scholarly journals Research of freak wave effect on a floating object in seakeeping tank

2021 ◽  
Vol 3 (397) ◽  
pp. 65-74
Author(s):  
V. Maslov ◽  
Keyword(s):  
Physical Modeling ◽  
Wind Waves ◽  
Interaction Process ◽  
Superposition Method ◽  
Floating Structure ◽  
Linear Superposition ◽  
Sea State ◽  
Wave Effect ◽  
Freak Wave ◽  
Acceleration Sensors

Object and purpose of research. This paper describes physical modeling of interaction process of abnormal wave (freak wave) with a marine floating structure in a seakeeping tank of the Krylov State Research Center. Freak wave is extremely dangerous because of the difference from wind waves by an unusually steep front slope and a gentle trough. Freak wave appears suddenly and collapses rapidly. Research of effect process features is necessary for understanding and analysis of the object behavior at extreme sea conditions. As experiment results it was necessary to obtain empirical data of sea object motions and accelerations at interaction with freak wave on different course angles and speeds. The obtained physical experiment results will be the foundation of theoretical studies and numerical calculation methods. Materials and methods. Physical modeling of the interaction process of freak wave with a marine floating structure was conducted in a deep seakeeping tank. Freak wave was generated by the linear superposition method of four twodimensional unidirectional regular waves with variable steepness in frequency range of 2 to 6 rad/s. To create a control signal was using special software. Wave packets were formed consisting of a sequence of a four harmonicas with a given frequency, height and duration. For parameters registration of freak wave were used string probes installed with a certain step along the length of the tank. A marine floating structure model was fixed by elastic fastening system in a window of a tow cart. For measure the motions of marine floating structure and its accelerations in define points at encounter with freak wave the contactless optic system and two-component acceleration sensors (accelerometers) were used. Cases of structure interaction with freak wave at different course angles and speeds were considered. Main results. As result of physical experimental data of floating structure motions in the interaction with freak wave in conditions of regular sea state at five course angles with speed and without speed were obtained. Dependencies of roll, pitch and heave motions at different course angles and various speeds were built. Similar dependencies of vertical and transverse accelerations on a stem also were built. Comparative analysis of results with data, which were obtained on intensive irregular sea state (spectrum JONSWAP) at identical experiment conditions, and also with foreign results was carried out. Conclusions. The greatest roll and maximum accelerations are registered at alongside position to abnormal wave, but cargo vessel has a sufficient reserve of dynamic stability to withstand such an impulse effect. The values of roll motion and accelerations on irregular sea state are close to the parameters measured at freak wave effect. This similarity is explained by rocking effect of periodic impact of irregular sea state, the proximity of natural period of roll oscillations to average period of waves and sufficiently high waves. In comparison with foreign researches, a wider range of heading angles and speeds is considered, and data about accelerations in a stem are obtained.

scholarly journals Study on Slamming Pressure Characteristics of Platform under Freak Wave

2021 ◽  
Vol 9 (11) ◽  
pp. 1266
Author(s):  
Fali Huo ◽  
Hongkun Yang ◽  
Zhi Yao ◽  
Kang An ◽  
Sheng Xu
Keyword(s):  
Three Dimensional ◽  
Experimental Tests ◽  
Air Gap ◽  
Offshore Platform ◽  
Superposition Method ◽  
Linear Superposition ◽  
Freak Wave ◽  
Wave Slamming ◽  
Run Up ◽  
Cfd Method

Freak waves have great peak energy, short duration, great contingency, and strong nonlinear characteristics, and can cause severe damage to ships and marine structures. In this study, numerical simulations in conjunction with experimental tests are applied to study air gap response and wave slamming loads of a semi-submersible offshore platform under a freak wave. A three-dimensional wave tank, which is created based on the computational fluid dynamics (CFD) method, is applied to study the hydrodynamic responses of a semi-submersible platform. The numerical model of the tank and offshore platform system are checked according to the experimental results. A typical freak wave is modelled in numerical wave tanks by the linear superposition method, and its significant wave height is 13.03 m. It is found that the freak wave is closely associated with the wave slamming. The appearance of the freak wave gives rise to a negative air, gap which appears on the side of the back wave surface at the bottom of the deck box, and considerable slamming pressure is generated. Furthermore, the wave run up at the junction of the column and the buoyancy tank is also seen due to the freak wave.

Evaluating Shakedown for Structures Based on the Element Bearing-Ratio

2011 ◽  
Vol 137 ◽  
pp. 16-23 ◽  
Author(s):  
Wei Zhang ◽  
Lu Feng Yang ◽  
Chuan Xiong Fu ◽  
Jian Wang
Keyword(s):  
Yield Criterion ◽  
Solution Procedure ◽  
Superposition Method ◽  
Linear Superposition ◽  
Non Linear Programming ◽  
Elastic Plastic ◽  
Non Linear ◽  
Efficiency And Effectiveness ◽  
Linear Programming Formulation ◽  
Element Bearing

Based on Melan’s theorem, an improved numerical solution procedure for evaluating shakedown loads by non-linear superposition method is presented, and the relationship between the classical non-linear programming formulation of shakedown problem and the numerical method is disclosed. The stress term in classical optimization problem is replaced by the element bearing-ratio (EBR) in the procedure, and series of residual EBR fields can be generated by the D-value of the elastic-plastic EBR fields and the elastic EBR fields at every incremental loading step. The shakedown load is determined by performing the incremental non-linear static analysis when the yield criterion is arrived either by the elastic-plastic EBR fields or residual EBR fields. By introducing the EBR, the proposed procedure can be easily used to those complex structures with multi-material and complicated configuration. The procedure is described in detail and some numerical results, that show the efficiency and effectiveness of the proposed method, are reported and discussed.

