bluuebunny/crossref_metadata_2025 huggingface dataset:
Constraints Implementation in the Application of Reinforcement Learning to the Reactive Control of a Point Absorber
Authors: Anderlini Enrico , Forehand David I. M. , Bannon Elva , Abusara Mohammad
Published: October 2017

Here, least-squares policy iteration, a reinforcement learning algorithm, is applied to the reactive control of a wave energy converter for the first time. Simulations of a linear point absorber are used for this analysis. The focus of this study is on the implementation of displacement constraints. The use of a penalty term is effective in teaching the controller to avoid the selection of combinations of the damping and stiffness coefficients that would result in excessive displacements in particular sea states. However, the controller can learn that the actions are bad only after trying them, as shown by the simulations. For this reason, a lower-level control scheme is proposed, which changes the sign of the controller force based on the magnitude of the float displacement and sign of its velocity. Its effectiveness is proven in both regular and irregular waves, although greater care is required for the determination of soft constraints.
Simulation of Wave Impacts at Belwind Offshore Wind Farm and Comparison With Full-Scale Measurements
Authors: Bunnik Tim , Weijtjens Wout , Devriendt Christof
Published: October 2017

The effects of operational wave loads and wind loads on offshore monopile wind turbines are well understood. For most sites, however, the water depth is such that steep and/or breaking waves will occur causing impulsive excitation of the monopile and consequently considerable stresses, displacements and accelerations in the monopile, tower and turbine.

At Belwind offshore wind farm (offshore Zeebrugge, Belgium) the waves and accelerations of a Vestas V90 3MW wind turbine have been monitored since November 2013, using wave radar and several accelerometers. During this period the wind turbine was exposed to several storms and experienced several wave impacts, resulting in vibrations in the monopile.

The measurements were compared with results from a numerical model for the flexible response of wind turbines due to steep waves. Previously this model was compared with scale model tests with satisfying results. The full-scale measurements provide an additional cross-check of the model.

The numerical model consists of a one-way coupling between a CFD model for wave loads and a simplified structural model based on mode shapes. An iterative wave calibration technique has been developed in the CFD model to ensure a good match between the simulated and measured incoming wave profile, obtained with the wave radar. This makes a deterministic comparison between simulations and measurements possible. This iteration is carried out in a 2D CFD domain (assuming long-crested waves) and is therefore relatively cheap. The calibrated numerical wave is then simulated in a 3D CFD domain including a (fixed) wind turbine. The resulting wave pressures on the turbine have been used to compute the modal excitation and subsequently the modal response of the wind turbine. The mode shapes have been estimated from the measured accelerations at the Belwind turbine. A grid refinement study was done to verify the results from the numerical model. The horizontal accelerations resulting from this one-way coupling are in fair agreement with the measured accelerations.
The Feasibility of OpenFOAM-Based One-Dimensional Prediction of Slugging Induced Pressure Fluctuations in Pipeline-Riser Systems
Authors: Meng Xiangyin , Tao Longbin
Published: October 2017

A quasi one-dimensional OpenFOAM-based transient solver is proposed to investigate slugging-induced pressure fluctuations for multi-phase flow in pipeline-riser systems. Following the two-fluid Eulerian approach, the governing equations of mass, momentum and energy are expressed for each phase of gas and liquid. The frictions and energy exchanges between the two phases are estimated by several empirical enclosure models. Gmsh is employed for geometrical models and one-dimensional mesh generations, while ParaView is used for data collection and necessary processing. By two of previously published experimental investigations, the newly proposed open source approach is validated and demonstrated capable of carrying out the study of transient multi-phase flow dynamics in pipeline-riser systems. Using the OpenFOAM-based one-dimensional way in the present work, the multi-phase slugging-related analysis for subsea product transportations in future deep ocean projects could be much more economical due to the approach natures of ‘open source’ and ‘one-dimensional’.
Simulations of Dynamic Interaction Between a Bluff Body and Installation Vessel During Launch and Recovery in Rough Seas
Authors: Bashir Musa B. , Evans Mathew , Benson Simon D. , Murphy Alan J.
Published: October 2017

Simulation of marine operations for launch and recovery of bluff bodies such as autonomous underwater vehicles (AUV), remotely operated vehicles (ROV) or subsea templates is traditionally performed in calm to moderate sea conditions. The reason for doing so is partly due to the interaction between the complex dynamic response of an installation vessel, a moving bluff body and the wave kinematics of the rough sea condition. This is in addition to the need for accurate hydrodynamic coefficients that would enable proper simulation and modelling of the launch and recovery process. The key objective of the current methodology is to minimize risks of damage to the vessel and total loss of assets during the deployment and recovery process for marine operations in rough sea conditions.

