ASME Conference Presenter Attendance Policy and Archival Proceedings

2017;():V07BT00A001. doi:10.1115/OMAE2017-NS7B.

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presenter attendance policy, if a paper is not presented at the Conference by an author of the paper, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Ocean Engineering: Marine Environment and Very Large Structures

2017;():V07BT06A001. doi:10.1115/OMAE2017-61041.

The current floating bridge concepts of Norwegian Public Roads Administration (Statens vegvesen, NPRA) use a flange shape part at the bottom part of the pontoons. The flange is in principle similar to the damping plates used in the offshore industry for SPAR type of structures. The project group initiated the flange part based on the requirement of extra added mass for tuning the bridge system Eigen-modes. Thus, the important modes can be shifted out of the main wave energy zone. The current study will focus on the damping effects of such structure. The damping effects on weak axis bending moment prediction is studied. The modelling of such damping is first proposed according to relevant literature based on both numerical and experimental studies. Since the reference studies were mainly focused on cylindrical structures, it is difficult to obtain an accurate estimation of the damping coefficient for the current bridge pontoon design, which contains a rectangular part between two half-cylindrical parts. In addition, the estimation of pontoon motions needs the input of damping coefficient, which means that the evaluation of damping coefficient is an iteration process. In order to include the uncertainties, a conservative value was adopted to represent the damping effect. The comparison of accounting for the damping effects or not has been given for all the bridge pontoons. The results show that the damping effects are important at the peaks of the responses; in addition, the reduction of the predicted maximum bending moments can be expected around 10–15 percent along different positions of the bridge. However, a further investigation also shows that viscous excitation would increase the bending moments slightly. The comparison also indicates the value of further investigating the effects by CFD or model test methods.

Commentary by Dr. Valentin Fuster
2017;():V07BT06A002. doi:10.1115/OMAE2017-61264.

SAR (Synthetic Aperture Radar ) systems are special types of radar that produce high resolution images (comparable to optical sensors) in all weather conditions, night and day. SAR sensors have many applications in marine and arctic applications. In this paper a compact SAR prototype system is developed for UAV (Unmanned Aerial Vehicle) platform. The radar is based on FMCW (Frequency-Modulated Continuous-Wave) radar mode. The system integrates a high performance RTK (Real Time Kinematic) GPS and IMU (inertial measurement unit) based motion compensation module, FPGA (Field Programmable Gate Array) based controller and signal processing module. It has a resolution of 0.3 meter with the weight below 2 kg. It has been test and verified on the guide rail, car and integrated on a rotary UAV. The system will extend the capability of UAV in the marine and arctic remote sensing area.

Commentary by Dr. Valentin Fuster
2017;():V07BT06A003. doi:10.1115/OMAE2017-61606.

As a strong western-boundary current, the Kuroshio Current has significant effects on the ship navigation in the East China Sea (ECS). To quantitatively know more about its influence, we present simulations of the ocean current in the North Pacific Ocean using the well-known Princeton Ocean Model (POM). The high-resolution current distributions could be applied to conduct numerical simulations of the ship navigation, which utilized a ship maneuvering model known as the Mathematical Maneuvering Group (MMG). Calculation of a container ship as well as a training ship have been conducted. The simulation results of both ships can show the significant effects of ocean currents on ship’s drifting as well as speed change, which could be used to optimize cost of both fuel and time by properly utilizing the current in ship routing.

Topics: Ships
Commentary by Dr. Valentin Fuster
2017;():V07BT06A004. doi:10.1115/OMAE2017-62228.

Liquid inside a floating structure influences both hydrostatic and hydrodynamic response of the floating structure. Some examples of floating structure with internal liquid are: vessels with roll damping tanks, floating hydrocarbon storage facility, LNG tankers and etc. A floating oil storage tank is considered in this study, which has a cuboid-shaped external wall, cylinder-shaped internal wall and simple structure configurations for its roof and bottom. Influence of internal liquid on the hydrostatic response of floating structures is well established and must be taken into consideration. The internal liquid reduces the stability of floating structures. The focus of this study is the influence of internal liquid on the hydrodynamic response of floating tank. Frequency domain analysis is performed with WAMIT, for partially filled tank with both solid mass and liquid mass. By comparing the different cases, the force induced by the internal liquid on the floating tank is illustrated. Based on the WAMIT calculated radiation damping force for the external flow, Impulse Response Function (IRF) connecting frequency domain and time domain solution is constructed and the force and moment induced by internal liquid is considered as an excitation force. By assuming linear tank motion, the internal liquid induced force is related to the incoming wave by a set of force transfer functions and it is moved to the right hand side of the tank motion equation. In practice, this set of force transfer function due to internal liquid has to be combined with the wave excitation force transfer function and it is the total force transfer function (internal liquid plus wave excitation) to be imported to SIMO. SIMO time domain simulation is performed in regular waves. Motion transfer functions from WAMIT frequency domain and SIMO time domain calculations are compared and reasonable agreement is achieved.

Commentary by Dr. Valentin Fuster
2017;():V07BT06A005. doi:10.1115/OMAE2017-62606.

In 2016, the International Maritime Organization (IMO) has enforced stricter nitrogen oxide (NOx) emission standards. Exhaust gas recirculation (EGR) technology is an effective way to achieve IMO Tier III standards for two-stroke marine diesel engines. This paper selected the 6S50ME-C8.2 diesel engine for the study, by making use of GT-POWER simulation software. In this paper, three different types of EGR were built to investigate the effects of EGR on engine performance and NOx emissions.

The results show that both the high pressure EGR system and the low pressure EGR system can reduce NOx emissions with the power drop and BSFC risen.

While in the high pressure EGR system combined with EGB, more NOx can be reduced with less power drop and BSFC risen. What is more, the running points of the compressor are still in the high efficient area and away the surge margin.

Based on the conclusions, the results obtained in this paper can offer reference for the turbocharged diesel engines with EGR system to reduce NOx emissions and improve engine performance.

Commentary by Dr. Valentin Fuster

Ocean Engineering: Model Tests

2017;():V07BT06A006. doi:10.1115/OMAE2017-61034.

Monopile support structures for offshore wind turbines may experience ringing-type responses in steep wave conditions. In order to experimentally capture the statistical distribution of the hydrodynamic loads and structural responses, traditional practice is to generate many 3-hour (full scale) realizations of the relevant sea states. An experimental campaign at 1:48 scale was carried out in the Lilletanken wave tank at NTNU/MARINTEK in order to examine the possibility of using shorter time windows to recreate irregular wave ringing events. Wave elevations and hydrodynamic loads on a rigid vertical circular cylinder in 27 m water depth were measured for a variety of 3-hour, 450 s (7.5-minute), 800 s (13.3-minute), 1150 s (19.2-minute), and 1500 s (25-minute) wave realizations, where all durations are listed in full scale. Wavelet transformations and a single degree-of-freedom oscillator were used to investigate the magnitude and repeatability of the high-frequency content of the wave loads. Large variations in the repeatability were seen among events. On average, the repeatability in the ringing response was 4.2 % for 3-hour tests, while 13.3-minute tests reproduced the same events within 9.1 %. The maximum deviation was, nonetheless, much higher when only 13.3 minutes were used.

Commentary by Dr. Valentin Fuster
2017;():V07BT06A007. doi:10.1115/OMAE2017-61154.

Assessment of hydrodynamic performance of submerged floating tunnel (SFT) under wave loading is one of important factor in the design of the structure. In this study, physical experiments were conducted in a two-dimensional wave flume to investigate hydrodynamic characteristics of a twin circular SFT model under the action of regular waves having different heights and periods. Five different configurations of the twin SFT model was tested in the experiment. The experimental results showed that the three-degree motions of the twin SFT and the associated tensile forces on the tension legs greatly vary with the configurations of the model. It was found that the CD0 configurations are most adequate for satisfactorily restricting the horizontal and vertical motions of the SFT model.

