bluuebunny/crossref_metadata_2025 huggingface dataset:
An Overview of Theories Describing Head Losses in Slurry Transport: A Tribute to Some of the Early Researchers
Authors: Miedema Sape A.
Published: November 2013

In dredging, the hydraulic transport of solids is one of the most important processes. Since the 50’s many researchers have tried to create a physical mathematical model in order to predict the head losses in slurry transport. One can think of the models of Durand and Condolios (1952) and Durand (1953), Worster and Denny (1955), Newitt et al. (1955), Gibert (1960), Fuhrboter (1961), Jufin and Lopatin (1966), Zandi and Govatos (1967) and Zandi (1971), Turian and Yuan (1977), Doron et al. (1987) and Doron and Barnea (1993), Wilson et al. (1992) and Matousek (1997). Some models are based on phenomenological relations and thus result in semi empirical relations, other tried to create models based on physics, like the two and three layer models. It is however the question whether slurry transport can be modeled this way at all. Observations in our laboratory show a process which is often non-stationary with respect to time and space. Different physics occur depending on the line speed, particle diameter, concentration and pipe diameter. These physics are often named flow regimes; fixed bed, shearing bed, sliding bed, heterogeneous transport and (pseudo) homogeneous transport. It is also possible that more regimes occur at the same time, like, a fixed bed in the bottom layer with heterogeneous transport in the top layer. It is the observation of the author that researchers often focus on a detail and sub-optimize their model, which results in a model that can only be applied for the parameters used for their experiments.
Experimental Results of Cooperative Control Applied to Multi-Vessel DP Operations
Authors: Queiroz Filho Asdrubal N. , Tannuri Eduardo A.
Published: November 2013

Offshore operations involving several floating units are becoming more frequent nowadays. Such operations are used for sub-sea equipment installation and undersea structures launching, for example. This kind of operations requires a high level of coordination between the vessels, which today is made without the ship’s information exchange, being each ship individually commanded. Therefore, in those cases a cooperative control could be applied, ensuring that the relative distance between the ships are maintained in limited range, controlling operational parameters such as the lifting line traction. The benefits of this control are shown when compared to the non cooperative control by means of a experimental setup with two DP vessels.
Simulation of Strong Inertial Currents to Support Preliminary Engineering Offshore Namibia
Authors: Jeans Gus , Wade Ian
Published: November 2013

The vertical profile of ocean current needs to be characterised for preliminary engineering applications such as exploration drilling. The acquisition of suitable current profile data can be a key challenge, especially in relatively unexplored deepwater frontier regions. Recent years have seen a dramatic increase in the use of ocean current models for this purpose, frequently involving freely available data from the HYCOM consortium. These data have proven reliable in some locations, but the accuracy remains questionable, or unproven, in many others. This paper describes a study in which it was clear from the outset that HYCOM would not represent dominant features of the local current regime. This concerned the region offshore Namibia, near 30°S, at which latitude the inertial period of natural ocean oscillation equals the diurnal period of 24 hours. This region is also subjected to relatively strong diurnal sea breezes, which can drive nearly resonant inertial responses in the ocean, which can, in turn, dominate the current regime. The spatial resolution of the wind field used to drive the global HYCOM model is insufficient to resolve this critical wind forcing, so the resulting model currents cannot represent the dominant features. Fortunately some relevant measurements were well documented in the public domain, from which a pragmatic inertial current simulation methodology was developed. Lack of inertial energy was actually beneficial for using HYCOM daily spot values to represent other, longer term, components of the flow. If inertial currents had been represented in the model, then more frequent values would be required to capture them. HYCOM data proved useful for representing long term inter-annual variability in the features it does represent, including meanders in the seasonal Benguala current.
Two Variational Stereo Methods for Space-Time Measurements of Ocean Waves
Authors: Gallego Guillermo , Yezzi Anthony , Fedele Francesco , Benetazzo Alvise
Published: November 2013

In recent years, the study of the dynamics of ocean waves via stereo vision systems has focused on investigating sea states with wavelengths in the range of 0.01 m to 1 m. In this work, we present two modern stereo variational approaches, viz. the disparity and graph methods, for the estimation of the ocean surface displacements over larger spatial scales of 10 to 100 m. The disparity method estimates the disparity between images as an intermediate step toward retrieving the depth of the waves with respect to the cameras, and the graph method estimates the ocean surface displacements directly in 3-D space. The pros and cons of both methods are compared and their performance is evaluated using experimental video data collected at an offshore platform in the Black Sea.
Drilling Riser Analysis During Installation of a Wellhead Equipment
Authors: Sevillano Lucas C. , Morooka Celso K. , Mendes Jose Ricardo P. , Miura Kazuo
Published: November 2013

In offshore petroleum drilling, the BOP stack is run into the sea by a string, composed of long and wide-bore pipes, called the marine drilling riser. It also connects the BOP stack (BOP + LMRP) on the wellhead to the drilling vessel at the sea surface, serving as conduit between them.

