bluuebunny/crossref_metadata_2025 huggingface dataset:
TRM Versus FRP as Strengthening Material for Improving Impact Resistance of RC Slabs
Authors: Abbas Husain , Almusallam Tarek , Al-Salloum Yousef , Siddiqui Nadeem , Abadel Aref
Published: October 2016

This study experimentally investigates the effectiveness of textile reinforced mortar (TRM) in controlling the local damage in reinforced concrete (RC) slabs subjected to impact loads. The results are then compared with that of conventional FRP strengthening of RC slabs. The impact tests were conducted using the strike of hemispherical nosed steel projectile on TRM- and FRP-strengthened RC slabs. The unstrengthened specimens considered as control were 600 mm square, 90 mm thick RC slabs. The slabs were reinforced with 8 mm diameter steel rebars at 100 mm c/c spacing both-ways. The slabs were strengthened by externally bonding single layer of Carbon FRP (CFRP) and two layers of TRM on the rear face of the slabs. In TRM strengthening, the textile contained equal quantity of high strength carbon fiber roved in two orthogonal directions. They were simply placed on the top of each other and bonded on a secondary polypropylene line. A commercial polymer modified cement mix was used as mortar in TRM strengthening. The velocity of 40 mm diameter steel projectile was varied within sub-ordinance range and the projectile was made to strike normal to the slab using a gas gun.

Based on the experimental testing conducted in this study, it was observed that CFRP and TRM strengthening of RC slabs are effective in reducing the local damage in RC slabs from front and rear faces for the same impact velocity. Both TRM and CFRP strengthening cause increase in the ballistic limit velocity and the perforation energy of RC slabs. Moreover, the CFRP sheet and TRM layer considerably reduce the generation of secondary projectiles which are in the form of flying debris in un-strengthened RC slabs. Overall, among the two strengthening methods studied, the TRM strengthening of RC slabs is found to be either equally good or better than the CFRP strengthening in resisting impact loads.
Experimental Study on a 3-Dimensional Hydro-Elastic Deformation of “Underwater Platform” With Multi-Towers in Waves
Authors: Haneda Ken , Murai Motohiko , Yamanoi Jun
Published: October 2016

Underwater platform was proposed in OMAE 2015 for the purpose of enhancing productivity of various types of renewable energy converter on the sea and its feasibility study was carried out through 2 types of tank experiment [1]. The underwater platform which is a very large frame shape structure connects several floaters under the sea to share power cables and mooring lines and to keep relative distances between the floaters. In the experiment, 1/200 scale elastic model with three spar buoys was used. The buoys imitated spar type floating offshore wind turbines (FOWTs). From the experiment, it was shown that the platform with large draft can reduce its response in waves.

In this paper, we report new result and knowledge obtained by additional model experiments use the 1/200 model. In the experiment, we changed the arrangement and draft of the model and measured hydro-elastic deformation of the underwater platform in waves. From the last experiment, relationship between draft settings and response was shown. In the experiment, relationship between wave angles and response was surveyed.

From the experiment, we have confirmed followings: 1. Rigid-body motion is remarkable in beam waves, 2. Elastic response is remarkable in head waves, and 3. Remarkable torsional motion is occurred in 45 degrees’ waves. The more important thing, however, is that the experimental result indicated that the platform of large draft decreases its motion in the all the wave angles.
Determination of Short Term Extreme Response Values for Temporary Riser Systems: A Practical Approach
Authors: Cetin Ali , Pytte Trond , Eriksrud Sveinung
Published: October 2016

Operation limits for temporary riser system are determined according to some probability of exceedance of a relevant variable. Accordingly, consistent statistical analysis and probability modelling of the data is required. The common industry approach is to rely on the classical narrow-banded Gaussian process assumption when considering time series of variables of interest. Thus, the time series peaks are characterized by means of the Rayleigh distribution and the relevant extreme values are estimated based on this. However, non-linearities present in riser systems may yield non-Gaussian (wide-banded) processes, rendering the classical approach inappropriate. In the present work, an approximate and practical method is presented to address above issue. It is demonstrated that the approximate method is capable of consistently estimating the relevant extreme values, even where the classical method comes short.
Human Factor Study for Maritime Simulator-Based Assessment of Cadets
Authors: Liu Yisi , Hou Xiyuan , Sourina Olga , Konovessis Dimitrios , Krishnan Gopala
Published: October 2016

Maritime accident statistics show that the majority of accidents/incidents are attributed to human errors as the initiating cause. Some studies put this as high as 95% of all accidents (collision, grounding, fire, occupational accidents, etc). The traditional way to investigate human factors in maritime industry is the statistical analysis of accident data. Although this analysis can provide key findings, it cannot capture the causal relationship between performance shaping factors and human performance in the everyday routine work, and is not suitable to be used in the individual assessment of cadets.

