bluuebunny/crossref_metadata_2025 huggingface dataset:
An Analytical Model Study of a Flapping Hydrofoil for Wave Propulsion
Authors: Lopes D. B. S. , Falcão de Campos J. A. C. , Sarmento A. J. N. A.
Published: September 2018

Flapping foils have been increasingly studied and applied as thrusters. Their use to propel Unmanned Surface Vehicles (USV) with ocean waves is a solution to overcome endurance and range limitations. To support experimental evidence and concept development, many studies have been carried out, employing methods based on linear, non-linear theories and numerical approaches. This work presents an analytical model intended to provide a better understanding on the physics affecting foil propulsion when oscillating in incident waves that can be used as an engineering tool to improve this concept. The formulation of the problem is introduced and the theoretical model is described. Several simulations are carried out, for different combinations of parameters and results are compared with data available in the literature. Results suggest that the simple model captures with enough precision oscillating foil lift and thrust forces for the conditions and ranges of interest. However further development is required, namely to include free surface effects and to study flow separation. A practical example of a vehicle propelled by a foil oscillating in waves is given.
Semi-Submersible Floater’s VIM Simulation Method for Mooring Line Safety Assessment
Authors: Fujiwara Toshifumi
Published: September 2018

The author proposed the Vortex-induced Motion (VIM) simulation method of a semi-submersible type offshore floating structure using the wake oscillator model based on the potential theory and model test data. This method is easy to use for the time-domain simulation of the VIM amplitude, that is in-line, transverse and yaw motions, of the semi-submersible floater in case of being demented mooring safety assessment of that. The simulation method presented in this paper was modified the single circular floater simulation method with the wake oscillator model for a semi-submersible floater. Some empirical parameters, obtained from the systematic model tests used many semi-submersible floaters, are only decided from external form of the semi-submersible floaters, that is the column / lower hull ratio etc. This simulation method is able to indicate general VIM trend and to be used for the assessment of mooring lines safety in the design stage. Using the VIM amplitude simulation, fatigue damage of mooring lines on one sample semi-submersible floater was investigated as an example.
Long-Term Area Statistics for Maximum Crest Height Under a Fixed Platform Deck
Authors: Hagen Øistein , Birknes-Berg Jørn , Grue Ida Håøy , Lian Gunnar , Bruserud Kjersti , Vestbøstad Tone
Published: September 2018

As offshore reservoirs are depleted, the seabed may subside. Furthermore, the extreme crests estimates are now commonly higher than obtained previously due to improved understanding of statistics of non-linear irregular waves. Consequently, bottom fixed installations which have previously had sufficient clearance between the deck and the sea surface may be in a situation where wave impact with the deck must be considered at relevant probability levels.

In the present paper, we investigate the long-term area statistics for maximum crest height under a fixed platform deck for 2nd order short crested and long crested sea based on numerical simulations as a function of platform deck dimension for jackets. The results are for one location in the northern North Sea, but some key results are also reported and verified for a more benign southern North Sea location. Time domain simulations for long crested and short crested waves over a spatial domain with dimension of a platform deck are performed, and relevant statistics for airgap assessment determined.

Second order waves are simulated for the different cells in the (Hs, Tp) scatter diagram for Torsethaugen two-peak wave spectrum for long-crested and short-crested sea. A total of 1000 3-hour sea states are generated per cell, and time series generated for 160 spatial points under a platform deck.

Short-term and long-term statistics are established for the maximum crest height as function of platform dimension; inline and transverse to the wave direction, and over the area. Results are given for the linear sea and for the second order time series.

The annual q-probability estimates for the maximum crest height over area as a function of platform dimension is determined for a location at the Norwegian Continental Shelf by weighting the short-term statistics for the individual cells in the scatter diagram with the long-term probability of occurrence of the sea state. To reduce the number of numerical second order simulations, the effect of excluding cells that have a negligible effect on the long term extreme crest estimate is discussed. The percentiles in the distribution of maximum crest (over area) in design sea states that corresponds to the extreme values obtained from the long-term analysis are determined for long crested and short crested sea. The increase in the extreme crest over an area compared to the point in space estimate is estimated for both linear and second order surface elevation.
Investigation of Fatigue Damage for Stiffened Plates in Splash Zone for a Semi-Submersible
Authors: Zhu Suji , Rentsch Hans C. , Lefranc Marc , Haaheim Helene
Published: September 2018

