bluuebunny/crossref_metadata_2025 huggingface dataset:
Alternative Environmental Contours for Marine Structural Design: A Comparison Study
Authors: Vanem Erik , Bitner-Gregersen Elzbieta M.
Published: September 2014

A new approach to estimating environmental contours has recently been proposed, where the contours are estimated in the original physical space by Monte Carlo simulations from the joint distribution directly rather than applying the Rosenblatt transformation. In this paper, the new and the traditional approach to estimating the contours are presented and the assumptions on which they are based are discussed. The different results given by these two methods are then compared in a number of case studies. Simultaneous probability density functions are fitted to the joint distribution of significant wave height and wave period for selected ocean locations and, for each area, environmental contours are estimated for both methods. The chosen locations are characterised by different wave climates. Thus, the practical consequences of the choice of approach are assessed. Particular attention is given to mixed sea systems, i.e. a combination of wind sea and swell. In these situations, the new approach for environmental contours may fail to identify realistic conditions along some parts of the contours while for other wave conditions the contours are quite similar. The paper also briefly discusses possible ways of amending the new approach to estimating the contours to obtain more realistic conditions all along the contour lines.
Promoting Safety and Managing Risks in Arctic Marine Transportation: A Best Practices Approach and the Polar Code
Authors: Parsons James , Progoulaki Maria
Published: September 2014

Currently there are no mandatory, internationally accepted rules written specifically to govern ship operations in Arctic waters. The Arctic is recognized by the IMO Guidelines for Ships Operating in Polar Waters as a significant area for international shipping that requires special attention to crew training and operational procedures. A legally binding Polar Code which should identify and unify the commonalities of what it takes, and what is currently enforced by flag states, to safely operate in all Arctic nation waters, is viewed by many as a sensible way forward. This paper presents a synopsis of the results of a funded research project that aimed to identify best practices currently in place by Arctic ship operators. It also aimed to identify possible new ways in which shipping operations in the Arctic could maximize benefits and reduce risks to all stakeholders. A review of the scientific literature clearly highlighted the chasms of knowledge regarding the impacts of marine related activity in Arctic waters. With respect to a methodological approach, a website search was conducted to look for shipping companies informing that they were involved in Arctic operations. The website search of Arctic shipping companies helped with the creation of a questionnaire aimed at identifying “best practices” currently in place with Arctic shipping operators. The questionnaire was administered electronically. Subsequently, a number of companies, identified from the website search and representing a pan-Arctic sample of operators, were contacted by email to inquire if they were interested in taking part in the online questionnaire.
Wave-Induced Accelerations of a Fish-Farm Elastic Floater: Experimental and Numerical Studies
Authors: Li Peng , Faltinsen Odd M. , Greco Marilena
Published: September 2014

Numerical simulations and experiments of an elastic circular collar of a floating fish farm are reported. The floater model without netting structure is moored with nearly horizontal moorings and tested in regular deep-water waves of different steepnesses and periods without current. Local overtopping of waves were observed in steep waves. The focus here is on the vertical accelerations along the floater in the different conditions. The experiments show that higher-order harmonics of the accelerations matter. A 3D weak-scatter model with partly nonlinear effects as well as a 3D linear frequency-domain method based on potential flow are used. From their comparison against the measurements, strong 3D and frequency dependency effects as well as flexible floater motions matter. The weak-scatter model can only partly explain the nonlinearities present in the measured accelerations.
Hydrodynamic Analysis of Flapping-Foil Thruster Operating in Random Waves
Authors: Filippas E. S. , Belibassakis K. A.
Published: September 2014

Oscillating foils located beneath the ship’s hull are investigated an unsteady thrusters, augmenting the overall propulsion of the ship in rough seas and offering dynamic roll stabilization. The foil undergoes a combined oscillatory motion in the presence of waves. For the system in the horizontal arrangement the vertical heaving motion of the hydrofoil is induced by the motion of the ship in waves, essentially ship heave and pitch, while the rotational pitching motion of the foil about its pivot axis is set by an active control mechanism. In previous works, a potential-based panel method has been developed for the detailed investigation of the effects of free surface in harmonic waves, and the results are found to be in good agreement with numerical predictions from other methods and experimental data. Also, it has been demonstrated that significant energy can be extracted from the waves. In the present work we examine further the possibility of energy extraction under random wave conditions using active pitch control. More specifically, we consider operation of the foil in head waves characterized by a given frequency spectrum, corresponding to specific sea states. The effects of the wavy free surface are taken into account through the satisfaction of the corresponding boundary conditions. Numerical results concerning thrust coefficient are shown, indicating that significant efficiency can be obtained under optimal operating conditions. Thus, the present method can serve as a useful tool for the preliminary design, assessment and optimum control of such systems extracting energy from sea waves and augmenting marine propulsion.
Simulation and Validation of a Numerical Model of a Full Aquaculture Net-Cage System
Authors: Endresen Per Christian , Birkevold Jens , Føre Martin , Fredheim Arne , Kristiansen David , Lader Pål
Published: September 2014