To reveal the effects of human factors in maritime and assess the performance of cadets, a full-mission simulator is widely used. Different scenarios such as bad weather, day and night environment, different traffic load, etc. can be simulated. The fine details of the cadet performance can be recorded in the simulator during the assessment. As a result, other than performance failure, the near misses can also be detected. Additionally, a number of cadets can go through the same scenarios at the same time and between-subjects comparison is enabled.

Besides the operations recording provided by the simulator, biosignal-based tools can additionally help in the human factors study in maritime. The existing methods include palmer perspiration, electrocardiography, etc. However, the psychophysiological states that can be recognized by these methods are limited. Electroencephalogram (EEG) biosignals can be used to directly assess the “inner” mental states of subjects. Nowadays, since the EEG devices become portable, easy to setup, and affordable in price, EEG-based tools can be used to assess psychophysiological state of subjects. Using the sensors during performing the task we can recognize the cadet/captain’s emotions, attentiveness/concentration, mental workload, and stress level in real time. In this work, we propose a real-time brain state recognition system using EEG biosignals to monitor mental workload and stress of cadets during simulator-based assessment. Currently, the proposed and implemented system includes stress and mental workload recognition algorithms. The EEG-based mental state monitoring can reflect the true “inner” feelings, stress level and workload of the cadets during the simulator-aided assessment. The time resolution is up to 0.03 second. As a result, we can analyze the recognized brain states and the corresponding performance and behavior recorded by the simulator to study how human factors affect the subject’s performance. For example, we can check is there any correlation of the cadet’s stress level and performance results. Finally, the proposed EEG-based system allows us to assess whether a cadet is ready to perform tasks on the bridge or needs more training in the simulator even if he/she navigated with few errors during the assessment.
A Study on Tsunami Protection Measures by the Floating Tsunami Protection Wharf and the Tsunami Mooring Bitt for Mooring Vessel at a Wharf
Authors: Masuda Mitsuhiro , Minami Kiyokazu , Masuda Koichi
Published: October 2016

On the vessels moored a wharf, the situation such as drifting and runs on a wharf due to the breaking of the mooring lines is generated by the tsunami. The authors are clarified for applicability of the proposed floating tsunami protection wharf (FTPW). The effect of FTPW is most promising as disaster prevention and mitigation measures for the vessels moored a wharf, but the effect cannot be confirmed in the backwash. Therefore, it is necessary to go in combination in FTPW and tsunami countermeasures in consideration of the backwash, such as new tsunami protection measures. The objectives of present study are to consider valid mooring method for backwash. In this paper, the tsunami mooring bitt is proposed, and the effectiveness of using the tsunami mooring bitt is shown.
A Fundamental Study on the Applicability of the Floating Large Size Tsunami Shelter
Authors: Kishi Takuma , Minami Kiyokazu , Masuda Mitsuhiro
Published: October 2016

A floating large-size tsunami shelter (FLTS) has been proposed. The shelter can evacuate more than 1500 people to evacuate from run-up tsunami. The floating tsunami shelter can be lifted by buoyancy when tsunami hitting. In this study, First, Authors assumed Principal specifications of the FLTS from estimation of damage and second we evaluated the validity and safety of a floating large size tsunami shelter using the mooring dolphin by water tank experiment and numerical simulation that is moving particle semi-implicit method (MPS). As a result, enough performance was recognized. The mooring of dolphin was reduced the motion against 10m height tsunami than no moored. However it became clear about the numerical simulation that improvement was more necessary. In particular, the collision model was necessary improved by the existing model.
Experimental Determination of Resonant Response in the Narrow Gap Between Two Side-by-Side Fixed Bodies in Deep Water
Authors: Zhao Wenhua , Wolgamot Hugh , Draper Scott , Taylor Paul H. , Eatock Taylor Rodney , Efthymiou Michalakis
Published: October 2016

