Journal of Offshore Mechanics and Arctic Engineering

Published by American Society of Mechanical Engineers
Print ISSN: 0892-7219
This study investigated various aspects of a fatigue crack growth analysis, ranging from the stress intensity factor solutions to the simulation of a fatigue crack coalescence process of a tubular joint weld toe surface flaw. Fracture mechanics fatigue crack growth analyses for offshore structural tubular joints are not simple, because of the difficulty to calculate the stress intensity factors due to their geometric complexity. The fully mixed-mode stress intensity factors of nine weld toe surface cracks of an X-shaped tubular joint under tension loading were calculated by detailed three-dimensional finite element analyses. Using these stress intensity factor solutions, a fatigue crack growth study was performed for the X-joint until (the crack surface length grew to two times the tube thickness. Through this study, the crack shape change during the fatigue crack propagation was investigated in detail. Fatigue life calculations were also performed for a range of crack geometries using the stress intensity factor solutions of the nine flaws. These calculations indicate that the natural fatigue crack growing path for a crack is its quickest growing path. The study demonstrated that detailed fracture mechanics fatigue analyses of tubular joints can be practical using the finite element method.
Safety of shipping is an ever growing concern. In a summary on shipping safety Douglas Faulkner investigates the causes of shipping casualties [1] and concludes that the number of unexplained accidents are far too high in comparison to other means of transport. From various sources, including insurers data over 30% of the casualties are due to bad weather ( a fact that ships should be able to cope with) and a further 25% remain completely unexplained. The European project MAXWAVE aimed at investigating ship and platform accidents due to severe weather conditions using different radar and in Situ sensors and at suggesting improved design and new safety measures., Heavy sea states and severe weather conditions have caused the loss of more than 200 super cargo vessels within the last 20 years [1]. In many cases single 'rogue waves' of abnormal height as well as groups of extreme waves have been reported by crew members of such ships. The European Project MAXWAVE deals with both theoretical aspects of extreme waves as well as new techniques to observe these waves using different remote sensing techniques. The final goal is to improve the understanding of the physical processes responsible for the generation of extreme waves and to identify geophysical conditions in which such waves are most likely to occur. Two dimensional sea surface elevation fields are derived from marine radar and space borne synthetic aperture radar (SAR) data. Individual wave parameters like maximum to significant wave height ratios and wave steepness are derived from the sea surface topography. Several ship and offshore platform accidents are analyzed and the impact on ship and offshore design is discussed. Tank experiments are performed to test the impact of designed extreme waves on ships and offshore constructions. This article gives an overview of the different work packages performed.
The influence of atmospheric conditions (specifically precipitation rate and external heat flux) on the freezing rain ice accretion forming on a non-rotating, horizontal cylinder is studied, using an analytical model based on a simple form of the equations for conservation of mass and heat balance. In keeping with the freezing rain application, but in order to simplify this first step, we have assumed vertical incidence of precipitation (no wind) and no dripping from the accretion (hence light to moderate precipitation rates with relatively low air temperatures). The initial ice accretion shape and the location of its center of mass are examined as a function of the ratio of the precipitation mass flux to the total heat flux lost from the ice surface. An increase in the flux ratio leads to a quantifiable downward displacement of the accretion center of mass. We complement this analysis with numerical simulations, using an improved, two-dimensional version of the Szilder-Lozowski morphogenetic model that predicts the evolution of the accretion shape. For the first time, the freezing probability, which is the critical model parameter, is expressed as a function of location and atmospheric conditions for an accretion shape evolving with time. Using the morphogenetic model, we examine the influence of atmospheric conditions on the accretion shape and ice load. In particular, we address the question of what gives rise to extreme ice loads by identifying the range of atmospheric conditions that tends to maximize (or minimize) the ice load for a given amount of precipitation. The results of this research are applicable to predicting ice formation on overhead transmission lines.
Anisotropic and rate-sensitive characteristics of the ratio of lateral strain to axial strain in addition to the rate-sensitive effective modulus for columnar-grained freshwater ice and sea ice from the Arctic, have been investigated. Tests were carried out at-20°C, for conditions of no microcracking under uniaxial loads (normal to the length of the grains) in the stress rate range of 1 × 10-3 MN · m-2 s-1 to 1 × 102 MN · m-2 s-1 or an equivalent strain rate range 1 × 10-7 s-1 to 1 × 10-2 s-1. With increase in stress rate, the ratio increased from about 0.2 to 0.3 in the plane parallel to the columns, whereas it decreased from about 0.65 to 0.3 in the plane normal to the columns.
