ASME Conference Presenter Attendance Policy and Archival Proceedings

2017;():V07AT00A001. doi:10.1115/OMAE2017-NS7A.

This online compilation of papers from the ASME 2017 36th International Conference on Ocean, Offshore and Arctic Engineering (OMAE2017) represents the archival version of the Conference Proceedings. According to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference by an author of the paper, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Ocean Engineering: Advanced Ship Hydromechanics and Marine Technology

2017;():V07AT06A001. doi:10.1115/OMAE2017-61011.

Modern ships are supported by internet of things (IoT) to collect ship performance and navigation information. That should be utilized towards digitalization of the shipping industry. However, such information collection systems are always associated with large-scale data sets, so called Big Data, where various industrial challenges are encountered during the respective data handling processes. This study proposes a data handling framework with data driven models (i.e. digital models) to cope with the shipping industrial challenges as the main contribution, where conventional mathematical models may fail. The proposed data driven models are developed in a high dimensional space, where the respective ship performance and navigation parameters of a selected vessel are separated as several data clusters. Hence, this study identifies the distribution of the respective data clusters and the structure of each data cluster in relation to ship performance and navigation conditions. An appropriate structure into the data set of ship performance and navigation parameters is assigned by this method as the main contribution. However, the domain knowledge (i.e. vessel operational and navigation conditions) is also included in this situation to derive a meaningful data structure.

Topics: Vessels
Commentary by Dr. Valentin Fuster
2017;():V07AT06A002. doi:10.1115/OMAE2017-61051.

The added resistance of a ship was calculated using Maruo’s formula [1] involving the three-dimensional Kochin function obtained using the source and normal doublet distribution over the wetted surface of the ship. The density of the doublet distribution was obtained as the solution of the three-dimensional frequency-domain forward-speed Green integral equation containing the exact line integral along the waterline. Numerical results of the Wigley ship models II and III in head seas, obtained by making use of the inner-collocation 9-node second-order boundary element method have been compared with the experimental results reported by Journée [2]. The forward-speed hydrodynamic coefficients of the Wigley models have shown no irregular-frequencylike behavior. The steady disturbance potential due to the constant forward speed of the ship has also been calculated using the Green integral equation associated with the steady forward-speed free-surface Green function since the so-called mj-terms [3] appearing in the body boundary conditions contain the first and second derivatives of the steady potential over the wetted surface of the ship. However, the free-surface boundary condition was kept linear in the present study. The added resistances of the Wigley II and III models in head seas obtained using Maruo’s formula showing acceptable comparison with experimental results, have been presented. The added resistances in following seas obtained using Maruo’s formula have also been presented.

Topics: Ships
Commentary by Dr. Valentin Fuster
2017;():V07AT06A003. doi:10.1115/OMAE2017-61110.

The development of loading and unloading operations is an essential part of the logistical chain of maritime transportation. One of the determining factors in this process is the movement of moored ships, which has a notable influence over the efficiency of the operation. The implementation of systems to characterize their real behavior would provide an important tool for the management of these activities.

In this context the Water and Environmental Engineering Group (GEAMA) at the University of A Coruña, Spain (UDC), through different projects executed in collaboration with the Port Authority of A Coruña (Spain), has developed an integrated system for monitoring the dynamic behavior of moored vessels. This system includes different devices which allow to characterize their six degrees of freedom and tensions in certain mooring lines.

The analysis of motions is carried out using three combined technologies. Ship rotations (roll, pitch and yaw) are registered by means of an Inertial Measurement Unit (IMU) developed by the Research Group. The translations of heave and surge are analyzed with a visual tracking system using fixed cameras deployed near the location. Finally, the evaluation of sway motion and an additional measure of yaw are obtained using two laser-distance meters located over the dock.

All devices integrated in the system were calibrated in laboratory at the R+D Centre CITTEC-UDC with several series of tests. Once the methodology was validated, the system has been employed in different projects carried out in Inner and Outer Ports of the city of A Coruña, with more than 30 monitored ships.

Commentary by Dr. Valentin Fuster
2017;():V07AT06A004. doi:10.1115/OMAE2017-61137.

Frequency-domain methods are proven efficient and reliable, especially for zero forward speed, in early design stage for the prediction of ship motions and wave-induced wave loads. There are still challenges for ships with forward-speed due to the inaccuracy in the computation of m-terms. In this paper, the panel-free method is further improved to predict motions and wave-induced loads on real ships with forward speeds. A simple algorithm has been developed to re-arrange the control points for Non-Uniform Rational B-Splines (NURBS) surfaces. This method led to reliable and accurate m-term computations and therefore improved ship motion and load predictions. Validation studies have been carried out for a hydroelastic model of a frigate. Computed motions and loads were compared with experimental data.

Topics: Stress , Waves , Ships
Commentary by Dr. Valentin Fuster
2017;():V07AT06A005. doi:10.1115/OMAE2017-61146.

It is important to predict the propeller tip vortex flow and its effect on hull vibration and noise. In our previous work, the tip vortex flow of the David Taylor Model Basin (DTMB) 5168 propeller model has been studied based on the Reynolds Averaged Navier-Stokes equation (RANS) solution using various eddy viscosity and Reynolds Stress turbulence models. A set of structural grids were used, however, large Jacobian values of the structural grids around the propeller tip region led to the convergence problem and inaccurate solutions. In the present work, the numerical prediction of the same propeller model was improved by using a steady-state RANS solver simpleFoam in OpenFOAM with locally refined unstructured grid along the tip vortex trajectory. The computed thrust and torque coefficients and the velocity components across the vortex core are compared with experimental data and results in the previous studies. Improvement in the prediction of velocity components across the tip vortex core were achieved.

Commentary by Dr. Valentin Fuster
2017;():V07AT06A006. doi:10.1115/OMAE2017-61199.

The ship wave resistance can be evaluated by two alternative methods after solving the boundary value problem. One is the far field method e.g. Havelock’s formula, and another is the near field method based on direct pressure integration over the wetted hull surface. As is well known, there exist considerable discrepancies between wave resistance results by far field method and by near field method.

This paper presents a Lagally expression in consistency with Havelock’s formula. In order to derive the Lagally expression, the symmetry of Havelock’s Green function is used in the same manner as Yeung et al (2004). Another expression to examine the relation with water pressure integrations or to ensure physical consistency is also derived by slightly deforming that expression. Some numerical comparisons of wave resistance of Wigley, KCS and KVLCC2 models among by Havelock’s formula, some direct pressure integration methods and present two new near field expressions, are shown to demonstrate consistency numerically.

Topics: Waves , Ships
Commentary by Dr. Valentin Fuster
2017;():V07AT06A007. doi:10.1115/OMAE2017-61219.

On-board Energy systems in vessels have to use fuel efficiently to maintain ship speed at lowest possible costs. This paper describes how to use ship operational data to improve and maintain efficiency of the vessel’s power production with respect to the condition and performance of equipment. The paper describes an overview of an automated Technical Operations Performance (TOP) Monitoring service. TOP monitors the performance of marine main and auxiliary diesel engines by use of the information collected onboard vessels at regular intervals. Performance data are stored in xml-reports sent as email attachments from ship to shore. This communication is reliable and cost efficient for merchant ships that are on-line only for shorter periods. Load, ISO and environmental corrections make results valid for benchmarking and trending. The service aggregates the hierarchical information obtained from different sources by transferring measurement readings into unified indicators, the Technical Condition Index (TCI) [1]. Experts manually check the automatically generated performance reports and add additional guidance on options to improve power production and machinery conditions analyzing the available data with respect to different targets, such as low engine degradation and high fuel efficiency. The performance reports then influence business processes indicating possible causes for loss of performance in equipment and possible erroneous instrumentation, and the need for maintenance actions. The obtained TCI values show the performance of individual units, or for a fleet/class of equipment and vessels.

Commentary by Dr. Valentin Fuster
2017;():V07AT06A008. doi:10.1115/OMAE2017-61400.

This paper presents some outcomes from a four years (2013 – 2016) long research project investigating validation of ship specific simulation models. In contrast to the other initiatives such as SIMMAN 2008 [1] and SIMMAN 2014 [2], the R&D project “Sea Trials and Model Tests for Validation of Shiphandling Simulation Models” (SIMVAL [3]) investigates model development and validation methods for specific vessels. For these vessels, the yard’s documentation of manoeuvring characteristic was available for the researchers. In addition, the project has been given permission to perform different sets of sea trials on selected case vessels. As far as possible, these sea trials were designed to document vessel-specific operations such as low-speed manoeuvres and dynamic positioning. Other papers presenting results from the SIMVAL project will be given in a separate session, Session 12–14, in the Torgeir Moan Symposium at OMAE2017.

Topics: Performance , Ships
Commentary by Dr. Valentin Fuster
2017;():V07AT06A009. doi:10.1115/OMAE2017-61860.

There has been a lot of interest in trim optimisation to reduce fuel consumption and emissions of ships. Many existing ships are designed for a single operational condition with the aim of producing low resistance at their design speed and draft with an even keel. Given that a ship will often sail outside this condition over its operational life and moreover some vessels such as LNG carriers return in ballast condition in one leg, the effect of trim on ships resistance will be significant. Ship trim optimization analysis has traditionally been done through towing tank testing. Computational techniques have become increasingly popular for design and optimization applications in all engineering disciplines. Computational Fluid Dynamics (CFD), is the fastest developing area in marine fluid dynamics as an alternative to model tests. High fidelity CFD methods are capable of modelling breaking waves which is especially crucial for trim optimisation studies where the bulbous bow partially emerges or the transom stern partially immerses. This paper presents a trim optimization study on the Kriso Container Ship (KCS) using computational fluid dynamics (CFD) in conjunction with towing tank tests. A series of resistance tests for various trim angles and speeds were conducted at 1:75 scale at design draft. CFD computations were carried out for the same conditions with the hull both fixed and free to sink and trim. Dynamic sinkage and trim add to the computational cost and thus slow the optimisation process. The results obtained from CFD simulations were in good agreement with the experiments. After validating the applicability of the computational model, the same mesh, boundary conditions and solution techniques were used to obtain resistance values for different trim conditions at different Froude numbers. Both the fixed and free trim/sinkage models could predict the trend of resistance with variation of trim angles; however the fixed model failed to measure the absolute values as accurately as the free model. It was concluded that a fixed CFD model, although computationally faster and cheaper, can find the optimum trim angle but cannot predict the amount of savings with very high accuracy. Results concerning the performance of the vessel at different speeds and trim angles were analysed and optimum trim is suggested.