The aim of this paper is to present the results of experimental and numerical investigation on the prediction of dynamic response of a bluff body during launch and recovery from a surface vessel in rough sea condition. Experimental measurements of hydrodynamic coefficients and responses of a large scale bluff body using a scaled model were completed. Further studies using a time-domain numerical tool have been undertaken to measure the response characteristic of bluff bodies in rough seas. The study also predicted the contributions of vessel motion in rough seas to the dynamic response of the bluff bodies. The results obtained have shown that simulation of launch and recovery operations in rough seas can be carried out efficiently if their hydrodynamic coefficients through the wave active regions of the rough seas are predicted and then adequately implemented in the simulations.
Impact of Climate Modes on Shoreline Evolution: Southwest Coast of India
Authors: Chowdhury Piyali , Behera Manasa Ranjan
Published: October 2017

Coastal geomorphology is a complex phenomenon which is governed by nearshore wave and tidal climate. Change in climate indices (like sea surface temperature, sea level, intensified cyclone activity, among others) and climate modes (like El Nino Southern Oscillation (ENSO), Southern Annular Mode (SAM), Indian Ocean Dipole (IOD)) affect the wave climate and modify many coastal processes thereby altering the geomorphology of shorelines. In countries like India where tropical and sub-tropical cyclones are common, the coastal geomorphology is under constant threat. Coasts are also vulnerable to anthropogenic factors like offshore structures, harbours, wave farms and other constructional activities along the shoreline. It is thus necessary to understand the evolution of coastlines under the changing climate scenario. The rapidly growing socio-economic development in south-west coast of India has generated the need to investigate the longshore sediment transport (LST) regime in this region under the influence of variable climate factors like the wave characteristics. The presence of numerous river deltas, estuaries and mud banks makes the situation worse especially during the south-west monsoon season (June-September). The investigation on the contemporary evolution of this coastline has not been undertaken and the knowledge of the climate factors that influence the shorelines of the southern tip of India are unknown. This study attempts to understand the temporal dynamics of the longshore sediment transport in this region.
Transportation of Hydrogen Gas From a Local Plant to Remote Markets via High Pressure Submarine Pipelines
Authors: Hval Morten , Gråberg Stig Vemund
Published: October 2017

Hydrogen is about to become a very important energy carrier. It is regarded as clean since its only bi-product is water when it is converted to electric energy in a fuel cell. Hydrogen gas needs, however to be transported to the end user from the hydrogen plant. If the end user is remote from the plant and the total energy demand is high, the hydrogen gas is best transported through a pipeline. Low pressure hydrogen gas pipeline distribution systems already exist onshore and have been in operation for years both in Europe and USA. REINERTSEN has performed a study to evaluate the possibilities for transporting pure hydrogen gas or natural gas/hydrogen mixtures under high pressure through a submarine pipeline from an onshore plant to an end-user either offshore or overseas from where the hydrogen gas is produced. Hydrogen under high pressure is known to have some possible detrimental effects with respect to material brittleness and enhancement of fatigue crack growth which must be carefully considered before a steel submarine pipeline is constructed and installed. The severity of the detrimental effect may be attributed to both the material type and welding procedure, something which must also be taken into the consideration.

This paper is devoted to the possible changes and limitations in material properties caused by pressurized hydrogen gas that must be considered when designing and installing a submarine transport pipeline or converting an existing natural gas pipeline to hydrogen service.
Return Volumes of Sudden Siltation Occurred in Outside Waterway of Huanghua Port
Authors: Dong Sheng , Jiao Chunshuo , Tao Shanshan , Zhang Ri
Published: October 2017

Strong wind often raises waves and result in the sediment siltation. The navigation of outside waterway of Huanghua Port is seriously influenced by sudden siltation induced by strong wind. Before outside waterway treatment, the return sudden siltation volumes must be determined previously. The point estimation of return values can be obtained by distribution curve fitting, meanwhile, interval estimation can provide ranges of return values under some fix confidence levels. Based on the sequences of annual maximum sudden siltation of Huanghua Port from 1979 to 2003, and by using curve fittings of lognormal distribution and maximum entropy distribution, and maximum likelihood interval estimation method, the point and interval estimations of return sudden siltation volumes are presented. The results given by the maximum entropy distribution have higher precision, so they can be applied as the reference of the treatment of sediment accumulation.
On-Bottom Stability Analysis of Subsea Pipelines Under Combined Irregular Waves and Currents
Authors: Ji Guomin , Li Lanjing , Ong Muk Chen
Published: October 2017