Fatigue damage at side structures are widely reported for ships because the external wave pressure reaches maximum at still-water level. At the same time, the ship side structures also withstand noticeable hull girder loads (vertical and horizontal bending moments). Correct assessment of the fatigue damage due to global and local load effects is of vital importance for side structures of marine vessels. Little research, however, has been carried out for the side structures of semi-submersibles. Like ships, the semi-submersible side structures experience both global and local load effects. Especially for external pressure, the effect of intermittent wet and dry surfaces on the side shell near or above the waterline needs to be considered. In this paper, a global analysis model of the Gjøa semi-submersible, shown in Figure 1, is built and the local response for the side structure is investigated. The contribution of fatigue damage due to global and local load effects are compared and presented.
Preventing the Added-Mass Instability in Fluid-Solid Interaction for Offshore Applications
Authors: Veldman Arthur E. P. , Seubers Henk , van der Plas Peter , Zahraei Matin Hosseini , Wellens Peter R. , Huijsmans Rene H. M.
Published: September 2018

Simulating the hydrodynamics of deformable, floating structures using a partitioned strategy poses a major challenge when the ratio of the added mass to the structural mass is considerate. Existing computational procedures for fluid-structure interaction become less efficient or even unstable. In these situations, it is advisable to modify the coupling to allow the fluid to respond better to the solid motions. A simultaneous solution of the equations governing fluid and solid-body would be a stable choice but is often not feasible. Usually the numerical problems are taken care of with subiterations between fluid and structure, but their convergence can be slow. In this paper we present a more powerful, quasi-simultaneous approach, which tries to mimic a fully simultaneous coupling in an affordable way. It makes use of a simple approximation of the body dynamics, based on the (6 DOF) solid-body modes and the main elastic modes of the structure. The method will be demonstrated in offshore practice, with a falling life boat, a floating CALM buoy, an elastic membrane and a rubber gate.
Scale Effect in Freshwater Ice Flexural Strength
Authors: Aly Mohamed , Taylor Rocky , Bailey Dudley Eleanor , Turnbull Ian
Published: September 2018

Ice flexural strength is an important parameter in the assessment of ice loads on the hulls of ice-class ships, sloped offshore structures or sloped bridge piers. While scale effects are well known for compressive ice strength, there has been debate as to whether or not scale effects in ice flexural strength exist. To investigate scale effects during flexural failure of freshwater ice, a comprehensive up-to-date database of beam flexural strength measurements has been compiled. The data show a considerable decrease in flexural strength as the specimen size increases, when examined over a large range of scales. An empirical model of freshwater ice flexural strength as a function of beam volume has been developed using regression analysis.
Optimization of the Vibration Response of a Longitudinal-Transverse Stiffened Conical Shell Based on an Ensemble of Surrogates
Authors: Qian Jiachang , Yu Enen , Zhang Jinlan , Zhan Dawei , Cheng Yuansheng
Published: September 2018

The acceleration responses at certain points of the longitudinal-transverse stiffened conical shells in special frequency region are major matters of concern. Because the finite element models of the longitudinal-transverse stiffened conical shells have to be employed to calculate the vibration response of the structure at all frequencies under consideration, it requires a large amount of computational cost when the optimization is conducted. In order to optimize the vibration response of the longitudinal-transverse stiffened conical shell, the surrogate modeling method is used in this study to approximate the frequency-acceleration response function which makes the vibration response optimization affordable. Since different surrogate models often perform differently in different regions of the design space, an ensemble of surrogate models is utilized to maximize the overall accuracy over the whole design space. The ensemble of surrogates is a weighted combination of Kriging model, radial basis function (RBF) and support vector regression (SVR). The weights of the ensemble of surrogates vary in different regions and are determined by the estimated errors of the surrogate models at the study point. The smaller the estimated error is, the higher the weight is. Then the prediction of ensemble of surrogates is compared to the individual surrogate’s, and the results show that the accuracies of the ensemble of surrogates in peak regions are significant higher than its components. Based on the ensemble of surrogates, a vibration optimization of a longitudinal-transverse stiffened conical shell is conducted using genetic algorithm (GA). The design variables of the optimization are the thickness of the longitudinal-transverse stiffened conical shell and the height of stiffened structure. The objective is to minimize the highest acceleration of the shell and the calculations of the peak accelerations are approximated by the built ensemble of the surrogates. The constraints include the weight of the stiffened conical shell and structure size combination. The optimization results show that the proposed approach is efficient in optimization of the vibration response of longitudinal-transverse stiffened conical shells.
Numerical Simulation of Subsea Cluster Manifold Installation by the Sheave Method
Authors: Zhao Yu , Wang Yingying , Li Liwei , Yang Chao , Du Yang , Chen Haoran , Duan Menglan
Published: September 2018

The sheave installation method (SIM) is an effective and non-conventional method to solve the installation of subsea equipment in deep water (>1000m), which has been developed to deploy the 175t Roncador Manifold I into 1,885 meters water depth in 2002. With the weight increment of subsea cluster manifold, how to solve its installation with the high reliability in the deep sea is still a great challenge. In this paper, the installation of the 300t subsea cluster manifold using the SIM is studied in the two-dimensional coordinate system. The mathematical model is established and the lumped mass method is used to calculate the hydrodynamic forces of the wireropes. Taking into account the complex environment loads, the numerical simulation of the lowering process is carried out by OrcaFlex. The displacement and vibration of the subsea cluster manifold in the z-axis direction and the effective tension at the top of the wireropes can be gotten, which can provide guidance for the installation of the cluster manifold in the South China Sea.
Implosion Tests of Aluminium Alloy Tubes Under External Hydrostatic Pressure
Authors: Muttaqie Teguh , Sohn Jung-Min , Cho Sang-Rai , Park Sang-Hyun , Choi Gulgi , Han Soonhung , Lee Phill-Seung , Cho Yoon Sik
Published: September 2018

