bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonlinear Piecewise Restoring Force in Hydrokinetic Power Conversion Using Flow Induced Motions of Two Tandem Cylinders
Authors: Ma Chunhui , Sun Hai , Bernitsas Marinos M.
Published: October 2017

Flow Induced Motions (FIMs) of two tandem, rigid, circular cylinders with piecewise continuous restoring force are investigated for Reynolds number 24,000≤Re≤120,000 with damping, and restoring force function as parameters. Selective roughness is applied to enhance FIM and increase the hydrokinetic energy captured by the VIVACE (Vortex Induced Vibration for Aquatic Clean Energy) Converter. Experimental results for amplitude response, frequency response, interactions between cylinders, energy harvesting, and efficiency are presented and discussed. All experiments were conducted in the Low Turbulence Free Surface Water (LTFSW) Channel of the MRELab of the University of Michigan. The main conclusions are: (1) The nonlinear-spring, Converter can harness energy from flows as slow as 0.33 m/s with no upper limit. (2) The nonlinear-spring Converter has better performance at initial galloping than its linear-spring counterpart. (3) The FIM response is predominantly periodic for all nonlinear spring functions used. (4) The influence from the upstream cylinder is becoming more dominant as damping increases. (5) Optimal power harnessing is achieved by changing the linear viscous damping and tandem spacing L/D. (6) Close spacing ratio L/D = 1.57 has a positive impact on the harnessed power in VIV to galloping transition. (7) The interactions between two cylinders have a positive impact on the upstream cylinder regardless of the spacing and harness damping.
Analysis of the Blockage Effect on a Cavitation Tunnel Using CFD Tools
Authors: Tadashi Katsuno Eduardo , Dantas Joao Lucas Dozzi
Published: October 2017

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.
Wave Energy Extraction From Multiple Buoys Supporting a Flexible Runway
Authors: Zhang H. C. , Xu D. L. , Li Q. H.
Published: October 2017

Integrating an array of buoys type converters with a flexible runway can be a viable option for cost-sharing between wave energy capturing devices and ocean space utilization structures, and thus enhance the cost-effectiveness of wave energy utilization. In this study, a configuration of multiple buoys supporting a runway is proposed. Hydrodynamic interactions among the buoys are analyzed using an exact algebraic method based on linear wave theory in the frequency domain. A parametric governing equation of compound wave energy converter referred to as a wave farm is formulated by using Hamilton’s principle which can be discretized by using Galerkin method. The effects of wave condition and the parameters of PTO on the wave energy absorption and dynamic characteristics of a runway are analyzed. This research work is aimed to provide a theoretical guideline for wave energy converters design.
Study on Application of CFD and FEM Coupling Method to Evaluate Dynamic Response of Ship Under Severe Wave Condition
Authors: Takami Tomoki , Oka Masayoshi , Iijima Kazuhiro
Published: October 2017

In this study, a series of validations of a weakly one-way CFD-FEA coupling method for the estimation of the dynamic response of a container ship under severe wave condition is made. By comparing the prediction with linear/non-linear strip method, 3D panel method and towing tank test results under various wave conditions in terms of rigid body motions and structural loads, the effectiveness of the CFD-FEA coupling method is confirmed. Furthermore, the hydro-elastic behavior obtained by 3D panel method and the CFD-FEA coupling method are validated regarding the whipping moment by comparing the experiment.
Corrosion Fatigue Mechanisms and Fracture Mechanics Based Modelling for Subsea Pipeline Steels
Authors: Cheng Ankang , Chen Nian-Zhong
Published: October 2017

Subsea structures such as pipelines are vulnerable to environment-assisted crackings (EACs). As a type of EAC, corrosion fatigue (CF) is almost inevitable. For such a process, stress corrosion (SC) and hydrogen-assisted cracking (HAC) are the two mainly driving mechanisms. And it was further pointed out that slip dissolution (SD) and hydrogen embrittlement (HE) should be responsible for SC and HAC respectively. Based on such a fact, a two-component physical model for estimating the CF crack propagation rate was proposed. The proposed model was built in a frame of fracture mechanics integrated with a dissolution model for C-Mn steel and a newly established model by the authors accounting for the influence from HE upon crack propagation. The overall CF crack propagation rate is the aggregate of the two rates predicted by the two sub-individual models, and then the crack propagation time is calculated accordingly. The model has been proven to be capable of capturing the features of HE influenced fatigue cracking behaviour as well as taking mechanical factors such as the loading frequency and stress ratio into account by comparison with the experimental data of X42 and X65 pipeline steels.
Optimization in Testing Techniques for Fracture Resistance Evaluation in Sour Environments
Authors: Youyou Wu , Tse-Ven Steven Chong , Daqin Xu , Tronskar Jens P.
Published: October 2017