Topics: Containers , Ships
Commentary by Dr. Valentin Fuster
2017;():V07AT06A010. doi:10.1115/OMAE2017-61878.

Station keeping analysis is an important activity in the early stages of any vessel/DP project that eventually determines the machinery and thruster configuration and thruster size selection. In order to obtain reliable results, it is crucial to apply engineering tools that realistically represent the flow physics and resulting hydrodynamic forces. Present computer tools are based on the assumption that wave drift- and current forces can be superimposed. However, there are also mutual interaction effects between waves, current and hulls that should be accounted for in the evaluation of the wave drift forces. In MULDIF, a 3D diffraction/radiation panel code developed by SINTEF Ocean within the framework of a JIP, this wave-current-body interaction is taken into consideration by a new potential flow numerical model. A case study with offshore vessels and general cargo ships of different main dimensions has been performed to assess the capabilities of MULDIF for station keeping purposes in wave and current environments. The first-order vessel motions as well as mean second-order drift forces for 0 kn forward speed without current have been calculated. Through an interface to SINTEF Ocean’s vessel response code VERES, MULDIF offers the possibility to include viscous roll damping due to hull friction, flow separation at bilge keels, lift effects as well as normal forces acting on bilge keels and hull pressure created by the presence of bilge keels. This reduces roll motions to a realistic extent as shown by the comparison of RAOs from MULDIF calculations and model tests. Roll reduction tank effects can currently only be considered through the external damping matrix. Model tests for the selected vessels have been performed in SINTEF’s Ocean Basin in a soft-mooring arrangement in different irregular sea states and headings in deep water. The models were equipped with two two-component force transducers, measuring the x- and y- components of the forces. The yaw moments have been calculated from the y-force measurements. In order to measure the vessel motions in six degrees of freedom, an optoelectronic position measuring system has been used. Selected cases illustrate the significant influence of wave-current interaction on motions and drift forces.

Topics: Waves
Commentary by Dr. Valentin Fuster
2017;():V07AT06A011. doi:10.1115/OMAE2017-61949.

Ships travel in waves most of the time. The horsepower increase or speed loss in waves can become large in adverse sea. Speed loss needs to be compensated for by increasing the power. Moreover, the associated speed-loss can possibly not be compensated due to the limit of installed power. This will cause delays, and may even lead to safety concerns in maneuvering. Thus, there is a need for minimizing added resistance due to both economical and safety reasons. Ships with better performance in waves even with reduced power are desired. Latest researches on advanced ship optimization are taking added resistance into account. Ship bow optimization has been the main measure to reduce added resistance in waves.

Based on analyzing the working principles and potential benefit of the different kinds of novel ship bows, a novel measure is proposed to reduce the ship resistance in waves is proposed. A novel measure to reduce the ship resistance in waves of existing ships by installing a simple structure at the ship bow. The structure is designed to sharpen the ship bow and therefore reduce added resistance due to wave reflection. Thus, it is suitable for the large tankers and bulk carriers, which have blunt bows. The volume above free surface was expected to reduce ship motions and the added resistance due to ship motions consequently. This measure does not change the original ship design. It can be retrofitted on existing ships fairly easily at low cost. In order to verify the benefit due to the retrofitted structure, CFD simulations are performed in both head and oblique waves, which can take 3D effect into account. Four different retrofitted structures are designed and the numerical simulations are performed with the same numerical and mesh settings in each wave condition. The CFD simulation results confirm that the novel measure can reduce ship added resistance efficiently and it also has benefits in oblique waves.

Topics: Waves , Ships
Commentary by Dr. Valentin Fuster
2017;():V07AT06A012. doi:10.1115/OMAE2017-62053.

Parametric resonance (PR) in roll is of concern for fishing vessels, especially in head-sea waves. Here the effect of passive anti-roll free-surface tank is investigated experimentally and numerically on realistic fishing-vessel geometry at zero forward speed. On the numerical side, the onboard tank is simulated using an open source computational fluid dynamic (CFD) development platform, OpenFOAM (Open Field Operation and Manipulation). The internal flow solver is coupled with the seakeeping solver, which is based on the weakly nonlinear method proposed in [1]. Experimentally, two different relevant scenarios were examined: 1) a 2D rectangular tank with shallow-water filling depth was forced to oscillate in roll and the loads induced on the tank were measured; 2) a fishing vessel was tested in a towing tank, prescribing regular head-sea waves. The vessel was examined both without and with anti-roll tanks and a mooring-line system was designed so to control the horizontal motions with limited effect on the parametric occurrence. The experiments on the fishing vessel are considered to assess the seakeeping solver. Both model tests and numerical results confirmed the effectiveness of an on-board tank in avoiding PR. For the examined cases with tank, the parametric resonance did not occur without forcing an initial roll. Moreover, the initial roll amplitude and roll phase relative to the heave motion matter for triggering the instability.

Topics: Resonance , Vessels
Commentary by Dr. Valentin Fuster
2017;():V07AT06A013. doi:10.1115/OMAE2017-62151.

A fast method for optimizing the configuration of a di-hull system is to take advantage of the wave-cut signatures of each hull and evaluate the combined resistance of the hull system using analytical expressions that portray the interference effects of the hull-generated waves. This interference formula is available in Yeung et al. [1] and can be used in conjunction with the wave-cut signatures. The Longitudinal Wave-cut Method (LCM) is utilized to acquire the wave-making spectrum for each monohull. Then the di-hull interference wave resistance is deduced by substituting these experimentally-acquired information into analytical expressions for resistance computation. The pre-acquired wave-spectrum information can be stored and used for a combination of any component hulls, identical or not. This hybridization procedure of theory and experiments is tested and evaluated. Its merits and deficiencies are discussed.

Topics: Waves , Hull
Commentary by Dr. Valentin Fuster
2017;():V07AT06A014. doi:10.1115/OMAE2017-62480.

Numerical simulations are carried out for gap resonance problem between two side-by-side non-identical boxes. The linear potential model over-predicts the resonant amplitude in the narrow gap because it not only neglects the energy dissipation due to vortical motion, but also neglect the nonlinearity due to free surface. More relative energy are reflected with the increase of incident wave amplitude, leading to the decrease of relative resonant amplitude and relative energy dissipation in the narrow gap at resonant frequency. When the incident wave frequency is outside a little band to resonant frequency, relative energy dissipation becomes the dominant factor for the decrease of relative wave amplitude in the narrow gap with the increase of incident wave amplitudes. In a word, both the free surface nonlinearity and fluid viscosity play the important, but different, roles on wave resonances in the narrow gap.

Topics: Resonance , Fluids , Viscosity
Commentary by Dr. Valentin Fuster
2017;():V07AT06A015. doi:10.1115/OMAE2017-62490.

Parametric rolling of a post-Panamax C11 class containership in regular and irregular waves is numerically investigated using body nonlinear time domain methods based on strip theory. The Froude-Krylov and the hydrostatic forces are calculated for the exact wetted surface area under the undisturbed incident wave profile. Two kinds of formulations are used for calculation of the radiation forces. The first one employs a linear radiation force in which the frequency dependent hydrodynamic coefficients are calculated for mean position of the sections at mean water level. The second formulation calculates the hydrodynamic coefficients for the exact submerged depth of ship sections under the undisturbed incident wave profile, and hence called as body nonlinear radiation force. The numerical results from the aforementioned formulations are compared with each other, and also with experimental results obtained from a wave tank in both regular and irregular waves. For all the cases in regular waves, the vulnerability to parametric rolling is clearly identified by the numerical models, even though a few discrepancies are observed in the estimation of the severity (maximum roll angle) of the problem. In this paper, the effects of the linear and body nonlinear radiation forces on the numerical calculation of parametric rolling of a container ship and the ability of the numerical methods to identify parametric rolling are investigated.

Topics: Containers , Waves , Ships
Commentary by Dr. Valentin Fuster
2017;():V07AT06A016. doi:10.1115/OMAE2017-62613.

Dynamic vertical bending moments are determined for a military vessel hull in still water and under head waves, with a weakly nonlinear method. The domain for hydrostatic and undisturbed pressures integration is time-variant and generated with a quad-tree adaptive mesh algorithm, on which exact formulations for pressure on polygonal elements are used. Linear radiation and diffraction pressures, on another mesh superimposed with the aforementioned, are calculated with a frequency domain code. Results are compared with published experimental ones for small and large wave heights.

Topics: Pressure , Stress , Ships
Commentary by Dr. Valentin Fuster

Ocean Engineering: Advanced Underwater Vehicles and Design Technology

2017;():V07AT06A017. doi:10.1115/OMAE2017-61066.

In this work, we numerically studied the steady swimming of a pufferfish driven by the undulating motion of its dorsal, anal and caudal fins. The simulations are based on experimentally measured kinematics. To model the self-propelled fish swimming, a Computational Fluid Dynamics (CFD) tool was coupled with a Multi-Body-Dynamics (MBD) technique.

It is widely accepted that deformable/flexible or undulating fins are better than rigid fins in terms of propulsion efficiency. To elucidate the underlying mechanism, we established an undulating fins model based on the kinematics of live fish, and conducted a simulation under the same operating conditions as rigid fins. The results presented here agree with this view by showing that the contribution of undulating fins to propulsion efficiency is significantly larger than that of rigid fins.

Topics: Fins
Commentary by Dr. Valentin Fuster
2017;():V07AT06A018. doi:10.1115/OMAE2017-61124.

