bluuebunny/crossref_metadata_2025 huggingface dataset:
Freely Vibrating Two Side-by-Side Square Columns With Combined Translational Motions
Authors: Guan Meng-Zhao , Jaiman Rajeev K. , Kang Chang-Wei , Lim Teck-Bin Arthur
Published: October 2016

This paper presents a set of numerical simulations of flow-induced vibrations (FIV) and coupled wake flow behind two identical square columns in a side-by-side configuration. To observe the four regimes as a function of different gap ratios, the computational results of the configuration at low Reynolds number Re=ρfUDμf in stationary condition are firstly compared with existing experimental data of moderate Reynolds number. We next investigate the configuration of elastically mounted square columns, which are free to oscillate in both streamwise and transverse directions. The simulations are performed by the Petrov-Galerkin finite-element method and Arbitrary Lagrangian-Eulerian technique to account for the fluid mesh motion. The four regimes of stationary side-by-side configuration follow the same trend of the experimental data conducted at moderate Reynolds number, while the ranges of each regime differ due to the turbulent wake properties. For the freely vibrating condition, all the simulations are computed at low Reynolds number (Re = 200), mass ratio equal to 10m*=Mmf and reduced velocity in the range of Ur ∈ [1,50] where Ur=UfND and in free-damping condition ζ = C2KM = 0. The four regimes in vibrating condition are investigated as a function of gap ratios g* = g/D, which is the ratio of spacing between the inner column surfaces to the diameter of the column. The effects of reduced velocity on the force variations, the vibration amplitudes and the vorticity contours are analyzed systematically to understand the underlying FIV physics of side-by-side columns in the four regimes. Finally, we present a FIV study of the full semi-submersible model at moderate Reynolds number Re = 20,000 which can be considered as the application of side-by-side configuration.
Hydrodynamic Study on Water Column Oscillation of Varying Cross-Section Moonpool and its Effect on Resistance of a Drill Ship
Authors: Ponnappan Sivabalan , Sankunny Surendran , Balakrishnan Rajesh Reguram
Published: October 2016

A moonpool is meant for access to the underwater part of the ship from onboard. It is a vertical opening along the depth having an effect on the performance of the floating platform. Inside the moonpool, water motions in horizontal plane is called sloshing and in vertical planes it is called piston mode. Moonpool causes deck wetness and sometimes result in the downtime of the platform. It is the necessity of the operator to be at the safe conditions of platform facing varied environmental conditions. In the present study, vessel response in the region of moonpool resonance was investigated with different shapes of moonpool and comparison is made with Molin’s (2001) theoretical and Fukuda’s (1977) empirical formulas. It is seen that there is a shift in the frequency of resonance based on moonpool shapes. The effect of moonpool on the ship motion with forward speed is also attempted in this paper. Proven packages are used to calculate the calm water resistance of the ship with moonpool of various cross-sections. Wave making coefficient of the ship is modified due to opening to accommodate the moonpool. The openings to accommodate moonpool causes further entry of water both zero and non-zero Froude number especially in the presence of waves.
Novel Experimental Investigation on Vortex Induced Motions of a Tension Leg Platform
Authors: Ramirez Miguel A. M. , Fernandes Antonio Carlos
Published: October 2016

Recently, a growing interest has been seen in vortex induced motions of offshore units. These induced motions are significantly relevant for the design of mooring systems and risers of offshore platforms.

This work analyzes the vortex induced motions (VIM) of a Tension Leg Platform (TLP) when submitted to currents. The model tests were carried out in the fKN@LOC/COPPE-UFRJ laboratory facilities. Furthermore, a novel arrangement to represent the TLP tendons in shallow current channels is presented and discussed. This experimental set-up is innovative since, to avoid the set down, a tower has been implemented. TLP vertical motions are restrained and necessary stiffness for horizontal modes is provided by springs.

The ultra-reduced model with four square columns and four pontoons in closed configuration was built using a scale of 1:200. The range of current velocities was from 0.33 to 2.59 m/s. and five angles of attack were considered: 0, 11.25, 22.50, 33.75, 45 degrees. The reduced velocity reached a very high value of 32 for 45 degrees of heading.

Results have shown that the induced motions are dependent on the angle of attack and the current speed. For zero degrees of heading a typical bell curve of VIM was observed. On the other hand, for 45 degrees, motions increase steadily with current speed.
Impact of Pore Topology on Gas Diffusion and Productivity in Barnett and Haynesville Shale Plays
Authors: Davudov Davud , Moghanloo Rouzbeh Ghanbarnezhad , Dadmohammadi Younas , Curtis Mark , Javadpour Farzam
Published: October 2016

This study evaluates gas transport through matrix pores in Barnett and Haynesville shale gas plays. Gas is mainly stored in a compressed phase in small pores or as a desorbed phase on the surface of the pores. At the early stage of production, gas decompression/expansion within networks of large fractures connected directly to the wellbore is the dominant production mechanism; however, after some time (a couple of weeks up to few months) diffusive flux from the body of kerogen is triggered in response to the pressure drop realized at the fracture face. In this study we present a comprehensive study of the importance of various diffusion patterns to predict the productivity of shale reservoirs at the later stage of production.