As the working temperatures in ice are very close to its melting point, it behaves visco-elastically and experiences what is commonly known as high temperature embrittlement. Its mechanical properties are rate and temperature sensitive and analysis must include load and displacement history. Borehole jack tests can be improved by the use of an electrohydraulic pump in conjunction with simultaneous recording of pressure and diametral displacement as a function of time. These small but significant modifications in the test procedures permit analysis of the response of a borehole jack test system under operational winter conditions in the High Arctic in first-year and multi-year sea ice. Les températures de service de la glace étant très proches de son point de fonte, la glace se comporte de facon visco-élastique et présente ce qu'on appelle communément une fragilité aux températures élevées. Ses propriétés mécaniques sont sensibles à la vitesse et à la température, et l'analyse doit prendre en compte les données concernant les charges et les déplacements. On peut améliorer les essais au vérin de forage en utilisant une pompe électro-hydraulique tout en enregistrant simultanément la pression et le déplacement diamétral en fonction du temps. Ces légères mais importantes modifications du mode opératoire permettent d'analyser la réponse d'un système d'essais au vérin de forage sur les glaces de mer de l'année ou pluriannuelles dans des conditions hivernales opérationnelles, dans l'Extrême Arctique. RES
An array of nine fabricated models comprised of three different shapes, with three model sizes for each shape, has been utilized in tow tank tests to investigate the hydrodynamic interaction between glacial ice masses and a transiting tanker. A generic model tanker was towed past free-floating ice mass models at various speeds and proximities. The ice masses were either spherical, pyramidal or cylindrical in shape. The influence of waves of various periods and wave heights was also investigated. Sway and surge of the ice masses in response to the tanker passage were measured as the primary indicators of the hydrodynamic interactions. Notable amongst the many observed behaviors was that waves tended to enhance the degree of sway. Also, in the scenarios tested the magnitude of surge speed and sway speed were <10% of the tanker speed, and would therefore not significantly reduce impact speed during collisions. The program results are intended for use primarily in validation of IOT?s numerical simulations of bergy bit/ship collisions, but can also serve as a validation database for simulation studies by other researchers. y
This paper presents results of a numerical and laboratory investigation into the mooring line forces and slow drift oscillations of large floating structures in multi-directional waves. A procedure for computing the spectral density of the second-order forces in random multi-directional waves based on the concept of a bidirectional, bifrequency quadratic transfer function is presented. Laboratory tests were carried out with a floating barge model, restrained horizontally by soft linear springs. The barge was subjected to random multi-directional waves with different degrees of directional spreading. The influence of wave directionality on the mooring line forces and low frequency motions is investigated by comparing results in undirectional and multi-directional sea states with an identical frequency spectrum.
This paper describes the generation and verification of four realistic sea states in a multidirectional wave basin, each representing a different storm wave condition in the Gulf of Mexico. In all cases, the degree of wave spreading and the mean direction of wave propagation are strongly dependent on frequency. Two of these sea states represent generic design wave conditions typical of hurricanes and winter storms and are defined by JONSWAP wave spectra and parametric spreading functions. Two additional sea states, representing the specific wave activity during hurricanes Betsy and Carmen, are defined by tabulated hindcast estimates of the directional wave energy spectrum. The Maximum Entropy Method (MEM) of directional wave analysis paired with a single wave probe/bi-directional current meter sensor is found to be the most satisfactory method to measure multidirectional seas in a wave basin over a wide range of wave conditions. The accuracy of the wave generation and analysis process is verified using residual directional spectra and numerically synthesized signals to supplement those measured in the basin. Reasons for discrepancy between the measured and target directional wave spectra are explored. By attempting to reproduce such challenging sea states, much has been learned about the limitations of simulating real ocean waves in a multidirectional wave basin, and about techniques which can be used to minimize the associated distortions to the directional spectrum. (A)
Thrusters working at low advance coefficients are employed in a wide range of offshore and marine applications on Floating, Production, Storage, and Offloading (FPSO) systems; shuttle tankers; tug boats; and mobile offshore units. Therefore, an understanding of the flow around the thrusters is of great practical interest. Despite this interest, there is lack of knowledge in the description of the hydrodynamic characteristics of a ducted thruster's wake at bollard pull and low advance coefficient values. This work was aimed at providing detailed data about the hydrodynamic characteristics of a Dynamic Positioning (DP) thruster near wake flow at different low advance coefficient values. Wake measurements were made during cavitation tunnel tests carried out on a ducted propeller model at the Italian Ship Model Basin (INSEAN), Rome, Italy. Through these experiments, the DP thruster near wake velocity components at different downstream axial planes, up to 1.5 diameters downstream, were obtained using a Stereoscopic Particle Image Velocimetry (SPIV) system. These experiments were carried out at different advance coefficient (J) values [bollard pull (J=0), J=0.4 and J=0.45]. y
Combatting oil spills in the Arctic is a major challenge. Drilling or producing oil or gas in the marginal ice zone (MIZ) may allow booms to be deployed upstream of an offshore structure to clear the water of ice, thereby enabling conventional oil spill countermeasures to be used. Such a boom would be kept in place by two ice-going service vessels or by moored buoys. SINTEF NHL and NRC have performed a number of small-scale tests with a flexible boom in the NRC ice basin in Ottawa. The purpose of the tests was to measure the effectiveness of using a flexible boom for collecting ice, and to determine the loads associated with collecting the ice. In the tests, various boom configurations were towed against a broken ice field consisting of ice pieces typically 50–100 mm across and 30 mm thick. The ice concentration was usually 10/10, but it was reduced to 8/10 and 5/10 for two tests. The boom was towed at speeds of 20 and 50 mm-s−1. Both the width of the boom and the slackness of the boom were varied over reasonable ranges. Two six-component dynamometers were used to support the boom. Thus, the force components on each end of the boom were measured. Further, two video cameras were used to record the effectiveness of each boom configuration. In this paper, the full results of this test program are presented and the application of the test results to the full-scale situation are discussed. The tests show that, under certain conditions, the use of boom is feasible for ice management in oil-contaminated water.
A series of indentation and penetration tests have been performed by edge loading of a vertical indentor into a floating sheet of columnar S2 freshwater ice. The load on the indentor was measured as a function of interaction speed (v equals 0. 1-60 cm-s** minus **1), indentor width (D equals 2. 54-12. 7 cm), ice thickness (h equals 0. 6-3. 3 cm), strain rate ( epsilon equals v/2D equals 10** minus **2-10**1 s** minus **1) and aspect ratio (D/h equals 0. 5-22). In total, 66 tests were performed. In this paper, a description of the test procedures is given along with the full results in both graphical and tabular form. Five different ice fracture modes are identified and described. From the test results, an ice-failure mode map is derived which indicates the conditions in which each ice fracture mode predominates.
