ASME Conference Presenter Attendance Policy and Archival Proceedings

2016;():V007T00A001. doi:10.1115/OMAE2016-NS7.

This online compilation of papers from the ASME 2016 35th International Conference on Ocean, Offshore and Arctic Engineering (OMAE2016) 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presenter attendance policy, if a paper is not presented at the Conference by an author of the paper, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.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

2016;():V007T06A001. doi:10.1115/OMAE2016-54163.

Rankine panel methods have been studied for solving 3D seakeeping problems of a ship with forward speed and oscillatory motions. Nevertheless, there is a drawback in the numerical method for satisfying the radiation condition of outgoing waves at low frequencies, because the waves generated ahead of a ship reflect from the outward computational boundary and smear the flow around the ship. The so-called panel shift technique has been adopted in the frequency-domain Rankine panel method, which is effective when the generated waves propagate downstream of a ship. In this paper, in addition to this conventional panel shift method, Rayleigh’s artificial friction is introduced in the free-surface boundary condition to suppress longer wave components in a computational region apart from the ship. With this practical new method, it is shown that there is no prominent wave reflection from the side and/or upstream computational boundaries even in the range of low frequencies. As a consequence, the unsteady pressure, hydrodynamic forces, wave-induced ship motions, added resistance are computed with reasonable accuracy even in following waves and in good agreement with measured results in the experiment using a bulk carrier model which is also conducted for the present study.

Topics: Ships
Commentary by Dr. Valentin Fuster
2016;():V007T06A002. doi:10.1115/OMAE2016-54250.

We analyze the interactions between a subsurface shear current of uniform vorticity and a moving surface disturbance of anisotropic shape which generates surface gravity waves. The problem extends previous analysis of ship waves in the presence of a shear current varying linearly with depth, now also accounting for the three dimensional shape of real ships, in order to study the interplay of aspect ratio and the shear current. Based on general solutions derived previously, we apply an elliptical Gaussian pressure disturbance at the surface moving at constant velocity as a model for a real “ship”. Wave contributions in the far field and expressions for the Mach angle (of maximum wave amplitude) based on asymptotic expressions for high Froude numbers, are derived thereafter. Through numerical calculations we present wave patterns, as well as Kelvin and Mach angles, at moderate Froude numbers under different shear strenghts and aspect ratios. Results show that the aspect ratio has negligible effect on the value of the critical shear vorticity and Kelvin angle, whereas a subtle interplay of aspect ratio and shear strenght is found to affect the Mach angle at moderate Froude numbers.

Commentary by Dr. Valentin Fuster
2016;():V007T06A003. doi:10.1115/OMAE2016-54276.

This paper is devoted to validation of ship manoeuvring models. An alternative approach to validation is proposed. It is based on comparison of accelerations measured in a validation experiment and calculated using a simulation model. It can be applied to arbitrary trials and in-service collected data. The approach is applied to assess three versions of a manoeuvring model of a twin-screw, twin-rudder vessel. Case trials are the 20° turning circle and the 20°/20° zigzag, performed with a free sailing model. Full simulations are performed as well. Conclusions made from two approaches are compared.

Topics: Ships
Commentary by Dr. Valentin Fuster
2016;():V007T06A004. doi:10.1115/OMAE2016-54311.

Ship maneuvering performance is usually predicted in calm water conditions, which provide valuable information about ship’s turning ability and its directional stability in the early design stages. Investigation of maneuvering simulation in waves is more realistic since the ship usually sails through waves. So it is important to study the effect of waves on the turning ability of a ship. This paper presents the maneuvering simulation for a container ship in presence of regular waves based on unified state space model for ship maneuvering. Standard maneuvers like turning circle and zigzag maneuver are simulated for the head sea condition and the same are compared with calm water maneuvers. The present study shows that wave significantly affects the maneuvering characteristics of the ship and hence cannot be neglected.

Topics: Waves , Ships
Commentary by Dr. Valentin Fuster
2016;():V007T06A005. doi:10.1115/OMAE2016-54312.

Evaluation of maneuverability of a ship at the early design stages is necessary for ensuring safety of its voyage. IMO recommends the test speed or approach speed for the maneuvering predictions as 90–100% of the service speed of the vessel. The confined model tests for ship maneuvering assessment are usually conducted at low speeds and the hydrodynamic derivatives obtained from these tests are used in the equation of motion even when vessel operates at much higher speeds. But the hydrodynamic derivatives and consequently the trajectory predicted using these derivatives differ substantially from the actual maneuvering conditions. Hence the dependency of the derivatives on vessel speed needs to be understood properly to get the correct estimate of the vessel trajectory prediction. This paper investigates the effect of vessel speed (Fn) on hydrodynamic characteristics of a container ship. Straightline test and horizontal planar motion mechanism (HPMM) tests are conducted for a container ship model for different speeds in a CFD environment.

Topics: Containers , Ships
Commentary by Dr. Valentin Fuster
2016;():V007T06A006. doi:10.1115/OMAE2016-54316.

Parametric roll resonance, as a nonlinear phenomenon related to ship stability, is particularly apt to happen when a ship is sailing in longitudinal waves. It can generate sudden oscillation with large amplitude up to 30–40 degrees of roll and put the ship and crew in danger. To predict the parametric roll resonance of ships, a suitable model for describing this phenomenon is needed. In this paper, a nonlinear mathematical model considering the strong nonlinear coupling among the heave, roll, and pitch motions of ships is established, and support vector regression (SVR) is applied to identify the unknown damping and restoring coefficients in the mathematical model. To verify the accuracy and validity of SVR in parametric identification, a container ship is considered, and the coupled heave, roll, and pitch motions of the ship in longitudinal regular waves are simulated. Based on the simulated responses, the unknown coefficients in the mathematical model are identified by SVR. Then the coupled heave-roll-pitch motion of the container ship in regular waves is predicted by using the identified coefficients in comparison with the simulated data, and satisfactory agreement is achieved. From this study, it is concluded that SVR can be applied to identify the unknown coefficients in the nonlinear mathematical model for predicting the parametric roll resonance of ships in longitudinal regular waves.

Topics: Resonance , Waves , Ships
Commentary by Dr. Valentin Fuster
2016;():V007T06A007. doi:10.1115/OMAE2016-54317.

Due to the random nature of ship motion in an open sea environment, the ship related maritime operations such as landing on an aircraft carrier, ship-borne helicopter recovery, cargo transfer between ships and so on, are usually very difficult. An accurate prediction of the motion will improve the operation safety and efficiency on board ships. This paper presents a research on the application of artificial neural network methods in the short-time prediction of ship pitching motion. The radial basis function (RBF) neural network is applied to develop a model for short-time prediction of ship pitching motion, and the other two kinds of artificial neural networks, i.e., back propagation (BP) neural network, Elman neural network are also applied for the same purpose. A comparative analysis among them is presented. It is shown that RBF neural network provides a more effective and accurate tool for predicting the ship pitching motion.

Commentary by Dr. Valentin Fuster
2016;():V007T06A008. doi:10.1115/OMAE2016-54334.

Evaluation of added resistance in short waves is critical to the assessment of the global performance of a ship traveling in a seaway. In this paper, three methods of added resistance evaluation in short waves are briefly reviewed, including those proposed by Fujii & Takahashi [1], Faltinsen et al. [2], and Kuroda et al. [3]. Based on the experimental data collected by Kuroda et al., a new method is developed for the estimation of added resistance in short waves. The proposed method is validated by comparing the obtained numerical results with experimental data and other numerical solutions for different types of hulls, including the Wigley hull I, KVLCC2 hull, Series 60 hull with CB = 0.7, and the S-175 hull. The present study confirms that the developed method can well predict the added resistance in short waves and complement the three-dimensional Rankine panel method developed in a previous study focusing on intermediate and long waves.

Topics: Waves , Computation
Commentary by Dr. Valentin Fuster
2016;():V007T06A009. doi:10.1115/OMAE2016-54353.

A time domain Rankine source solver is extended to compute the wave added resistance of ships. The proposed approach applies the momentum conservation principle on the near field fluid volume enclosed by the wet surface of a floating body, the free surface and a control surface. The wave added resistance is then calculated by the integration over the control surface of the fluid velocities and free surface elevations. To be able to incorporate the proposed method with the Rankine source code, an interpolation scheme has been developed to compute the kinematics for the off-body points close to (or on) the free surface. Two Wigley ship models, a containership model S175 and a tanker model KVLCC2 are used to validate the present method. In general good agreement is found comparing with the model test data. The convergence behavior is examined for the proposed method including the selection of the time step and location of the control surface. Both Neumann-Kelvin and double body linearization methods are evaluated with the proposed method. It is found that the Neumann-Kelvin linearization can only be applied for slender ship hull, whereas double body method fits also for blunt ships. It is suggested to apply the proposed method with double body linearization to evaluate the wave added resistance of ships with a control surface close to the ship hull.

Topics: Waves , Computation
Commentary by Dr. Valentin Fuster
2016;():V007T06A010. doi:10.1115/OMAE2016-54391.

Fast mono-wave-piercing craft (Fast Mono-WPC) use a tumblehome design and run at high speeds. Efforts have been made to study hull wetness problems because of the special hull shape and running state of Fast Mono-WPC. Numerical and experimental methods were adopted to study hull wetness problems of Fast Mono-WPC both in calm water and waves. Numerical methods were based on RANS. Dynamic mesh methods were used to simulate model motions. Influence of velocity, spray strips and hull motions on hull wetness of Fast Mono-WPC in calm water was studied. Results of CFD and EFD in calm water show that hull wetness of Fast Mono-WPC is sensitive to hull motions such as heave and pitch, and spray strips on the bow can reduce hull wetness. Model tests in regular and irregular waves and numerical simulations in regular waves of a free Fast Mono-WPC model were conducted at a high Froude number, Fn = 0.815. Numerical results show that wetness is slight when Fast Mono-WPC runs in 1L-long and 4L-long waves, and water climbs up the freeboard to the deck when Fast Mono-WPC cuts through 2L-long waves. Serious hull wetness was also experimentally observed both in regular and irregular waves.

Commentary by Dr. Valentin Fuster
2016;():V007T06A011. doi:10.1115/OMAE2016-54407.

When solving the forward speed hydrodynamic problem in frequency domain adopting the matching method with a meshless cylinder surface as the control surface, the simple Green function is used in the interior domain. To tackle the integration containing the first and second order derivatives of velocity potential on free surface about x, a method in which the velocity potential on the free surface and its derivatives are fitted by the cubic B spline is given, and the regular wave is chosen as the incident wave, and the theory solutions of its velocity potential and the first and second order derivatives about x are compared to the numerical solutions get from the cubic B spline.

Commentary by Dr. Valentin Fuster
2016;():V007T06A012. doi:10.1115/OMAE2016-54523.

A moonpool is meant for access to the underwater part of the ship from onboard. It is a vertical opening along the depth having an effect on the performance of the floating platform. Inside the moonpool, water motions in horizontal plane is called sloshing and in vertical planes it is called piston mode. Moonpool causes deck wetness and sometimes result in the downtime of the platform. It is the necessity of the operator to be at the safe conditions of platform facing varied environmental conditions. In the present study, vessel response in the region of moonpool resonance was investigated with different shapes of moonpool and comparison is made with Molin’s (2001) theoretical and Fukuda’s (1977) empirical formulas. It is seen that there is a shift in the frequency of resonance based on moonpool shapes. The effect of moonpool on the ship motion with forward speed is also attempted in this paper. Proven packages are used to calculate the calm water resistance of the ship with moonpool of various cross-sections. Wave making coefficient of the ship is modified due to opening to accommodate the moonpool. The openings to accommodate moonpool causes further entry of water both zero and non-zero Froude number especially in the presence of waves.

Commentary by Dr. Valentin Fuster
2016;():V007T06A013. doi:10.1115/OMAE2016-54591.

For numerous offshore operations, wave induced vessel motions form a limitation for operability: Installation of wind turbines, removal and placements of top sites on/from jackets, landing of helicopters etc. can only be done safely in relatively benign wave conditions. In many cases the actually critical phase takes no more than some tens of seconds. An on-board prediction of vessel motions would enable crew to anticipate on these near future vessel motions and avoid dangerous situations resulting from large ship motions. This paper presents results from a field campaign in which non-coherent raw X-band navigation radar data was used as input for a procedure that inverts the radar data into a phase resolved estimation of the wave elevation. In combination with a wave propagation and vessel response model, this procedure can compute a prediction of phase resolved vessel motions, some tens of seconds up to minutes into the future, depending on radar range and sea state. We compare predictions obtained this way with actual measurements of a well intervention vessel that were obtained during a sea trial performed at the North Sea. It was concluded that the method results in very accurate predictions: correlations between 0.8–0.9 were obtained for predicted ship motions of/around the COG and the vertical motions of the helicopter deck.

Topics: Waves , Ships
Commentary by Dr. Valentin Fuster
2016;():V007T06A014. doi:10.1115/OMAE2016-54594.

Model experiments and numerical simulations on the surf-riding and broaching in following seas of a 42.5m long purse seiner are conducted. Firstly, the free running model experiments with various ship speeds and wave heights are performed in the towing tank to reproduce the phenomena of surf-riding and broaching. Then, the 6-DOF weakly nonlinear unified model is applied to simulate the motions of the purse seiner with the same cases as the model experiments. Through the comparison between results of model test and numerical simulation, the occurrence conditions of periodic motion, surf-riding and broaching are roughly determined. Finally, it is found that although it is difficult for the numerical simulations to get the same time histories as model tests, the modes of motion (periodic motion, surf-riding or broaching) obtained from the numerical simulations agree well qualitatively and quantitatively in part with the model test results.

Commentary by Dr. Valentin Fuster
2016;():V007T06A015. doi:10.1115/OMAE2016-54596.

In the present study, the added resistance of a containership in parametric roll motion is investigated. The numerical simulation is carried out using a three dimensional Rankine panel method along with the weakly nonlinear formulation. The added resistance is evaluated by a near-field method, namely, the direct integration of the 2nd-order pressure on a body surface. To calculate the component resulting from the large-amplitude roll motion, the higher-order restoring and Froude-Krylov forces on wetted hull surfaces are taken into account. With or without parametric roll in regular waves, the components of added resistance classified with respect to integral terms are compared to figure out the important of each term. Through the investigation, the correlation between the added resistance and parametric roll is derived from coupling and decoupling the components of roll motion and vertical motions.

Commentary by Dr. Valentin Fuster
2016;():V007T06A016. doi:10.1115/OMAE2016-54793.

