bluuebunny/crossref_metadata_2025_split huggingface dataset:
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.
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.
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.
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.
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].
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.
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.
Collision Study Between a Ship Section Moving Sideways and an Oil Platform
Authors: Pericard Yaël , Halse Karl H. 
Published: October 2017

Over the last decades, the need for overseas transportation of both people and goods has gradually increased. At the same time, several offshore activities as oil and gas exploration, wind power parks as well as fish farms have bloomed. As a consequence of the increased traffic and activities, the risk of accidents became noticeable. This paper presents a study of the collision between a Platform Supply Vessel (PSV) and an oil platform. Even if such accidents are seldom, their consequences are disastrous, for the ship owner as well as the oil-platform company. Beyond the economic consequences, some lives have been lost due to the ship - structure collisions. These problems are inducing to investigate the possibility of developing protective systems in order to save lives of the concerned persons.
Structural impact analyses can be carried out to give a scientific foundation for an evaluation of the consequences of a possible collision. This study is based on Non-Linear Finite Element Analysis (NLFEA) of the interaction between the PSV and the platform. Due to heavy calculations, only the ship section located close to the crash zone has been modelled and additional masses have been included on each side of this section in order to get the correct ship inertia. These additional masses correspond to the mass of the remaining part of the ship structure. Moreover, the sideways ship motion is modelled with a prescribed initial velocity, and does not contain hydrodynamic response calculations. The NLFEA software package HyperWorks is used to perform the numerical simulations.
As a way to reduce the consequences of a ship platform collision, a shock absorber called “crash box” is proposed mounted on the platform column to absorb the kinetic energy and thus decrease the effect in both ship and platform. 4 case studies are performed in order to compare the results with and without the crash box. Presented results are based on the deflection, resultant forces due to the contacts and absorption of the kinetic energy. The results demonstrate that penetration of the oil platform into the ship structure can be decreased by 96% if the crash box is used. Similar systems such as tires are currently used but this crash box should be investigated with an inner structure using “honeycomb” shape in order to maximise the absorption.