der Pol wake oscillator is improved by introducing a local reference coordinate system and relative flow velocity. The comparative analysis between the numerical and experimental results demonstrates the numerical model effectively reproduces the multimodal VIVs of curved cylinders with a satisfactory agreement. The VIV response of a real-scale riser is then explored parametrically by varying the flow velocity for incident angles of 0°, 30°, 60°, and 90°. The effects of incident angle on the structural multidirectional response, spatially varying dominant frequency, lock-in position, trajectory, phase evolution, and fluid-structure energy transfer are highlighted. The results of the present study suggest that the three-dimensional excitation coefficients are synchronized and share the same power-in region under oblique flow conditions, which predisposes VIVs of the riser to travelling wave pattern. [Display omitted] [ABSTRACT FROM AUTHOR]

A novel structural dynamics model for the lowering process of a J-lay pipeline is established by combining the absolute nodal coordinate formula with the arbitrary Lagrangian–Eulerian description. The constraint on the top of the pipeline is introduced into the dynamic equation in the form of generalized coordinates by using a material transport boundary to simulate the lowering process of the top end of the pipeline. A mesh scheme is adopted to control the element length and prevent numerical disturbances. The accuracy of the material transport boundary and mesh scheme is verified for the case of a pendulum with a time-varying length. Then, a coupled analysis program is developed to investigate J-lay deployment by a vessel using dynamic positioning. The tensioner is modeled as a damper to adjust the pay-out speed of the upper end of the pipeline. The numerical analysis is carried out on an explanatory example of a 16-inch pipeline being laid at a 1000-m water depth. The results show that the lowering process strongly affects the dynamic characteristics of the touch-down zone. • A novel and feasible ALE-ANCF model is established for the offshore pipeline J-lay lowering process. • A reasonable damping tensioner is developed for adjusting the pay-out speed. • Investigation of the dynamic response by implementing the ALE-ANCF model coupling with the DP vessel. • Study the effect of the lowering process on the dynamic behaviors of TDZ. [ABSTRACT FROM AUTHOR]

A nonuniform distribution of internal flow along the offshore riser can lead to transient in-plane excitation. To predict the global riser response due to slug flow by nonlinear time domain analyses, a finite element method is developed in this study by combining the absolute nodal coordinate formula (ANCF) and arbitrary Lagrange-Eulerian (ALE) description. This proposed method has the capacity to effectively simulate both the nonuniform gravity distribution of slug flow and geometric nonlinearity of the riser. The nodal position and slope vectors described in the global coordinate system avoid the complex coordinate transformation of the elements while the internal fluid is effectively integrated into the framework. After the accuracy of the proposed program is verified, a kind of steep wave riser (SWR) with internal flow is taken as an example to discuss the effect of slug flow. The results show that slug flow can result in large local deformations and lead to variable pattern changes in the bending moment. • A modeling method based on ALE-ANCF is proposed to simulate oscillations of offshore risers conveying the slug flow. • This formulation effectively describes the slug-flow and avoids the complex coordinate transformation. • The methodology is suitable for the slug-flow-riser interaction problem undergoing large deformations. • The effect of slug-flow on the global response is analysed for a steep wave riser. • Large fluctuation amplitudes of the global bending moment are observed when considering the slug-flow load. [ABSTRACT FROM AUTHOR]

The paper presents a fatigue assessment method for offshore steel catenary risers (SCR) under vortex-induced vibrations (VIV) with multi-directional incoming flow. Firstly, the structural responses are obtained using the finite element method (FEM) in conjunction with a coupled cross-flow (CF) and in-line (IL) wake model. Subsequently, the cyclic stress associated with fatigue life is transformed into the frequency domain, and a corresponding Kriging model is constructed. Further, the structural fatigue life is correspondingly computed using the Miner-Palmgren damage rule and the structural fatigue reliability model is established. To validate the effectiveness of the FEM model and the frequency-domain-based Kriging model, comparisons between experimental and numerical results are conducted. Through the analysis of the structural response, the paper elucidates the mechanism underlying the impact of the incident angle on the fatigue distribution, while emphasizing the contribution of higher harmonics to structural fatigue. Moreover, a case study demonstrates the superiority of the proposed method in predicting the probability of failure and factor of safety compared to models that disregard the incident angle and IL responses. Overall, the proposed fatigue analysis methodology contributes to the reliability and availability of SCRs during the design stage, and to reduce potential fatigue-related failures. [ABSTRACT FROM AUTHOR]