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 discussed by some researchers in the past. In this study, the viscous damping of spar structures is explicitly calculated, and incorporated to the potential solution. It is concluded that the heave resonant response can be considerably reduced by alternative hull shapes via increased damping mechanism and the natural heave period being kept outside the range of the wave energy. Yes Yes
The dynamic collapse of submerged cylindrical shells subjected to lateral impulsive pressure loads caused by underwater explosions is studied via coupled experimental and numerical work. The parent problem of the dynamic collapse of such structures under hydrostatic pressure is also investigated. Two sets of experiments were performed. Initially, 50.6mm outside diameter aluminum tubes with diameter-to-thickness ratio of 32.3 were tested inside a pressure vessel. Hydrostatic pressure was applied quasi-statically up to the onset of collapse in order to obtain the collapse pressure of the tubes tested. Subsequently, similar tubes were tested in a 5m x 5m x 1.6m deep water tank under various explosive charges placed at different distances. Explosive charges and standoff distances were combined so as to eventually cause collapse of the specimens. In both sets of experiments, dynamic pressure and strain measurements were recorded using a fit-for-purpose data acquisition system with sampling rates of up to 1 Mega Samples/sec per channel. In parallel, finite element models were developed using commercially available software to simulate underwater explosion, pressure wave propagation, its interaction with a cylindrical shell and the subsequent onset of dynamic collapse. The surrounding fluid was modeled as an acoustic medium, the shells as J2 flow theory based materials with isotropic hardening, and proper fluid-structure interaction elements accounting for relatively small displacements of the boundary between fluid and structure were used. Finally, the physical explosion experiments were numerically reproduced with good correlation between results.
The emphasis of this paper is on nonlinear ship roll motion, because roll is the most critical ship motion of all six modes of motion. However, coupling between roll and the other modes of motion may be important and substantially affect the roll. Therefore, the complete six-degrees-of-freedom Euler’s equations of motion are studied. In previous work (Falzarano et al., 1990, 1991), roll linearly coupled to sway and yaw was studied. Continuing in this direction, this work extends that analysis to consider the dynamically more exact six-degrees-of-freedom Euler’s equations of motion and associated Euler angle kinematics. A combination of numerical path-following techniques and numerical integrations are utilized to study the steady-state response determined using this more exact modeling. The hydrodynamic forces are: linear frequency-dependent added-mass, damping, and wave-exciting, which are varied on a frequency-by-frequency basis. The linearized GM approximation to the roll-restoring moment is replaced with the nonlinear roll-restoring moment curve GZ(φ), and the linear roll wave damping is supplemented by an empirically derived linear and nonlinear viscous damping. A particularly interesting aspect of this modeling is the asymmetric nonlinearity associated with the heave and pitch hydrostatics. This asymmetric nonlinearity results in distinctive “dynamic bias,” i.e., a nonzero mean in heave and pitch time histories for a zero mean periodic forcing, and a substantial second harmonic. A Fourier analysis of the nonlinear response indicates that the harmonic response is similar to the linear motion response.
A method for the identification of the damping, restoring, and coupling parameters in the equations describing the coupled heave and pitch motions for an underwater robotic vehicle (URV) sailing near sea surface in random waves using only its measured responses at sea is presented. The random decrement equations are derived for the URV performing coupled heave and pitch motions in random waves. The hydrodynamic parameters in these equations are identified using a new identification technique called RDLRNNT, which uses a combination of a multiple linear regression algorithm and a neural networks technique. The combination of the classical parametric identification techniques and the neural networks technique provides robust results and does not require a large amount of computer time. The developed identification technique would be particularly useful in identifying the parameters for both moderately and lightly damped motions under the action of unknown excitations effected by a realistic sea. Numerically generated data for the coupled heave and pitch motion of a URV are used initially to test the accuracy of the technique. Experimental data are also used to validate the identification technique. It is shown that the developed technique is reliable in the identification of the parameters in the equations describing the coupled heave and pitch motions for an URV. y
We are studying numerically the problem of generation and propagation of long-crested gravity waves in a tank containing an incompressible inviscid homogeneous fluid initially at rest with a horizontal free surface of finite extent and of infinite depth. A nonorthogonal curvilinear coordinate system, which follows the free surface, is constructed and the full nonlinear kinematic and dynamic free surface boundary conditions are utilized in the algorithm. "Wavemakers" are modeled using both the Dirichlet and Neumann lateral boundary conditions and a full comparison is given. Overall, the Dirichlet model was more stable than the Neumann model, with an upper limit steepness S=2A/ of 0.08 using good resolution compared with the Neumann's maximum of 0.05. y
The static equilibrium position and its associated dynamic stability of a cylinder situated in the wake of an upstream cylinder is investigated in this paper Both the upstream and downstream cylinders are elastically mounted on springs to allow for streamwise and transverse displacements. Due to the wake effect the downstream cylinder is subject to a lift force as well as a drag. It is shown that under certain flow conditions there exist multiple stable and unstable equilibria for the downstream cylinder. There also exist a critical flow velocity and once this velocity is exceeded no equilibrium positions of the downstream cylinder can be found, which suggests a likely occurrence of clashing between the two cylinders.