We present transient diffusion models to address gas diffusion from kerogen bodies through connected pores with different shapes (slit or cylindrical) toward fractures. In this model we use a new set of boundary conditions as opposed to traditional Dirichlet and Neumann types of boundary conditions. The analytical solution is validated using a set of experimental data.

In the next step, we choose wells from gas producing parts of Barnett and Haynesville shale plays. Based on detailed studies of scanning electron microscope (SEM) images, we identified two different governing patterns for diffusive flux in the shale matrix: inter diffusion in Barnett and intra diffusion in Haynesville. We used production data to differentiate the matrix contribution using conventional methods of rate transient analysis. Using the analytical model, we compared the matrix contribution parts of production data between Barnett and Haynesville shale plays. We found an excellent match between observed small-scale pore shapes and field production data in our transient diffusion model. Based on results, the inter diffusion pattern provides greater flux; while everything else remains equal, Barnett wells benefit more from matrix contribution than those of Haynesville. Results indicate that the distribution of connected pores in the shale matrix has significant effect on the matrix contribution.

The main contribution of this work is providing a transient flow model for gas transport through shale matrix accounting for the impacts of dominant shape of the connected pores and their abundance in the matrix texture (namely: organic or inorganic part). Our results suggest that the inter diffusion in Barnett shale play outperforms the intra diffusion mechanism in the Haynesville field. In addition, the more cylindrical pores, the better the performance. In general, the topology of the connected pores as well as their abundance within kerogen or inorganic matters can greatly influence long-term productivity of gas wells.
CFD Simulations of Non-Linear Sloshing in a Rotating Rectangular Tank Using the Level Set Method
Authors: Grotle Erlend Liavåg , Bihs Hans , Pedersen Eilif , Æsøy Vilmar
Published: October 2016

In this paper, numerical simulations of non-linear sloshing in rectangular tanks are presented. Model implementations in the open source software REEF3D are tested and results compared with experimental data. Three different conditions are compared with experiments in 2D. First, the free surface time-evolution is compared for both linear and non-linear sloshing. In the last case, video images from the SPHERIC project are compared with simulations images of the free surface. A condition with lateral wave impacts in sloshing, with a frequency closer to the natural frequency of the first mode, can be found in this case. The non-linear sloshing, case 2, is also simulated in 3D. The numerical model is solving the RANS equations with the k-ω turbulence model. The level set method is used to capture the interface. Higher order discretization schemes are implemented to handle time-evolution and convective fluxes. A ghost cell method is used to account for solid boundaries and multiple grids for parallel computations. It is found that the limiting factor for the eddy-viscosity has significant influence in case 2 and 3. As the sloshing becomes more violent, the increased strain at the gas-liquid interface overproduces turbulence energy with unrealistically high damping of the motion. 3D simulations are only performed in case 2, which shows slightly better comparison than with 2D. Due to non-linearities and small damping, the time to reach steady-state may take several cycles, but no information is given in the paper [1]. The last case shows promising results for the global motion. As expected, the break up of the liquid surface makes it difficult to resolve each phase. But overall, the numerical model predicts the sloshing motion reasonably well.
Numerical Flow Analysis of Hydrate Formation in Offshore Pipelines Using Computational Fluid Dynamics (CFD)
Authors: Turco Neto Eugenio , Rahman M. A. , Imtiaz Syed , Ahmed Salim
Published: October 2016

Hydrate formation is one of the major challenges faced by the Oil and Gas industry in offshore facilities due to its potential to plug wells and reduce production. Several experimental studies have been published so far in order to understand the mechanisms that govern the hydrate formation process under its thermodynamic favorable conditions; however, the results are not very accurate due to the uncertainties related to measurements and metastable behavior observed in some cases involving hydrate formation. Moreover, thermodynamic models have been proposed to overcome the latter constraints but they are formulated assuming thermodynamic equilibrium, which such condition is difficult to be achieved in flow systems due to the turbulence effects. Due to the low solubility of methane in water, the mass transfer effects can possibly control several mechanisms that are still unknown about the hydrate formation process. Also, the reaction kinetics plays a major rule in minimizing hydrate formation rate. The objective of this work is to develop a mechanistic Computational Fluid Dynamics (CFD) model in order to predict the formation of hydrate particles along the pipeline from a hydrate-free gas dominated stream constituted by methane and water only. The transient simulations were performed using a commercial CFD software package considering the multiphase hydrate chemical reaction and mass transfer resistances. The geometry used was a straight pipe with 5 m length and 0.0254 m diameter. The results have shown the appearance of regions in the pipeline at which hydrate formation is controlled either by the mass transfer or reaction kinetics. The rate of hydrate formation profile has shown to be high at the inlet even though the temperature at that regions was high, which can be a possible explanation for metastable region encountered in most of recent phase diagrams.
Analysis of Variance to Determine the Effect of Hull Form Parameters on Resistance and Seakeeping Performance for PSV Hulls
Authors: Boyd Nicholas , Molyneux David
Published: October 2016