Numerical simulation models are useful tools for the design and capacity analyses of cage-based fish farm systems. To ensure that such tools produce realistic estimates on forces and deformations experienced by fish farms, it is important to validate the models through comparison with experiments. A recent experiment investigated the response of a scaled model of a full aquaculture net cage placed in a mooring system when exposed to waves and current. In this study, a numerical model of this system containing the main components used in the physical experiments was set up and simulated. After simulations the tension in anchor lines, bridles and buoys were compared to the corresponding data series obtained in the experiments. The comparison indicated that FhSim was able to reproduce the main dynamics and responses of the physical model when exposed to currents and waves. Furthermore, a sensitivity analysis was conducted, aimed at investigating how much model output is affected by variations in the stiffness of the mooring system.
Experimental Study on Vortex-Induced Vibration of Floating Circular Cylinders With Low Aspect Ratio
Authors: Gonçalves Rodolfo T. , Fujarra André L. C.
Published: September 2014

Experiments regarding vortex-induced vibration on floating circular cylinders with low aspect ratio were carried out in a recirculation water channel. The floating circular cylinders were elastic supported by a set of linear springs to provide low structural damping on the system. Eight different aspect ratios were tested, namely L/D = 0.2, 0.3, 0.4, 0.5, 0.75, 1.0, 1.5 and 2.0. These aspect ratios were selected to cover the aspect ratio range of the main offshore circular platforms, such as spar and monocolumn. The aims were understanding the VIM of such platforms; due to this, the cylinders were floating, or m* = 1. The range of Reynolds number covered 2,800 < Re < 55,400. The amplitude results showed a decrease in amplitude with decreasing aspect ratio in both directions. The frequency results confirm a different behavior for cylinders with L/D ≤ 0.5; in these cases, the cylinder free-end effects were predominant. The resonant behaviour was no longer observed for L/D ≤ 0.2. The decrease in Strouhal number with decreasing aspect ratio is also verified. All the results presented here complement the work presented previously for stationary circular cylinder with low aspect ratio presented by Gonçalves et al. (2013), Experimental Study on Flow around Circular Cylinders with Low Aspect Ratio, OMAE2013-10454.
Analysis of Hydrodynamic Resonant Effects in Side-by-Side Configuration
Authors: Dinoi Pasquale , Watai Rafael A. , Ramos-Castro Hugo , Gómez-Goñi Jesus , Ruggeri Felipe , Souto-Iglesias Antonio , Simos Alexandre N.
Published: September 2014

Seakeeping behavior of a multibody system in side-by-side configuration in head sea condition is discussed in this paper. The system, which can be assimilated to a FLNG and LNG carrier during an offloading operation is composed of a barge and a prismatic geosim with two gap values. Seakeeping tests in regular waves have been performed in the model basin of CEHINAV-Technical University of Madrid (UPM). The movements for the geosim were restricted to the surge, heave and pitch motions (on the vertical plane), whereas the barge was kept fixed. In this way the gap remained constant during the tests. Numerical modeling has been undertaken using WAMIT and an in-house time-domain Rankine Panel Method (TDRPM). Response amplitude operators in terms of movements and wave amplitude in the gap obtained from seakeeping test and numerical models are documented in the paper, illustrating the limitation of the numerical codes regarding the modeling of this hydrodynamic problem. Numerical results indicate a resonant behavior of the waves in the gap for a range of frequencies, with amplitudes much higher than those observed during the tests. Due to the small distances considered in the experiments, these resonant waves are related to longitudinal wave modes in the gap. In order to overcome this problem, a procedure for introducing an external damping factor that attenuates the wave amplitude along the gap in the time-domain RPM is evaluated based on the experimental data.
Density and Drag Reduction With Hollow Glass Additives
Authors: Kutlu Bahri , Ozbayoglu Evren M. , Miska Stefan Z. , Takach Nicholas , Yu Mengjiao , Mata Clara
Published: September 2014

This study concentrates on the use of materials known as hollow glass spheres, also known as glass bubbles, to reduce the drilling fluid density below the base fluid density without introducing a compressible phase to the wellbore. Four types of lightweight glass spheres with different physical properties were tested for their impact on rheological behavior, density reduction effect, survival ratio at elevated pressures and hydraulic drag reduction effect when mixed with water based fluids. A Fann75 HPHT viscometer and a flow loop were used for the experiments. Results show that glass spheres successfully reduce the density of the base drilling fluid while maintaining an average of 0.93 survival ratio, the rheological behavior of the tested fluids at elevated concentrations of glass bubbles is similar to the rheological behavior of conventional drilling fluids and hydraulic drag reduction is present up to certain concentrations. All results were integrated into hydraulics calculations for a wellbore scenario that accounts for the effect of temperature and pressure on rheological properties, as well as the effect of glass bubble concentration on mud temperature distribution along the wellbore. The effect of drag reduction was also considered in the calculations.
Numerical Simulation and Experimental Research on Dewatering Hydrocyclone
Authors: Li Feng , Zhang Xiaoli , Liang Wenyi , Ren Lixia
Published: September 2014