While being lowered down, a special riser spider is used to support the riser. During this operation, the hanging riser is under the effects of sea current, waves and displacements induced by sea surface drilling vessel motions.

The discovery of petroleum fields in deeper waters, usually in remote and harsh environments, makes it a requirement for the riser to withstand more severe conditions. Nevertheless, riser design and failure analysis during this operation is a seldom studied topic, even though drilling in ever increasing water depths imply the riser is subject to risks inherent to running a BOP for a longer period of time. It is of utmost importance the adequate modeling of the system’s response during this operation, in order to safely evaluate failure cases due to extreme static and dynamic stresses.

As such, the present paper describes and proposes a methodology developed for the analysis of extreme stresses that act over the riser during the BOP running. Case studies were conducted for a water depth up to 2000 m. Numerical simulations were run to evaluate system’s static and dynamic behavior due to environmental loading.

As a result conclusions were drawn regarding operation limits in ultra-deep waters. Results are shown through charts that delimit ocean conditions and vessel’s response that are secure to operate under.
Engineering Critical Assessment in Sour Service by Fracture and Fatigue Small Scale Sour Testing
Authors: Nakamura Jun , Hisamune Nobuyuki , Fukuba Tetsuya , Scott Colin Stuart , Thodla Ramgopal , Gui Feng
Published: November 2013

Engineering critical assessments (ECA) have increasingly become a routine part of pipeline design to determine tolerable flaw sizes for weld defects. These assessments are now being applied to pipeline systems in deeper water with increased loadings arising from responses to thermal and pressure cycling. Often these are flowline systems in which fatigue damage is exacerbated by the presence of aggressive internal conditions.

Deep water flowlines operating at high temperatures and pressures need to be designed to accommodate issues such as significant end expansion, walking and lateral buckling. Offshore flowlines are subject to large thermal transients particularly during shut down. These thermal transients tend to vary from about 200°F to ambient temperature and place a large fatigue demand on the pipelines. The transients typically are very low frequency events with high amplitudes, which can lead to lateral buckling. The loading rates associated with the transients are very slow, hence the toughness properties of the material in environments at low loading rates is important. The pipelines are also subject to high stress and thus need to have good toughness properties. Riser materials are also subject to significant fatigue loading, though under conditions different from flow lines. They are typically pre-loaded and operate under smaller amplitudes of loading and at higher frequencies associated with wave motion.

Currently for a number of offshore sour service pipelines, Engineering Critical Assessment (ECA) for flaw acceptance criteria are performed using knockdown factors up to 50X on fatigue crack growth rates. The toughness values used for the analysis are performed assuming a CTOD of 0.1 mm. In order to develop material chemistry and heat treatment procedures for offshore pipelines, it is important to be able to understand the fracture toughness behavior as well in sour environments.

In order to evaluate the safety of offshore flowlines and risers applications fatigue and fracture toughness properties of X80 parent pipe material in sour environments were determined. The data developed was used to develop an ECA framework for flowlines and riser applications.
Long-Term Probability Distribution of Extreme Offshore Structural Response via an Efficient Time Simulation Method
Authors: Abu Husain M. K. , Mohd Zaki N. I. , Lambert L. , Wang Y. , Najafian G.
Published: November 2013

Offshore structures are exposed to random wave loading in the ocean environment and hence the probability distribution of the extreme values of their response to wave loading is required for their safe and economical design. To this end, the conventional simulation technique (CTS) is frequently used for predicting the probability distribution of the extreme values of response. However, this technique suffers from excessive sampling variability and hence a large number of simulated response extreme values (hundreds of simulated response records) are required to reduce the sampling variability to acceptable levels. A more efficient method (ETS) was recently introduced which takes advantage of the correlation between the extreme values of linear response and their corresponding response extreme values. The method has proved to be very efficient for both low and high-intensity sea states. In this paper, further development of this technique, which leads to more accurate estimates of the long term probability distribution of the extreme response, is reported.
Misalignment Effect on a Three-Dimensional Cruciform Welded Joint
Authors: Yamatoki Shuichi , Arimatsu Seitaro , Gotoh Koji
Published: November 2013

In the ship-hull design phase, the distance of a misalignment of a cruciform welded joint is enlarged, by up to 100 mm for example, until its effect is thought to be minimized. To control tolerance, we used a technique to estimate the coefficient of the misalignment effect: the ratio of the stress with misalignment to stress without misalignment. Current methods, created using the two-dimensional (2D) misalignment model and focusing on small misalignments, cannot be applied in the design phase. Misalignment models are needed, as there have been no studies of these joints and it is impractical to do finite element (FE) analysis in each case. We propose an estimation method of the coefficient of misalignment effect in three dimensions (3D). First, we created a new equation to estimate the coefficient in 2D. The equation accommodates larger misalignments. Second, we found the membrane component of a misaligned member stress is reduced as a 3D effect and the coefficient converges given a certain misalignment value. Third, a modified equation taking the effect into account indicates the coefficient in 3D.
Unique Long Term Simulated Service Testing of Selected Deep Water Wet Insulation Coating Systems
Authors: Haraldsen Kristian
Published: November 2013