During the drilling operation of a Spar platform with a tender semisubmersible platform, the motion of the floating platforms is complex and quite different from that of the separate conditions due to the influence of multibody hydrodynamics and the connecting system. This can lead to potential collision accidents. In this paper, the motion characteristics of a tender assistant drilling (TAD) system under drilling scenarios are analyzed numerically and experimentally. Based on three-dimensional potential flow theory, the numerical model of a multiple floating body system is established to obtain the hydrodynamic parameters, the first-order wave force, and the mean drift force, and then a time domain analysis is conducted. Both single-body and coupled multibody conditions are observed to analyze the motion performance of the gangway and the tension response of the connecting lines. The results show that the Spar platform affects the low-frequency motion of the semisubmersible platform more intensely when the platforms work side by side without hawser lines. The phenomenon of hydrodynamic interaction is weakened under fully coupled conditions due to this connection. In addition, the stroke of the gangway is positively related to the relative motion, which reflects the relative motion response of the platforms. • The surge-pitch coupled motion characteristics of the Spar platform and its effect on TDU are observed in detail. • The motion response of the two platforms with and without connection is discussed. • Response of hawser lines and the gangway is analyzed to study the relative motion. [ABSTRACT FROM AUTHOR]

The dynamic cables are usually designed to present a complex layout to improve service life and safety in commercial scale floating applications. It is urgent to establish consistent design philosophy and analysis methodologies to provide cost-efficient design requirement. A stepwise scheme for the optimization implementation of lazy wave dynamic cables is proposed and performed with a mass-damping equivalence model. A complicated multi-objective optimal procedure is implemented progressively under hydrostatic state, regular wave state and stochastic sea state. It is validated through a lazy wave dynamic cable (LWDC) from a cylindrical FPSO system under extreme conditions. Stepwise upgraded calibrations based on the coupling of multi-objective optimization modular NSGA-II and lumped mass FEM are applied to assess every sub-objective in defined steps. It is shown that the result is far beyond expectation than initial design after the application of the progressive solution system. • A stepwise scheme for the optimization of lazy wave dynamic cable is proposed. • A mass-damping equivalence model is applied. • A multi-objective optimization is implemented under stochastic dynamic state. [ABSTRACT FROM AUTHOR]

A numerical model for computing the vortex-induced vibration (VIV) and fatigue damage of steel catenary risers (SCRs) was developed. The structural dynamics were accurately simulated using an absolute nodal coordinate formulation (ANCF). The Van der Pol wake oscillator is applied to generate the fluctuating lift, which is further transformed into the cross-flow direction by considering the structural deformation. The Randolph-Quiggin (RQ) model and the Coulomb friction 'bilinear' model are employed to simulate the vertical and lateral riser-soil interactions, respectively. After case validations, the effects of riser-soil interaction on the VIV amplitude, frequency, mode, and fatigue of the SCR at different current angles are investigated, and a sensitivity study of different seabed model parameters is discussed. The bands of significant VIV frequencies were broadened by riser-soil interactions, accompanied by more frequency components of disturbance and more abundant vibration modes. Severe fatigue damage cannot be captured by the truncated model, and seabed models that require improvement are ignored. It is suggested that vertical and lateral riser-soil interactions should be considered in the evaluation of VIV fatigue damage for SCRs. • A time-domain VIV model of SCR considering vertical and lateral riser-soil interactions is proposed. • The severe fatigue damage near TDP cannot be captured by the truncated model ignoring riser-soil interaction. • The frequency components and mode of VIV at different current angles are affected the by the riser-soil interaction. • A parameter sensitivity investigation regarding seabed model properties has been carried out. [ABSTRACT FROM AUTHOR]