Offshore oilfield generally has the characteristics such as high oil content of produced liquid and limited platform space etc, so it is urgent to design a kind of dewatering processing equipment with the following advantages such as small volume, high efficiency and low energy consumption etc. And the dewatering hydrocyclone studied in this paper is such a device which is mainly applied for the dehydration of crude oil in offshore oilfield exploration and development. Hydrocyclone named as the Thew type with double cones double inlets is taken as the initial model, meanwhile the physical medium parameters close to oil pipe of Bohai Sea is selected. Applying computational fluid dynamics (CFD) method, and by means of the Reynolds Stress Model and SIMPLEC algorithm in the FLUENT analysis software, the influence of structural parameters and operating parameters on the pressure field, velocity field and oil phase distribution of dewatering hydrocyclone is analyzed systematically. Some structural parameters, including swirl chamber diameter, underflow pipe length, large cone angle, small cone angle, the diameter of the overflow pipe, overflow pipe length and inlet section size etc, are optimized. A kind of main structure of dewatering hydrocyclone is determined, which can deal with about 40% of the oil-water mixtures for dewatering processing, and the comprehensive separation efficiency is higher. The numerical simulations and the experimental research results in this paper have a great influence on the advanced study and application of dewatering hydrocyclone and provide certain theory evidence for further research on flow field analysis and design improvement.
An Analysis of Slurry Transport at Low Line Speeds
Authors: Miedema Sape A.
Published: September 2014

In Deep Sea Mining, material will be excavated at the sea floor and transported to the surface. This transport always consists of horizontal and vertical transport and can be carried out mechanically or hydraulically. If the transport is hydraulically, during the horizontal transport there is a danger of bed formation and plugging the line. This is similar to the horizontal transport in dredging with the difference that in deep-sea mining the line length is much smaller, but also the line speeds may be smaller. To avoid plugging the line, the line speed has to be higher than a certain critical line speed. In literature there are many theories about this critical line speed and about bed forming in the pipe, but these theories are usually empirical and cannot be applied under all circumstances. Different particles sizes, pipe diameters and line speeds require different equations, although a generic theory should cover everything. For the critical velocity different definitions exist. Some researchers use the definition that above the critical velocity no bed, either stationary or sliding, exits. This definition is also referred to as the limit deposit velocity. Others use the transition between a stationary bed and a sliding bed as the definition of the critical velocity. Whatever definition is used, the Moody friction factor on the bed always plays an important role. Since in literature no explicit formulation for this Moody friction factor exists, an attempt is made to find an explicit formulation for the Moody friction factor for the interface of the fluid flow and the bed, where this interface consists of sheet flow.
Sea-Keeping Analysis of an Offshore Wind Farm Installation Vessel During the Jack-Up Process
Authors: Augener Philip H. , Hatecke Hannes
Published: September 2014

Offshore wind farms are not planned in sheltered and shallow waters any longer. Especially in the North Sea there exist many approved offshore wind farm projects at water depth between 30 and 50 meters. In particular the installation process of these projects is strongly influenced by weather conditions and the sea-keeping capabilities of the installation vessels. For reliable planning of the entire project, not only the weather statistics, but also the vessel’s sea-keeping capabilities need to be known accurately. For this purpose different kinds of sea-keeping analyses can be conducted. In this paper a sea-keeping analysis is presented, where the focus is upon the jack-up process. For the numerical computation the sea-keeping code E4ROLLS is applied. The results of this sea-keeping analysis are operational limitations for the jack-up process, caused by two different criteria derived from jack-up classification requirements.
Case Studies From Implementation of the First Commercial Leasing Processes for Areas of the Outer Continental Shelf (OCS) Under the U.S. Renewable Energy Regulations, 30 CFR 585
Authors: Trager Erin C.
Published: September 2014

President Barack Obama announced on April 22, 2009 that the U.S. Interior Department (USDOI) had completed the Final Renewable Energy Framework or rulemaking process (i.e., regulations) to govern management of the Renewable Energy Program for the U.S. Outer Continental Shelf (OCS). The Bureau of Ocean Energy Management (BOEM) within USDOI is responsible for overseeing the implementation of this framework. It establishes a program to grant leases, easements, and rights-of-way for orderly, safe, and environmentally responsible renewable energy development activities, such as the siting and construction of offshore wind generating facilities on the OCS, as well as other forms of renewable energy such as wave, current, and solar.