In this study, the authors developed the dynamic routing algorithm combining an image detection technique to support the optimal route plan of Autonomous Underwater Vehicle (AUV) inspecting an offshore wind farm affected by ocean currents. A modular structure is applied to program design by the graphical language, LabVIEW (Laboratory Virtual Instrument Engineering Workbench). The modular structure is composed of 6-DOF (Six Degrees-of-Freedom) motion module, a self-tuning fuzzy control module, a stereo-vision detection module, and a dynamic routing module. In terms of path planning for inspection, several Pareto frontiers are solved iteratively according to two objectives, namely, cruise time and energy consumption. Performances obtained from MOPSO (Multi-Objective Particle Swarm Optimization) -based dynamic routing algorithm would be in comparison with those from SOPSO (Single-Objective Particle Swarm Optimization) -based dynamic routing algorithm. In addition, selections of fixed weight and dynamic weight of MOPSO-based dynamic routing algorithms would be discussed in the environment with or without ocean currents. Eventually, the image inspection mode is not only beneficial for optimizing feasible routes but it can also identify features of obstacles for positioning.

Commentary by Dr. Valentin Fuster
2017;():V07AT06A019. doi:10.1115/OMAE2017-61170.

This paper presents 3D path tracking simulations and path-following by experimental testing with a small inspection class ROV. The ROV is underactuated, but moves freely in surge, heave and yaw (4 DOF). Paths used are a spiral for path-tracking and a lawnmower pattern for path-following, where both paths are related to a proposed structural setting and a mission. The structure is a cylindrical fishing cage used for fish farming in the aquaculture, and the task is to perform inspection with equipped cameras and sensors while tracking or following the path.

Commentary by Dr. Valentin Fuster
2017;():V07AT06A020. doi:10.1115/OMAE2017-61461.

This paper presents a method for three dimensional dynamic analysis of a complete deep-water Remotely Operated Vehicle (ROV) system. The presented formulation utilizes Euler-Bernoulli beam theory to represent the cable and the Tether Management System (TMS) Module is represented as a rigid body moving in 6 degrees of freedom. The model is applied to a real ROV-system and compared to experimental data collected from deep-water ROV operations. The ability to estimate TMS position, velocity and acceleration (state estimation) is the key feature of this analysis. This is because acoustic positioning may not always be available hence other means of state estimation is needed to ensure safe and efficient operations. Two different ROV-deployments are simulated and show good correspondence with the experimental data, limitations in acoustic positioning is also shown on the basis of signal-loss. Based on simple current estimates and measured surface-ship position, the relative position of the TMS can be estimated with fairly good accuracy. The ability to simulate the ROV system in real time is also an important feature making the procedure feasible to incorporate in controller design. The cable forces are estimated and show that horizontal ship motions are of minor importance with regards to cable tension.

Commentary by Dr. Valentin Fuster
2017;():V07AT06A021. doi:10.1115/OMAE2017-61651.

Floating bodies such as oil rig/production platform and wind turbine in ocean need to be fixed or controlled at expected position by its supporting system which includes tension tendon and catenary mooring-line. Recently, the later one, catenary mooring-line, is increasingly used in deeper water due to its lower cost and easier installment. As the floating platform are developed toward deeper water depth, the length of the mooring-line become larger and consequently the dynamic behaviors such as the structural inertia and hydrodynamic inertia/damping of the mooring-line become more obvious.

In this paper, the dynamic behaviors of the mooring-line are considered, and compared with the traditional quasi-static method where only the static restoring force is involved, so as to comprehensively examine the non-linearly restoring performance of catenary mooring-lines. Firstly, the nonlinear dynamic model of the mooring system is developed based on our 3d dynamic catenary equations along with the modified finite element simulations. Compared with the static restoring force, essentially depending on structural gravity and overall shape based on static catenary theory, the dynamic restoring force is analyzed based on our 3d curved flexible beam approach where the structural curvature changes with its spatial position and time in terms of vector equations. In our modified finite element simulations, the rotation degree of freedom between neighboring beam elements is released and bending stiffness of individual element is set to be zero, and the statically original shape and top tension according to the traditional static catenary theory are used as the initial conditions. Moreover, the hydrodynamic force is loaded as depending on structural motion.

Based on our numerical simulations, the influences of the amplitude and frequency of the catenary’s top-end motion, along with the structural parameters (including the mass density and initial tension ratio), on mooring line’s temporal-spatial evolution of displacement and dynamic tension are studied. Also, the slack-taut phenomenon caused by structural /hydrodynamic inertia and damping are presented. Our results show: 1) Generally, the displacement distribution along the mooring-line is characterized as a stable stand wave. The additional part of restoring tension due to the dynamic effects is up to 20% of the quasi-static method, and the tension amplitude difference (between the maximum tension and minimum tension) is around three times of the quasi-static value. Particularly, as the mooring-line becomes slack, the response is characterized as travelling wave, the maximum tension amplitude is up to 9 times of the static method. 2) As the amplitude/frequency of the catenary’s top-end motion increases, the value of catenary displacement firstly drops and then rises. The displacement distribution along catenary length changes with the motion of top end. Interestingly, the maximum displacements occur at the middle point of the catenary for case of surge while the maximum displacement moves up along the catenary as the top end motion gets larger for case of heave. 3) The magnification factor of top tension drops with increase of mooring-line mass density but rises with the increase of the initial tension ratio. It is also noted the velocity amplitude at higher frequency in the velocity spectrum may increase as the top end motion increases.

Topics: Mooring
Commentary by Dr. Valentin Fuster
2017;():V07AT06A022. doi:10.1115/OMAE2017-61742.

This paper presents a filter for underwater positioning in an aquaculture environment with demanding weather conditions. The positioning system is based on acoustic transponders mounted at a net pen on the sea surface. The transponders are exposed to oscillations due to wave disturbance. This will have an impact on the accuracy of the positioning system. An extended Kalman filter (EKF) solution has been proposed including a wave motion model integrated with the pseudo-range measurements from the transponders. Simulations show that the proposed filter compensates well for the disturbances.

Topics: Wave motion
Commentary by Dr. Valentin Fuster
2017;():V07AT06A023. doi:10.1115/OMAE2017-61773.

A vision based underwater localization system using fiducial markers is proposed. The system is implemented as a new control mode in a shared control system for ROVs and tested experimentally in a pool. A high accuracy underwater motion capture system in the pool is used as ground truth for performance assessment of the proposed system. The main objective has been to assess the feasibility for using a vision system and fiducial markers for localization of an underwater vehicle performing automated manipulation at subsea facilities. This research show that it is feasible to perform autonomous manipulation using the proposed system. The assessment of the performance in a set of experiments show that all degrees of freedom have a an approximate Gaussian error distribution with zero mean. The largest position errors are experienced in the Y-direction. This is seem to be caused by inherent coupling between yaw and lateral position in the camera projection of the fiducial marker and errors in the yaw direction. The main contribution of this paper is an experimental performance assessment of a localization method commonly used for terrestrial robots in a marine environment.

Commentary by Dr. Valentin Fuster
2017;():V07AT06A024. doi:10.1115/OMAE2017-62056.

The present study is focused on performance issues of underwater vehicles near the free surface and gives insight into the analysis of a speed loss in regular deep water waves. Predictions of the speed loss are based on the evaluation of the total resistance and effective power in calm water and preselected regular wave fields w.r.t. the non-dimensional wave to body length ratio. It has been assumed that the water is sufficiently deep and that the vehicle is operating in a range of small to moderate Froude numbers by moving forward on a straight-line course with a defined encounter angle of incident regular waves.

A modified version of the Doctors & Days [1] method as presented in Skejic and Jullumstrø [2] is used for the determination of the total resistance and consequently the effective power. In particular, the wave-making resistance is estimated by using different approaches covering simplified methods, i.e. Michell’s thin ship theory with the inclusion of viscosity effects Tuck [3] and Lazauskas [4] as well as boundary element methods, i.e. 3D Rankine source calculations according to Hess and Smith [5]. These methods are based on the linear potential fluid flow and are compared to fully viscous finite volume methods for selected geometries.

The wave resistance models are verified and validated by published data of a prolate spheroid and one appropriate axisymmetric submarine model. Added resistance in regular deep water waves is obtained through evaluation of the surge mean second-order wave load. For this purpose, two different theoretical models based on potential flow theory are used: Loukakis and Sclavounos [6] and Salvesen et. al. [7]. The considered theories cover the whole range of important wavelengths for an underwater vehicle advancing in close proximity to the free surface. Comparisons between the outlined wave load theories and available theoretical and experimental data were carried out for a submerged submarine and a horizontal cylinder.

Finally, the effective power and speed loss are discussed from a submarine operational point of view where the mentioned parameters directly influence mission requirements in a seaway.

All presented results are carried out from the perspective of accuracy and efficiency within common engineering practice. By concluding current investigations in regular waves an outlook will be drawn to the application of advancing underwater vehicles in more realistic sea conditions.

Commentary by Dr. Valentin Fuster

Ocean Engineering: Coastal Engineering

2017;():V07AT06A025. doi:10.1115/OMAE2017-61010.

Building design codes in the US do not include any consideration of tsunami design, even though past tsunamis have caused significant structural damage in coastal communities. In February 2011 the American Society of Civil Engineers (ASCE) formed a new Tsunami Loads and Effects subcommittee, which spent four years to develop a new chapter for inclusion in the ASCE7-16 Standard, Minimum Design Loads and Associated Criteria for Buildings and Other Structures. This new chapter has now been approved by the ASCE7 Main Committee and ASCE7-16 has been published with a new Chapter 6, Tsunami Loads and Effects. In December 2016, ASCE 7–16 was officially adopted by the International Code Council, with the new chapter on Tsunami Loads and Effects, for inclusion in the US model code, IBC 2018. The tsunami design provisions will apply to all coastal communities in California, Oregon, Washington State, Alaska and Hawaii. This paper presents an overview of the new ASCE7-16 Tsunami Loads and Effects design provisions and how they were developed based on field survey observations and laboratory experimentation.

Topics: Design , Tsunamis
Commentary by Dr. Valentin Fuster
2017;():V07AT06A026. doi:10.1115/OMAE2017-61206.

This study evaluates the influence of coastal structures on coastal morphology near Hsin-Chu fishery harbor in the northwest of Taiwan. As a result, the downdrift side has undergone local erosion due to the longshore sediment was impounded at updrift breakwaters resulting the enhancement of coastal flood risks as well as deterioration of the biological environment. Process and trend analyses were used to assess the effects the coastal exploitation made on the coastal morphology. Environmentally-friendly remedial measure, beach nourishment is proposed. The sediment source supply and the location of beach nourishment were also suggested for achieving the goal of sustainable use in the area.