Use of Single Edge Notch Tension (SENT) specimen geometry to determine the fracture toughness of the pipeline material in sour service environment has earlier been established in OMAE2009-79305(1). Optimization of the testing techniques has been made with use of implementation of Direct Current Potential Drop (DCPD). DCPD allows for the simultaneous monitoring of crack growth throughout the loading process and subsequently the fracture resistance J /CTOD R-curve can be obtained with just a single test specimen. In this paper the implementation of DCPD method in a wet sour environment using the single-specimen procedure is described and the results have been compared with those obtained using the multi-specimen approach and the results showed good agreement. Some other challenges encountered during the testing process are also addressed in this paper.
Maximum Wave Load Cycles on Submerged Rotating Tidal Energy Turbines: Identification of Worst Case Scenarios
Authors: Kosleck Sascha , Sprenger Florian
Published: October 2017

In recent decades, significant research and development has been invested in techniques that harvest renewable energy from an ocean environment. Starting from offshore wind energy we also see developments in devices that extract energy from sources such as waves and tides. Although a lot can be learned by transferring existing on- and offshore technology, a multitude of new challenges arouse. The offshore wind industry for example has struggled with wave induced loads on their wind turbine foundations structures, which resulted in intense nacelle and rotor vibrations. These vibrations have a significant impact a turbine’s lifetime. The specific problem of oscillating wave loads worsens for tidal energy turbines as no longer only the foundation but also the rotor itself is submerged and directly exposed to wave induced forces.

Therefore, the best possible determination of these loads is a key prerequisite for any holistic vibration and fatigue analysis. During operation, the turbine rotor will, depending on the actual tidal current velocity, rotate at different speeds whilst the waves propagate over it. The number of ratios between wave period Tw and rotational period of the rotor Tr is therefore infinite. This raises the question what load changes a revolving rotor blade experiences and which combination of Tw/Tr and wave encounter angle will generate the maximum loads per cycle. This study presents a comprehensive, general approach to identify maximum possible wave induced forces and moments on stationary and turning rotor blades, for any turbine design, position in the water column and orientation in the wave field. The procedure is exemplified for a generic 3-bladed horizontal axis turbine which is fixed to the seabed. The approach identifies loads depending on wave period and height, the period of rotor rotation and the wave encounter angle, utilising diffraction theory. Forces and moments are firstly calculated in a global coordinate system and subsequently transferred into a blade fixed coordinate system. This allows for an examination of the load changes as the blades rotate about the turbine axis. Thus, the worst-case scenarios in terms of load changes for each combination of wave parameters can be identified. Those maximum load cycle events are then combined to transfer functions, for loads and moments respectively. Transfer functions such as these will later allow for a quick identification of maximum load changes and cycle periods, depending on any given environmental condition. This information will help identifying structural loads and immanent fatigue and vibration issues during the actual turbine design process.
Numerical Simulation on the Dynamic Installation of the OMNI-Max Anchors in Clay Using a Fluid Dynamic Approach
Authors: Liu Jun , Zhang Yuqin
Published: October 2017

The OMNI-Max anchors, which are used as foundations for mooring deep-water offshore facilities, are raised recent years for their dynamically installation. ANSYS CFX 17.0 is a computational fluid dynamic (CFD) program, capable of simulating the dynamically installation process of the OMNI-Max anchor. In the simulation, soft clay with linearly increasing shear strength is modeled as Eulerian fluid material. The clay is subjected to high shear strain rate during the dynamical installation procedure, hence the H-B model is proposed as it is applicable to a wide range of shear strain rate. Different anchor impact velocity levels are modeled to investigate their effects on the anchor final penetration depths. To improve the anchor impact velocity and final penetration depth, a booster, which is retrievable and renewable, is attached to the tail of the anchor. The results demonstrate that the anchor would achieve deeper penetration depth with the increase in impact velocity. Also the anchor with a booster could reach a deeper penetration depth than that of the single anchor owing to the increase of the anchor total energy.
Robust Control for Heave Compensator With the Use of Kalman Filter-Based Disturbances Estimator
Authors: Cuellar William H. , Linhares Tassio Melo , Limaverde Filho Jose Oniram de A. , Vargas Jose A. R. , Fortaleza Eugenio
Published: October 2017