The oil and gas industry is moving to deeper water developments and thermal insulation of pipelines at 2–3000 meter water depths is required. Wet thermal insulation systems are exposed directly to the seawater and large hydrostatic water pressure in combination with high fluid temperatures challenge the integrity of the insulation systems. The thermal insulation properties of the insulation system can be gradually decreased by thermal and physical strains and challenge the flow assurance if not taken into account in the design. Wet thermal insulation systems have traditionally been qualified for long term use by a combination of long-term small scale aging tests and shorter term full scale simulated service tests (typically 7–28 days). To evaluate the long performance, simulated service tests have been run for one full year at high water pressure and internal pipe temperatures. A selection of commercially available wet insulation systems have been tested together with system specific field joint coatings under the maximum internal pipe temperature specified for the individual coating systems. The test has been run in natural seawater at 300 barg pressure and temperature of 2–4 °C. The coating thicknesses have been selected to give U-values of 3–4 W/(m2K). Before the simulated service testing all test pipes were run through a simulated reel installation to impose realistic strains on the coating systems before testing. All tested coatings performed according to expectations during the simulated service test. The syntactic systems (PP and PU based) gave gradual decrease in thermal insulation efficiency with time but the reduction was close to linear and can be accounted for in the design. The coating systems based on solid polymer materials did not show the same reduction of thermal insulation properties with time. Tests of natural cool-down performance of the insulated test pipes showed that the cool down times were maintained during the one year simulated service tests. Post-test investigations of the insulation materials did not reveal significant degradation of the materials as result of the tests but coating systems application related defects were identified which had developed over the test period. None of the pre-existing defects developed to such an extent as to affect the overall U-value of the coating system.
Model Testing of Turret-Based Drill Ship in Ice Conditions
Authors: Kovalyov Maksym V. , Dobrodeev Aleksey A. , Blagovidova Iryna L. , Sazonov Kirill Ev. , Kolchenko Leonid V. , Klementieva Nataliya Yu. , Blagovidov Lev B.
Published: November 2013

Recently, process platforms and drill ships featuring anchor sea-keeping system equipped with a turret arrangement have increasingly been proposed for deep-water offshore applications to be operated in freezing seas. A possibility of environmentally-oriented yawing with the ship’s bow ahead of the resulting vector of environment actions provides the major advantage of the turret system use. The turret system allows passive positioning of the ship without activation of any propulsion means or thrusters, which is particularly important in ice conditions where ship controls can be difficult to operate.

CDB Corall PJSC designed a promising turret-based drill ship (TBDS) intended to perform drill operations in seas of Russian Arctic region. The design is substantially different from drill ships and platforms conventionally considered for similar purposes. The adopted design solutions ensure significantly higher drill ship performances in ice conditions.

The report includes results of TBDS model tests performed in the model basin of the Federal State Unitary Enterprise “Krylov State Research Centre”. Tests of ice resistance and tension of the anchor sea-keeping system have been incorporated in the test program, while the model was being interacted with the variable-thickness ice sheet at a 0° heading. Testing to investigate behavior of the interacted with ice ridge formations have also been performed at the same heading.

The study of passive model yawing at 90° and 170° conditions made up a highly important part of the program. Such tests were performed both for model/ice sheet and model/ice ridge interaction types. In addition to measuring force parameters during the tests, changes in heading, heel and trim, as well as model displacements were also determined. Testing proved high ice resistance and maneuverability of the test model.

The report includes comparison of the results obtained in model basin tests with other experimental data for models fitted with turret arrangement and anchor sea-keeping system features. The comparison suggests considerable advantages of the new solution versus previously considered options.
Estimation of Constraint Factor on the Relationship Between J Integral and CTOD for Offshore Structural Steel Weldments
Authors: Moon Dong Hyun , Lee Jeong Soo , Lee Jae Myung , Kim Myung Hyun
Published: November 2013

As ships and offshore structures become larger than before the usage of high strength steel and ultra thick plate has been increased. However, the thick plates have disadvantages with respect to brittle fracture and fatigue strength. Elastic plastic fracture mechanics (EPFM) is the domain of fracture analysis which considers extensive plastic deformation at crack tip prior to fracture. The J integral and crack tip opening displacement (CTOD) have been commonly used as parameters for EPFM analysis. The relationship between these parameters has been studied by industry and academia. The plastic constraint factor can serve as a parameter to characterize constraint effects in fracture. Therefore, the characteristics of plastic constraint factor are important in EPFM analysis.

