bluuebunny/crossref_metadata_2025 huggingface dataset:
Non-Linear Dynamics of Parametric Roll of Container Ship in Irregular Seas
Authors: Somayajula Abhilash S. , Falzarano Jeffrey M.
Published: September 2014

Parametric motion is the phenomenon where a structure is excited into large amplitude motion even when there is no direct excitation. A well-known example of this type of motion is the parametric roll of ships in head or following seas. Parametric roll of container ships in head seas is relatively a new problem which has gained much importance after the catastrophic incidence of APL China in 1998. Many studies have investigated this phenomenon in the case of a ship being excited in regular waves. However, ships do not encounter regular waves in the actual ocean. So, it is imperative to study the importance of parametric roll in irregular seas.

In this paper the analysis of roll equation of motion is performed by nonlinear modeling. The problem of parametric roll is approached as a non-linear dynamics problem with due consideration to nonlinear time varying hydrostatics as well as the nonlinear damping. A nonlinear damping model is used to approximate the actual viscous damping in the system. The variation of the roll righting arm with time has been modeled using a Volterra series representation which includes the hydrostatic non-linearity.

Various realizations of the roll motion have been simulated and analyzed to study the ergodic behavior of the phenomenon. The paper also discusses future ideas of how to analyze parametric roll in irregular seaways.
Study of Bending Capacity of an HPHT CRA-Lined Seamless Pipeline
Authors: Gil Cyprian , Tørnes Knut , Damsleth Per
Published: September 2014

A study has been performed to better understand ultimate bending moment and strain capacities of pipelines in relation to criteria defined in the design codes. An 18″ HPHT flowline was designed to undergo global buckling on uneven seabed and to resist trawl gear interference. The high temperature (155 degC) and pressure (300 bar) posed considerable design challenges for material selection and design criteria. A CRA-lined X60 CMn pipeline was selected for the project. The pipeline was of seamless manufacture for which the stress/strain characteristics are subject to the effect of Lüders bands. The DNV-OS-F101 code covers a wide range of D/t but does not specifically address Lüder’s material behaviour which could significantly reduce the bending moment capacity of pipe.

The global buckling and trawl pull-over FE analysis results indicated the pipe was highly utilized, requiring excessive amounts of seabed intervention at great cost to meet the DNV LCC criteria. Detailed FE simulation of limit states for local buckling and strain localization of a 3D solid element pipe model was performed, with both Roundhouse and Lüders material properties, to investigate pipe capacity in relation to that stipulated by the design codes. The pipe moment capacity was established by obtaining the moment curvature relationship by bending the local pipe section subject to internal pressure until the maximum resistance was reached. Imperfections were introduced to initiate local buckling at the desired location. To determine strain concentration factors and strain localization, the effects of thickness changes and weld misalignment were also studied.

The DNV OS-F101 LCC moment criterion formulation computes a decreasing moment capacity for increasing internal pressure. It has been suggested in the literature that this is correct for higher D/t but the criterion may be conservative for pipes with lower D/t. The combination of Lüders material with low D/t is not specifically addressed by any design code. Clarification of these aspects will provide a better understanding of the risk of failure for highly utilized seamless pipelines and allow for modified design criteria that will reduce seabed intervention costs.

The results of the study showed that a higher bending moment criterion and associated strain criterion could be adopted for the design that allows for the higher initial strain caused by Lüder’s plateau. The ultimate bending moment capacity of low D/t pipe with Lüder’s material was found to be similar to that of Roundhouse material due to work hardening. In addition, it was demonstrated that the potential strength of the CRA liner could enhance the moment capacity of the seamless pipe.
Numerical Prediction of the Added Resistance of Ships in Waves
Authors: Ley Jens , Sigmund Sebastian , el Moctar Ould
Published: September 2014

The added resistance in waves is computed for different ship types using two different Reynolds-Averaged Navier-Stokes Equations (RANSE) solvers, namely Comet and interFoam (OpenFOAM). Hence, the RANS equations are implicitly coupled with the non-linear equations of motions for six degrees of freedom and the solvers are extended by algorithms for mesh morphing to account for ship motions. The computational effort for these simulations is high compared to potential flow based simulations, especially for short waves. However, to understand the physics related to added resistance of ships and to investigate influencing parameters, field methods based on RANS equations may be suitable. The prediction of the added resistance in waves consists of two steps; the computations of the calm water resistance and the total resistance in waves. The discretisation errors as well as the influence of the surge motions on the added resistance are investigated. Further, the added resistance is decomposed in diffraction and radiation problems as it is commonly done in potential theory.
Refinement of Fatigue-Life Estimates by Interpolation on Metocean Conditions
Authors: Mathisen Jan , Jia Huirong
Published: September 2014