This paper presents a backstepping approach for an active heave compensator actuated by a double acting cylinder. For practical applications, is desired to stabilize the system around the equilibrium point when subjected to unknown external disturbances. Knowing the governing equations, a robust backstepping control design is proposed by introducing a well-defined smooth function and using a Nussbaum function together with a Kalman filter-based disturbance estimator. The efficiency of the proposed control scheme is demonstrated through numerical simulations and compared with related works.
Prediction of Low Failure Probabilities With Application to Marine Risers
Authors: Zhang Xiaodong , Low Ying Min , Koh Chan Ghee
Published: October 2017

Offshore riser systems are subjected to wind, wave and current loadings, which are random in nature. Nevertheless, the current deterministic based design and analysis practice could not quantitatively evaluate the safety of structures taking random environmental loadings into consideration, due to high computational costs.

Structural reliability method, as an analysis tool to quantify probability of failure of components or systems, can account for uncertainties in environmental conditions and system parameters. It is particularly useful in cases where limited experience exists or a risk-based evaluation of design is required. Monte Carlo Simulation (MCS) method is the most widely accepted method and usually used to benchmark other proposed reliability methods. However, MCS is computationally demanding for predicting low failure probabilities, especially for offshore dynamic problems involving many types of uncertainties. Innovative structural reliability methods are desired to perform reliability analysis, so as to predict the low failure probabilities associated with extreme values.

Variety of structural reliability methods are proposed in the literature to reduce the computational burden of MCS. The post processing methods, which recover PDF or tail distribution of random variable from sample data to perform structural reliability analysis, have great advantages over the methods from other categories on solving engineering problems. Thus the main focus of our study is on post processing structural reliability methods. In this paper, four post processing reliability methods are compared on the prediction of low failure probabilities with applications to a drilling riser system and a steel catenary riser (SCR) system: Enhanced Monte Carlo Simulation (EMCS) assumes the failure probability follows the asymptotic behavior and uses high failure probabilities to predict low failure probabilities; Multi-Gaussian Maximum Entropy Method (MGMEM) assumes the probability density function (PDF) is a summation of Gaussian density functions and adopts maximum entropy methods to obtain the model parameters; Shifted Generalized Lognormal Distribution (SGLD) method proposes a distribution that specializes to the normal distribution for zero skewness and is able to assume any finite value of skewness for versatility; and Generalized Extreme-Value Distribution method (GEV) comprises three distribution families: the Gumbel-type, Frechet-type and Weibull-type distribution. The study compares the bias errors (the difference between the predicted values and the exact values) and variance errors (the variability of the predicted values) of these methods on the prediction of low failure probabilities with applications to two riser systems. This study could provide offshore engineers and researchers feasible options for marine riser system structural reliability analysis.
Effect of Sacrificial Anodes on Wave Loads on Jacket Structure
Authors: Nallayarasu Kasthuri , Rajamanickam Panneer Selvam
Published: October 2017

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

This paper focuses on the evaluation of wave loads on jackets due to the presence of anodes on jacket legs and braces, and comparing them to that of a jacket without anodes. The evaluation of wave loads is done by both numerical modelling and theoretical evaluation. The numerical model is based on frame analysis using SACS software which has the facility to simulate the wave load on space frame structures. Three different anode-to-jacket weight ratios (total weight of anode to total weight of jacket) are considered. The anodes are modelled as per design requirements and distributed throughout the structure. Recommended hydrodynamic coefficients from codal provisions are used. The overturning moments and base shear are evaluated for design regular waves and current. Results are presented in terms of comparison of base shear and overturning moment to ensure consistency, for three different cases. The recommendations for design engineers within the depth range and region studied can be drawn from this study.
Ship Routing Based on the Kuroshio Current
Authors: Chen Chen , Kashiwagi Masashi
Published: October 2017

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.
A Numerical Simulation of Rapid Depressurization in Pressure Vessels Incorporating Nucleate Boiling of Hydrocarbon Mixture
Authors: Park Ahmin , Ko Yoonae , Lim Youngsub
Published: October 2017