Throughout the world many Platform Supply Vessel designs have been proposed as the optimal form for their given operating environment, but evaluating these claims has been difficult due to a poor understanding of the relationships between hull form shapes and performance for these vessels. This paper presents the results of analysis aimed at determining these relationships.

Results of CFD calculations to determine the Effective Horsepower/tonne for a series of PSV designs were presented in the paper A step towards an optimum PSV Hull form. This paper presents results for 16 separate hull forms, which were designed as each possible combination of four two-level hull form parameters. The hull form features considered were bow shape (vertical stem or bulbous), flat of bottom (flat or deadrise), length of parallel mid body (short or long), and stern shape (convention or integrated); resistance was calculated at two typical operating speeds (10 and 14 knots). This set of results was favourable for analysis using the statistical design of experiments technique: analysis of variance, which was used to determine the relationship between the hull and resistance performance.

The same hull form series was used to study the effects of the hull form parameters on motions in head waves. A 2 level factorial experiment was designed based on the hull parameters with the heave and pitch response calculated using the potential flow ship motion prediction code Shipmo3D, for each of two representative wave conditions (summer light seas and winter heavy seas) at the zero speed and 10 knot operating speed. Analysis of variance was used to analyze the heave and pitch responses measured, and was used to determine the relationship between each hull parameter and each response.

In both cases a 5% F-test was used to determine the significance of each parameter studied, and the significant effects were analyzed to determine their contributions to the overall model of the data.

The results have found the relationships between the hull design parameters and the Effective Horespower/tonne, heave, and pitch response of the vessel, indicating which factors provide the largest contribution to minimizing each response. The interaction effects between factors were also examined to allow for a generalized understanding of the resulting effect of selecting one hull parameter over another. A numerical model combining all significant factors was fitted to the data, allowing for multiple objective optimization to determine which hull forms provide the most desirable performance for each response.
Scheduling of Offshore Support Vessels on the Grand Banks
Authors: Molyneux David , Boyd Nicholas
Published: October 2016

The Grand Banks off the coast of Newfoundland and Labrador present a challenging environment for the offshore oil industry. There are currently three oil producing developments, with an average distance offshore of approximately 185 nautical miles. The environment is characterized in winter by strong winds, high waves and the presence of pack ice. In the summer the area is prone to heavy fog and low visibility. Offshore support vessels are a key element of any marine based drilling and production system and they must function effectively in this environment. Their key role is to provide a timely delivery of supplies (and if necessary personnel) to and from the offshore platform.

The main challenge for the existing vessels operating on the Grand Banks is that there is a high level of seasonal variation in the weather conditions between winter and summer. The research results presented in this paper were a preliminary attempt to understand the relationships between the distance offshore of the production or drilling platform, the number of supply vessels serving a platform, vessel speed, weather conditions and the required slack in the vessel schedule to provide a reliable service. This analysis was undertaken as there is significant potential for offshore development within the Flemish Pass Basin, which is even further offshore (270 nautical miles) and in a more severe winter wave regime.

The results show that although the current Offshore Support Vessel fleet is satisfactory for 185 nautical miles offshore, there will need to be some changes in order to provide effective service when developments move further offshore. This may include a combination of more ships to service an offshore platform, faster ship speeds, reduced frequency of delivery, or increasing the limiting wave height for lifting operations.
Local Strain and Stress Calculation Methods of Irregular Honeycombs Under Dynamic Compression
Authors: Yu Jilin , Wang Peng , Liao Shenfei , Zheng Zhijun
Published: October 2016