The fatigue life of ships and offshore structures is commonly based on the Miner-Palmgren hypothesis and S-N curves. Fatigue damage rates can be calculated for each short-term sea state, and then combined with the probabilities of the short-term sea states to obtain the long-term damage rate and the estimated fatigue life. Each short-term stress response is usually computed for one point within the corresponding bin of the scatter diagram. In order to minimize uncertainty arising from the choice of a single representative point within a relatively large bin, a refinement procedure is proposed by interpolation on metocean conditions. The proposed procedure is applied to the most heavily utilized girth weld of a TLP tether and discussed.
Support Structure Optimization for Offshore Wind Turbines With a Genetic Algorithm
Authors: Bárcena Pasamontes Lucía , Gómez Torres Fernando , Zwick Daniel , Schafhirt Sebastian , Muskulus Michael
Published: September 2014

This study considers the use of a genetic algorithm for the structural design optimization of support structures for offshore wind turbines. Member diameters, thicknesses and locations of nodes are jointly optimized. Analysis of each design is performed with a complete wind turbine simulation, for a load case in the time domain. Structural assessment is in terms of fatigue damage, evaluated for each joint using the hot-spot stress approach. This defines performance constraints. Designs are optimized with respect to their weight. The approach has been tested with the modified 4-legged UpWind jacket from the OC4 project. The weight is quickly reduced, convergence slows after about 100 iterations, and few changes occur after 250 iterations. Interestingly, the fatigue constraint is not active for any member, and it is the validity of stress concentration factors that determines the best design, which utilizes less than 90 percent of the available fatigue lifetime. These results of the preliminary study using the genetic algorithm demonstrate that automatic optimization of wind turbine support structures is feasible under consideration of the simplified load approach. Even for complex, multi-member structures such as the considered jacket a weight reduction was achieved.
Detecting Stationary Intervals for Random Waves Using Time Series Clustering
Authors: Euán Carolina , Ortega Joaquín , Alvarez-Esteban Pedro C.
Published: September 2014

The problem of detecting changes in the state of the sea is very important for the analysis and determination of wave climate in a given location. Wave measurements are frequently statistically analyzed as a time series, and segmentation algorithms developed in this context are used to determine change-points. However, most methods found in the literature consider the case of instantaneous changes in the time series, which is not usually the case for sea waves, where changes take a certain time interval to occur.

We propose a new segmentation method that allows for the presence of transition intervals between successive stationary periods, and is based on the analysis of distances of normalized spectra to detect clusters in the time series. The series is divided into 30-minutes intervals and the spectral density is estimated for each one. The normalized spectra are compared using the Total Variation distance and a hierarchical clustering method is applied to the distance matrix. The information obtained from the clustering algorithm is used to classify the intervals as belonging to a stationary or a transition period We present simulation studies to validate the method and examples of applications to real data.
Numerical Calculation of Residual Stress Corrected J-Integral Using ABAQUS
Authors: Zhu Xian-Kui
Published: September 2014

Residual stresses exist in welded structures due to thermal stresses. Without temperature change, large plastic deformation can result in “cold” residual stresses in a wrinkle or dent in a metallic pipe. For a crack in residual stress field, residual stresses might have strong effect on fracture parameter, the J-integral. In order to ensure its path-independence, different correction methods have been developed in consideration of residual stress effect. Recently, the finite element commercial software ABAQUS adopted one of the correction methods, and is able to calculate the residual stress corrected J-integral.

A brief review is first given to the J-integral definition, the conditions of path-independence or path-dependence, and the modifications to consider the residual stress effect. A modified single edge-notched bend (SENB) specimen is then used, and a numerical procedure is developed for ABAQUS to evaluate the path-independence of the residual stress corrected J-integral. Detailed elastic-plastic finite element analyses are performed for the SENB specimen in three-point bending. The residual stress field, crack-tip stress field, and J-integral with and without consideration of residual stresses are discussed.
Hydrokinetic Energy Resource Assessment of the Gulf Stream Near Cape Hatteras, North Carolina
Authors: Kabir Asif , Lemongo Ivan J. , Fernandez Arturo
Published: September 2014

The Gulf Stream near the coasts of North Carolina is considered a promising source of hydrokinetic energy. A statistical analysis is conducted to assess the energy available for extraction in this region. Weibull distribution is used as the Probability Density Function (PDF) for this purpose. The ocean current velocity data are collected from the ‘HYbrid Coordinate Ocean Model (HYCOM)’. The data are collected at a depth of 20 m from the sea surface which is considered a good position for energy extraction. The Weibull parameters from the analysis are calculated using the maximum likelihood method. The direction of the ocean current was found to be mostly uniform in this region. The theoretical power density of this region was estimated to be more than 275 W/m2 around 70% of the time and exceeded 2000 W/m2 around 10% of the time.
Leg-Seabed Interactions of Jack-Up Vessels due to Motions in Irregular Waves
Authors: Kreuzer Edwin , Solowjow Eugen , Qiu Gang , Hamann Thorben , Grabe Jürgen
Published: September 2014