Topics: Waves , Tunnels , Seas
Commentary by Dr. Valentin Fuster
2017;():V07BT06A008. doi:10.1115/OMAE2017-61158.

The authors have developed a wind loads simulator (WiLS) that enables us to carry out free-running model tests for investigating wind effects on ship performance. WiLS provides a free-running model ship with simulated wind loads taking account of supposed true wind speed and direction, and instantaneous model ship speed, drift angle, and heading angle. It does not generate environmental wind but exerts forces and moment on a model ship using three pairs of duct fans. A control PC calculates time varying longitudinal and lateral wind forces and yaw moment using wind loads coefficients estimated beforehand and ship motion data, and distribute them to the three pairs of duct fans. Feedback control ensures the intended wind loads using data from load cells on which the duct fans are mounted and those from accelerometers for correcting inertia forces of the duct fans. This paper reports the concept, configuration, and control procedure of WiLS, and presents tank test data, which confirms effectiveness and usefulness of WiLS as a device in free-running model tests to evaluate ship performance at actual seas.

Topics: Stress , Ships , Wind
Commentary by Dr. Valentin Fuster
2017;():V07BT06A009. doi:10.1115/OMAE2017-61229.

Green water impact due to extreme waves impinging on a fixed, rectangular shaped model structure was investigated experimentally. The experiment was carried out in the large wave basin of the Offshore Technology Research Center at Texas A&M University. In the study, two wave conditions were considered: a plunging breaking wave impinging on the frontal vertical wall (referred as wall impingement) and a breaking wave directly impinging on the deck surface (referred as deck impingement). The aerated flow velocity was measured by employing the bubble image velocimetry (BIV) technique with high speed cameras. The pressure distribution on the deck surface was measured by four differential pressure sensors. The fiber optic reflectometer (FOR) technique was employed to measure the void fraction in front of each pressure sensor end face. The flow velocity, void fraction, and impact pressure, were synchronized and simultaneously measured. Comparisons between an earlier study by Ryu et al. (2007) and the present study were performed to examine the scale effect. Results between Song et al. (2015) and the present results were also compared to investigate the influence of structure geometry on green water flow and impact pressure. To examine the role of air bubbles during the impact, the velocity, pressure, and void fraction were correlated. Correlation between the peak pressure and the aeration level shows a negative trend before the wave impingement but a positive linear relationship after the impingement.

Commentary by Dr. Valentin Fuster
2017;():V07BT06A010. doi:10.1115/OMAE2017-61256.

Wind resistance is an important part of the container ship compared with the other types of commercial vessels because of the containers piled on the deck, and it is significant to investigate the wind load of the container ship and the approach to minimize it in order to reduce the wind resistance and save energy. Two approaches are adopted in the investigation to minimize the wind load, the first one of which is smoothing the corners of the superstructure while the second one is adding a forecastle fairing in the front of the deck. Three different superstructures and six different fairings are designed and tested with the ship in the wind tunnel. It is suggested by the comparison of the experimental results that the optimization of the superstructure and forecastle fairing is helpful in wind resistance reduction.

Commentary by Dr. Valentin Fuster
2017;():V07BT06A011. doi:10.1115/OMAE2017-61338.

This paper presents the results of an experimental investigation of green water loads on a wave-piercing tumblehome ship. A water tank experiment was carried out in head regular waves by using a self-propelling segmented ship model. Wave probes and pressure sensors were arranged on the bow deck along the longitudinal and transverse directions. The height of water and the impact pressure on the deck were measured and their distributions in different wave conditions studied. The motion of the water flowing on the deck was recorded by a high-speed video system. Based on the experimental results, it was found that the green water is more serious with the increase of incident wave height and ship speed. The bow shape has little effects on the occurrence of green water, but it influences the green water loads to some extent. The distribution of green water pressure is different from that of green water height due to the strong nonlinearity of green water pressure.

Topics: Stress , Waves , Ships , Water
Commentary by Dr. Valentin Fuster
2017;():V07BT06A012. doi:10.1115/OMAE2017-61404.

We investigate a new concept for wave and current generation. It consists of axial-flow pumps driven such as to generate an oscillatory flow through an orifice located at one end of the flume. Oscillations of the flow lead to the generation of water waves at the free surface. If the average of the flow is different from zero, a current is generated that superposes on the waves. In this study, we explored the technical capabilities of this concept and the influence of geometric parameters on wave and current generation. We used numerical and experimental modelling. Most noticeably, the numerical results indicate that this concept is well suited for the generation of long and high waves. An experimental setup has been designed and built. We used it to make an experimental proof of concept for the wave and current generation, including waves propagating against the current.

Topics: Waves , Flumes , Pumps , Axial flow
Commentary by Dr. Valentin Fuster
2017;():V07BT06A013. doi:10.1115/OMAE2017-61429.

The breakwater is one of the most important structures in coastal areas and the rubble mound breakwaters have been most widely designed and constructed. When designing the rubble mound structures, the determination of the stable weight of concrete armor units is the most significant and critical problems. The stable weight of concrete armor units could be calculated by Hudson or van der Meer formula. The breakwater is assumed to be straight and the waves are propagating normally to the structures when using this formula. The weight of concrete armor units at the round breakwater head should be protected by 1.3∼1.6 times of that at the trunk part because the coastal engineers think it to be vulnerable. Recently the damage of concrete armor units at the convex corner of breakwater trunk was reported several times in Korea. However, there were no any design standards for this area. In this study, three-dimensional hydraulic model tests were performed to investigate the stable weight of concrete armor unit at convex corner area. The tetrapod was used to the target concrete armor unit in the study. The stable weight is suggested by stability coefficient, Kd by using Hudson formula. From this study, the stable armor weight at the convex corner is similar with that at the round breakwater head and this means that about 1.4 times of weight for trunk area was needed at convex corner area.

Commentary by Dr. Valentin Fuster
2017;():V07BT06A014. doi:10.1115/OMAE2017-61514.

As part of the test campaign led by the 27th ITTC Ocean Engineering Committee to produce reliable experimental benchmark data for multiple-body interactions in waves, model tests of two identical models in close proximity were carried out at the towing tank of Memorial University, Canada and at the wave basin of Ecole Centrale de Nantes, France. This paper presents the experimental results for the two bodies in regular waves with various gaps. The experimental data, including motions of two bodies, wave elevations in the gap and drift forces, were compared with numerical solutions based on the potential-flow solution.

Topics: Waves
Commentary by Dr. Valentin Fuster
2017;():V07BT06A015. doi:10.1115/OMAE2017-61542.

The LiquefAction project is dedicated to the study of cargo liquefaction, especially to understand the inception of this phenomenon and its effects on bulk carrier stability. Both numerical and experimental approaches are set-up to simulate the behaviour of specific ores in various modes of motions.

This paper is dealing with forced motion tests of a tank filled with two highly viscous fluids. The tank is a bulk carrier hold model and a hexapod platform is reproducing the ship motions. Free surface evolutions as well as global loads are analysed.

Some preliminary conclusions are given with respect to the filling ratio, the fluid viscosity and type of motions.

Topics: Stress , Ships
Commentary by Dr. Valentin Fuster
2017;():V07BT06A016. doi:10.1115/OMAE2017-61543.

Recent studies suggest that the vortex-induced motions (VIM) of a semi-submersible found in model tests over-predicts the response in the field, which may lead to an over conservative design of the mooring and riser system. Within the Vortex Induced Motion Joint Industry Project (VIM JIP), run by MARIN and University of São Paulo (USP), possible reasons for this over-prediction are investigated using model tests and CFD [1–6]. A model test campaign was carried out at MARIN to test different candidates that might explain the observed differences. The results obtained with an air bearing setup regarding damping, mass ratio, draft variations and sinusoidal tow velocities, have been published elsewhere [6]. The present publication focuses on the influence of waves on the VIM response.