The influence of regular head waves on the propulsion characteristics of a twin screw cruise ship is investigated using RANS based flow-solver. Propeller open water characteristics are determined at first by computing the propeller forces in homogeneous inflow. Then, computations of the towed model without propeller and of the self-propelled model in calm water are performed to obtain the propulsion characteristics in calm water. Afterwards, the total resistance as well as the forces of the self-propelled model in regular head waves are computed. All computations are performed using a RANS based flow-solver coupled with the six-degrees-of-freedom equations of motion. The sliding interface method is used to allow the rotation of the geometrically modelled propeller, when needed. Computations are performed using the same numerical grids to keep errors originating on different spatial and temporal discretization as small as possible. Grid studies are performed to evaluate discretization errors of each mesh region, namely the hull region and the rotating propeller region, separately. The numerical results are compared with experimental results obtained from physical tests. It is shown, that RANS is capable of investigating the propulsion behavior of a ship in regular head waves, but to the cost of high computational effort. Fair agreement between numerical and experimental results is obtained. All results show that the propulsion characteristic change in waves. It is also found, that this is mainly caused by the change of the propeller efficiency due to a different propulsion point as a consequence of the added resistance in waves.

Topics: Propulsion , Waves , Ships
Commentary by Dr. Valentin Fuster
2016;():V007T06A017. doi:10.1115/OMAE2016-54861.

In this study, numerical computation was carried out for evaluating the effects of the design parameter variations on the added resistance of Aframax tanker in head seas. The design of experiments (DOE) was used to efficiently conduct the numerical simulations with the hull form variations and save computational resources. A computational fluid dynamics (CFD) code based on the continuity and Reynolds averaged Navier-Stokes (RANS) equation was used for the numerical simulation. The simulation was performed in a short wave condition where the wave length was half of the ship length, which is expected to be most frequent in the vessel operation. Five design parameters of fore-body hull form were selected for the variations: design waterline length (DWL), bulbous bow height (BBH), bulbous bow volume (BBV), bow flare angle (BFA) and bow entrance angle (BEA). Each parameter had two levels in the variations, thus total 32 cases were designed initially. The results of the numerical simulations were analyzed statistically to determine the main effects and correlations in the five design parameters variations. Among them, the most significant parameter that influences on the added resistance in waves was DWL, followed by BBV and BEA. The other parameters had little effects on the added resistance in waves. By the computations, it was revealed that Extending DWL and decreasing BEA promoted the reflection of waves more toward the side than forward. In addition, there existed two-way interactions for the following two-factor combinations: DWL-BFA, DWL-BEA, DWL-BBV, BBH-BBV.

Commentary by Dr. Valentin Fuster
2016;():V007T06A018. doi:10.1115/OMAE2016-54886.

While in aerospace and automotive industry, airplanes and cars are built in quantity, in maritime industry ships and offshore platforms are built uniquely such that even sister ships can be significantly different from each other. Hence, building a full scale prototype to test, verify, and demonstrate effectiveness of new innovative solutions, is not an option in maritime sector. Model testing and simulation of separate modules have been practiced in many applications successfully, however, capturing the complete interaction of different modules in a maritime system is not straight forward. To best of our knowledge, the modeling and simulation of a maritime system to the extent where the complete system, including the mutual interactions, is not accomplished yet. A maritime system incorporates a wide variety of components from different engineering fields and in order to develop a simulation framework for such a complex system, an interdisciplinary effort is needed from different branches of science including but not limited to hydrodynamics, machinery and power systems, structural engineering, navigation and control. This paper aims to introduce a joint effort from different research institutes, universities, and industrial partners to shed a light on the different issues in virtual prototyping in maritime systems and operations. It summarizes some of the available frameworks for virtual prototyping, and ends with a numerical simulation of a generic hull model coupled with propellers, propeller actuators, DP controller, thrust loss calculations, wind, waves and current, performed with the current implementation of our Virtual Prototyping Framework (VPF).

Commentary by Dr. Valentin Fuster
2016;():V007T06A019. doi:10.1115/OMAE2016-54896.

The ship wave resistance can be estimated by two alternative methods after solving the boundary integral equation. One is the far field method e.g. Havelock’s formula based on momentum conservation in fluid domain, and another is the near field method based on direct pressure integration over the wetted body surface. Nakos and Sclavounos (1994) had shown a new near field expression of ship wave resistance from the momentum conservation law in the fluid domain with linearized free surface condition. Their new expression differs slightly from the traditional near field form. This problem of near field expression is reconsidered in terms of Green’s second identity. After linearization of the free suface condition and some transformation of equations, the present paper will agree with the Nakos and Sclavounos’ near field expression for the ship wave resistance. Some numerical calculations of wave resistance from the far field method and from the near field method are shown using the classical Kelvin sources distributed on the centerplane of thin ship but solving the different boundary integral equation. Numerical results suggest that the problematic run-up square integration along the waterline is to be omitted as a higher order small quantity. If this run-up term is omitted in each method except for far field, the traditional direct pressure integrtaion is equal to the Nakos and Sclavounos’ near field expression.

Commentary by Dr. Valentin Fuster
2016;():V007T06A020. doi:10.1115/OMAE2016-54964.

Active anti-rolling tank (ART) is sophisticated equipment on a floating vessel to reduce roll motion for the slender ship-shape vessel. Three-dimensional panel based diffraction and radiation linear potential program employed to obtain hydrodynamic coefficients of floating vessel. For the ship motion, a BEM (Boundary Element Method)-based ship motion program was used and inner sloshing effects were conducted by a particle-based CFD (Computational Fluid Dynamics) program which is the Moving Particle Semi-implicit (MPS). By using panel program, the hydrodynamic coefficients were obtained in frequency domain, and then were converted into time domain ship motion simulation program. In this procedure, time memory effect was considered by Volterra series expansion. The ship motion program and sloshing program was coupled dynamically; inner tank received displacement, velocity and acceleration data from ship motion program and use them for inner tank motion, while the ship motion program was waiting external forces due to sloshing impact loads and inertia forces/moments from sloshing simulation program. Thus, two programs run simultaneously and allowed real time coupling effects of inner sloshing on vessel motion. By comparing response amplitude operator (RAO) of the vessel without anti-rolling tank, it was shown both values have good agreement. And then comparing between vessels with and without anti-rolling tank, it is shown that the effects of ART changed and shift RAOs. Furthermore, by changing the location of ART, location effects of ART were also investigated.

Topics: Vessels
Commentary by Dr. Valentin Fuster
2016;():V007T06A021. doi:10.1115/OMAE2016-54982.

This paper presents small-scale low-speed maneuvering tests with an oceanographic research vessel and the comparison with mathematical model using the real time maneuvering simulator developed by the University of São Paulo (USP). The tests are intended to verify the behavior of the vessel and the mathematical model under transient and low speed tests. The small-scale tests were conducted in deep and shallow waters, with a depth-draft ratio equal to 1.28, in order to verify the simulator ability to represent the vessel maneuverability on both depth conditions. The hydrodynamic coefficients used in the simulator model were obtained by CFD calculations and wind tunnel model tests carried out for this vessel. Standard turning circle and accelerating turn maneuvers were used to compare the experimental and numerical results. A fair agreement was achieved for shallow and deep water. Some differences were observed mainly in the initial phase of the accelerating turn test.

Commentary by Dr. Valentin Fuster
2016;():V007T06A022. doi:10.1115/OMAE2016-55133.

This project defines a methodology for analyzing the proposition of expanding the operational sector of DP shuttle tankers in offloading operations in Spread Moored Platforms and still guaranteeing the operations’ safety.

The methodology consists at first in an evaluation of the reduction of the operation’s downtime as a function of sector angle increase by performing static analysis. These simulations are calibrated and validated by performing fast time dynamic simulations. Then, a Preliminary Risk Analysis is made to assess the potential hazards associated with the operation on the new expanded sector and, if necessary, some quantitative analyses take place. Finally, the methodology proposes that real time simulations are performed so that most critical conditions are recreated in the presence of an experienced Captain so he can give his opinion on how safe the operation would be.

Topics: Mooring
Commentary by Dr. Valentin Fuster

Ocean Engineering: Advanced Underwater Vehicles and Design Technology

2016;():V007T06A023. doi:10.1115/OMAE2016-54440.

An important role among machines for sea bottom exploration is assigned to the autonomous ground devices. Some rescue tasks also require subsea robotic devices. The main purpose of the work is to investigate and improve adaptive characteristics, traction properties and control methods of cyclic walking movers in underwater conditions. Traction properties of walking machines, which moves at sea bottom was analyzed. Some experience of development and experimental tests of the walking robot “Vosminog”, designed for work at weak and waterlogged grounds. Dynamic model of a walking machine has been shown. Studied an opportunity to increase adaptive characteristics and shape passableness of walking machines. Also design and results of underwater tests of subsea walking unit MAK-1 are discussed. During tests the performance of a walking unit has been checked and the influence of design features of a walking mover on its traction characteristics and ground passability has been investigated. Some details about control system, power system and energy usage, vertical motions and accelerations for different types of walking and conditions of movement has been given. Also, certain attention was given to testing of methods of standalone movement control of subsea unit in conditions of incomplete and ambiguous vision of current situation. Tests have shown that walking movers in subsea conditions can provide higher traction properties, in comparison with wheeled and tracked ones. The unit can be used for exploration of seabed resources and for rescue tasks.

Commentary by Dr. Valentin Fuster
2016;():V007T06A024. doi:10.1115/OMAE2016-54631.

Precise vertical positioning has a very significant impact on the accuracy and reliability of the Remotely Operated Vehicle (ROV) working process. This paper establishes an experimental system which is capable of controlling and measuring the vertical position of ROV. The control signal output system is developed by integrating certain modules such as video recording, image-analog-digital signal converting, timing depth calculating and force signal outputting modules. The effects of Proportional-Integral-Derivative (PID) method on ROV depth control in still water and under various regular waves was tested and compared. The causes of difference of control effect between in still water and in waves were analyzed. Giving insight into the hydrodynamic characteristics and consequent movement of ROV, a practical underwater vehicle maneuvering mathematical model was established. The space motion equations were set up to solve the underwater vehicle maneuvering problem. The effects acted on the carrier by the waves were analyzed in order to develop a feasible control strategy that is applicable to the practical operation. Unsatisfactory effects of thrust output are optimized with MATLAB simulation based on ITAE criterion, using the simplex method for re-tuning PID parameters. Experimental results show that the system for measuring and controlling the vertical position of the ROV is successful. With an appropriate time interval for data collecting, the ROV model’s vertical depth can be effectively determined by the program written in VB language in the still water and small wave height situation. While the wave height increases or the wave period changes, the program needs to be adjusted. Simulation results also show that the wave frequency response abates as the wave period decreases. Compared to the original PID parameters, the new ones can rapidly give response to the vertical position change and guarantee the stability of ROV depth controlling. It is of obvious importance to actual control of the underwater vehicles’ vertical movement with the help of mathematical optimization method for the controller parameters. The proposed controlling algorithm can be an alternative for other similar working conditions.

Commentary by Dr. Valentin Fuster
2016;():V007T06A025. doi:10.1115/OMAE2016-54642.

In this paper the hydrodynamic parameters that characterize the behavior of a typical unmanned underwater vehicle are evaluated. A complete method for identifying these parameters is described. The method is developed to give a brief and accurate estimate of these parameters in all six degrees of freedom using basic properties of the vehicle such as dimensions, mass and shape. The method is based on both empirical and analytical results for typical reference geometries (ellipsoids, cubes, etc.). The method is developed to be applicable for a wide variety of UUV designs as these typically varies substantially. The method is then applied to a small observation class ROV. The results are first verified using an experimental method in which the full scale ROV is towed using a planar motion mechanism. An additional verification is performed with numerical simulations using Computational Fluid Dynamics and a radiation/diffraction program. The method shows promising results for both damping and added mass for the tested case. The translational degrees of freedom are more accurate than the rotational degrees of freedom which are expected as most empirical and analytical data are for translational degrees of freedom. The case study also reveals that the relative difference between the numerical simulations and the experimental results are similar to the relative difference between the proposed method and the experiment.

Commentary by Dr. Valentin Fuster
2016;():V007T06A026. doi:10.1115/OMAE2016-54714.

With the aim of aiding mitigation efforts, in mapping and simulating the transport of the discharged hydrocarbon this paper proposes an autonomous surface vehicle (ASV), propelled by wind and water currents for the long-term monitoring of spilled oil on the ocean surface. This paper makes a unique contribution to the literature in proposing a cluster-based decision-making algorithm for sailing the ASV based on a complete scanning history of the area surrounding the vehicle by the oil detection sensor. A Gaussian-based oil cluster filtering algorithm is introduced to identify the largest oil slick patch. The physical constraints of the ASV have been taken in account to allow for the computation of feasible maneuvering headings for sailing to avoid sailing upwind (i.e., in the direction from which the wind is coming). Finally, using neoprene sheets to simulate oil spills, field test experiments are described to validate the operation of the ASV with respect to oil spill tracking using a guidance, navigation, and control system based on onboard sensor data for tracking the artificial oil targets.

Commentary by Dr. Valentin Fuster
2016;():V007T06A027. doi:10.1115/OMAE2016-54830.

Autonomous underwater vehicles (AUVs) depend primarily on electrical power. Monitoring the deep-sea ocean floor efficiently requires easily accessible recharging techniques. In contrast to conservative ship-dependent charging processes on the surface, the Subsea Monitoring via Intelligent Swarms (SMIS) consortium uses a seabed station (SBS) as a power supply for AUVs. This paper describes the fully automated secure landing procedure of the SBS followed by a ground mooring maneuver on sandy sediments or soft clays. The unmanned seabed station is equipped with various sensors for status detection and environmental observation. In order to control the freefall and grounding without endangering the valuable components, an innovative procedure was implemented. Once landed, the nearly buoyancy-neutral station is moored using down-scaled suctions buckets that are normally used in immobile offshore oil rig foundations.

Laboratory full-scale experiments in modified test tanks for different length-to-diameter ratios endorse the design process. The feasibility of landing, mooring and ascending techniques was proven in field-tests in the Baltic Sea and the Atlantic Ocean.

Topics: Suction , Vehicles , Mooring , Seas
Commentary by Dr. Valentin Fuster
2016;():V007T06A028. doi:10.1115/OMAE2016-54841.

The efforts to discover the world’s oceans — even in extremely deep-sea environments — have grown more and more in the past years. In this context, unmanned underwater vehicles play a central role. Underwater systems that are not tethered need to provide an apparatus to ensure a safe return to the surface. Therefore, positive buoyancy is required and can be achieved by either losing weight or expanding volume. A conservative method is the dropping of ballast weight. However, nowadays this method is not appropriate due to the environmental impact.