The objective of the paper is to study the effect of different parameters regarding on-bottom stability of subsea pipelines under combined irregular waves and currents. The effect of friction coefficient is first investigated. The development of lateral displacement and penetration for three different friction coefficients are compared for sandy and clayey seabed respectively when applied wave and current conditions are kept same for all the cases. The friction coefficient affects the soil resistance force and further changes the initial time when the pipeline starts to move in the lateral direction. The accumulated displacement reduces for large friction coefficient and it results in less penetration. The total effect of the increasing friction coefficient depends on the competition between the increased friction force and the reduced passive soil resistance force. The pipeline usually crosses different types of soil along the route. Hence, different combinations of soil types along the route are applied in the analysis. The soil property at middle of the pipeline is found to be important when the boundary conditions at both ends are fixed. Three analyzing procedures, namely standard 3-hour procedure, the procedure recommended by PONDUS and the procedure recommended by DNV, could be used for on-bottom stability analysis under storm conditions. The comparison of these procedures shows that the procedure recommended by PONDUS is the most appropriate for the storm conditions. The procedure recommended by DNV considers the build-up of initial penetration before the storm; and it could be applied in the analysis when the penetration is stabilized after the start -up time (20% of 3-hour).
Advanced Approaches for Coupled Deformation-Seepage-Analyses of Suction Caisson Installation
Authors: Stapelfeldt Marc , Bienen Britta , Grabe Jürgen
Published: October 2017

In this paper the installation procedure of suction caissons is investigated by means of coupled seepage large deformation analysis performed with finite element methods. The modelling techniques employed to enable simulations of the penetration of a caisson into the soil under offshore conditions, i. e. several tens of meters below the water level. The numerical model includes a u-p-formulation, which is used to calculate the excess pore pressures and effective stresses from the total stresses. The Coupled-Eulerian-Lagrangian (CEL) approach available in conjunction with the Abaqus/Explicit solver is used. The calculation results are compared to centrifuge tests that were carried out recently at the Centre for Offshore Foundation Systems (COFS). This sheds light on the potential and the limitations of the presented numerical techniques. This paper concludes with a brief discussion of alternative numerical approaches that could be capable of the simulation of caisson installation.
Reliability Optimization Design of the Steel Tube Umbilical Cross Section Based on Particle Swarm Algorithm
Authors: Yan Jun , Yang Zhixun , Zhao Panpan , Lu Qingzhen , Wu Wenhua , Yue Qianjin
Published: October 2017

Umbilical is one of key control equipment in the under-sea production system. The cross section at the top of configuration could undertake the most dangerous load cases during operation. Firstly, the uncertainty of components in the section is investigated, and the strength failure is analyzed under this working condition. Based on above content the limit state equation is established. Then, the reliability of the umbilical is calculated by improved first order second moment method and Monte Carlo method. Finally, strength reliability optimization equation is established with the objective of minimum mass of unit length, which takes the external diameter and wall thickness of steel tubes as variables, with the consideration of the reliability constraint. The particle swarm optimization algorithm is adopted to solve the problem, the results of which show that reliability index is 67.43% higher than the original design. The effectivity of this method is verified, which is greatly engineering significant to guide the high-performance design of the umbilical.
Determination of Environmental Conditions Relevant for the Ultimate Limit State at an Exposed Aquaculture Location
Authors: Bore Pål Takle , Amdahl Jørgen
Published: October 2017

Methods for determination of environmental conditions relevant for the ultimate limit state at an exposed aquaculture location are proposed. The considered location is in Frohavet, northwest of Trondheim, Norway. Wind, waves and current are considered, but as more severe wave conditions are expected to be the most influential environmental factor when going from sheltered to more exposed locations, particular focus has been given on the description of wave conditions. The estimated long term description of wind and waves are based on 16 year of SWAN hindcast data, where the wave data have been calibrated against one year of in-situ measurements. One month of current measurements were available. The initial distribution method is used to determine the long term distribution of wind and wave conditions. The data fit the applied distributions well, but the sea state characteristics are seen to be highly directional dependent, which is important to take into account. The effect of using only one year of wave data is also investigated, showing that this can lead to significant underestimation of the extreme values. Concepts from the environmental contour line method are applied for determination of sea states corresponding to a given annual probability of exceedance. A deterministic approach from NS 9415 is used to determine the extreme current velocities. Comments on the use of directional criteria for design are also given.
Experimental Evaluation of the Natural Frequency of an Offshore Wind Turbine’s Scaled Model
Authors: Kerner Laura , Benfeddoul Selim , Dupla Jean-Claude , Cumunel Gwendal , Canou Jean , Pereira Jean-Michel , Argoul Pierre
Published: October 2017