The model was a generic bare hull semi submersible with four rounded square columns at scale 56.5. A simplified mooring system consisting of four springs was designed to match the desired natural period in the sway and yaw direction. The model was towed at different velocities, corresponding to the range of reduced velocities where the highest response is expected. A VIM tow test campaign was carried out in calm water as a benchmark. The model was then tested at 7 different wave-current conditions, and the results are compared with the benchmark case.

The results suggest that two factors are important for VIM response in waves: the wave height, and the relation between wave and current direction. Comparing to calm water conditions, a reduction of 15% on the peak nominal response was found for a smaller sea state (HS = 2m, TP = 10s), however with a higher significant wave height (HS = 4m, TP = 10s) the peak nominal response was reduced by 30%. Depending on the combination of current-wave direction, the influence of the same sea state (HS = 4m, TP = 10s) on VIM response can be negligible (transverse seas) or result in a 30% reduction of the peak nominal response for collinear sea and current. This is a relevant finding since most research on the topic has focused on collinear conditions, and VIM tests in waves with transverse or oblique conditions are rare [7–9].

Comparing the calm water VIM response obtained with the air bearing setup, published in [6], with the soft mooring configuration reported here, the latter shows a generally smaller response, with a narrower lock in region. Nevertheless the peak response is found to be similar for both experimental setups.

Commentary by Dr. Valentin Fuster
2017;():V07BT06A017. doi:10.1115/OMAE2017-61623.

When an ocean current encounters a vertical structure, such as an artificial reef (AR), it can create a plankton-rich upwelling that provides a reliable feeding spot for fish. The fish attracted into the AR zones depends largely on the size, structure, and layout of the ARs. Thus, this study was conducted to demonstrate the effect of different layouts on the flow field around cubic ARs, a common type of AR in China. Water tank tests and numerical simulations were used to study the flow field around the cubic ARs. Experiment of hydrogen bubble was conducted in a water tank to observe the effect of the reef spacing on the flow field around the reefs. Using computational fluid dynamics, a three-dimensional numerical model was established based on the Navier-Stokes equation. The standard k-ϵ model was adopted to simulate the flow field around the ARs. On comparing the streamlines of the flow field around the ARs, it was found that the numerical results were in good agreement with the hydrogen bubble results. Using the numerical model, the flow around the ARs with different transverse or longitudinal spacing was simulated. When the transverse spacing distance is equal to the width of the AR, the upwelling becomes stronger. When the distance increases to 7L, the interaction between the reefs can be ignored. When the longitudinal spacing distance is equal to 1L, the effect of the interaction of the reefs on the back eddy is quite significant. However, as the distance increases, the interaction between two reefs gradually weakens.

Commentary by Dr. Valentin Fuster
2017;():V07BT06A018. doi:10.1115/OMAE2017-61629.

Model tests of a drillship with a rectangular moonpool opening were conducted in regular and irregular waves from the bow and bow-quarter. Most tests were conducted at zero speed, the rest was performed with the model towed to a speed of 10 kn. From the video-recordings and transfer functions of the measured relative water elevation inside the moonpool, the typical piston and first sloshing modes are well captured, for wave frequencies that agree relatively well with relevant formulations. A few tests conducted at varying wave amplitudes show that the water elevation is non-linear by nature, while repeat tests conducted with the moonpool fitted with two layers of side wall flanges shows that these can reduce the water motions by nearly 40 %.

Topics: Water
Commentary by Dr. Valentin Fuster
2017;():V07BT06A019. doi:10.1115/OMAE2017-61896.

A thruster-assisted position mooring (TAPM) system includes different control functions for stationkeeping and motion damping for a moored offshore vessel with assist from thrusters. It consists of a conventional mooring system and a dynamic positioning (DP) system. The thrusters are used to provide damping and some restoring to the vessel motion and compensate if line breakage occurs. The mooring system absorbs the main loads to keep the vessel in place. This paper presents a complete modeling, parameter identification, and control design for a 1:90 scaled TAPM model vessel. The numerical values for the different model parameters are identified from towing tests.

State-of-the-art TAPM control algorithms have been tested on the vessel in the Marine Control Laboratory (MC Lab), to see the behavior resulting from the different control algorithms. The presented experiments focus on the setpoint chasing algorithm, where the position setpoint slowly moves to the equilibrium position where the environmental loads are balanced by the mooring loads. This avoids conflicts between the mooring system and the control actions. If the environmental loads are too large so that the setpoint exceeds a user-defined safety radius, the setpoint is set to this radius and thruster forces grow to support the mooring system in counteracting the environmental loads to avoid line breakage. The experiments show that the vessel and setpoint chasing control algorithm behaves as expected, minimizing thruster usage and maximizing utilization of mooring system.

Commentary by Dr. Valentin Fuster
2017;():V07BT06A020. doi:10.1115/OMAE2017-62052.

Physical scale model testing is an important development tool, used extensively to study the behaviour of marine devices, vessels and structures in a controlled environment prior to deployment at sea. Whilst specific guidance on developing and testing marine renewable energy devices has been published over the past decade, it has limitations in terms of advanced environmental conditions for testing. The body of existing guidance is reviewed, and initial suggestions offered for additional test conditions that may be considered in later stages of model testing. This focuses on testing in combined waves and currents, particularly the multi-directional aspect thereof, which is now possible in facilities such as FloWave.

Commentary by Dr. Valentin Fuster
2017;():V07BT06A021. doi:10.1115/OMAE2017-62203.

During tests in MARIN’s wave basins, it was observed that large-scale current patterns may develop under the influence of wave generation and absorption. The velocity of these currents is very low, so they generally do not influence the behaviour of models. However, for specific experiments at low speeds — wave added resistance tests with small models or current drag tests — a residual current may influence the results significantly.

A good understanding of the residual circulation in a wave basin is essential to improve the quality of the tests performed. The wave-induced current patterns were observed during tests in MARIN’s Seakeeping and Manoeuvring Basin (SMB). The patterns may develop in several ways under the influence of waves in a basin. End effects of Stokes drift (mass transport due to second-order wave effects) can play a role, as the water has to return at the end of the basin. The SMB has the capability to generate oblique waves. It therefore has a wave-damping beach along two sides of the basin. Similar to ‘real’ beaches, they may cause alongshore currents and rip currents under the influence of oblique and perpendicular waves respectively. During the tests, floaters in the form of oranges were distributed in the basin after wave generation. They were tracked using a camera system. The images were processed such that the surface current patterns in the basin were visualized, and an estimate of the velocities was obtained. Additional local acoustic current meter measurements were used to check the order of magnitude of these velocities.

Based on these tests, it was concluded that different patterns may occur in the basin, with the largest velocities after oblique wave generation. Typical surface velocities are in the order of 1 to 2 cm/s, non-uniformly distributed over the basin. Due to this non-uniformity and because decay is slow (memory effects), very sensitive added resistance and current drag tests may have to be corrected for a measured current velocity in the future.

Topics: Waves
Commentary by Dr. Valentin Fuster
2017;():V07BT06A022. doi:10.1115/OMAE2017-62555.

Evaluation of the response amplitude operator (RAO) function for ship wave frequency motions by means of scale model tests in regular waves is a standard procedure conducted by hydrodynamic model testing institutions. The resulting RAO function allows for evaluating sufficiently reliable seakeeping predictions for low to moderate sea states. However, for standard hull forms, correct prediction of roll motion in irregular wave (and also in regular waves different than these used in the experiment) on the basis of RAO function presents a substantial challenge due to considerable contribution of viscous damping to roll response. In other words, the RAO values depend strongly on the amplitude of the waves used in the experiment, so the final prediction requires careful application of relevant correction of RAO, dependent on the actual significant wave height, for which the prediction is computed. Thus, in order to collect complete data for ship roll prediction, the roll decay test is usually also required. Additional drawback of evaluating the seakeeping prediction on the basis of RAO is the fact that the experiment in regular waves is quite time-consuming, which refers to the experiment itself as well as to the processing. The following paper presents a proposal of the alternative method for experimental evaluation of response amplitude operator of roll motion in beam waves, consisting in exposing the ship model to irregular wave characterized by white noise spectrum, i.e. the spectrum of uniform energy density. In theory, RAO function is equivalent to the square root of the spectrum of the response to white noise wave. The results of experiments in white noise waves were verified on the basis of the results of comprehensive experiments conducted in usual way. Additionally, the effect of non-linearity of viscous damping was widely studied by comparing the calibrated RAO-based predictions with actual response to irregular waves of different heights. As a result, a method for including the non-linear effects in prediction based on white noise was proposed. It was proved that the proposed method is capable of providing equally valuable information in significantly shorter time.

