OC6 Phase Ia: CFD Simulations of the Free-Decay Motion of the DeepCwind Semisubmersible

Currently, the design of floating offshore wind systems is primarily based on mid-fidelity models with empirical drag forces. The tuning of the model coefficients requires data from either experiments or high-fidelity simulations. As part of the OC6 (Offshore Code Comparison Collaboration, Continued, with Correlation, and unCertainty (OC6) is a project under the International Energy Agency Wind Task 30 framework) project, the present investigation explores the latter option. A verification and validation study of computational fluid dynamics (CFD) models of the DeepCwind semisubmersible undergoing free-decay motion is performed. Several institutions provided CFD results for validation against the OC6 experimental campaign. The objective is to evaluate whether the CFD setups of the participants can provide valid estimates of the hydrodynamic damping coefficients needed by mid-fidelity models. The linear and quadratic damping coefficients and the equivalent damping ratio are chosen as metrics for validation. Large numerical uncertainties are estimated for the linear and quadratic damping coefficients; however, the equivalent damping ratios are more consistently predicted with lower uncertainty. Some difference is observed between the experimental and CFD surge-decay motion, which is caused by mechanical damping not considered in the simulations that likely originated from the mooring setup, including a Coulomb-friction-type force. Overall, the simulations and the experiment show reasonable agreement, thus demonstrating the feasibility of using CFD simulations to tune mid-fidelity models.

ON THE ROLL DAMPING OF AN FPSO WITH RISER BALCONY AND BILGE KEELS

Although the topic of roll damping of vessels at sea is already brought to the attention of naval architects by Froude more than 100 years ago, the physics of it remain intriguing, even today. An accurate prediction of the motions of offshore structures in harsh environments, designed for 25 years continuous operation, is the topic of this paper. Model test experiments for two FPSO’s developed by SBM for Petrobras are discussed. It is shown that the FPSO submerged riser balcony on one side of the vessel contributes to the roll damping through similar physics as the bilge keel does. Flow memory effects are discussed in detail since these are shown to have a noticeable effect on the roll damping coefficients. The paper further employs 3D CFD simulations to enhance the understanding of the fluid behaviour around the FPSO appendages, necessary to construct a rational and accurate roll damping model in the future.

Static and dynamic characteristics of a Rayleigh-steps mechanical seal with reverse steps

This paper presents a Rayleigh-steps mechanical seal with reverse steps (RS-MS), and the governing equation was solved by the finite difference method (FDM). The effects of angular misalignment, working condition parameters, and film thickness on sealing performance were discussed, including the opening force, cavitation ratio, leakage rate, frictional torque, stiffness and damping coefficients. The results indicate that the cavitation phenomenon in the reverse step groove can restrain the leakage, while it also affects the stability of the seal. The angular misalignment makes the seal have greater stiffness and damping coefficients. The stiffness and damping coefficients decrease rapidly with the increase of the film thickness, and the dynamic stability of the mechanical seal decreases with the increase of the film thickness, which is not conducive to the stable operation of the seal. The research results can guide the optimization design of mechanical seals.

Bifurcation analysis of rotor/bearing system using third-order journal bearing stiffness and damping coefficients

Hydrodynamic journal bearings are used in many applications which involve high speeds and loads. However, they are susceptible to oil whirl instability, which may cause bearing failure. In this work, a flexible Jeffcott rotor supported by two identical journal bearings is used to investigate the stability and bifurcations of rotor bearing system. Since a closed form for the finite bearing forces is not exist, nonlinear bearing stiffness and damping coefficients are used to represent the bearing forces. The bearing forces are approximated to the third order using Taylor expansion, and infinitesimal perturbation method is used to evaluate the nonlinear bearing coefficients. The mesh sensitivity on the bearing coefficients is investigated. Then, the equations of motion based on bearing coefficients are used to investigate the dynamics and stability of the rotor-bearing system. The effect of rotor stiffness ratio and applied load on the Hopf bifurcation stability and limit cycle continuation of the system are investigated. The results of this work show that evaluating the bearing forces using Taylor’s expansion up to the third-order bearing coefficients can be used to profoundly investigate the rich dynamics of rotor-bearing systems.