Strain field reconstruction of crossbeam structure based on load-strain linear superposition method

2021 ◽  
Author(s):  
Yangyang Cheng ◽  
Zhaohua Li ◽  
Guangjun Wang ◽  
Chang Peng ◽  
Lei Zhang ◽  
...  
Keyword(s):  
Strain Field ◽  
Superposition Method ◽  
Linear Superposition ◽  
Field Reconstruction

scholarly journals Hydraulic Fracture Propagation in a Poro-Elastic Medium with Time-Dependent Injection Schedule Using the Time-Stepped Linear Superposition Method (TLSM)

Energies ◽  
2020 ◽  
Vol 13 (24) ◽  
pp. 6474
Author(s):  
Tri Pham ◽  
Ruud Weijermars
Keyword(s):  
Hydraulic Fracture ◽  
Fracture Propagation ◽  
Fracture Network ◽  
Time Dependent ◽  
Reservoir Rock ◽  
Hydraulic Fractures ◽  
Superposition Method ◽  
Stress Concentrations ◽  
Linear Superposition ◽  
Dynamic Fractures

The Time-Stepped Linear Superposition Method (TLSM) has been used previously to model and analyze the propagation of multiple competitive hydraulic fractures with constant internal pressure loads. This paper extends the TLSM methodology, by including a time-dependent injection schedule using pressure data from a typical diagnostic fracture injection test (DFIT). In addition, the effect of poro-elasticity in reservoir rocks is accounted for in the TLSM models presented here. The propagation of multiple hydraulic fractures using TLSM-based codes preserves infinite resolution by side-stepping grid refinement. First, the TLSM methodology is briefly outlined, together with the modifications required to account for variable time-dependent pressure and poro-elasticity in reservoir rock. Next, real world DFIT data are used in TLSM to model the propagation of multiple dynamic fractures and study the effect of time-dependent pressure and poro-elasticity on the development of hydraulic fracture networks. TLSM-based codes can quantify and visualize the effects of time-dependent pressure, and poro-elasticity can be effectively analyzed, using DFIT data, supported by dynamic visualizations of the changes in spatial stress concentrations during the fracture propagation process. The results from this study may help develop fracture treatment solutions with improved control of the fracture network created while avoiding the occurrence of fracture hits.

Dynamic Response of Spar-Type Floating Offshore Wind Turbine in Freak Wave

2019 ◽  
Author(s):  
Yougang Tang ◽  
Yan Li ◽  
Peng Xie ◽  
Xiaoqi Qu ◽  
Bin Wang
Keyword(s):  
Dynamic Response ◽  
Wind Turbine ◽  
Offshore Wind ◽  
Modulation Method ◽  
Power Coefficient ◽  
Offshore Wind Turbine ◽  
Floating Structure ◽  
Mooring Lines ◽  
Freak Wave ◽  
Floating Offshore Wind Turbine

Abstract 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.

Can swell increase the number of freak waves in a wind sea?

2010 ◽  
Vol 650 ◽  
pp. 57-79 ◽  
Author(s):  
ODIN GRAMSTAD ◽  
KARSTEN TRULSEN
Keyword(s):  
Nonlinear Evolution ◽  
Wind Waves ◽  
Short Wave ◽  
Freak Waves ◽  
Freak Wave ◽  
Short Waves ◽  
Term Evolution ◽  
Wind Sea ◽  
Modified Equation

The effect of a swell on the statistical distribution of a directional short-wave field is investigated. Starting from Zakharov's spectral formulation, we derive a new modified nonlinear Schrödinger equation appropriate for the nonlinear evolution of a narrow-banded spectrum of short waves influenced by a swell. The swell-modified equation is solved analytically to yield an extended version of the result of Longuet-Higgins & Stewart (J. Fluid Mech., vol. 8, no. 4, 1960, pp. 565–583) for the modulation of a short wave riding on a longer wave. Numerical Monte Carlo simulations of the long-term evolution of a spectrum of short waves in the presence of a monochromatic swell are employed to extract statistical distributions of freak waves among the short waves. We find evidence that a realistic short-crested wind sea can on average experience a small increase in freak wave probability because of a swell provided the swell is not orthogonal to the wind waves. For orthogonal swell and wind waves we find evidence that there is almost no significant change in the probability of freak waves in the wind sea. If the short waves are unrealistically long crested, such that the Benjamin–Feir index serves as indicator for freak waves (Gramstad & Trulsen, J. Fluid Mech., vol. 582, 2007, pp. 463–472), it appears that the swell has much smaller relative influence on the probability of freak waves than in the short-crested case.

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