In offshore operations, overpressure of pressure vessels can arise in case of emergencies like fire or malfunction of valves. This situation can cause physical damages of the vessel and, operation break. Thus, managing overpressure is important in terms of safety of offshore facilities. To handle the overpressure problems, the rapid depressurization, so-called ‘Blowdown’, is used. During depressurization, temperature of internal fluids in a vessel get decreases by the expansion of the fluids. Predicting decrease of the temperature is critical to choose the material of a pressure vessel. Overdesign without the prediction leads to the rapidly decreasing profit margins. For these reasons, the analyzing dynamic behavior of thermodynamics properties like temperature is required for material selection and design verification during depressurization. In this study, a dynamic model for depressurization was developed to simulate thermodynamics behavior in a vessel during depressurization including low temperature phenomena. The model contains non-equilibrium zone between phases, heat transfer between walls and fluids in the vessels. The heat transfer coefficient between internal vapor and wall was calculated from a combined convection that includes the both natural and forced convection. This study includes the calculation of liquid/wall heat transfer coefficient. During depressurization, liquid in the vessel becomes boiling closed to surface of the wall because the temperature of the wall is higher than the boiling point of the liquid. This phenomenon can be described as ‘nucleate boiling’, causes decreasing convective heat transfer coefficient from inner wall to the liquid in the vessel. Using the proper correlations about this phenomenon, the calculated coefficient made this study get closer to reality. The results were compared to experimental and simulation data from literature and it shows this model can properly estimate the thermodynamic property change in a vessel.
Assessing Mechanical Stresses in Dynamic Power Cables for Floating Offshore Wind Farms
Authors: Leroy Jean-Marc , Poirette Yann , Brusselle Dupend Nadège , Caleyron Fabien
Published: October 2017

Floating offshore wind offers a promising potential in the development of renewable energies. One of the key component of offshore wind farms projects is the inter-array power cable. Additionally, the dynamic behavior of floating support under offshore environmental conditions requires dynamic power cable structures which have to withstand both fatigue and extreme loads over its lifetime.

Both global and local analyses are required to properly design a dynamic power cable. Indeed, the axial, bending and torsion loads due to environmental conditions (waves and current) and the floating support motion are calculated from a global model by using a dedicated software. Then, the global loading has to be transferred to local sub-models in order to calculate stresses acting on the different components of the power cable.

This paper describes a 3D finite elements (FE) model dedicated to a detailed prediction of stresses in an armoured power cable. The loads under extreme environmental conditions are first evaluated from a global analysis. The local model, developed in a commercial FE software, uses periodic boundary conditions to reduce the computational costs and accurately model cables with constant curvatures in space. The model includes contact pressure and friction effects between all cable components, as well as potential lateral contacts between adjacent armour wires, and the radial stiffness of the cable core.

The applicative example, focusing on the amour layers, illustrates the potential of the model.
Water Tank and Field Tests on the Performance of a Submergible Fish Cage for Farming Silver Salmon
Authors: Kitazawa Daisuke , Mizukami Yoichi , Kanehira Makoto , Takeuchi Youto , Ito Sho
Published: October 2017

Silver salmon is usually cultured around the eastern coast of Tohoku Region in Japan. The smolt of silver salmon begins to be cultured in a cage from November. Currently, the aquaculture of silver salmon is terminated until the end of July due to high water temperature since silver salmon will die in a few days if the diurnal minimum water temperature becomes 21 or 22°C. Live salmon cannot be obtained around August because wild salmon is captured from September or October. In the present study, a submergible cage using flexible tubes is proposed to farm silver salmon in deeper and cooler waters in August. The cage was submerged and floated up by ejecting air from and injecting air into the flexible tubes, respectively. The flexible tubes were inserted into the polyethylene pipes. First, water tank test using the 1/3.64 scaled model was carried out in the Ocean Engineering Basin, Institute of Industrial Science, The University of Tokyo. The cage was submerged or floated up, changing the methods of air ejection or injection. The cage was submerged and floated up faster in case of two vents to increase the rate of air flow than in case of only one vent on flexible tubes. The submersion was also faster if the length of an injection tube between the vents and air compressor was shortened to reduce the pressure loss. However, the maximum inclination angle of the cage was determined by the diameter of the cage and the submerged depth, not depending on the methods of air injection or ejection. Similar results were observed in the field test. Consequently, silver salmon could be farmed in deeper and cooler waters until the middle of August. The inclination of the cage was not the problem for silver salmon, while it may have unfavorable effects on the other species through the deformation of netting. Hence, the methods to reduce the inclination of the cage in submerging or floating up operation should be considered and validated in field test in the future.
Applications of the Static Condensation Technique to Nonlinear Structural Analysis of Floating Offshore Structures
Authors: Oh Min-Han , Boo Seung-Hwan , Lee Phill-Seung , Kim Jong-Min , Moon Joong-Soo , Sim Woo-Seung
Published: October 2017