Several continuum-based shock models have been proposed to understand the dynamic compressive behavior of cellular materials, but they are mainly based on the quasi-static stress–strain relation and thus lack sufficient dynamic stress–strain information. A virtual ‘test’ of irregular honeycombs under constant-velocity compression is carried out using the finite element method. A method based on the optimization of local deformation gradient by using the least square method is employed to calculate the one-dimensional strain distribution in the loading direction of the specimen. Meanwhile, a method based on the cross-sectional engineering stress is developed to obtain the one-dimensional stress distribution in the loading direction. The two typical features of cellular materials under dynamic crushing, namely deformation localization and strength enhancement, can be characterized by the strain and stress distributions, respectively. The results also confirm the existence of plastic shock front propagation in cellular structures under high-velocity impact, from which the shock wave speed can be estimated. The shock wave speed obtained from the local strain field method coincides with that from the cross-sectional stress method. The results of shock wave speed are also compared with those predicted by continuum-based shock models. It is shown that the shock wave speed predicted by the R-PP-L (rate-independent, rigid–perfect plastic–locking) shock model or the R-LHP-L (rate-independent, rigid–linearly hardening plastic–locking) shock model is overestimated, but that predicted by the R-PH (rate-independent, rigid–plastic hardening) shock model is close to those obtained from the local strain and cross-sectional stress calculations using the cell-based finite element model.
Numerical Study on Through-Thickness Cone Cracking of Reinforced Concrete Slabs Struck Normally by Flat-Ended Projectiles
Authors: Xu P. B. , Cheng J. S. , Wen H. M.
Published: October 2016

Through-thickness cone cracking is one of major failures which needs to be considered in the safety calculations and assessment of containment structures in a nuclear power plant. In this paper, numerical study is performed on the through-thickness cone cracking of reinforced concrete slabs subjected to impact by flat-ended projectiles at normal incidence. First, a recently proposed 3D meso-mechanical model together with a recently developed computational concrete constitutive model are briefly described and then employed to study numerically the problem. The present numerical predictions are compared with some available experimental results. Furthermore, the 3D meso-mechanical model predictions are also compared with the numerical results obtained from FEM (finite element method) model. It transpires that the present numerical simulations are in good agreement with available experimental observations for the through-thickness cone cracking of reinforced concrete slabs struck normally by flat-nosed missiles. It also transpires that the 3D meso-mechanical model can predict more accurately than the FEM model in terms of the details of crack patterns though these two models produce similar results for relatively low velocities.
Economic Seafloor Massive Sulfide Mining by Japan’s Model
Authors: Yamazaki Tetsuo , Takeda Yosuke , Arai Rei , Nakatani Naoki
Published: October 2016

Because of the higher Au, Ag, and Cu contents, seafloor massive sulfides (SMS) have received much attention as future commercial mining targets by private companies and nations. One of them, Solwara 1 Project in Papua New Guinea (PNG), is scheduled to start the commercial mining operation from 2018. Because the mining site is inter-island area and almost no cost is necessary for the waste disposal in PNG, the economy of the mining is expected very well. In contrast with this, because all the SMS distribution sites in Japan locate outer ocean areas and the waste disposal cost on land in Japan is very expensive, the economy of SMS mining in Japan is quite negative. In order to overcome the problems, a self-standing riser with flexible link to the sea surface platform and a primary ore separation on the seafloor prior to the ore lift-up are proposed. The improved SMS mining concept named Japan’s model is examined.
Vortex Shedding Control Using Jets: A Computational Study With Lattice Boltzmann Method
Authors: Fu Guoqiang , Younis Bassam A. , Sun Liping , Dai Shaoshi
Published: October 2016

The use of the Lattice Boltzmann Method (LBM) for fluid-flow simulation has been the subject of several recent studies where it was reported that the method offers many advantages such as high accuracy coupled with computational efficiency. In this paper, we report on the use of this method, in conjunction with Large-Eddy Simulations, to study an interesting phenomenon related to the suppression of vortex shedding from circular cylinders. Specifically, it has been observed in experiments that vortex shedding from a cylinder can be drastically reduced by the injection of a fluid jet into the approach flow. We first present results for a cylinder without jet injection in order to quantify the suitability of the LBM for use in such flows. Thereafter, we present results for the case with jet injection where we consider the case where Re = 55,440. Preliminary results conclusively demonstrate that the presence of jet injection does indeed lead to substantial reduction in the magnitude of lift forces on the cylinder. This strongly argues in favor of further research to understand the dynamics of this phenomenon, and the range of parameters needed to maximize its beneficial effects.
First- and Second-Order Wave-Induced Dynamic Response of Submerged Floating Tunnels
Authors: Leira Bernt J.
Published: October 2016

Submerged floating tunnel concepts have been developed for the purpose of crossing wide straits with a span length of many kilometers. The frequencies of the first dynamic modes of these structures may typically be of the order of one minute or more. This implies that amplified dynamic response effects need to be assessed both for the wave-frequency and the low-frequency regimes. The first order (i.e. wave frequency) dynamic response has been found to depend strongly on wave direction and wave spreading. This is also be expected to apply for the second-order response. Furthermore, the small inherent damping levels for the low-frequency response will imply that high dynamic amplification may occur.

These issues are addressed by the present paper. Computational methods for evaluation of the hydrodynamic loading and the associated dynamic response are addressed. Both simplified assessment as well as more refined calculation procedures are considered.