Topics: Waves , Ships , White noise
Commentary by Dr. Valentin Fuster

Ocean Engineering: Ocean Engineering Technology

2017;():V07BT06A023. doi:10.1115/OMAE2017-61022.

Safety and energy efficiency are two of the key issues in the maritime transport community. A sail plan system, which combines the concepts of weather routing and voyage optimization, are recognized by the shipping industry as an efficient measure to ensure a ship’s safety, gain more economic benefit, and reduce negative effects on our environment. In such a system, the key component is to develop a proper optimization algorithm to generate potential ship routes between a ship’s departure and destination.

In the weather routing market, four routing optimization algorithms are commonly used. They are the so-called modified Isochrone and Isopone methods, dynamic programming, threedimensional dynamic programming, and Dijkstra’s algorithm, respectively. Each optimization algorithm has its own advantages and disadvantages to estimate a ship routing with shortest sailing time or/and minimum fuel consumption. This paper will present a benchmark study that compare these algorithms for routing optimization aiming at minimum fuel consumption. A merchant ship sailing in the North Atlantic with full-scale performance measurements, are employed as the case study vessels for the comparison. The ship’s speed/power performance is based on the ISO2015 methods combined with the measurement data. It is expected to demonstrate the pros and cons of different algorithms for the ship’s sail planning.

Commentary by Dr. Valentin Fuster
2017;():V07BT06A024. doi:10.1115/OMAE2017-61031.

This paper describes a method for providing real time decision support based on measurements rather than optimizing a mathematical model. The proposed method is thus beneficial for systems for which the modelling would be inaccurate, the dynamics and complexity of the system would make it difficult to optimize in real time, or the risk of returning local minima is not acceptable.

The proposed method is implemented on four fishing vessels. These vessels are complex and give the skipper many choices related to how the vessel is operated. The developed tool advises the crew on in real time on operational decisions, particularly on the use of various diesel electric and diesel mechanic propulsion modes, including decisions such as the use use of shaft generator, direct coupling between main engine and propeller or not, propeller pitch, etc. This will presumably reduce both fuel consumption and emissions of CO2 and NOX.

Some examples of obtainable results from both onshore analyses and the onboard application are presented to demonstrate the methods applicability.

Commentary by Dr. Valentin Fuster
2017;():V07BT06A025. doi:10.1115/OMAE2017-61167.

Multi-rig bottom trawling can negatively impact the seabed, and the bottom contact forces contribute to the total towing resistance. Lifting the doors from the seabed can reduce some of the trawl gear’s influence on the seabed and save fuel costs. In this study we evaluated a simple reference controller designed to control a trawl door’s distance from the seabed. This was achieved by providing desired warp lengths to a controller, which then regulated the distance via a low-pressure hydraulic winch. The reference controller algorithm uses an anti-windup proportional-integral controller with saturation of winch acceleration and speed. Herein, we present results of a full-scale sea trial conducted using a triple shrimp trawl with trawl doors towed close to the seabed. Our results demonstrate the feasibility of controlling the trawl doors’ altitude above the seabed using only winch-actuated feedback.

Topics: Control systems , Seas
Commentary by Dr. Valentin Fuster
2017;():V07BT06A026. doi:10.1115/OMAE2017-61171.

Ship maneuvering in close-range maritime operations is challenging for pilots, since they have to not only prevent the ship from collisions and compensate environmental impacts, but also steer it close to the target towards a proper heading. This paper presents a path planner to assist the pilots to foresee the optimal trajectory in the scenario. The path planning is formatted as an optimizing problem to minimize the turning variation fluctuation and the fuel consumption of the ship through ocean current while satisfying the constraint of orientations at the start and the end positions. Taking advantages of Bézier curves’ smoothness and adjustability, feasible trajectories are divided into two categories based on the location of the intersection between the start and end directions, and are designed as a set of parameterized Bézier curves. The variables in the Bézier curves become the state space. By searching the space using an evolutionary technique, the candidate of the Bézier curve that has the best turning and the minimized fuel consumption can be obtained. Through two case studies, the feasibility and effectiveness of the proposed planner is verified.

Commentary by Dr. Valentin Fuster
2017;():V07BT06A027. doi:10.1115/OMAE2017-61309.

A submerged floating tube bridge (SFTB) concept is one of the three alternatives for the Bjørnafjord crossing in Norway. Two SFTB designs have been developed: one pontoon-stabilized and one tether-stabilized. There are no submerged floating tube bridges yet built and installed. Consequently there is no direct practical experience with assembly and installation of such massive string-like structures. The purpose of this article is to describe possible means of fabrication, assembling, jointing and installation of SFTB elements and the tube-bridge itself. All major project activities have been divided into stages. Further, these stages have been subdivided into factual steps to demonstrate feasibility based on various criteria. The major construction challenge is to assembly fabricated SFTB elements into one 4580 m string in a floating condition and then to tow and install it in the fjord. The focus was to utilize existing technologies and experiences available from offshore industry to the fullest extent, so that conventional tools and methods could be applied to this new application. Possible solutions both for the tether-stabilized and pontoon-stabilized alternatives — with differences in the foundation/tether and pontoon installations and associated SFTB interfaces — have been developed.

Commentary by Dr. Valentin Fuster
2017;():V07BT06A028. doi:10.1115/OMAE2017-61368.

The conditions of competition within ship yards are changing. The current market situation requires a new orientation of the Pella Sietas ship yard with flexible solutions for new ship types. Complex, heavy and ice-going ships show one way for future designs. In view of all the technical difficulties involved in such challenging projects, the first question must be how to handle these heavy constructions with the yards building facilities available. The Pella Sietas yard is using a floating platform for newbuildings. The question arises whether or not this platform is still capable and suited for this kind of ship types.

The docking procedure is a complex multi-body interaction that copes with hydrostatic and structural challenges. The docking operation is regulated by the sequence of flooding and emptying ballast water tanks of the dock. At any time of this dynamic operation the hydrostatic stable equilibrium of ship and dock must be ensured.

When the ship becomes afloat the keel block system transfers the ships weight on the structure of the dock. It must be ensured that the resulting tensions and deformations do not exceed the maximum permissible values.

This paper describes a fast calculation method that determines the mentioned hydrostatic as well as the structural investigations during the docking procedures.

The method implies a numerical progressive flooding simulation that calculates the hydrostatics of ship and dock under consideration of their interaction by dock blocks together with the ballasting sequence in the time domain. Furthermore it calculates the block forces distribution by applying the deformation method. In the calculation process ship and dock are modeled as Timoshenko beams and the dock blocks as non-linear spring elements. Moreover the shear force and bending moment distributions of ship and dock are calculated and the deflection lines are presented.

Therefore, the described method enables the ship yard to evaluate quickly the possibility of building new types of ships on the existing building platform and allows evaluating which modifications are useful to enlarge the capacity of the platform even further.

It provides a useful tool to minimize local and global stresses and deformations of the interacting bodies during the whole docking procedure by fast optimization of the block system arrangement and the ballasting sequences.

As a result the described method could expand the range of flexibility of a given floating dock structure. In addition, the whole hydrostatic and structural integrity of docking sequences can be computed faster and more accurate even at a very early project stage.

Topics: Hydrostatics
Commentary by Dr. Valentin Fuster
2017;():V07BT06A029. doi:10.1115/OMAE2017-61771.