A cost effective finite element (FE) procedure is proposed for analysis of load-carrying structures with nonlinear contact and frictional behaviors between large floating offshore structures. The key of the procedure is to use the static condensation technique developed from the Guyan model reduction method. The time for computing contact and friction forces on contact interface areas can be dramatically reduced compared to nonlinear analysis with a full FE model. Two representative applications to offshore projects are presented. One is a problem with nonlinear contact of independent tank support in FLNG hull structures and the second is a sea-fastening system used during offshore dry transportation. The reliability and computing efficiency of the proposed analysis procedure are investigated. It is conclusively confirmed that the proposed procedure is practical for application to actual design of offshore projects.
Impact of New Slamming Wave Design Method on the Structural Dynamics of a Classic, Modern and Future Offshore Wind Turbine
Authors: Peeringa Johan M. , Hermans Koen W.
Published: October 2017

In the WiFi-JIP project, the impact of steep (and breaking) waves on a monopile support structure was studied. Observations during model tests showed that large tower top accelerations occur due to a slamming wave. Using experiments and simulations results, a new formulation of the design load for a slamming wave was developed. Instead of the embedded stream function, as applied in industry, the wave train is generated with the nonlinear potential flow code Oceanwave3D. On the wave train a set of conditions is applied to find the individual waves, that are closest to the prescribed breaking wave and most likely cause a slamming impact.

To study the effect of the new slamming load formulation on different sized offshore wind turbines, aero-hydroelastic simulations were performed on a classic 3MW wind turbine, a modern 4MW wind turbine and a future 10MW wind turbine. The simulations are performed with and without a slamming wave load. The slamming has a clear effect on the tower top acceleration. Accelerations due to the wave impact are highest for the 3MW model at the tower top and at 50m height. A serious tower top acceleration of almost 7m/s2 due to wave slamming is found for the 3MW turbine. This is an increase of 474% compared with the case of Morison wave loads only.
Numerical Study on Expected Electrical Power of Linear Wave Energy Converter in Arrangement Condition
Authors: Li Qiao , Murai Motohiko , Kuwada Syu
Published: October 2017

A linear electrical generator is a kind of device which can be used to wave energy converter, for directly converting mechanical energy of a floating structure into electrical energy. A wave farm consists of multiple wave energy converters which equipped in a sea area. In the present paper, a numerical model is proposed considering the interference effect in the multiple floating structures, and the controlling force of each linear electrical generator. Especially, the copper losses in the electrical generator is taken into account, when the electrical power is computed. At first, the controlling force coefficients are discussed to find their physical effects on heaving motion. In a case study, the heaving motions and electrical powers of the three floating structures are estimated in the straight arrangement and triangle arrangement. And the average electrical power is analyzed in different distances of the floating structures.
Classification of Aquaculture Locations in Norway With Respect to Wind Wave Exposure
Authors: Lader Pål , Kristiansen David , Alver Morten , Bjelland Hans V. , Myrhaug Dag
Published: October 2017

In Norway there are a total of 1070 registered sites for salmon farming all along the coast. Trends in the aquaculture industry in the recent decade are that salmon farming sites tend to gradually relocate to more wave and current exposed locations. This trend is mainly motivated by the good water quality found in more exposed areas, as well as a lack of available sheltered locations. On the other hand, the increased exposure puts higher loads on the structures and this needs to be addressed by the development of more robust technology. The first step in order to address an increased exposure is to quantify the level of exposure of waves and current, and in this paper a method to easily estimate the level of wind wave exposure on a large number of sites are presented, and subsequently used to analyse all Norwegian sites.