This paper analyzes the leg-seabed interaction due to motions of a 3rd generation jack-up vessel in irregular waves. The proposed model considers hydrodynamic forces on the hull as well as on the legs. The sea floor reaction forces are analyzed with a Coupled Eulerian-Lagrangian (CEL) method. The results are used to develop a mechanical rheological model for the sea floor. With computational efficiency in mind a state-space representation in time domain is derived. The excitation of the system is caused by irregular waves, which are described by the JONSWAP spectrum. The proposed method allows the efficient computation of short term (2–15 min) scenarios and events through sampling of realizations. It is used to compute the impact forces on the legs from the seabed induced by irregular vessel motions. The presented framework is suitable for the planning of jack-up operations in the offshore wind industry. It can also be applied for the design of dynamic positioning and jacking systems.
Impact Forces of Submarine Landslides on Free-Span Pipelines
Authors: Liu Jun , Tian Jianlong
Published: September 2014

Submarine landslides are perhaps the most significant geohazard on the deep water oil and gas developments, especially on the pipelines. The impacted forces of submarine landslides on pipelines are usually expressed in terms of drag factors within a fluid mechanics framework. The drag factors mainly depend on an equivalent Reynolds number for the non-Newtonian debris material. In this paper, Computational Fluid Dynamics (CFD) numerical analysis was employed to analyze the impact forces of submarine landslides on free span pipelines. In these numerical simulations, influences on impact forces of debris material composition, attack angles and velocity of debris flow were discussed in detail. Based on the simulation results we find that the impact angle also has apparent influence on normal drag factors, but has little influence on longitudinal drag factors. Prediction formula are improved to better estimate normal and axial impact forces on pipelines at any attack angle within the fluid mechanics framework.
Modeling the Dynamic Lateral Buckling Response of Subsea Pipelines Using Rate-Dependent Seabed Friction
Authors: Westgate Zack , Chee Kin Yin , Walker Alastair
Published: September 2014

As subsea pipeline installation methods become increasingly sophisticated through use of increased vessel power and advanced stability and positioning controls, pipelines are likely to exhibit lower levels of horizontal out-of-straightness on the seabed than currently considered to occur during the lay process. This has resulted in an increased tendency for the problematic phenomenon of snap-buckling to occur as part of the controlled lateral buckling design.

A key parameter affecting the pipeline response during lateral buckling is the friction between the pipeline and the seabed, which is strongly influenced by the rate of pipe displacement. Under stable buckle formation, the relatively slow rate of pipe displacement is more predictable and leads to acceptable strains in the pipeline. During the formation of less stable snap-through buckles, the rate of pipe displacement can mobilize a wide range in friction factors that can cause high strains in the pipeline. Quasi-static methods cannot model the dynamic transition between a high friction (drained) response and a low friction (undrained) response. To circumvent this problem in design, a fully dynamic finite element approach is required which captures the rate-dependent frictional response.

This paper presents the results of a series of FE analyses that illustrate how the changing seabed friction affects the lateral buckling response of typical subsea pipelines. A contractile seabed is modeled, chosen to capture the general case where high values of slow drained friction are followed by low values of fast undrained friction, leading to changes in the buckle shapes compared to those obtained from static buckle modeling. The results are used to examine the conditions under which dynamic buckle analysis should be used and the effects of dynamic pipeline response on the levels of strain and deformation induced in buckles.
Motion of a Large Disconnectable Buoy Within FPSO Turret Moon-Pool
Authors: Bian Sean , Leverette Steve
Published: September 2014

The disconnectable buoy turret mooring (BTM) for an FPSO has been a proven technology for areas where hurricane and seasonal cyclones are predominant, or are subject to ice features which need to be avoided. With the requirements of large buoy payload from deep water moorings and heavy risers, the disconnectable buoy size can be substantial. Recent projects are considering buoy net displacements well above 5000Te. The increased buoy size imposes large inertia loads on the supporting structure and mechanical connectors when connected, and possibly more so on the structural integrity and winching system during disconnect and reconnect operations. In order to provide structure design loads as well as to estimate safe operational windows and procedures, it is desirable to understand the physical properties of 2-body motions when the buoy and ship are close enough to have significant interactions.