Floating liquefied natural gas (FLNG) facilities are a new type of offshore structure, which have been developed as a game changer in offshore hydrocarbon development for unlocking stranded gas reserves. One of the key challenges associated is offloading from FLNG facilities to LNG carriers. Offloading may proceed with vessels in a side-by-side configuration, which allows offtake by un-modified vessels and minimizes requirements for new hardware or procedures (e.g. compared to a tandem operation). Significant challenges remain, however, and reliable offloading is critical for successful FLNG implementation. In this scenario, the two vessels are separated by a narrow 4 m wide gap. The resonant response of the sea surface in the gap has been predicted by numerical simulations [1] to be a few times that of the incident waves at particular frequencies. As a consequence, the gap resonant response may play a role in determining the operational window for side-by-side offloading operations, and thus has attracted a lot of attention recently. There have been studies on this topic both numerically and experimentally. However, many of these studies are in 2 dimensions (2D), for relatively large gaps and relatively shallow water depth, which may pose difficulties when extending the results to a real project. It is unclear what will happen for a gap resonance if the gap width gets narrower (say 4 m in full scale) and the water depth gets deeper (say 600 m in full scale). In this study, we conducted a series of model tests at a scale of 1:60 in a large wave basin, and focused on deep water and, crucially, narrow gaps, which are closer to a real project geometry. To facilitate future numerical simulations, we used two identical fixed bodies in the model tests and the vessels were simple barge-like shapes. Using white noise waves as the excitation, which covers a broad brand, the response of the fluid in the gap has been measured at several points. In these experiments, different modes of the gap resonance have been observed. Response amplitude operators (RAOs) of the gap resonance have been obtained through spectral analyses, which provide valuable information for the design of side-by-side operations and will benefit future numerical simulations. Test runs in white noise waves with different significant wave heights were also performed, to study the nonlinearities of the gap resonance phenomenon.
Hydroelastic Analysis on a Flexible Barge Adjacent to a Floating Mobile Harbor
Authors: Wei Wei , Fu Shixiao , Li Runpei , Li Jun , Ou Shaowu
Published: October 2016

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.
Experimental Study on Flow-Induced Vibration of Floating Squared Section Cylinders With Low Aspect Ratio: Part II — Effects of Rounded Edges
Authors: Gonçalves Rodolfo T. , Gambarine Dennis M. , Momenti Aline M. , Figueiredo Felipe P. , Fujarra André L. C.
Published: October 2016

Experiments regarding flow-induced vibration on floating rounded squared section cylinders with low aspect ratio were carried out in an ocean basin equipped with a rotating-arm apparatus. Floating squared section cylinders with rounded edges and aspect ratios of L/D = 2.0 were elastically supported by a set of linear springs in order to provide low structural damping to the system. Two different incidence angles were tested, namely 0 and 45 degrees. The Reynolds numbers covered the range from 2,000 to 30,000. The aim was to understand the flow-induced vibrations around single columns, gathering information for further understanding the causes for the Vortex-Induced Motions in semi-submersible and TLP platforms. Experiments on circular and squared sections cylinders (without rounded edges) were also carried out to compare the results with the rounded square section cylinders (with rounded edges). The amplitude results for in-line, transverse and yaw amplitude for 0-degree models showed to be higher for squared section cylinders compared to those for the rounded square section cylinders. No significant difference between the 45-degree models was observed. The results of ratio between frequency of motion in the transverse direction and natural frequency in still water confirmed the vortex-induced vibration behavior for the squared and rounded square section cylinders for 45-degree incidence; and also the galloping characteristics for 0-degree incidence cases. The rounded effect on the square section cylinders showed to be important only for reduced velocity larger than 8, which is probably related to the position of the separation point that changes around the rounded edge, behavior that did not occurr for the squared edge that fixed the separation point for any reduced velocity.
Experimental Study of Breaking Wave Impinging and Overtopping a Deck Structure
Authors: Wu Yun-Ta , Chang Kuang-An
Published: October 2016

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.
Soil Characterisation for Installing and Operating Deep-Water Pipelines
Authors: Borges Rodriguez Antonio , Dantal Vishal , Smith Victor Bjorn , Carroll Roselyn
Published: October 2016

