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The flow past a cylinder in proximity to a plane wall is investigated numerically for small gap ratios. Three vortex dynamic processes associated with different hairpin vortex generation mechanisms are identified for the first time, and the wake-induced turbulent transition is analysed. The vortex shedding is suppressed at $G/D = 0.1$ , while the spanwise vortex is generated via a Kelvin–Helmholtz instability and evolves into hairpin vortices. For $G/D= 0.3$ , the upper and lower rollers alternatively shedding from the cylinder, interact with the secondary vortex. The split secondary vortex merges with the upper roller and results in a new vortex downstream, which develops into hairpin vortices. When $G/D = 0.9$ , the secondary vortex interacts with the lower roller and then evolves into hairpin vortices. A tertiary vortex induced by the secondary vortex is observed, rotating in the opposite direction to the secondary vortex the wake-induced transitions share the same route. The velocity fluctuations deviate from the optimal growth theory in the pre-transitional region. In the transitional region low-frequency disturbances penetrate the sheltering edge to generate streaks where the disturbance energy declines. In the turbulent region the logarithmic layer is formed, indicating that the turbulent equilibrium is established.

Committee Mandate Concern for advances in structural model testing and full-scale experimentation and in-service monitoring and their role in the design, construction, inspection and maintenance of ship and offshore structures. This shall include new developments in: best practice and uncertainty analysis; experimental techniques; full field imaging and sensor systems; big data applications for ship and offshore structures; and correlation between model, full-scale and numerical datasets. Introduction Design methods and rule requirements have to be benchmarked with experimental or service history data. The Goal Based Standards (GBS) approach by IMO, see e.g., IMO Res. 296(87), 454(100) or MSC.1/Circ.1394, require benchmarking of standards by measuring the performance of methodologies, assessments, criteria and requirements by using indicators that can be compared with an accepted standard or with experimental and/or service history data, performance levels or outcomes known to be reliable. This approach calls for a systemic use of experimental results and monitoring of data from ships during construction, in-service and possibly from the end of the life cycle. Thus, the elaboration of experimental outcomes and the handover of information into engineering and management practice need to be further addressed and standardized to fully exploit their potential. Further, the continues growth of complexity in ship and offshore structures potentially designed outside the range of empirical references calls for reliable means of validation. While computer simulations are continuously improving at the same time, their improvement to a large extent depends on a physical understanding of the underlying phenomena to be captured. Therefore, experimental investigations are essential in driving further developments and innovations through their accurate feedback when performed correctly and meaningful. The latter is however challenging, especially with increased complexity of the phenomena to be investigated. Therefore, this report summarizes the expertise of the contributing authors with respect to their corresponding research fields. The report seeks to give a general overview of the specific experiments dealt with therein, presents the latest publications, possible obstacles to be addressed and closes with future recommendations for further developments and integration of the experiments into engineering practice and scientific developments. The report at first covers a section on scaling laws relevant to any experiment followed by specific testing concerning: digital image correlation (DIC), hydrodynamics of flexible structures, wave-in-deck, hybrid models, friction, vibrations, fatigue at low temperatures, corrosion, large scale impact, large scale wind turbine blades, full-scale ice loads, health monitoring and digital twin models. Further, a benchmark study on free vibrations of a cantilever beam is carried out to demonstrate the vast possibilities to carry out such seemingly simple experiment followed by a discussion of the results achieved. The report is concluded with a summary and an outline of further research and development recommendations.

This paper proposes a stress analysis method for three-dimensional structures, with the consideration of the global hydroelasticity which is calculated through the beam connected discrete modules method. First, the continuous structure is discretized into rigid modules connected by elastically equivalent beams. The hydroelastic responses are solved by the coupling of the hydrodynamics of rigid modules and the structural stiffness of elastic beams. Then, according to six degrees of freedom motions of each module extracted from the hydroelasticity, the pressure distributions of rigid modules can be obtained based on the three-dimensional potential theory. Finally, the inertia force and the pressure distributions are statically loaded on three-dimensional finite element model. The process is applied to a barge under the regular wave. In rigid condition, the local stress using the proposed method is verified against the result from the Quasi-static method. Furthermore, the springing effect on the local stress is investigated. In the test model, the result shows that the springing effect induces an obvious difference.