In this study, the relationship between J Integral and CTOD was investigated by conducting 3-point bending tests using single edge notched bend (SENB) specimens. Two types of specimens were fabricated. One is API 2W Gr.50 welded by means of flux cored arc welding (FCAW) with heat input 15kJ/cm, and the other is API 2W Gr.50 welded by means of submerged arc welding (SAW) with heat input 45kJ/cm. The plastic constraint factor was estimated considering heat input, temperatures at weldment and heat affected zone (HAZ).
At-Sea Experiment of a Hybrid SPAR Type Offshore Wind Turbine
Authors: Ishida Shigesuke , Kokubun Kentaroh , Nimura Tadashi , Utsunomiya Tomoaki , Sato Iku , Yoshida Shigeo
Published: November 2013

Ministry of the Environment of Japan started the national demonstration project of Floating Offshore Wind Turbine (FOWT) in 2010 FY. In this project two SPARs with different scales are used. The first half-sized model, supporting a 100kW wind turbine, was installed in 2012 and is currently in operation. It is characteristic that the floater has a hybrid structure, i.e., the upper part is constructed of steel and the lower part of pre-stressed concrete and that the turbine is downwind type of horizontal axis.

The FOWT is equipped with many sensors for measuring motions of 6 DOF, the strain of the steel / concrete parts and other factors. In this paper the measuring system is presented. In September 2012, the FOWT was attacked by a huge typhoon and it survived, although the measured wave height exceeded the designed value. The measured record of the floater in the storm is also presented.
Coupled Multi-Phase CFD and Transient Mooring Analysis of the Floating Wave Energy Converter OWEL
Authors: Nicholls-Lee Rachel , Walker Adam , Hindley Simon , Argall Richard
Published: November 2013

Floating wave energy converters are surface based thus facilitating installation and maintenance. They tend to be moored offshore and consequently have less of an impact than other devices both visually and audibly. Mooring these devices is a challenging task, as not only are they subject to drift forces due to the aggressive environment, but they are also designed to operate at their resonant frequency in order to obtain as much power as possible. Such operational parameters require heavy duty mooring systems, capable of coping with the dynamic environment. These moorings will, in turn, affect the performance of the device by restraining the motions and thus modifying the energy absorption characteristics. In this paper a free floating representation of the Offshore Wave Energy Ltd. device (OWEL) has been modeled in RANS CFD in order to obtain initial mooring loads. Subsequently, a preliminary mooring arrangement for OWEL was developed, and using these loads it was modeled using OrcaFlex. The dynamic, non-linear loads were then coupled to a fully transient, multiphase CFD analysis of the device in order to obtain performance characteristics for further detailed design. The numerical results have been compared to results obtained through physical model scale tests of the device and show a good degree of correlation.
Efficient Mooring Line Fatigue Analysis Using a Hybrid Method Time Domain Simulation Scheme
Authors: Christiansen Niels Hørbye , Torbergsen Voie Per Erlend , Høgsberg Jan , Sødahl Nils
Published: November 2013

Dynamic analyses of mooring line systems are computationally expensive. Over the last decades an extensive variety of methods to reduce this computational cost have been suggested. One method that has shown promising preliminary results is a hybrid method which combines finite element analysis and artificial neural networks (ANN). The present study presents a novel strategy for selecting, arranging and normalizing training data for an ANN. With this approach one ANN can be trained to perform high speed dynamic response prediction for all fatigue relevant sea states and cover both wave frequency motion and slow drift motion. The method is tested on a mooring line system of a floating offshore platform. After training a full fatigue analysis is carried out. The results show that the ANN with high precision provides top tension force histories two orders of magnitude faster than a full dynamic analysis.
Steep Wave Loads From Irregular Waves on an Offshore Wind Turbine Foundation: Computation and Experiment
Authors: Paulsen Bo Terp , Bredmose Henrik , Bingham Harry B. , Schløer Signe
Published: November 2013

Two-dimensional irregular waves on a sloping bed and their impact on a bottom mounted circular cylinder is modeled by three different numerical methods and the results are validated against laboratory experiments. We here consider the performance of a linear-, a fully nonlinear potential flow solver and a fully nonlinear Navier-Stokes/VOF solver. The validation is carried out in terms of both the free surface elevation and the inline force. Special attention is paid to the ultimate load in case of a single wave event and the general ability of the numerical models to capture the higher harmonic forcing. The test case is representative for monopile foundations at intermediate water depths.