Offshore Wind Turbines (OWT) are slender structures with sensitive dynamics, strongly influenced by the soil-structure interaction. The structure is subjected to cyclic and dynamic loads with frequencies close to the first natural frequency of the offshore wind turbine. To avoid any resonance phenomenon, a precise evaluation of the initial first natural frequency of the wind turbine is essential. The present work deals with the evaluation of the natural frequency of an OWT’s scaled model with monopile foundation. The main factor influencing the natural frequency is the soil-structure interaction which needs to be assessed precisely. To do so, a simple method presented by [Adhikari and Bhattacharya, 2012] assimilates the offshore wind turbine as an Euler-Bernoulli beam on a flexible foundation with lateral and rotational springs. The key factor in the evaluation of the natural frequency is the value of the stiffness of these springs. In this way, this paper presents a method combining experimental measurements and a finite element model on Abaqus which allows a precise evaluation of the stiffness of the springs. The proposed method is compared to the existing methods used to evaluate the soil’s stiffness (such as [Eurocode 8, 2003]). The suggested method gives a fine evaluation of the response of the structure with a mean deviation below 1%, compared to the average errors obtained for the previous methods ranging from 6.6 to 17.4%.
Determination of Parameters for the Damage Mechanics Approach to Ductile Fracture Based on a Single Fracture Mechanics Test
Authors: Coppejans O. J. , Walters C. L.
Published: October 2017

The local approach to modelling ductile tearing is a useful technique to give insight into fracture mechanics. However, applications of the local approach have been stymied by the high cost of finding the parameters that characterize it because of the number of specimens and expensive post-processing that the testing requires. In this paper, a novel iterative method to extract a failure locus from one Crack Tip Opening Displacement (CTOD) specimen is presented. Material points fail under various different stress states in a CTOD specimen, so many different points on the failure locus can be found through thoughtful post-processing in FEA. A phenomological ductile failure locus is fitted through the stress triaxiality, Lode angle, and plastic strains that cause failure at material points in the CTOD test. Simulating a CTOD test with a different aspect ratio has shown that the failure locus found by this method can be predictive, giving both accurate force versus Crack Mouth Opening Displacement (CMOD) curves and realistic fracture surfaces featuring separate tunnelling and shear lips.
Modelling of Complex DEH Systems
Authors: Hanisch Øyvind Pharo
Published: October 2017

Direct Electric Heating (DEH) is an increasingly attractive method for flow assurance that has been in use for more than 15 years. All current systems in operation consist of a single flowline or have subsea architecture such that interaction between systems as well as components in the proximity is limited. DEH is selected as flow assurance for two ongoing field developments where the layout is such that there will be interaction with other subsea architecture. One system has numerous flowline systems in close interaction and the other has unfavorable crossing. This paper presents some of the design aspects which do arise for fields where the flowlines are in close proximity to each other. The paper also discusses how to handle interference with existing infrastructure when installing DEH in a complex field layout. The aspects studied are effect on power rating, ac corrosion and thermal rating of cables and flowlines.
Experiments on Stability of Concrete Armor Units at Convex Corner Trunk
Authors: Kim Young-Taek , Lee Jong-In
Published: October 2017

The breakwater is one of the most important structures in coastal areas and the rubble mound breakwaters have been most widely designed and constructed. When designing the rubble mound structures, the determination of the stable weight of concrete armor units is the most significant and critical problems. The stable weight of concrete armor units could be calculated by Hudson or van der Meer formula. The breakwater is assumed to be straight and the waves are propagating normally to the structures when using this formula. The weight of concrete armor units at the round breakwater head should be protected by 1.3∼1.6 times of that at the trunk part because the coastal engineers think it to be vulnerable. Recently the damage of concrete armor units at the convex corner of breakwater trunk was reported several times in Korea. However, there were no any design standards for this area. In this study, three-dimensional hydraulic model tests were performed to investigate the stable weight of concrete armor unit at convex corner area. The tetrapod was used to the target concrete armor unit in the study. The stable weight is suggested by stability coefficient, Kd by using Hudson formula. From this study, the stable armor weight at the convex corner is similar with that at the round breakwater head and this means that about 1.4 times of weight for trunk area was needed at convex corner area.
A Study on the Method to Estimate Ship Hull Girder Ultimate Strength Considering Biaxial Compression in Bottom Stiffened Plates
Authors: Naruse Yoshiaki , Kawamura Yasumi , Okada Tetsuo
Published: October 2017