The method can be called fetch analysis, and use long term wind data connected with the fetch length in order to estimate wind wave conditions. The method is divided into four steps: 1) Fetch analysis, 2) Wind data, 3) Estimating wave parameters Hs and Tp and 4) Wave statistics. Significant wave height Hs with return period 1 year and 50 years are estimated for each site. Hs 50 year is often used for design, and the analysis shows that for 38% of the sites Hs 50 year exceeds 1 meter, for 17% of the sites Hs 50 year exceeds 1.5 meter, while 1.4% of the sites have Hs 50 year larger than 2.5 meter. The most exposed site has a Hs 50 year of 2.9 meter. Thus there are large differences in Hs 50 year in the various coastal regions of Norway.
CFD Verification and Validation Study for a Captive Bullet Entry in Calm Water
Authors: Maximiano António , Vaz Guilherme , Scharnke Jule
Published: October 2017

As a step towards complex impact loads cases, e.g. lifeboat drop tests or ship/platform slamming in waves, a verification and validation (V&V) study is carried out with an open-usage community based CFD code ReFRESCO for a simple impact load test case: a captive axisymmetric generic lifeboat shape (bullet) that penetrates the water surface at a constant velocity and angle of attack. The quantities of interest are the body fixed longitudinal force FX, vertical force FZ, and pitch moment MYY.

The influence of the iterative convergence level, domain size and free surface modelling are investigated. Seven different grids and four time steps were used to assess the grid and time step sensitivity, in a total of 28 calculations. For the tested grids and time steps it was found that the results are more sensitive to the grid resolution than to the time step. The pressure distribution on the hull is correlated with the trends observed in the loads, and the relation between between relative and static pressure is found to be important for the calculated loads. An experimental test campaign was previously carried out by MARIN, and its results are used to validate the simulations performed. A very good match between experiments and simulations is found.

A V&V study is performed for the quantities of interest at nine different time instants covering the impact phase. The numerical uncertainties are obtained from a solution verification procedure [1]. The experimental uncertainties are estimated, and a validation exercise carried out according to the ASME standards [2]. The outcome of the validation exercise is an estimated 95 % confidence interval for the modelling error, δM. For FX the modelling error is below 15 N, for 8 out of 9 time instants. For FZ the modelling error is below 14 N, except at the time instants where, due to vibrations in the experimental setup, a larger value (up to 23 N) is found. For MYY the modelling error is under 5N m. These results provide confidence in ReFRESCO for the simulation of free surface impact flows.
Modulational Instability in JONSWAP Sea States Using the Alber Equation
Authors: Gramstad Odin
Published: October 2017

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].
Effect of Uni- and Bi-Directional Coupling of Ocean-Met Interaction on Significant Wave Height and Local Wind
Authors: Rasheed Adil , Süld Jakob Kristoffer , Tabib Mandar
Published: October 2017

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.
Wind Farm Modeling in a Realistic Environment Using a Multiscale Approach
Authors: Rasheed Adil , Tabib Mandar , Kristiansen Jørn
Published: October 2017

We present a multiscale approach to model a windfarm under real meteorological conditions. The multiscale model consists of a mesoscale atmospheric code coupled to a stochastic ocean wave model, a microscale model and a super-microscale model. The mesoscale model (with 2.5km × 2.5km horizontal resolution) forces the microscale model (with finer 100m × 100m horizontal resolution). The microscale model is capable of resolving surface variations both on wavy and complex terrain surfaces. Finally, the computed wind, temperature and turbulent kinetic energy from microscale model are used to provide boundary conditions to a super-microscale model, which has features to resolve turbine wakes using actuator line model. The three classes of models are validated using a very diverse array of observational data obtained from satellites, radiosondes and a wind tunnel. Towards the end, performance of an operational onshore wind farm under realistic meteorological conditions is evaluated.
Ultimate Strength Assessment of Semi-Submersible Platform Under Different Load Conditions
Authors: Yu Yangzhe , Feng Guoqing , Ren Huilong
Published: October 2017