This paper focuses on the hydrodynamic and structural (Winch, Mooring/riser, Fender/bumper etc.) interactions with the buoy operating in close proximity to the turret. Since the readily available offshore engineering tools have limitations to deal with close body hydro-interaction, particularly with the presence of moon-pool resonances, rational approximations are derived for hydrodynamic forces during pull-in and disconnection of the buoy. To analyze fully coupled motions and loads in combined environmental conditions, a typical diffraction/convolution based numerical tool (AQWA) is extended by a custom defined external force function. This analysis allows simulating the real time motions during disconnect, reconnect, or at suspended elevations. Various findings from dedicated model tests and analyses are presented and discussed with implications for configuring and operating the disconnectable buoy system.
Layout Method of a FPSO (Floating, Production, Storage, and Off-Loading Unit) Using the Optimization Technique
Authors: Ku Namkug , Jeong Se-Yong , Roh Myung-Il , Shin Hyun-Kyoung , Ha Sol , Hong Jung-woo
Published: September 2014

In the case of a floating offshore plant such as FPSO (Floating, Production, Storage, and Off-loading unit), many equipment should be installed in the limited space, as compared with an onshore plant. At this time, special conditions, such as the movement due to external force by wind and wave, salt content, and so on, should be also considered because the floating offshore plant should be operated in the special environment of ocean. The requirement for an optimal layout method of the plant has been raised due to much considerations for layout design. Thus, a layout method of the floating offshore plant was proposed in this study. For this, an optimization problem for layout was mathematically formulated, and then an optimization algorithm was implemented based on the genetic algorithm or mixed integer linear programming in order to solve it. To evaluate the applicability of the proposed method, it was applied to an example of FPSO and LNG FPSO topsides. As a result, it was shown that the proposed method can be applied to layout design of the floating offshore plant.
Pipe Soil Interaction During Cyclic Buckling: The Importance of Site-Specific Seabed Properties
Authors: Borges Rodriguez A. , Draper S. , Boylan N. P. , Begaj L. , Bransby M. F.
Published: September 2014

Knowledge of how an unburied pipeline interacts with the seabed during operational buckling is critical both to check the fatigue life of a pipeline in the buckling zone and to make accurate predictions of the pipeline end-expansions and walking. This paper explains how site-specific seabed conditions affect pipe-soil interaction (PSI) for buckling pipelines during their operational life.

The paper first demonstrates that for many soil conditions, cyclic PSI models that only consider the ‘berm push’ resistance fail to capture all the key elements of the pipeline-seabed interaction during cyclic buckling. An additional mechanism associated with the changing pipeline trajectory which develops after many pipeline lateral movements (‘sweeps’) is identified. For some soil and pipeline conditions this second mechanism provides the main component of the seabed resistance after many operational cycles. Appropriate analysis methodologies are discussed which allow the ‘berm-push’ and trajectory mechanisms to be considered in light of soil conditions and pipeline input conditions.

Secondly, published design approaches to assess cyclic PSI during buckling have typically assumed that the seabed around pipelines is not mobile (i.e. unscourable). However, in some situations, the combination of pipeline geometry, seabed properties and metocean conditions may lead to seabed scour around the pipeline and surrounding soil berms. This may therefore affect how the lateral soil resistance develops during the operational lifetime of a pipeline and should be considered in design. The paper discusses this issue, presenting results from CFD scour analyses of a particular berm geometry for different metocean conditions. The implications of berm scour on pipe-soil interaction for this geometry is explained.
Norwegian Work on Search and Rescue in Barents Sea
Authors: Dyrkoren Erik , Berg Tor Einar
Published: September 2014

Increased shipping and offshore activities in the Barents Sea need improved emergency response capability in Norway and Russia. In both countries there are several projects and initiatives that aim towards mitigating the consequences from small accidents and larger catastrophes, some coordinated across borders, others not.

In this paper we aim towards giving an overview of the current and near future state of emergency response in the Barents Sea. First we describe the emergency response preparedness that is fully operational today. Then we give a brief description of operations and activities we see today and in the foreseen future, and discuss types of challenges and risks associated with them in this particular environment. Using this as a background, we look on Norway’s ambitions for future search and rescue preparedness in the Norwegian Arctic waters and the ways emerging technologies can improve emergency response operations.

Finally we give some recommendations on what needs to be done by Norway, Russia and the industrial operators in order to achieve an improved level of emergency response preparedness.
Semi-Submersible Hub Platform With an Internal Dock Model Testing
Authors: Malta Edgard Borges , Ruggeri Felipe , Cardozo de Mello Pedro , Vilameá Eduardo M. , de Oliveira Allan C. , Nishimoto Kazuo
Published: September 2014

