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Spar platforms with cylindrical shape and constant cross-section area may experience resonant heave motions in sea states with long peak periods, which are probably excessive for riser integrity due to its low damping and relatively low natural heave period. Changes to hull shape and cross-section that produce more benign heave behavior were discus...
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Citations
... A non-linear mechanism defines Heave-resonance associated with the critical state. Tao et al. [80] revealed that the hull form geometry plays a key role in the heave-resonant reduction. A hull shape's geometry increases damping and modulates the natural heave period. ...
Offshore oil and gas drilling operations are going to remote and harsh arctic environments with demands for heightened safety and resilience of operational facilities. The remote and harsh environment is characterized by extreme waves, wind, storms, currents, ice, and fog that hinder drilling operations and cause structural failures of critical offshore infrastructures. The risk, safety, reliability, and integrity challenges in harsh environment operations are critically high, and a comprehensive understanding of these factors will aid operations and protect investment. The dynamics, environmental constraints, and the associated risk of the critical offshore infrastructures (COI) for safe design, installation, and operations are reviewed to identify the current state of knowledge.
This paper introduces a systematic review of harsh environment characterization by exploring the metocean phenomena prevalent in harsh environments and their effects on the floating offshore structures performance and supporting systems. The dynamics of the floating systems are described by their six degrees of freedom and their associated risk scenarios. The systematic methodology further explores the qualitative, quantitative, and consequences modelling techniques for risk analysis of floating offshore systems in a harsh environment. While presenting the current state of knowledge, the study also emphasizes a way forward for sustainable offshore operations. The study shows that the current state of knowledge is inexhaustive and will require further research to develop a design that minimizes interruption during remote harsh offshore operations. Resilient innovation, IoT and digitalization provide opportunities to fill some of the challenges of remote Arctic offshore operations.
... The experimental results are based on forced vibration or free decay tests (Thiagarajan and Troesch 1998;Tao and Dray 2008;Yang et al. 2014;Lopez-Pavon and Souto-Iglesias 2015;Tian et al. 2017;Abazari et al. 2020a, b;Bezunartea-Barrio et al. 2020). The numerical studies mostly utilize tools of computational fluid dynamics (CFD), e.g., finite-difference or boundary element codes to accurately simulate the real physics of the problem for deriving the proper damping and added mass (Tao et al. 2004;Tao et al. 2007;Zhang and Ishihara 2019;Wang et al. 2020;Yue et al. 2020;Abazari et al. 2021). Furthermore, the effect of the heave plate on platform response has been investigated both numerically and experimentally (Mello et al. 2020). ...
A submerged oscillating heave plate is utilized as a mechanism in some marine structures for vibration reduction. Quantifying the hydrodynamic properties in axial (heave) oscillations has been extensively archived in published research. However, the rotational hydrodynamic properties of an isolated plate under forced oscillation have not been investigated, which may be of relevance when such structures undergo roll or pitch oscillations. This paper reports on an experimental study that aims to find the trends of the rotational hydrodynamic coefficients versus vibration amplitude for an isolated disk. Furthermore, formulae for nondimensional rotational coefficients based on empirical fits to experimental data will help to generalize these coefficients to full scale. The experimental tests were conducted in two sets. In the first set, disks of different sizes were excited in rotation around their central axis, whereas in the second set the disks were oscillated at a distance from the rotational axis. Generally and in accordance with results for heave oscillations, the results of the hydrodynamic coefficients show an increasing linear trend versus vibration amplitude.
... This is for dominating the damping forces in the measured signal while the inertia forces are offset by restoring force [3]. It is concluded that in the other excitation frequencies which are away from the natural frequency the effect of damping plate on RAO is lower than for the natural frequency [32] while the damping due to the vortex shedding is still applied to the structures. For experimental tests on isolated FRD models without a specified structure, the excitation period has been approximated regarding the scaling of the dominant period of the sea waves (8-10) [12,24]. ...
Heave plates are structural components used for reducing the vibrations caused by environmental forces on marine and offshore structures by changing the hydrodynamic properties. The fact that the added mass increase via heave plates does not always lead to the structural response reduction underscores the role of damping in maintaining the vibration amplitude within allowable limits. In the present experimental study, a novel combined rigid-elastic design is used to improve the damping through the velocity increase in the elastic part and added mass creation in the central rigid part. The desired percentage of total added mass and damping can be adjusted by changing the rigid-to-elastic parts diameter ratio, which is the main scope of this experimental research. Frequency of vibration, which affects the elastic edge excited mode shapes, also affects the forming of the vortex shedding. Experimental tests show that the frequency increase generally causes high damping performance provided that the excited mode shapes are axisymmetric, which strongly depends on equivalent stiffness and mass.