Deep-water developments rely on pipeline and riser systems to transfer hydrocarbon products to floating facilities or potentially longer tie-back pipelines to shallow water platforms/onshore facilities. Depending on the nature of the product and operational conditions, the pipeline and riser system design may need to consider a range of dynamic processes during operation such as (i) controlled lateral buckling of the pipeline in order to relieve excessive constrained axial forces induced by temperature and pressure changes in the system; (ii) the accumulation of pipeline axial displacement or ‘walking’; and (iii) evolution of the pipe-soil interaction at the riser seabed touchdown point due to the dynamic behaviour of the riser. Under these conditions, the reliable structural assessment of the pipeline system relies upon accurate assessment of the pipeline-soil interaction (PSI), from the initial lay embedment of the pipeline to the evolution of the lateral and axial response over the lifetime of the facilities. Accurate assessment of these PSI parameters requires adequate characterisation of the seabed topography, seabed processes (e.g. geohazards) and the soil properties. This paper proposes ways for efficient planning of the geophysical and geotechnical site investigation activities and subsequent soil element and physical model testing for the assessment of relevant PSI parameters in deep-water.
A Ballast System for Automated Deep-Sea Ascents
Authors: Golz Matthias , Boeck Florin , Ritz Sebastian , Holbach Gerd
Published: October 2016

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.
Numerical and Experimental Investigations of Ship Maneuvers in Waves
Authors: el Moctar Ould , Sprenger Florian , Schellin Thomas E. , Papanikolaou Apostolos
Published: October 2016

Assuring a ship’s maneuverability under diverse conditions is a fundamental requirement for safe and economic ship operations. Considering the introduction of the Energy Efficiency Design Index (EEDI) for new ships and the related decreasing installed power on ships, the necessity arose to more accurately predict the maneuverability of ships in severe seas, strong winds, and confined waters. To address these issues, extensive experimental and numerical investigations were performed within the European funded Project SHOPERA. Here, second order forces and moments for a containership and a tanker were measured in model tests and computed by solving the Reynolds-Averaged Navier-Stokes (RANS) equations. Generally, these measured and computed second order loads (drift forces and yaw moments, added resistance) compared favorably. Furthermore, the effects of waves on zig-zag and turning circle maneuvers were investigated.
Numerical Modelling of Floating and Submerged Bridges Subjected to Wave, Current and Wind
Authors: Lie Halvor , Fu Shixiao , Fylling Ivar , Fredriksen Arnt G. , Bonnemaire Basile , Kjersem Geir Lasse
Published: October 2016

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.
Combined Optical and Acoustic Monitoring of Fishes in the Demonstration Site of Marine Renewable Energy Development
Authors: Kitazawa Daisuke , Mizukami Yoichi
Published: October 2016

Before the installation of marine renewable energy devices, fish species and abundance should be examined for selecting the proper site where the effects of the devices on the environment and fish will be as small as possible. Fish species and abundance can be examined in a variety of methods such as a fish finder using an acoustic wave and fishing gears such as a gill net. However, the fish finder cannot specify the species of fish that is sometimes estimated from the experience of fishermen or scientific researchers. Some amounts of fish must be removed from the target sea area in case of using the fishing gear, while the species of fish can be specified. In the present study, an underwater optical video camera is combined with the fish finder using an acoustic wave to specify the species of fish. A circular fish-eye digital video camera is inserted into a waterproof container. A part of the container is made of glass in a dome shape for the circular fish-eye lens. The container is attached to polyethylene ropes and is towed by a fishing vessel. First, the hydrodynamic characteristics of the container was examined by a towing test with the three kinds of towing speed in a water tank. Then the container was towed in the real sea, which is the demonstration site of offshore wind and wave energy developments off Kamaishi of Iwate Prefecture. The depth of the video camera with the container was not constant since the moving speed of the fishing vessel was slow and fluctuating. The image of video camera could be captured successfully together with that of the acoustic video camera, while fish could not be found in both the optical and acoustic measurements in the present investigation. The investigation will be continued, and the effects of transparency of water should be discussed as future works. Also the actual or model fish should be captured by the underwater video camera to evaluate if it can specify the species of fish.
Parametric Investigation and Mechanism of Low-Drag Fairing Devices for Suppressing Vortex-Induced Vibration
Authors: Law Yun Zhi , Benjamin Tan Jui Hang , Jaiman Rajeev K.
Published: October 2016