In this study, ship hull girder ultimate strength considering biaxial compression in bottom stiffened plates is investigated. Firstly, elasto-plastic finite element analysis is carried out for the stiffened plates under biaxial compression by using the commercial FEM software, LS-DYNA. From the observation of collapse behavior of the stiffened plates, it is found that the decrease of the effective width of the stiffened plate under biaxial compression is more rapid compared with the formula in the Common Structure Rules (CSR). This is because the buckling in one half-wave mode occurs when the transverse compressive load becomes larger, while the buckling with multi half-waves mode usually occurs in longitudinal direction when only longitudinal compressive load is applied. Based on the observation, a new formula to estimate ultimate strength of stiffened plates considering biaxial compression is proposed in which three new parameters are introduced which can represent the decrease of the effective width. Next, a method to estimate ultimate strength of hull girder considering the biaxial compression of ship bottom is developed based on the conventional Smith method provided in the CSR, by introducing the above new formula to estimate the ultimate strength of stiffened plates, and by improving the conventional formula of the hard corner element in the Smith method. Finally, the hull girder strength of a container ship under biaxial compression estimated by the proposed method is compared with the result of FEM analysis to show validity of the method.
Does More Tension Reduce VIV?
Authors: Vandiver J. Kim , Ma Leixin
Published: October 2017

In this paper it is shown that for very long risers in sheared flow there is a surprising outcome — the VIV response amplitude in the power-in region does not depend at all on the amount of damping in the power out region, as long as it is sufficient to prevent waves from reflecting at the boundary and returning to the power-in region. In these cases, the response in the power-in region depends on the wave radiation damping and not on the damping in the power-out regions. Tension plays a major role in the determining the radiation damping and in some cases, but not all, pulling harder will indeed reduce response in the power-in region. Numerical simulations are presented in which a finite element model of a long riser is used to compute the VIV response in a sheared flow for which the power-in region is at one end of the riser. The radiated waves are shown to diminish with distance traveled as expected. When ζoutnout, the product of the number of wavelengths to reach the far termination and the damping ratio in the power-out region is greater than 0.18, it is shown that no significant vibration energy returns to the power-in region and the response in the power-in region is independent of the damping in the power-out region. The numerical simulation is used to illustrate the effect of changing tension on the radiation damping and therefore on the VIV response. The VIV response prediction program SHEAR7 is used to evaluate the effect of increasing tension on a realistic deepwater drilling riser in 3000 m water depth. A 20% increase in tension leads to a 12% reduction in fatigue damage rate.
State Estimation of Deep-Water Tether Management System
Authors: Eidsvik Ole A. , Schjølberg Ingrid
Published: October 2017

This paper presents a method for three dimensional dynamic analysis of a complete deep-water Remotely Operated Vehicle (ROV) system. The presented formulation utilizes Euler-Bernoulli beam theory to represent the cable and the Tether Management System (TMS) Module is represented as a rigid body moving in 6 degrees of freedom. The model is applied to a real ROV-system and compared to experimental data collected from deep-water ROV operations. The ability to estimate TMS position, velocity and acceleration (state estimation) is the key feature of this analysis. This is because acoustic positioning may not always be available hence other means of state estimation is needed to ensure safe and efficient operations. Two different ROV-deployments are simulated and show good correspondence with the experimental data, limitations in acoustic positioning is also shown on the basis of signal-loss. Based on simple current estimates and measured surface-ship position, the relative position of the TMS can be estimated with fairly good accuracy. The ability to simulate the ROV system in real time is also an important feature making the procedure feasible to incorporate in controller design. The cable forces are estimated and show that horizontal ship motions are of minor importance with regards to cable tension.
Limitations in Scaling Towing Tests for Simple Pontoon Shapes
Authors: Krijger Jan Willem , Chalkias Dimitris
Published: October 2017

For merchant vessels full scale resistance is determined by extrapolating the towing tank tests using Hughes’ method. For blunt and simple shapes this approach might not be valid due to significant Reynolds dependency.

In this paper a study is presented on two semi-submersible pontoon shapes and one drillship. The resistance for these hulls was calculated using URANSE simulations. Four different scales: 1:1, 1:10, 1:20 and 1:40 were investigated for all the different hull shapes. The goal of the study was to determine a form factor to use when scaling towing test results to full scale. However due to the simple shapes of the pontoons it was found that there was a significant Reynolds dependency at all the different scales. This Reynolds dependency had much more influence on the full scale resistance than a form factor. For the drillship no significant Reynolds dependency was found and applying ITTC 1978 method [1] with a form factor improved the accuracy of the resistance prediction compared to using the Froude method which is not using a form factor.

The simple pontoon shapes used were based on a combination of boxes, cylinders and spheres. The cylindrical and spherical shapes have a Reynolds regime where a drag crisis occurs. For all the shapes the drag crisis regimes were right in the same Reynolds number range at which typical model tests would be performed. Therefore performing towing tests and making accurate full scale resistance predictions for these simple shapes is not straightforward. Applying the ITTC 1978 method does not provide the same level of accuracy as it does for merchant vessels.