Nonlinear roll damping has a profound influence on ship motions and stability in ocean waves. In this study, an experimental investigation is conducted on the nonlinear roll damping of a ship in regular and irregular waves. The random decrement method, previously used in linear roll damping prediction, is Extended to nonlinear roll damping estimation in the data process. The accuracy of the nonlinear roll damping obtained by using the random decrement method is found to be dependent on the values of the threshold and segment number. Yes Yes
To provide experimental data on the hydrodynamic characteristics and features of dynamic positioning (DP) thrusters under variable operating conditions, wake measurements were performed on a DP thruster model using 2D laser Doppler velocimetry (LDV) and stereoscopic particle image velocimetry (SPIV). These tests were performed with and without a nozzle and over a range of advance coefficient values including the bollard pull condition. In this paper, a detailed analysis of the hydrodynamic characteristics of the wake at a plane equal to a distance of 0.5 diameters downstream from the thruster, at advance coefficient values of 0, 0.4, and 0.45 are presented for both the LDV and SPIV measurements showing a comparison between the results of each technique. The effect of the duct and of changes in the advance coefficient values is presented in this paper. y
The Wave Crest Sensor Inter-comparison Study (WACSIS) employed many different types of sensors to measure ocean surface waves. These sensors were attached to a steel jacket platform located in 18 m deep water about 9 km from the Dutch coast. To investigate the suitability and consistency of different wave sensors, wave characteristics at the locations of some sensors were deterministically predicted based on three other wave measurements using a Directional Hybrid Wave Model (DHWM). The comparisons between the predictions and related measurements were applied to the examination of the consistency among different sensors. As an example, a mistake of the orientation of a current meter has been found through the comparisons. This study also demonstrates that consistency among different types of wave sensors is satisfactory and the DHWM is valuable to the analysis of field wave measurements.
The evaluation of wave characteristics has been widely studied by ocean engineers in the past. However conventional investigations for determining wave characteristics have been focused on the nonlinear wave effects in a rigid seabed. On the other hand, most previous investigations for the wave-induced seabed response in a porous seabed have been only concerned with the soil response after wave pressure penetrate into seabed. In this paper, employing a Complex wave number, the whole wave-seabed interaction problem will be re-examined. Based on the new closed-form analytical solution, a new wave dispersion equation is derived, including the seabed characteristics. The numerical results indicate that the wave characteristics (such cis the wavelength, wave pressure and wave profile) are affected by the soil permeability and shear modulus in a shallow water.
A dynamic model for the ice-induced vibration (IIV) of structures is developed in the present study. Ice properties have been taken into account, such as the discrete failure, the dependence of the crushing strength on the ice velocity and the randomness of the ice failure. The most important prediction of the model is to capture the resonant frequency lock-in, which is analogue to that in the vortex-induced vibration (VIV). Based on the model, the mechanism of resonant IIV is discussed. It is found that the dependence of the ice crushing strength on the ice velocity plays an important role in the resonant frequency lock-in of IIV. In addition, an intermittent stochastic resonant vibration is simulated from the model. These predictions are supported by the laboratory and field observations reported. The present model is more productive than the previous models of IIV. y
"a… Screenshots of top and bottom views of the CPF colliding with an ice wall in EnSight; "b… Screenshot of the CPF isosurfaces "top… and vector arrows "bottom…
"a… Plot of the velocity of a node in the x direction versus time; "b… von Mises impact stresses in the ship
Both computational structural dynamics ~CSD! and computational fluid dynamics ~CFD! have come together to empower offshore structural engineers to forecast and enhance the performance of various structures designs. Equally important, they enable researchers and scientists to experiment with a wide range of ??what-if?? scenarios for risk assessments, accident scenario investigations, and fragility analyses. The true value of any computational model is determined by both the accuracy of the results of the simulations and our ability to interpret all of the significant information contained in those results. To a large extent, the accuracy of the results can be assured via verification and validation analyses ~V&V analyses!, also known as numerical uncertainty analyses. The ability to find out and understand the effects of the physical phenomena/parameters that control the overall behavior of an offshore system depends, to a large extent, on the visualization tools used to view the results. Without strong visualization tools, it might be difficult to recognize the existence of problems or inefficiencies within a given design. y
For most design and recertification work on tubular structures, it is not practical to determine the joint hot spot stresses experimentally or using detailed finite element analysis. Therefore, parametric equations were developed in the past for various joint geometries to relate stress concentration factors around the joints to basic joint geometrical parameters. Such parametric equations have limited applications to reinforced joints because the nature of reinforcement varies and may be difficult to represent by a generic set of geometric parameters. In this paper, a new method is introduced to include local joint stiffness in the parametric equations represented by a generic characteristic of the joint determined by modal analysis. The parametric equations produced in this paper can be applied to any reinforced T-joint regardless of the nature of reinforcement. This is especially useful in those cases where the exact nature of reinforcement is not known, for example, hidden in the interior or deteriorated through age or fabrication error The dimensionless joint stiffness parameter can be calculated by a simple modal test and a beam model without needing to know the nature and details of the reinforcement.
A test program has been performed in an ice modeling basin to measure the load apportioning through ice rubble around Gulf's Molikpaq, a steel caisson offshore structure. A model of the Molikpaq and its supporting submarine berm was built at a 1:75 scale. The Molikpaq and berm were instrumented independently, so the load apportioning could be determined. Thirty-six ice-loading events, including rubble formation from level ice as well as impacts through the rubble by extreme ice features, were analyzed. The results of the tests show that the ice rubble can deform and transmit load to the structure at force levels well below those predicted by a rigid-body analysis of the rubble.