To date, five commercial wind energy leases have been issued for areas on the U.S. OCS and several other areas are at various stages in the process leading to leasing. The leases issued for two states in particular — Delaware and Virginia — help serve as case studies for how the U.S. regulatory process for ocean wind leasing operates. This paper will discuss the process by which these leases were issued and next steps, which will illustrate the implementation and best practices of the U.S. regulatory process for offshore wind leasing.
The Key Mechanics Principles for Vortex-Induced Vibrations
Authors: Zueck Robert F. , Palo Paul A.
Published: September 2014

Vortex-Induced Vibrations (VIV) are cyclic motions in flexible slender structures that are induced by the shedding of vortices mostly transverse to the length of the structural member. The authors contend that the complete three-dimensional (3D) geometric changes that evolve in the structure are too often overlooked when investigating the basic physical nature of VIV. In this paper, we use a physics-based numerical model of a cable (a specific type of slender structure) to demonstrate the following mechanics principles:

• Fluid drag (or other field loading) results in 3D geometric changes in a suspended cable.

• Those changes necessarily include a lack of structural stiffness in the cable transverse to fluid flow.

• In those instances, the structure will inevitably deflect to any kind of transverse action, particularly fluid vortices.

• The nonlinear nature of this mechanism allows VIV to occur over a range of time periods, often called “lock-in.”

Although the vortex shedding in the fluid provides the necessary repetitive inducement action, the evolving geometry of flexible slender structure appears to be the dominant factor concerning the actual nature (amplitude, pattern, etc.) of the resulting VIV behavior.
Experimental Study of Bubbles Sweep-Down on Oceanographic Research Vessels
Authors: Delacroix Sylvain , Germain Grégory , Billard Jean-Yves , Gaurier Benoit , Facq Jean-Valery , Bacchetti Thomas
Published: September 2014

In this work, a specific experimental protocol is developed to study the bubble sweep-down phenomenon on oceanographic research vessel. The combination of the utilization of a wave and current flume tank and an hexapod allows the simulation of conditions under which the phenomenon can be observed at full scale. Visualizations and PIV measurement results both obtained in the bow vicinity of the Ifremer oceanographic vessel Pourquoi pas? are presented in this paper.
Coupled Dynamic Response of a Spar Type Floating Offshore Wind Turbine
Authors: Peng Cheng , Yan Fasuo , Zhang Jun
Published: September 2014

FOWTs (Floating Offshore Wind Turbine) are feasible renewable devices to harness the wind energy in the near future. However, because of the complicated interactions among wind turbine, mooring system and the hull, the motion of a FOWT under the impact of severe wind, wave and current has not been well studied yet. This research focuses on the coupled numerical analysis of a FOWT. A numerical code COUPLE-FAST is developed by integrating two existing codes, namely, COUPLE and FAST, to carry out the task.

In this study, a particular FOWT model is chosen for the numerical simulation, which consists of a NREL 5-MW baseline wind turbine and OC3-Hywind Spar. Although the numerical simulation is limited to this particular type of FOWTs, the results and related code (COUPLE-FAST) may be helpful to the design of FOWTs in the future.
Coupled Analysis of SCR and Flowline Under High Pressure and High Temperature
Authors: Luk C. H. , Qi Xinhai , Zhong Jianxia
Published: September 2014

This paper presents a coupled design analysis for a Pipe-in-Pipe (PIP) Steel Catenary Riser (SCR) and Flowline (FL) system in 5000ft of water under high pressure and high temperature (HP/HT) conditions in the Gulf of Mexico (GOM).

The finite element program ABAQUS is used to model the inner and outer pipes of the PIP system, the centralizer, the SCR hangoff, the flowline lateral and end supports, and pipe-soil interactions on the seabed. Thermal stresses and stress ranges caused by repeated temperature transients, axial and lateral displacements of SCR and flowline, as well as by the dynamic responses of the coupled riser and flowline system, are presented. Comparisons are also made with results by ABAQUS models with beam and pipe elements, and results obtained by coupled and de-coupled analysis. The sample vessel motion was obtained for an example Spar platform in GOM.
Flexible Flapping Dynamics of Parallel Elastic Plates in a Uniform Flow: Application to Energy Harvesting Devices
Authors: Gurugubelli Pardha S. , Jaiman Rajeev K. , Khoo Boo Cheong
Published: September 2014