Autonomous marine vessels are the way forward to revolutionize maritime operations. However, the safety and success of autonomous missions depend critically on the availability of a reliable positioning system and time information generated using global positioning system (GPS) data. GPS data are further used for guidance, navigation, and control (GNC) of vehicles. At a mission planning level GPS data are commonly assumed to be reliable. From this perspective, this article aims to highlight the perils of maritime navigation attacks, showing the need for the enhancement of standards and security measures to intercept any serious threats to marine vessels emanating from cyber attacks and GPS spoofing. To this end, we consider a case where a cyber attacker blocks the real GPS signals and dupes the GPS antennas on board the marine vehicle with fake signals. Using the Nomoto model for the steering dynamics of a marine vessel and exploiting tools from linear control theory we show analytically, and verify using numerical simulations, that it is possible to influence the state variables of the marine vessel by manipulating the compromised GPS data.

Commentary by Dr. Valentin Fuster
2017;():V07BT06A030. doi:10.1115/OMAE2017-61945.

There are increasing concerns and regulations regarding the emission of pollutants from shipping. Therefore, regulations such as the Ship Energy Efficiency Management Plan (SEEMP) and Energy Efficiency Design Index (EEDI) have been made mandatory to cope with climate change concerns. To put these efforts into practice, the Energy Efficiency Operational Indicator (EEOI) was introduced in 2009 to account for the fuel consumption, distance travelled by the vessel and cargo mass. However, it is stated that these do not apply to ships that are not engaged in transport work such as research vessels and tugboats. These short sea shipping vessels have been neglected under current indexes and it is not possible for their properties to be quantified since current indices are for vessels carrying loads. The numbers of these specialised vessels are increasing in local waters, and are closer to coastal communities where concerns and impact from these pollutants would be more direct.

In the IMO greenhouse gas study, options for improving energy efficiency in terms of design includes the concept, design speed and capability, hull and superstructure, power and propulsion whilst the principle of energy efficiency in terms of operation includes fleet management, logistics and incentives, voyage optimisation and energy management. A reliable energy flow breakdown architecture and diagnostics for these smaller vessels is important and will contribute to an understanding of the energy production, distribution and consumption on-board. This feeds into the IMO plan to encourage energy management.

A systematic approach consisting of five distinct stages is recommended to accomplish a holistic approach for energy efficiency management. This includes understanding of energy flow breakdown architecture, vessel survey to understand operation and conduct, review existing sensors and new sensor installation, sensor communication and data processing, and finally data analysis. These stages are addressed in this paper to provide an overall understanding of a robust energy efficiency audit procedure and sensor matrix. This includes unifying the existing on-board sensors with the proposed new sensors for additional data collection where primary parameters are not readily available. Inferred secondary parameter calculations are also applied where direct data collection is not possible. This will allow information from the vessel to be transmitted to a common platform to enable detailed data analysis. The aim of this work is to improve energy management and monitoring, which leads to understanding and managing consumption of energy. A case study of this methodology has been carried out on the Princess Royal, a Newcastle University research vessel. Recommendations for further testing and optimisation of this methodology will be applied to tugboats and Offshore Supply Vessels (OSV).

Commentary by Dr. Valentin Fuster
2017;():V07BT06A031. doi:10.1115/OMAE2017-62027.

Nowadays, there are more ships equipped with on-board monitoring systems and vessels with many sensors are becoming a standard. Available measurements can be employed for system optimization which play an important role in the vessel power plant configuration. The improvements in the power system design can be based on theoretical (modeling based on physical and empirical laws — for simulation purposes, for example [1]) or data-driven modeling (machine learning, statistical approach [2]). The data-driven models can be supportive confirming theoretical assumptions or simplifications from simulations. They are also helpful to understand the real systems, including vessel dynamic behavior and interactions. Therefore, the combination of simulation and data-driven modeling will be beneficial by identifying relationships that help explain unidentified variations. This approach is recommended when aiming for a more reliable tool for design optimization and to overcome the limitation of the simulation models that all system properties and dynamic effects must be known beforehand. The scope in this work is to present a potential synergy between the simulation and the machine learning approach. A data-driven method can be complementary to a model based on physical and empirical laws. This is shown in the example of power plant model connected with a thruster and vessel model to simulate the typical transit scenario and the data-driven model. The paper proposes a simultaneous analysis of the theoretical and machine learning models to predict the vessel power/speed and study the complex systems’ interactions in more detail, which are essential while exploring the system behavior.

Commentary by Dr. Valentin Fuster

Ocean Engineering: Ocean Measurement and Data Interpretation

2017;():V07BT06A032. doi:10.1115/OMAE2017-61118.

Ocean internet of things (IoT - onboard and onshore) collects big data sets of ship performance and navigation information under various data handling processes. That extract vessel performance and navigation information that are used for ship energy efficiency and emission control applications. However, the quality of ship performance and navigation data can play an important role in such applications, where sensor faults may introduce various erroneous data regions and that may degrade to the outcome. This study proposes visual analytics, where hidden data patterns, clusters, correlations and other useful information are visually from the respective data set extracted, to identify such erroneous data regions. The domain knowledge (i.e. ship performance and navigation conditions) has also been used to interpret such erroneous data regions and identify the respective sensors that relate to the same situations. Finally, a ship performance and navigation data set of a selected vessel is analyzed to identify erroneous data regions for three selected sensor fault situations (i.e. wind, log speed and draft sensors) under the proposed visual analytics. Hence, this approach can be categorized as a sensor specific fault detection methodology by considering the same results.

Commentary by Dr. Valentin Fuster
2017;():V07BT06A033. doi:10.1115/OMAE2017-61120.

Ship performance and navigation data are collected by vessels that are equipped with various supervisory control and data acquisition systems (SCADA). Such information is collected as large-scale data sets, therefore various analysis tools and techniques are required to extract useful information from the same. The extracted information on ship performance and navigation conditions can be used to implement energy efficiency and emission control applications (i.e. weather routing type applications) on these vessels. Hence, this study proposes to develop data visualizing methods in order to extract ship performance and navigation information from the respective data sets in relation to weather conditions. The relative wind (i.e. apparent wind) profile (i.e. wind speed and direction) collected by onboard sensors and absolute weather conditions, which are extracted from external data sources by using position and time information a selected vessel (i.e. from the recorded ship routes), are considered. Hence, the relative wind profile of the vessel is compared with actual weather conditions to visualize ship performance and navigation parameters relationships, as the main contribution. It is believed that such relationships can be used to develop appropriate mathematical models to predict ship performance and navigation conditions under various weather conditions.

Topics: Visualization , Ships , Wind
Commentary by Dr. Valentin Fuster
2017;():V07BT06A034. doi:10.1115/OMAE2017-62383.

In this paper, we study statistics of ocean currents using data collected from a subsurface autonomous moored (moving) vehicle and from a nearby oceanographic mooring. Among different sensors mounted on the moored subsurface buoy, we only use an upward-looking Nortek Aquadopp (AQD) and an Acoustic Doppler Velocimeter (ADV) to measure the near surface currents (for the upper ∼8-m below the surface) and surface gravity waves from moving sensors. Using a simplified stochastic model, a theoretical representation is derived for the Probability Density Function (PDF) of ocean currents, i.e. one-parameter Rayleigh distribution. Extension of this solution to two-parameter Weibull distribution is then utilized to determine the analytical representations for the higher-order statistical moments (i.e. skewness, and kurtosis) of currents throughout the water column. The current observations from moving ADV and AQD systems near the sea surface confirm previous speculation that two-parameter Weibull distribution can reasonably provide a well approximation to the probability distribution of the upper ocean currents. The non-Weibull structures in the results can, however, be attributed to different sources of uncertainty in measurements near the sea surface such as wave-induced platform instability and effects of various scales of motions interacting in the upper ocean boundary layer. Using data from oceanographic mooring-based upward-looking Acoustic Doppler Current Profiler (ADCP) and single-point AQDs, the statistics of currents from these quasi-fixed profiling systems at depths below 8-m are observed to have relatively smaller departures from the theoretical Weibull predictions than those estimated from moving sensors.