As part of an INSROP project, large-scale hull loading of first-year sea ice, two series of experiments were carried out to simulate ice loading on a ship's hull. The first, Phase I, was a preliminary series on freshwater lake ice near Calgary, Alberta, and the second, Phase II, took place in Tuktoyaktuk Harbour in the Canadian Arctic also on essentially freshwater ice. Loading was generated by hydraulic actuators impressing a rigid indentor against an ice edge, and by using flatjacks. A finite element analysis of the test geometry was carried out to assess the deformation and stress distributions in the ice edge for cases with both undamaged and varying degrees of damage. The calculated and measured stiffness of the ice edge agreed for a realistic selection of elastic modulus of the parent ice and damaged ice. The field results did not show conclusively any influence of damage on the failure strength of the ice. A review of these results, and those from Resolute Bay sea ice obtained earlier, showed that the nature of the ice loading, depending on whether it was uniform pressure or uniform deformation, significantly affected the results. The failure stress for uniform pressure tests did not have any dependence on area or aspect ratio. The measured field results gave average ice pressures less than those recommended by the Arctic Pollution Prevention Regulations. y
A stepwise response surface approach is proposed in this paper. The response surface is determined by modified stepwise regression, so that the square and cross terms can be absorbed into the model automatically according to their actual contribution, which is calculated by repeated variance analysis. Besides, by applying a weighting factor to the statistical value of contribution and changing the thresholds of introduction and rejection, the entry of each term can be controlled in a fairly flexible manner. None other criteria than those in the traditional statistics are needed to check the goodness of fit. Considering the relatively small sample set at the beginning, the algorithm starts with a linear response surface. As the adaptive iteration proceeds, the bar to quadratic terms is lifted gradually to allow ordered entry. Since the sampling points in one step of iteration are recycled in the succeeding ones, a simple experimental design is enough to fit a robust response surface. A double bottom hull system is analyzed with randomized Young's modulus, load distribution, and geometric properties. The sensitivity analysis is also performed with respect to the random variables and the parameters in their distributions.
The uniform flow over a nominally two-dimensional normal thin flat plate with blockage ratio 0.214 was numerically investigated in three dimensions by three methods: unsteady Reynolds-averaged Navier–Stokes (URANS) based on the realizable k-epsilon (RKE) turbulence model, URANS based on the k–omega shear stress transport (SST) turbulence model and detached eddy simulation (DES). The Reynolds number based on the inlet flow velocity and the chord width of the plate was 117000. A comprehensive comparison against earlier experimental results showed that URANS-SST method only could give a correct Strouhal number but overestimated the mean base pressure distribution and mean drag coefficient, while URANS-RKE and DES methods succeeded in giving accurate prediction of all. Moreover, by comparing the instantaneous vorticity contours and 3D turbulent flow structures, it is found that DES is better suited for the present case because it can capture irregular small-scale structures and reproduce the three-dimensionality and low-frequency unsteadiness of the vortex shedding. Finally, through the volume-of-fluid (VOF) based simulation of the free surface, it is demonstrated that the free surface has no significant effect on mean drag coefficient and Strouhal number.
The work by Viselli et al. (2022, “LiDAR Measurements of Wind Shear Exponents and Turbulence Intensity Offshore the Northeast United States”, ASME J. Offshore Mech. Arct. Eng., 144(4), p. 042001) presents results on wind shear exponents and turbulence intensity obtained from analysis of field data collected offshore the Northeast United states. This discusson is focused on the wind shear exponent which is obtained by combining the logarithmic mean wind speed profile and the power law mean wind speed profile, containing the sea surface roughness as a parameter. Here the sea surface roughness is specified using three formulae given in terms of significant wave height and spectral wave steepness. The discussion points out how the sea surface roughness can be included in future applications of the authors’ analysis method.
This paper numerically investigates the flow-induced vibration of a circular cylinder attached with front and/or rear splitter plates at Re = 120. The effects of plate length and plate location on the hydrodynamic coefficient, vibration response, and flow wake are examined. The results reveal that the hydrodynamic coefficient of the cylinder with a single rear plate is significantly reduced at Ur = 8 (Ur is the reduced velocity), resulting in the VIV suppression. However, the galloping is excited at Ur > 8 due to the hydrodynamic instability, accompanying with the rapid jump of response amplitude and hydrodynamic force as well as the abrupt drop of response frequency. The alternate reattachment of shear layers on the plate surface introduces an extra lift force that strengthens the vibration response. By introducing an individual front plate, the separation point of cylinder-plate body moves downstream, leading to the delayed vortex shedding and narrowed wake flow and hence the VIV suppression. The vibration exhibits variable patterns when the cylinder is equipped with bilateral plates, including the typical VIV mode, weak VIV-galloping coupling response mode, and IB-galloping-DB mode (IB and DB represent the initial branch and desynchronization branch, respectively). The galloping branch in IB-galloping-DB mode is observed with an abrupt drop of response frequency as well as a tiny time lag between displacement and lift force. The vibration response is significantly suppressed by the cylinder with 1D front plate and 1–2D rear plate due to the more streamlined profile.
A passive control means to suppress flow-induced motions (FIM) of a rigid circular cylinder in the TrSL3, high-lift, flow regime is formulated and tested experimentally. The method developed uses passive turbulence control (PTC) consisting of selectively located roughness on the cylinder surface with thickness about equal to the boundary layer thickness. The map of “PTC-to-FIM”, developed in previous work, revealed robust zones of weak suppression, strong suppression, hard galloping, and soft galloping. PTC has been used successfully to enhance FIM for hydrokinetic energy harnessing using the VIVACE Converter. The same technology revealed the potential to suppress FIM to various levels. The map is flow-direction dependent. In this paper, the “PTC-to-FIM” map is used to guide development of FIM suppression devices that are flow-direction independent and hardly affect cylinder geometry. Experiments are conducted in the Low Turbulence Free Surface Water Channel of the University of Michigan on a rigid, horizontal, circular cylinder, suspended on springs. Amplitude and frequency measurements and broad field-of-view visualization reveal complex flow structures and their relation to suppression. Several PTC designs are tested to understand PTC direction, roughness, thickness, and coverage. Gradual modification of PTC parameters, leads to improved suppression and evolution of a design reducing the VIV synchronization range, fully suppressing VIV in a wide range, and reducing the maximum occurring near the system’s natural frequency by about 60% compared to the maximum amplitude of the smooth cylinder.
