Dynamic Response Analysis of Semi-Submersible Floating Wind Turbine with Different Wave Conditions

To address the problem of poor wave resistance of existing offshore floating wind turbines,a new type of semisubmersible platform with truncated-cone-type upper pontoons is proposed by combining the characteristics of offshore wind turbine semi-submersible floating platforms.Based on the coupled hydrodynamic,aerodynamic,and mooring force physical fields of FAST,the surge,heave,pitch,and yaw motions responses of the floating wind turbine under different wave heights and periods are obtained,and the mooring line tension responses are also obtained;and compare the dynamic response of the new semi-submersible platform with the OC4-DeepCwind platformat six degrees of freedom.The results show that different wave conditions have obvious effects on the heave and pitch motions of the new floating wind turbine,and fewer effects on the surge and yaw motions;the tensegrity response of the mooring system is more affected by the wave conditions;compared with the OC4-DeepCwind floating wind turbine,the pitch and roll response of the new floating wind turbine has been significantly reduced and has good stability.

Conceptual Design of a Deep Draft Semi-Submersible Platform with a Moveable Heave-Plate

Based on the principle of turned mass damper(TMD) systems,the conceptual design of semi-submersible platform with a moveable heave-plate(MHS) has been put forward.The heave motion response amplitude operator(RAO) and viscous damping of the MHS platform are calculated by iteration,and the coupling stiffness between the MHS hull and the heave-plate is optimized to decrease the maximum heave motion response of the MHS hull under 10-year survival conditions in the South China Sea.The nu-merical results indicate that the heave motion RAO of the MHS hull can be decreased in the range of predominant wave frequencies,which may provide some reference to the heave motion control of offshore platforms.

Research on the Effects of in-Line Oscillatory Flow on the Vortex-Induced Motions of A Deep Draft Semi-Submersible in Currents

A Deep Draft Semi-submersible （DDS） under certain flow conditions could be subjected to Vortex-Induced Motions （VIM）, which significantly influences the loads on and life fatigue of the moorings and the risers. To investigate the VIM of a DDS with four rectangular section columns in waves coupled with a uniform current, a numerical study using the computational fluid dynamics （CFD） method was conducted. The issues of the VIM of multi-column floaters can be con＇,＇eniently converted to the issues of oscillating cylinders in fluid cross flows. This paper looks into the CFD numerical simulation of infinite cylinders having rectangular sections in a two-dimensional sinusoidal time- dependent flow field coupled with a uniform current. The resulted hydrodynamic forces and motion responses in different oscillatory flows plus currents both aligned in the same direction for the incidence of 135° of the DDS relative to the flow are compared with the ones in current only cases. The results show that the VIM response of this geometric arrangement of a DDS with four rectangular columns in a current combined with oscillatory flows is more evident than that in the current only case. The oscillatory flows and waves have the significant influence on the VIM response, forces and trajectory, in-plane motions of the DDS.

Impact of Crosswind on Steady-State and Dynamic Performance of Natural Draft Dry Cooling Tower Group:A Numerical Analysis

This study investigates the performance of a natural draft dry cooling tower group in crosswind conditions through numerical analysis.A comprehensive three-dimensional model is developed to analyze the steady-state and dynamic behavior of the towers.The impact of wind speed and direction on heat rejection capacity and flow patterns is examined.Results indicate that crosswinds negatively affect the overall heat transfer capacity,with higher crosswind speeds leading to decreased heat transfer.Notably,wind direction plays a significant role,particularly at 0°.Moreover,tower response time increases with higher crosswind speeds due to increased turbulence and the formation of vortices.The response times are generally similar for wind directions of 45°and 90°,but differ when facing 0,where the leeward tower exhibits a shorter response time compared to the windward tower.These findings provide valuable insights into the performance of natural draft dry cooling tower groups under crosswind conditions,which can inform the design and operation of similar systems in practical applications.