The paper identifies limitations involved in scaling towing test results for typical offshore hull shapes. Further possible solutions are proposed to improve the accuracy of the resistance predictions.
Analytical Solutions of Bimodal Gaussian Processes’ Fatigue Damages
Authors: Huang Wenbo
Published: October 2017

Based on the rain-flow counting technique, a frequency domain method is developed for calculating the fatigue damages caused by the bimodal Gaussian loads. Firstly, by considering the reduction effect of low frequency loads on high frequency ones, the amplitudes of the small rain-flow cycles are obtained. Secondly, the amplitudes of the large rain-flow cycles are determined by means of Turkstra’s rule of load combination. Moreover, based on the numerical solutions developed, the two analytical formulas for the damage estimates of small and large cycles are developed. Both numerical and analytical solutions are benchmarked against the rain-flow damage estimates and compared with the existing ones. The numerical analyses show that the damages estimated by the new method are close to the rain-flow damages.
Passive Control of a Pentapod Offshore Wind Turbine Under Earthquakes by Using Tuned Mass Damper
Authors: Wang Wenhua , Gao Zhen , Li Xin , Moan Torgeir , Wang Bin
Published: October 2017

The finite element model (FEM) of a pentapod offshore wind turbine (OWT) is established in the newly compiled FAST. The dynamic responses of the OWT are analyzed in detail. Further, a tuned mass damper as a passive control strategy is applied in order to reduce the OWT responses under seismic loads. The influence of the tuned mass damper (TMD) locations, mass and control frequencies on the reduction of OWT responses are investigated. A general configuration of TMD can effectively reduce the local and global responses to some degree, but due to the complexity of characteristics of the OWT structure and seismic waves, the single TMD can not obtain consistent controlling effects.
Three-Dimensional Finite Element Analysis of a Mixed Mode I/II Fracture Test Specimen: Asymmetric Four-Point Shear Specimen
Authors: Cohen Mark , Wang Xin
Published: October 2017

In this paper, extensive three-dimensional finite element analysis is conducted to study the asymmetric four-point shear (AFPS) specimen: a widely used mixed mode I/II fracture test specimen. Complete solutions of fracture mechanics parameters KI, KII, KIII, T11, and T33 have been obtained for a wide range of a/W and t/W geometry combinations. It is demonstrated that the thickness of the specimen has a significant effect on the variation of fracture parameter values. Their effects on crack tip plastic zone are also investigated. The results presented here will be very useful for the toughness testing of materials under mixed-mode loading conditions.
Performance Assessment of the Anaconda WEC in Regular Waves at 1:50 Model Scale
Authors: Mendes A. C. , Braga F. P. , Paredes L. M. A. , Chaplin J. R.
Published: October 2017

The Anaconda WEC belongs to a new generation of wave-energy converters that are currently on their way to reach a pre-commercial stage. It consists of a long rubber tube that is designed to float head to waves. The tube is filled with water and its stern is connected to a power take-off system (PTO). As a result of the interactions with the incoming ocean waves, the tube conveys internal pressure bulges whose intensity grows in the direction of the PTO. A pneumatic system is considered herein, in which electrical energy can be produced from a turbo-generator set.

The present research focuses on the performance assessment of a free-floating Anaconda model with air-flow PTO. The results of a series of tests with a 1:50 scale physical model in wave-tank are presented and discussed. In this model the pneumatic chamber connects with the atmosphere through an orifice plate. Several calibrated orifices of different diameters have been tested. The tests were undertaken in regular waves that translate to waves of 7 to 14 seconds in full-scale. Pressure in the pneumatic chamber and the water-column oscillations in the shaft were monitored. They provide estimates of the extracted power and energy capture efficiency of the system.

One of the aims of the study is to account for the effects of compressibility on the power output of the system, as well as properly assessing the impact of scale effects upon performance estimates. The results, presented as a function of the wave frequency, are ultimately used to predict prototype performance.
Evolution of Residual Stress in Tensile Armour Wires of Flexible Pipes During Pipe Manufacture
Authors: Fernando Upul S. , Davidson Michelle , Yan Kun , Roy Matthew J. , Pirling Thilo , Withers Philip J. , Francis John A.
Published: October 2017

Tensile armour layers in unbonded flexible pipes are constructed by the parallel helical wrapping of several rectangular wires. Pairs of layers, wound in opposite directions and with different helical shapes are used to provide the necessary axial strength, water depth capacity and torsion balance.