The VIVACE (Vortex Induced Vibration for Aquatic Clean Energy) Converter was introduced at OMAE2006 as a single, smooth, circular-cylinder module. The hydrodynamics of VIVACE is being improved continuously to achieve higher density in harnessed hydrokinetic power. Inter-cylinder spacing and Passive Turbulence Control (PTC) through selectively located roughness are effective tools in enhancement of Flow Induced Motions (FIMs) under high damping for power harnessing. VIVACE Converters consist of multi-cylinder modules. Single cylinders harness energy at high density even in 1knot currents. For downstream cylinders questions are raised on energy availability and sustainability of high-amplitude FIM. Through PTC and inter-cylinder spacing, strongly synergetic FIM of 2/3/4 cylinders is achieved, harnessing hydrokinetic energy with increased footprint density. Two-cylinder smooth/PTC and four-cylinder PTC systems are tested experimentally. Using the “PTC-to-FIM” map developed in previous work at the Marine Renewable Energy Laboratory (MRELab), PTC is applied and cylinder response is measured for the following parameter ranges: In-flow center-to-center distance 1.63•D–5.00•D (D = diameter), transverse center-to-center distance 0.5•D–1.5•,D, Re ∊[28,000–120,000], m* ∊[1.677–1.690], U ∊[0.36m/s–1.45m/s], aspect ratio l/D = 10.29, and m*ζ ∊[0.0283–0.0346]. All experiments are conducted in the Low Turbulence Free Surface Water (LTFSW) Channel of MRELab. Amplitude spectra and broad filed-of-view (FOV) visualization help reveal complex flow structures and cylinder interference undergoing VIV, interference/proximity/wake/soft/hard galloping.
In the Marine Renewable Energy Laboratory of the University of Michigan, selectively located surface roughness has been designed successfully to suppress vortex-induced vibrations of a single cylinder by 60% compared to a smooth cylinder. In this paper, suppression of flow-induced motions of two cylinders in tandem using surface roughness is studied experimentally by varying flow velocity and cylinder center-to-center spacing. The two identical cylinders are rigid, suspended by springs, and allowed to move transversely to the flow direction and their own axis. Surface roughness is applied in the form of four roughness strips helically placed around the cylinder. Results are compared to smooth cylinders also tested in this work. Amplitude ratio A/D, frequency ratio fosc/fn,water, and range of synchronization are measured. Regardless of the center-to-center cylinder distance, the amplitude response of the upstream smooth-cylinder is similar to that of the isolated smooth-cylinder. The wake from the upstream cylinder with roughness is narrower and longer and has significant influence on the amplitude of the downstream cylinder. The latter is reduced in the initial and upper branches while its range of VIV-synchronization is extended. In addition the amplitude of the upstream rough cylinder and its range of synchronization increase with respect to the isolated rough cylinder.
Two-dimensional (2D) Unsteady Reynolds-Averaged Navier-Stokes equations (URANS) equations with the Spalart-Allmaras turbulence model are used to simulate the flow and body kinematics of the transverse motion of spring-mounted circular cylinder. The flow is in the high-lift TrSL3 regime of a Reynolds number in the range 35,000< Re< 130,000. Passive turbulence control (PTC) in the form of selectively distributed surface roughness is used to alter the cylinder flow induced motion (FIM). Simulation is performed using a solver based on the open source Computational Fluid Dynamics (CFD) tool OpenFOAM, which solves continuum mechanics problems with a finite-volume discretization method. Roughness parameters of PTC are chosen based on tests conducted in the Marine Renewable Energy Lab (MRELab) of the University of Michigan. The numerical tool is first tested on smooth cylinder in vortex-induced vibration (VIV) and results are compared with available experimental measurements and URANS simulations. For the cylinder with PTC cases, the sandpaper grit on the cylinder wall is modeled as a rough-wall boundary condition. Two sets of cases with different system parameters (spring, damping) are simulated and the results are compared with experimental data measured in the MRELab. The amplitude ratio curve shows clearly three different branches, including the VIV initial and upper branches, and a galloping branch. The numerical branches are similar to those observed experimentally. Frequency ratio, vortex patterns, transitional behavior, and lift are also predicted well for PTC cylinders at such high Reynolds numbers.
Instrumented Pier 31, Confederation Bridge (Brown and Croasdale [1]) 
Load uncertainty in average and trigger data (Tibbo [4])
Event attributes for 10 events in Table 4
Idealized Rubble Pile with horizontal upper surface 
The Confederation Bridge spans across the Northumberland Strait in Eastern Canada connecting Prince Edward Island to mainland Canada through New Brunswick. Due to the presence of ice during each winter, the bridge piers are subjected to ice loads. A comprehensive permanent monitoring program has been implemented to observe and measure the ice-structure interaction events at two piers since the start of the bridge operations in 1998. This study uses the derived ice loads on one pier, and the associated event attributes for 100 selected events. Flexural failure models are used to determine theoretical loads of the selected interaction events. It is found that the weight of the total ice rubble pile and the physical and mechanical properties of the ice sheet are the dominant parameters affecting the ice load exerted on the conical structure. A semi-empirical correlation is developed to relate the ice load with those parameters for the Confederation Bridge.