This paper reports the implosion tests of aluminium alloy 6061-T6 tube models under external hydrostatic pressure. The investigations took an emphasis on how to replicate the deep-ocean pressure environment of the implosion phenomenon on a laboratory scale. The parameters which affected the implosion pressure pulse were also observed. Two kinds of implosion tests were conducted, namely, dynamic implosion test and quasi-static implosion test. The pressure drops in the post ultimate regime was negligible in the dynamic implosion test which performed using compressed nitrogen gas. The pressure and strain histories of the both implosion tests were compared and analysed. In addition, non-linear FE analyses for quantitative validation and comparison of the imploded tubes were conducted. The numerical quantities including the initial ovality, thickness unevenness, and air backed fluid cavity parameters are investigated to propose more realistic imploded tubes induced by the external hydrostatic loading.
Residual Longitudinal Strength of Damaged Box Girder Structures
Authors: Cho Sang-Rai , Park Sang-Hyun , Cho May Thu , Shin Hyun Kyoung
Published: September 2018

A series of collapse tests were performed on box girder models to evaluate the effect of collision damage on the residual longitudinal strength of the hull girder. The experiments were carried out under pure bending moment using, so-called 4-point bending test method, the load, deflection and strain were measured. Ten experimental models were fabricated, two of them were intact, four were plastically damaged and four were fracture damaged. To generate the damages, lateral collision tests were firstly performed on the box girder models. To consider the effects of ship speed and relative draft on extents of damage due to collision, the drop height and impact location of striker were changed for each model. Numerical analyses were also performed by applying nonlinear finite element analysis. In the numerical analysis, the measured initial imperfection data and the welding residual stresses were considered.
Dynamic Characteristics of Deep-Water Risers Carrying Multiphase Flows
Authors: Ma Bowen , Srinil Narakorn
Published: September 2018

Deep-water flexible risers conveying hydrocarbon oil and gas flows may be subject to internal dynamic fluctuations associated with the spatial variations of phase densities, velocities and pressure drops. Many studies have focused on single-phase flows in pipes whereas understanding of multiphase flow effects is lacking. This study aims to investigate the planar free-vibration characteristics of a long flexible catenary riser carrying the steady-state, multiphase slug oil-gas flows in order to understand how the inclination-dependent internal slug flows affect riser natural frequencies and modal shapes. The influence of slug characteristics such as phase velocities on the riser vibration is also studied. The catenary riser planar motions are mathematically described by a two-dimensional continuum model capturing coupled horizontal and vertical responses. Based on the selected two-phase flow rates at the wellhead, riser geometric configurations and specified slug unit lengths, a steady-state slug flow model is considered by taking into account several empirical closure correlations and riser mechanical properties, solving for the multiphase flow aspects including pressure, velocities, liquid holdup and gas fraction.

By assigning an undamped free-vibration shape of an empty catenary riser as initial displacement conditions, the space-time numerical simulations are performed using a finite difference approach. Comparisons of oscillation frequencies, time histories, phase planes, time-space varying responses and dynamic stresses of catenary risers with and without slug flows are presented, identifying the dynamic modifications arising from the internal slug-induced mass momentum change and pressure loss. To understand the influence of slug flow properties, parametric studies are carried out with different gas velocities. Numerical results highlight the reduced riser tensions, decreased oscillation frequencies, multiple oscillation modes, amplified amplitudes and stresses. These key observations will be useful for the forced vibration analysis of catenary risers subject to combined internal (multiphase) and external (vortex-shedding) flow excitations.
Study on the Lateral Bearing Capacity of Single-Helix Pile for Offshore Wind Power
Authors: Ding Hongyan , Wang Le , Zhang Puyang , Le Conghuan
Published: September 2018

To support a lighthouse, a trestle and other structures, screw pile has been used as the foundation for more than a hundred years. Because of its efficient installation, high bearing capacity and other advantages, increasing attention has been paid to use it as the offshore wind power foundation. In this research, laboratory test was conducted to study the influence of screw pitch and the diameter of helix on the lateral bearing capacity of single-helix pile, considering the effect of the installation. The results of the single-helix pile were compared with that of steel pipe pile and other methods. According to the test, helix diameter has negligible effect on the lateral bearing capacity of helical pile, but the pitch of helical influence that significantly because of more disturbance of soil.
Anchor Sharing in Sands: Centrifuge Modelling and Soil Element Testing to Characterise Multi-Directional Loadings
Authors: Herduin Manuel , Gaudin Christophe , Johanning Lars
Published: September 2018