A specific case study is considered for the purpose of illustrating the relative effects of the various response components. The example structure is a tunnel with a double cross-section which was developed for crossing of the Sognefjord located at the West Coast of Norway. The same type of floating tunnel is also relevant for crossing of other fjord spans.
The Temperature Distribution Analysis of the Large LNG-FSRU
Authors: Wang Chong , Qin Hongde , Shen Jing , Hao Xiuzi
Published: October 2016

LNG-FSRU (Liquefied Natural Gas-Floating Storage and Regasification Unit) can adopt the structure of double-row cargo tanks. Meanwhile, its heat maintenance system has the characteristics of the special structure, the large temperature difference between inside and outside and the complicated heat transfer process. Aiming at this particular structural form and considering the hull structure and some effect of convection in cavity, this paper establishes the mathematical model and utilizes the finite element method to conduct the deep analysis of ultra-low temperature field in order to obtain its temperature distribution in the ultra-low temperature field. Moreover, the results indicate that the temperature distribution of the double-row-tank structure is linear from the inside out with the relatively uniform overall distribution; the temperature difference above and below the waterline has affects the temperature distribution of the inner shells; the temperature of the middle longitudinal cofferdam zone is relatively lower than that of other areas.
Slam Induced Loads on a 3D Bow With Various Pitch Angles
Authors: Wang Yiwen , Wu Weiguo , Guedes Soares C.
Published: October 2016

A 3D water entry of a typical bow model of sea-river link ship is studied using both experimental measurements and numerical predictions. A large number of systematic experiments have been performed with different pitch angles. The slamming process is simulated through finite element method with LS-DYNA. The distribution and magnitude of slam induced loads is determined from experiments and is calculated. The effect of the pitch angle and impact velocity is discussed based on the comparison between the predicted results and the experiments values.
Responses of Submarine Pipelines to Impacts in Soft Clay
Authors: Wang Yi , Yue Jixiang , Duan Menglan , Yu Zhang , Zhao Yi
Published: October 2016

Subsea pipelines have been widely used to transfer oil from platforms to the mainland near harbor anchorage zone, and they become increasingly susceptible to risks stemming from dropped object impact damages. The impact from dropped objects may lead to local deformation and fracture in the pipeline and vast economic and environmental consequences. In this research, the responses of continuously supported offshore pipelines subjected to transverse impacts caused by dropped objects are studied. For this, the impact on an internally pressurized pipeline resting on a flexible bed has been numerically simulated. A relatively extensive parametric study has then been carried out to examine effects from variations in the cement coating thickness, internal pressure, indenter shape, impact velocity and subsoil mechanical properties on the pipeline response. It has been noticed that the presence of internal pressure results in substantial decrease in the impact dent depth, causing the deformation to become spatially more localized. It has also been shown that the flexibility of pipe bed plays an important role in the impact energy dissipation.
An Empirical Procedure for Fatigue Damage Estimation in Instrumented Risers
Authors: Shi C. , Manuel L.
Published: October 2016

Vortex-induced vibration (VIV) can lead to significant fatigue damage accumulation in deepwater marine risers. In order to assess the effects of VIV and to ensure riser integrity, field monitoring campaigns are often conducted wherein riser response is recorded by a few data sensors distributed along the length of the riser. It is possible to empirically estimate the fatigue damage at “key” critical fatigue-sensitive locations, where sensors may not be available as part of the spatially distributed discrete measurements. In this study, two empirical techniques — Proper Orthogonal Decomposition (POD) and Weighted Waveform Analysis (WWA) — are sequentially applied to the data; together, they offer a novel empirical procedure for fatigue damage estimation in an instrumented riser. The procedures are briefly described as follows: first, POD is used to extract the most energetic spatial modes of the riser response from the measurements. Often, only a few dominant POD modes preserve most of the riser motion kinetic energy; other modes are less important. Identified POD mode shapes are discrete as they are defined only at the available sensor locations. Accordingly, a second step in the proposed procedure uses WWA to express each dominant POD mode as a series of riser natural modes that are continuous spatial functions defined over the entire riser length. Based on the above empirically identified modal information, the riser response over the entire length is reconstructed using backward procedures — i.e., compose identified natural modes into the POD modes and, then, assemble all these dominant POD modal response components into the derived riser response. The POD procedure empirically extracts the energetic dynamic response characteristics without any assumptions and effectively cleans the data of noisy or less important features, which makes it possible for WWA to identify dominant riser natural modes — all this is possible using the limited number of available measurements from sensor locations. Application of the entire procedure is demonstrated using experimental data from the Norwegian Deepwater Programme (NDP) model riser.
Experimental Study About the Influence of the Free End Effects on Vortex-Induced Vibration of Floating Cylinder With Low Aspect of Ratio
Authors: Gambarine Dennis M. , Figueiredo Felipe P. , Fujarra André L. C. , Gonçalves Rodolfo T.
Published: October 2016