It is well known that fairing devices are better alternatives than helical strakes due to their low-drag performance while suppressing vortex-induced vibration (VIV). Our objective is to present a systematic numerical study to understand the hydrodynamic performance and physical mechanism of fairing configurations and then propose a new device for suppressing VIV and reducing drag force. In this work, we simplify our investigation by allowing the cylinder-fairing system to oscillate in cross-flow direction without rotation. Firstly, we present a set of simulations of vortex-induced vibration for Short Crab Claw (SCC) fairings [1] with different nondimensional length (Lf/D), where Lf is the length of fairing and D denotes the diameter of cylinder. To establish the relation between the length of fairing and the performance with respect to VIV suppression and drag reduction, we consider the length ratio Lf/D = 1.25, 1.50, 2.00. The underlying VIV suppression mechanism is investigated with the aid of force and amplitude variations, wake flow structures and frequency ratios. Our results show that the SCC fairing with longer length performs better by suppressing the amplitude up to 84% and reduces the drag coefficient by 40%. This finding implies that by offsetting the vortices shed away from the main cylinder, it lowers the influence of vortex interactions, which leads to the suppression of VIV and net reduction in the drag force generation. Based on this mechanism, we propose a new design of fairing, namely the “Hinged C-shaped”, which consists of a thin splitter plate (connected at the base of main cylinder) bifurcating into a C-shaped geometry after a certain distance. Through our numerical study on its hydrodynamic performance, it is shown to be efficient with respect to VIV suppression and drag reduction. To understand the VIV suppression physics, the numerical study is conducted in two-dimension for the cylinder-fairing mounted elastically with mass ratio m* = 2.6 and the damping ξ = 0.001 at low Reynolds number. We further demonstrate the performance of the new fairing device in three-dimension at sub-critical Reynolds number.
Expert Knowledge Based Statistical Experimentation for Investigating Optimal Arc Welding Parameter Combination Under Noise
Authors: Ratnayake R. M. Chandima , Dyakov Daniel
Published: October 2016

An expert knowledge based (EKB) statistical approach was used for investigating the optimal parameter settings of an arc welding process (AWP) with experimentation. The experiments were performed to investigate the effect of welding factors (i.e. gap geometry, different welding techniques, material transfer and welding processes) on the material properties in the weld and heat affected zone (HAZ). Based on the experts’ knowledge and published research, key factors and factor levels were identified. Orthogonal arrays proposed in the engineering robust design approach (ERDA) were utilized to perform the experiments. Joint performance was assessed in relation to the tensile strength. Microstructures were examined in selected areas. This manuscript illustrates the mathematical approach, experimental design, parameter settings that resulted in the optimal joint performance, microstructure, and results of the verification experiment.
Effects of Wave-Current Interaction on Floating Bodies
Authors: Hermundstad Elin Marita , Hoff Jan Roger , Stansberg Carl Trygve , Baarholm Rolf
Published: October 2016

Wave-current interaction effects may significantly influence the mean wave drift forces on a structure as well as the motion responses and wave elevation around the structure. Additionally, the drift force may be used to estimate the wave drift damping of a moored structure.

A new numerical potential theory code for industry applications (MULDIF) has been recently developed, where the hydrodynamic interaction between waves and current of arbitrary direction with large volume structures is consistently included. The code also handles multiple bodies and finite water depth including wave-current interaction effects. The aim has been to create a robust and easy-to-use practical tool.

Initial validation studies against model tests have been conducted. The numerical results show a strong heave-pitch coupling due to the presence of the current. Preliminary results for a semi-submersible show good agreement for the motions provided that the mooring used in the model tests are accounted for. The free surface elevation around the semi-submersible is presented in contour plots.
CFD Reproduction of Model Test Generated Extreme Irregular Wave Events and Nonlinear Loads on a Vertical Column
Authors: Pakozdi Csaba , Östman Anders , Bachynski Erin E. , Stansberg Carl Trygve
Published: October 2016

Recently, a method for numerical reproduction of measured irregular wave events has been developed. The measured motion of the wave maker flaps defines the wave kinematics at the boundary of the numerical simulation in order to generate the waves as described in (Pakozdi, Kendon, & Stansberg, 2011) and (Ostman, Pakozdi, Stansberg, Fagertun, & Vestbostad, 2015).