A new high-order finite element Coupled Field with Explicit Interface (CFEI) code has been developed for simulating flapping motion of a thin flexible body in a uniform flow with strong add-mass effects. In the first part, we study the flapping dynamics of a single cantilevered plate for wide range of mass ratios and maintaining relatively low bending rigidity through our direct fluid-structure simulations. As a function of mass-ratio, the flapping dynamics reveals three distinct regimes: (i) fixed-point stable; (ii) limit-cycle flapping; and (iii) chaotic flapping. The changes associated with regime transition with increasing mass ratio are analyzed by vortex wake patterns and tip-displacement responses. Dependencies of stability predicted by the theoretical analysis are confirmed by the nonlinear fluid-structure simulations. In the second part, two parallel cantilevered plates will be investigated as a function of spacing between the parallel plates to assess the flapping motion and the net energy transfer. The flow-induced vibrations of this kind of coupled system have a potential to extract energy from the surrounding fluid flow for generation of electric power.
Numerical and Experimental Study of the Flow Around Two Ship Sections Side-by-Side
Authors: Arslan Tufan , Visscher Jan , Pettersen Bjørnar , Andersson Helge I. , Muthanna Chittiappa
Published: September 2014

This paper reports calculations of three dimensional (3D) unsteady cross flow over two ship sections in close proximity and compares the results with measurements. The ship sections have different breadth and draft, and represent typical situations in a ship-to-ship marine operation in a cross current. The behavior of the vortex-shedding around the two different ship hull sections is investigated numerically by CFD methods and experiments. For the two sections, simulations are done for several Reynolds numbers by using the dynamic Smagorinsky Large Eddy Simulation (LES) turbulence model. Finally the cross flow past the ship sections in side-by-side position is simulated and vortex interaction between the sections is found by using the software (Ansys) FLUENT. The numerical predictions are compared with PIV results taken in a circulating water tunnel.
Fusion Zone Microstructure Associated With Embrittlement of Subsea Dissimilar Joints
Authors: Dodge M. F. , Dong H. B. , Gittos M. F. , Mobberley T.
Published: September 2014

Within subsea oil and gas systems, nickel alloy filler metals are commonly used in the joining of ferritic steels with different mechanical properties. An example of this is the joining of low alloy steel (LAS) forged manifold hubs (e.g 8630 or F22) to low hardenability pipelines steels, such as X65). The joint is part of a two stage welding process that simplifies offshore installation. Initially, a weld deposit is made on the forging using a multi-pass ‘buttering’ technique, providing intermediate layers of a suitable material, such as Alloy 625. A postweld heat treatment (PWHT) is applied to the buttered forging, onshore, to temper the hard heat affected zone (HAZ). After machining a bevel into the buttering layer, a closure weld, applied offshore, is employed to join the pipeline to the forging. As the buttering layer and linepipe are not critically hardened by the closure weld, no PWHT is required.

To prevent corrosion, subsea systems of this kind, are subjected to cathodic protection, via aluminium based anodes. Whilst successful in protecting the ferritic parts within the manifold structure, a number of high profile failures has been attributed to the evolution and ingress of hydrogen, and its diffusion to, the fusion zone of the dissimilar weld.

To investigate the susceptibility of fusion zone microstructures to hydrogen embrittlement, three dissimilar weld samples were fabricated: 8630-Alloy 625, F22-Alloy 625 and F22-309LSi. The latter is not a combination normally used to complete joins of this type. Each specimen was the subject of thermodynamic and kinetic modelling studies using Thermo-Calc™ and Dictra™, with microscopic examination by scanning electron microscopy (SEM) combined with energy-dispersive X-ray (EDX) and electron backscattered diffraction (EBSD). Nano-scale features were also investigated by transmission electron microscopy (TEM). By comparing the fusion zones of the three different joints in both as-welded and heat-treated conditions, the metallurgical aspects relating to hydrogen embrittlement are revealed. The results indicate that precipitates, which form in a zone of carbon diffusion during PWHT, are particularly harmful in the presence of hydrogen. The formation of this hydrogen susceptible region is partly due to the initial solidification structure morphology, the formation of which is also discussed.
Evaluation of Pressure Change While Steady-State Tripping
Authors: Podryabinkin Evgeny , Ahmed Ramadan , Tarasevich Vladimir , May Roland
Published: September 2014

Excessive tripping speed in an uncased borehole increases the risk of having formation damage or influx of formation fluid (kick). However, if the tripping is performed at lower speeds, the operation requires more rig time. Hence, increased trip speed cuts expensive rig time. These opposing goals require thorough planning and optimization of the tripping operation to avoid operational problems and reduce financial expenditures. To maximize the tripping speed, accurate prediction of the pressure change occurring due to the axial pipe movement (surge or swab pressure) is necessary. The pressure change is influenced by the hole and string diameters, eccentricity, fluid properties and trip speed. The tripping speed is one of the operational parameters, which are regularly adjusted at the rig site.

Analytical solutions exist only for special scenarios. The semi-analytical models for calculation of the steady-state pressure change cannot provide accurate predictions. They are mostly based on disputable assumptions which make the model to underestimate the pressure change. Most of the existing models are based on the parallel-plate approximation of the annular geometry. In another approach, the parameter, which reflects the amount of fluid which is dragged the direction of the string, assumed to be constant or calculated independent of the fluid viscosity.