The recent discoveries and development of the Pre-salt reservoir in Brazilian coast require a new logistical model for crew transportation and transhipment to the drilling and oil rigs due to the large distance from coast, harsh environment conditions and large amount of workers to be transported against the actual model adopted considering only transportation by helicopters in order to reduce overall costs. The adoption of a logistic model with maritime transportation in these scenarios could provide several advantages, however there are several challenges from the technical point of view in transhipment between ship-shaped vessels, that could represent a great limitation in terms of operational window. Previous works showed the feasibility of monocolumn platforms with an internal moonpool as a Logistic HUB [1], allowing the boat docking in sheltered conditions. This work shows an overview of the model testing of a semi-submersible with an internal dock and the comparison of the free-surface elevation and RAOs (Response Amplitude Operators) between experimental results and potential flow computations. The tests were performed for 5 headings considering 10 regular, 5 irregular and 1 transient waves under a single draft and 5 different devices to reduce wave energy in the interior region.
Turbulence Modeling, Local Grid Refinement and Absorbing Boundary Conditions for Free-Surface Flow Simulations in Offshore Applications
Authors: Veldman Arthur E. P. , Luppes Roel , van der Heiden Henri J. L. , van der Plas Peter , Düz Bülent , Huijsmans René H. M.
Published: September 2014

To study extreme hydrodynamic wave impact in offshore and coastal engineering, the VOF-based CFD simulation tool ComFLOW is being developed. Recently, much attention has been paid to turbulence modeling, local grid refinement, wave propagation and absorbing boundary conditions. The turbulence model has to cope with coarse grids as used in industrial applications. Thereto a blend of a QR-model and a regularization model has been designed, in combination with a dedicated wall model. Local grid refinement is based on a semi-structured approach. Near refinement interfaces special discretization stencils have been designed. The computational domain is restricted to the close environment of the objects studied. To suppress unphysical reflections, special generating and absorbing boundary conditions have been designed. The combined performance of the new ingredients will be demonstrated with several applications. For validation, experiments have been carried out at MARIN.
Implementing Nonlinear Buoyancy and Excitation Forces in the WEC-Sim Wave Energy Converter Modeling Tool
Authors: Lawson Michael , Yu Yi-Hsiang , Nelessen Adam , Ruehl Kelley , Michelen Carlos
Published: September 2014

Wave energy converters (WECs) are commonly designed and analyzed using numerical models that combine multibody dynamics with hydrodynamic models based on the Cummins equation and linearized hydrodynamic coefficients. These modeling methods are attractive design tools because they are computationally inexpensive and do not require the use of high-performance computing resources necessitated by high-fidelity methods, such as Navier-Stokes computational fluid dynamics. Modeling hydrodynamics using linear coefficients assumes that the device undergoes small motions and that the wetted surface area of the devices is approximately constant. WEC devices, however, are typically designed to undergo large motions to maximize power extraction, calling into question the validity of assuming that linear hydrodynamic models accurately capture the relevant fluid-structure interactions.

In this paper, we study how calculating buoyancy and Froude-Krylov forces from the instantaneous position of a WEC device changes WEC simulation results compared to simulations that use linear hydrodynamic coefficients. First, we describe the WEC-Sim tool used to perform simulations and how the ability to model instantaneous forces was incorporated into WEC-Sim. We then use a simplified one-body WEC device to validate the model and to demonstrate how accounting for these instantaneously calculated forces affects the accuracy of simulation results, such as device motions, hydrodynamic forces, and power generation.

Other aspects of WEC-Sim code development and verification are presented in a companion paper [1] that is also being presented at OMAE2014.
Insights in the Application of Structural Reliability Analysis (SRA) for Challenging Pipeline Lateral Buckling Design
Authors: Gerginov Elena , Rathbone Andrew , Sullivan Chris , Griffiths Terry
Published: September 2014

Lateral buckling of pipelines is influenced by, and sensitive to, a complex combination of factors associated with considerable uncertainty. Key elements such as pipe to soil friction, vertical and horizontal imperfections can vary significantly not only along sections of the same pipeline, but at the same location over time. These factors carry a probabilistic likelihood of occurrence that results in a probabilistic distribution of buckle formation forces and locations, as well as post buckle loads, as described by Rathbone et. al. (2008).

To effectively capture all possible combinations of inputs and results, and thus gain a full understanding of potential buckle behaviour and interaction, would require many individual finite element analyses to be undertaken, which is not viable. The paper describes new insights into challenging pipeline lateral buckling design gained through use of a highly adaptable Structural Reliability Analysis (SRA) method. It discusses the difficulties in defining pipeline and seabed complexities while still retaining adequate detail to fully describe the pipeline system.

A range of realistic lateral buckling design scenarios will be discussed, illustrating how to reduce existing limitations in probabilistic design through the use of the SRA method. The method is flexible enough to enable varying levels of detail to be modelled, from simple idealised checks of buckling susceptibility, which are commonly performed during the early phases of design, to the more complex analysis of pipeline failure, performed during detailed design. The paper will demonstrate the ability for the method to capture complex real life scenarios such as the effect of a bi-modal distribution in observed pipeline embedment, as well as the influence of covariance between frictions, on the probability of buckling and resulting strains.
Numerical Evaluation of the Straight-Line Stability of a VLCC-Tanker Using Computational Fluid Dynamics
Authors: Lamas Francisco , Fernandes Antonio Carlos
Published: September 2014

The main objective of the present work is to propose a methodology to evaluate the Directional Stability index of a displacement ship using Computational Fluid Dynamics to evaluate its linear hydrodynamic derivatives.