Topics: Ocean currents
Commentary by Dr. Valentin Fuster
2017;():V07BT06A035. doi:10.1115/OMAE2017-62396.

One important parameter in reconstructing and predicting the sea surface elevation from radar images is the surface current. The common method to derive the current is based on 3DFFT with which the (absolute) frequency is derived from a series of images and is fitted to the encounter dispersion relation that consist of the intrinsic exact dispersion relation for linear waves with an additional term that contains the current velocity to be found. The derived dispersion relation will be inaccurate because the images contain many inaccuracies from noise, shadowing, and other radar effects. This paper proposes an alternative method to determine the surface current. Following the method of the Dynamic Averaging and Evolution Scenario (DAES) as presented in [1], the idea is to choose the current velocity that minimizes the difference between an image at a previous time that has been evolved to the time of another image. In order to reduce inaccuracies, an averaging procedure over various images is applied. The method is tested on synthetic data to quantify the accuracy of the results. The robustness of the method will be investigated for several cases of different current parameters (speed and direction) for ensembles of seas with different peak frequency of characteristic sea states.

Topics: Radar , Seas
Commentary by Dr. Valentin Fuster

Ocean Engineering: Offshore Industry: Structures and Design

2017;():V07BT06A036. doi:10.1115/OMAE2017-61789.

Due to increased energy demand and thrive for clean energy, offshore wind energy has become popular these days. A large number of offshore wind turbines supported by fixed type substructures have been installed, among which jacket structures are getting popular in recent times. The forces from breaking waves are a major concern in the design of offshore structures installed in shallow waters. However, there are only limited studies available regarding breaking wave forces on jacket structures and still there exist many uncertainties in this area. During the WaveSlam experiment carried out in 2013, a jacket structure of 1:8 scale was tested on a large number of breaking wave conditions. Wave properties and the forces on the structure were measured during the experiment. The total wave slamming forces are being filtered from the experimental measured force using the Empirical Mode Decomposition method and local slamming forces are obtained by the Frequency Response Function method. Based on these results, the peak slamming force and slamming coefficients on the jacket members are estimated. The wave parameters (wave height and period) and wave front asymmetry are obtained from measured wave properties. The variation of slamming forces and slamming coefficients with respect to these parameters are also investigated.

Topics: Waves
Commentary by Dr. Valentin Fuster
2017;():V07BT06A037. doi:10.1115/OMAE2017-62277.

Soil damping is usually modeled as a contribution to global proportional or Rayleigh damping in industry and academia. However, this soil damping model does not represent the physical behavior. In this paper, soil damping for offshore wind turbine foundation is modeled by means of dashpots with damping parameters associated with the soil stiffness. The case study used in this paper is a 5MW NREL wind turbine resting on a jacket foundation as the effect of soil damping in this type of foundation is more significant due to its structural nature and number of piles. A number of dashpots for each pile are used with six degrees of freedom to represent soil damping.

First, free decay vibrations are assesed for different soil damping levels. Next, the aerodynamic loading is applied at the interface level and the integrated analysis is performed in dynamic mode for stochastic loading. Displacements at top of the foundation, base shear and base moment forces for a number of design load cases are evaluated to assess the effect of quantifying soil damping with dashpots. Also, fatigue damages of different hotspots are assessed and compared with the damages from models with no soil damping. It can be concluded that soil damping modeled by dashpots may provide some additional damping and it is more realistic. It leads to load reductions especially mudline moment, with the maximum of 20% for the peak values. Therefore, this type of soil damping modeling can be used to optimize the design of offshore wind turbine especially for the critical hotspots near the mudline.

Commentary by Dr. Valentin Fuster
2017;():V07BT06A038. doi:10.1115/OMAE2017-62377.

In order to fully exploit the potential of FPSOs in the development of offshore oil field, a new concept of sandglass-type FPSO has been put forward recently. In this paper, a novel approach is proposed for designing the main dimensions of the new sandglass-type floating body. With the application of the strip method, the wave-free frequency in heave motion is intensively investigated. The resulting expression shows that the wave-free frequency has close connection with the water-plane area and the corresponding added mass. Then a uniform approximation of the relationship between the added mass and the main dimensions of structure below the waterline is discussed. By comparing with the numerical results of minimum heave RAO of heave motion, the validity and rationality of the proposed method are verified. Besides, experiments are carried out for the sandglass-type floating model and the results support the numerical results and the proposed method. Finally, combining with other requirements in the configuration of the structure above the waterline for the operation at sea, the design scheme for the main dimensions of the sandglass-type FPSO is established.

Topics: Dimensions , FPSO
Commentary by Dr. Valentin Fuster
2017;():V07BT06A039. doi:10.1115/OMAE2017-62381.

This paper takes the presented concept of sandglass-type floating body as the research object. The shape of new sandglass-type floating body has extended oblique characteristic, which may result in special performance and the problem of pitch (or roll) motion. From the experimental tests, it can be found that its pitch natural frequency is small and the second order slowly varying pitch motion in irregular wave is significant, which may cause an unintentional interaction between the pitch and surge motions. Additionally, the floating body with small-waterplane-area and low metacentric height may have obvious mean pitch angle with wind moment under harsh sea condition. To solve the above two problems, numerical simulations have been conducted with two sea conditions. Lastly, based on the existing technique, the problems are solved and the positioning accuracy can be satisfied well for the harsher sea condition.

Commentary by Dr. Valentin Fuster

Ocean Engineering: Unsteady Hydrodynamics, Vibrations, Acoustics and Propulsion

2017;():V07BT06A040. doi:10.1115/OMAE2017-61075.

Due to the resonance behavior of roll motions, roll damping is an important consideration for vessel motions and associated extreme and fatigue loading on the hull, topsides and risers of an FPSO. In many cases radiation damping is limited and passive damping devices such as bilge keels are installed to spur viscous eddies and hence limit the roll motions. This contributes nonlinear damping to an already complex problem. Designers often rely on model tests to assess this damping. Based on test results, empirical and semi-empirical estimation models have been developed for different ship types and are available in current literature, but examples of benchmark validation with real world data are limited. These benchmarks are often hindered by uncertainty in the observed weather conditions, vessel loading conditions and vessel heading with respect to the waves. This paper discusses these challenges and introduces a novel approach used to characterize the actual roll damping for an FPSO under real world conditions. The assumptions, methodology and results will be discussed in this paper. In this study, 5 years of hindcast weather data is examined along with FPSO heading and roll motion measurements. The roll damping characteristics of this FPSO was expected to change over the course of the measurements and the study documents the actual variation of roll damping under various conditions over this period.

Topics: Damping , FPSO
Commentary by Dr. Valentin Fuster
2017;():V07BT06A041. doi:10.1115/OMAE2017-61194.

The hydrodynamics of a flexible foil, which is inspired by the thunniform kind of propulsion system, is studied numerically. The NACA0012 foil with a chord length of 0.1m is selected for the study. The foil performance parameters such as force coefficients and efficiency are estimated for different flexibility of the foil and Strouhal numbers. The results show that oscillating foil generated thrust increases with increase in Strouhal number up to 0.4 and a maximum efficiency is achieved at St = 0.15.

Commentary by Dr. Valentin Fuster
2017;():V07BT06A042. doi:10.1115/OMAE2017-61470.

The operation of ships with “slow-steaming” poses new problems for the torsional vibration analysis of the drive train. It is well known that the propeller determines the essential part of the mass moment of inertia and the system damping. Both values are determined during the initial design phase by semi-empirical methods with have originally been developed by Schwanecke and Grim between 1970 and 1980. Since then, propeller designs have changed significantly and it is unclear if modern propeller designs are still covered by these calculation methods. The paper suggests an extension of Grim’s and Schwanecke’s method for modern screw propellers in homogeneous and unsteady flow.

