Figure 10 - uploaded by Harrif Santo
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Force-time history comparison between forced oscillations at three scales (black lines) and forced oscillations at two scales combined with the expanded form of the Morison relative-velocity term (red lines) for a range of cases.
Source publication
This paper aims to model the hydrodynamic forces on grids of perforated flat plates undergoing forced motions at three scales, namely steady (current), and combined low (wave) frequency and high (structural) frequency oscillatory motion. The intended application is the design and re-assessment of dynamically-responding offshore platforms. A recent...
Citations
... Following the earlier work by Santo et al. (2018d), Li et al. (2022) developed a one-way hydro-structural coupling model using OpenFOAM as the CFD solver providing the hydrodynamic forces and damping, and ABAQUS as the Finite Element (FE) solver providing the corresponding structural response. In this section, the developed coupling model is briefly described, using jack-up as the space-frame structure of interest. ...
... Ways for modeling the hydrodynamic forces on grids of perforated plates are presented in the article [11] for the case of combined steady, low frequency, and high-frequency motion. As a result, it was proved that the porous block model is capable of capturing the global large-scale wake structures, which are responsible for the reduction in fluid flow velocity and associated forces on a structure. ...
... The proposed approach allows determining the parameters of the model by the experimental data. Particularly, formulas (6) and (17) allow determining the average velocity and hydraulic losses in the air distribution device using the regression dependencies (11) and (20) for evaluating the unknown parameters. ...
The article is focused on the comprehensive analysis of the aerodynamics of air distribution devices with the combined heat and mass exchange, with the aim to improve the following hydro-and thermodynamic parameters of ventilation systems: flow rate, air velocity, hydraulic losses, and temperature. The inadequacy of the previously obtained characteristics has confirmed the need for more rational designs of air distribution systems. Consequently, the use of perforated plates was proposed to increase hydraulic losses for reducing the average velocity and ensuring a uniform distribution of the velocity field on the outlet of the device. The effectiveness of one of the five possible designs usage is confirmed by the results of numerical simulation. The coefficient of hydraulic losses decreased by 2.5-3.0 times, as well as the uniformity of the outlet velocity field for the air flow being provided. Based on the three-factor factorial experiment, the linear mathematical model was obtained for determining the dependence of the average velocity on the flow rate, plate's area, and diameter of holes. This model was significantly improved using the multiparameter quasi-linear regression analysis. As a result, the nonlinear mathematical models were obtained, allowing the analytical determination of the hydraulic losses and average velocity of the air flow. Additionally, the dependencies for determining the relative error of measuring the average velocity were obtained.
... The hydrodynamic loading according to Morison's equation is expressed in terms of the undisturbed fluid-particle velocity and accelerations directly which allows Morison's equation to be applied in the case of nonlinear wave and current kinematics models Santo et al. (2018). ...
The influence of fully nonlinear wave effects on floating wind turbine has been studied in this paper by comparing both floater motions and structural responses of tower base and mooring lines exposed to linear and fully nonlinear long-crested irregular waves. Wave kinematics of the linear and nonlinear wave are calculated in a 2D Harmonic Polynomial Cell wave tank. The wave kinematics are further processed by a polynomial fitting method to scale down the data size so that it fulfills the memory requirement of HAWC2 where coupled dynamic analysis is carried out. The external DLL used to provide wave kinematics to HAWC2 is extended from one dimensional (Wkin.dll 2.4) to two dimensional (Wkin.dll 2D) wave field so that fully nonlinear wave can be implemented on floating wind turbine through reading pre-generated wave kinematics manner. The whole work procedure including wave generation, polynomial fitting and implementation in HAWC2 has been verified with a linear regular wave case. Two extreme irregular wave conditions are focused to study the nonlinear wave effects regarding critical responses, such as wave elevation, floater motions and mooring line tension. The results have not only proved the accuracy and applicability of the polynomial fitting method and the extended Wkin.dll 2D for HAWC2 but also revealed the importance to consider fully nonlinear wave model in hydrodynamic analysis compared with linear wave theory especially for high sea states in shallow and intermediate water. As a result of development of computer capacity and numerical wave tank, this paper has demonstrated that fully nonlinear wave effect can be considered in an engineering manner with acceptable efficiency.
... This CFD-based approach has been implemented in a numerical wave tank based on the open-source software OpenFOAM (www.openfoam.org) and waves2foam (Jacobsen et al. [8]), see Figure 1 and Santo et al. [9] - [14]. ...
This paper summarises extensive research work on the accurate calculation of extreme loads from waves and current on space-frame offshore structures. Although relevant to new builds, improved prediction of extreme loads is also key to the re-assessment of old and ageing offshore platforms.
Current blockage is a field effect. Due to the presence of the rest of the structure, the flow velocity on each structural member is reduced on average leading to smaller overall loads. The first model to account for this ‘current blockage’ was first proposed by Taylor [1], and incorporated into standard industry practice (API, DNV and ISO). This is a simple improvement to the original Morison equation (Morison et al. [2]), which predicts forces using the undisturbed open ocean flow properties.