Offshore foundations are typically designed to resist mooring loads coming from a single direction. This paper provides some insights into the geotechnical challenges associated with anchor sharing (i.e. where mooring loads are coming from multiple directions to one anchor) for Offshore Renewable Energy (ORE) applications. Firstly, the multidirectional mooring load conditions on shared anchors are characterised for cases with one, two and three mooring lines connections. Secondly, multi-directional loadings applied on a caisson foundation in a geotechnical centrifuge facility showed large capacity reduction and large displacement of the foundation at large loading angle. Thirdly, multi-directional simple shear tests have showed that volumetric contraction increases as the degree of shear stress reversal increases. Lastly, the results from the soil element tests are brought into discussion with the physical modelling results to identify the foundation’s response to multi-directional loading.
Progress in Coupling Potential Wave Models and Two-Phase Solvers With the SWENSE Methodology
Authors: Li Zhaobin , Bouscasse Benjamin , Gentaz Lionel , Ducrozet Guillaume , Ferrant Pierre
Published: September 2018

This paper presents the recent developments of the Spectral Wave Explicit Navier-Stokes Equations (SWENSE) method to extend its range of application to two-phase VOF solvers. The SWENSE method solves the wave-structure interaction problem by coupling potential theory and the Navier-Stokes (NS) equations. It evaluates the incident wave solution by wave models based on potential theory in the entire computational domain, leaving only the perturbation caused by the structure and the influence of the viscosity to be solved with CFD. The method was proven in previous studies to be accurate and efficient for wave-structure interaction problems, but it was derived for single-phase NS solvers only. The present study extends the SWENSE method by proposing a novel formulation which is convenient to implement in two-phase NS solvers. A customized SWENSE solver is developed with the open source CFD package Open-FOAM. An improvement in accuracy and stability is observed in wave simulations compared with conventional two-phase VOF solvers. The horizontal force on a vertical cylinder in regular waves is also calculated. First results show a good agreement with the experiment on the first harmonic component.
Numerical Simulation of Hydrodynamic Performance of Dolphin Fluke Motion
Authors: Li Rijie , Chen Jiajun , Huang Yushen , Liu Liwei , Wang Xianzhou
Published: September 2018

The dolphins’ cruising, generally, with an extremely high thrust efficiency and low drag, which attracted many researchers’ wide attention. It is hoped that we can improve the hydrodynamic performance of underwater vehicle by studying the thrust characteristics of dolphin’s kick and the relationship between the formation of vortex and the thrust generation. However, previous work is mostly focused on investigation of hydrodynamic performance of dolphin fluke motion with a rigid tail which means that the locomotion of caudal fin is defined only by the oscillating motion, without the chordwise deformation. In this paper, the dolphin’s fluke motion is realized by a flexible caudal fin which is defined by a combination of oscillating motion and chordwise deformation.

The simulation of the dolphin fluke motion is achieved by STRA-CCM, and dynamic moving mesh is implemented for different stroke functions. This paper primarily analyzed the thrust characteristics and the formation of vortex of dolphin fluke motion, then compared with the available data from previous work with rigid tail. It can be found that the structure of the vortex generated by the dolphin fluke motion with flexible caudal fin is different from a rigid one. Finally, by analyzing the instantaneous flow condition behind the dolphin caudal fin, it can get the reason why the thrust generated by the flexible caudal fin is larger than rigid one.
Code Comparison of a NREL-FAST Model of the Levenmouth Wind Turbine With the GH Bladed Commissioning Results
Authors: Serret Jordi , Rodriguez Carlos , Tezdogan Tahsin , Stratford Tim , Thies Philipp
Published: September 2018

This paper describes the process adopted to set up a FAST model to produce relevant design load cases (DLCs) for the Levenmouth (Samsung Heavy Industries - S7.0-171) demonstration foreshore wind turbine owned by the Offshore Renewable Energy Catapult (ORE Catapult). The paper does not take into account hydrodynamic forces.

Existing literature has carried out FAST studies predominantly using reference turbines (e.g. NREL-5MW, DTU-10MW) instead of real prototype or commercial turbines. This paper presents the results for the Levenmouth wind turbine, a real, operating demonstration wind turbine. The paper explores and simulates the critical loads for the turbine, which will be very valuable validation case for industrial and academic use. Moreover, the Levenmouth wind turbine exhibits a new generation of extremely flexible blades that conflict with the previous approaches used by most common aero-elastic codes and makes this simulation a challenge.

The study is divided into three steps. It starts with building the model and fine-tuning it until it matches the natural frequencies of the blades and tower. The second step encompasses the comparison of the commissioning results with the relevant NREL FAST simulations to match the dynamic behaviour of the turbine. The final step comprises a load comparison for the interface between the tower and transition piece, in order to validate the new aero-elastic model with the commissioning loads.