Topics: Screws , Damping , Propellers
Commentary by Dr. Valentin Fuster
2017;():V07BT06A043. doi:10.1115/OMAE2017-61545.

This paper aims to analyze the tests of blockage effect on Cavitation Tunnel propellers from Institute for Technological Research, IPT, using Siemens commercial CFD (Computational Fluid Dynamics) software STAR-CCM+. The tests presented a relation between the propeller load, the tunnels geometry, the cavitation pattern and the blockage effect. This investigation is conducted in three parts. Firstly, a numerical model without the influence of the walls is conducted to investigate several numerical parameters, such as mesh and turbulence model. In the second part, the numerical model is expanded to include different cases of blockage ratio and advance ratio. A steady-state simulation is conducted, without cavitation model. The results are corrected with potential analysis of blockage correction, showing that this correction is satisfactory. In the last part, blockage simulations with cavitation model are conducted and blockage correction with cavitation is verified, resulting, again, in satisfactory results. However, it can be noted that there is a high influence of blockage ratio in the cavitation area, not contemplated in the classical blockage correction.

Commentary by Dr. Valentin Fuster
2017;():V07BT06A044. doi:10.1115/OMAE2017-61678.

For the well-defined bodies of revolution, the viscous flow past Prolate spheroids has fascinated scientists in fluid mechanics, marine hydrodynamics and aero-dynamical communities for decades. Previous experiment in different yaw and pitch angles of a prolate-spheroid like submarine mode suggests that the asymmetry is a feature of the high Reynolds wake of the symmetric body of revolution with incline angle. The objective of this paper is to examine the similar phenomenon — the asymmetric wake flow — in the relatively low Reynolds number. The present paper focuses on flow field with LES model for a 45° inclined 6:1 prolate spheroid. LESs of the flow and wake have been conducted at Reynolds numbers ReD = 3000, and 10000 (ReD based on the free-stream velocity U0 and the minor axes length D). The maximum grid points reaches 30 million. Results from simulation show the dominant structure of the wake to be a pair of counter-rotating vortices. Detailed observations and analyses show strong leeward side axial flow. Simulation results from LES were compared with the DNS results for ReD = 3000. The wake at ReD = 3000 and 10000 shown was complex with numerous disordered vortical structures. Strong asymmetric wake and large side-force were surprisingly observed and the asymmetry in near field and far field were analyzed and compared.

Commentary by Dr. Valentin Fuster
2017;():V07BT06A045. doi:10.1115/OMAE2017-62465.

A number of new closed-form fundamental solutions for the two-dimensional generalized unsteady Oseen and Stokes flows associated with arbitrary time-dependent translational and rotational motions have been developed. As an example of application, the hydrodynamic force acting on a circular cylinder translating in an unsteady flow field at low Reynolds numbers is calculated using the new generalized fundamental solutions.

Commentary by Dr. Valentin Fuster
2017;():V07BT06A046. doi:10.1115/OMAE2017-62629.

In this paper, the Mixture multiphase flow model and the Schnerr-Sauer cavitation model are used to simulate the tip vortex cavitation of the propeller and then to predict the sound pressure level of the propeller. The structured and unstructured grids are adopted in stationary domain and rotating domain, respectively. The moving reference frame model is used in the rotating domain. The open water characteristics of the propeller are calculated by the SST k-ω turbulence model and the isosurface of vorticity magnitude is clearly presented. The results of the calculation are compared with the non-cavitating condition. It shows that the efficiency of the propeller is reduced when the tip vortex cavitation appears. The tip vortex cavitation will lead to increases both in the overall sound pressure in time histories and in the sound pressure level in the frequency domain.

Commentary by Dr. Valentin Fuster

Ocean Engineering: Wave Mechanics and Wave Effects

2017;():V07BT06A047. doi:10.1115/OMAE2017-61231.

For waves generated by a wave source which is simultaneously moving and oscillating at a constant frequency ω, a resonance is well known to occur at a particular value τres of the nondimensional frequency τ = ωV/g (V: source velocity relative to the surface, g: gravitational acceleration). For quiescent, deep water, it is well known that τres = 1/4. We study the effect on τres from the presence of a shear flow in a layer near the surface, such as may be generated by wind or tidal currents. Assuming the vorticity is constant within the shear layer, we find that the effects on the resonant frequency can be significant even for sources corresponding to moderate shear and relatively long waves, while for stronger shear and shorter waves the effect is stronger. Even for a situation where the resonant waves have wavelengths about 20 times the width of the shear layer, the resonance frequency can change by ∼ 25% for even a moderately strong shear VS/g = 0.3 (S: vorticity in surface shear layer). Intuition for the problem is built by first considering two simpler geometries: uniform current with finite depth, and Couette flow of finite depth.

Commentary by Dr. Valentin Fuster
2017;():V07BT06A048. doi:10.1115/OMAE2017-61273.

A wind-wave hybrid power generation system is a floating offshore energy platform which is equipped with a number of wind turbines and wave energy converters (WECs) to harvest energy from various resources. This wind-wave hybrid platform is moored by eight catenary lines to keep its position against wind-wave-current environment. In most cases, the resonant frequency of horizontal motion of moored platform is very low, so a resonance is hardly seen by numerical simulation with linear wave assumptions. However, the incident waves with different frequency components are accompanied by sum and difference frequency loads due to the nonlinearity of the waves. Typically, the magnitude of the second-order wave loads are small and negligible, but once the second-order wave loads excite the platform at its natural frequency, the resonance can take place, which results in adverse effects on the platform. In this paper, the second-order difference frequency wave load on the wind-wave hybrid platform is numerically assessed and time domain simulation by coupled platform-mooring dynamic analysis is carried out. As a result, the horizontal motions of the platform was highly excited and the increased motions led higher top tension of the mooring lines compared with the case of linear wave environment. Especially, the combination of the wind and wave loads excited the horizontal motions more and made the mooring top tension far higher than wave load was only applied. With regards to the second-order difference frequency wave load, the result with the Quadratic Transfer Function (QTF) is compared to the one with Newman’s approximation. As the simulation results between them was insignificant, the Newman’s approximation can be used instead of the complete QTF to reduce the computational effort.

Topics: Stress , Waves , Wind waves
Commentary by Dr. Valentin Fuster
2017;():V07BT06A049. doi:10.1115/OMAE2017-61530.

Within ship design it is of great importance to predict the vessels movement while at sea so the designer can ensure safety and comfort of passengers and crew. In early stages of the design process, where main dimensions are not yet set, it is useful to have a qualified estimate of what the vessel behavior will be. In 2004 Jensen et.al proposed a series of closed form expressions to calculate the wave induced motions for a monohull ship. Jensen’s formulas were based on a semi analytical approach and the input data is restricted to main dimensions together with forward speed and heading.

The main idea behind a simplified vessel response estimate is to be able to make qualified evaluations of a vessel’s hydrodynamic characteristics at an early stage in the design process. Recently, several applications have been developed based on Jensen’s method. An independent comparison of the method was therefore sought after, in order to increase the understanding of the accuracy level of the simplified method. In this paper we compare the ship motion estimated with Jensen’s simplified method with the vessel prediction from a state-of-the-art linear hydrodynamic vessel prediction module in ShipX.

J.J. Jensen’s simplified method delivers significant differences in response amplitude and resonance range. While more general trends seem to be well captured. The differences originate mainly from Jensen’s assumptions about constant added mass, the definition of the damping coefficient and that the ship is considered box-shaped. These differences give that the simplified method is not suitable as a tool for detail design of ships, but more suitable as support during concept development of ship designs.

Topics: Computers , Vessels
Commentary by Dr. Valentin Fuster
2017;():V07BT06A050. doi:10.1115/OMAE2017-61664.