The forming of armour wires as supplied by the vendor into a helix shape on the pipe involves significant plastic straining; twisting and repeated bending of the wires in different planes. The wires that are wrapped on the pipe are not unloaded. Therefore the armour wires in flexible pipes are considered to contain residual stress (RS). Knowledge of RS in the wires of the manufactured pipe is essential in making appropriate design decisions with high confidence to meet material utilization requirements and subsequently predict the integrity and fatigue durability of the pipe.

This paper describes an investigation performed to examine the evolution of RS in the tensile wires during various stages of the pipe manufacturing process. To this end, different methods including a relatively simple and inexpensive stress relaxation method termed the contour method, as well as diffraction methods were used to evaluate RS in the wire.

A finite element (FE) model has been developed to simulate the wire deformation involved in the pipe manufacturing process. The procedure was used to predict the evolution of RS in a tensile wire and change in material response during the pipe manufacturing process. A comparison of FE model predictions and measured data is given.

The results show that the RS measured by the contour method give comparable values to those obtained from more advanced methods such as high energy synchrotron X-ray and neutron diffraction. The need for using representative material properties and deformation boundary conditions in FE models to predict RS accurately is highlighted.
Development and Testing of a Friction-Based Post-Installable Fiber-Optic Monitoring System for Subsea Applications
Authors: Bentley Nicole L. , Seaman Calvin H. , Brower David V. , Tang Henry H. , Le Suy Q.
Published: October 2017

This paper presents the design and development of a friction-based coupling device for a fiber-optic monitoring system capable of measuring pressure, strain, and temperature that can be deployed on existing subsea structures. A summary is provided of the design concept, prototype development, prototype performance testing, and subsequent design refinements of the device. The results of laboratory testing of the first prototype performed at the National Aeronautics and Space Administration (NASA) Johnson Space Center (JSC) are also included. Limitations of the initial concept were identified during testing and future design improvements were proposed and later implemented. These new features enhance the coupling of the sensor device and improve the monitoring system measurement capabilities.

A major challenge of a post-installed instrumentation monitoring system is to ensure adequate coupling between the instruments and the structure of interest for reliable measurements. Friction-based devices have the potential to overcome coupling limitations caused by marine growth and soil contamination on flowlines, risers, and other subsea structures. The work described in this paper investigates the design and test of a friction-based coupling device (herein referred to as a friction clamp) which is suitable for pipelines and structures that are suspended in the water column as well as for those that are resting on the seabed. The monitoring elements consist of fiber-optic sensors that are bonded to a stainless steel clamshell assembly with a high-friction surface coating. The friction clamp incorporates a single hinge design to facilitate installation of the clamp and dual rows of opposing fasteners to distribute the clamping force along the structure. The friction clamp can be modified to be installed by commercial divers in shallow depths or by remotely operated vehicles in deep-water applications. NASA-JSC was involved in the selection and testing of the friction coating, and in the design and testing of the prototype clamp device. Four-inch diameter and eight-inch diameter sub-scale friction clamp prototypes were built and tested to evaluate the strain measuring capabilities of the design under different loading scenarios. The testing revealed some limitations of the initial design concept, and subsequent refinements were explored to improve the measurement performance of the system.

This study was part of a collaboration between NASA-JSC and Astro Technology Inc. within a study called Clear Gulf. The primary objective of the Clear Gulf study is to develop advanced instrumentation technologies that will improve operational safety and reduce the risk of hydrocarbon spillage. NASA provided unique insights, expansive test facilities, and technical expertise to advance technologies that will benefit the environment, the public, and commercial industries.
Extreme Value Analyses of Dynamic Response Parameters of a Wind Tower Structure Under Short-Term Nonlinear Irregular Seastate
Authors: Nascimento Leonardo , Sagrilo Luis , Ellwanger Gilberto
Published: October 2017

In the assessment of marine structures in shallow waters domain it is important to take into account the nonlinear (or non-Gaussian) nature of the irregular waves when predicting short and long-term responses of such structures. Other sources of nonlinearities in the response are also present due to some nonlinear effects such as: wet-dry surface effects, wind force on dry parts of the structure, drag term in Morison hydrodynamic force equation, etc. The estimation of the characteristic short-term extreme responses requires the extreme value analysis of a non-Gaussian stochastic process. There are many approaches available in literature which can be employed, such as: Hermite-based model, Weibull-fitting model, etc. In this paper two distinct Weibull fitting models (one based on the first two and other based on the first three moments of the response peaks sample) and Hermite-based models using both conventional and linear moments (L-moments) are investigated for the prediction of extreme short-term response of mono-column wind tower installed in a water depth of 20m and subject to wave, current and wind loading. The tower responses (load effects) time-histories are obtained by means of a time-domain finite element-based program using 3-D geometric nonlinear beam elements developed for the dynamic analysis of this type of structure. In this program, the nonlinear behavior of the irregular waves is modelled by means of the second order Sharma and Dean theory [1] and the wind forces are represented by a very simplified load model based on wind velocity simulated time-series and the obstruction area of the tower and blades.
Benchmark Tests of Two Body Interaction in Waves
Authors: Qiu Wei , Rousset Jean-Marc , Peng Heather , Meng Wei , Horel Boris
Published: October 2017