This paper presents a ballast system for an automated ascent of a deep-sea seabed station in up to 6000 m depth. The ballast system uses a DC motor driven modified hydraulic pump and a compressed air auxiliary system inside a pressure vessel. With regard to the environmental contamination in case of a leakage, only water is used as ballast fluid. The modification of an ordinary oil-hydraulic radial piston pump and the set-up of the ballast system is introduced.

Results from sea trials in the Atlantic Ocean are presented to verify the functionality of the ballast system.

Topics: Seas
Commentary by Dr. Valentin Fuster
2016;():V007T06A029. doi:10.1115/OMAE2016-54849.

We present a method to directly predict the hydrodynamic response of a Remotely Operated Vehicle (ROV) as it transitions through the wave-affected splash zone during launch or recovery. ROVs are commonly used in offshore operations and for many different purposes. They are usually deployed from a purpose built Launch and Recovery System (LARS) located on the open deck of a surface ship. Current industry practice used for LARS design is to use a pre-defined dynamic amplification factor (DAF) together with high factors of safety. However, this does not properly account for the influence of different sea states and operational profiles. We therefore propose an improved method to directly predict the hydrodynamic response of an ROV as it transitions through the wave-affected splash zone. We apply the approach to a trencher ROV and validate using physical experiments completed in a wave tank. We demonstrate the need to for an accurate definition of the added mass and damping characteristics of the ROV. The method is incorporated into OrcaFlex for direct use within the design and analysis of a LARS system.

Commentary by Dr. Valentin Fuster

Ocean Engineering: Coastal Engineering

2016;():V007T06A030. doi:10.1115/OMAE2016-54162.

The wave-induced erosion processes involved in the interaction of revetments and dunes are investigated by means of small scale experiments performed in the 3D wave basin of Leichtweiss-Institute for Hydraulic Engineering and Water Resources (LWI) in Braunschweig, Germany. Two adjacent revetments (R1 and R2) were constructed with an angle of δ = 22° in plane. Irregular JONSWAP waves were generated at two water depths, which resulted in two different wave overtopping rates. The influences of the wave attack angle and wave overtopping rate on dune erosion behind the revetments are investigated by means of laboratory tests and the results are discussed.

Topics: Waves , Erosion
Commentary by Dr. Valentin Fuster
2016;():V007T06A031. doi:10.1115/OMAE2016-54237.

The response amplitudes of different resonant modes in an elongated harbor with constant depth induced by N-waves with different amplitudes and different types are calculated using the Normal Mode Decomposition (NMD) method. Wave conditions inside the harbor are simulated with a fully nonlinear Boussinesq model FUNWAVE-TVD. It is found that, for the small amplitude of the incident N-wave, wave energy inside the harbor is dominated by the lowest few modes, while only a small proportion of the wave energy is distributed over the higher modes. However, with the increase of the incident wave amplitude, most of the wave energy inside the harbor tends to concentrate on the higher resonant modes, hence the relative wave energy distributed over the lowest few modes decreases. On the other hand, the wave energy distribution inside the harbor is also significantly affected by the type of the N-wave. The wave energy inside the harbor excited by the MS-type incident N-wave is more concentrated than that excited by the TS-type N-wave. The MS- and TS-type N-waves correspond to the N-wave expressions proposed by Madsen and Schäffer (Madsen, Schäffer, 2010. Analytical solutions for tsunami runup on a plane beach single waves, N-waves and transient waves. Journal of Fluid Mechanics 645, 27–57) and Tadepalli and Synolakis (Tadepalli, Synolakis, 1994. The run-up of N-waves on sloping beaches. Proceedings of the Royal Society London A: Mathematical, Physical & Engineering Sciences 445, 99–112), respectively.

Commentary by Dr. Valentin Fuster
2016;():V007T06A032. doi:10.1115/OMAE2016-54298.

Coastal vegetation can not only provide shade to coastal structures but also reduce wave run-up. Study of long water wave climb on vegetation beach is fundamental to understanding that how wave run-up may be reduced by planted vegetation along coastline. The present study investigates wave period influence on long wave run-up on a partially-vegetated plane slope via numerical simulation. The numerical model is based on an implementation of Morison’s formulation for rigid structures induced inertia and drag stresses in the nonlinear shallow water equations. The numerical scheme is validated by comparison with experiment results. The model is then applied to investigate long wave with diverse periods propagating and run-up on a partially-vegetated 1:20 plane slope, and the sensitivity of run-up to wave period is investigated based on the numerical results.

Topics: Waves
Commentary by Dr. Valentin Fuster
2016;():V007T06A033. doi:10.1115/OMAE2016-54514.

In this study, the net bottom sediment transport pattern is measured and compared with flow pattern in the Pearl River Estuary (PRE). Based on 106 bottom sediment samples taken from the PRE, the spatial distribution of bottom sediment of mean grain size μ (mm), sorting coefficient σ, and skewness SK is calculated. Then the grain size trend analysis (GSTA) was used to detect the net bottom sediment transport characteristics. The bottom sediment is transported southward and southeastward in the upper part of the estuary and northward in the lower part, while a clockwise rotation trend occurs in the central part. Furthermore, a numerical flow model, based on Princeton Ocean Model (POM), is used to modulate the hydrodynamic conditions in the PRE. The simulated long-term bottom residual currents correlate well with the GSTA results, implying that the net bottom sediment transport is mainly controlled by the residual currents.

Topics: China , Currents , Sediments , Rivers
Commentary by Dr. Valentin Fuster
2016;():V007T06A034. doi:10.1115/OMAE2016-54535.

In this work an scaling analysis of the liquefaction phenomena is presented. The characteristic scales are obtained by balancing term by term of the well known partial dynamics governing equations (approximation UP). The order of magnitude of the horizontal displacement are very smaller compared with the vertical displacement and therefore the governing equation are only a function of the dependent vertical variables. The UP approximation is reduced and presented in its dimensionless version. This scaling analysis can be used to obtain analytical solutions of the liquefaction phenomena under the action of the water waves.

Commentary by Dr. Valentin Fuster
2016;():V007T06A035. doi:10.1115/OMAE2016-54641.

Surface waves are simulated and extreme parameters are calculated in Weifang Port in this paper. Firstly, wind data of every 3 hours from 1985 to 2010 in the Bohai Sea are simulated by Weather Research Forecast Model (WRF). Then wave elements such as significant wave height, wave period and wave direction are simulated every 1 hour from 1985 to 2010 by SWAN model in the whole of Bohai Sea. By comparison and validation, both the simulation wind data and wave data show a good agreement with the observation data. Extreme parameters of wind speed, wave height and wave period in every direction are calculated according to Pearson-III distribution at the −15m isobath in Laizhou Bay. The results show that the strong wind and wave are in direction N, NNE and NE. Surface waves are calculated using MIKE21 SW to study wave propagation from the open sea to the coast. The wave elements inside of the submerged breakwaters are calculated by three empirical formulas. The results from the three formulas are different, and the conservative result is chosen considering engineering safety.

Commentary by Dr. Valentin Fuster
2016;():V007T06A036. doi:10.1115/OMAE2016-54644.

This paper provides a practical stochastic method by which the burial and scour depths of short cylinders and truncated cones exposed to long-crested (2D) and short-crested (3D) nonlinear random waves plus currents can be derived. The approach is based on assuming the waves to be a stationary narrow-band random process, adopting the Forristall [1] wave crest height distribution representing both 2D and 3D nonlinear random waves. Moreover, the formulas for the burial and the scour depths for regular waves plus currents presented by Catano-Lopera and Garcia [2, 3] for short cylinders and Catano-Lopera et al. [4] for truncated cones are used.

Topics: Waves , Currents , Cylinders
Commentary by Dr. Valentin Fuster
2016;():V007T06A037. doi:10.1115/OMAE2016-54837.

This paper presents an experimental work on a breaking wave impinging and overtopping a deck structure. Because the Particle Image Velocimetry (PIV) technique is known of unsuitable of applying in highly aerated flows such as breaking waves, a technology named Bubble Image Velocimetry (BIV) is adopted to quantify the turbulent velocity characteristics in the bubbly region. A high-speed camera is used to capture images during the wave-structure interaction event in the framework of BIV methodology and the images are subsequently processed using cross-correlation for velocity determination. A wave focusing method is employed to generate plunging breaker in a laboratory-scale wave flume, and the model structure is a horizontal, flat and rigid deck that can be considered as a representative of a coastal bridge or an offshore floating platform. The goal is to gain physical insight from the breaking wave interaction with a simplified structure through measuring the kinematics of the bubbly flow.

Topics: Waves
Commentary by Dr. Valentin Fuster
2016;():V007T06A038. doi:10.1115/OMAE2016-54840.

A study of the wave conditions in the North Sea, the Norwegian Sea and the Barents Sea is presented in this paper. For each region, one reference location for which there are buoy measurements is selected. For the selected locations, WAM10 hindcast data are obtained from the Norwegian Meteorological Institute (MET Norway). The hindcast data for each location cover the period from 1957 to 2014.

First, the hindcast datasets were validated against available buoy measurements — both for extreme value predictions and for application of hindcast data for planning of marine operations. The validation was carried out considering the winter season and the summer season separately. For each season, the datasets for two consecutive months were used. A comparison of the time-series of the hindcast datasets against the buoy measurements showed that the hindcast datasets compared relatively well with the buoy measurements.

However, a comparison of the statistical parameters of the hindcast datasets against the buoy measurements showed that the hindcast datasets are slightly conservative in the estimate of the significant wave height for the Barents Sea and the Norwegian Sea. Overall, the data compared well, and the hindcast datasets are therefore considered in the following analysis.

Hindcast data from these 57 years show that the wave conditions in the selected Norwegian Sea location is harsher than the wave conditions in both the North Sea and the Barents Sea locations. This is in agreement with the general expected spatial trend in the wave climate on the Norwegian Continental Shelf (NCS). It was also observed that the wave conditions in the selected Barents Sea location are harsher than the wave conditions in the North Sea. These findings are also reflected in the NORSOK N-003 standard on “Actions and Action effects” (NORSOK, 2015).

The weather windows for weather-sensitive marine operations, that is, operations with operational reference period not exceeding 72 hours, were established from the hindcast dataset for each of the locations. It was observed that the Norwegian Sea has shorter weather windows, especially in the winter seasons, compared to both the Barents Sea and the North Sea. It was expected that the operational windows would be shorter in the winter seasons in the Barents Sea, due to the occurrence of polar lows. However, the polar lows are few and cause more concern related to forecasting of the weather conditions to start actual marine operations. Generally, the month with the highest probability of weather windows exceeding 72 hours was found to be July for all three locations.

Commentary by Dr. Valentin Fuster
2016;():V007T06A039. doi:10.1115/OMAE2016-54862.

Southern China has been subject to some of the deadliest typhoons in history with records going back over a thousand years. Before the large waves associated with a typhoon reach the mainland of China there is a delay between the typhoon reaching landfall and the time of the extreme waves arriving. This paper focuses on an approach to simulate this lag in the waves reaching landfall in the Qiongzhou Strait in southern China. A numerical approach has been adopted to simulate the typhoon and wave processes using a parametric typhoon model and the SWAN wave model. Two typhoon events are simulated (typhoon Kai-Tak in 2012 and typhoon Jebi in 2013) and used to tune the parameters for the numerical models. The simulated wind speeds and significant wave heights of the typhoon are compared with measured data. For the simulation of typhoon Kai-Tak, the correlation coefficient gives an 87% agreement between the simulated and measured values of wave height with a standard deviation of 0.29 m. For typhoon Jebi the fit is less good (66%). However, the simulation results have provided insight into improving the parametric typhoon model.

Commentary by Dr. Valentin Fuster
2016;():V007T06A040. doi:10.1115/OMAE2016-54956.

In recent years, considerable effort has been made in order to validate different methods that aim at estimating the wave spectra from the motions recorded on a ship or on an offshore platform. For more than ten years now, the University of São Paulo has been working on a wave inference method for moored oceanic systems, such as Floating Production Storage and Offloading (FPSO) vessels.

This paper brings the first results from an ongoing field campaign, started in December 2014, for the estimation of wave statistics by means of this system, which is based on a Bayesian inference approach. The performance of the motion-based method is checked against the wave estimations provided by a commercial marine radar system. The radar is installed in a fixed platform close to the FPSO that is being monitored, which is moored in a turret configuration. Comparison between both systems allows one not only to evaluate the performance of the method but also to evaluate the inherent limitations that exist when the estimations are based on a very large vessel, especially one whose heading may be subjected to fast variations.

In this work, a new algorithm for the partition of the wave spectrum is used, differing from the previous works. This technique allows the identification and combination of energy peaks of the directional spectrum into wave systems, providing better modal wave statistics and less noise in the final spectrum.

Previously validation of the wave inference method was made through numerical and small-scale experimental analysis. Also, in previous studies, a 9-month field campaign was also used to validate the Bayesian method using a spread-moored FPSO, but no comparison to other measurement technique was made, instead, numerical forecasts were adopted. In this work this objective is achieved: so far, 10 months of estimates from the Bayesian inference algorithm have already been compared to data supplied by a radar system.

Comparisons with the marine radar readings attest an adequate identification of mean wave directions, with the Bayesian model estimating mean wave directions that are in close agreement to those provided by the radar system most of the time. There are, however, a few cases when the discrepancy is large and the reasons for this are still under investigation.

The comparison between both sources of data provides some insightful results, which are discussed in this paper. First, they confirm and quantify the main biases of the applied method, which are related to the filtering of the energy from high-frequency waves. On the other hand, they attest that the performance of the method for more severe sea states is good overall, showing that the method is able to provide good estimates of wave height and direction and also to capture situations when wave conditions are changing fast, such as the ones that occur with the incoming of cold fronts from South.

Topics: Waves , FPSO
Commentary by Dr. Valentin Fuster
2016;():V007T06A041. doi:10.1115/OMAE2016-55066.

Periodic seabed undulations, such as nearshore sandbars, are known to reflect incoming surface waves of twice the wavelength by the so-called Bragg resonance mechanism. In view of this property, longshore seabed-mounted bars were proposed long ago as a means of coastal protection against the high momentum of incident oceanic waves. Many theoretical, computational, experimental and field measurements were conducted to understand their effectiveness in shielding the shore. The idea, nevertheless, proved impractical when Yu and Mei (JFM 2000, [1]) showed that due to an inevitable finite reflection from the shoreline, energy can get trapped in the area between the shoreline and the patch of bars eventually resulting in a much higher wave energy flux impinging the shoreline. Here we propose an arrangement of oblique bars that shelters the shore by diverting, rather than reflecting, shore-normal incident waves to the shore-parallel direction. A protected buffer zone is thus created at the shoreline. We show that this novel arrangement can very efficiently shelter the shore, is almost insensitive to the distance between the bottom corrugations and the shoreline, is relatively robust against frequency detuning, and will discuss that it can be designed to protect the shore against almost the entire broadband spectrum of incident waves.