Flow-induced vibration (FIV), primarily vortex-induced vibrations (VIV), and galloping have been used effectively to convert hydrokinetic energy to electricity in model-tests and field-tests by the Marine Renewable Energy Laboratory (MRELab) of the University of Michigan. It is known that the response of cylinders with passive turbulence control (PTC) undergoing vortex shedding differs from the oscillation of smooth cylinders in a similar configuration. Additional investigation on the FIV of two elastically mounted circular cylinders in a staggered arrangement with low mass ratio in the TrSL3 flow-regime is required and is contributed by this paper. The two PTC-cylinders were allowed to oscillate in the transverse direction to the oncoming fluid flow in a recirculating water channel. The cylinder model with a length of 0.895 m and a diameter of 8.89 cm, a mass ratio of 1.343 was used in the tests. The Reynolds number was in the range of 2.5 × 10⁴ < Re < 1.2 × 10⁵, which is a subset of the TrSL3 flow-regime. The center-tocenter longitudinal and transverse spacing distances were T/D=2.57 and S/D=1.0, respectively. The spring stiffness values were in the range of 400 < K (N/m) < 1200. The values of harnessing damping ratio tested were ζharness=0.04, 0.12 and 0.24. For the values tested, the experimental results indicate that the response of the upstream cylinder is similar to the single cylinder. The downstream cylinder exhibits more complicated vibrations. In addition, the oscillation system of two cylinders with stiffer spring and higher ζharness could initiate total power harness at a higher flow velocity and obtain more power.
Flow-induced vibrations (FIVs) of two elastically mounted circular cylinders in staggered arrangement were experimentally investigated. The Reynolds number range for all experiments (2.5×10⁴ < Re < 1.2×10⁵) was in the transition in shear layer 3 (TrSL3) flow regime. The oscillator parameters selected were: mass ratio m* = 1.343 (ratio of oscillating mass to displaced fluid mass), spring stiffness K=250 N/m, and damping ratio ζ=0.02. The experiments were conducted in the low turbulence free surface water (LTFSW) channel in the MRELab of the University of Michigan. A closed-loop, virtual spring-damper system (Vck) was used to facilitate quick and accurate parameter setting. Based on the characteristics of the displacement response, five vibration patterns were identified and their corresponding regions in the parametric plane of the in-flow spacing (1.57 < L/D < 4.57) and transverse cylinder spacing (0 < T/D < 2) were defined. The hydrodynamic forces and frequency characteristics of the vibration response are also discussed.
The Deepwater Horizon Mobile Offshore Drilling Unit (MODU) was one of several classes of floatable drilling systems. The explosion on April 20, 2010 led to fatalities and the worst oil spill in the U.S. We present an independent estimate of the oil-flow rate into The Gulf caused by the drill-pipe rupture. We employed the NASA Moderate-Resolution Imaging-Spectroradiometer (MODIS) satellite photographs, starting from the days immediately following the disaster, to determine the magnitude of spill. From these images, we obtained the surface area of the spill and calculated the oil flow rate by two different methods based on contrasting luminance within that area. The first assumes a constant thickness for the total area with upper and lower bounds for the thickness. The second separates the area into different patches based on the luminance levels of each. The probability density function (PDF) of such luminance plots showed natural groupings, allowing patches be identifiable. Each patch maps to a specific thickness. This second approach provides a more accurate average thickness. With the assumption that evaporation and other loss amounted to a 1/440% of the spill, we obtained, from the first method, a flow rate ranging from 9,300 barrels per day (BPD) to 93,000 BPD. A value of 51,200 BPD was obtained using patch-separation method. This latter estimate was a plausible value, obtained from the current analysis, but with no details presented in an Extended Abstract in OMAE2012, is remarkably consistent with the "official U.S.-Govt. estimates.".
An improved understanding of the present and future marine climatology is necessary for numerous activities, such as operation of offshore structures, optimization of ship routes and the evaluation of wave energy resources. To produce global wave information, the WW3 wave model was forced with wind and ice-cover data from an RCP8.5 EC-Earth system integration for two 30-year time slices. The first covering the periods from 1980 to 2009 represents the present climate and the second, covering the periods from 2070-2099, represents the climate in the end of the 21st century. Descriptive statistics of wind and wave parameters are obtained for different 30-year time slices. Regarding wind, magnitude and direction will be used. For wave, significant wave height (of total sea and swell), mean wave period, peak period, mean wave direction and energy will be investigated. Changes from present to future climate are evaluated, regarding both mean and extreme events. Maps of these statistics are presented. The long-term monthly joint distribution of significant wave heights and peak periods is generated. Changes from present to future climate are assessed, comparing the statistics between time slices.
Design of a steel catenary riser (SCR) requires the use of connection hardware to decouple the large bending moments induced by the host floater at the hang-off location. Reliability of this connection hardware is essential, particularly in applications involving high pressure and high temperature fluids. One option for this connection hardware is the metallic tapered stress joint. Titanium (Ti) Grade 29 has been identified as an attractive material candidate for demanding stress joint applications due to its “high-strength, low weight, superior fatigue performance and innate corrosion resistance”2. Titanium stress joints for deep-water applications are typically not fabricated as a single piece due to titanium ingot volume limitations, thus making an intermediate girth weld necessary to satisfy length requirements. As with steel, the potential effect of hydrogen embrittlement induced by cathodic and galvanic potentials must be assessed to ensure long term weld integrity. This paper describes testing from a joint industry project (JIP) conducted to qualify titanium stress joint (TSJ) welds for ultra-deepwater applications under harsh service and environmental conditions. Corrosion-fatigue crack growth rate (CFCGR) results for Ti Grade 29 1G/PA gas tungsten arc welding (GTAW) specimens in seawater under cathodic potential and sour brine under galvanic potentials are presented and compared to vendor recommended design curves.