The nonlinear finite-element method has been widely used in evaluating the ultimate strength of stiffened plates and part of hull girders, considering the effect of boundary conditions, geometrical initial imperfection and welding-induced residual stress in recent years. However, available research on the ultimate strength of large-sized structures, especially of semi-submersible platform is limited. New large-sized semi-submersible platform has been designed with lateral brace structure and square cross-section columns. The investigation of ultimate strength of the whole structure is of paramount importance in assessing the safety and design of such large structure. Therefore, in this paper, a three-dimensional nonlinear finite element model was developed to investigate the ultimate strength of a new generation of semi-submersible platform under different load conditions and its behavior after collapse using explicit dynamic solvers. Results showed that the time dependent dynamic explicit method was reliable and feasible for the calculation of ultimate strength of such complicated structure. For the target platform, the bracings and upper hull structure were the main bearing component and were critical for the ultimate strength of the whole structure. High stress occurred in connection areas and special attention shall be paid for.
Model Tests to Assess Wave and Current Loads on Ocean Cleanup’s Conceptual Plastic Capturing Barrier
Authors: Sterenborg Joost , Paalvast Mark , van Schoten Willem , Boot Lourens , Tjallema Arjen
Published: October 2017

Lot of plastics enters the ocean every day with negative effects on the environment, economy and health. A large portion of the floating plastics accumulate in so-called gyres, where currents converge. One of the aims of the Ocean Cleanup is to develop technologies to extract plastic pollution from the oceans. The idea is to install a flexible barrier in the ocean that is supposed to concentrate the plastic at the Great Pacific Garbage Patch. The design of the barrier is still in the conceptual phase and the model tests described in this report are conducted to assist in the development of the barrier.

The model tests were carried out in MARIN’s Offshore Basin and served two main goals: 1) provide loads and displacements for numerical model calibrations and 2) examine the 3D fluid-structure interactions and the barrier performance for three different design concepts. A 360m prototype length barrier was considered that consists of a floater with a diameter of 1.5m and a screen with a height of 2m. To model larger lengths of the barrier, various pretensions were applied at the ends of the barrier. For the secondary mooring concept the barrier was moored each 60m via the bottom of the screen to a submerged tension line. For the low mooring and high mooring concepts, the model was only moored at both ends of the bottom of the screen or bottom of the floater.

In general mooring loads were found to be the largest for the low mooring configuration. For this same configuration the mooring loads increment with increasing current velocity was the largest. Mooring load fluctuations seemed to be not strongly influenced by the amount of applied pretension.

Vertical screen orientations, which are expected to be beneficial for the plastic capturing efficiency, were mostly observed for the secondary mooring and high mooring configurations. For the low mooring concept the offsets along the flow direction were the largest and the screen was more tilted. Additional ballast for the high mooring concept promoted a more vertical orientation of the screen with as downside increasing mooring loads.

Overtopping or bridging (air gap exists below the barrier) negatively impact the plastic capturing efficiency and are important to consider. Both events are most likely to happen for shorter wave conditions and higher current velocities. The number of occurrences of overtopping and bridging was the lowest for the secondary mooring and the high mooring setup with a low pretension. For increasing pretension the number of overtopping and bridging events increased.
Key Contributors to Lifetime Accumulated Fatigue Damage in an Offshore Wind Turbine Support Structure
Authors: Smilden Emil , Bachynski Erin E. , Sørensen Asgeir J.
Published: October 2017

A simulation study is performed to identify the key contributors to lifetime accumulated fatigue damage in the support-structure of a 10 MW offshore wind turbine placed on a monopile foundation in 30 m water depth. The relative contributions to fatigue damage from wind loads, wave loads, and wind/wave misalignment are investigated through time-domain analysis combined with long-term variations in environmental conditions. Results show that wave loads are the dominating cause of fatigue damage in the support structure, and that environmental condtions associated with misalignment angle > 45° are insignificant with regard to the lifetime accumulated fatigue damage. Further, the results are used to investigate the potential of event-based use of control strategies developed to reduce fatigue loads through active load mitigation. Investigations show that a large reduction in lifetime accumulated fatigue damage is possible, enabling load mitigation only in certain situations, thus limiting collateral effects such as increased power fluctuations, and wear and tear of pitch actuators and drive-train components.
Loading Sequence Effects on Fatigue Damage Accumulation of Offshore Structures: A Deterministic Approach
Authors: Pavlou Dimitrios G.
Published: October 2017