The vortex-induced vibration (VIV) response performances of a two-degree-of-freedom circular cylinder with different surface roughnesses were numerically investigated in this work. Four typical roughnesses values of 1 × 10−4, 5 × 10−3, 1 × 10−2, and 2 × 10−2 were examined for the range of the reduced velocity ( Vr = 1–20). The VIV response characteristics including the vibration amplitude, vibration frequency, VIV trajectory, vortex shedding flowing pattern, and hydrodynamic force for different rough cylinders were systematically compared. The numerical results showed that the VIV response of roughness cylinder experiences five evolutions as Vr increases, including five typical X–Y trajectories: centrosymmetry, “M” or “W,” “dough twists” or “candy,” “∞,” and asymmetrical ∞, exhibiting three vortex shedding flow pattern: S mode, C + S mode, and SS mode (a pair of symmetric vortices along the x-axis) under certain conditions. The VIV response was sensitive to the roughness in regime IV and regime V. Furthermore, the normalized vibration frequency of the rough cylinder was an integer multiple of the oscillatory flow frequency, except for the Vr ranged from 6 to 7. The vibration frequency in in-line (IL) direction was consistent with the frequency of the oscillatory flow, which was immune to the change of roughness. Additionally, when roughness was equal to 2 × 10−2, the reduction of maximum vibration amplitude in the cross-flow direction reached 31.2%, comparing with smooth cylinder, whereas the maximum vibration amplitude in the IL direction increased only by 6.2%.

The vortex induced vibration (VIV) of marine risers has been investigated by many researchers in experimental studies of a straight flexible riser model as well as a rigid cylinder to reveal the dynamic response characteristic and the mechanics behind it. However, due to the limitation of experimental apparatus, very few studies are about the VIV of a steel catenary riser (SCR) which is with a complex geometry. To investigate the VIV features and to further develop the corresponding numerical predictions of a SCR under steady current, a large-scale model test of a SCR was towed in an ocean basin at various speeds. Fiber Bragg grating strain sensors are instrumented on the riser model to measure both in-plane and out-of-plane responses. The characteristics of oscillating amplitude and dominating frequency response, the phenomenon of mode competition and travelling wave and the fatigue damage of the steel catenary riser in inline and cross-flow direction under steady current are analyzed.

Vortex-induced Vibrations ( VIVs ) of a flexible cylinder subjected to multi-directional flows have been studied based on a wake oscillator model. The multi-directional flow comprises two slab of flows in different directions, with each slabs having uniform uni-directional profile. The dynamics of the flexible cylinder is described based on the linear Euler-Bernoulli beam theory and a wake oscillator model is uniformly distributed along the cylinder to model the hydrodynamic force acting on it. The dynamics of the coupled system has been solved numerically using the finite element method and simulations have been conducted with the cylinder subjected to multi-directional flows with different angles between the two slabs. A large number of different initial conditions have been applied and more than one steady state responses have been captured for each flow condition. The steady state responses exhibit two different patterns, one is characterized by two waves travelling in the opposite directions, while the other is dominated by a single travelling wave. It has been found that the cross-flow VIV primarily occurs in the local cross-flow direction and a transition of its vibrating direction happens at the interface of the two flows. Such transition is not observed in the inline VIV and significant vibrations at the double frequency appear in both local cross-flow and inline directions. Energy analysis shows that this transition is boosted by a specific energy transfer pattern between the structure and the flow, which excites the vibration of the cylinder in some directions while damps it in others. It has also been found that this energy transfer is related to certain motion trajectories.

The semisubmersible platform is one of the most important equipment for the exploitation of deep-sea oil and gas. And its survival and operational capacities depend on the towing and motion performances. The present work investigates the properties of different platform component designs based on their towing and motion performances under wind and waves by using an integrated computational fluid dynamics (CFD) method. The joint application of RANS and a VOF/6-DOF solver compiled in a new Fluent UDF Library is adopted for the main calculation. Numerical simulations and results demonstrate that the VOF/6-DOF solver is accurate and stable, which could achieve the same functions as the previous OpenFOAM scheme. In order to acquire a better towing performance, different shapes are suggested for the pontoon end shape, the column cross section, and the brace longitudinal section, respectively. As to the motion response, among all the three design selections, the column cross section has the greatest influence on the whole platform. The simulation results reveal that circle section increases the roll and pitch response and reduces the heave response, to which the square section is adverse.