We examine the implementation of two different wave breaking models into the nonlinear potential flow solver HOS-NWT. HOS-NWT is a computationally efficient, open source code that solves for surface elevation in a numerical wave tank using the High-Order Spectral (HOS) method [1]. The first model is a combination of a kinematic wave breaking onset criteria proposed by Barthelemey, et al. [2] and validated by Saket, et al. [3], and an energy dissipation mechanism proposed by Tian, et al. [4, 5]. The wave breaking onset parameter is based on the ratio of local energy flux velocity to the local crest velocity. Once breaking is initiated, an eddy viscosity parameter is estimated based on the pre-breaking local wave geometry, as described in [4, 5]. This eddy viscosity is then added as a diffusion term to the kinematic and dynamic free surface boundary conditions for the duration of wave breaking. Results implementing this wave breaking mechanism in HOS-NWT have shown that the model can successfully calculate the surface elevation and corresponding frequency spectra, as well as the energy dissipation associated with breaking waves [6–8].

The second model implemented to account for wave breaking in HOS-NWT is based on the method proposed by Chalikov, et al. [9–11]. This model defines wave breaking onset by the curvature of the water surface and defines the wave as broken if it exceeds a certain value. A diffusion term is added to the kinematic and dynamic free surface boundary conditions which dissipates energy based on the local curvature of the water surface, which is consequently not constant in space nor time.

Calculations made using the two models are compared with large scale experimental measurements conducted at the Hydrodynamics, Energetics and Atmospheric Environment Lab (LHEEA) at Ecole Centrale de Nantes. Comparison of calculations with measurements suggest that both models are successful at predicting wave breaking onset and energy dissipation. However, the model proposed by Barthelemy, et al. [2] and Tian, et al. [4] can be applied without knowing anything about the breaking waves a priori, whereas the model proposed by Chalikov [9] requires tuning to specific conditions.

Topics: Waves
Commentary by Dr. Valentin Fuster
2017;():V07BT06A051. doi:10.1115/OMAE2017-61671.

An investigation of the instability of JONSWAP wave fields is carried out within the framework of the Alber equation [1]. The Alber equation describes the weakly nonlinear evolution of an inhomogeneous wave spectrum, and by linear stability analysis of this equation [1–3] the instability of an arbitrary wave spectrum subject to inhomogeneous perturbation is investigated.

We are solving the equations for instability using a numerical method based on the Levenberg-Marquardt algorithm for solving systems of nonlinear equations, as implemented in the FORTRAN library MINPACK. Results from previous works addressing related topics [4, 5] are verified and refined, providing new results for the stability of JONSWAP wave spectra.

Based on the results of the instability analysis we propose more optimal parameters for parameterizing the effects of modulational instability and probability of rogue waves in JONSWAP sea states. The results from the stability analysis of the Alber equation as well as the proposed parameters for parameterizing the effect of modulational instability are verified and tested by performing phase-resolving numerical simulations with the Higher Order Spectral Method [6, 7].

Topics: Seas
Commentary by Dr. Valentin Fuster
2017;():V07BT06A052. doi:10.1115/OMAE2017-61681.

Accurate prediction of near surface wind and wave height are important for many offshore activities like fishing, boating, surfing, installation and maintenance of marine structures. The current work investigates the use of different methodologies to make accurate predictions of significant wave height and local wind. The methodology consists of coupling an atmospheric code HARMONIE and a wave model WAM. Two different kinds of coupling methodologies: unidirectional and bidirectional coupling are tested. While in Unidirectional coupling only the effects of atmosphere on ocean surface are taken into account, in bidirectional coupling the effects of ocean surface on the atmosphere are also accounted for. The predicted values of wave height and local wind at 10m above the ocean surface using both the methodologies are compared against observation data. The results show that during windy conditions, a bidirectional coupling methodology has better prediction capability.

Commentary by Dr. Valentin Fuster
2017;():V07BT06A053. doi:10.1115/OMAE2017-61698.

Breaking waves have been a popular research area among scientists and engineers since they present a strongly nonlinear and turbulent phenomenon. When these waves encounter an offshore or coastal structure, they exert significant amount of loads and stresses, which may result in a catastrophic consequence. Therefore, it is of utmost importance to study breaking waves and associated phenomena. Inspired by this need, in a recent MARIN experiment kinematics of breaking waves were measured with Particle Image Velocimetry (PIV). Among different types of breaking waves, spilling breakers were selected in this initial campaign. First, a summary of the measurement results will be given. These results will then be used for validation of a Computational Fluid Dynamics (CFD) tool. In numerical simulations two methods were followed in order to reproduce the focused wave: in the first method, the CFD tool was coupled to a nonlinear wave model, and in the second method an iterative scheme was used with the CFD tool. Results from these methods were then compared with the measurements.

Commentary by Dr. Valentin Fuster
2017;():V07BT06A054. doi:10.1115/OMAE2017-61845.

The aquaculture industry in Norway produced 1.3 million metric tons of fish in 2014, and further expansion is expected if the main sustainability challenges related to production and operation are mitigated. Major biological, operational and environmental challenges are parasite infection, fish escape, fish health, human injuries and fatalities. The larger farms, exposed locations, and sustainability challenges related to more production of salmon increases the need for efficient decision support methods and risk management. The combined effect of the technological development, with increased remote operation, autonomy and automation, and the production and operational challenges related to sustainability means that an interdisciplinary and systemic approach integrating risks to the environment, as well as to fish welfare and human safety, is needed. Therefore, the main contents of such a risk management approach are outlined in this paper. Potential users are fish farming companies, but the paper also addresses the need for an industry standard for sustainability and risk performance monitoring, which should be of interest to authorities and the whole industry. The paper concludes that risk management and sustainable development are complementary concepts that benefit each other because efficient risk management is decisive for achieving sustainability in aquaculture.

Commentary by Dr. Valentin Fuster
2017;():V07BT06A055. doi:10.1115/OMAE2017-62233.

Tidal energy has the advantages of high predictability, high energy density, and limited environmental impacts. As tidal turbines are expected to be used in the most energetic waters where there might be significant waves, the assessment of unsteady hydrodynamic load due to surface waves is of great concern. The objective of this paper is to assess the effects of surface waves and submergence of the turbine on the power performance and loads of a tidal turbine by experimental approach. The experiments on a 1 : 25th model tidal turbine were carried out in a towing tank. A wide range of regular waves with periods from 1.0 s to 3.0 s at model scale were generated. Different submergence conditions were considered to investigate the effects of the presence of free surface. The cases with blade tip partly going out of water were also performed. The regular waves did not have significant influences on average loads and power output in the present experiments, but caused large amplitude of the cyclic variation of the loads. The amplitudes of the cyclic load were proved to be proportional to the incident wave height, and to be sensitive to the wave frequency and submergence of the rotor. As the tidal turbine getting close to free surface, significant waves were induced by the underwater rotating blade. The effects of surface waves and submergence need to be taken into account in design.

Commentary by Dr. Valentin Fuster
2017;():V07BT06A056. doi:10.1115/OMAE2017-62706.

With the increasing demand for marine structures, including ships and wave energy devices, to operate in energetic, high seastates, the need for modeling and simulation of nonlinear ocean wave fields in large-scale wave basins is becoming essential. In response to this demand, a number of large-scale wave basins have been placed into operation over the years and larger and more sophisticated new ones are under planning and construction. In this article, the current state of practice and technical issues in modeling and simulation of high seastate ocean waves are summarized. A novel methodology for quantitative evaluation of the suitability of competing linear and nonlinear wave theories for a given wave field with multi-spatial measurements is presented. Preliminary results of an on-going study on wave modeling and analysis of measured data from a wave simulation performance study of the Oregon State University directional wave basin, using nonlinear wave theory (e.g. the nonlinear Schrödinger equation), nonlinear Fourier analysis and inference to the existence of rogue waves, are presented. Suggestions on future development of nonlinear wavemaker theories and numerical modeling and simulation of large-scale wave basin nonlinear wave generation are proposed. The article concludes with some observations and remarks on the importance of using an appropriate wave theory to determine the existence of nonlinear coherence structures, including breathers and rogue waves.

Commentary by Dr. Valentin Fuster

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