Offshore structures are subjected to irregular loading spectra due to their exposure to waves and wind. The environmental loads cause variable amplitude stress histories on critical spots of the structures. The existing engineering methodology (adopted by most of the national standards) to estimate the accumulated fatigue damage is based on Miner’s rule for crack initiation. Paris rule and its modifications are used for crack propagation prediction. However, Miner’s rule is a linear model and does not take into account the sequence effect of loading blocks with different stress amplitude. On the other hand, the widely used Paris rule does not take into account the load interaction effects (e.g. overload-induced crack growth retardations). The prediction of the crack growth rate and the crack growth direction of mixed mode cracks is an important issue as well. Aim of the present paper is the analysis of the weaknesses of the engineering tools for fatigue analysis, and the demonstration of the advantages of non-linear damage functions and crack propagation models. A review of models for fatigue crack initiation and growth (for mode I or mixed mode loading) developed by the author is presented. Representative results are discussed and commented.
Review of Ice Load Standards and Comparison With Measurements
Authors: Kellner Leon , Herrnring Hauke , Ring Michael
Published: October 2017

Sea ice can interact with offshore structures in regions with at least seasonal ice coverage. Therefore the prediction of ice loads on offshore structures is required by many standards or classification rules and guidelines. In order to do this, empirical formulas are often prescribed. These are based on assumptions in combination with model or full scale tests. Yet there are very few publications where the results of the formulas are actually compared to measurements.

A case study is made for ice loads on the Norströmsgrund lighthouse. First of all current empirical formulas given by standards bodies or classification societies are reviewed with focus on applicability. Secondly, the ice loads predicted by the empirical formulas are compared to measurements. It was found that for the given case most methods significantly overestimate the load. The applicability of some methods is disputable.
Calibration of Long-Term Time-Domain Load Generation for Fatigue Life Assessment of Offshore Wind Turbine
Authors: Nelson Bryan , Quéméner Yann , Lin Tsung-Yueh , Huang Hsin-Haou , Chien Chi-Yu
Published: October 2017

This study evaluated, by time-domain simulations, the fatigue life of the jacket support structure of a 3.6 MW wind turbine operating in Fuhai Offshore Wind Farm. The long-term statistical environment was based on a preliminary site survey that served as the basis for a convergence study for an accurate fatigue life evaluation. The wave loads were determined by the Morison equation, executed via the in-house HydroCRest code, and the wind loads on the wind turbine rotor were calculated by an unsteady BEM method. A Finite Element model of the wind turbine was built using Beam elements. However, to reduce the time of computation, the hot spot stress evaluation combined FE-derived Closed-Form expressions of the nominal stresses at the tubular joints and stress concentration factors. Finally, the fatigue damage was assessed using the Rainflow Counting scheme and appropriate SN curves. Based on a preliminary sensitivity study of the fatigue damage prediction, an optimal load setting of 60-min short-term environmental conditions with one-second time steps was selected. After analysis, a sufficient fatigue strength was identified, but further calculations involving more extensive long-term data measurements are required in order to confirm these results. Finally, this study highlighted the sensitivity of the fatigue life to the degree of fluctuation (standard deviation) of the wind loads, as opposed to the mean wind loads, as well as the importance of appropriately orienting the jacket foundations according to prevailing wind and wave conditions.
Spatial Corrosion Wastage Modelling of Steel Plates Subjected to Marine Environments
Authors: Garbatov Yordan , Guedes Soares C.
Published: October 2017

The objective of the present study is to analyse and identify the most suitable corrosion degradation model, fitted with real corrosion depth measurement data sets and to reproduce the corroded steel plate surface as a function of time and spatial distribution using advanced statistical methods. An approach for properly identifying the best fitted model to real corrosion depth measurement data sets is employed. A sequence dependent data analysis is performed based on the fast Fourier transform, which is used as an input for a random field modelling of the corroded steel plate surfaces.
Nonlinear Droop Load Sharing to Minimize Gas Emissions and Fuel Consumption
Authors: Rejani Miyazaki Michel , Sørensen Asgeir Johan
Published: October 2017

In this paper, load sharing curves are generated for marine systems with multiple gensets, where the goal is to reduce both gas emissions and fuel consumption. Initially, the average emissions and fuel consumption for each engine are calculated based on the specific emission and Specific Fuel Oil Consumption (SFOC) curves of each generator set (genset). An optimal nonlinear load sharing subject to gas emission and fuel consumption minimization is found for each engine. One result is that identical gensets should not have the same droop curve on the optimum condition, since it would lead to equal load sharing among them and a suboptimal configuration. Cases with two identical engines, two different engines, and multiple different engines were studied. The results show that by modifying the usually linear droop curve of engines, it is possible to reduce the fuel consumption and the gas emission, and it is also possible to fine tune the solution such that the fuel consumption or gas emissions are reduced.