When such data are not available, the control signal of the wave maker can, instead, be generated from a given free surface elevation following the same procedure as in model tests. Following this procedure automatically gives the possibility to subsequently reproduce the numerical wave experimentally using the obtained control signal.

The latter procedure is applied to a model test case with extreme irregular wave events and resulting nonlinear global wave loads on a vertical cylinder (Stansberg, 1997), with the focus on higher-order ringing excitation. The purpose of the investigation is two-fold: 1) to validate the wave reconstruction procedure, and 2) to validate the resulting CFD ringing loads with the given waves. The results show generally good agreement both in the waves and in the loads; discrepancies found in the loads are considered to be mainly originating from corresponding uncertainties in the wave reconstruction. Wave breaking may be one source of uncertainty.
Impact Pressure, Void Fraction, and Green Water Velocity due to Plunging Breaking Wave Impingement on a 2D Tension-Leg Structure
Authors: Chuang Wei-Liang , Chang Kuang-An , Mercier Richard
Published: October 2016

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.
Shape Optimisation of Model Scale Geotextile Sand Containers (GSC) Regarding Sinking Behaviour: First Results of Physical Model Tests
Authors: Plenker Désirée , Heins Evelyn , Grabe Jürgen
Published: October 2016

Energy transition towards sustainable power generation affects the offshore wind energy sector greatly. Due to extensive research work and technological developments, the number of foundation types for offshore wind energy plants has increased significantly. Independent of foundation type, each structure influences the ecological and hydrodynamic regime surrounding the structure. As a consequence, local flow turbulences may cause scours at the seabed and can lead to a reduction of structure stability. Geotextile sand containers (GSC) are an approved method for scour protection. During installation of scour protection systems, the sinking behaviour of GSC is affected by translational and rotational movement, which impedes an accurate positioning of GSC. Physical model tests have been conducted to analyse the influence of container shape and material properties of GSC. This paper presents the results of these model tests.
Low Reynolds Flow Past a Circular Cylinder Close to a Free-Surface With Vertical Motion Dynamics
Authors: Cercos-Pita Jose L. , Colagrossi Andrea , Souto-Iglesias Antonio
Published: October 2016

Flow past a circular cylinder close to a free surface at low Reynolds number is investigated numerically in this paper extending the work done in previous 2014 and 2015 OMAE papers [1, 2]. In the former, the dependence of the flow with the submergence was discussed and in the latter the flow at high Froude numbers was investigated. It was found that shedding is blocked as the cylinder approaches de free surface with an increasing value of the net lift coefficient and a reduction of the lift alternate oscillations due to such shedding. This variation on the lift pattern suggests that if a VIVACE device [3] is set under these flow characteristics, its performance will significantly differ from that in deep water conditions. These devices take advantage of flow induced motions in bluff bodies (such as the cylinder) to generate electric power. Singh & Mittal [4] coupled VIV system is studied in present paper in the presence of a free surface, selecting a configuration with large response in unbounded flow and adding a free-surface to such case. Results are presented and discussed.
Comparison of Global Response of a 3-Span Floating Suspension Bridge With Different Floater Concepts
Authors: Fredriksen Arnt G. , Bonnemaire Basile , Lie Halvor , Munkeby Jan , Nesteby Anders , Buckholm Petter , Tan Xiang
Published: October 2016

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.
Hydrodynamic Analysis and Optimization of a Hinged Type Wave Energy Converter
Authors: Li Yuzhu , Peng Heather , Qiu Wei , Lundrigan Brian , Gardiner Tim
Published: October 2016

SeaWEED (Sea Wave Energy Extraction Device) is a multi-body floating wave energy converter (WEC) with hinged joints developed by Grey Island Energy Inc. (GIE) in Canada. Initial conceptual studies have been carried out to evaluate the performance of the first generation device by testing a 1:16 scale model in a wave basin. The experimental results were compared with the numerical solutions. Based on the experimental studies, improvements were made and a second generation model with a new geometry of the hull and a new connection structure was developed. This paper is mainly focused on the numerical analysis and optimization of the second generation SeaWEED model. In the numerical studies, the hydraulic power take-off (PTO) system was simulated by a linear spring damper system coupled with the motion of the hinged bodies. The vertical hinge motion was computed at a series of wave periods using WAMIT. Optimization was focused on the PTO damping and the geometrical parameters in terms of the draft and the length of the truss structure between hinged bodies by using the response surface method. An optimal combination of length, draft and PTO damping was recommended for an intended operation location.
Wave-Structure Interaction of Focussed Waves With REEF3D
Authors: Bihs Hans , Alagan Chella Mayilvahanan , Kamath Arun , Arnsten Øivind A.
Published: October 2016