Experiments regarding free-end effects on vortex-induced vibration (VIV) of floating circular cylinders with low aspect ratio were carried out in a towing tank. Four cylinders with low aspect of ratio, L/D = 2 (Length / Diameter) were tested with different free end corner shape types, namely by the relation between chamfer rounding radius (r) divided by the radius of cylinder (R) (r/R = 0.0, 0.25, 0.5 and 1.0). For the initial case, r/R = 0.0 represents flat tip and r/R = 1.0 the hemispherical tip. The aims were to understand the effect of different free-end types on VIV behavior of cylinders. The floating circular cylinders, i.e. unit mass ratio m* = 1(structural mass/displaced fluid mass) were elastically supported by a set of linear springs to provide low structural damping on the system and allow six degrees of freedom. The range of Reynolds number covered 3,000 ≤ Re ≤ 20,000. To conclude, cylinder with r/R = 0.25, shows lower amplitudes in transverse direction. The same occurs for the cylinder r/R = 0, but for amplitudes of vibration in in-line direction. Behaviors of the vibration frequencies in in-line and transverse direction don’t have significantly differences. Regarding force coefficient, flat tip cylinder (r/R = 0) presents higher values compared to the others however, for the lift coefficient, results converge in similar values for the same velocities that were observed higher transverse amplitudes. The visualization experiments show an expressive reduction of the recirculation bubble for r/R = 0.5 model compared with the flat tip, can therefore justify the lower values for this model obtained in draft amplitudes and drag coefficient compared with the flat tip model.
Parametric Study Related to the Collision Probability Between Two Risers
Authors: Fu Ping , Leira Bernt J. , Myrhaug Dag
Published: October 2016

Efficient and accurate methods for assessment of riser collision become increasingly important as offshore oil and gas exploration moves into deeper water. Collisions may occur as a result of the environmental loading (current and waves), flow interaction and surface floater motions, or combinations of these effects. This paper provides a parametric study on the influence of these factors, especially the influence of the fluid interference on the minimum distance statistics. The conditional collision probability for two flexible risers in a tandem arrangement is also estimated for sea-states with a 100 year return period.

The drag coefficient of the downstream riser is reduced due to flow-field interaction according to the relative position of the risers. The reduced drag force results in a decrease of the relative distance between two risers when current is applied. This leads to an increase of collision probability. In this paper, the drag coefficient is obtained by Blevins wake model and used for both static and dynamic analysis. The statistics of the minimum distance between two risers are calculated according to the relative position of Finite Element nodes which are obtained from dynamic analyses. Based on a set of such dynamic analyses, the probability distribution function of the minimum distance is estimated. The probability of riser collision can then readily be computed.
Environmental Regulations in Shipping: Interactions and Side Effects
Authors: Jafarzadeh Sepideh , Ellingsen Harald
Published: October 2016

The shipping sector progressively faces stricter environmental regulations. Several technical and operational measures are available to ship owners and operators in order to comply with these regulations. While some measures are successful in reducing emissions of a pollutant, they may increase emissions of other pollutants. In addition, there may be interactions between regulations on environmental impacts on sea and air. In some other cases, despite the success of these measures in reducing environmental footprints of ships, the environmental problem may shift elsewhere in the life cycle. Therefore, there may be a trade-off between different environmental controls and, in a broader context, a trade-off between environmental impacts at a life stage and overall environmental goals. Although several studies focus on addressing individual regulations, few acknowledge such interactions. This study aims at highlighting the side effects of different measures to address environmental regulations.
Towards an Improved Understanding of Green Water Exceedance at the Bow of an FPSO
Authors: van ‘t Veer Riaan , Boorsma Anne
Published: October 2016

When a permanently moored FPSO in deep draft condition finds itself in harsh weather conditions it most likely will experience freeboard exceedance at the bow and consequently green water on deck. Knowledge about how and how much water will come onto the deck, is relevant for both a turret moored and spread-moored FPSO since both are expected to experience the most severe design conditions in head to bow quartering waves.

In this paper we focus on green water phenomena observed on a Suezmax FPSO in severe head seas in the model test basin. Using an on-board video in combination with deck-mounted wave probes, green water events are recorded in detail. This provides unique insights in how the water floods onto the deck. Very different flooding phenomena are observed between different events and they can strongly deviate from a dam-breaking kind of event.