The first step is the review of the ship’s equations of motion, followed by the formulation of the straight-line stability problem, which depends basically on the evaluation of the ship’s linear hydrodynamic derivatives. After this, the experimental methods that are normally used to evaluate these characteristics during ship design are presented, since they will be simulated in a numerical environment.

Having reviewed the common experimental methods applied to determine the straight-line stability of a ship, the numerical code used to evaluate it in this work is presented, in order to show how the physical constitution of the experimental apparatus is adapted to a computational environment. Together with this will be presented the KVLCC2 model used or the computations, together with the numerical mesh where the computations were carried out. With this, it becomes possible to show the numerical results obtained in this work.

The last part of this paper consists in comparing the numerical results obtained in the step before with the Clarke statistical correlations in order to assess or not the accuracy of the methodology proposed.
VIV Assessment of Deepwater Lazy-Wave Risers
Authors: Constantinides Yiannis , Zhang Michelle
Published: September 2014

The steel lazy wave riser is an emerging solution for deepwater applications in harsh conditions. The addition of buoyancy to provide the unique “lazy wave” shape reduces the dynamic stresses at the touchdown zone resulting in improved performance due to vessel motions and waves. However as the buoyant region cannot be easily fitted with Vortex-Induced Vibration (VIV) suppression, VIV becomes a critical aspect of the design. The present study focuses on understanding the global response of a deepwater lazy wave riser with a combination of computational fluid dynamics (CFD) and semi-empirical software analysis. An industry first full scale CFD simulation with different buoyancy region geometries is presented and analyzed to understand the field response and provide guidance on important aspects of design. Results show a different response than what was expected based on previous testing of similar systems, introducing a new parameter related to the aspect ratio of the buoyancy modules.
Computations of Arbitrary Thin-Shell Vertical Cylinders in a Wave Field by a Hypersingular Integral-Equation Method
Authors: Hariri Nokob Mohamed , Yeung Ronald W.
Published: September 2014

We present a numerical formulation and computational results for the hydrodynamic loads on bottom-mounted thin-shell vertical cylinders of arbitrary cross-sectional shapes, including open bodies. Such cylinders may undergo prescribed or free motion or may be subjected to a wave load. The formulation is based on linear theory and a hypersingular integral-equation results from sectional contours of zero thickness. The method reduces the fully three-dimensional problem into a number of two-dimensional ones in the horizontal plane and is therefore much faster than the usual boundary integral method used for water wave problems. This traditional method of solution is also known to become ill-conditioned as the body thickness decreases. As an example of the current method, radiation and diffraction loads are presented for the cases of a circular and square closed and opened cylinders with the effect of the increased opening size discussed at different frequencies. The free-surface elevation associated with this method of solution is presented for some cases as well.
Residual Stresses in Weld Repairs and Mitigation by Design
Authors: Song Shaopin , Dong Pingsha
Published: September 2014

It is well known that residual stresses caused by weld repairs tend can be significantly higher than those caused by initial fabrication welds due to severe restraint conditions associated with typical weld repairs. This paper presents an in-depth investigation of repair weld geometry effects on residual stress distribution characteristics, particularly in terms of the through-thickness membrane and bending content that can be shown fundamentally different from that in initial fabrication welds. The results indicate that repair weld design parameters in terms of repair length, depth, and width are of critical importance to the mitigation of residual stresses. A 3D finite element residual stress modeling procedure developed and validated for numerous case studies in the past, after further validation on a selected component in the current study, is used throughout this investigation.
Accurate Estimation of the 100-Year Responses From the Probability Distribution of Extreme Surface Elevations
Authors: Mallahzadeh H. , Wang Y. , Abu Husain M. K. , Mohd Zaki N. I. , Najafian G.
Published: September 2014

Accurate estimation of the 100-year responses (derived from the long-term distribution of extreme responses) is required for the safe and economical design of offshore structures. However, 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, conventional Monte Carlo time simulation technique could be used for predicting the long-term 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 values of the 100-year responses without the need for extensive simulations. It is demonstrated that the technique could be used for both quasi-static and dynamic responses.
Transferring Wave Conditions From Offshore to Nearshore: The Case of Nordfold
Authors: Stefanakos Christos N. , Eidnes Grim
Published: September 2014

In the present work, an analysis of the wave climate in Nord-fold area in the northern Norway has been performed. The analysis was carried out by transferring offshore wave conditions to the nearshore area of interest by successive applications of the well-known third-generation wave model SWAN. The area presents a particular interest, since it has a very deep and complex bathymetry near the coast and a very complicated coastline. Analysis has been carried out using a very detailed bathymetry of the area provided by the Norwegian Mapping Authority. Moreover, as input, five year long time series of directional spectra of offshore wave parameters have been used, after being calibrated using the best available satellite altimeter dataset.
Probabilistic Red Limit Study for Mobile Offshore Drilling Units
Authors: Gjerde Tony , Chen Haibo
Published: September 2014