It is essential for a Navier-Stokes equations solver based on a projection method to be able to solve the resulting Poisson equation accurately and efficiently. In this paper, we present numerical solutions of the 2D Navier-Stokes equations using the fourth-order generalized harmonic polynomial cell (GHPC) method as the Poisson equation solver. Particular focus is on the local and global accuracy of the GHPC method on non-uniform grids. Our study reveals that the GHPC method enables use of more stretched grids than the original HPC method. Compared with a second-order central finite difference method (FDM), global accuracy analysis also demonstrates the advantage of applying the GHPC method on stretched non-uniform grids. An immersed boundary method is used to deal with general geometries involving the fluid-structure-interaction problems. The Taylor-Green vortex and flow around a smooth circular cylinder and square are studied for the purpose of verification and validation. Good agreement with reference results in the literature confirms the accuracy and efficiency of the new 2D Navier-Stokes equation solver based on the present immersed-boundary GHPC method utilizing non-uniform grids. The present Navier-Stokes equations solver uses second-order FDM for the discretization of the diffusion and advection terms, which may be replaced by other higher-order schemes to further improve the accuracy.
Moonpool resonance is investigated in a two-dimensional setting in terms of regular, forced heave motions of a model with moonpool with different rectangular-shaped recess configurations. A recess is a reduced draft zone in the moonpool. Dedicated experiments were carried out. The model consisted of two boxes of 40 cm width each, with a distance of 20 cm between them. Recess configurations varying between 5 cm to 10 cm in length and 5 cm in height were tested. Different drafts were also tested. A large number of forcing periods, and five forcing amplitudes were tested. A time-domain Boundary Element Method (BEM) code based on linear potential flow theory was implemented to investigate the resonance periods, mode shapes as well as the moonpool response as predicted by (linear) potential flow theory. Dominant physical effects were discussed, in particular damping due to flow separation from the sharp corners of the moonpool inlet and recess. The effect of the recess on the piston-mode behavior is discussed. The non-dimensional moonpool response suggests strong viscous damping at piston-mode resonance. The viscous BEM (VBEM) simulations demonstrate improvement over inviscid BEM, although further improvement of the method is needed. The VBEM simulations are, in general, in good agreement with the experiments. For the largest recess case, some discrepancies are observed in the amplitude-dependent RAOs. The piston mode shapes are clearly different from the near flat free-surface elevation for a moonpool without recess, consistent with recently published theory.
Extreme waves have led to many accidents and losses of ships at sea. In this paper, a two-dimensional (2D) hydroelastoplasticity method is proposed as a means of studying the nonlinear dynamic response of a container ship when traversing extreme waves, while considering the ultimate strength of the ship. On one hand, traditional ultimate strength evaluations are undertaken by making a quasi-static assumption and the dynamic wave effect is not considered. On the other hand, the dynamic response of a ship as induced by a wave is studied on the basis of the hydroelasticity theory so that the nonlinear structural response of the ship cannot be obtained for large waves. Therefore, a 2D hydroelastoplasticity method, which takes the coupling between time-domain waves and the nonlinear ship beam into account, is proposed. This method is based on an hydroelasticity method and a simplified progressive collapse method that combines the wave load and the structural nonlinearity. A simplified progressive collapse method, which considers the plastic nonlinearity and buckling effect of stiffened, is used to calculate the ultimate strength and nonlinear relationship between the bending moment and curvature, so that the nonlinear relationship between the rigidity and curvature is also obtained. A dynamic reduction in rigidity related to deformation could influence the strength and curvature of a ship's beam; therefore, it is input into a dynamic hydrodynamic formula rather than being regarded as a constant structural rigidity in a hydroelastic equation. A number of numerical extreme wave models are selected for computing the hydroelastoplasticity, such that large deformations occur and nonlinear dynamic vertical bending moment (VBM) is generated when the ship traverses these extreme waves. As the height and Froude number of these extreme waves are increased, a number of hydroelastoplasticity results including VBM and deformational curvature are computed and compared with results obtained with the hydroelasticity method, and then, some differences are observed and conclusions are drawn.
Passive turbulence control (PTC) in the form of two straight roughness strips with variable width, and thickness about equal to the boundary layer thickness, is used to modify the flow-induced motions (FIM) of a rigid circular cylinder. The cylinder is supported by two end springs and the flow is in the TrSL3, high-lift, regime. The PTC-to-FIM Map, developed in the previous work, revealed zones of weak suppression (WS), strong suppression (SS), hard galloping (HG), and soft galloping (SG). In this paper, the sensitivity of the PTC-to-FIM map to: (a) the width of PTC covering, (b) PTC covering a single or multiple zones, and (c) PTC being straight or staggered is studied experimentally. Experiments are conducted in the low turbulence free surface water channel of the University of Michigan, Ann Arbor, MI. Fixed parameters are: cylinder diameter D = 8.89 cm, m* = 1.725, spring stiffness K = 763 N/m, aspect ratio l/D = 10.29, and damping ratio ζ = 0.019. Variable parameters are circumferential PTC location αPTC ∈ (0-180 deg), Reynolds number Re ∈ (30,000-120,000), flow velocity U ∈ (0.36-1.45 m/s). Measured quantities are amplitude ratio A/D, frequency ratio fosc/fn,w, and synchronization range. As long as the roughness distribution is limited to remain within a zone, the width of the strips does not affect the FIM response. When multiple zones are covered, the strong suppression zone dominates the FIM.
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Carlos Guedes Soares
  • University of Lisbon
Torgeir Moan
  • Norwegian University of Science and Technology
Michael M. Bernitsas
  • University of Michigan
Jørgen Hals Todalshaug
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Krish Thiagarajan
  • University of Massachusetts Amherst