Commentary by Dr. Valentin Fuster
2017;():V07AT06A027. doi:10.1115/OMAE2017-61354.

Coastal geomorphology is a complex phenomenon which is governed by nearshore wave and tidal climate. Change in climate indices (like sea surface temperature, sea level, intensified cyclone activity, among others) and climate modes (like El Nino Southern Oscillation (ENSO), Southern Annular Mode (SAM), Indian Ocean Dipole (IOD)) affect the wave climate and modify many coastal processes thereby altering the geomorphology of shorelines. In countries like India where tropical and sub-tropical cyclones are common, the coastal geomorphology is under constant threat. Coasts are also vulnerable to anthropogenic factors like offshore structures, harbours, wave farms and other constructional activities along the shoreline. It is thus necessary to understand the evolution of coastlines under the changing climate scenario. The rapidly growing socio-economic development in south-west coast of India has generated the need to investigate the longshore sediment transport (LST) regime in this region under the influence of variable climate factors like the wave characteristics. The presence of numerous river deltas, estuaries and mud banks makes the situation worse especially during the south-west monsoon season (June-September). The investigation on the contemporary evolution of this coastline has not been undertaken and the knowledge of the climate factors that influence the shorelines of the southern tip of India are unknown. This study attempts to understand the temporal dynamics of the longshore sediment transport in this region.

Topics: Climate , Shorelines
Commentary by Dr. Valentin Fuster
2017;():V07AT06A028. doi:10.1115/OMAE2017-61356.

Strong wind often raises waves and result in the sediment siltation. The navigation of outside waterway of Huanghua Port is seriously influenced by sudden siltation induced by strong wind. Before outside waterway treatment, the return sudden siltation volumes must be determined previously. The point estimation of return values can be obtained by distribution curve fitting, meanwhile, interval estimation can provide ranges of return values under some fix confidence levels. Based on the sequences of annual maximum sudden siltation of Huanghua Port from 1979 to 2003, and by using curve fittings of lognormal distribution and maximum entropy distribution, and maximum likelihood interval estimation method, the point and interval estimations of return sudden siltation volumes are presented. The results given by the maximum entropy distribution have higher precision, so they can be applied as the reference of the treatment of sediment accumulation.

Commentary by Dr. Valentin Fuster
2017;():V07AT06A029. doi:10.1115/OMAE2017-61381.

This paper focuses on design aspects regarding breakwaters for cold climate terminals, in particular, the different types of berm breakwaters, their stability, armour mobility criteria and armour size.

A methodology is analyzed in order to determine the mean weight of the heaviest armour class as a function of wave parameters such as the significant wave height and the mean wave period, both for non-reshaping and reshaping stable berm breakwaters. The influence of the wave period on the stone mass required will be of special importance in the discussion. This methodology will enable us to determine the required median armour weight for a specific wave parameter, being easily able to compare the feasibility of different concepts or availability of the required stone size at the location.

As a case study, the breakwater selection for the Melkøya terminal in the Norwegian Barents Sea, is assessed through a comparison of the necessary armour unit masses for the different berm breakwaters. The armour mobility criteria currently established is reviewed and a recommendation for an updated criterion for the statically stable non-reshaped berm breakwater category is proposed.

Topics: Breakwaters , Design , Climate
Commentary by Dr. Valentin Fuster
2017;():V07AT06A030. doi:10.1115/OMAE2017-62283.

The characteristics of wave breaking over a fringing reef are considered using a set of laboratory experiments and the results are used to develop associated predictive models. Various methods are typically used to estimate the characteristics of nearshore wave breaking, mostly based on empirical, analytical and numerical techniques. Deo et al. (2003) used an artificial neural network approach to predict the breaking wave height and breaking depth for waves transforming over a range of simply sloped bottoms. The approach is based on using available representative data to train appropriate neural network models. The Deo et al. (2003) approach is extended here to predict other characteristics of wave breaking, including the type of wave breaking, and the position of breaking over a fringing reef, as well as the associated wave setup, and the rate of dissipation of wave energy, based on observations from a series of laboratory experiments involving monochromatic waves impacting on an idealized reef. Yao et al. (2013) showed that for such geometry, the critical parameter is the ratio of deep-water wave height to the depth of the shallow reef flat downstream of the position of wave breaking, H1/hs, rather than the slope of the reef. H1/hs, and the wave frequency parameter, Display FormulafH1/g, are provided as inputs to the neural network models of the feed-forward type that are developed to predict the above characteristics of wave breaking. The models are trained using the experimental data. The breaker type classification model has a success rate of over 95%, implying that the neural networks method outperforms previously used criteria for classifying breaker types. The numeric prediction model for the dimensionless position of wave breaking also performs well, with a high degree of correlation between the predicted and actual positions of wave breaking. The performance is higher when only the plunging breaker instances are considered, but lower when only the spilling breaker instances are considered. The corresponding neural network models for wave setup within the surf zone and the difference in energy flux between the incident and broken wave have success rates of approximately 89% and 94% respectively. The method may be extended to provide predictive models for consideration of a range of natural coastal conditions, random waves, and various bottom profiles and complex geometry, based on training and testing of the models using representative field and laboratory observational data, in support of accurate prediction of near-shore wave phenomena.

Commentary by Dr. Valentin Fuster
2017;():V07AT06A031. doi:10.1115/OMAE2017-62325.

Based on good simulation results during storm events in 2009, MIKE21 was used to study the extreme water level, current velocity and wave height in Laizhou Bay, China. 95 extreme weather processes during 1988–2012 were simulated. For each event, coupled hydrodynamic and wave modules of MIKE21 was chosen to calculate the maximum water level and current velocity. The Gumbel distribution method, commonly used for estimating return-period values of marine hydrodynamic variables, is adopted in this study. The extreme high water level of 50-year return period in Laizhou Bay can reach 2.6–3.8 m; and that of 100-year return period can be as high as 2.8–4.6 m. The 50-year and 100-year return-period values of current velocity can reach up to about 2.8 m/s and 3.2 m/s respectively, both around the Yellow River mouth. Wave height strongly depends on water depth, water level rise, wind speed and direction. The results provide parameter reference for structure design in the Laizhou Bay.

Topics: Waves , China , Water
Commentary by Dr. Valentin Fuster
2017;():V07AT06A032. doi:10.1115/OMAE2017-62341.

Consolidation is one of the most important processes that are needed to be accounted for in the model of cohesive sediment transport. Previous studies have shown that the consolidation paths for soil elements are not unique at low effective stress (here low effective stress means the effective stress is smaller than a threshold effective stress, which is short as TES and denoted as σb) In this study, a new theoretical constitutive equation for the effective stress of soft mud is derived to describe the non-unique consolidation paths at low effective stress.

Firstly, based on the concept of fractal dimension, the mud flocs are treated as self-similar fractal entities and the relationship between mud floc size and sediment volume fraction is established. Due to the fact that the fractal dimension decreases as the floc size increases, the variation of fractal dimension is accounted for. Based on the self-similar model, the theoretical constitutive equation for the effective stress of soft mud is derived.

The constitutive equation is validated using two data sets obtained from consolidation tests in a settling column for soft clay. At low effective stress, the model results curves follow the data trends and the non-uniqueness of consolidation paths are captured. The model results are also compared with existing numerical results and better agreement is presented especially when η is less than 0.5.

Commentary by Dr. Valentin Fuster
2017;():V07AT06A033. doi:10.1115/OMAE2017-62363.

In this paper, a fully nonlinear Boussinesq model is used to simulate the shoreward propagation of bichromatic wave groups over different fringing reef topographies and the subsequent low-frequency oscillations inside a harbor. Based on a low-frequency wave separation technique, the effects of the reef-face slope and the reef ridge on the bound and free long waves inside the harbor and their relative components under the condition of the lowest resonant mode are systematically investigated. For the given harbor, the given reef ridge and the range of the incident short wave amplitudes and the reef-face slopes studied in this paper, results show that the amplitude of the free long waves inside the harbor increases with the reefface slope, while the bound long waves inside the harbor is insensitive to the variation of the reef-face slope. The existence of the reef ridge can notably restrain the bound long waves inside the harbor when the incident short wave amplitudes are large, while it has little influence on the free long waves inside the harbor.

Commentary by Dr. Valentin Fuster
2017;():V07AT06A034. doi:10.1115/OMAE2017-62476.

A stochastic approach calculating the random wave-induced burial and scour depth of short cylinders and truncated cones on mild slopes is provided. It assumes the waves to be a stationary narrow-band random process and a wave height distribution for mild slopes is adopted, also using formulae for the burial and scour depths for regular waves on horizontal beds for short cylinders and for truncated cones. Examples of results are also provided.

Topics: Waves , Cylinders
Commentary by Dr. Valentin Fuster
2017;():V07AT06A035. doi:10.1115/OMAE2017-62543.

Harbours are important infrastructures for an offshore production chain. These harbours are protected from the actions of sea by breakwaters to ensure safe loading, unloading of vessels and also to protect the infrastructure. One of the important hydrodynamic processes in these regions is the interaction of water waves with permeable breakwaters such as rubble mound breakwaters or berm breakwaters. It is important to study the wave-breakwater interactions in order to have an optimal design of these structures. In current literature, research regarding the design of these structures is majorly based on physical model tests. Empirical formulations are derived based on these test, which can have a relatively narrow range of applicability. In this study a new tool, a three-dimensional numerical model is introduced. Physical and numerical models have limitations that can restrict their independent use. A combined use of both can lead to different forms of improvements: being able to model problems that cannot be modelled by either physical or numerical modelling alone; increasing quality at the same cost or obtaining the same quality at reduced cost.

In this study, the open-source CFD model REEF3D is used to study the design of berm breakwaters. The model uses the Volume averaged Reynolds Averaged Navier-Stokes (VRANS) equations to solve the porous flows. At first the VRANS approach in REEF3D is validated for flow through porous media. A dam break case is simulated for two different porous materials. Comparisons are made for the free surface both inside and outside the porous medium. The numerical model REEF3D is applied to show how to extend the database obtained with purely numerical results, simulating different structural alternatives for the berm in a berm breakwater. Different simulations are conducted with varying berm geometry. The influence of the berm geometry on the pore pressure and velocities are studied. The resulting optimal berm geometry is compared to the geometry according to empirical formulations.