In this paper, accurate solutions were obtained from direct numerical simulation of flow in a cylindrical annulus with axial movement of the inner cylinder. The numerical algorithm is based on finite volume method and incorporates laminar flows of Newtonian, Power Law, Bingham Plastic and Herschel-Bulkley fluids. The method predicts the pressure change occurring in concentric and eccentric annuli with and without rotation of the inner cylinder. The goals of this work are to: i) study the influence of the eccentricity, fluid properties and tripping speed on the pressure change; and ii) evaluate the accuracy of the simplified approaches by comparing experimental data and numerical solutions, and determine their validity ranges.

This paper presents a new method for finding trip-caused pressure change in the wellbore through systematic analysis of the numerical solutions. Parametric study was performed to examine the effects of different influential parameters on the pressure change. In addition, the results obtained from the numerical method are compared with the simplified solutions and the discrepancies are analyzed to show the improved accuracy of the new method.
Statistical Estimation of Extreme Roll in Head Seas
Authors: Kim Dae-Hyun , Belenky Vadim , Campbell Bradley L. , Troesch Armin W.
Published: September 2014

The objective of this paper is to present three probabilistic methods useful in estimating the severity of extreme roll responses in short crested irregular head seas. Underlying assumptions are examined and strengths and potential weaknesses are identified. The methods reviewed here are the statistical extrapolation of extreme value distributions, the Envelope Peaks Over Threshold method (EPOT), and the Design Loads Generator (DLG) approach.
Structural Analysis of a Hybrid Substructure With Multi-Cylinder for 5MW Offshore Wind Turbines
Authors: Park Min-Su , Jeong Youn-Ju , You Young-Jun
Published: September 2014

The substructure for offshore wind turbines is strongly influenced by the effect of wave forces as the size of substructure increases. Therefore, it is very important to reduce the wave force acting on substructures. In the present study the hybrid substructure, which is composed of a multi-cylinder having different radius near free surface and a gravity substructure at the bottom of multi-cylinder, is suggested to reduce the wave forces. The fluid domain is divided into two regions to calculate the wave forces acting on the hybrid substructure with multi-cylinder and the scattering wave in each fluid region is expressed by an Eigen-function expansion method. The comparison between the mono pile and the hybrid substructure is made for wave forces. Using the wave forces obtained from this study, the structural analysis of hybrid substructure is carried out through ANSYS mechanical. In order to investigate the resonance between the wind turbine and the hybrid substructure, the modal analysis is also carried out.
Ice Condition Database for the Arctic Sea
Authors: Tõns Tõnis , Erceg Sandro , Ehlers Sören , Leira Bernt Johan
Published: September 2014

Increasing trends in the arctic sea transport lead to the necessity to determine route specific ice conditions to ensure safe vessel transits. In order to achieve this, the comprehensive understanding of ice conditions from the past should be used to predict future trends for arctic sea ice conditions.

This paper presents the development of such ice conditions database with the implementation of a satellite data source, which will become a basis for the determination of route specific ice conditions. A case study is performed in order to show how the database could be used to collect the route specific ice data for statistical analyses.
RANS Simulation on Hydrodynamic Forces of a Horizontal Cylinder in Focused Steep Waves and Current
Authors: Bai Junli , Ma Ning , Gu Xiechong
Published: September 2014

The wave and current loads are the primary loads acting on the offshore structures. Rogue wave, a typical steep wave, is considered to occur due to wave focusing and wave-current interaction in the ocean and becomes one of the major causes of offshore structure damage. In this study, the hydrodynamic forces on horizontal cylinders exerted by the focused steep waves has been investigated considering wave-current interaction. The RANS equations solving by finite volume method are applied to evaluate the strongly nonlinear interaction between waves and current, in which the VOF method is adopted to capture the free surface. The velocity and water depth are given at the inlet boundary of the computation domain to generate deep water focused wave. In this paper, the wave forces acting on the cylinder in focused waves without current are investigated firstly. The wave forces are simulated for different horizontal distances between the cylinder and the pre-designed wave concentration point, and the maximal wave forces are analyzed. Meanwhile, the effects of the cylinder on wave deformation are also discussed. To validate the numerical model, the simulation results of wave forces on a cylinder by regular wave and the water surface elevation of a focused wave are compared with published experiment and simulated results. Then, the nonlinear interaction between the focused wave and current are investigated, the hydrodynamic forces acting on cylinder are simulated for different current velocities. Accordingly, the nonlinear effects of wave-current interaction on the hydrodynamic forces are discussed with respect to the results of wave deformation at concentration point, forces under actions of focused wave and combined wave-current conditions.
Challenges in Modelling Propeller-Ice Interaction
Authors: Vroegrijk Erik A. J. , Carlton John S.
Published: September 2014