Performance Evaluation of Solar Chimney Draft: Application to Ventilation

Ventilation is one of the factors contributing to energy consumption in buildings and food preservation. The solar chimney proves to be an alternative for reducing conventional energy consumption. Thus, in this study, the performance of a solar chimney with two active faces for thermally drawing air from a chamber for preserving agri-food products was evaluated. These performances were experimentally assessed through data measurements: temperatures and velocities within the chimney, and their analysis using Excel and MATLAB. The obtained results were compared with those from literature to verify their validity. From this study, it is found that the maximum temperature at the chimney outlet reaches 49.4˚C with an average value of 43.7˚C. Additionally, the heating evolution of the chimney air presents four (04) identical phases in pairs, reflecting the chimney’s operation throughout day. The temperature difference between the outlet and inlet of the chimney reaches a maximum of 17˚C with an average of 12.6˚C. Regarding airflow, the maximum air velocity at the chimney outlet is 0.8 m/s, and the average velocities have consistently been greater than or equal to 0.46 m/s. Thus, it can be concluded that the solar chimney is capable of providing ventilation for the preservation chamber through thermal draft.

Study on the general layout of semi-submersible offshore drilling platforms based on process flow

The general layout of 6th generation semi-submersible drilling platforms is the main factor impacting the efficiency of their drilling operations. This paper provides a compound/integrated algorithm based on process flow that is aimed at improving efficiency, while giving attention to stability and safety at the same time. The paper describes the process flow of dual drilling centers and a hierarchical division of rigs based on the different modes of transportation of various drilling support systems. The general layout-centripetal overall arrangement spatially was determined based on drilling efficiency. We derived our modules according to drilling functionality; the modules became our basic layout units. We applied different layout algorithm to mark out the upper and lower decks. That is, the upper deck was designed based on the lowest transportation cost while the lower deck＇s calculations were based on the best-fit scope. Storage configurations in columns and pontoons were also considered for the layout design. Finally the center of gravity was taken into consideration and the general layout was adjusted accordingly, to result in an optimal center of gravity. The methodology of the general layout can provide a reference for implementation of domestic designs of semi-submersible rigs.

An Investigation on Low Frequency Fatigue Damage of Mooring Lines Applied in a Semi-Submersible Platform

Assessing the fatigue life of mooring systems is important for deep water structures. In this paper, a comprehensive fatigue analysis is conducted on the mooring lines applied in a semi-submersible platform with special focus on the low frequency(LF) fatigue damage. Several influential factors, including water depth, wave spectral parameters, and riser system, are considered. Numerical simulation of a semi-submersible platform with the mooring/riser system is executed under different conditions, and the fatigue damage of mooring lines is assessed by using the time domain analysis method as a benchmark. The effects of these factors on the mooring line tension and the fatigue damage are investigated and discussed in detail. Research results indicate that the LF fatigue damage only accounts for a very small portion of the total damage, although the LF components dominate the global motion response and the mooring line tension of the semi-submersible platform. However, it is demonstrated that the LF fatigue damage is clearly affected by the influential factors. The increase in water depth and spectral peak periods, and the existence of risers can weaken the contribution of the LF components to the mooring line fatigue damage, while the fatigue damage due to the LF components increases with the increase of significant wave height.

Wave load computation in direct strength analysis of semi-submersible platform structures

A wave load computation approach in direct strength analysis of semi-submersible platform structures was presented in this paper. Considering the differences in shape of pontoon, column and beam, the combination of accumulative chord length cubic parameter spline theory and analytic method was adopted for generating the wet surface mesh of platform. The hydrodynamic coefficients of platform were calculated by the three-dimensional potential flow theory of the linear hydrodynamic problem for platform with low forward speed. The equation of platform motions was established and solved in frequency domain, and the responses of wave-induced loads on the platform can be obtained. With the interpolation method being utilized, the pressure loads on shell elements for finite element analysis (FEA) were converted from those on the hydrodynamic computation mesh, which pave the basis for FEA with commercial software.A computer program based on this method has been developed, and a calculation example of semi-submersible platform was illustrated.Analysis results show that this method is a satisfying approach of wave loads computation for this kind of platform.

Sensitivity Analysis of Air Gap Motion with Respect to Wind Load and Mooring System for Semi-Submersible Platform Design