Commentary by Dr. Valentin Fuster
2017;():V07AT06A036. doi:10.1115/OMAE2017-62622.

Salient and tombolo are common features found in the lee of detached breakwaters. The empirical parabolic bay shape equation (PBSE) can be applied when their planform is fully developed, whereas numerical model is required to simulate the dynamic shoreline evolution prior to the planform reaching static equilibrium. This paper reports the excellent performance of PBSE through the comparison with labaratory results and the development of a numerical model for dynamic shoreline change that utilizes the concept of PBSE and equilibrium beach profile. Formulation proposed for sediment transport rate is theoretically compared with that in GENESIS. The governing equation for the combined shoreline response model is based on the one-line beach model, which includes shoreline changes owing to longshore and cross-shore sediment transport. Finally, numerical results reveal, by comparing with an experimental case in the laboratory, that the model is adequate to successively simulating the dynamic evolutions of the shoreline behind a detached breakwater.

Commentary by Dr. Valentin Fuster
2017;():V07AT06A037. doi:10.1115/OMAE2017-62634.

Images of the 2004 Indian Ocean tsunami at landfall shows a leading edge marked by short waves (“fission” waves). These waves appear to be cnoidal in shape and of a temporal and spatial scale in line with the longest swell present in the region, and may interact with the longer waves in the background random wave spectrum. As part of a comprehensive series of experiments, the Large Wave Flume at Oregon State University (USA) was used to generate and measure the properties of cnoidal, random, and combined cnoidal-random wave trains. Both the nonlinear energy transfer characteristics (via bispectral analysis) and dissipation characteristics (via a proxy dissipation function) are studied for all generated wave conditions. It is generally determined that the characteristics of the cnoidal wave dominate the combined cnoidal-random wave signals if the energy of the cnoidal wave is at least equal to that of the random wave.

Topics: Waves
Commentary by Dr. Valentin Fuster

Ocean Engineering: Computational Mechanics and Design Applications

2017;():V07AT06A038. doi:10.1115/OMAE2017-61164.

In this work, a co-simulation case study of a marine offshore surface vessel in Dynamic Positioning (DP) operation, where the DP-controller is placed on an Arduino® micro-controller, is presented. The reasons for using co-simulation are that it is possible to distribute the model among different cores in one computer as well as among different computing members over a local area network. Also, it is possible to export submodels from different software and connect them together in a common simulation. This enables the use of suited modeling software for different types of dynamical systems, as well as hardware, such as micro-controllers for Hardware-In-the-Loop testing. Such an integrated and open simulation method facilitates the development of new products as well as shortening the iterative process in design phases. As for co-simulation standard, the Functional Mock-up Interface (FMI) for co-simulation will be used in this work, and a communication Functional Mock-up Unit (FMU) that communicates with hardware and handles the signal flow between the hardware and the co-simulation will be developed. In the case study, a DP-controller is implemented on the microcontroller and connected to a filter, a position reference system and an offshore vessel model, all implemented as FMUs in the total co-simulation. For simulation master algorithm, the open source software “Coral”, that was developed in the knowledge building project “Virtual Prototyping of Maritime Systems and Operations” (ViProMa), will be used. The simulation results show that even though the micro-controller is set to communicate with a lower frequency than the rest of the co-simulation submodels, the total c-simulation is stable and produces good results. It also show that the FMI standard facilitates hardware in the loop in the co-simulation, and that the co-simulation master algorithm Coral is suited for such simulation cases.

Commentary by Dr. Valentin Fuster
2017;():V07AT06A039. doi:10.1115/OMAE2017-61625.

Diesel electric propulsion has become industry standard for many marine applications. Typically, a significant part of the operations of vessels with diesel electric propulsion is done with low loads on the motors and generators. However, the efficiency of a drive train is typically only calculated for full load conditions. This underestimates the losses during low load conditions. This article presents modeling methods for the electric drive train, which can be used to estimate the efficiencies, also at low load. The models are established with limited parameter sets, as detailed information about of the components are seldom available. This article compares the estimated efficiency of the generator and motor with the given data from datasheets.

Commentary by Dr. Valentin Fuster
2017;():V07AT06A040. doi:10.1115/OMAE2017-61641.

Numerical simulations of wake flow generated by a surface-mounted square cylinder over a subsequent square cylinder are presented for incompressible low-Re turbulent flow. The behaviors of the square cylinder in the wake flow are investigated. A developed numerical method — Semi-Implicit Method for Pressure-Linked Equations (SIMPLE) is adopted to solve the governing equations in steady state. Turbulence effect is modelled by Reynolds Averaged Navier Stokes (RANS) equations with a standard k-ω turbulence model. The k-ω SIMPLE method is validated by comparison with experimental data. The results show that in wake flow, the square cylinder produces a much weaker wake effect and is subjected to an inverse drag force.

Commentary by Dr. Valentin Fuster
2017;():V07AT06A041. doi:10.1115/OMAE2017-61918.

Dynamic Positioning (DP) capability studies are used to assess if a vessel has sufficient thrust capacity to withstand environmental loads while keeping its position and orientation at a specified set-point or path. These studies are usually performed on ships and other DP-controlled surface vessels; consequently, standards and procedures for these are widely known. In this work, a methodology for conducting a DP capability study for Remotely Operated Vehicles (ROV) is presented. Due to the nature of ROV operations, a DP capability study should include different features that are not common to surface vessels. In this case, an ROV connected to a surface vessel through a tether is considered. During operation, the tether is subject to varying current loads that are accumulated along the water column and transferred to the vehicle. Therefore, the ROVs thrusters must be able to withstand, in addition to its own drag, three-dimensional loads due to three-dimensional currents and umbilical-related loads. To illustrate the methodology, two case studies are considered: the DP capability of an ROV that has to operate in the Colombian Caribbean and an existing ROV operating in the North Sea.

Commentary by Dr. Valentin Fuster
2017;():V07AT06A042. doi:10.1115/OMAE2017-62010.

This paper will present the derivation of dynamic equations describing the motion of a knuckleboom crane for marine vessels. The dynamics will be derived using Lagrange’s equation, the theory of virtual work and generalized forces. Simulations of the unforced and forced system will be carried out to verify the crane behaviour. The work is part of setting up a crane lab at the Norwegian University of Science and Technology, NTNU. The rig will be used as an experimental setup available for both students and researchers and is expected to create a solid foundation for further work regarding crane control. Examples of its application are research on heave compensation and soft landing. However, before the setup can be finished a dynamical model governing the system must be derived in order to construct simple crane controllers. As part of the modeling a step by step method for describing the crane motions will be presented. Using rotation matrices and moving reference frames the position of each point on the crane will be described in an inertial reference frame. An implementation of the model in Matlab’s Simulink will also be shown, along with some simple simulations verifying system behaviour. The main result will be a state space formulation of the crane dynamics, a Simulink model for anticipating crane behaviour and a discussion of some simple simulation scenarios.

Topics: Cranes
Commentary by Dr. Valentin Fuster
2017;():V07AT06A043. doi:10.1115/OMAE2017-62356.

Piping stress analysis is performed by the manipulations of support type, location and pipe arrangement based on many specific design criteria. A classical way to find good engineering solutions satisfying design criteria among lots of combinations is obviously time-consuming work. In field practice, it also highly depends on engineer’s experiences and abilities.

This paper proposes a hybrid method by combining several global search optimization algorithms and predication model generation in order to automatically control the combinations of support types as the engineering solutions. Here, we use some efficient and popular algorithms such as genetic algorithm, swarm intelligence and Gaussian pattern search to develop initial design of experiments. From the set of the initials, we build and update a prediction model by applying machine learning algorithm such as artificial neural network. As a result of using the hybrid method, the engineering solution is sufficiently optimized for the classical solution.

Design variables for this problem are the types of restraints (or the pipe support type). The nonlinearity conditions such as gaps and frictions are also treated as key design variables. Each restraint is initially identified as a binary set of design variables, and transformed to integer numbers to run on the n-dimensional design space. The number of dimension corresponds to the number of pipe supports. Currently, pipe stress analysis problems are divided into a certain size that is enough to run on one computer for project management purpose. If we have bigger system with more design variables to consider, the hybrid machine learning method plays a key role in saving computation time with the help of additional parallel computation technique.

Commentary by Dr. Valentin Fuster
2017;():V07AT06A044. doi:10.1115/OMAE2017-62485.

This study presents the performance investigation of a rudder bulb-turbine device (RBTD), which is designed to recover the rotational energy into torque to drive a generator inside the rudder. The blade of turbine is designed first by using lifting line theory, which is modified by lifting surface theory. The induced velocities between the forward propeller and turbine are obtained through the surface panel method (SPM). Hydrodynamic performance for the system is predicted by using SPM. An iterative calculation method is used until the hydrodynamic performance of the system converges. Through the design of turbine behind propeller of a large container ship, the influence of rotational speed of turbine on hydrodynamic performance of propeller-turbine system is observed. Then the size of rudder bulb is determined by calculating the influence of rudder bulb geometry on the hydrodynamic performance of the propeller-rudder system. Based on CFD technology, the hydrodynamic performance of the propeller-rudder system without/with RBTD is also simulated, and velocity distribution is observed. The results show that RBTD is successfully designed and as a ship energy saving device is feasible.

Topics: Design , Turbines
Commentary by Dr. Valentin Fuster

Ocean Engineering: Fluid-Structure, Multi-Body and Wave-Body Interaction

2017;():V07AT06A045. doi:10.1115/OMAE2017-61054.

The present paper focuses on modeling of green water loading on an oscillating body using CFD. The vessel motion is calculated a priory using time domain panel method code, then green water impact is computed based on that pre-calculated motion. The finite volume method is used to capture the green water impact, however in order to represent the free surface, volume of fluid method is used. A sophisticated dynamic mesh is used to handle the motion of the vessel in fluid domain. Several examples and case studies are considered to validate the present CFD model as well as to check the effect of global motion on green water loading such as effect of steepness and heading angle on green water impact. Results show that due to the motion, the impact loading phenomena changes significantly and there is a significant change in pressure on the deck after considering the motion effect and it deviates considerably with the results obtained from fixed vessel cases.