Topics: Seabed
Commentary by Dr. Valentin Fuster

Ocean Engineering: Computational Mechanics and Design Applications

2016;():V007T06A042. doi:10.1115/OMAE2016-54994.

The wave-current effects are very important in several offshore applications, for instance, the wave-drift-damping of a Turret moored FPSO. This papers presents the incorporation of current effects in the higher order time domain Rankine Panel Method on development in the Numerical Offshore Tank (TPN) at the University of São Paulo (USP) already introduced in [1]. The method is based on a perturbation theory to study first and second order effects, considering the geometry described using NURBS (Non Uniform Rational Basis Spline) and the potential function, free surface elevation, pressure etc by B-splines of arbitrary degree. The study is performed for a simplified geometry (sphere) and the results regarding a fixed hemisphere compared to other numerical methods considering both first and second order quantities are presented.

Topics: Waves
Commentary by Dr. Valentin Fuster
2016;():V007T06A043. doi:10.1115/OMAE2016-55037.

Due to the combination of the forward speed and the prevailing wind for surface ship traveling in the ocean, the airflow passing over the ship’s superstructure causes the formation of a disturbed flow region and the large speed gradients of the mean wind over the flight deck, known as the ship airwake. This airwake would cause significant influence on the performance of the helicopter rotor during its taking off or landing, increase the operation workload of the pilot and even cause safe-landing issues, especially when the wind sweeps over the deck. This paper presents a numerical simulation of flow across the ship superstructure using DES and LES turbulent model. The ship model used for simulation is the standard SF2 surface ship model with experimental measurement data which could be used for the CFD code validation. The simulation results are compared with the experimental measurement data, and the comparison with experimental results shows good match for both DES and LES turbulent models. Simulation results show that a series of vortex had been generated after the flow separation with asymmetric characteristics. From upstream to downstream, the vortex intensity decreases, but suddenly increases after encountering the chimney. The comparison between DES and LES turbulent models shows the similar flow field and vortex structure around the ship superstructure with same grid sets. Both DES and LES are superior to RANS in solving ship airwake. The comparisons of DES and LES turbulent models show that DES can reflect the separated flow with limited computational resource and LES simulation could get higher resolution of the fluid flow structure with enough computational resources.

Commentary by Dr. Valentin Fuster
2016;():V007T06A044. doi:10.1115/OMAE2016-55072.

The arrangement design of ships and offshore plants is normally being made based on data of the past design and experts’ experiences. When performing the arrangement design, experts refer the past data and add additional requirements to the data for a new design based on their experiences. During this task, delay in design can occur due to the data missing or the absence of experts. In addition, most of ships and offshore plants are operated in the ocean environment so that spaces that can be filled with components for them are restricted. In this sense, there is a need to accumulate data regarding the past design, experts’ experiences, and design rules as a systematic structure. And a demand for optimization technique for the arrangement deign of ships and offshore plants are increasing. For this, an arrangement design framework for ships and offshore plants based on expert system and optimization technique is proposed in this study. And the proposed framework is applied to a problem with regard to the arrangement design of a small size submarine, and an FPSO topside. The results shows that the proposed framework can be used as a new tool for the arrangement design of ships and offshore plants.

Commentary by Dr. Valentin Fuster
2016;():V007T06A045. doi:10.1115/OMAE2016-55110.

Recently offshore installations for the development of gas and oil resources have moved toward the deeper sea and harsher environment. In this regard, there are increasing operational demands for offshore structures performing in depth with long distance transportation. Drill ships are one of the representative structures commonly employed for the development of energy resources in deep sea. However, these drilling systems are exposed to the various loadings and harsh environment. Therefore, current design code and standards are very conservative to take account of such conditions. The riser stanchions for the staking the riser are one of the structure that installed on the riser deck platform of drill ship. In transit or operating conditions, the riser load is applied to the stanchion. Therefore, the required strength of stanchion enough to withstand the riser loads and should be designed to meet owner and classification requirements. However, the currently practice design loads, which has been widely used in the shipyard and manufacture industries are too conservative resulting in excessive structural weight. In this regard, the purpose of this study is to propose a rational calculation procedure for the design of cost effective and lightweight stanchion structures.

This study used a pyramid stacking type of arrangement for the investigation of the effects of stanchion by riser self-weight and hull acceleration. First, the riser loads based on the current conventional design practice is compared with the results obtained by non-linear finite element analysis. In the finite element simulation, contact and damping conditions of stacked risers are explicitly considered. Second, the calculation of riser loads in transverse direction is not easy due to the difficulty associated with considering transverse hull acceleration. Therefore, a new design guideline is presented in a strength calculation of the stanchion structure against to the estimated design loads.

Commentary by Dr. Valentin Fuster

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

2016;():V007T06A046. doi:10.1115/OMAE2016-54013.

This paper conducts a RANS solver with k-ε turbulent closure to simulate hydrodynamics of wave run-ups of three types of wind turbine foundations, including monopile, gravity-based and tripod support structures. In this study, a semi-empirical formula is developed and calibrated based on velocity stagnation head theory by means of a CFD model, FLUENT. The numerical results are validated by the experimental data, which were implemented in the Large Wave Flume (GWK) of the Coastal Research Centre (FZK) in Hannover and published by Mo et al. (2007) [1]. It is indicated that the difference of normalized run-up envelopes among these wind turbine foundations is smaller for higher wave steepness than those for lower wave steepness. It is also obvious that the tendency of maximum run-up heights is considerably correlated with higher nonlinearity, whereas an opposite trend is obtained for minimum run-up envelops. Eventually, a calibrated run-up parameter is obtained by the present numerical simulation and found that the value becomes smaller with respect to higher nonlinearity and run-up heights.

Commentary by Dr. Valentin Fuster
2016;():V007T06A047. doi:10.1115/OMAE2016-54030.

The European highway E39 ferry-free Project led by the Norwegian Public Roads Administration (Statens vegvesen, NPRA) is in its full momentum on solving the technological challenges to cross the deep (up to 1350 m) and wide (up to around 6 km) Norwegian fjords. Different types of crossing solutions have been explored up to now. The current paper is related to the submerged floating tube bridge (SFTB) solution. But the model itself can be applied to any other structure that may experience loads induced by a passing vessel. The method of Xiang and Faltinsen (2010) for evaluating the calm water interacting loads between two vessels up to the point of collision has been extended to include an SFTB. Equations of loads are extended to include also the vertical load and pitch moment on the body in question. The vertical load induced on the SFTB is investigated for the purpose of identifying the significance of this effect on the design. The investigation includes this effect under different maneuver scenarios of the passing vessels: a single ship passing, two ships advancing in side-by-side positions, encountering or overtaking maneuvers. In addition, parametric studies on the submergence of floating tube bridge, and vessel speeds are studied to get more understanding of this effect. A simple exercise shows that it is worth of further investigation on ship-induced surface waves effects on the structures in vicinity of the ship path. Finally, the evaluated loads are compared with other types of loads from the analysis results of the on-going NPRA SFTB design.

Commentary by Dr. Valentin Fuster
2016;():V007T06A048. doi:10.1115/OMAE2016-54121.

The wave-induced motions of vessels moored next to a fixed object and open to the sea impact the operability of many offshore operations, and should be assessed in order to avoid accidents and catastrophes. When analysing vessels moored by a fixed object (e.g. quay-side or platform), time domain simulations have shown numeric instabilities resulting in unreliable outcomes. The origin of the numerical instability might lie in the hydrodynamic added mass and wave radiation damping. This is typically calculated using potential flow methods and influenced by the existence of standing waves in the gap between the two bodies. For certain frequencies, these give negative values, potentially causing instabilities in non-linear (coupled) time domain simulations. In these cases, the vessel can behave unexpectedly, generating energy rather than dissipating it. As such, certain simulations have been disregarded as they are unlikely to accurately represent real-life scenarios.

This paper investigates and compares added mass and damping using two different tools and studies the gap effect when conducting diffraction analysis using 3D panel methods. The work covers a literature study into potential theory, multibody analysis, Computational Fluid Dynamics (CFD) and lid techniques.

This is followed by a study conducted using both panel method and CFD analyses. The results from both approaches have been compared, showing interesting information and the necessity of researching more into the problem addressed in this paper.

Commentary by Dr. Valentin Fuster
2016;():V007T06A049. doi:10.1115/OMAE2016-54161.

Hydroelasticity is an important problem in the field of ocean engineering. It can be noted from most of the works published as well as theories proposed earlier that this particular problem was addressed based on the time independent/ frequency domain approach. In this paper, we propose a novel numerical method to address the fluid-structure interaction problem in time domain simulations. The hybrid numerical model proposed earlier for hydro-elasticity (Sriram and Ma, 2012) as well as for breaking waves (Sriram et al 2014) has been extended to study the problem of breaking wave-elastic structure interaction. The method involves strong coupling of Fully Nonlinear Potential Flow Theory (FNPT) and Navier Stokes (NS) equation using a moving overlapping zone in space and Runge kutta 2nd order with a predictor corrector scheme in time. The fluid structure interaction is achieved by a near strongly coupled partitioned procedure. The simulation was performed using Finite Element method (FEM) in the FNPT domain, Particle based method (Improved Meshless Local Petrov Galerkin based on Rankine source, IMPLG_R) in the NS domain and FEM for the structural dynamics part. The advantage of using this approach is due to high computational efficiency. The method has been applied to study the interaction between breaking waves and elastic wall.

Commentary by Dr. Valentin Fuster
2016;():V007T06A050. doi:10.1115/OMAE2016-54221.

With the fluid assumed incompressible and inviscid, and the flow irrotational, the fluid motion can be described by a potential ϕ which satisfies the Laplace equation within the whole domain Ω. The seabed and wall are impermeable and grid plate is transversely permeable only. Then the relationship between horizontal velocity and pressure difference could be deduced without considering the vertical component of the fluid in the grid plate. Reflection coefficient, transmission coefficient and energy-loss coefficient could be calculated when regular waves travel across one grid plate or multi-grid plates, by matching velocity potential and boundary conditions at each grid plate. For the given case, if 0.8 < Gr < 4.6 and Gi < 1.6, the wave absorptivity is more than 90% for four grid plates wave absorbing device.

Topics: Waves
Commentary by Dr. Valentin Fuster
2016;():V007T06A051. doi:10.1115/OMAE2016-54333.

Much research has been directed at understanding and predicting water slamming loads for a range of geometries of varying rigidity and size. Analytical and numerical studies focused on slamming of cylindrical rigid bodies are present in literature but there are relatively few experimental studies useful for validation purposes, none of which methodically investigate a range of curvatures. Despite the current understanding of slamming loads and structural responses, high speed marine vehicles still experience slamming related failures in operation.

In this study, nominally rigid, singly curved prismatic specimens of varying curvature are subjected to constant velocity water impacts relevant to those encountered by high performance offshore racing yachts and other high-speed craft.

Peak impact forces of 14 to 52 kN were recorded while testing specimens with radii ranging from 0.300 to 5.000 m. Experimental peak impact force and event impulse are found to be significantly lower than predicted by numerical and small scale empirically derived methods. A modification is introduced which improves the empirical model.

Topics: Stress
Commentary by Dr. Valentin Fuster
2016;():V007T06A052. doi:10.1115/OMAE2016-54365.

Wave runup on a vertical wall has both fundamental and practical interest. The wave impact on a structure is an important aspect that must be taken into account in the design of coastal structures. From linear wave theory, it is known that the wave amplitude on a vertical fully reflecting wall is twice the amplitude of the incoming wave. The result may be different for nonlinear dispersive waves. Following [1], we show that the propagation of a short wave group of very long nonlinear monochromatic waves above a flat bottom, can increase the amplitude nonlinearly in the travel to a wall, and produce much higher amplitudes on the wall compared to the initial amplitude: the amplification of such a wave can reach six times the initial amplitude. The strong amplification due to the combined action of nonlinear steepening and dispersion before the wall leads to waves that resemble undular bores. Using the same principles, we show that the propagation of a focusing wave group to a wall that is located at the focusing point of the wave can produce an extreme runup with 11 times amplification of the maximum initial amplitude. The simulations are done by HAWASSI-AB, a spatial-spectral implementation of Analytic Boussinesq model [2].

Commentary by Dr. Valentin Fuster
2016;():V007T06A053. doi:10.1115/OMAE2016-54441.

Nonlinear bubble dynamics in a pipeline and its exciting force are investigated by a numerical model based on BEM. The bubble motion is one of the main causes that the pipeline vibrates and generates noise in modern ships. The numerical bubble dynamics model is established under the incompressible potential theory. Bubble motion with different incoming flow in a bended pipe is simulated. We found that the bubble develops jet when it passes by the bend, and adjoin to the pipe surface in the side of the fillet center. The pulsation and the direction change of the bubble apply an exciting force on the pipe which has a positive correlation with the incoming flow speed and may lead vibration and noise.

Commentary by Dr. Valentin Fuster
2016;():V007T06A054. doi:10.1115/OMAE2016-54467.

In this paper, potential-flow and viscous-flow (CFD) solvers are used to compute wave loads on a suspended surface piercing cylinder, for a Keulegan-Carpenter number of KC ≈ 5. After describing the numerical tools used (Wavec2Wire and Re-FRESCO), and the associated numerical settings, a calibration of the wave height and periods is made with basis on experimental data. On the viscous-flow CFD side, attention is also paid to discretization (space and time) and iterative errors, in order to keep the uncertainty of the calculations the lowest possible. Moreover, both a non-turbulent symmetric domain and a turbulent full domain CFD calculations are performed and compared. On the potential-flow side, calculations with and without considering additional-drag forces of the cylinder are conducted. In general, the comparison between potential and viscous-flow results show small differences, with a slightly better agreement between the viscous-flow results and experiments but with a significant increase on computational costs. For this particular case the consideration of turbulence and of additional-drag forces have negligible influence on the results obtained.

Commentary by Dr. Valentin Fuster
2016;():V007T06A055. doi:10.1115/OMAE2016-54510.