A design of semi-submersible platform is mainly based on the extreme response analysis due to the forces experienced by the components during lifetime. The external loads can induce the extreme air gap response and potential deck impact to the semi-submersible platform. It is important to predict air gap response of platforms accurately in order to check the strength of local structures which withstand the wave slamming due to negative air gap. The wind load cannot be simulated easily by model test in towing tank whereas it can be simulated accurately in wind tunnel test. Furthermore, full scale simulation of the mooring system in model test is still a tuffwork especially the stiffness of the mooring system. Owing to the above mentioned problem, the model test results are not accurate enough for air gap evaluation. The aim of this paper is to present sensitivity analysis results of air gap motion with respect to the mooring system and wind load for the design of semi-submersible platform. Though the model test results are not suitable for the direct evaluation of air gap, they can be used as a good basis for tuning the radiation damping and viscous drag in numerical simulation. In the presented design example, a numerical model is tuned and validated by ANSYS AQWA based on the model test results with a simple 4 line symmetrical horizontal soft mooring system. According to the tuned numerical model, sensitivity analysis studies of air gap motion with respect to the mooring system and wind load are performed in time domain. Three mooring systems and five simulation cases about the presented platform are simulated based on the results of wind tunnel tests and sea-keeping tests. The sensitivity analysis results are valuable for the floating platform design.

Hydrodynamic Comparison of a Semi-submersible,TLP,and Spar:Numerical Study in the South China Sea Environment

The South China Sea contains tremendous oil and gas resources in deepwater areas. However, one of the keys for deepwater exploration, the investigation of deepwater floating platforms, is very inadequate. In this paper, the authors studied and compared the hydrodynamics and global motion behaviors of typical deepwater platforms in the South China Sea environment. The hydrodynamic models of three main types of floating platforms, e.g. the Semi-submersible, tension leg platform （TLP）, and Truss Spar, which could potentially be utilized in the South China Sea, were established by using the 3-D potential theory. Additionally, some important considerations which significantly influence the hydrodynamics were given. The RAOs in frequency domains as well as global motions in time domains under time-varying wind, random waves, and current in 100-y, 10-y, and 1-y return period environment conditions were predicted, compared, and analyzed. The results indicate that the heave and especially the pitch motion of the TLP are favorable. The heave response of the Truss Spar is perfect and comparable with that of the TLP when the peak period of random waves is low. However, the pitch motion of Truss Spar is extraordinarily lar^er than that of Semi-submersible and TLP.

Dynamic Performance of a Semi-Submersible Platform Subject to Wind and Waves

By applying experimental and numerical simulations, the motion performance of a semi-submersible platform with mooring positoning system under combined actions of wind and waves is studied. The numerical simulation is conducted by the method of nonlinear time domain coupled analysis, and the mooring forces are calculated by the piecewise extrapolating method. The scale in the model experiment is 1:100, and the mooring system of the model is designed with the method of equivalent water-depth truncation by comparing the numerical and the experimental results, the platform motion and mooring forces subject to wind and waves are investigated. The results indicate that the numerically simulated mooring forces agree well with the experimental results in static equivalent field, but show some difference in dynamic equivalent field; the numerically simulated platform motions coincide well with the experimental results. The maximum motion of the platform under operating conditions is 20.5 m. It means that the horizontal displacement is 2% less than the water depth, which satisfies the operating requirements.

Strength Analysis on Brace Structure for Semi-Submersible in Consideration of Wave Slamming

Slamming on bracings of column stabilized units shall be considered as a possible limiting criterion under transit condition based on the requirements in DNV-OS-C103. However, the wave slamming loads under survival condition were ignored for the strength analysis of the brace structures in many semi-submersible projects. In this paper, a method of strength analysis of brace structure is proposed based on the reconstruction and extrapolation of numerical model. The full-scale mooring system, the wind, wave and current loads can be considered simultaneously. Firstly,the model tests of the semi-submersible platform in wind tunnel and wave tanker have been carried out. Secondly,the numerical models of the platform are reconstructed and extrapolated based on the results of model tests. Then, a nonlinear numerical analysis has been conducted to study the wave slamming load on brace in semi-submersible platform through the reconstructed and extrapolated numerical model. For the randomness of wave load, ten subcases under each condition have been carried out. The value of the 90% Gumble distribution values of the ten subcases are used. Finally, the strength on brace structure has been analyzed considering the wave slamming. The wave slamming loads have been compared between the survival condition and transit condition with the method. The results indicate that wave slamming under survival condition is more critical than that under transit condition.Meanwhile, the wave slamming is significant to the structural strength of the brace. It should be overall considered in the strength analysis of the brace structure.