With industry gearing up to sail more frequently in ice-infested waters, so rises the demand for Class Societies to have a deeper technical understanding of different ice-interaction scenarios. This paper focusses on propeller-ice interaction and is predominantly based on the author’s Master thesis at Delft University of Technology [9]. It discusses the different physical processes that govern propeller-ice interaction from an engineering perspective and highlights the areas in which further research is desired. Using conservative 2D Computational Fluid Dynamics (CFD) computations it is shown that the hydrodynamic forces acting on a propeller blade, during a propeller-ice interaction event, are at least an order smaller than the mechanical ice breaking forces. Given the comparability of propeller-ice interaction with bird strikes in jet engines, from a numerical perspective, it is argued to use explicit Finite Element Method (FEM) codes to solve the mechanical interaction loads. This paper proposes a basis for a new material model, potentially capable of describing the orthotropic rate-dependent properties of sea ice. It represents an engineering approach, aimed at delivering reliable answers within commercially acceptable time limits and expenditure.
Flow Energy Harvesting With Piezoelectric Flags
Authors: Doaré Olivier , Michelin Sébastien , Pineirua Miguel , Xia Yifan
Published: September 2014

In this article, energy harvesting with a fluttering cantilevered plate covered by piezoelectric patches in an axial flow is adressed. A theoretical model is presented which is then discretized and numerically integrated to perform a parametric study of the energy harvesting efficiency of the system. When one, two or three piezoelectric patches cover the plate, the optimal distributions of the patches that maximize the efficiency are obtained. Experimental results are presented, which are in good agreement with the model. When a significantly high number of patches of small size are considered, a continuous model is used to study the influence of a resonant harvesting circuit. A lock-in phenomenon is evidenced, which is able to significantly increase the efficiency.
The Shielding Effect of Drilling Fluids on MWD Downhole Compasses: The Effect of Drilling Fluid Composition, Contaminants and Rheology
Authors: Saasen Arild , Ding Songxiong , Amundsen Per Amund , Tellefsen Kristoffer
Published: September 2014

Materials such as added clays, weight materials, drill solids and metalic wear products in the drilling fluid are known to distort the geomagnetic field at the location of the Measurement While Drilling (MWD) tool magnetometers that are used to measure the direction of well path. This distortion contributes to substantial errors in determination of azimuth while drilling deviated wells. These errors may result in missing the target of a long deviated 12 ¼″ section in the range of 1–200m; representing a significant cost to be mitigated. The error becomes even more pronounced if drilling occurs in arctic regions close to the magnetic North Pole (or South Pole). The effect on the magnetometer readings is obviously linked to the kinds and amounts of magnetic materials in the drilling fluid. The problem has recently been studied by laboratory experiments and analyses of downhole survey data.

A series of experiments has been carried out to understand how some drilling fluid additives relate to the magnetic distortion. Experiments with free iron ions show that presence of iron ions does not contribute to magnetic distortion; while experiments with bentonite-based fluids show a strong effect of bentonite on magnetic shielding. Albeit earlier measurements showing a strong dependency of the content of organophilic clay, clean laboratory prepared oil-based drilling fluids show no increased shielding when adding organophilic hectorite clays. The anticipated difference between these two cases is outlined in the paper. When eroded steel from an offshore drilling site is added into the oil-based drilling fluid, it is found that these swarf and steel fines significantly increase the magnetic shielding of the drilling fluid. The paper outlines how the drilling direction may be distorted by the presence of these additives and contaminants and how this relates to the rheological properties of the drilling fluid.
Hybrid Riser Tower Design: Evolution, Operational Efficiency and Compliance to Ultra Deep Offshore Challenge
Authors: Gueveneux Hervé , Le Buhan Philippe
Published: September 2014

The riser system selection and sizing is one of the most challenging design issues of the subsea world specifically for deepwater applications. Bundle riser towers have been used for few deep water developments since the late 1990s and have demonstrated their capacity to meet the operational constraints in terms of field layout congestion, flow assurance, mechanical design, fabrication and installation etc.

From the pioneer application in the Gulf Of Mexico (Green Canyon 29 – 1988) to the 2nd generation systems in Western Africa (GIRASSOL to Rosa or CLOV), the design of Hybrid Riser Towers has evolved to improve mechanical robustness, installation efficiency and operability. This article reviews the past evolutions and gives an overall view of the performance achieved so far, mainly in West African sectors. The move of the offshore industry towards ultra-deep (WD > 1800m) and/or marginal fields may trigger some additional design evolutions which are highlighted in the article, with a focus on the water depth increase considering environmental conditions met on previous projects. The feasibility in harsher conditions is not addressed in this document.