Topics: Vessels , Water
Commentary by Dr. Valentin Fuster
2017;():V07AT06A046. doi:10.1115/OMAE2017-61136.

It is important to predict wave impact and structural responses of ship and offshore structures in extreme sea conditions. There are advantages to apply Lagrangian particle methods to simulate highly nonlinear breaking free surface flow and fluid-structure interactions (FSI). In this paper, an improved moving particle semi-implicit (MPS) method was developed to solve the FSI problems. At each time step, the fluid motions and the structural responses are solved. For flow computations, a modified mixed source term method and an improved free surface identification method were adopted to suppress pressure oscillations. Moreover, a particle collision model was used to enhance the numerical stability and avoid nonphysical solutions. The discretized Poisson equations for pressure were solved by a parallel version of the bi-conjugate gradient stabilized method based on the message passing interface (MPI) approach. For structural responses, solids were treated as isotropic elastic particles. Validation studies were carried out for cases of 2D dam breaking and its interaction with a rubber gate. The numerical solutions are in good agreement with experimental data and other published numerical results.

Commentary by Dr. Valentin Fuster
2017;():V07AT06A047. doi:10.1115/OMAE2017-61235.

Experiments have been carried out with a circular cylinder fitted with four different models of flexible shrouds with variable values of structural damping. Shroud models were derived from the geometry of the suppressor called “ventilated trousers”. VIV responses are presented in a range of reduced velocities from 2 to 15 and Reynolds number from 2,000 to 25,000. Influence of structural and hydrodynamic damping is considered and, since the meshes generate more hydrodynamic damping than a bare cylinder, this effect on VIV suppression is analyzed. With the minimal structural damping, the VT mesh reduced about 55% of the amplitude peak of bare cylinder, while the thick-sparse, thin-sparse and thin-dense meshes reduced about 50%, 45% and 50%, respectively. For the case with similar levels of total damping (structural plus hydrodynamic) the same models reduced about 50%, 25%, 40% and 50%, respectively. The results show that the effect of increasing the structural damping has an important role in the VIV suppression of the thick-sparse mesh. For the other models, this effect is less significant.

Commentary by Dr. Valentin Fuster
2017;():V07AT06A048. doi:10.1115/OMAE2017-61288.

In this paper two methods for modelling the damping in a narrow gap are investigated. The first method is called the Pressure Damping Model. This method has been used in studies of wave energy devices. An attractive feature of this model is that the modified input is directly related to the energy dissipation in the gap, which means that if this dissipation is estimated the input to the model can be obtained directly. The idea of the method is to add a pressure input in the gap to suppress the resonant motion. A challenge with the method is that it contains a non-linear term.

The second method is the Newtonian Cooling damping model. The method is based on introducing a dissipation term in the free surface boundary condition. This dissipation term contains a coefficient which is not directly related to the energy dissipation. Hence this method is not so easy to relate directly to the estimated energy dissipation. An advantage with this method is that it is linear and hence can be expected to be more robust.

In the first part of the paper a 2-dimensional problem is investigated using both methods. In addition to the numerical performance and robustness, much focus is put on investigation of the energy balance in the solution, and we attempt to relate both models to the energy dissipation in the gap.

In the second part the Newtonian cooling method is implemented in a 3-dimensional potential flow solver and it is shown that the method provides a robust way to handle the resonance problem. The method will give rise to a modified set of equations which are described.

Two different problems are investigated with the 3D solver. First we look at a side-by-side problem, where the 3D results are also compared with 2D results. Finally, the moonpool problem is investigated by two different 3D solvers, a classical Green’s function based method and a Rankine solver. It is also shown how this damping model can be combined with a similar model on the internal waterplane to remove irregular frequencies.

Topics: Resonance , Damping
Commentary by Dr. Valentin Fuster
2017;():V07AT06A049. doi:10.1115/OMAE2017-61319.

Simulation of marine operations for launch and recovery of bluff bodies such as autonomous underwater vehicles (AUV), remotely operated vehicles (ROV) or subsea templates is traditionally performed in calm to moderate sea conditions. The reason for doing so is partly due to the interaction between the complex dynamic response of an installation vessel, a moving bluff body and the wave kinematics of the rough sea condition. This is in addition to the need for accurate hydrodynamic coefficients that would enable proper simulation and modelling of the launch and recovery process. The key objective of the current methodology is to minimize risks of damage to the vessel and total loss of assets during the deployment and recovery process for marine operations in rough sea conditions.

The aim of this paper is to present the results of experimental and numerical investigation on the prediction of dynamic response of a bluff body during launch and recovery from a surface vessel in rough sea condition. Experimental measurements of hydrodynamic coefficients and responses of a large scale bluff body using a scaled model were completed. Further studies using a time-domain numerical tool have been undertaken to measure the response characteristic of bluff bodies in rough seas. The study also predicted the contributions of vessel motion in rough seas to the dynamic response of the bluff bodies. The results obtained have shown that simulation of launch and recovery operations in rough seas can be carried out efficiently if their hydrodynamic coefficients through the wave active regions of the rough seas are predicted and then adequately implemented in the simulations.

Commentary by Dr. Valentin Fuster
2017;():V07AT06A050. doi:10.1115/OMAE2017-61322.

The objective of this study was to develop a model for predicting spray droplet size and velocity distributions during spray cloud formation arising from a wave impact with an object. The study looked at scaling issues and developed a scaling model to relate the spray characteristics measured in a tow tank to large-scale spray formations arising from wave impact with vessels. Several phenomena related to spray scaling have been studied to develop the scaling rules in large enclosures. These are wave theories for deep water, air entrainment process during the wave impact, water sheet disintegration and droplet size distribution as well as the scaling of two-phase flow interfaces (water/air). The focus of this study was on atomization and particle motions, and the thermodynamic part of scaling was ignored. The formation of upstream droplets caused by a wave impact on the bow of a vessel is the result of sheet and droplet breakup. Scaling models related to the process of air entrainment, which is caused by the wave impact, water sheet breakup, and spray cloud formation, were investigated to implement a comprehensive scaling model. A mathematical formulation, considering the aforementioned phenomena, was developed to calculate the final average droplet diameter and maximum run-up velocity. The effects of initial wave characteristics, the geometrical characteristics of the water sheet at the moment of water impact, and a spray parameter, on the final average droplet diameter were investigated. Predictions of wave characteristics and final droplet diameter are compared with previously published field observation data.

Topics: Waves , Drops , Sprays
Commentary by Dr. Valentin Fuster
2017;():V07AT06A051. doi:10.1115/OMAE2017-61327.

This paper presents the applications of an efficient hybrid time-domain simulation model for predicting moored Sevan-floater motions in irregular waves and finite water depth. The irregular incident waves are modeled by the extended Boussinesq equations, which can capture wave-wave interactions and the low-frequency long waves accurately in finite and shallow water depth. By imposing the incident wave kinematics on the surface of the floater, a panel model based on Rankine source method is applied for the calculation of wave forces and corresponding floater motions. The contributions from low-frequency components in incident waves as well as their diffraction effects are included in the wave force calculations. Validation of the irregular waves simulated by the present numerical model are performed against experimental data. Then, the simulated moored floater motions are compared with model test results and results based on Newman’s approximation. The general good agreements with experimental results demonstrate the present model can be used as an alternative for this problem while Newman’s approximation shows non-conservative results.

Topics: Mooring , Water
Commentary by Dr. Valentin Fuster
2017;():V07AT06A052. doi:10.1115/OMAE2017-61528.

This paper is inspired by a recent numerical study (Shoele and Zhu, 2012, “Leading edge strengthening and the propulsion performance of flexible ray fins,” Journal of Fluid Mechanics, Vol. 693, pp. 402–432), which shows that, for a 2D flexible ray replicating the pectoral fins of live fish, undergoing a flapping motion in a viscous fluid, the performance can be significantly improved via the flexibility distribution on the rays. In present study, we investigate the propulsion capability of a 3D caudal fin undergoing a flapping motion. The embedded rays are modeled as linear springs and the soft membrane is modeled as a flexible plate being able to deform in span-wise direction. A finite-volume method based Navier-Stokes solver is used to solve the fluid-structure interaction problem. The present paper focuses on the effects of various distributions of the ray and the ray flexibilities, which can lead to different fin deformations. It is shown that the detailed ray distribution has significant influence on the propulsion performance. By distributing fin rays at the tips rather than the middle of fin, a less power expenditure is observed, leading to higher propulsion efficiency. However, larger thrust force is obtained through distributing the rays at the middle, which is attributed to larger effective flapping amplitude. Additionally, ray flexibilities also play a pivotal role in the thrust generation of the fin.

Topics: Propulsion
Commentary by Dr. Valentin Fuster
2017;():V07AT06A053. doi:10.1115/OMAE2017-61553.

In this study, a series of validations of a weakly one-way CFD-FEA coupling method for the estimation of the dynamic response of a container ship under severe wave condition is made. By comparing the prediction with linear/non-linear strip method, 3D panel method and towing tank test results under various wave conditions in terms of rigid body motions and structural loads, the effectiveness of the CFD-FEA coupling method is confirmed. Furthermore, the hydro-elastic behavior obtained by 3D panel method and the CFD-FEA coupling method are validated regarding the whipping moment by comparing the experiment.

Commentary by Dr. Valentin Fuster
2017;():V07AT06A054. doi:10.1115/OMAE2017-61591.

Wave and current loads on offshore structures are an important factor in the design of offshore structures. These forces are usually evaluated by semi-empirical Morison equation for tubular members wherein the ratio of characteristic dimension to wavelength is less than 0.2. In many cases, offshore structures such as jackets will have appurtenances such as anodes fitted on to them for various purposes, which may not contribute to the overall stiffness. However, these items will contribute to the wave and current loads in the order of magnitude of 10 to 20%. The calculation of hydrodynamic loads on such singular tubular members fitted with appurtenances can be done by taking to account their contribution towards drag and inertia. However, for complex structures, such as jacket structures with numerous members, it becomes practically very difficult and time consuming to do this calculation. In the industry, the general practice is to increase the overall loading due to presence of anodes by around 10 to 20%, based on experience and thumb rules.