Study on Global Performances and Mooring-Induced Damping of A Semi-Submersible

The harsh environmental conditions bring strong nonlinearities to the hydrodynamic performances of the offshore floating platforms, which challenge the reliable prediction of the platform coupled with the mooring system. The present study investigates a typical semi-submersible under both the operational and the survival conditions through numerical and experimental methods. The motion responses, the mooring line tensions, and the wave loads on the longitudinal mid-section are investigated by both the fully non-linearly coupled numerical simulation and the physical experiment. Particularly, in the physical model test, the wave loads distributed on the semi-submersible＇s mid-section were measured by dividing the model into two parts, namely the port and the starboard parts, which were rigidly connected by three six-component force transducers. It is concluded that both the numerical and physical model can have good prediction of the semi-submersible＇s global responses. In addition, an improved numerical approach is proposed for the estimation of the mooting-induced damping, and is validated by both the experimental and the published results. The characteristics of the mooring-induced damping are further summarized in various sea states, including the operational and the survival ~nvironments. In order to obtain the better prediction of the system response in deep water, the mooring-induced damping of the truncated mooring lines applied in the physical experiment are compensated by comparing with those in full length. Furthermore, the upstream taut and the downstream slack mooring lines are classified and investigated to obtain the different mooring line damping performances in the comparative study.

First Rocketsonde Launched from an Unmanned Semi-submersible Vehicle

The unmanned semi-submersible vehicle(USSV) developed by the unmanned surface vehicle team of the Institute of Atmospheric Physics is an unmanned, rugged, and high-endurance autonomous navigation vessel designed for the collection of long-term, continuous and real-time marine meteorological measurements, including atmospheric sounding in the lower troposphere. A series of river and sea trials were conducted from May 2016 to November 2017, and the first rocketsonde was launched from the USSV. Real-time meteorological parameters in the marine atmospheric boundary layer(MABL) were obtained, including sea surface temperature, and vertical profiles of the pressure, temperature, relative humidity, wind speed,and wind direction. These data are extremely useful and important for research on air–sea interactions, sea surface heat and latent heat flux estimations, MABL modeling, and marine satellite product validation.

Dynamics of Large-Truncated Mooring Systems Coupled with A Catenary Moored Semi-Submersible

With the floating structures pushing their activities to the ultra-deep water, model tests have presented a challenge due to the limitation of the existing wave basins. Therefore, the concept of truncated mooring system is implemented to replace the full depth mooring system in the model tests, which aims to have the same dynamic responses as the full depth system. The truncated mooring system plays such a significant role that extra attention should be paid to the mooring systems with large truncation factor. Three different types of large truncation factor mooring system are being employed in the simulations, including the homogenously truncated mooring system, non-homogenously truncated mooring system and simplified truncated mooring system. A catenary moored semi-submersible operating at 1000 m water depth is presented. In addition, truncated mooring systems are proposed at the truncated water depth of 200 m. In order to explore the applicability of these truncated mooring systems, numerical simulations of the platform’s surge free decay interacting with three different styles of truncated mooring systems are studied in calm water. Furthermore, the mooring-induced damping of the truncated mooring systems is simulated in the regular wave. Finally, the platform motion responses and mooring line dynamics are simulated in irregular wave. All these simulations are implemented by employing full time domain coupled dynamic analysis, and the results are compared with those of the full depth simulations in the same cases. The results show that the mooring-induced damping plays a significant role in platform motion responses, and all truncated mooring systems are suitable for model tests with appropriate truncated mooring line diameters. However, a large diameter is needed for simplified truncated mooring lines. The suggestions are given to the selection of truncated mooring system for different situations as well as to the truncated mooring design criteria.

Investigation of the Hydrodynamic Performance of a Novel Semi-Submersible Platform with Multiple Small Columns

This paper presents a novel semi-submersible(SEMI) platform concept, called the multiple small columns(MSC) SEMI that improves upon the hydrodynamic performance of the conventional SEMI. Unlike the conventional SEMI, the proposed MSC SEMI utilizes multiple small circular columns to support the deck and a large pontoon that increases the structural displacement. The novelty of the MSC SEMI is its reduction of the hydrodynamic load on the structure and suppression of its motion response, particularly in the heave direction. The MSC SEMI has the advantages of increasing the added mass, radiation damping, and natural period of the structure. A comprehensive investigation of the hydrodynamic performance of the novel MSC SEMI is conducted in both the time and frequency domains with a special focus on the resulting hydrodynamic load and motion response. Numerical simulation results demonstrate that the MSC SEMI concept can reduce the hydrodynamic load and motion response and improve the hydrodynamic performance of SEMIs as expected.