The potential flow computations are carried out in a two-dimensional vertical plane and the inline force on the cylinder is evaluated by the Morison equation. The Navier-Stokes/VOF computations are carried out in three-dimensions and the force is obtained by spatial pressure integration over the wettet area of the cylinder. In terms of both the free surface elevation and the inline force, the linear potential flow model is shown to be of limited accuracy and large deviations are generally seen when compared to the experimental measurements. The fully nonlinear Navier-Stokes/VOF computations are accurately predicting both the free surface elevation and the inline force. However, the computational cost is high relative to the potential flow solvers. Despite the fact that the nonlinear potential flow model is carried out in two-dimensions it is shown to perform just as good as the three-dimensional Navier-Stokes/VOF solver. This is observed for both the free surface elevation and the inline force, where both the ultimate load and the higher harmonic forces are accurately predicted. This shows that for moderately steep irregular waves a Morison equation combined with a fully nonlinear two-dimensional potential flow solver can be a good approximation.
Assessment of Hull Monitoring Measurements for a Large Blunt Vessel
Authors: Storhaug Gaute , Hareide Ole Jakob
Published: November 2013

A large blunt vessel was designed for a longer target life than usual in world-wide trade. Due to experience with whipping and springing, special attention to these effects was also made during the design and approval in order to ensure satisfactory strength of the vessel. The vessel was consequently strengthened beyond the minimum industry standard, and it has now been operated for several years.

The owner/operator who observed wave induced vibrations (whipping/springing) onboard, equipped the vessel with a hull monitoring system from a recognized supplier. After a few years of measurements, the data was sent to DNV for assessment of the effect of the vibrations and the consequence for the particular design.

The data shows that the vessel has been trading in more demanding areas than assumed during design, but the environment is less severe than the North Atlantic. The measured fatigue life based on a stress concentration factor of 2.0 has been estimated to be well below the design life, so special attention to cracks need to be taken from now on if the trade remains the same in the future. No cracks have, however, been identified so far during inspection.

The vessel has also experienced two severe storms. The maximum loading level has been higher than ever assessed by DNV before based on hull monitoring data of blunt vessels. The rule of thumb value of 20% increase on extreme loading for blunt vessels due to whipping has been exceeded. The wave bending moment according to IACS URS11 has also been exceeded without whipping. The ultimate collapse strength has been assessed and compared to the measured dynamic loading and allowable still water loading. When whipping is assumed fully effective to contribute to collapse, the safety margin is still above 1.0, but on the borderline of what is desirable. However, if the vessel had not been strengthened beyond the original design due to the concern of whipping and springing, the safety margin would have been below 1.0. This may be the first documentation of a vessel that has been saved from breaking in two due to addressing springing and whipping properly during design.
Experimental Investigation of the Effect of Waves, Ventilation and Cavitation in Bollard Pull Conditions
Authors: Hagesteijn G. , Brouwer J.
Published: November 2013

Ventilation is a feared working condition of ship propulsors, especially in Dynamic Positioning operation, since it leads to the lost of thrust of the propeller resulting into an uncontrollable ship. Experience showed that the risk of ventilating propellers was negligently underestimated in traditional towing tank experiments but better predicted in depressurised towing tanks, where the ambient pressure is scaled down according to Froude similarity.

In 2012 MARIN’s Depressurized Wave Basin (DWB) has taken into service. This unique facility is the only one in the world that is able to generate waves in a large depressurized towing tank. This ensures correct representation of the pressure inside the enclosed ventilation bubbles and vortices, resulting into a correct physic behaviour. The EU-funded Streamline project was the first project for which ventilation inception measurements were carried out in the DWB. Tests were carried out with a fully instrumented podded ship model, sailing and in bollard pull condition, in waves and depressurised conditions.

In order to acquire detailed load measurements, MARIN used their in house developed 6 component and 5 component transducers. The 6 component transducer was used for measuring the omnidirectional propeller loads, while the 5 component transducer was used for measuring 2 blade forces and 3 blade moments. At the same time synchronised high speed video recordings were made to acquire insight in the occurring phenomena. In the present paper a description of the test set up will be presented briefly, followed by a discussion of the recordings and the observations that were made for bollard pull condition in waves.
Summary of Conclusions and Recommendations Drawn From the DeepCwind Scaled Floating Offshore Wind System Test Campaign
Authors: Robertson Amy N. , Jonkman Jason M. , Goupee Andrew J. , Coulling Alexander J. , Prowell Ian , Browning James , Masciola Marco D. , Molta Paul
Published: November 2013

The DeepCwind consortium is a group of universities, national labs, and companies funded under a research initiative by the U.S. Department of Energy (DOE) to support the research and development of floating offshore wind power. The two main objectives of the project are to better understand the complex dynamic behavior of floating offshore wind systems and to create experimental data for use in validating the tools used in modeling these systems. In support of these objectives, the DeepCwind consortium conducted a model test campaign in 2011 of three generic floating wind systems: a tension-leg platform (TLP), a spar-buoy (spar), and a semi-submersible (semi). Each of the three platforms was designed to support a 1/50th-scale model of a 5-MW wind turbine and was tested under a variety of wind/wave conditions.