A three-dimensional numerical model is established to simulate the interaction between waves and net cages. The porous-media fluid model is introduced to model the net cage in waves. An oscillating-boundary method is used to generate waves at one end of a tank partially filled with water. The flow motion of an incompressible, viscous fluid is described by Navier-Stokes equations and the free surface is tracked by volume of fluid (VOF) method. Validation of the numerical model is conducted by comparing the numerical results with the corresponding physical-model measurements of a net-cage model. In order to visualize the wave field around in-line net cages, numerical results of both wave elevation and water-particle-velocity distribution on a vertical plane of a transient field are presented. Compared with the undisturbed wave field, the effects of the net cages on both the wave elevation and the water-particle-velocity distribution around the net cages are noticeable. The study will contribute to understanding of the damping effect of a large fish farm on wave propagation.

Topics: Waves
Commentary by Dr. Valentin Fuster
2016;():V007T06A056. doi:10.1115/OMAE2016-54518.

The membrane-type tank is widely used in LNG storage devices, which include FLNG, FSRU, LNGC, and so on, and sloshing is an internationally important research focus. The run-up along the side wall of the tank and impact will be produced. Many researchers have proposed a variety of suppressing measures to ensure the safety of the tank. Based on the principle of wave eliminating, a new idea is put forward in the vicinity of the liquid surface mounting the floating plate and the original free surface is divided into three sub-surfaces. In the present paper, we make a series of model experiments on the Six-DOF motion platform through changing the frequency of the excitation. As a result, the sloshing mode is separated into the U-tube mode, the separated-tank mode and the diffraction mode. The experiments indicate that the range of the amplitude as for the slamming pressure is much smaller than without the floating plate.

Commentary by Dr. Valentin Fuster
2016;():V007T06A057. doi:10.1115/OMAE2016-54525.

As the sea transport demand increases constantly, marine corporations around the world are pursuing solutions with large scale and low cost, which makes ultra large containerships’ construction consequentially. Ultra large containerships are more flexible relatively, and the 2-node natural frequency can easily fall into the encountered spectrum frequency range of normal sea state. Meanwhile, as the speed of containerships is high and its large bow flare, when sailing with high speed, the bow structures may suffer severe slamming forces which can increase the design wave loads’ level and the fatigue damage. The importance of hydroelastic analysis of large and flexible containerships of today has been pointed out for structure design. Rules of Many Classification Society have made changes on design wave loads’ value and fatigue influence factor modification. The paper firstly introduced 3-D linear hydroelasticity theory to calculate the Response Amplitude Operator (RAO) in frequency domain, and then described 3-D nonlinear hydroelasticity theory to obtain the nonlinear wave loads time history in irregular waves in time domain, considering large amplitude motion and slamming force due to severe relative motion between ship hull and wave. Based on the theories, computer programs are made to conduct the calculations under specified load case, and some calculation and statistical results are compared with experimental results to verify the accuracy and stability of the programs secondly. The paper focused on the influence of springing and whipping on fatigue damages of 8500TEU and 10000TEU containerships in different loading cases, using spectrum analysis method and time domain statistical analysis method. The spectrum analysis method can calculate fatigue damage due to low-frequency wave loads and high-frequency springing separately, while the time domain statistical analysis can calculate fatigue damage due to the high-frequency damping whipping additionally, based on 3-D time domain nonlinear hydroelasticity wave loads’ time series simulation in irregular waves and rain flow counting method. Finally, discussions on influence factor of springing and whipping with different loading cases are made. Based on these two containerships in example, the fatigue damage due to whipping can be the same as the fatigue damage due to springing and even sometimes can be larger than the springing damage. According to the wave loads influence factor, the fatigue assessment of different position on midship section is done on the basis of nominal stress. Besides, some suggestions on calculating load case selection are made to minimize the quantity of work in frequency and time domain. Thus the tools for fatigue influence factor modification are provided to meet the demand of IACS’ UR[1].

Commentary by Dr. Valentin Fuster
2016;():V007T06A058. doi:10.1115/OMAE2016-54558.

Immersed tunnel technique has been developed and successfully applied to the installation of immersed tunnel. During the installation, the tunnel is connected to the dual pontoon floating structures (DPFS) by cables and lowered slowly to the tunnel trench. Such installations generally involve multi-body dynamics with wave-induced motions and nonlinear constraints. This paper establishes a time domain model for the immersed tunnel installation based on a linear potential flow approach. The mooring lines are simplified as linear undamped springs with constant stiffness. More attention has been paid to simulate the marine cables, which are modelled by the lumped mass method as well as by bilinear undamped spring. The effects of cable model on the wave-induced dynamics of the given system are discussed. In the numerical simulation, the slack phenomena and snap load of cables are observed and reveal the rich and complex dynamical behaviors of the immersed tunnel installation.

Commentary by Dr. Valentin Fuster
2016;():V007T06A059. doi:10.1115/OMAE2016-54576.

The dynamic analysis of a Tension Leg Platform (TLP) is investigated by combining wind and wave loads within the typhoon area. By considering different typhoon parameters, such as the tangential velocity, the radius of maximum velocity, the translational direction and velocity, the model of typhoon is established. The wave height and period are obtained by the empirical formula related to the parameters of typhoon. The nonlinear restoring stiffness of TLP is derived with the set-down motion of platform and the coupled motion of the tension leg and platform. The results in this paper indicates that typhoon has a major impact on the safety of the platform in production operation, and it is also a threat to the strength of tension legs and risers.

Commentary by Dr. Valentin Fuster
2016;():V007T06A060. doi:10.1115/OMAE2016-54583.

Safety of cargo transfer operations between side-by-side vessels depends on accurate modelling of hydrodynamic behavior, especially in terms of predicting the gap free surface elevations between the two vessels. The common industry practice of using linear potential flow models to study these interactions over-predicts the free surface elevations, due to the fact that potential flow does not include viscous dissipation effects such as flow separation at hull corners and skin friction. This may result in inaccurate projections of the time-window when these operations can safely take place. This is an important aspect for developments such as Floating Liquefied Natural Gas (FLNG) platforms, where side-by-side cargo offloading is an essential operation.

In a recent research [1], an approach of splitting the amount of energy lost through viscous dissipation (calculated from three-dimensional viscous CFD simulations) into components representative of the flow phenomena has been proposed. Using the approach, referred to as component energy dissipation, the amount of energy lost due to vortex shedding and skin friction can be estimated. Modifications to linear potential flow were also proposed in the referenced research, such that the energy loss components can be converted into dissipative coefficients that are used in terms added to the free surface and body boundary conditions. By combining use of the component energy dissipation approach and the modified dissipative potential flow model, better predictions of gap hydrodynamic interaction can be obtained, compared to using conventional potential flow.

In this paper, results from viscous simulations of two identical fixed-floating side-by-side barges of 280m (length) × 46m (breadth) × 16.5m (draught) under excitation from regular incident waves are presented, and compared with corresponding results from the modified dissipative potential flow model. Two types of side-by-side hull configurations were investigated, the first using rectangular barges with sharp bilge corners at varying gap distances and the second using barges with rounded bilge corners of varying radii at a fixed gap distance. Estimation of the dissipative coefficients used in the modified potential flow model, calculated from the viscous results, will also be discussed. The comparison of results serves both as a validation of the modified potential flow model, and to highlight the importance of including viscous dissipation when analyzing hydrodynamic interactions.

Commentary by Dr. Valentin Fuster
2016;():V007T06A061. doi:10.1115/OMAE2016-54610.

A 3D water entry of a typical bow model of sea-river link ship is studied using both experimental measurements and numerical predictions. A large number of systematic experiments have been performed with different pitch angles. The slamming process is simulated through finite element method with LS-DYNA. The distribution and magnitude of slam induced loads is determined from experiments and is calculated. The effect of the pitch angle and impact velocity is discussed based on the comparison between the predicted results and the experiments values.

Topics: Stress
Commentary by Dr. Valentin Fuster
2016;():V007T06A062. doi:10.1115/OMAE2016-54625.

Typically, in some side-by-side offshore operations, the speed of vessels is very low or even 0 and the headings are manually maneuvered. In this paper, the hydrodynamic responses of a two-body system in such operations under irregular seas are investigated. The numerical model includes two identical PSVs (Platform Supply Vessel) as well as the fenders and connection lines between them. A horizontal mooring system constraining the low frequency motions is set on one of the ships to simulate maneuver system. Accounting for the hydrodynamic interactions between two bodies, 3D potential theory is applied for the analysis of their hydrodynamic coefficients. With wind and current effects included, these coefficients are further applied in the time domain simulations in irregular waves. The relevant coefficients are estimated by experiential formulas. Time-varying loads on fenders and connection lines are analyzed. Meanwhile, the relative motions as well as the effects of the hydrodynamic interactions between ships are further discussed, and finally an optimal operation scheme in which operation can be safely performed is summarized.

Commentary by Dr. Valentin Fuster
2016;():V007T06A063. doi:10.1115/OMAE2016-54636.

Prediction of hydrodynamic loads during water exit of a body is of a great importance in designing the marine vehicles that take off from the free water surface such as sea planes and wing-in-ground effect vehicles (WIG), and that pierce through the free surface like missiles and submarines. The results of an experimental investigation on water exit of two different geometries, sphere and flat plate, with hydrophobic surfaces are presented in this paper. With and without the hydrophobic effects present, different fluid dynamics phenomena like free surface evolution, deformation and break up of free surface, wave generation, splash, air entrapment and water detachment from the solid surfaces during a water exit event have been examined. The non-dimensional exit coefficient, Ce is a function of the total vertical hydrodynamic force which depends on the geometry of the object and the hydrodynamic conditions along with the water parameters. Our study is aimed at understanding and modeling the nonlinear free surface effects and the dynamics of water exit under an extended range of parameters including hydrophobic effects.

In this study, due to lack of the experimental data on the water exit problem in literature, water exit tests have been set up, first for initially partially immersed spheres and flat plates, with their center above the free surface, to be towed vertically from the water surface at various speeds. Secondly, buoyancy driven water exit of a fully immersed sphere is investigated. It is observed that when the sphere rises up, it first starts deforming the free surface, and then pierces into it. The thin water layer attached to the surface of the sphere is drawn back to the test tank as the sphere moves further upward. This causes breaking of the free surface, air entrapment and wave generation in the water tank.

From digital images captured using a high speed camera, free surface breakup and water detachment at different velocities are observed and the time evolution of the water detachment and the exit characteristics are measured during a water exit event. The position of the sphere and its velocity are plotted against time. A detailed measurement of the global loads on the test objects during exit is carried out by employing strain gauges.

We also showed the effects of water detachment on the test bodies during exit and after fully exited via strain gauges. All this data is also collected under the hydrophobic effects, to show how the change in surface characteristics would have significant impacts on the water exit phenomenon. Analyzing the difference in occurrence of water flow separation, the change in kinetic energy of the fluid and the free surface deformation under the hydrophobic effects may help give a better explanation of the phenomena observed during water exit and improve the design characteristics of marine structures for a water exit event.

Topics: Water
Commentary by Dr. Valentin Fuster
2016;():V007T06A064. doi:10.1115/OMAE2016-54809.

The increasing scale of the ship structure calls for hydroelastic analysis due to the growing flexibility compared with the regular ships which are regarded as rigid bodies traditionally. In the berthing stage of a barge, the motion responses and bending/torsional forces of the barge will be strongly affected by the flexible deformations of the barge itself and the wave elevation, which will be further affected by both the neighboring floating mobile harbor’s rigid motions and flexible deformations. In this paper, based on 3D potential theory, the coupling analysis model of a flexible barge adjoining a mobile harbor for the predictions of the motion responses and forces of the flexible barge is built. The effects of the flexibility of the barge, the distance between two adjacent structures, and the angles of incoming waves are numerically investigated. Wave elevation in the gap is also analyzed in order to explain the resonant peaks of the motion response.

Commentary by Dr. Valentin Fuster
2016;():V007T06A065. doi:10.1115/OMAE2016-54872.

Violent impacts due to plunging waves impinging on a 2D tension-leg model structure were experimentally investigated in a laboratory. In the experiment, velocities, pressures, and void fraction were simultaneously measured and the relationship among them was examined. The nonintrusive bubble image velocimetry technique was employed to quantify the instantaneous bubbly flow velocities and structure motion. Pressures on the structure vertical wall above the still water level were measured by four differential pressure sensors. Additionally, four fiber optic reflectometer probes were used to measure the void fraction coincidently with the pressure sensors. With repeated simultaneous, coincident velocity, pressure and void fraction measurements, temporal evolution of the ensemble-averaged velocities, pressures, and void fraction were demonstrated and correlated. Relationship between the peak pressures and their rise time was examined and summarized in dimensionless form. Impact coefficients that relate the impact pressure with flow kinetic energy were obtained from the ensemble-averaged measurements. Finally, the impact coefficients with and without the consideration of the fluid density variation due to bubbles were examined and compared.

Topics: Pressure , Waves , Porosity , Tension , Water
Commentary by Dr. Valentin Fuster
2016;():V007T06A066. doi:10.1115/OMAE2016-54892.

Several bridge concepts for crossing deep and wide fjords along E39 at the west coast of Norway have been developed the last years. One of the most challenging fjord crossing is suspected to be the crossing of Sulafjord, 3 to 5 km wide, 400 m deep and with presence of relatively large swell waves.

A suspended floating bridge concept is a marine slender flexible structure with large volume elements as floating support. The hydrodynamic actions on the floaters is an additional excitation compared to a traditional suspended bridge with fixed piles. In order to assess the effects of this excitation, it is important to consider the whole system and accurate hydrodynamic methods.

While the superstructure type — a suspended bridge — is set, the type of floating foundation remains open. From the offshore experience, it is seen that different types of floaters are used for moored platforms, and these floaters have significantly different characteristics in particular with regards to wave response and stability.

The design requirements for an offshore platform differ greatly from those of a suspended floating bridge crossing a fjord. For a floating bridge, the payload requirements are not the most challenging, while it is more difficult to limit the tilting and dynamic excitation of the tower (mounted on the floaters). The bridge beam is suspended at the top of the towers and will respond to any excitation due to motions of the tower tops.

A global numerical model of the bridge to simulate nonlinear dynamic response due to regular and irregular waves is built. The numerical model of the bridge is simplified from a structural point of view. However, the dynamical properties and eigenmodes are verified against a more detailed structural model. Together with a 50-year long continuous time-series of wind, wind waves and swells a study of the bridge operability and extreme responses for different floater concepts is conducted.

Normally the design phase should aim at avoiding any natural periods to fall within the wave frequency domain. This seems difficult for the proposed 3-span floating suspension bridge, instead solutions to minimize the excitation from waves for wave periods around the given bridge eigenperiods are sought.

Commentary by Dr. Valentin Fuster
2016;():V007T06A067. doi:10.1115/OMAE2016-54937.