The results of the study show 98% agreement in the natural frequencies and dynamic performance, and a 78% agreement in the loads between the aero-servo-elastic model and the certification results.

The present study is framed into an engineering doctorate project and a more comprehensive turbine virtualisation project. We anticipate that our work to be the initial point for more sophisticated aero-elastic models adapted to the unique properties of the Levenmouth demonstration wind turbine.
Stress Intensity Factors in Fitness-for-Service Assessment of Cracks in Mooring Chains
Authors: Arredondo Alberto , Altuzarra Jorge , Mena Alex , Fernández Jonathan
Published: September 2018

Chain corrosion-fatigue often governs the design of the mooring systems of floating production units, and it is assessed based on a stress-life or S-N approach. For integrity management along the lifetime of the assets, a Fracture Mechanics (FM) approach can be more appropriate, especially if cracks were detected during in-service inspections.

An essential parameter of FM is the Stress Intensity Factor (SIF), which becomes fundamental to evaluate the response of a cracked link. The aim of this paper is to present SIF results for chains under different combinations of cracks and loads. Analytic or handbook solutions exist and provide accurate SIFs for simple geometries and test specimens. However, the particular geometry of chain links makes the Finite Element Method (FEM) more appropriate.

The authors utilized the contour integral method together with the general purpose nonlinear FEM code Abaqus, to carry out a large amount of analyses to obtain SIFs for different chain sizes, material grades, crack shapes, crack depths and crack locations along the links. In addition to this, SIFs were derived in combination with other degradation phenomena such as large pitting or interlink wear.

As an outcome, empirical equations were developed to predict the SIFs of propagating cracks in mooring chains under a large variety of scenarios. This allows engineers to assess the criticality of fatigue cracks using suitable crack growth models, and hence evaluate their fitness-for-service or need to implement remedial actions.
Late Stages in the Development of Modulation Instability of Waves
Authors: Shugan Igor , Kuznetsov Sergey , Saprykina Yana , Chen Yang-Yih
Published: September 2018

We conducted experimental and theoretical studies on Benjamin-Feir (BF) instability and revealed a number of new features of the development of instability on the late stages of wave’s evolution. We employ the reduced (truncated) version of Zakharov equations — the multi-wave near-neighbor resonance model (NN model), which takes into account the most effective quasi-resonances with minimum detuning from exact resonance conditions.

We show that near-neighbor model for wave interactions can adequately describe the number of new prominent features of BF instability observed in experiments and it is much simpler than Zakharov equation for computation and analysis. Numerical simulations of the full Zakharov equations confirm the main predictions obtained by the NN modeling and both reasonably correspond to the results of available physical experiments.

Strong permanent downshifting of spectral maximum for gentle waves without wave breaking is revealed for twice as narrow side band spectral width in comparison with the most unstable case. Regime of multiple downshifting accompanied by wave breaking is discovered for steep waves. Discrete energy flow to higher spectral components takes a place in breaking and no breaking regimes. Results of numerical simulations of Zakharov and NN models reasonably correspond to each other and to our experimental and field observations on wave modulation.
Resonances of Floating Elastic Structures in Enclosed Shallow-Water Basins
Authors: Papathanasiou T. K. , Belibassakis K. A.
Published: September 2018

Water wave interactions with floating deformable bodies is an interesting coupled problem finding important applications, as in the case of the responses of large floating structures and platforms of shallow draft and wave-ice sheet interaction. In this work we consider interactions of waves with floating elastic structures in enclosed or partially enclosed basins, as e.g. lakes, bays, reservoirs and harbors. Related applications include the wave induced deflection of floating marinas, solar energy platforms, ice covered lakes, etc. When enclosed or partially enclosed basins are considered, the hydroelastic interactions may be triggered due to the formation of standing waves. These standing waves or seiches have been documented in several occasions. In many cases the seiche effects in terms of wave amplitudes are small, but extreme catastrophic seiches have also occured. This fact suggests that the integrity of a floating structure might be compromised due to an extreme seiche. The main aim of this study is the analysis and simulation of hydroelastic standing waves. Towards this aim a configuration comprising of a constant depth basin, partially covered by a large, thin, floating elastic plate is studied. Shallow-water conditions, typical for the analysis of long waves are considered. The study focuses on the identification of the main resonant frequencies, which is important concerning the design of the considered floating structures. The problem is treated by a semi-analytical method based on the shallow water model by Stoker (1957), in conjunction with modal series expansions. Indicative results are presented illustrating the effects of the main parameters characterising the system, like the dimensions and rigidity of the structure, the clearances and the bathymetry.
Performance Assessment of the Overset Grid Method for Numerical Wave Tank Experiments in the OpenFOAM Environment
Authors: Windt Christian , Davidson Josh , Akram Benazzou , Ringwood John V.
Published: September 2018