The focus of this paper is to summarize the work done by consortium members in analyzing the data obtained from the test campaign and its use for validating the offshore wind modeling tool, FAST.
Design and Estimated Economic Performance of a Floating Axis Marine Current Turbine in Kuroshio Ocean Current
Authors: Akimoto Hiromichi , Tanaka Kenji , Hara Yutaka , Uzawa Kiyoshi
Published: November 2013

Although ocean currents are more stable than wind, the present cost of energy produced by ocean current turbine is considered to be higher than that of offshore wind turbines. It is caused by the expensive cost of submerged devices and difficulties of their maintenance. To solve the problem, the authors are developing a new concept of a cross flow turbine for high economic performance. The turbine axis of the concept is not fixed in the upright position for the reduction of supporting structures and device cost.

Since a firm foundation is not available in ocean environment, discarding the idea of fixed turbine axis position is a very natural approach in ocean engineering. The economic performance of the proposed turbine is estimated based on the existing cost analysis of floating wind turbines and ocean plants. Although there are remaining problems of high mooring tension, possible changes of ocean current path and survivability in rough sea conditions, the result shows favorable economic performance of the concept.
Efficient Hybrid-Spectral Model for Fully Nonlinear Numerical Wave Tank
Authors: Christiansen Torben B. , B. Bingham Harry , Engsig-Karup Allan P. , Ducrozet Guillaume , Ferrant Pierre
Published: November 2013

A new hybrid-spectral solution strategy is proposed for the simulation of the fully nonlinear free surface equations based on potential flow theory. A Fourier collocation method is adopted horisontally for the discretization of the free surface equations. This is combined with a modal Chebyshev Tau method in the vertical for the discretization of the Laplace equation in the fluid domain, which yields a sparse and spectrally accurate Dirichlet-to-Neumann operator. The Laplace problem is solved with an efficient Defect Correction method preconditioned with a spectral discretization of the linearised wave problem, ensuring fast convergence and optimal scaling with the problem size. Preliminary results for very nonlinear waves show expected convergence rates and a clear advantage of using spectral schemes.
A Numerical Parameter Study on Current Forces on Circular Aquaculture Net Cages
Authors: Kristiansen Trygve
Published: November 2013

The focus in this paper is on current forces on circular aquaculture nets, and how they vary with different parameters. The main parameters are current velocity and profile, solidity ratio of the net, sinker weights and net depth. The drag force on the net is calculated by a newly developed screen type force model. A subset of the calculations are validated by recent experimental data. The presented results are meant to represent a basis for discussing how current loads on a net cage vary with different environmental and strucural parameters. Scaling effects are discussed.
Efficient Derivation of the Probability Distribution of Extreme Responses due to Random Wave Loading From the Probability Distribution of Extreme Surface Elevations
Authors: Mallahzadeh H. , Wang Y. , Abu Husain M. K. , Mohd Zaki N. I. , Najafian G.
Published: November 2013

Offshore structures are exposed to random wave loading in the ocean environment and hence the probability distribution of the extreme values of their response to wave loading is required for their safe and economical design. Due to nonlinearity of the drag component of Morison’s wave loading and also due to intermittency of wave loading on members in the splash zone, the response is often non-Gaussian; therefore, simple techniques for derivation of the probability distribution of extreme responses are not available. To this end, the conventional Monte Carlo time simulation technique is frequently used for predicting the probability distribution of the extreme responses. However, this technique suffers from excessive sampling variability and hence a large number of simulated response records are required to reduce the sampling variability to acceptable levels. This paper takes advantage of the correlation between extreme responses and their corresponding extreme surface elevations to derive the probability distribution of the extreme responses accurately and efficiently, i.e. without the need for extensive simulations.
Use of Overset Mesh to Allow Dynamic Deflection of Tight-Fitting Control Surfaces in CFD Simulations
Authors: Coe Ryan G. , Neu Wayne L.
Published: November 2013

The development of vehicle maneuvering simulations based within computational fluid dynamics (CFD) environments demands that vehicle control surfaces be dynamically deflected during such simulations. This paper details the process of developing and testing CFD simulation methods that allow for the deflection of a specific AUV’s control surfaces. This task is made particularly challenging by the geometry of the AUV, as its moving control surfaces fit very closely to stationary fixed strakes and the AUV’s hull (a fairly common trait among this class of vehicles). After ruling out embedded and deformable mesh approaches, an overset mesh method is applied. Steady-state simulations with this overset mesh show general agreement with static mesh simulations. The two approaches do, however, highlight the mesh sensitivity of CFD simulations in their ability to predict the onset of stall.
Employing Visual Image Correlation for the Measurement of Compressive Strains for Arctic Onshore Pipelines
Authors: Ozkan Istemi F. , Bandstra Daryl J. , Timms Chris M. J. , Zielinski Arthur T.
Published: November 2013