... A number of numerical studies on the hydrodynamic coefficients has been conducted for simple heave plates or cylinders, and compared with the water tank tests by Tao et al. [5], [6], [7]. Lopez-Pavon and Souto-Iglesias [8] predicted the hydrodynamic coefficients for a heave plate by RANS model, and provided reasonable results for plain plate, but there were some errors for reinforced plate. 2 Benitz et al. [9] predicted the hydrodynamic coefficients for the simplified OC4-DeepWind semisubmersible, and highlighted the role of free surface in the fluid flow near the structure. ...
Hydrodynamic coefficients for predicting hydrodynamic loading is significant for the design of offshore wind turbine floaters. In this paper, hydrodynamic coefficients for a semi-submersible floating offshore wind turbine are investigated experimentally and numerically. In the water tank test, added mass and drag coefficients of the semi-submersible model are identified by the forced oscillation tests. In numerical simulation, large eddy simulation (LES) with volume of fluid method (VOF) is adopted to predict added mass and drag coefficients. Firstly, numerical errors in the predicted hydrodynamic coefficients are systematically studied, and Richardson extrapolation is employed to obtain the grid independent solution. The predicted added mass and drag coefficients varying with KC number are then validated by the water tank tests and the mechanism of the hydrodynamic force are clarified by vortex shedding patterns. The hydrodynamic coefficients for each element are also investigated. Finally, effect of free surface on the hydrodynamic forces is discussed and the predicted added mass and drag coefficients are compared with those obtained from the water tank tests.
... varies in a similar manner; however, contrary to the rest exciting forces, it obtains a local zero value at ω≈0.3 rad/sec. This local minimum is attributed to "counteracting effects" (Tao et al., 2004) resulting from the existence of the taper with the linearly increasing diameter (Fig. 1b). With regard to the generalized exciting forces corresponding to the flexible modes (Fig. 3b), it is worth to note that Fi, i=7~10, have values comparable to the ones of both F1 and F3 (Fig. 3a). ...
In this paper, linear frequency-domain hydroelastic analysis is performed for investigating the behavior of a spar-type supporting platform of a floating offshore wind turbine considering the platform's flexibility. A "dry" mode superposition approach is utilized, where the flexible mode shapes are determined through the application of a FEM based structural model. The diffraction/radiation problem is solved using the boundary integral equation method. Focus is given on the flexible modes' generalized hydrodynamic forcing and responses, the spar's hydroelastic response and the coupling effects between the flexible and the rigid-body modes. For irregular waves, the effect of the peak period on the response spectra and the hydroelastic response is analyzed.
... Damping is also a problem for Spar platform that concerns engineers, in 2004, the viscous damping coefficients of spar platforms with changing geometries was calculated by Tao et. al [3] the research team found that the heave-resonant response can be significantly decreased by employing alternative hull shapes to obtain higher damping and change the natural heave frequency. Hong et al. [4] studied the Mathieuin stability of the spar platform as well as the pitch that is associated with heave motion, they have proved that a coupling effect exists between heave and pitch for the spar platform. ...
This paper provides comprehensive review on heave motion of rigid floating structure due to wave impacts. To specify and explain the structure response, this review firstly provides a brief introduction on ocean sea wave theory, floating structure motion interpretation. Then the floating body motion in regular waves was demonstrated using a superposition method of the oscillated motion in still water and the restrained motion in waves. Meanwhile, added mass and damping coefficient, these two frequency-dependent terms are brought into discussion to generate the motion response with given wave amplitude, which is known for response amplitude operator( RAO). Based on the study in regular waves, RAO of floating structure in irregular waves is introduced while no longer in time domain but in frequency domain. The whole review covers the literatures from the early 1980s up to nowadays, based on the review, it is recommended that more experimental work regarding to frequency characteristic and relative response of larger floating body should be carried out to improve the accuracy of this method.
... The change rule of the heave amplitude is in good agreement with the previous theoretical and experimental studies of Tao. 38 When the loads are very great, the growth of the heave amplitude of the substructure is increased significantly. For example, the instantaneously maximum amplitude reaches 11.2 m in load condition 11, which is increased by 166% compared with load condition 10. ...