To maximise the energy output of wave energy converters (WECs), large structural motions are desired. When simulating WEC performance in Computational Fluid Dynamics (CFD) based numerical wave tanks, these motions must be explicitly accommodated in the computational domain. Using well established mesh morphing (MM) methods, this explicit accommodation results in deformation of control volumes (CVs)/mesh. Thus, large amplitude WEC oscillations may lead to highly distorted CVs and push MM models beyond the limits of numerical stability. While advanced numerical mesh motion methods, such as overset grids, have been developed in commercial CFD codes to overcome these issues, little use of these methods can be found in WEC analysis. However, recently the overset grid method (OSG) has been made available to a wider user community through its release in the open source CFD environment OpenFOAM [1,2]. To evaluate the performance of the OSG, this paper will compare the classical MM method and the OSG against experimental tank test data of the WaveStar device [3].
Experimental and Numerical Modelling of an Offshore Aquaculture Cage for Open Ocean Waters
Authors: Jurado Alfonso , Sánchez Patricia , Armesto Jose A. , Guanche Raúl , Ondiviela Bárbara , Juanes Jose A.
Published: September 2018

An innovative offshore aquaculture cage design and its hydrodynamic behavior for different environmental conditions is presented. The work involves the following blocks: (i) design of the aquaculture cage, (ii) experimental tests performed at IHCantabria facilities (Cantabria Coastal and Ocean Basin, CCOB), (iii) brief description of the ad-hoc time domain numerical model developed, and (iv) calibration of decay tests and sea states cases, in order to be able to simulate different locations of interest for aquaculture stakeholders.

The offshore aquaculture cage is a floating cylindrical structure, designed according to different requirements from standards and needs of contacted aquaculture stakeholders. The cage is focused on the growing and farming of fish species in open ocean conditions. It has been specially designed to be able to withstand different environmental conditions, even waves of more than 5m height.

Through numerical and physical modeling, it has been shown that it has a good seakeeping and accessibility, to reduce O&M costs. Motions and loads have been registered during the tank testing to calibrate and validate a specific coupled time domain numerical model developed for multi-body structures to simulate the behavior of the assembled structure (cage and sinker connected by tendons), including non-linear damping forces and a FEM approach to model the mooring and the tendon system.
Development of New Methodologies to Assess the Structural Integrity of the Grouted Joint of a 10 MW Wind Turbine Substructure
Authors: Santos Benjamin , Rodriguez Alvaro , Fernandez David , Gintautas Tomas , Sorensen John , Natarajan Anand , Njomo Wilfried
Published: September 2018

Monopiles are currently the most commonly used substructure in the offshore wind market due to their ease of installation in shallow to medium waters. The monopile and the transition piece are connected by a grouted joint.

Fatigue and corrosion are two of the most important degradation mechanisms in this type of support structures. These mechanisms increase the costs and compromise the reliability of the structures. The development of new models and methodologies for the analysis of these degradation mechanisms is crucial. For this reason, a deterministic methodology to analyze the behavior of grouted joints has been developed considering the effect of corrosion on the steel parts of the grouted joints and the reliability of the joint under fatigue. Also, a probabilistic model for concrete grout fatigue strength and two probabilistic models for fatigue reliability assessment of steel components of the grouted connection have been developed.
Numerical Study on the Response of Functionally Graded PVC Foam Core Sandwich Panels Subjected to Non-Contact Underwater Explosion
Authors: Zhou Tianyu , Zhang Pan , Cheng Yuansheng , Liu Manxia , Liu Jun
Published: September 2018

In this paper, the numerical model was developed by using the commercial code LS/DYNA to investigate the dynamic response of sandwich panels with three PVC foam core layers subjected to non-contact underwater explosion. The simulation results showed that the structural response of the sandwich panel could be divided into four sequential regimes: (1) interaction between the shock wave and structure, (2) compression phase of sandwich core, (3) collapse of cavitation bubbles and (4) overall bending and stretching of sandwich panel under its own inertia. Main attention of present study was placed at the blast resistance improvement by tailoring the core layer gradation under the condition of same weight expense and same blast load. Using the minimization of back face deflection as the criteria for evaluating the blast resistant of panel, the panels with core gradation of high/middle/low or middle/low/high (relative densities) from the front face to back face demonstrated the optimal resistance. Moreover, the comparative studies on the blast resistance of the functionally graded sandwich panels and equivalent ungraded ones were carried out. The optimum functionally graded sandwich panel outperformed the equivalent ungraded one for relatively small charge masses. The energy absorption characteristics as well as the core compression were also discussed. It is found that the core gradation has a negligible effect on the whole energy dissipation of panel, but would significantly affect the energy distribution among sandwich panel components and the compression value of core.
Mooring System Design Approach: A Case Study for MARMOK-A Floating OWC Wave Energy Converter
Authors: Touzon Imanol , de Miguel Borja , Nava Vincenzo , Petuya Victor , Mendikoa Iñigo , Boscolo Francesco
Published: September 2018