This paper focuses on the evaluation of wave loads on jackets due to the presence of anodes on jacket legs and braces, and comparing them to that of a jacket without anodes. The evaluation of wave loads is done by both numerical modelling and theoretical evaluation. The numerical model is based on frame analysis using SACS software which has the facility to simulate the wave load on space frame structures. Three different anode-to-jacket weight ratios (total weight of anode to total weight of jacket) are considered. The anodes are modelled as per design requirements and distributed throughout the structure. Recommended hydrodynamic coefficients from codal provisions are used. The overturning moments and base shear are evaluated for design regular waves and current. Results are presented in terms of comparison of base shear and overturning moment to ensure consistency, for three different cases. The recommendations for design engineers within the depth range and region studied can be drawn from this study.

Topics: Anodes , Stress , Waves
Commentary by Dr. Valentin Fuster
2017;():V07AT06A055. doi:10.1115/OMAE2017-61682.

To study the hydrodynamic characteristics of the submersible mussel raft in waves and currents, the numerical model of the submersible raft was established based on the finite element method and kinematics theory. The finite element program Aqua-FE™ was applied to simulate the impacts of waves and currents on the hydrodynamic responses of the surface and submerged rafts, respectively. Morison Equation was applied to compute the tension of the mooring lines. Apart from the wave condition, the flow has a significant effect on the mooring line tension of the submersible raft. The submerged raft is useful for reducing the mooring loads. The submergence depth of the mussel raft can be adjusted depending on the marine environment. The results show that the submerged raft wave response was found to be reduced relative to the surface raft. The vertical motion of mussel rope connection points was significantly reduced by submergence, resulting in reduced potential for mussel drop-off. Compared the performance of the submerged raft in the same condition, the motion amplitude of the framework of the raft decreased significantly while increasing the submergence depth. At the same period, the trend of the decrease followed by levelling off with an increasing wave height. However, the submergence depth had no significant effect on the mooring line tension.

Commentary by Dr. Valentin Fuster
2017;():V07AT06A056. doi:10.1115/OMAE2017-61787.

This work deals with an experimental and numerical study on the horizontal and vertical hydrodynamic forces induced by tsunami-like waves on horizontal cylinders. The laboratory investigation has been performed in the wave flume of the University of Calabria. Twelve pressure transducers have been mounted along the external contour of a cylinder, while four wave gauges have been located close to the cylinder and an ultrasonic sensor behind the paddle to measure its displacement. Tests have been carried out for Keulegan-Carpenter numbers, KC, ranging from about 4 to 7. By the numerical viewpoint, a diffusive weakly-compressible SPH model has been adopted. To prevent spurious flows near the cylindrical contour, a packing algorithm has been applied before SPH simulations. The acoustic components occurring in the numerical pressure field have been filtered through the application of Wavelet Transform. By using different calibration methods, experimental and SPH forces and kinematics at the cylinder have been used to calculate the hydrodynamic coefficients in the Morison and transverse semi-empirical equations for engineering purposes.

Commentary by Dr. Valentin Fuster
2017;():V07AT06A057. doi:10.1115/OMAE2017-61852.

In this paper an existing time domain panel method, which was originally developed for propeller flow simulations, is extended by implementing the mixed Eulerian-Lagrangian approach for the computation of the non-linear free water surface. The three-dimensional panel method uses a constant source and doublet density distribution on each panel and a Dirichlet boundary condition to solve the velocity potential in every time step. Additionally, a formulation for the acceleration potential is included in order to determine the hydrodynamic forces accurately. The paper gives an overview on the governing equations and introduces the numerical approach. Validation results of the developed method are presented for the wave resistance of a submerged spheroid and a wigley hull. Additionally, the wave diffraction due to a surface piercing cylinder in regular waves is validated regarding the forces and the water surface elevation around the body. Here, the computations are compared with other numerical methods as well as tank test results. Apart from this, the paper deals with an application example showing simulations of an artificial service vessel catamaran in waves. The forces on the hull with and without forward speed are presented. The paper concludes with a discussion of the presented results and a brief outlook on further work.

Commentary by Dr. Valentin Fuster
2017;():V07AT06A058. doi:10.1115/OMAE2017-61867.

When floating structure with internal fluid compartment is close to other structures, the multibody interaction problem needs to be addressed in addition to the internal fluid influence. Furthermore, shallow water effects become important, especially when the gap between the floating structure and the sea floor is small. These issues are encountered when designing a novel floating oil storage facilities in nearshore area. To investigate these issues, floating models under 1:50 scale are built to perform model tests.

The test set-up uses a set of flexible constraints working as fenders placed on frames to restrain the motions of the models in the horizontal plane. Various tests in waves are carried out to measure motion responses of single model in waves with different filling levels and stiffness of “fenders”. The reaction forces on the “fenders” are also measured. Several regular wave conditions are selected to perform tests on double model system to investigate multibody interactions under the influence of internal fluid and effects of waves between the tanks. The drag forces for both single model and double models are measured by performing model tests under constant current from different directions, to check the shielding effects. The tests are performed in shallow-water wave basin, and the constant currents tests are performed by towing the models in a flume tank. Both facilities are located at National University of Singapore (NUS).

This paper presents the detailed setting of the model tests. The single model’s RAOs with 20% filling level of internal fluid are given to demonstrate the influence of internal fluid on the motions. The performances of a single tank, including six DOF motions are shown. The results will be used for validation of numerical analysis results in the near future.

Topics: Fluids , Storage tanks
Commentary by Dr. Valentin Fuster
2017;():V07AT06A059. doi:10.1115/OMAE2017-61948.

Three-dimensional numerical simulation of regular waves passing over cylindrical monopile has been conducted to investigate the vortex dynamics. To do so the rectangular wave flume and monopile is modeled on a solver, available in the open-source CFD toolkit OpenFOAM®. The solver applied RANS equations with VOF method for tracking free surface. Model validation has been done by comparison numerical results with the experimental ones and admissible agreement has been seen. Computations have been done for three cases with different pile diameters consequently for different Keulegan-Carpenter numbers (KC).

The vorticity field around the pile was investigated as well as vortices by means of Q criterion. It was seen that by increasing KC number, horseshoe vortices will be formed and vortex shedding will be happened. Moreover, Bed shear stress around the pile has been extracted and it has been seen that, the bed shear stress is influenced by KC value which result of existence of horseshoe vortices and vortex shedding.

Commentary by Dr. Valentin Fuster
2017;():V07AT06A060. doi:10.1115/OMAE2017-61966.

We consider a comprehensive mathematical and numerical strategy to couple water-wave motion with rigid ship dynamics using variational principles. We present a methodology that applies to three-dimensional potential flow water waves and ship dynamics. For simplicity, in this paper we demonstrate the method for shallow-water waves coupled to buoy motion in two dimensions, the latter being the symmetric motion of a crosssection of a ship. The novelty in the presented model is that it employs a Lagrange multiplier to impose a physical restriction on the water height under the buoy in the form of an inequality constraint. A system of evolution equations can be obtained from the model and consists of the classical shallow-water equations for shallow, incompressible and irrotational waves, and relevant equations for the dynamics of the wave-energy buoy. One of the advantages of the variational approach followed is that, when combined with symplectic integrators, it eliminates any numerical damping and preserves the discrete energy; this is confirmed in our numerical results.

Commentary by Dr. Valentin Fuster
2017;():V07AT06A061. doi:10.1115/OMAE2017-62002.

The motions of floating moored structures are affected by first order wave loads which are proportional to the wave amplitude and associated with the wave frequency. On the other hand, second order wave loads are proportional to the square of the wave amplitude and related to the sum or difference of a pair of frequencies of the irregular sea. Although the second order loads are usually smaller compared to the first order loads, these loads can excite resonance motions in frequencies that the system has very low damping. Therefore, second order wave loads have particular importance in the design of mooring systems.

The multi-body system composed by Tension Leg Platform (TLP) and Tender Assisted Drilling (TAD) is particularly susceptible to the second order effects, due to the very low natural frequencies of their horizontal modes and the very high natural frequencies of the vertical modes of the TLP.

This work presents a numerical study of second order wave loads on the TLP-TAD multi-body system. An extensive hydrodynamic analysis focus on the hydrodynamic interactions between the floaters and how these effects modify the second order loads on the platforms was performed. The second order quadratic transform functions (QTFs) were evaluated using the indirect and the direct method. Moreover, the importance of the free surface integral was checked. Finally, the accuracy of Newman approximation for the low-frequency QTF was evaluated.

Commentary by Dr. Valentin Fuster
2017;():V07AT06A062. doi:10.1115/OMAE2017-62070.

In this work, the solitary wave loads on a submerged horizontal circular cylinder are studied by comparing new analytical results with ad-hoc experimental data. The proposed analytical solution has been recently proposed by Gurnari and Filianoti (2017) and represents an extension of the formulation developed by Filianoti and Piscopo (2008) to assess the solitary wave loads acting on a submerged breakwater and tested in a wave flume on a small-scale model (Filianoti and Di Risio, 2012). Here we deal with a submerged horizontal cylinder following the concept that a solitary wave is subjected to a slowdown passing over a submerged cylinder. A laboratory investigation was addressed to calibrate the adopted semi-analytical method. The speed of the solitary wave celerity crossing the solid submerged cylinder was measured for several wave amplitudes. In the adopted wave flume at the University of Calabria equipped by a piston-type wavemaker, an horizontal cylinder with diameter D = 0.127 m was posed with its center at a depth of 0.2 m. Twelve transducers measured the instantaneous pressures along the external contour of the body. A battery of wave gauges measured the free surface elevation to evaluate the celerity crossing the equivalent water cylinder. Tests confirmed the existence of the slowing down of the celerity of the wave pressure. In other words, we found that the pressure wave is nearly double the time necessary to cross the cylinder with respect to the time necessary to cover the same distance in the undisturbed field, for solitary waves amplitudes ranging from about 0.08 and 0.19 times the water depth. The slowing down increases the horizontal wave force on the solid body with respect to the Froude-Krylov one. Moreover, it appears that in the adopted experimental range the wave force is essentially inertial if compared to the drag one, enabling us to rely upon a simplified analytical model to obtain an effective estimate of the horizontal force produced by a solitary wave on a cylinder.