Calculation of hydrodynamics for semi-submersibles based on NURBS

Accurate hydrodynamic calculations for semi-submersibles are critical to support modern rapid exploration and extraction of ocean resources. In order to speed hydrodynamic calculations, lines modeling structures were separated into structural parts and then fitted to Non-uniform Rational B-spline (NURBS). In this way, the bow and stern section lines were generated. Modeling of the intersections of the parts was then done with the universal modeling tool MSC.Patran. Mesh was gererated on the model in order to obtain points of intersection on the joints, and then these points were fitted to NURBS. Next, the patch representation method was adopted to generate the meshes of wetted surfaces and interior free surfaces. Velocity potentials on the surfaces were calculated separately, on basis of which the irregular frequency effect was dealt with in the calculation of hydrodynamic coefficients. Finally, the motion response of the semi-submersible was calculated, and in order to improve calculations of vertical motion, a damping term was affixed in the vertical direction. The results show that the above methods can generate fine mesh accurately representing the wetted surface of a semi-submersible and thus improve the accuracy of hydrodynamic calculations.

A Prediction Method of Internal Solitary Wave Loads on the Semi-Submersible Platform

An investigation into the prediction method for internal solitary waves(ISWs)loads on the columns and caissons of the semi-submersible platform found on three kinds of internal solitary wave theories and the modified Morison Equation is described.The characteristics of loads exerted on the semi-submersible platform model caused by the ISWs have been observed experimentally,and the inertial and drag coefficients in Morison Equation are determined by analyzing the forces of experiments.From the results,it is of interest to find that Reynolds number,KC number and layer thickness ratio have a considerable influence on the coefficients.The direction of incoming waves,how-ever,is almost devoid of effects on the coefficients.The drag coefficient of columns varies as an exponential function of Reynolds number,and inertia coefficient of columns is a power function related to KC number.Meanwhile,the drag coefficient of caissons is approximately constant in terms of regression analysis of experimental data.The results from different experimental conditions reveal that the inertia coefficient of caissons appears to be exponential correlated with upper layer depths.

Numerical and Experimental Study on Hydrodynamic Performance of A Novel Semi-Submersible Concept

The Multiple Column Platform(MCP) semi-submersible is a newly proposed concept, which differs from the conventional semi-submersibles, featuring centre column and middle pontoon. It is paramount to ensure its structural reliability and safe operation at sea, and a rigorous investigation is conducted to examine the hydrodynamic and structural performance for the novel structure concept. In this paper, the numerical and experimental studies on the hydrodynamic performance of MCP are performed. Numerical simulations are conducted in both the frequency and time domains based on 3D potential theory. The numerical models are validated by experimental measurements obtained from extensive sets of model tests under both regular wave and irregular wave conditions. Moreover, a comparative study on MCP and two conventional semi-submersibles are carried out using numerical simulation.Specifically, the hydrodynamic characteristics, including hydrodynamic coefficients, natural periods and motion response amplitude operators(RAOs), mooring line tension are fully examined. The present study proves the feasibility of the novel MCP and demonstrates the potential possibility of optimization in the future study.

Structural Stress Monitoring and FEM Analysis of the Cutting Operation of the Main Bracket of A Semi-Submersible Platform

For a semi-submersible platform in repair, the eight old main brackets which connect columns with pontoons need to be replaced by new ones. In order to ensure the safety of the cutting operation of the old main bracket and calculate the initial stress condition of new main bracket, the structural stress monitoring of eight key spots is carried out, and then the calibrated finite element model is established according to the field monitoring results. Before cutting the main bracket and all associated structures, eight rectangular rosettes were installed, and a tailored cutting scheme was proposed to release the initial stress, in which the main bracket and associated column and pontoon plates were partly cut. During the cutting procedure, the strains of the monitoring spots were measured, and then the structural stress of the monitored spots were obtained. The stress variation characteristics at different spots during the initial cutting operation were shown and the initial stress condition of the monitored spots was figured out. The loading and support conditions of the semi-submersible platform were calibrated based on the measured initial stress condition, which made the finite element model more credible. The stress condition with the main bracket and associated structures being entirely cut out is analyzed by the Finite Element Method (FEM), which demonstrates the cutting operation to be safe and feasible. In addition, the calibrated finite element model can be used to calculate the initial stress condition of the new main bracket, which will be very helpful for the long-term stress monitoring on the main bracket.