The Arctic onshore environment contains regions of discontinuous permafrost, where pipes may be subject to displacement-controlled bending in addition to high hoop stresses due to the pressurized fluids being transported. Considering the displacement-controlled nature of the deformations, strain-based design methodologies have been developed for permafrost pipelines when they are subject to bending and tension, which limit the longitudinal compressive and tensile strains. The widely accepted methodology in the industry to obtain the compressive strain capacity of line pipes subject to bending is to conduct Finite Element Analysis, incorporating material and geometrical nonlinearity calibrated against benchmark full-scale tests (bend tests) [1,2]. During these tests, compressive strains can be measured by various methods. The seemingly obvious choice is to apply strain gauges along the compression face of the specimen with respect to bending (intrados). This method will provide reasonable results until the compressive strain pattern begins to vary due to the initiation of buckle formation, which typically occurs shortly after yield. In order to measure average compressive strain beyond yield and up to buckling, the method used by C-FER Technologies (C-FER) involves using rotation measurement devices (inclinometers) to calculate the strain change between the most compressive and tensile fibres of the specimen (intrados and extrados, respectively) with respect to the bending direction. This value is then subtracted from the tensile strain gauge readings as measured by the strain gauge(s) located on the extrados of the specimen. The average compressive strain values derived from the inclinometer and extrados strain gauge measurements are based on the assumption that the plane sections remain plane.

Recently, five large diameter pipes were bend-tested at C-FER’s testing facility in Edmonton, Alberta. In addition to the compressive strain measurement method used by C-FER described above (C-FER method), a visual image correlation (VIC) camera system was used to survey the strain distribution on the compressive face of the specimens. This paper gives a brief description of the test setup and instrumentation of this test program. The VIC camera setup and measurement technique are described and the overall strain distribution on the bending intrados as measured by the VIC cameras is presented. Strain measured by the VIC system is compared with gauge measurements at local points as well as the average compressive strain behaviour of the specimens obtained through the C-FER method described above. The results show that the VIC system can be a candidate to replace the conventional measurement techniques employed for compressive strain limit testing in support of strain-based design of arctic pipelines.
Efficient Computation and Modeling of Viscous Flow Effects in ComFLOW
Authors: van der Heiden Henri J. L. , van der Plas Peter , Veldman Arthur E. P. , Verstappen Roel W. C. P. , Luppes Roel
Published: November 2013

In offshore applications, details of viscous flow effects can become relevant when predicting e.g. drag forces on the columns of oil drilling rigs, or the flow around a semisubmersible in figure 1. This motivates a novel approach for efficiently simulating viscous flow effects at high Reynolds numbers with the CFD simulation tool ComFLOW.

In ComFLOW, the Navier–Stokes equations can be solved for one-phase and for two-phase flow. The equations are discretized second-order in space, and second-order in time. An Improved Volume-of-Fluid (IVOF) algorithm is used for free-surface advection and reconstruction [1, 2].

Modeling viscous flow effects in high Reynolds number flows requires a turbulence model that provides accurate results on coarse grids. We pursue to achieve a high local grid resolution in a computationally efficient manner. Both approaches are tested for flows around a square cylinder: grid refinement at Reynolds numbers 10 and 100, and the turbulence model at Reynolds number 22,000.
Probabilistic Modeling of Roughness Effects Caused by Bio-Colonization on Hydrodynamic Coefficients: A Sensitivity Study for Jacket-Platforms in Gulf of Guinea
Authors: Ameryoun Hamed , Schoefs Franck
Published: November 2013

Nowadays, challenge for requalification of existing offshore platforms through the reassessment process leads to consider the importance of updating new information (e.g. environmental data, new regulations, etc.). Regarding to this information and depending on offshore fields, data of marine growth colonization is shown to have a dominant effect. This is a real challenge in the Gulf of Guinea that this study focuses on. Marine growth is known to cause adverse effects on the performance of offshore structures. Its presence can change the roughness and the diameter of structural members and hence change the level of hydrodynamic coefficients. Moreover, modifying the added mass can change the natural period and hence dynamic responses of structures. Even platforms with the best protection schemes against marine organisms will after few weeks at least few months start to be covered by various types of marine growth. Generally, it was also recognized that the most important source of loading exerted on offshore structures comes from hydrodynamic actions which are influenced by hydrodynamic coefficient values. The colonization process is very complex and results are in a large diversity of marine growth type (animal, vegetal — hard, soft) and species. This study therefore proposes a stochastic modeling of marine growth and the roughness of hard species based on Response Surface Methodology. A geometrical description of nth order of Stokes model, formed by a random linear combination of deterministic vectors is employed. Finally, the complexity level of roughness modeling is analyzed and the results are discussed.
Weather Window Analysis of Irish and Portuguese Wave Data With Relevance to Operations and Maintenance of Marine Renewables
Authors: O’Connor Michael , Lewis Tony , Dalton Gordon
Published: November 2013

This paper presents the results of a weather window analysis of wave data from the west coast of Ireland and the Atlantic coast of Portugal in order to quantify the levels of access to ocean energy renewables, which may be deployed there, for operation and maintenance activities. In order to operate and maintain offshore marine renewables, a device will have to be accessible for a certain period of time. This will require a weather window consisting of a consecutive period of wave heights low enough and long enough for the device to be accessed. It is important to quantify what the levels of access are off the Irish west coast and Portuguese Atlantic coast given their high wind and wave resource.