For the stability of offshore structures, such as offshore wind foundations, extreme wave conditions need to be taken into account. Waves from extreme events can become critical from design perspective. In a numerical wave tank, extreme waves can be generated through focussed waves. Here, linear waves are generated from a wave spectrum. The wave crests of the generated waves coincide at a pre-selected location and time. In order to test the generated waves, the time series of the free surface elevation are compared with experimental benchmark cases. The numerically simulated free surface shows good agreement with the measurements from experiments. In further computations, the wave impact of the focussed waves on a vertical circular cylinder is investigated. The focussed wave generation is implemented in the numerical wave tank module of REEF3D, which has been extensively and successfully tested for various wave hydrodynamics and wave-structure interaction problems in particular and for free surface flows in general. The open-source CFD code REEF3D solves the three-dimensional Navier-Stokes equations on a staggered Cartesian grid. Solid boundaries are taken into account with the ghost cell immersed boundary method. For the discretization of the convection terms of the momentum equations, the conservative finite difference version of the fifth-order WENO (weighted essentially non-oscillatory) scheme is used. For temporal treatment, the third-order TVD (total variation diminishing) Runge-Kutta scheme is employed. For the pressure, the projection method is used. The free surface flow is solved as two-phase fluid system. For the interface capturing, the level set method is selected. The level set function can be discretized with high-order differencing schemes. This makes it the appropriate solution for wave propagation problems based on Navier-Stokes solvers, which requires high-order numerical methods to avoid artificial wave damping. The numerical model is fully parallelized based on the domain decomposition, using MPI (message passing interface) for internode communication.
Development of a Simulation Tool to Accurately Predict Tidal Turbine Reliability
Authors: Waldron Benson , Zymaris Alexandros , Bittencourt Claudio , Kakalis Nikolaos
Published: October 2016

The pre-commercial development of the tidal energy sector is characterized by a diverse range of technology concepts. Within the spectrum of proposed and developed solutions the three-bladed Horizontal Axis Tidal Turbine (HATT) is one of the more dominant configurations. However, even within this “narrow” classification technology developers have chosen different design solutions and strategies. Major influences on design decisions are the maintenance and repair costs and their impact on Levelised Cost of Energy (LCOE); therefore it is critical to accurately determine reliability to support the engineering and financial decision making. This paper is based upon work done to develop techniques for the reliability analysis of tidal turbines Power Take Off (PTO) systems and proposes a simulation tool for improved reliability prediction. In brief the proposal is to utilize available methodologies and software to demonstrate how the different loading and environmental conditions experienced in tidal impact on PTO reliability. The approach utilizes a system engineering model of the powertrain augmented with physics-of-failure models. Using available surrogate reliability data and the output from the systems engineering modelling a modified probability of failure for a system or component can be generated. The model is capable to simulate the system under nominal, off-design and faulty conditions. By utilizing this approach, the reliability of the turbine can be quantitatively analyzed taking into account realistic operating conditions. An indicative case-study is presented to demonstrate the proposed approach.
Determining the Hydrodynamic Coefficients of a BOP
Authors: Holmes Samuel , Calver William , Tognarelli Michael , Constantinides Yiannis , Russo Massimiliano
Published: October 2016

Blow-out preventers (BOPs) can be subjected to vortex induced vibration and also oscillating wave-induced loads from attached risers. Typically, the response of the BOP is estimated using heuristic codes and assumed hydrodynamic coefficients (added mass and drag) of the BOP and attached structures. In current practice, complex structures such as a BOP are tested in a tow basin using physical models to find these coefficients. An alternative, which is explored here, is to use computational fluid dynamics (CFD). Two BOP designs have been tested in 1:12 scale. We use CFD to simulate these experiments and to calculate forces on the models the resulting force histories. Added mass and drag coefficients are calculated from the CFD results at various Keulegan-Carpenter numbers and the results are compared with data derived from tow tank experiments. It is shown that the CFD simulations can provide an alternative to tow tank experiments over a wide range of experimental parameters. CFD simulations are also used to predict the hydrodynamic coefficients of a full scale structure and the results compared with the model scale results.
Optimization of Flexible Pipes Dynamic Analysis Using Artificial Neural Networks
Authors: Chaves Victor , Sagrilo Luis V. S. , Ribeiro Machado da Silva Vinícius
Published: October 2016