Floating wind turbines (FWTs) have become the preferred structures to exploit offshore winds since a large part of the offshore wind resource is located in deep water zones. In the harsh marine environment, a spar-type substructure is the most promising substructure for FWTs with good structure stability and economic advantages. In this study, the surge, heave, and pitch motions of the spar-type substructure under combined action of wind, wave, and current are analyzed by using the AQWA module of ANSYS. Besides, the effects of different parameters, namely, the mooring line distribution form, fairlead position, and line number of mooring systems, on the spar-type substructure motion response are also investigated. Based on the calculation results, a new spar-type substructure with an optimized mooring system is put forward and analyzed. It can be found that both the dynamic response and the mooring tension of the new spar-type substructure are significantly improved relative to the non-optimized structure. The results of this paper could offer reference for further development of spar-type FWTs in real marine environments.
... In order to effectively increase the hydrodynamic damping in heave direction and reduce heave response 13) , 14) , appendage such as a disk (heave plate) are usually added to the keel of a vertical cylinder such as the disk chosen in WindFloat and heave plate employed in Fukushima MIRAI15 ) . Ishihara et al. 15) proposed Morison like equation to evaluate hydrodynamic force on heave plate in axial direction. ...
One reliable simulation tool was developed to be able to predict dynamic response of Floating Offshore Wind Turbines (FOWTs) to various sea states. To represent real environmental condition in field site, multidirectional wave simulation was carried out in this research to investigate effect of wave spreading on dynamic response of FOWT. On the other hand, effect of tower frequency change due to flexible foundation on fatigue load was studied. It was found that unidirectional assumption is conservative in terms of prediction of dynamic motion of platform and multidirectional sea state should be employed to reach cost-effect design. In addition, when wind turbine is supported by flexible foundation, the change of tower natural frequency will result in significant increase of fatigue load. Therefore, Effect of flexible foundation on tower frequency change needs to be taken into consideration in the design of platform and commercial wind turbine. Keyword: Floating Offshore
... A is cross-sectional area; u is the maximum water particle velocity, T is incident wave period, D is diameter of cylinder and is the kinematic viscosity of water. In order to effectively increase the hydrodynamic damping in heave direction and reduce heave response [13,14,15], appendage such as a disk (heave plate) are commonly added to the keel of a vertical cylinder such as the disk used in WindFloat and heave plate employed in Fukushima MIRAI [7]. Ishihara et al. [7] proposed a Morison like equation to evaluate hydrodynamic force on heave plate in axial direction. ...
A fully coupled nonlinear simulation tool (CAsT) using Morison based theory was developed to predict dynamic response of Floating Offshore Wind Turbine (FOWT). Performance of the simulation tool in prediction of dynamic response to sea states was validated through one water tank experiment in this paper. Besides, hydrodynamic coefficients were evaluated from numerical simulation and were validated with another water tank experiment. Significance of radiation damping, axial Froude-Krylov force on slender members and dynamic behavior of mooring system were investigated and clarified. In addition, systematic comparison of results from FAST and CAsT was carried out and discussed in this research.
... The alternative hull shapes with higher natural period and counteracting effects in the heave excitation force, illustrated by arrows in Figure 1b-1e, reduce the heave resonant response. Tao et al. (2003) calculated the viscous damping for the alternate hull shapes proposed by Haslum and Faltinsen (1999) by iterative post-processing procedure (solving the linear wave frequency response and the Navier-Stokes equation for unsteady incompressible viscous flow problem alternatively). Spar shown in Figure 1b has higher heave damping. ...
Spar platforms are used for drilling, production, storage and offloading of oil in deep water. Spar with its deep draft and large inertia experience low heave and pitch motions in operating conditions. However, the heave motions can be large when encountered by long-period swells or near resonant period. The heave motion of the Spar platforms can be reduced by decreasing the water plane area, increasing the draft, the added mass and/or damping. Various alternatives to reduce the heave motion in long-period swells have been in the fore front of research for the last two decades. An alternate hull form of a shape similar to a buoy with deep draft has been proposed in this study. The buoy form Spar is a cylindrical floating vessel with curved surface near the water plane. The present study focus on the efficiency of the buoy form Spar in reducing the heave motion and increasing the heave natural period. A classic Spar of 31 m diameter and deep draft buoy form Spars with 25 and 20 m diameter at the water plane area has been considered. The moon pool diameter of 12.5 m and the displacement of 63,205 tonnes are maintained for all Spars. The experimental investigations are conducted using 1:100 scale models in the wave flume. The natural period and the damping ratio for the heave and pitch motions were obtained by conducting free decay tests. Numerical simulations have been carried out using panel method. Based on the study, it is concluded that the reduction in water plane area is effective in reducing the hydrodynamic response of the buoy form Spar and increase in the heave natural period is noted.