The objective of this comparison study is to verify and clarify whether the energy competitive force model has significant benefits in eliminating the influence of certain factors on the hydrodynamic coefficient. The typical cases with the smaller Keulegan–Carpenter (KC) number (KC = 31) and larger KC number (KC = 178) under the maximum reduced velocity of 6.5 with the prominent VIV responses are selected for analysis. The characteristics of the ortex-induced vibration (VIV) response, the hydrodynamic force, and the corresponding spatiotemporal coefficients in typical cases are revealed, compared, and discussed. The results suggest that the energy competition force model cannot avoid spatial-temporal distribution characteristics of the hydrodynamic coefficients. Drag and excitation coefficients with energy roles under Sarpkaya's and energy competitive force models are affected by the VIV response and wake effects in an oscillatory flow. The larger coefficients are witnessed under the smaller KC number. When the KC number increases from 31 to 178, the mean drag coefficients decrease from 2.25 to 1.73, and the mean excitation coefficients in in-line (IL) and cross-flow (CF) directions decrease from 1.42 and 1.44 to 0.60 and 0.81, respectively, under the energy competition force model. Changes in wake effects under a larger KC number lead to distinctive differences in drag and excitation coefficients during the acceleration and deceleration phases. Through a quantitative comparative analysis, the current identified drag and excitation coefficients under the energy competition force model can obtain a more accurate prediction result. The present work shows that there exists the blindness in the selection of coefficients within the framework of the energy competition force model.

A frequency capture phenomenon in which the dominant response frequency of the downstream cylinder is the same as that of the upstream cylinder despite the differences in their physical characteristics was recently experimentally identified. The mechanism of this phenomenon is investigated by flow around two cylinders with unequal diameters undergoing flow-induced vibrations (FIV) using the open-source code OpenFOAM. Two FIV systems, a large stationary/vibrating upstream cylinder and vibrating downstream cylinder, are used for the simulation. The cylinders are free to vibrate in both the in-line and cross-flow directions. The forgetting factor least squares algorithm is applied for the time-varying excitation mechanism analysis. Simulation results show that the response of the downstream cylinder has a larger amplitude and contains multi-frequency components than the vortex-induced vibration of an isolated cylinder, making its trajectory more complicated. Simulations confirm that the frequency capture phenomenon is induced by the action of the upstream shedding vortex on the downstream cylinder. The energy transfers from vorticity to the structure when the frequency capture phenomenon occurs. The response of the downstream cylinder comprises a significant component of the vortex shedding frequency of the upstream cylinder.

The hydrodynamic forces on the stationary partially submerged cylinder are investigated through towing test with Reynolds number ranging from 5 × 104 to 9 × 105. Three test groups of partially submerged cylinders with submerged depths of 0.25 D, 0.50 D, and 0.75 D and one validation group of fully submerged cylinders are conducted. During the experiments, the hydrodynamic forces on the cylinders are measured using force sensors. The test results show a considerable difference in the hydrodynamic coefficients for the partially submerged cylinders versus the fully submerged cylinders. A significant mean downward lift force is first observed for the partially submerged cylinders in a steady flow. The maximum of the mean lift coefficients can reach 1.5. Two distinct features are observed due to the effects of overtopping: random distributions in the mean drag coefficients and a clear quadratic relationship between the mean lift coefficients and the Froude number appear in the non-overtopping region. However, the novel phenomenon of a good linear relationship with the Froude number for the mean hydrodynamic coefficients is clearly shown in the overtopping region. In addition, fluctuating hydrodynamic coefficients are further proposed and investigated. These results are helpful to have a better understanding of the problem and to improve related structural designs.

The influences of helical strake geometry and vortex-induced vibration (VIV) on the drag coefficients of a long flexible pipe with helical strakes are investigated through uniform flow experiments with the Reynolds (Re) number ranging from 1.1 × 104 to 9.5 × 104. The strains caused by drag forces of a bare pipe and six different straked pipes are measured by fiber bragg grating (FBG) sensors. According to the inverse analysis method and Morison formula, the drag forces and coefficients at each section of the pipe models are identified. Then, the mean drag coefficients of the bare and straked pipes are compared. The results show that helical strakes can effectively stabilize the fluctuation of drag coefficients with Re numbers. An unexpected phenomenon is that helical strakes with pitches of 5 D can reduce the mean drag coefficients. Moreover, the mean drag coefficients of straked pipes are less sensitive to VIV response. Furthermore, an empirical mean drag coefficient prediction formula for straked pipes is proposed, which accounts for the VIV response and geometry of helical strakes. Its applicability is well verified via comparison between predicted and experimental results.