More and more mobile offshore drilling units (MODUs) are equipped with dynamic positioning (DP) system to maintain vessel position during drilling and well operations. A failure in the DP system might in worst case lead to position loss in terms of drive-off or drift-off. In a position loss scenario, the operator needs to activate the emergency disconnect sequence (EDS) of the marine riser at the time when the vessel reaches a site specific critical distance, denoted the red limit. The red limit distance takes into account the time it takes for the EDS sequence to complete, in addition to the vessel position loss speed and the physical excursion limit for the marine riser, BOP and wellhead system.

The red limit is a site-specific parameter, which is dependent on many factors. This paper presents an alternative probabilistic methodology to determine the red limit based on probabilistic modeling of position loss scenarios, using a case study of DP class 3 drilling vessels at specific well sites and environmental conditions.
Strongly Coupled Method for Predicting the Response of Flexible FOWT With Mooring and its Experimental Validation
Authors: Ma Chong , Iijima Kazuhiro , Nihei Yasunori
Published: September 2014

In this research, a numerical simulation method for a coupled system of a Floating Offshore Wind Turbine (FOWT) and its mooring system is developed. Flexibility of the platform and the nonlinear properties of mooring can be accounted for by the proposed method. A series of scaled model experiments which include the TLP and SPAR types of FOWT are also performed to evaluate the response of the FOWTs under combined wind and wave loads. Steady wind and regular waves are applied to the models. Measurements are made on strains in the structure, tension variation in the mooring as well as the rigid body motions of the platform. For validating the numerical model, comparison between the experimental and simulation results is made. An acceptable correlation between the experimental and the simulation results is obtained. It is shown that the flexibility of the platform may affect the tension variation in the mooring.
Influence of Wave-Induced Skid Motions on the Launch of Free-Fall Skid Lifeboats From Floating Hosts: Experimental and Numerical Investigations
Authors: Luxcey Neil , Reinholdtsen Svein-Arne , Sauder Thomas , Fouques Sébastien , Jin Jingzhe , Kauczynski Wojciech , Hovland Geir
Published: September 2014

The evacuation of personnel from offshore installations in severe weather conditions is generally ensured by free-fall lifeboats. Their performance can be assessed by means of numerical simulations to estimate accelerations loads on occupants, structural loads on the lifeboat hull, as well as forward speed after water-exit. These parameters strongly depend on the water entry conditions of the lifeboat, which in turn are very sensitive to the previous phases of the launch that starts on the skid. On floating production, storage and offloading (FPSO) vessels in the Norwegian Sea, lifeboats are often installed on skids at the bow so that waves may induce large skid motions with typical extreme vertical amplitude of fifteen to twenty meters in a 100-year storm condition. Moreover, wave-induced motions may also cause trim and list of the skid, which initiates more complex six degrees-of-freedom trajectories during free-fall. In such conditions, a proper modelling of the lifeboat trajectory on the moving skid is necessary in order to assess the performance of the lifeboat with numerical simulations.

This paper investigates the effects of the wave-induced skid motion on the launch of free-fall lifeboats from floating hosts. The first part of the paper describes the six-degrees-of-freedom numerical skid model used in MARINTEK’s lifeboat launch simulator VARUNA. The second part presents two model test campaigns aimed at validating the numerical skid model. The model test results are compared to those obtained from the numerical simulations. Finally, the importance of the skid motion on the lifeboat trajectory is discussed.
Fatigue Strength Assessment of Propellers by Means of Weakly Coupled CFD and FEM Analyses
Authors: Bertoglio Chiara , Gaggero Stefano , Rizzo Cesare M. , Viviani Michele
Published: September 2014

Propeller design considerably evolved during years, continuously imposing new challenges to the designer, from usual high efficiency and avoidance of erosive cavitation up to reduction of radiated noise and pressure pulses for “high added value” ships. New more stringent limits, together with a general requirement for performances optimization, leads to the need of a continuous review of design procedures which, in some cases, still rely on rather “old-fashioned” approaches.