Through a detailed description of three typical flooding events, the present paper improves the understanding of how green water flows onto the deck. It highlights the benefit of detailed model tests and emphasizes that new and more detailed modelling is required since the dam-breaking theory has limited value for the present problem.
On the Simulation of Wind, Waves and Currents During Hurricane Sandy for Assessing the Performance of Jacket-Supported Offshore Wind Turbines
Authors: Kim E. , Manuel L.
Published: October 2016

The simulation of wind and wave fields for the evaluation of jacket-supported offshore wind turbines during Hurricane Sandy of 2012 is the focus of this study. For realistic load assessment of offshore wind turbines, it is important that coupled wind, wave, and current fields appropriately resolved spatially are provided throughout the evolution of the hurricane. A numerical model describing the hurricane track and intensity variation with time is used to generate the coupled wind, wave, and current fields. Time series of turbulent wind fields and coupled wave kinematics are generated using the hurricane simulation output and a detail procedure is presented. These coupled wind, wave, and current fields form the inputs for the analysis of a 5-MW offshore wind turbine supported on a jacket platform sited in 50 meters of water and assumed located in the path of Hurricane Sandy. The turbine’s and support structure’s performance under the simulated loading conditions is the subject of a companion paper.
Fatigue Modelling of Semi-Elliptical Surface Cracks in Welded Pipe Geometries
Authors: Hoh Hsin Jen , Pang John H. L. , Tsang Kin Shun
Published: October 2016

Offshore pipelines and risers transfer oil and gas across long distances, from seabed to production facility to the surface. The long pipelines are formed by welding together pipe segments. The welded joints formed are a source of stress concentration and defects from which fatigue cracks can grow. Hence, it is imperative that the effect of the weld geometry on the stress concentration be understood so that appropriate measures can be taken to assess the potential remaining service life of the welded structure.

The effects can be understood by the linear elastic fracture mechanics approach, where the stress intensity factors quantify the stress concentration. While the classical equations of Newman and Raju have been long available for semi-elliptical surface cracks in plates, no similarly elegant stress intensity factor solutions are available for pipes. There have been solutions in tabular form which can be cumbersome in practice. Moreover, solutions of welded pipe geometries have not been developed. The objectives of the current work are to develop closed-form solutions for stress intensity factors for external semi-elliptical surface cracks in plates. The welded pipe geometry will also be studied to develop solutions for the weld toe magnification factors of welded pipe geometries. The stress intensity factors can be used to determine the propagation rate of cracks in pipe or welded pipe geometries.

The stress intensity factors are obtained by the J-integral output of the three-dimensional finite element method. First, a plate with a circular crack is modelled. The initial step transforms the model to a plate with a semi-elliptical crack with the appropriate crack aspect ratio and width. A second transformation follows to transform the geometry to pipe form. The main parameters studied are the relative crack depth to thickness, crack aspect ratio, radius and thickness. The developed stress intensity factor solutions can be reduced to the classical equations. The new solutions show good agreement compared to previous work. A similar approach is developed to study the welded pipe geometry to develop weld toe magnification factor solutions. The weld toe magnification factor solutions for certain geometries are presented as a function of the relative crack depth.

The stress intensity factor solutions are then applied to predict the crack growth rates of cracks in pipe geometries. The prediction was conducted by a program written to assess the fatigue life of single and multiple cracks in pipes and welded pipes. The fatigue life assessment of welded pipes using the weld toe magnification factor solutions shows how significantly the weld geometry affects fatigue life.
Effect of Nano-MoS2 Particles on Properties of Micro-Arc Oxidation Coating Prepared on the Surface of Aluminum Alloy Drill Pipe for Offshore Platform
Authors: Pan Yipeng , Shen Yan , Sahoo Prasanta K.
Published: October 2016

In order to improve the corrosion resistance of the drill pipe, the alumina ceramic membrane is coated on the surface of aluminum alloy drill pipe of offshore platform by micro arc oxidation technology. This paper mainly focuses on the experimental work to determine the effect of the nano-MoS2 content in the solution on the micro morphology, element content and corrosion resistance of the coating. The results show that the amount of nano-MoS2 particles has big influence on the making of micro-arc oxidation (MAO). The thickness of MAO coating is increased with the adding of nano-MoS2 particles. Besides, the size of the coating is first increased then decreased, and the surface of coating become more compact and smooth with the increase of the amount of nano-MoS2 particles. Also, the corrosion resistance of MAO coating is improved as the amount of MoS2 increases. The corrosion rate is decreased from 0.032g.m−2.h−1 to 0.024g.m−2.h−1 when the addition amount of nano-particles increased from 0.5g/L to 2g/L.
Influence of Different Wave Load Sequence Models on Fatigue Life Prediction of Ship Structures Based on Fracture Mechanics Approach
Authors: De Gracia Luis , Osawa Naoki , Mao Wengang , Ichihashi Daichi
Published: October 2016