In this study, an efficient time-domain prediction model is developed to predict unsteady flow vortex-induced vibrations (VIV) of flexible risers. The hydrodynamic forces on flexible risers are calculated on the basis of forced oscillation experiments on rigid cylinders. A period identification criterion, based on the spatial and temporal variations of reduced velocity, is proposed to divide the entire vibration process into exciting and damping periods of each exited mode. In exciting periods, assuming that VIV enters an ideal lock-in stage, a non-iterative solving model is established under modal space for response calculations, which efficiently predicts time domain VIV responses. In damping periods, free-decay vibration theory based recurrence formulas are established under modal space, and they get solved stepwise for modal responses. After some slight response adjustments to smooth period transitions, the VIV response time history can be obtained efficiently. This model is validated by steady flow VIV prediction cases, and further applied to predict oscillatory flow VIV experimental results. The prediction cases reveal that this model is able to realize high-speed VIV predictions with satisfactory results and no convergence problems. This model, with high efficiency and stability, is highly suitable for unsteady flow VIV prediction in engineering applications.

A large-scale model test of a truncated steel catenary riser (SCR) was performed in an ocean basin to investigate the riser responses under pure steady uniform current and pure vessel motions separately. Out-of-plane vortex-induced vibration (VIV) was confirmed to have occurred under both test conditions. A comparative analysis and discussion were carried out on selected cases in terms of out-of-plane VIV responses, VIV developing mechanisms and the fatigue damage contribution. Results indicate that both steady current-induced and vessel motion-induced VIV responses are dominated by strong travelling waves, but vessel motion-induced VIV responses are more ‘intermittent’ with respect to the response amplitude and frequency owing to its space- and time-varying shedding frequency. ‘Power-in’ regions are further estimated to understand the VIV developing mechanisms for both test conditions. Finally, fatigue damages are evaluated showing the damage by vessel motion-induced VIV is comparable and at the same level as uniform current-induced VIV, which highlights the great importance of vessel motion-induced VIV, which cannot be neglected in the design and analysis for SCR systems.

The Norwegian Public Road Administration is planning to upgrade Coastal Highway E39 by replacing ferry connections with floating bridges or submerged floating tunnels (SFTs). This study considers a potential pontoon-supported curved SFT designed for crossing Sognefjorden at a submergence of 12 m. It consists of two identical tubes with a diameter of 12.6 m each in a tandem configuration and with a length of approximately 4 km. The natural frequencies of the low-order modes are well within the energy content in the spectra of the second-order difference-frequency wave excitation forces and the vortex shedding-induced forces. In this paper, numerical simulation of wave-induced hydroelastic response and flow-induced vibrations of the twin-tube SFT is performed. Long- and short-crested waves, the first and second order wave loads, are considered. A time-domain approach to simulate crossflow vortex-induced vibration (VIV) and VIV-amplified inline drag forces, partly based on the coefficients obtained experimentally, is established and applied. The focus is on extreme conditions – relating to ultimate strength limit states. The second-order wave load substantially affects the lateral motion and lateral bending moment, as expected. The short-crested waves influence the response in both the lateral and vertical directions by exciting asymmetric eigenmodes. In strong flow conditions, once VIV is excited, the standard deviation of the vertical motion (of about 30% of the diameter) and the bending moment about the horizontal axis is more that an order of magnitude larger than that induced by the wave loads. The simulation of the wave- and flow-induced load effects provides a good reference for the design of SFTs.

This work experimentally investigated the performance of hydrodynamic forces on a semi-submerged cylinder under an oscillatory flow. To generate the equivalent oscillatory flow, the semi-submerged cylinder is forced to oscillate in several combinations of different periods and amplitudes. The mean downward lift force was observed to be significant and the fluctuating lift forces show dominant frequency is twice that of oscillatory flow and amplitude that is the same as the mean lift force. Based on this main hydrodynamic feature, a novel empirical prediction formula for the lift forces on semi-submerged cylinder under oscillatory flow is proposed where the lift forces expression is proportional to the square of oscillatory flow velocity. This novel empirical formula directly assigns the fluctuating lift force with frequency twice of oscillatory flow and the amplitude that is the same as the mean lift force. This assignment of empirical lift force formula reduces parameters required to determine a dynamic lift force but is demonstrated to well predict the fluctuating lift force. The lift coefficient can reach 1.5, which is larger than the typical value 1.2 of the drag coefficient for a fully submerged cylinder with infinite depth. Moreover, relationships among hydrodynamic coefficients, Keulegan-Carpenter (KC) number, Stokes number and Froude number are studied. With the increase of KC number, the Froude number has a more significant influence on the distribution of hydrodynamic coefficients. As Froude number is increasing, the drag coefficient shows a nonlinear decay (KC &lt, 20) but a linear increase (KC &gt, 20), while the added mass coefficients show a nonlinear (KC &lt, 20) and a linear (KC &gt, 20) increase trend. The present work can provide useful references for design of the relevant marine structures and serve as the useful guideline for future research.