We report on the mathematical and numerical modelling of amplified rogue waves driving a wave-energy device in a contraction. This wave-energy device consists of a floating buoy attached to an AC-induction motor and constrained to move upward only in a contraction, for which we have realised a working scale-model. A coupled Hamiltonian system is derived for the dynamics of water waves and moving wave-energy buoys. This nonlinear model consists of the classical water wave equations for the free surface deviation and velocity potential, coupled to a set of equations describing the dynamics of a wave-energy buoy. As a stepping stone, the model is solved numerically for the case of linear shallow water waves causing the motion of a simple buoy structure with V-shaped cross-sections, using a variational (dis)continuous Galerkin finite element method.

Commentary by Dr. Valentin Fuster
2016;():V007T06A068. doi:10.1115/OMAE2016-54957.

Multibody operations are routinely performed in offshore activities. One classical example is the FLNG and LNGC side-by-side offloading case. To understand the phenomenon occurring inside the gap is of growing interest to the offshore industry. One important issue is the existence of the irregular frequency effect. The effect can be confused with the physical resonance. Thus it needs to be removed.

An extensive survey of the previous approaches to the irregular frequency problem has been undertaken. The matrix formulated in the boundary integral equations will become nearly singular for some frequencies. The existence of numerical round-off errors will make the matrix still solvable by a direct solver, however will result in unreasonably large values in some aspects of the solution, namely the irregular frequency effect.

The removal of the irregular effect is important especially for multi-body hydrodynamic analysis in identifying the physical resonances caused by the configuration of floaters. This paper will mainly discuss the lid method on the internal free surface. To reach a higher accuracy, the singularity resulting from the Green function needs special care. Each term in the wave Green function will be evaluated using the corresponding analysis methods. Specifically, an analytical integral method is proposed to treat the log singularity. Finally, results with and without irregular frequency removal will be shown to demonstrate the effectiveness of our proposed method. The validation cases include mini-boxbarge, boxbarge and cylindrical dock, which has apparent irregular frequency effect in their output results.

Commentary by Dr. Valentin Fuster
2016;():V007T06A069. doi:10.1115/OMAE2016-55055.

A semi-analytical method is developed to investigate water-wave radiation and diffraction by an array of truncated vertical cylinders as a model for a point-absorber wave farm. Each cylinder can have independent movements in six modes. The method of matched eigenfunction expansions is applied to obtain the velocity potential for the fluid. To achieve fast computation, the effects of evanescent modes of locally scattered waves from one cylinder are neglected in the near fields of the neighboring cylinders. Wave-exciting forces and moments on an individual cylinder or a group of cylinders, situated among an array, are evaluated by a new, generalized form of Haskind relation that is applicable to an array configuration. In results, hydrodynamic coefficients and wave-exciting loads are presented for arrays of different configurations. Comparisons between wave-exciting loads obtained from the generalized Haskind relation and those from direct diffraction solutions show excellent agreements.

Topics: Waves , Cylinders
Commentary by Dr. Valentin Fuster
2016;():V007T06A070. doi:10.1115/OMAE2016-55071.

Ship Classification societies have rule requirements for assessment of propeller strength in terms of minimum blade thickness, however these rules generally covers only propellers of conventional geometry. In case of nonconventional (e.g. highly skewed) propellers, class societies ask for direct and detailed stress analysis for examination and approval. This paper introduces a procedure/strategy for assessing the strength of highly skewed propeller by determination of hydrodynamic load and consequent structural stress. The hydrodynamic analysis is performed separately using Reynolds Averaged Navier Stokes (RANS) solver STARCCM+ and structural analyses by finite element solver ANSYS. The hydrodynamic pressures as computed by CFD are transferred to the propeller model to calculate the resulting stationary stresses for maximum and minimum blade loading, using a quasi-static approach. The significance of root fillet in propeller design is demonstrated by comparing blade stress results with and without fillet at blade root. The stress concentration as a function of geometry and loading is also represented by the presented examples. The proposed method can assist the designer or reviewer to assess adequacy of strength of solid propellers as well as composite propellers of any geometrical configuration.

Commentary by Dr. Valentin Fuster
2016;():V007T06A071. doi:10.1115/OMAE2016-55087.

Ultra large vessels such as recently engineered FPSOs and FLNG terminals require careful consideration of the effect of water depth in determining their motion characteristics. An efficient numerical implementation of the finite depth Green function, utilizing a series and integral form, has been developed. This is then incorporated in a three dimensional frequency domain panel method program capable of considering forward speed effects. The effect of water depth on the first and second order wave exciting forces and induced motions are studied.

An optimization framework is then developed combining an automated parametric FLNG hull form, the aforementioned in-house developed hydrodynamic analysis tool and an evolutionary optimization solver. In this paper, the principal methodologies used in the development of the finite depth Green function, solving the boundary value problem and key learning from the FLNG optimizations will be discussed.

Topics: Optimization , Water
Commentary by Dr. Valentin Fuster

Ocean Engineering: Marine Environment and Very Large Structures

2016;():V007T06A072. doi:10.1115/OMAE2016-54056.

A submerged floating tube bridge (SFTB) concept has been developed for the Bjørnafjord crossing. Two designs are currently considered feasible: pontoon stabilized and tension leg stabilized SFTB. Previous studies have found that the vertical motions and accelerations are largest for the pontoon stabilized concept, although both designs are within the requirements. With respect to ship impact, seabed mooring is favorable. No ship impact is likely at the current depth of the bridge, and this is considered a great advantage. Submarine impact on the tethers must be considered, however. Using a failsafe link between the pontoons and the tube itself significantly lowers the stress in the bridge due to ship impact. The link will break at impact energies above a given threshold, and release the pontoon. The bridge is designed to survive loosing one pontoon. It is seen that the bridge survives impact even without the failsafe, thus it is not a critical element, but the stress reduction is beneficial. The cost of the tension leg moored alternative is substantially higher than that of the pontoon stabilized one. Both concepts are considered feasible, but the final decision depends on more accurate knowledge of the soil conditions in the fjord.

Commentary by Dr. Valentin Fuster
2016;():V007T06A073. doi:10.1115/OMAE2016-54579.

As the development of offshore marginal field becomes a heated topic, various design selections of offshore platform have already been put forward in the community such as Sevan SSP, IQFP-FPSO, octagonal FPSO or sandglass-type FDPSO. In this paper, a new Muliti-Cylinder FPSO (MCPSO) with section pile foundation is proposed for developing offshore marginal field in South China Sea. And the structure design in detail has been described.

The MCPSO consists of six cylindrical concrete tanks and six suction piles. The suction piles can be raised up during platform towing process. Considering the effects of tanks and piles on the towing stability, both intact and damage stability analysis of MCPSO were performed under different towing conditions. The influence of ballast ratio, towing direction and different damaged modes on the towing stability is analyzed.

The results indicate that the MCPSO has the characteristics of self-installation, movable, reusable and so on, which is suitable for the development of marginal oil field. The MCPSO filled with 40% seawater in ballast tank and towed in 90 direction has the best intact and damage stability characteristic.

Commentary by Dr. Valentin Fuster
2016;():V007T06A074. doi:10.1115/OMAE2016-54711.

The measured data are often required and taken as the input of different kinds of algorithms for modal parameter identification, model updating and/or damage detection of offshore platforms. This paper investigates the application performance of the approach to offshore platforms, by using real measured data. Considering the fact of a recorded loading is always finite in duration, most likely aperiodic, and even damped because of the existence of damping of the structures, a recent developed complex exponential decomposition method [1] was employed to deal with real measured data. Therefore, in this article, three kinds of measured data will be used. The first one is a measured wave force that was collected through an experiment conducted in the lab of Ocean University of China, to investigate the feasibility of original external loading decomposition. To study the performance of the proposed method when applied to offshore structures, two sets of typical signals are used: one set was collected from the JZ20-2MUQ offshore platform when it was excited by ice, and the other was collected from the WZ11-4D platform when it was excited by waves. The results show that the approach can decompose the real measured data into a series of non-harmonic components, and a large model order is often suggested.

Commentary by Dr. Valentin Fuster
2016;():V007T06A075. doi:10.1115/OMAE2016-54851.

We employed maturely developed methods and software, RIFLEX/SIMO/SIMA to look into the feasibilities of different floating tunnel/bridge design concepts. The global hydroelastic responses of two concepts, i.e. tether/pontoon supported hybrid tunnel concept and floating foundation supported girder concept and have been investigated. The distributions of maximum values of the deflection, bending moment and stress along the bridges under different sea conditions are presented.

Commentary by Dr. Valentin Fuster
2016;():V007T06A076. doi:10.1115/OMAE2016-54898.

Abundant oil and gas resources are stored in the continental slope region of northern South China Sea. However, due to the submarine landslide disasters, submarine pipelines and cables and other submarine facilities were damaged, which seriously affected the development of oil and gas resources, leading to a huge economic losses. In order to ensure the safe operation of the submarine gas pipeline, it is necessary to carry out some researches in the submarine slope stability. In this paper, an uncertainty analysis method is applied and a distribution model which is suitable for slope stability influencing factors of this region is put forward. In addition, the risk of submarine slope is analyzed, plotting submarine landslide risk evaluation maps. Different stochastic characteristics of variables such as water depth, angle of slope, shear strength and earthquake acceleration etc can be reflected reasonably, which is helpful for the safety of oil and gas transportation.

Commentary by Dr. Valentin Fuster
2016;():V007T06A077. doi:10.1115/OMAE2016-55154.

Due to the serious results of an oil leakage of a subsea pipeline, the prediction of the numerical simulation must be modeled accurately and quickly. To select a proper numerical model to simulate the complex flow, a comparative study with different grid models is carried out. A two dimensional oil diffusion model under the ocean current is established using the finite volume method. Based on the structured grid, the volume of fluid method is employed to track the interface of the multiphase flow. Two kinds of grid models, the sparse and the dense grid, are simulated under the same conditions. Three cases are selected from the typical experiments in Fan (1967). The available experimental data in the oil spill test of Fan (1967) are employed as a criterion for the comparison between different grid models. According to the calculation results, a comparative analysis of the oil diffusing form, the arriving time to the surface, and the lateral drifting distance is studied between the two grid models, respectively. The surface diffusion range and the simulated time are also introduced to do the comparison for the grid density.

Commentary by Dr. Valentin Fuster

Ocean Engineering: Model Tests

2016;():V007T06A078. doi:10.1115/OMAE2016-54020.

During steady state measurements, erratic oscillations can often be observed in the measured signals. For example, when measuring the ship-model resistance in a towing tank, oscillations might originate from vortex shedding, carriage control, rail imperfections, etc. These oscillations contribute to the random uncertainty of the observed mean value. The total observed random uncertainty can be caused by rapidly or slowly evolving oscillations. Rapidly evolving means that the periodicity is typically within the time frame of measurement, e.g. vortex shedding. Slowly evolving means that the periodicity is typically not contained within the available time frame of measurement, e.g. creep in a force sensor. The random uncertainty of the mean of a single measurement can be evaluated accurately when the rapidly evolving effects are dominant. The observed autocovariance of the signal can be used to find an uncertainty estimate for the signal as a whole [1]. In this paper, a new methodology to quantify the individual contribution of various spectral components to the random uncertainty is presented.

The new methodology is based on the power spectrum of a signal. The power spectrum only describes the energy content of all the rapidly evolving effects, while energy of slowly evolving effects is absent. For many stationary processes, individual spectral components of the power spectrum can be regarded uncorrelated or weakly correlated with each other. Each individual frequency component may be regarded as an uncorrelated band pass noise process. The uncertainty contribution of such processes to the mean value can be evaluated accurately, hence the spectral density of the process is directly correlated to the uncertainty of the mean; lowering the spectral density lowers the uncertainty. By quantifying the uncertainty contribution for each spectral component, various effects can be distinguished in the frequency domain and their individual contribution to the uncertainty of the mean can be determined. Using this information, sound decisions can be made to what effects should receive effort to lower the amplitude of oscillation and hence the random uncertainty.

This paper presents the newly developed power spectrum based method, called uncertainty spectrum, to determine the spectral contribution to the uncertainty of the mean and demonstrates the application of this method to ship-model resistance measurement data.

Topics: Uncertainty
Commentary by Dr. Valentin Fuster
2016;():V007T06A079. doi:10.1115/OMAE2016-54134.

Modeling shallow-water waves in a basin with a finite length and width introduces challenges related to low-frequency (LF) waves, especially for testing of moored vessels with long natural periods. Waves in this frequency range are also present in reality, as for instance bound set-down waves and unbound free waves formed by the geometry bathymetry. In model basins, additional unwanted LF wave components will be formed as a side-product of the wave generation and due to the basin geometry though. Standing waves over the basin length and width (basin modes) can generally be identified, which are difficult to dampen using beaches. This is the case for every wave basin, as they all have finite dimensions.

Moored structures generally have natural frequencies in the LF range, which may be excited by basin modes with similar frequencies. It is therefore important to understand the natural modes of a basin before tests with moored structures in shallow water are done. The energy of these basin modes increases and their natural frequency decreases with decreasing water depth (waves travel slower in shallow water). Generally, it can be said that the issues with basin modes are present on very shallow water (typically ∼15–30 m water depth full-scale for structures with a length around 200 m at a scale around 1 to 40). The smaller the basin for the same water depth, the higher the basin mode frequencies and the higher the likelihood of resonance problems. The energy and frequencies of the basin modes and their relevance for specific tests depend on the effective length and width of the basin, the water depth, wave conditions and the (mooring stiffness of) the structure under consideration. The influence of these variables is evaluated in the current study. Tests were done in MARIN’s Offshore Basin (OB), but most of the results are also expected to be applicable to other basins. The observed basin mode frequencies during these tests were compared to the theoretical values, and an overview of the unwanted LF wave content as a function of water depth, wave height and period is presented. The energy and shape of individual basin modes is also discussed. Considering these results, a practical approach for future basin projects on shallow water is described.

Topics: Waves , Water
Commentary by Dr. Valentin Fuster
2016;():V007T06A080. doi:10.1115/OMAE2016-54262.

As a multifunction floating platform, Floating Drilling, Production, Storage and Offloading (FDPSO) combining the well-known Floating Production, Storage and Offloading (FPSO) with a drilling unit. For the environment condition of deep-water oilfield is very severe, the motion response and mooring line tension of FDPSO is a worthy topic of studying. In this study, the numerical time-domain coupled prediction method for the mooring line tension and motion response of FDPSO system is constructed by ANSYS AQWA software. Furthermore, the results of a model test conducted in Harbin Engimeering University are used to investigate the feasibility and validity of the commercial simulation. The effect of mooring line pre-tension on the response of FDPSO is studied by varying the pre-tension of mooring line during the calculation. The time series curve of the mooring line tension and motion response, and the comparison of motion spectrum and mooring line tension spectrum are provided in this article.