In addition, the challenges raised by the engineering of such deepwater Hybrid Riser Tower concepts are discussed:

• Structural models of different scales shall be used to properly account for potential couplings between the global behavior of the whole tower with the local responses of each individual component;

• Top and bottom assemblies as well as the rigid or flexible spools, the piping and connectors shall be introduced into the FE models to define correct boundary conditions to the HRT;

• Slug or severe internal flows shall be considered for fatigue issues requiring complex models which combine CFD and structural calculations;

• Risks of hydrodynamic instabilities shall be assessed like plunge and torsion galloping for non-cylindrical sections and VIV for bare peripherals;

• A specific attention shall be paid to the fatigue budget spent from towing and Installation to the In-place configuration.
An Experimental Study for Fatigue Performance of 7% Nickel Steels for Type B LNG Carriers
Authors: Kim Young Woo , Lee Jae Myung , Kim Myung Hyun , Noh Byeong Jae , Sung Hee Joon , Ando Ryuichi , Matsumoto Toshiyuki
Published: September 2014

The most important issue associated with liquefied natural gas storage systems, such as LNG carriers, LNG FPSO and FLNG, is the structural safety. One of the most common materials for the LNG storage systems has been 9% nickel steel over the last 50 years as it has excellent mechanical properties under cryogenic temperature. Recently, the consideration for lowering the nickel content becomes necessary due to the increase of the nickel price and the high price of nickel based welding consumables. In this respect, 7% nickel steels are developed for cryogenic applications. Nippon Steel and Sumitomo Metals Corporation have developed 7% nickel steels with improved toughness comparable to that of 9% nickel steels by TMCP and micro-alloying technology. The major objective of this study is to evaluate the fatigue performance of 7% nickel steels with a special attention to Type B LNG carrier applications. Cyclic fatigue and fatigue crack growth rate (FCGR) tests for 7% nickel steels were conducted at room and cryogenic temperature. Fatigue tests were carried out with three types of specimens such as base metal, butt weld and fillet weld to characterize the fatigue properties. Also FCGR tests were carried out using compact tension specimens. The difference of FCGR characteristics among base, weld and HAZ (Heat Affected Zone) are investigated for three types of specimens. The results of 7% nickel steels are evaluated and compared with those of 9% nickel steels.
A Fully Nonlinear RANS-VOF Numerical Wavetank Applied in the Analysis of Green Water on FPSO in Waves
Authors: Östman Anders , Pakozdi Csaba , Sileo Lucia , Stansberg Carl-Trygve , de Carvalho e Silva Daniel Fonseca
Published: September 2014

This paper presents numerical simulations of Green Water events and wave impact on a FPSO. The simulations are performed at model scale and the results are compared against experimental model test results. The commercial Star-CCM+ CFD software is used in the simulations. The incoming waves are modeled using 5th order Stokes theory, as implemented in the CFD software. Both fixed and free floating FPSO are considered. The moving FPSO are modeled using Chimera overset mesh technology. The vessels is free to move in heave and pitch at 180 (head sea), roll and heave at 270 (beam sea), while roll, pitch and heave is released at 225 (quartering sea). The computed water height on the deck and the relative wave height in vicinity the vessel are compared against model test results at several positions. Also the impact force on load cells blocks located at the deck of the vessel is computed and compared against model test results. The comparison of the time histories of the water elevation and load histories are in reasonable agreement with the measured time series. The number of grid cells range from 7M for the simulations at head sea, where flow is assumed to be symmetric, to 21M for the simulations at quartering sea. Total wall clock simulation time was about 10days for the most computationally demanding cases, which are the quartering sea simulations. This includes simulation of 12 wave periods with the ship fixed, and thereafter 8 wave periods of the free floating vessel. The computations show that CFD tools can be used as a research tool when studying the physics of green water and wave impact events. However, due to time CPU demanding simulations, this type of CFD analysis are not yet a practical tool for parametric design studies and deck structure optimizations. This work is a part of the research project “Green Water and Wave Impact on FPSO” carried out for and in cooperation with PETROBRAS.
Coupled Simulation of Oscillatory Flow, Sediment Transport and Morphology Evolution of Ripples Based on the Immersed Boundary Method
Authors: Leftheriotis Georgios A. , Dimas Athanassios A.
Published: September 2014

In the present study, numerical simulations of oscillatory flow over a rippled bottom, coupled with bed and suspended sediment transport, as well as the resulting morphology evolution, are performed. The simulations are based on the numerical solution of the Navier-Stokes equations and the advection-diffusion equation for the suspended load, while empirical formulas are used for the bed load. The bed morphological evolution is obtained by the numerical solution of the conservation of sediment mass equation. A fractional time-step scheme is used for the temporal discretization, while finite differences are used for the spatial discretization on a Cartesian grid. The Immersed Boundary method is implemented for the imposition of fluid and sediment boundary conditions on the ripple surface. Two types of ripples are examined, i.e., ripples of parabolic shape with sharp crests and sinusoidal ripples, and cases of ripple length to orbital motion amplitude ratio of 1.6 and ripple height to orbital motion amplitude ratios of 0.16, 0.20 and 0.24, at Reynolds number equal to 5×103. The effect of ripple steepness and ripple shape on suspended sediment and ripple migration is discussed.