Abstract 13048: A Simple Model to Describe the Relationship Between Compression Force and Depth During CPR

Aim: The relationship between force and depth during manual chest compressions depends on the patient and on the dynamics with which the rescuer applies the force. Force-depth models with many fitting parameters have been proposed making physical interpretation complicated. The aim of this work was to design a simpler force-depth model, accommodating anticipated differences in compression and recoil phases. Materials and Methods: Force and acceleration signals were extracted from out-of-hospital-cardiac arrest (OHCA) defibrillator recordings (TVF&R, OR, USA), equipped with CPR technology. Compression depth and velocity signals were computed from acceleration. We analyzed intervals of 20-s within the 1st min of chest compressions. Our model decomposes the applied force as the sum of an elastic and a damped term, considering different damping coefficients for the compression and recoil phases. Coefficient of elasticity was calculated at the instant of maximum compression depth (null velocity) and damping coefficients at the instants of maximum compression and recoil velocities. The estimated depth signal is shown in the figure. The goodness of the model was assessed through the determination coefficient R 2 . Results: We analyzed 1,074 compressions from 30 OHCA recordings. Median (IQR) compression depth was 4.6 (4.0-5.4) cm; compression rate was 107 (102–113) cpm; coefficient of elasticity was 100.67 (78.95–125.01) N/cm; compression damping coefficient was 2.57 (1.84–3.29) N/(cm/s) and recoil damping coefficient was 3.59 (2.58–4.90) N/(cm/s). Median R 2 was 0.993 (0.984–0.996). Conclusions: This model, derived using fewer parameters, could help with the interpretation of the mechanical properties of the chest during CPR. It may also be useful for the assessment of inter-patient differences with age, sex, and body constitution, as well as of the evolution of elasticity and damping of patient’s chest during the course of resuscitation.

Determination of ship roll damping coefficients by a differential evolution algorithm

The execution of the so-called extinction tests represents the classical experimental method used to estimate the damping of an oscillatory system. For the specific case of ship roll motion, the roll decay tests are carried out at model-scale in a hydrodynamic basin. During these tests, the vessel is posed in an imbalance condition by an external moment and, after the release, the motion decays to the equilibrium condition. When the damping is far below the critical one, the transient decay is oscillatory. Here a new methodology is presented to determine the damping coefficients by fitting the roll decay curves directly, using a least-square fitting through a differential evolution algorithm of global optimisation. The results obtained with this methodology are compared with the predictions from standard methods. This kind of approach seems to be very promising when the motion model of the system under investigation is established with any level of non-linearities included. The usage of the fitting procedure on the approximate analytic solution of the differential equation of motion demonstrates the flexibility of the method. As a benchmark example, two experimentally measured roll extinction curves have been considered and suitably fitted. The newly predicted results, compared with the ones obtained from standard roll decay analysis, show that the algorithm is capable to perform a good regression on the experimental data.

Numerical Simulation and Modelling Convention of Unsteady Fluidelastic Forces of Tube Arrays

This paper presents a numerical investigation on the unsteady fluidelastic forces of tube arrays. The key focus is on the consistency between the unsteady fluidelastic force model and the quasi-steady model for tube arrays at large reduced flow velocities, as well as comparing two well-known conventions for the unsteady model. Two-dimensional unsteady Reynolds-averaged Navier-Stokes (URANS) simulations are used to prove that the viscous damping coefficients of Tanaka's convention (Tanaka and Takahara, 1981) approach their quasi-steady values as the reduced flow velocity approaches infinity, whereas the hysteretic damping coefficients of Chen's modified convention (Chen et al., 1983) always approach zero and hence result in low-resolution data plots as the reduced flow velocity becomes large. The non-constant viscous damping coefficients of Tanaka's experimental data at high reduced flow velocities (which motivated the introduction of Chen's modified convention) might be induced by a systematic identification error in the phase of the fluidelastic force. A row of three flexible cylinders is used as a numerical example to analyse the effect of systematic phase error on the predicted stability boundary of the fluidelastic instability. Although identical fluidelastic forces are simulated by using the two conventions, Tanaka's convention is recommended due to its compatibility with the quasi-steady theory and optimal resolutions of data plots over any range of reduced flow velocities.

Hydrodynamic coefficients of a floater near a partially reflecting seawall in the presence of an array of caisson blocks

In the present study, surface gravity wave scattering and radiation by a freely floating rectangular buoy placed near a partially reflecting seawall and in the presence of an array of caisson blocks are analyzed. Various hydrodynamic parameters related to the wave scattering and radiation, such as the added mass and radiation damping coefficients, correspond to sway, heave and roll motions of the floating buoy, horizontal force, vertical force and moment acting on the floating structure, and horizontal wave force acting on the partially reflecting seawall are studied for a variety of wave and structural parameters. The study reveals that the resonating pattern in various hydrodynamic coefficients occurs for moderate values of the wavenumber. Further, when the distance between the floater and the sidewall is an integral time of half wavelength, the resonating behavior in the sway, heave and roll added masses, and associated damping coefficients appears, and the aforementioned hydrodynamic coefficients change rapidly around this zone. These resonance phenomena can be diminished significantly with appropriate positioning of the floater with respect to the sidewall and in the presence of partially reflecting seawall.