Topics: Mooring , Tension
Commentary by Dr. Valentin Fuster
2016;():V007T06A081. doi:10.1115/OMAE2016-54336.

The authors report how to estimate stopping ability of full-scale ships using free-running model equipped with an auxiliary thruster. Theoretical analysis based on a modular mathematical model clarifies the similarity of three-degree-of-freedom stopping manoeuvre of a model ship to full-scale cannot be ensured by the use of auxiliary thruster. The authors, however, propose JSC, propeller advance ratio J and speed correction, ensuring the equality of J and the similarity of speed response of free-running model ship to full-scale during stopping manoeuvre. JSC is a control method of free-running model ship equipped with an auxiliary thruster for estimating stopping ability of full-scale ship from the viewpoint of safety. Numerical simulation confirms JSC gives safe side estimates of stopping ability with regard to the track reach and stopping time with acceptable margin. The analysis shown here proves the advantage of JSC comparing with possible other control methods of free-running model ships.

Topics: Ships
Commentary by Dr. Valentin Fuster
2016;():V007T06A082. doi:10.1115/OMAE2016-54356.

For ultra large ore carriers, springing response should be analyzed in the design stage since springing is the steady-state resonant vibration and has an important effect on the fatigue strength of hull structure. The springing response of a 550,000 DWT ultra large ore carrier has been studied by using experimental and numerical methods. A flexible ship model composed of nine segments was used in the experiment. The model segments were connected by a backbone with varying section, which can satisfy the request of natural frequency and stiffness distribution. The experiments in regular waves were performed and the motions and wave loads of the ship were measured. The experimental results showed that springing could be excited when the wave encounter frequency coincides with half or one-third the flexural natural frequency of the ship. In this paper, the analysis of the hydroelastic responses of the ultra large ore carrier was also carried out using a 3-D hydroelastic method. Comparisons between experimental and numerical results showed that the 3-D hydroelastic method could predict the motions and the vertical bending moments quite well. Based on this numerical method, the fatigue damage was estimated and the contribution of springing was analyzed.

Commentary by Dr. Valentin Fuster
2016;():V007T06A083. doi:10.1115/OMAE2016-54550.

Trimaran, as a high performance ship, its special ship form is different from regular monohull ship. The particular characteristics of longitudinal and transverse wave loads are concerned greatly by ship structure designers. Theoretical methods for forecasting the wave loads of trimaran are under developing. And the relative comprehensive model test study of trimaran’s wave loads is rare in the publication. In this paper, the trimaran model test had been carried out in the tank. According to similarity theory and simulation of stiffness, a scale model of trimaran with measurement system had been manufactured. Then the segmented model had navigated in regular waves of different sea conditions. Through analysis of the experimental data, the wave loads characteristics of this trimaran are presented. A comparison of test results and theoretical values is also made to show the similarities and differences. This study can provide some useful information for further design of high-speed trimaran in the point of wave loads, which maybe a reference for research of trimaran.

Topics: Stress , Propulsion , Waves
Commentary by Dr. Valentin Fuster
2016;():V007T06A084. doi:10.1115/OMAE2016-54734.

Jackets are structures used in the offshore industry as a bottom supported platform for oil and gas production. The jackets have to be built in order to withstand the harsh sea environment. Such designs demand in depth analysis to predict the loads acting on the structure and its response. Depending on the sea states in which the structure needs to be installed, breaking load can be important. Estimation of breaking load for single cylinder exists in literature, since the breaking load on the jacket structure needs a lot more clarity. The aim of this paper is to estimate the impact force on a model jacket using Duhamel integral, which was not explored before. The impact load so far analyzed was compared with theoretical explanations given by Goda, et al. (1966), Wienke and Oumeraci (2005). The scope of the study is limited to plunging type of breakers. Five loading cases include wave breaking at far-front of a structure, in front of structure, on the front leg, on the rear leg and a non-breaking case was considered.

Topics: Stress
Commentary by Dr. Valentin Fuster
2016;():V007T06A085. doi:10.1115/OMAE2016-54842.

Model tests at ballast and design draught are used to convert the sea trial results from the ballast trial draught to the contractual design draught. Correlation allowances in model test results and their effect on the trial performance prediction are of major importance. Nowadays it is not only typical to verify the contract speed but also the EEDI certification requires a verification of the speed power performance of the vessel. The use of a to favorable CA-value may lead to attractive performance figures, but also leads to higher fuel consumption figures than expected. Furthermore the design point of the propeller is affected, which leads to a too low light running margin and in some cases to erosive cavitation.

During a study, large spreading in the values of the correlation allowances for design draughts have been found for merchant vessels tested at different model test institutes, but at ballast trial draught the spreading is much less. Can it happen that some institutes select favorable correlations allowances on the basis of inaccurate trial data of shipyards? Or should we accept a large spreading in correlation allowances and have these indeed been confirmed by sea trials at design draught? This paper will present a discussion using the experience of a large full scale trial database as well as the accuracy of model and full scale tests.

Commentary by Dr. Valentin Fuster

Ocean Engineering: Ocean Engineering Technology

2016;():V007T06A086. doi:10.1115/OMAE2016-54093.

Various emission control regulations enforce vessels to collect performance and navigation data and evaluate ship energy efficiency by implementing onboard sensors and data acquisition (DAQ) systems. These DAQ systems are designed to collect, store and communicate large amounts of performance and navigation information through complex data handling processes. It is suggested that this information should eventually be transferred to shore based data analysis centers for further processing and storage. However, the associated data transfer costs introduce additional challenges in this process and enforce to investigate cost effective data handling approaches in shipping. That mainly relates to the amount of data that are transferring through various communication networks (i.e. satellites & wireless networks) between vessels and shore based data centers. Hence, this study proposes to use a deep learning approach (i.e. autoencoder system architecture) to compress ship performance and navigation information, which can be transferred through the respective communication networks as a reduced data set. The compressed data set can be expanded in the respective data center requiring further analysis. Therefore, a data set of ship performance and navigation information is analyzed (i.e. compression and expansion) through an autoencoder system architecture in this study. The compressed data set represents a subset of ship performance and navigation information can also be used to evaluate energy efficiency type applications in shipping. Furthermore, the respective input and output data sets of the autoencoder are also compared as statistical distributions to evaluate the network performance.

Commentary by Dr. Valentin Fuster
2016;():V007T06A087. doi:10.1115/OMAE2016-54168.

This study proposes marine engine centered data analytics as a part of the ship energy efficiency management plan (SEEMP) to overcome the current shipping industrial challenges. The SEEMP enforces various emission control measures, where ship energy efficiency should be evaluated by collecting vessel performance and navigation data. That information is used to develop the proposed data analytics that are implemented on the engine-propeller combinator diagram (i.e. one propeller shaft with its own direct drive main engine). Three marine engine operating regions from the initial data analysis are noted under the combinator diagram and the proposed data analytics (i.e. data clustering methodology) to capture the shape of these regions are implemented. That consists of implementing the Gaussian Mixture Models (GMMs) to classify the most frequent operating regions of the marine engine. Furthermore, the Expectation Maximization (EM) algorithm is used to calculate the respective parameters of the GMMs. This approach can also be seen as a data clustering algorithm that facilitated by an iterative process for capturing each operating region of the marine engine (i.e. the combinatory diagram) with the respective mean and covariance values. Hence, these data analytics can be used in the SEEMP to monitor the performance of a vessel with respect to the marine operating regions. Furthermore, it is expected to develop advanced mathematical models of ship performance monitoring under these operational regions of the marine engine as the future work.

Topics: Engines , Ships
Commentary by Dr. Valentin Fuster
2016;():V007T06A088. doi:10.1115/OMAE2016-54180.

An increasing number of ships are equipped with vessel monitoring systems logging ship data during normal operation. We developed a framework for multivariate time series data mining to extract the information of vessel behavior from an in-service dataset. The approach is established on unsupervised data clustering using Self-Organizing Map (SOM), K-means, and k-Nearest Neighbors Search (K-NNS) for searching specific maneuvers. The results are based on ship monitoring data of NTNU’s research vessel, Gunnerus. It is shown that this approach is effective to detect prior unknown ship states with acceptable accuracy. The framework proposed and the results of this work can be of interest to those involved in ship administration, marine traffic flow engineering, ship maneuvering studies and assessment of ship design.

Topics: Data mining , Ships
Commentary by Dr. Valentin Fuster
2016;():V007T06A089. doi:10.1115/OMAE2016-54646.

The shipping sector progressively faces stricter environmental regulations. Several technical and operational measures are available to ship owners and operators in order to comply with these regulations. While some measures are successful in reducing emissions of a pollutant, they may increase emissions of other pollutants. In addition, there may be interactions between regulations on environmental impacts on sea and air. In some other cases, despite the success of these measures in reducing environmental footprints of ships, the environmental problem may shift elsewhere in the life cycle. Therefore, there may be a trade-off between different environmental controls and, in a broader context, a trade-off between environmental impacts at a life stage and overall environmental goals. Although several studies focus on addressing individual regulations, few acknowledge such interactions. This study aims at highlighting the side effects of different measures to address environmental regulations.

Topics: Regulations
Commentary by Dr. Valentin Fuster

Ocean Engineering: Ocean Measurement and Data Interpretation

2016;():V007T06A090. doi:10.1115/OMAE2016-54176.

Image based methods, particularly those applying stereoscopic approaches, have difficulty in measuring waves produced in laboratory facilities, mainly because, in such conditions, the water surface is smooth, translucent and highly specular. The present study introduces a method to estimate regular wave parameters (particularly the amplitude) that is capable of dealing with such characteristics of the surface. The method is based on image intensity and its relationship with the surface normal. Although the concepts behind the method are well known since the 1960 decade, we present a novel framework of algorithms and experimental setup which produces promising results even using cheap cameras and under regular laboratory illumination conditions. The specific set of results reported herein was obtained by a single camera; however, multiple cameras can be used both to improve the quality of calibration parameters and to reduce the overall estimation errors. To conclude, we present conjectures regarding the expansion of the method to measure irregular waves and to deal with more unrestricted illumination conditions.

Topics: Waves
Commentary by Dr. Valentin Fuster
2016;():V007T06A091. doi:10.1115/OMAE2016-54282.

In this paper, we interrogated wave data collected by US Army Corps of Engineers at their well-known Field Research Facility, Duck, North Carolina and SHELL Corporation at Lagos, Nigeria. Both measurements were designed to collect wind waves with a conventional wave sampling configuration and not a dedicated infragravity wave sampling regime. Here, we developed a new approach to obtain directional information of and explored the potential to model infragravity waves in the spectral domain. It was found that infragravity wave heights had a strong dynamic relationship with an inverse relative depth parameter and that directional spreadings were moderately correlated with wind wave spreadings and wave energy. Further, infragravity directional spreadings were typically broader compared to their wind wave directional spreading counterparts.

Topics: Waves , Modeling
Commentary by Dr. Valentin Fuster
2016;():V007T06A092. doi:10.1115/OMAE2016-54363.

This paper concerns the field measurements of wave crest elevations and wind speed obtained from fixed platforms in the North Sea during the period of 2011 to 2015. An improved quality control process is proposed by the authors aiming to maximise the amount of waves that are not discarded, whilst maintaining a reliable database. Applying this revised methodology, the total number of waves in the quality-controlled dataset is approximately 184 million. In total, the measurements correspond to 32 years of continuous data and contain approximately 0.0054% of rogue waves, according to the definition of Haver (2000) [1]. As the sea-state steepness increases, it is observed that the measured crest elevations are poorly modelled by the Forristall (2000) [2] distribution; similar behaviour is observed for sea-states with high wind speeds. The model by Tayfun and Fedele (2007) [3] is shown to be a better alternative in these cases, demonstrating that non-linear amplification beyond second-order cannot be neglected.

Commentary by Dr. Valentin Fuster
2016;():V007T06A093. doi:10.1115/OMAE2016-54496.

The use of remotely wave sensing by a marine radar is increasingly needed to provide wave information for the sake of safety and operational effectiveness in many offshore activities. Reconstruction of radar images needs to be carried out since radar images are a poor representation of the sea surface elevation: effects like shadowing and tilt determine the backscattered intensity of the images. In [1], the sea state reconstruction and wave propagation to the radar has been tackled successfully for synthetic radar images of linear seas, except for a scaling in the vertical direction. The determination of the significant wave height from the shadowed images only has been described in [2]. This paper will summarize these methods, and provides the first results for the extension to nonlinear seas.

Commentary by Dr. Valentin Fuster

Ocean Engineering: Offshore Industry: Structures and Design

2016;():V007T06A094. doi:10.1115/OMAE2016-54347.

With the development of ocean energy exploration, reliable and low cost semi-submersible platforms are expected to develop. The maximum pitching amplitude of a floater for floating offshore wind turbine should be less than a few degrees to avoid fatigue failure. In this paper, a novel conceptual design of a new type semi-submersible with suspensions for suppressing the pitch motion is presented. Many wave energy dissipation devices, such as add-on wave energy converters to a floating platform, could be regarded as the suspension system in our design. Firstly, linear models are applied to approximate the radiation forces and wave exciting forces so that the whole motion system is represented by a state-space model. Then, we show that design of suspensions leads to synthesize a controller via solving a constrained H optimization problem. Finally, numerical examples are performed to verify the design and it can be shown that the pitch motion of the semi-submersible platform is remarkably reduced.

Commentary by Dr. Valentin Fuster
2016;():V007T06A095. doi:10.1115/OMAE2016-54448.

The paper presents a flexible and lightweight fast prototyping approach for marine crane concept design based on WebGL. The goal is to develop a user-friendly tool that facilitates the communication between designers, system engineers and customers during the early product and system design stage. In this study, we focus on the design of the mechanical parts of the crane in addition with the actuation systems, typically the hydraulic power systems. The degrees of freedom, main geometric dimensions of the crane links and joints, and the configurations of the actuators can be evaluated according to the requirements of workspace, load capacity, etc. The implementation and user interface of the designer tool are developed based on the application of WebGL. In this way, it also enables remote communications between departments in different locations over the web. The development of the parameterization and visualization tool will bring in the customer directly into the product and system design process, thus to improve the work efficacy and reduce the overall time and cost. The results from the designer tool also provide primary information and data for later design processes and simulations in the virtual prototyping environment.

Commentary by Dr. Valentin Fuster
2016;():V007T06A096. doi:10.1115/OMAE2016-54607.