As part of the test campaign led by the 27th ITTC Ocean Engineering Committee to produce reliable experimental benchmark data for multiple-body interactions in waves, model tests of two identical models in close proximity were carried out at the towing tank of Memorial University, Canada and at the wave basin of Ecole Centrale de Nantes, France. This paper presents the experimental results for the two bodies in regular waves with various gaps. The experimental data, including motions of two bodies, wave elevations in the gap and drift forces, were compared with numerical solutions based on the potential-flow solution.
Response Variability in Flexible Cylinder VIV Model Test Data
Authors: Resvanis Themistocles L. , Vandiver J. Kim
Published: October 2017

This paper presents some results from the 2011 SHELL tests at the MARINTEK basin. The tests involved towing densely instrumented 38m long flexible cylinders at Reynolds numbers up to 220,000. The main objective it to present the experimental results in a manner that describes the response variability that exists in the measured response data.

Despite the fact that VIV is known to be a stochastic process, this is rarely addressed in the literature and currently there is no framework or ‘best-practice’ in the VIV community that can address statistically non-stationary data. In this paper, the experimental measurements are treated like non-stationary time-series and all statistical quantities which are typically of interest are computed with the use of short duration moving windows (or time-gates). A novel way of plotting and presenting VIV response data for flexible cylinders is introduced that is capable of revealing the inherent variability that exists in the cylinder’s response.
Tensile Strength of Nylon Netting Subjected to Various Concentrations of Disinfecting Chemicals
Authors: Moe Føre Heidi , Dahle Stine Wiborg , Gaarder Rune H.
Published: October 2017

This paper presents a study of traditional netting materials subjected to disinfecting chemicals during fish farming and treatment of net cages. A series of tests were performed in order to study the effect of various concentrations of disinfecting chemicals on the tensile strength of Raschel knitted Nylon netting materials.

Simulated spill of diluted hydrogen peroxide to the jump fence during de-lousing did not affect the strength of the applied new and used knotless nylon netting samples. Hydrogen peroxide reacted with biofouling forming gas bubbles, but this did not result in reduced netting strength.

The performed tests did not indicate any effect on netting strength from a simulated single, traditional bath disinfection as performed at service stations applying the disinfectant Aqua Des containing peracetic acid. However, increasing the Aqua Des concentration from 1 to 10 % resulted in a strength reduction of 3–6 %. Simulated spill of concentrated Aqua Des on the jump fence of a net with copper coating residuals resulted in a severe reduction in strength of 45 %. This strength loss was probably a consequence of chemical reaction between copper and Aqua Des, and uncoated netting did not experience any loss in strength subjected to the same chemical exposure. These findings from application of Aqua Des should also apply to other peracetic acid disinfection chemicals with trade names as for example Perfectoxid and Addi Aqua.
A Preliminary Study of a Rigid Semi-Submersible Fish Farm for Open Seas
Authors: Li Lin , Ong Muk Chen
Published: October 2017

The development of reliable fish farm structures for open seas becomes more and more important. One of the challenges is to design a robust structure to withstand the harsh offshore environmental loads. This paper investigates a semi-submersible type offshore fish farm system for open seas. This system consists of a semi-submersible support structure with pontoons and braces, a catenary mooring system and net cages. The support structure is designed to be rigid to resist severe offshore conditions. A preliminary hydrodynamic and response analysis is carried out for this concept. The linear hydrodynamic properties using different composite models with panel and Morison elements are computed. Based on the hydrodynamic analysis, linearised frequency-domain and coupled time-domain analysis are performed to predict the extreme motions of the support structure and the extreme tensions in the mooring lines. The results indicate that the frequency-domain method underestimates the extreme responses, and the couplings between the structure and the mooring system need to be considered in the time-domain. Responses using various hydrodynamic models are also compared to evaluate the influences of the viscous effects from the pontoons and the nets of this fish farm concept.
Comparison of a Simplified Vessel Response Estimation With a State of the Art Vessel Response Prediction Computer Tool
Authors: Ildstad Jens B. , Kolstad Thomas M. , Halse Karl H.
Published: October 2017

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

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

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