Irregular wave dynamic analysis is an extremely computational expensive process on flexible pipes design. One emerging method that aims to reduce these computational costs is the hybrid methodology that combines Finite Element Analyses (FEA) and Artificial Neural Network (ANN). The proposed hybrid methodology aims to predict flexible pipe tension and curvatures in the bend stiffener region. Firstly using short FEA simulations to train the ANN, and then using only the ANN and the prescribed floater motions to get the rest of the response histories. Two approaches are developed with respect to the training data. One uses an ANN for each sea state in the wave scatter diagram and the other develops an ANN for each wave incidence direction. In order to evaluate the accuracy of the proposed approaches, a local analysis is applied, based on the predicted tension and curvatures, to calculate stresses in tension armour wires and the corresponding flexible pipe fatigue lifes. The results are compared to those from full nonlinear FEM simulation.
Advanced Finite-Element Modeling for Creep Simulation on Flexible Pipe Pressure Sheath
Authors: Naturel Jérôme , Epsztein Thomas , Gavouyère Thierry
Published: October 2016

Unbounded Flexible pipe used for offshore fields development are usually composed of different layers of polymer and steel, each layer having a specific function during the product service life. This multi-layer characteristic enables to tailor the cross-section of the pipe to meet project-specific requirement, and optimize the cost of the product for each application. In particular, the main function of the thermoplastic pressure sheath is to guaranty the sealing of the product. The material and the thickness of this pressure sheath mainly depend on the pressure and temperature of the bore, and the design choice is driven by the creeping of the sheath in the interstices of the pressure vault: it must be limited with regard to sheath thickness reduction, as per API17J design requirement.

Consequently, when developing new material for pressure sheath application, the early prediction of the creep performance over the full range of the targeted application is crucial. For this reason, before any full-scale test, a test campaign is required to evaluate the creeping of the material on small-scale material sample. In this development context, the use of advanced finite-element simulation for predicting the creeping behavior is quite useful to amplify the benefit of tests campaign results, and to give additional information on material performances. As far as the modelling is validated by correlation with small-scale tests, the numerical tool is used to multiply virtual creep tests configurations.

This paper will focus on the numerical challenges for developing such creeping simulation, based on ABAQUS commercial software. Firstly, the identification of the viscoelastoplastic parameters for polymer material law will be presented. This material law is a nonlinear viscoelastoplastic model consisting of multiple networks connected in parallel. The number of parameters of such law is not limited, but a compromise between law precision and identification robustness must be found. Then, the correlation process between small-scale test and finite-element results will be detailed. In particular, the influence of the experimental protocol has to be determined. Finally, a sensitivity study of the most influent parameters, based on parametric FEA model, will be presented to highlight the benefice of such model. The benefice of such model does not only consist on correlation with small-scale test. As the material modeling is intrinsic, it is also possible to use the same law for studying the creep behavior on very different geometrical configurations.
Steel Lazy Wave Riser Tests in Harsh Offshore Environment
Authors: Constantinides Yiannis , Cao Peimin , Cheng Jingyun , Fu Shixiao , Kusinski Greg
Published: October 2016

The Steel Lazy Wave Risers (SLWR) is a newly utilized riser concept with increasing popularity due to its improved structural performance in harsh environment. However, SLWR technology is still new, and has very limited qualification test data and field experience to establish a consistent technology readiness and maturity level through industry. The present study, championed by the DeepStar® consortium, planned and executed a comprehensive test campaign of a SLWR in an offshore model basin. The objective was to better understand the global dynamic behavior of SLWR due to vessel induced motions and currents to advance the concept maturity and quantify the design margins. The paper describes the experiment and highlights key findings from the tests that are essential in addressing the challenges around SLWR design. The findings of the study progress the technology readiness level of the riser concept and provide essential guidelines for the design of SLWRs.