This paper presents a methodology and a flowchart of steps to take for a, consistent and rapidly convergent design of catenary mooring systems. It is subsequently applied for a floating Oscillating Water Column WEC MARMOK-A developed by Oceantec Energías Marinas, in order to fulfill the technical requirements of such dynamic systems. The approach, based on the catenary equations, considers the water depth as a design scale factor for the mooring system, leading to an equivalent static mooring performance. In general, a mooring system configuration is described by the number and distribution of lines; thus, as a preprocess in the herein described procedure, a database is built for different line lengths. The main advantage of the procedure is that once that, after characterizing a mooring system configuration at a specific water depth with a specific line mass and axial stiffness, the database built can be used for any other water depth with any line mass and axial stiffness, accelerating the design optimization process. Mooring static properties are derived for a given material elastic modulus, lines’ mass and water depth. The mean offset and horizontal stiffness are afterwards derived with lines pretension and steady environmental forces (mean wave drift, current and wind) as well as maximum offset and characteristic line tensions. Finally, the process is applied for different lines pretensions to achieve an objective horizontal stiffness of the structure.

The introduced procedure is presented through its application to the MARMOK-A device at a 90m depth site moored by means of a Karratu named mooring configuration. Results are presented in terms of total lines mass, device maximum expected excursion and required footprint for different horizontal stiffness and lines mass in order to give an insight of the impact on total plant cost indicators.
Economic Potential of Industrializing Floating Wind Turbine Foundations
Authors: Andersen Morten Thøtt , Tetu Amelie , Stiesdal Henrik
Published: September 2018

The potential of offshore wind is enormous. It could meet Europes electric energy demand seven times over, and the United States energy demand four times over. However, much of the offshore potential is at water depths that can only be served by floating systems. In order to truly enable floating offshore wind, the cost of energy needs to reach the level of fixed-bottom offshore wind. At the present time, a number of suppliers are offering floating offshore wind foundations, but at cost levels that are prohibitive for large-scale application. The root cause of the high cost levels is that existing designs have emerged from the offshore oil and gas sector; they are manufactured using conventional, non-industrialized methods, weights are measured in thousands of tons and manufacturing times are measured in months.

In contrast, the TetraSpar concept is based on the application of proven design and manufacturing technologies from the highly competitive wind industry. As a result, the weight is only a fraction of the weight of other floating wind turbine foundations, manufacturing takes place in factories using industrialized methods, and assembly and installation is measured in days or weeks, not months. The foundation and wind turbine can be installed in any port of a reasonable size using a standard, land-based crane, and the complete assembly can be towed to site and hooked up to the moorings and the electrical cable using standard tugs.

This paper presents how these desirable economic traits of the TetraSpar design are achievable, and how the near future feasibility of offshore floating wind turbines may develop as a consequence of this radical change in cost levels.
Numerical and Experimental Modelling of Mooring Systems: Effects of Wave Groupiness on Extreme Loads
Authors: Barrera Sánchez Carlos , Guanche Raúl , Losada Iñigo J. , Armesto José A. , de los Dolores Daniel
Published: September 2018

Mooring systems constitute an important element to secure the stability and survival of floating structures. In the last years, its use has increased potentially linked to the growth of marine renewable energies, about all, offshore wind and waves. Traditionally, fixed foundations such as gravity, monopiles or jackets have been installed. However, new alternatives have been developed based on floating structures looking to take advantage of the potential resource in deep waters.

This paper involves an exhaustive mooring system study based on catenary configuration from different points of view. Physical modelling was performed by means of different experimental tests under different loading conditions including prescribed movements and natural forcing (waves and currents) considering two different types of seabed: a rigid seabed using the glass of the flume simulating a rocky bottom and a deformable seabed through sandy seabed. Different types of numerical approaches were implemented and they were validated with laboratory tests. Quasi-static and dynamic models were included. Also, a commercial software called SESAM was used to verify the results. Finally, the effect of wave groupiness on extreme loads was analysed using a Floating Offshore Wind Turbine (FOWT).
Mechanical Behavior After Stress Relaxation in PVDF
Authors: Menezes Freitas Rafael Luis , Costa Celio , Chaves Erica Gervasoni , Teixeira Sylvia
Published: September 2018

This study presents the mechanical properties evaluation of two commercial grades of PVDFs, which were extruded with the same parameters but with different cooling temperatures. After processing, stress relaxation with 7% strain was imposed and tensile properties were measured.

The cooling temperature after extrusion were 4°C, 23°C and 80°C. Then, the PVDFs were submitted to stress relaxation at 23°C and 7% strain. The as processed and after relaxation samples were characterized by FTIR, XRD, DSC and tensile tests at 23°C.