LNG-FSRU (Liquefied Natural Gas-Floating Storage and Regasification Unit) can adopt the structure of double-row cargo tanks. Meanwhile, its heat maintenance system has the characteristics of the special structure, the large temperature difference between inside and outside and the complicated heat transfer process. Aiming at this particular structural form and considering the hull structure and some effect of convection in cavity, this paper establishes the mathematical model and utilizes the finite element method to conduct the deep analysis of ultra-low temperature field in order to obtain its temperature distribution in the ultra-low temperature field. Moreover, the results indicate that the temperature distribution of the double-row-tank structure is linear from the inside out with the relatively uniform overall distribution; the temperature difference above and below the waterline has affects the temperature distribution of the inner shells; the temperature of the middle longitudinal cofferdam zone is relatively lower than that of other areas.

Commentary by Dr. Valentin Fuster

Ocean Engineering: Unsteady Hydrodynamics, Vibrations, Acoustics and Propulsion

2016;():V007T06A097. doi:10.1115/OMAE2016-54092.

The analysis of the influence of risers on the motions of a floating platform is often conducted and simplified by uncoupled method. As the number of risers and water depth increase, this method would not predict system motion accurately. Coupled analysis method in time domain becomes a very convenient approach in response calculation since it automatically includes the interaction among platform, mooring lines and risers.

This paper introduces a full coupled approach by AQWA-NAUT to include viscous damping of the semi submersible and effects of steel catenary risers on the wave frequency response of platform in time domain motion analysis.

The main conclusion of this paper is that full coupled method can accruately predict semi submersible Response Amplitude Operator (RAOs) comparing to the case without risers. Other conclusions are that risers have an important effect on the wave frequency motion of semi submersible and also lead to a phase lag with respect to platform motions.

Commentary by Dr. Valentin Fuster
2016;():V007T06A098. doi:10.1115/OMAE2016-54183.

In this paper, we present a high-voltage electric-spark bubble-generating method which can generate a bubble with its maximum radius reaching up to ∼35 mm at a room pressure. Vertical migration and clear liquid jet inside the bubble are captured by a high speed photography. With this method, a series of experiments on bubbles collapse above a solid boundary are carried out under different non-dimensional standoff distances γ (= s/Rm, where s is the vertical distance from the bubble center to the solid boundary and Rm denotes the maximum bubble radius). It is found when bubble is extremely close to the solid boundary (γ < 0.6), the lower surface of the bubble will cling to the solid boundary, which causes the cone-shaped liquid jet to impact on solid boundary directly without buffering of the water layer. With the increase of γ, the bottom of the bubble is gradually away from the solid boundary with an increasing curvature, but the jet inside the bubble remains conical all along. The speed of the jet tip and the migration of the bubble top are also discussed subsequently, aiming to provide a reference for the numerical study. Finally, the critical value of γ is investigated, at which the effect of the buoyancy will compensate the attraction of the solid boundary when the buoyancy parameter of bubble is bout 0.06.

Topics: Bubbles , Collapse
Commentary by Dr. Valentin Fuster
2016;():V007T06A099. doi:10.1115/OMAE2016-54858.

Propeller boss cap fins (PBCF) is one of the most popular ESDs in the industry. The present study aims to investigate effects of design variations of PBCFs on the propulsive efficiency and propeller wake field, with special attention on hub vortex dynamics. The wake fields and force on the whole propulsive system were measured by a towed underwater stereoscopic particle image velocimetry (SPIV) system and a propeller open water (POW) test dynamometer, respectively. Design parameters of PBCFs, i.e., the fin surface area and the angle of attack onto the fins, were varied to control fin loading on the PBCF. In the wake field, root vortices generated from the propeller blades were separated by PBCF and did not form a strong hub vortex, which caused pressure drop on the propeller boss cap. The hub vortex reduction practically increased total thrust, as evidenced in the global force measurement results. In PBCF design variations, Total efficiency increased linearly as the pitch angle and fin chord length decreased. The global force measurement results implied that PBCF in light loading separated root vortices efficiently. Hub vortex reduction by PBCF in light loading was also confirmed by the wake field measurement. In the case of low fin height, however, root vortices were not blocked and actually merged to form a hub vortex. Therefore, the primary function of PBCF, i.e., reducing hub vortex, was not effective anymore and the total efficiency decreased. In heavy loading conditions, axial velocity near the center retarded further, causing greater drag and diminishing the total efficiency. The model tests were also conducted in self propulsion condition, to reveal that the new PBCF with reduced loading also improves the energy saving performance when it works in the wake of the ship.

Commentary by Dr. Valentin Fuster
2016;():V007T06A100. doi:10.1115/OMAE2016-55073.

When a marine propeller with the wing shape rotates at high speed underwater, local pressure on the blade decreases and various types of cavitation inevitably occur in where the local pressure falls below the vapor pressure. Cavity reduces the efficiency, erodes the propeller surface, and generate vibration and serious noise. Especially, underwater noise caused by cavitation is directly connected to the comfort of commercial ships and also the survivability of naval vessels. In order to reduce the occurrence of the cavitation and to design low noise propeller, it is demanded to figure out the correlation of noise characteristics with growth patterns of the cavity. In this paper, we observed global behavior of partial cavities generated on two-dimensional hydrofoils and made a map of cavity patterns. We also measured pressure fluctuations and investigated noise characteristics directly connected with the process of occurrence of the cavity.

Commentary by Dr. Valentin Fuster

Ocean Engineering: Wave Mechanics and Wave Effects

2016;():V007T06A101. doi:10.1115/OMAE2016-54084.

An analytical weakly nonlinear model of Benjamin–Feir instability of a Stokes wave on nonuniform unidirectional current is presented. The model describes evolution of a Stokes wave and its two main sidebands propagating on a slowly-varying steady current. In contrast to the models based on versions of the cubic Schrodinger equation the current variations could be strong, which allows us to examine the blockage and consider substantial variations of the wave numbers and frequencies of interacting waves. Interaction with countercurrent accelerates the growth of sideband modes on a short spatial scale. An increase in initial wave steepness intensifies the wave energy exchange accompanied by wave breaking dissipation, results in asymmetry of sideband modes and a frequency downshift with an energy transfer jump to the lower sideband mode, and depresses the higher sideband and carrier wave. Nonlinear waves may even overpass the blocking barrier produced by strong adverse current. The frequency downshift of the energy peak is permanent and the system does not revert to its initial state. We find reasonable correspondence between the results of model simulations and available experimental results for wave interaction with blocking opposing current.

Topics: Waves
Commentary by Dr. Valentin Fuster
2016;():V007T06A102. doi:10.1115/OMAE2016-54207.

Linear random wave theory (LRWT) is frequently used to simulate water particle kinematics at different nodes of an offshore structure from a reference surface elevation record. However, it is well known that LRWT leads to water particle kinematics with exaggerated high-frequency components in the vicinity of mean water level (MWL). A number of empirical techniques have been suggested to provide a more realistic representation of near surface wave kinematics. The empirical techniques popular in the offshore industry include Wheeler stretching, linear extrapolation, delta stretching, and vertical stretching. Each of these methods is intended to calculate sensible kinematics above the MWL, yet they have been found to differ from one another in the results yielded. In this paper, two new methods of simulating water particle kinematics are introduced. In this study, the values of 100-year responses derived from different methods of simulating wave kinematics are compared.

Commentary by Dr. Valentin Fuster
2016;():V007T06A103. doi:10.1115/OMAE2016-54214.

An important step in the development of new ocean structures is to conduct tests on reduced scale models in tanks that reproduce the environmental conditions of their operation. For the wave modeling in a 2D channel, waves are generated in one of its end by the wave-maker and they should not be reflected back to the tested vessel. If these reflected waves were not absorbed they would interfere into the wave field and consequently interact with the body or structure under investigation, which can reduce considerably the experiment duration and the quality of its results. The simplest solution is the use of a passive beach to absorb the reflected waves, composed by a long ramp at the far end of the channel. As an alternative to these conventional beaches, wave-makers equipped with a designed control system can be used as active absorbers. These so-called active beaches have the advantage that they do not extend into the experimental domain of the basin and, at least in linear theory, low or zero reflection could be achieved. The objective of the present work is to design and construct an active wave absorption system into LOC, Laboratory of Waves and Currents, at COPPE/UFRJ, considering a wave field over the channel with no body or structure interaction. First results showed an absorption level comparable with the passive beach, however a careful tuning process is necessary to minimize the reflection coefficient.

Topics: Construction , Waves , Design
Commentary by Dr. Valentin Fuster
2016;():V007T06A104. doi:10.1115/OMAE2016-54263.

The boundary element method (BEM) which can solve the boundary integral equations is used to calculate the velocity potential on the floating bodies. The equation is discretized by the higher order BEM or the constant panel method. The constant panel method is relatively easy to compute the velocity potential. However the near field method cannot evaluate the wave drift forces and moment accurately, when the velocity potential is computed by the constant panel method.

In the article, a new numerical technic of the constant panel method is proposed. Then it is easy to take advantage of the near field method to calculate the wave drift forces and moment, especially considering two floating system. In addition, the results of the fluid forces calculated by new method are compared to the other methods results. At last the hydrodynamic interaction between two floating bodies is assessed in the calculation of the wave exciting forces and the wave drift forces.

Commentary by Dr. Valentin Fuster
2016;():V007T06A105. doi:10.1115/OMAE2016-54668.

In this paper, Fast Multipole Boundary Element Method (FMBEM) is developed to build a numerical wave tank, which is used to simulate a linear irregular wave train. Comparison is illustrated between the simulated and theoretical waves to validate the precision of FMBEM. Furthermore, the time consumptions of FMBEM are also compared with the traditional BEM to demonstrate its high efficiency. Besides, this effective numerical method is used to simulate the wave propagation with the influence of a fixed surface piercing body. All the results demonstrate that the proposed method is an effective numerical technique to study the linear irregular wave propagation with both high accuracy and efficiency.

Commentary by Dr. Valentin Fuster
2016;():V007T06A106. doi:10.1115/OMAE2016-54704.

The accuracy of a wave model for simulating waves in deep and coastal areas is highly determined by the dispersive properties as well as by the nonlinearity of the model. The Variational Boussinesq Model (VBM) for waves [1–4], available publicly as HAWASSI-VBM software [5], is based on the Hamiltonian structure of surface gravity waves. The model has tailor-made dispersive properties, which can be set to be sufficiently accurate for simulating a desired wave field. In this paper, we extend the nonlinear property of the HAWASSI-VBM from weakly nonlinear to be fully nonlinear. To show the improvement in nonlinearity, simulations of the model with a Finite Element implementation is tested against laboratory experiments, of regular and irregular waves propagating above a submerged bar and the dam-break problem.

Commentary by Dr. Valentin Fuster
2016;():V007T06A107. doi:10.1115/OMAE2016-54783.

Mooring offshore floating structures such as offshore platforms in large waves and winds, against the drift force and rotational moments are challenging in offshore engineering and ocean engineering. To investigate these kind of problems named positioning problems accurately, not only in hydrodynamic forces of first order but also in time-averaged steady forces of second order named wave drift force need to be taken into account. That arranging of several small cylinders regularly on a circle concentric with a fixed floating body is considered to reduce the wave drift force. Fortunately, a Cloaking phenomenon occurs at certain conditions with proper geometrical dimensions of floating bodies, which can reduce the wave drift force acting on bodies, perfectly even to zero. In this paper, with a combination of higher-order boundary element method (HOBEM) and wave-interaction theory, the influences of geometrical parameters of outer surrounding cylinders on the wave drift force of floating bodies are systematically investigated and discussed.

Commentary by Dr. Valentin Fuster
2016;():V007T06A108. doi:10.1115/OMAE2016-54856.

In this paper, the Homotopy Analysis Method (HAM) is applied to solve the fully nonlinear partial differential equation for the steady propagating periodic gravity wave of finite water depth. The series solution of the wave elevation and the velocity potential function are obtained. And then the velocity and pressure fields are plotted and discussed carefully. In order to overcome the drawback of the integral calculations with complex free surface elevation, the discrete integration and fitting procedure based on high-order Fourier series is developed. Based on the accurate HAM solution and fitting technique, the mass, momentum and energy conservation equations are validated. At last, the corresponding mean fluxes are calculated and the velocities of the mass transport and energy transport are supplied accurately.

Topics: Momentum , Water waves
Commentary by Dr. Valentin Fuster
2016;():V007T06A109. doi:10.1115/OMAE2016-55018.

Submarine surfacing in waves is three dimensional unsteady motion and includes complex coupling between force and motion. This paper uses computational fluid dynamics (CFD) to solve RANS equation with coupled six degrees of freedom solid body motion equations. RANS equations are solved by finite difference method and PISO arithmetic. Level-set method is used to simulate the free surface. Computations were performed for the standard DARPA SUBOFF model. The structured dynamic overset grid is applied to the numerical simulation of submarine surfacing (no forward speed) in regular waves and computation cases include surfacing in the calm water, transverse regular waves with different ratio of wave height and submarine length (h/L = 0.01, 0.02, 0.03, 0.04) and transverse regular waves with different ratio of wave length and submarine length (λ/L = 0.5, 1, 1.5). The asymmetric vortices in the process of submarine surfacing can be captured. It proves that roll instability is caused by the destabilizing hydrodynamic rolling moment overcoming the static righting moment both under the water and in regular waves. Relations among maximum roll angle, surfacing velocity fluctuation and wave parameters are concluded by comparison with variation trend of submarine motion attitude and velocity of surfacing in different wave conditions. Simulation results confirm that wave height h/L = 0.04 and wave length λ/L = 1.5 lead to surfacing velocity fluctuation significantly. Maximum roll angle increases with the increase of wave height and wave length. Especially the law presents approximate linear relationship. Maximum roll angle with wave height (h/L = 0.04) can reach to 7.29° while maximum roll angle with wave length (λ/L = 1.5) can reach to 5.79° by contrast with 0.85° in calm water. According to the above conclusions, maneuverability can be guided in the process of submarine surfacing in waves in order to avoid potential safety hazard.

Commentary by Dr. Valentin Fuster
2016;():V007T06A110. doi:10.1115/OMAE2016-55091.

Rogue waves or freak waves are very large amplitude waves compare to the ambient waves in a given sea state. There are very few recording of such waves and therefore, despite years of research, very little is known about their properties. Here we invoke a statistical approach to find out about the typical shape of these giant waves. We consider different sea states and unidirectional vs crossing seas and study how each environment affects the morphology of oceanic rogue waves.

Topics: Waves , Seas
Commentary by Dr. Valentin Fuster

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