Under vessel motion, dynamic responses of compliant risers, like a steel catenary riser, are no longer small displacement and small deformation problems but characterised with large displacement and small deformation instead. It is therefore difficult to directly obtain the riser global motion based on experimental or field-measured local strain or acceleration data. This paper proposes a generalised global motion reconstruction method for large displacement but small deformation problems, assuming that the global motion can be divided into quasi-static motion and dynamic vibration. The proposed methodology is validated numerically and experimentally with satisfactory accuracy. Parameters like sensor location and mode number to be used in the motion reconstruction are optimised and recommended. Uncertainty analysis considering different noise levels is performed to evaluate the robustness of the proposed method. Most importantly, further comparative hydrodynamic analyses indicate the significance ...

This paper proposes the Forgetting Factor Least Squares (FF-LS) method for identification of time-varying hydrodynamics of a flexible riser under multi-frequency vortex-induced vibrations (VIV). Differing from the traditional least squares method used to identify hydrodynamics, with equal weights on all sampled data, this method introduces a forgetting factor to give higher weight to data closer to the present moment. FF-LS allows the possibility of handling the time-varying parameters. By following this procedure for all sampled data of a flexible riser undergoing multi-frequency VIV, the corresponding time-varying hydrodynamics in the Cross-Flow (CF) direction will be obtained considering multi-frequency coupling. The results show that, under multi-frequency coupling, vortex-induced force coefficients of flexible risers change periodically and differ from coefficients at the basic frequency usually used in VIV prediction. This difference is a result of the coupling effect between the basic frequency and high frequency. Time-varying coefficients considering multi-frequency coupling contain steady components and vibration components whose frequencies are the sum of coupling frequencies and the difference of coupling frequencies. To show the advantages of time-varying hydrodynamics, we compare forces reconstructed from different vortex-induced force coefficients, assuming vortex-induced force obtained from the inverse analysis is the real force. The results show that time-varying vortex-induced force coefficients considering a coupling effect between multiple frequencies can accurately reconstruct vortex-induced force, while the sum of vortex-induced force coefficients under multiple frequencies will produce the cross-term that overestimates the vortex-induced force.

In this paper, a time domain prediction method from experimental data is proposed for vortex-induced vibration (VIV) of flexible risers. The nonlinear factors, couplings among axial tension, VIV response in cross flow (CF) direction and the hydrodynamic force, have been taken into account in this method, with a simplified tension variation model and empirical hydrodynamic force model. The hydrodynamic force, including the excitation force in the excitation region and the damping force in the damping region are the function of excitation coefficients, non-dimensional VIV amplitude and frequency based on vibration experiment data. Iterations are performed to achieve balances between the hydrodynamic forces and the VIV responses of a riser. Moreover, a new added mass coefficient of 2.0 from model tests of flexible pipes is applied, where the predicted VIV response frequencies reveal higher accuracy. Comparison between the predicted results and the experimental results under uniform flow of 2.8 m/s and shear flow of 2.0 m/s are conducted, which verifies the feasibility and reliability of the proposed method. In addition, by comparing the prediction results with and without coupling between axial tension and VIV responses, it is found that this coupling effect is of importance to VIV prediction and can improve VIV prediction accuracy, especially under the case of high flow velocity and high vibration mode.

A modal space based direct method is developed for vortex-induced vibration (VIV) prediction of flexible risers. A VIV response solution model is established in modal space, consisting of a hydrodynamic force equation and a dynamic response equation. Adopting a non-iterative root-search method in modal space, the equation set gets solved directly without power-balance iterations. This direct method is compared against conventional empirical models, with its validity and applicability verified. This method entirely gets rid of convergence problems during predictions, which is conducive to practical engineering applications and puts forward fresh ideas for further development of empirical models for VIV prediction.