Wave data from two wave buoys, the M3 buoy located 56km off the west coast of Ireland and the Leixoes buoy located 19km off the Portuguese coast, are analysed to quantify the levels of access that exist. The data is used to quantify the general regimes at both sites by presenting the wave energy resource, the mean annual exceedance and the wave height frequency at both sites. The levels of access are quantified at operations and maintenance (O/M) access limits of Hs 1.0, 1.5, 2.0 and 2.5m wave height, by presenting the number of windows and the percentage of the year that these windows make up as well as the total number of hours, monthly and annual, that the wave heights are below these limits. Also presented are the waiting periods between windows by showing both the longest individual waiting periods between windows in a year and also the total intervals between windows in a year. The levels of access observed off Ireland and Portugal are then compared to levels of access observed at other marine renewable locations, namely the North Sea, Irish East Coast and Pacific North-western US coast.

The results indicate that the levels of access off Ireland and Portugal are far below those observed at other marine renewable locations, and at the lower wave height access limits, there are very few suitable weather windows and considerable winter waiting periods between these windows. The implications of these low levels of access suggest that maintaining wave energy converters, off the west coast, may not be feasible and devices will need to be brought ashore for O/M activities.
Water-Driven Debris Impact Forces on Structures: Experimental and Theoretical Program
Authors: Riggs H. R. , Cox D. T. , Naito C. J. , Kobayashi M. H. , Piran Aghl P. , Ko H. T.-S. , Khowitar E.
Published: November 2013

Water-driven debris generated during tsunamis and hurricanes can impose substantial impact forces on structures that are often not designed for such loads. This paper presents the design and results of an experimental and theoretical program to quantify these potential impact forces. Two types of prototypical debris are considered: a wood log and a shipping container.

Full-scale impact tests at Lehigh University were carried out with a wooden utility pole and a shipping container. The tests were carried out in-air, and were designed to provide baseline, full-scale results. A 1:5 scale shipping container model was used for in-water tests in the Oregon State University large wave flume. These tests were used to quantify the effect of the fluid on the impact forces.

Results from both experimental programs are presented and compared with theoretical predictions. The analytical predictions are found to be in sufficient agreement such that they can be used for design. A fundamental takeaway is that the impact forces are dominated by the structural impact, with a secondary affect provided by the fluid. Both forces are quantified in the paper.
The Effect of T-Stress on Crack-Tip Plastic Zones Under Cyclic Mixed-Mode Loading Conditions
Authors: Nazarali Qays , Wang Xin
Published: November 2013

In this paper, the influence of T-stress on crack-tip plastic zones under mixed-mode I and II loading conditions under cyclic loading is examined. The crack-tip stress field is defined in terms of the ranges of mixed-mode stress intensity factors and the T-stress using William’s series expansion. The crack-tip stress field is incorporated into the Von Mises yield criteria to develop an expression that determines the cyclic crack-tip plastic zone. Using the resultant expression, the cyclic plastic zone is obtained for various combinations of mode II to mode I stress intensity factor ratios and levels of T-stress. For the purpose of demonstrating the significance of the T-stress, this paper further analyses the plastic zone size for center cracked plate (CCP) specimen subjected to bi-axial mixed-mode cyclic loading.
Critical Assessment of PVDF Multilayer Barriers in Unbonded Flexible Risers: Applications and Benefits
Authors: Fernando Upul S. , Davidson Michelle , Sheldrake Terry
Published: November 2013

The polymer barrier is the key component in a flexible pipe structure that is specifically designed to contain hydrocarbon media within the pipe. Maintaining the integrity of the barrier for the total service life of the pipe is therefore an essential requirement to prevent any leaking or spilling of the hydrocarbon into the external environment. For high temperature (> 100°C) service PVDF is commonly used as a barrier material, and the barrier designs are based on either single layer or multilayer (two or three layers) structures depending on design strategy, manufacturing constraints and service application.

This paper presents a critical review of the barrier design strategies, considering both single and multilayer structures. The main parameters in deciding a reliable barrier structure are discussed and the advantages/disadvantages of multilayers over single layers are highlighted. The different pipe failure modes/mechanisms that may be related to different barrier structures, the analyses and test requirements to evaluate and overcome these failures are discussed.