The stress relaxation at 23 °C resulted in no change in volume fraction of crystallinity for PVDF A and B. The XRD and FTIR, for both PVDFs, showed that the crystalline phases were the same, for all cooling conditions and did not change after the stress relaxation. The tensile properties at room temperature showed that the yield stress was a little affect by the cooling temperature, while Young’s Modulus and yield strain were insensible to the cooling temperature. After the stress relaxation, these three tensile properties were slightly affected for both grades.
An Overview of Data Veracity Issues in Ship Performance and Navigation Monitoring
Authors: Perera Lokukaluge P. , Mo Brage
Published: September 2018

An overview of data veracity issues in ship performance and navigation monitoring in relation to data sets collected from a selected vessel is presented in this study. Data veracity relates to the quality of ship performance and navigation parameters obtained by onboard IoT (internet of things). Industrial IoT can introduce various anomalies into measured ship performance and navigation parameters and that can degrade the outcome of the respective data analysis. Therefore, the identification and isolation process of such data anomalies can play an important role in the outcome of ship performance and navigation monitoring. In general, these data anomalies can be divided into sensor and data acquisition (DAQ) faults and system abnormal events. A considerable amount of domain knowledge is required to detect and classify such data anomalies, therefore data anomaly detection layers are proposed in this study for the same purpose. These data anomaly detection layers are divided into several levels: preliminary and advanced levels. The outcome of a preliminary anomaly detection layer with respect to ship performance and navigation data sets of a selected vessel is presented with the respective data handling challenges as the main contribution of this study.
A Framework for Offshore Load Environment Modeling
Authors: Glavind Sebastian T. , Faber Michael H.
Published: September 2018

In the present paper, we propose a novel decision analytical framework for systems modeling in the context of risk informed integrity management of offshore facilities. Our focus concerns the development of system models representing environmental loads associated with storm events. Appreciating that system models in general serve to facilitate the optimal ranking of decision alternatives, we formulate the problem of systems modeling as an optimization problem to be solved jointly with the ranking of decision alternatives. Taking offset in recent developments in structure learning and Bayesian regression techniques, a generic approach for the modeling of environmental loads is established, which accommodates for a joint utilization of phenomenological understanding and knowledge contained in databases of observations. In this manner, we provide a framework and corresponding techniques supporting the combination of bottom-up and top-down modeling. Moreover, since phenomenological understanding as well as analysis of databases may lead to the identification of several competing system models, we include these in the formulation of the optimization problem. The proposed framework and utilized techniques are illustrated on a principal example. The example considers systems modeling and decision optimization in the context of possible evacuation of an offshore facility in the face of an emerging storm event.
Nonlinear Wave-Body Interaction Using a Mixed-Eulerian-Lagrangian Spectral Element Model
Authors: Monteserin Carlos , Engsig-Karup Allan P. , Eskilsson Claes
Published: September 2018

We present recent progress on the development of a new fully nonlinear potential flow (FNPF) model for estimation of nonlinear wave-body interactions based on a stabilised unstructured spectral element method (SEM). We introduce new proof-of-concepts for forced nonlinear wave-body interaction in two spatial dimensions to establish the methodology in the SEM setting utilising dynamically adapted unstructured meshes. The numerical method behind the proposed methodology is described in some detail and numerical experiments on the forced motion of (i) surface piercing and (ii) submerged bodies are presented.
Inherently Safer Choices in Early Design of Offshore Oil and Gas Installations: A Multi-Target KPI Approach
Authors: Crivellari Anna , Tugnoli Alessandro , Martina Costanza , Bonvicini Sarah , Cozzani Valerio
Published: September 2018

As offshore oil & gas activities are currently evolving towards more productive yet complex situations, the availability of efficient safety metrics has become essential in the early stages of offshore oil & gas projects to underline potential major accidents hazards and clearly communicate safety criticalities. Inherent safety has demonstrated to be a widespread concept in offshore risk management strategies, but there are few preliminary studies in the existing literature about systemic indexing to orient the conceptual and basic design stages of the project lifecycle. In the present work, a methodology for the selection of inherently safer solutions was developed as a support tool for decision-making in early design activities of offshore oil & gas installations. The expected inherent safety performance of alternative design options is assessed by means of a comprehensive set of key performance indicators (KPIs) based on the simulation of consequences of offshore accident scenarios and credit factors of the possible loss of containment events from offshore equipment. The proposed KPIs aim to capture the hazard level of single units and to address selectively multiple targets of the potential threats than personnel and process equipment on the installation, such as marine organisms on the sea environment. Moreover, overall aggregated KPIs were introduced as a sound synthetic measure of the inherent safety performance of the offshore system. The method was applied to the assessment of alternative designs of an offshore production facility, particularly characterized by environmental and safety concerns. The results from the case study evidenced the capability of the proposed method in ranking the potential and credible critical units of each alternative configuration and identifying the relative magnitude of targets contributions to the global safety profile of the installation.