Topics: Stress , Waves , Flumes , Cylinders
Commentary by Dr. Valentin Fuster
2017;():V07AT06A063. doi:10.1115/OMAE2017-62075.

The physical level of interaction between fluid and structure can be either one-way or two-way depending on the direction of information exchange at the interface of fluid and solid. The former can be solved by a partitioned approach and weak coupling. In problems involving two-way fluid-structure interaction, using a partitioned approach and strong coupling, sometimes stability restriction is encountered. This is an artificial added mass effect, which is independent of the numerical time step. Unfortunately an accurate and efficient method to deal with all the different levels of interaction is scarce. Conventionally, relaxation is applied to remedy this problem. The computational cost is directly related to number of sub-iterations between fluid and structural solver at each time step. In this study, the source of this instability is investigated. A discrete representation of a basic added mass operator is given and instability conditions are assessed. A new method is proposed to relax this restriction, the idea essentially is to remove the instability source from the structure and move it to the fluid and solve it monolithically with the fluid. We call this an interaction law. An estimate of the structural response is derived from structural mode shapes. As a test case, a 2D dam break problem interacting with an elastic vertical flexible beam is selected. The interaction of fluid with the beam undergoes several stages. The breaking waves on the beam can increase the added mass drastically, therefore the added mass ratio increases as well. In such a cases, the asset of interaction law is better elaborated, while the stability condition requires very high relaxation without interaction law, but the relaxation can be lowered by only using first five beam mode shapes. As a consequence, the number of sub-iterations reduces by one order. The numerical observations confirm the reduction in computational time due to utilization of the interaction law.

Topics: Fluids , Mode shapes
Commentary by Dr. Valentin Fuster
2017;():V07AT06A064. doi:10.1115/OMAE2017-62083.

Several authors have studied the solitary wave load on a submerged horizontal cylinder. In the present work, a semi analytical expression of the horizontal force exerted by a solitary wave on a horizontal cylinder is derived. The formula is based on the speed drop factor ƒr, that is the ratio (greater than one) between the time needed by the wave pressure to cross the solid body and the travel time across a transparent cylinder. The ƒr, is calculated numerically by means of the Boundary Element Method on assuming that a solitary wave and a periodic wave having the same wavelength undergoes the same slowing down. (Clearly the wavelength for the solitary wave is estimated approximately.) Abaci for the ƒr, as a function of the ratio between the diameter and the wavelength, for assigned A/d (= amplitude of the solitary wave / bottom depth) have obtained. In order to check the appropriateness of BEM, we carried out an experiment in a numerical wave flume, using the Computational Fluid Dynamics (CFD) technique.

Topics: Waves , Cylinders
Commentary by Dr. Valentin Fuster
2017;():V07AT06A065. doi:10.1115/OMAE2017-62113.

Green water occurs when an incoming wave exceeds the freeboard and propagates on the deck of naval/offshore structures, such as FPSO’s and platforms. The water on deck can affect the integrity of facilities and equipments installed on it, compromise the safety of the crew and affect the dynamic stability of the structure. Traditionally, regular or irregular waves generated by different types of wave-makers have been used to reproduce green water events. This is a good practice to study consecutive events. However, to study isolated events, an alternative could be the use of the wet dam-break approach to generate the incoming flow. The purpose of this paper is to investigate experimentally the use of the wet dam-break approach to generate isolated green water events. Tests were carried out in a rectangular tank with a fixed structure. Different freeboard conditions were tested for one aspect ratio of the wet dam-break (h0/h1 = 0.6). High speed cameras were used to investigate the initial phases of green water. Results demonstrated the ability of this approach to represent different types of green water events.

Topics: Dams , Water
Commentary by Dr. Valentin Fuster
2017;():V07AT06A066. doi:10.1115/OMAE2017-62193.

Water breakup affects the variety of droplet sizes and velocities in a cloud of spray resulting from a sea wave striking a vessel bow. The Weber and Reynolds numbers of droplets are the main parameters for water breakup phenomena. “Stripping breakup” is a faster phenomenon than “bag breakup” and occurs at higher velocities and with larger diameters of droplets. A water breakup model employs droplet trajectories to develop a predictive model for the extent of spray cloud. The governing equations of breakup and trajectories of droplets are solved numerically. Stripping breakup is found as the major phenomenon in the process of the formation of wave-impact sea spray. Bag breakup acts as a complementary phenomenon to the stripping breakup. The extent of the spray as well as wet heights, for a Mediumsized Fishing Vessel (MFV), are obtained by numerical solutions. The results show that bag breakup occurs at higher heights. In addition, there is no breakup when droplets move over the deck.

Topics: Sprays , Vessels , Water
Commentary by Dr. Valentin Fuster
2017;():V07AT06A067. doi:10.1115/OMAE2017-62227.

Hydroelastic response of Very Large Crude Carrier (VLCC), such as springing and whipping, is very important for the fatigue strength analysis. In the present paper, numerical simulation and model test are conducted simultaneously on the springing responses of a 350,000DWT VLCC. The numerical simulation results are obtained by combining 2-D strip method and 3-D Finite Element Method (FEM) in the frequency domain. Meanwhile, the segmented model tests are conducted in three conditions. Additionally, the experimental time histories of wave bending moments in regular and irregular waves are presented. In conclusion, the methods and results presented in this paper have shown that the springing would be much easier when the VLCC is in ballast condition or the full loading conditions with reduced stiffness. The springing response will happen, when the wave encounter frequency is near to the half and one third of the ship’s flexural natural frequency. The numerical method presented is available for springing response analysis quickly with acceptable accuracy.

Commentary by Dr. Valentin Fuster
2017;():V07AT06A068. doi:10.1115/OMAE2017-62321.

Accurate computation of the drift forces and moments is required to study the higher order steady excitations on the floater in waves. One of the components of the drift forces and moments is regarding the relative wave elevation. To obtain this term, we need an accurate estimation of the wave elevation. If the field point is exactly on the waterline, we will encounter some difficulty to get the potential value.

This paper will provide a more accurate method to calculate the wave elevation around the body and discuss the mathematical justifications. The accuracy will be validated by the comparison against the commercial software. When using more panels in the commercial software, the results will converge to those from our in-house program. We observe a large discrepancy in the relatively higher frequency range. The sharp spikes in the original method are not observed in the improved method. Therefore it is necessary to adopt the proposed method in order to provide more reasonable predictions of the mean drift forces.

Commentary by Dr. Valentin Fuster
2017;():V07AT06A069. doi:10.1115/OMAE2017-62322.

We study the multi-body problems with a nonzero speed or with current-wave interactions using the Green function method. The two problems are equivalent when the current is along the longitudinal direction of the ship.

The formulations of a single ship for the zero-speed and nonzero-speed cases are derived. The concepts, the motivations and the assumptions are explained. We also provide the derivations of the added mass and damping for the asymmetric ship profile. Then we extend the conclusions to the multiple ships and formulate the multi-body problems. The in-house program MDL Multi DYN is validated through the study of two Wigley hulls with zero speed in waves. After that, the results for two moving Wigley hulls are generated. The two ships are assumed to advance in parallel at the same speed. Finally, the effects of the separation distances and the different speeds are discussed.

Commentary by Dr. Valentin Fuster
2017;():V07AT06A070. doi:10.1115/OMAE2017-62355.

Vortex-induced motions (VIM) of offshore floaters occur when the frequency of the vortex shedding is close to the natural frequency in the transverse direction of the unit subjected to the current, and this can severely affect the fatigue life of mooring lines and risers. Literatures in recent years have shown an increasing interest in the VIM behavior of semi-submersibles (SS), the geometry of which implies a more complex VIM phenomenon. In the case of multi-column semi-submersibles, the vortices shed around each column. Thus the shape of the columns, circular or square, and the wake interference different for each current heading, characterize the VIM of the unit.

Based on verification by the VIM towing tests for a semi-submersible with four rounded square columns, VIM simulations were conducted using CFD method in the model scale for four semi-submersibles with different column designs: a SS model with four rounded square columns (SRC-SS), a SS model with four circular columns (CC-SS), a SS model with two tandem rounded square columns and two tandem circular columns (SRCT-CCT-SS), and a SS model with two staggered rounded square columns and two staggered circular columns (SRCS-CCS-SS). The current headings ranged from 0° to 180° and reduced velocities of 4 up to 14 were considered.

Different semi-submersibles present different VIM performances owing to the influence of the column design. The most significant transverse motions of the CC-SS model occur at the 0° current heading with the largest nominal transverse amplitudes around 74% of the column diameter. On the other hand, the maximum amplitudes in the transverse direction of the SRC-SS model, approximately 63% of the column width, are observed at the 45° current heading. Additionally, the VIM responses could be mitigated when the semi-submersible consists of combined circular-section and square-section columns.

Commentary by Dr. Valentin Fuster
2017;():V07AT06A071. doi:10.1115/OMAE2017-62391.

Experimental impact tests were performed using a shock machine and aerated water by means of an air-bubble generator. High speed shock test machine allows carrying out tests of impact on water (slamming). This machine permits to stabilise velocity with a maximal error equal to 10% during slamming tests. The air volume fraction in the bubble was measured by optical probe technique. The present work is aimed at quantifying the effects of the aeration on the hydrodynamic loads and pressures during the entry of a rigid body at constant speed in an air-water mixture. The impact tests were conducted with a rigid pyramid for an impact velocity equal to 15 m.s−1 and for two average void fractions, 0,46% and 0,84%. The reduction of the impact force and pressure due to aeration has been confirmed by these experiments.

Topics: Impact testing , Water
Commentary by Dr. Valentin Fuster
2017;():V07AT06A072. doi:10.1115/OMAE2017-62445.

In impacts of breaking waves on offshore structures, it is still not well-known how the air entrainment phenomenon affects the exerted loads. In this paper, a developed CFD solver capable of simulating the air entrainment process was employed to reproduce an experimental investigation on the impact of a spilling wave against a circular cylinder. The exerted in-line force was computed with and without the inclusion of dispersed bubbles. Results showed that the magnitude of the computed force was affected when the entrainment of bubbles was simulated.

Commentary by Dr. Valentin Fuster

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