The aim of this paper is to analyse in detail the importance of considering fatigue phenomena in the propeller design, proposing a procedure for the evaluation in time of the stress field acting on a blade during one revolution, which is then considered for fatigue assessment.
Benchmark of CFD Modeling of TLP Free Motion in Extreme Wave Event
Authors: Wu Guangyu , Jang Hyunchul , Kim Jang Whan , Ma Wei , Wu Muo-Chung , O’Sullivan Jim
Published: September 2014

In this study, a numerical wave tank was set up to simulate the free motion of a Tension Leg Platform (TLP) in extreme wave event. For better computational efficiency, a nonlinear potential flow solver is coupled with a CFD software, with the former to simulate the far-field wave domain and the latter to simulate the near-field wave domain and platform motion. In order to benchmark against model test, a five-minute time window of interest was selected from the extreme sea state in model test. The incoming irregular wave was firstly reconstructed from the measured wave time history using the nonlinear potential flow solver and then applied as input to CFD simulations for two different headings to the platform. Static offset tests and free decay tests were simulated in CFD initially to confirm that the platform and tendon properties were properly modeled. The 6-DOF platform motions were then obtained from the CFD simulations and the time histories of motion, air gap, and tendon tension were compared with model test measurements. Good agreements were achieved except for the initial transient period and low-frequency motions. In particular, the air gap or relative wave elevation compared well for all the locations around the platform. The high frequency response in tendon tension and the different tension characteristics of weather side tendons and leeside tendons were also well captured.
The Effect of Notch Sharpness on the Fracture Toughness Determined From SENT Specimens
Authors: Moore Philippa
Published: September 2014

Fracture toughness test standards require specimens to be fatigue precracked to generate conservative values of fracture toughness. Nonetheless, it is believed that for ductile steels on the upper-shelf, whether or not the electro-discharge machined (EDM) notch is subsequently fatigue precracked does not affect the value of fracture toughness obtained. Avoiding fatigue precracking during single edge notched tension (SENT) specimen preparation would reduce the testing time, and improve notch placement accuracy and straightness. However, there are circumstances when using EDM notches causes the fracture toughness to be overestimated. It is important to know when fatigue precracking affects the fracture toughness and when it does not. In the work presented here, SENT testing was performed on EDM notched specimens, and on specimens which were subsequently fatigue precracked. Tests were conducted at +20°C, −20°C and −80°C to compare ductile and brittle behaviour. The full tearing resistance curves (R-curves) were reasonably independent of the fatigue precracking, but the initiation value of δ0.2 was higher when EDM notches were used. At lower test temperatures, the difference in fracture behaviour between both notch types was more significant. EDM notches can therefore be most justified for the assessment of fracture toughness determined from the maximum load in the load-displacement curve, i.e. upper shelf behaviour. The upper shelf can be determined from standard fracture toughness testing or estimated using Charpy data, without performing additional fracture toughness tests. Several Charpy-based criteria for determining the temperature of the upper shelf were also evaluated in this work.
Long-Term Fatigue Analysis of Deepwater Risers in the Time Domain
Authors: Gao Yidan , Low Ying Min
Published: September 2014

A floating production system is exposed to many different environmental conditions over its service life. Consequently, the long-term fatigue analysis of deepwater risers is computationally demanding due to the need to evaluate the fatigue damage from a multitude of sea states. Because of the nonlinearities in the system, the dynamic analysis is often performed in the time domain. This further compounds the computational difficulty owing to the time consuming nature of time domain analysis, as well as the need to simulate a sufficient duration for each sea state to minimize sampling variability. This paper presents a new and efficient simulation technique for long-term fatigue analysis. The results based on this new technique are compared against those obtained from the direct simulation of numerous sea states.
Effect of Tube Wall Wettability on the Onset of Churning in Upward Gas-Liquid Annular Flow
Authors: Belfroid S. P. C. , Khosla V. , Nennie E. D. , Alberts G. J. N. , Veeken C. A. M.
Published: September 2014

The influence of hydrophobic and hydrophilic tube walls on the pressure drop and liquid loading behavior was investigated in previous projects in the past. In this paper, results of visualization experiments and of experiments at different liquid-to-gas ratios and inclination angles are presented for hydrophobic tube walls. Experiments were performed in coated and uncoated steel and Perspex pipes of different diameters: 20 mm and 50 mm. Experiments with different deviation angles ranging from 0° (vertical) to close to horizontal (≈80°) were performed for the 20mm diameter tube. The impact of the wall wettability on the flow patterns was examined by performing flow visualizations with a high speed camera in coated and uncoated Perspex tubes. From the experiments it becomes clear that the hydrophobic coating prevents the formation of a liquid film on the tube wall. As a result, the transport of the liquid phase is solely in the form of droplets/ligaments. In the hydrophobic coated tube, the onset of churning is at a lower gas velocity than in the uncoated tube. The presence of the coating reduces the critical gas rate corresponding to the minimum pressure drop by as much as 40%. This decrease reduces for high liquid rates and for larger inclination angles. However, for a liquid content of LGR = 1000 Sm3/106 Sm3, the benefit is still 33% at a vertical flow and a reduction in the critical rate remains up to a deviation angle of 80°. The hydrophobic coating increases the friction at higher gas rate but reduces the hydrostatic head at lower gas rate with a 70% reduction at low liquid content.