New work shows that unsteady large waves on top of a steady current introduces additional blockage, interpreted as wave-current blockage. Large-scale laboratory experiments have been used to validate numerical force calculations. This paper describes a numerical Computational Fluid Dynamics (CFD) model of a porous block with embedded Morison drag and inertia stresses distributed over the enclosed volume of the space-frame as a global representation. At a local member scale, the standard Morison equation is used, but on the local flow. This local flow speed is reduced because of overall interaction between the structural members interpreted as resulting from a distributed array of obstacle. Since the Morison equation is semi-empirical, drag and inertia coefficients are still required, consistent with present industry practice. This new method should be useful for assessing the overall structural load resistance and integrity in extreme wave and current conditions when survivability is in question.
Results are presented from recent large-scale experiments on a scaled (1:80) jacket model in the Kelvin Hydrodynamics Laboratory in Glasgow. These tests cover force measurements on both a jacket (stiff, statically-responding) and the same model restrained on springs to mimic structural dynamics (the first mode of a deep-water jacket, the second mode of a compliant tower or the first mode of a jack-up). For a jacket structure under all range of wave and current conditions, only a single pair of values of Morison drag and inertia coefficients is required to reproduce the complete total force-time histories on the jacket model. This is in contrast to the present industry practice whereby different Morison drag coefficients are required in order to fit the measured peak forces over the wide range of cases considered. For the dynamic tests, we find that the relative velocity formulation of the Morison equation for space-frame structures is valid for dynamically sensitive structures. All of these effects can be captured using our numerical porous block model.
... The hydrodynamic loading according to Morison's equation is expressed in terms of the undisturbed fluid-particle velocity and accelerations directly which allows Morison's equation to be applied in the case of nonlinear wave and current kinematics models (Santo et al., 2018). ...
From the view of cost efficiency, floating wind turbine becomes more competitive than bottom fixed wind turbine when the water depth comes above 50 m. Semi-submersible floater concepts have been proposed to be deployed in shallow waters (50-100 m) because of its smaller draft compared to other floater types.
However, mooring system design is extremely challenging for floating structures in shallow water. The relationship between mooring line tension and offset becomes nonlinear after an initial linear part for small offsets, which could lead to extreme large tension during harsh environment when the floater motions have large offsets. Moreover, the nonlinear response of the platform in shallow water becomes more critical than deep water.
The present thesis focuses on the mooring system design and analysis for the 5-MW-CSC semi-submersible floater in 100 m and 50 m water depth using the mooring system design concept in 200 m water depth as a reference. Natural periods of the platform, horizontal and vertical mooring line stiffness, ultimate strength and fatigue life of mooring lines and tower are basic factors that have been considered and examined. Initially static design has been carried out in SIMA for determining the optimal mooring system design and afterwards fully coupled time-domain dynamic analysis was performed with the tool Simo-Riflex-AeroDyn to study and check the mooring system performance.
This paper documents large laboratory-scale measurements of hydrodynamic force time histories on a realistic 1:80 scale space-frame jacket structure, which is allowed to respond dynamically when exposed to combined waves and in-line current. This is a follow-on paper to Santo, Taylor, Day, Nixon and Choo (2018a) which used the same jacket structure but very stiffly supported. The aim is to investigate the validity of the Morison equation with a relative velocity formulation when applied to a complete space-frame structure, and to examine the fluid flow (and the associated hydrodynamic force) reduction relative to ambient flow due to the presence of the jacket structure as an obstacle array as well as the dynamic structural motion, interpreted as wave–current-structure blockage. Springs with different stiffness are used to allow the jacket to respond freely in the incident wavefield, with the emphasis on high frequency modes of structural vibration relative to the dominant wave frequency. Transient focussed wave groups, and embedded wave groups in a smaller regular wave background are generated in a towing tank. The jacket is towed under different speeds opposite to the wave direction to simulate wave loading with different in-line uniform currents. The measurements are compared with numerical predictions using Computational Fluid Dynamics (CFD), with the actual jacket represented in a three-dimensional numerical wave tank as a porous tower and modelled as a uniformly distributed Morison stress field derived from the relative velocity form. A time-domain ordinary differential equation solver is coupled internally with the CFD solver to account for feedback from the structural motion into the Morison distributed stress field. An approximate expanded form of the Morison relative-velocity is also tested and is recommended for practical industrial applications. Reasonably good agreement is achieved in terms of incident surface elevation, dynamic model displacement as well as total hydrodynamic force time histories, all using a single set of Morison drag (Cd) and inertia (Cm) coefficients, although the numerical results tend to slightly overpredict the total forces. The good agreement between measurements and numerical predictions and the generality of the results shows that the Morison relative-velocity formulation is appropriate for a wide range of space-frame structures. In these tests, this gives rise to additional damping of the dynamic system which is equivalent to 8% of critical damping. This is significantly larger than both the structural and hydrodynamic damping combined (which is about 1%) as quantified through free vibration (push test) in otherwise stationary water.