FCP (Fatigue Crack Propagation) based fatigue assessments of a welded joint in a 2800 TEU container ship which sails on North Atlantic routes are performed. The Great-circle course with small variation of relative heading angle and a southerly course with large variation of relative heading angle are considered. Short sea sequences are generated by using ‘storm model’ developed by Osaka University and ‘spatio-temporal model’ developed by Chalmers University. Sea keeping analyses are performed for both cases wherein the variation in wave direction’s occurrence probability is considered (‘real headings model’) and not (‘all-headings model’). FCP analyses are performed considering plasticity-induced crack closure by using FASTRAN-II. Fatigue crack propagation lives and characteristics of crack propagation retardation due to excessive loads are compared. Based these results, the influence of the difference in load sequence model on FCP-based fatigue assessment result is discussed.
Drag Reduction and VIV Suppression Behaviour of LGS Technology Integral to Drilling Riser Buoyancy Units
Authors: Marcollo H. , Potts A. E. , Johnstone D. R. , Pezet P. , Kurts P.
Published: October 2016

Drilling risers are regularly deployed in deep water (over 1500 m) with large sections covered in buoyancy modules. The smooth cylindrical shape of these modules can result in significant vortex-induced vibration (VIV) response, causing an overall amplification of drag experienced by the riser.

Operations can be suspended due to the total drag adversely affecting top and bottom angles. Although suppression technologies exist to reduce VIV (such as helical strakes or fairings), and therefore reduce VIV-induced amplification of drag, only fairings are able to be installed onto buoyancy modules for practical reasons, and fairings themselves have significant penalties related to installation, removal, and reliability.

An innovative solution has been developed to address this gap; LGS (Longitudinally Grooved Suppression)1. Two model testing campaigns were undertaken; small scale (sub-critical Reynolds Number flow), and large scale (post-critical Reynolds Number flow) to test and confirm the performance benefits of LGS.

The testing campaigns found substantial benefits measured in hydrodynamic performance that will be realized when LGS modules are deployed by operators for deepwater drilling operations.
Effects of Disturbance of Current Field on Power Characteristics of a Floating Type Pitch-Controlled VAMT in a Real Sea
Authors: Ikoma Tomoki , Masuda Koichi , Eto Hiroaki , Rheem Chang-Kyu , Enomoto Osamu
Published: October 2016

While a type of marine turbine for tidal current generation can be chosen from several types, a vertical axis marine turbine (VAMT) should be better in Japan because sea areas around Japanese islands where current velocity is sufficient are limited. This study conducted a sea test of a VAMT of a floating type installed with six straight pitch-controllable blades. The cycloidal mechanism was adapted for the pitch control. The purpose of the study is to understand effects of ocean waves and motion of a floating body on turbine performance and behaviours of the VAMT in unideal current conditions. Besides, the data taken should be effective to consider that effects in order to design VAMTs. The setup with the setting angle of −30 degrees suggested highest performance from the sea tests, then 15% in maximum turbine power and maximum output was 40W. Ocean waves strongly affected on the turbine performance because fluid velocity changes due to ocean waves and it was unable to neglect the variation of the velocity in spite of small. The characteristics of the turbine sensitively varied because of ocean waves. The results suggested that during accelerating and decelerating incoming fluid speed, characteristics of the turbine were different in each case.
A Dual-Limit-Erosion Approach to Estimate Fracture Zone of RC Slabs Subjected to High Velocity Impact Loads
Authors: Itoh Masaharu , Matsuzawa Ryo , Beppu Masuhiro
Published: October 2016

This paper presents a novel erosion approach to reproduce fracture of concrete subjected to impact loadings. An erosion method is a numerical technique to remove highly distorted Lagrangian meshes which, if not deleted, tend to decrease the accuracy of numerical results obtained and often cause to terminate the calculation prematurely. Existing erosion criteria generally use a single thermo-mechanical variable for the deformation measure such as strain, stress tensor or damage. In this study we propose to utilize two measures to activate the erosion function with a view to better reproducing fracture zones of concrete by distinguishing the element deformation in compression or tension state. Test calculations in two and three dimensions are carried out to investigate the applicability of the proposed erosion method, and the numerical results are discussed with reference to available experimental data.
Structure Design of Large Blocks Lifting Tooling
Authors: Ji Zeng , Yuhan Wang , Wei Zhang , Chunhui Li , Chao Wang , Ruijia Zhou
Published: October 2016

Gantry crane is used frequently in the dock. How to reduce the number of lifting and improve construction efficiency are given more attention by shipyard. According to site and equipment conditions of a shipyard, a tooling which is used for the large blocks lifted is designed to improve construction efficiency. Moreover, the processes of lifting the large blocks are described. Tooling structure strength is checked by FEM. And the influence of different rails arrangement for tooling structure is analyzed. The successful tooling design plan is of great significance for improving construction efficiency, reducing building period and saving construction cost.
Fully Nonlinear Dispersive HAWASSI-VBM for Coastal Zone Simulations
Authors: Adytia Didit , Lawrence
Published: October 2016

The accuracy of a wave model for simulating waves in deep and coastal areas is highly determined by the dispersive properties as well as by the nonlinearity of the model. The Variational Boussinesq Model (VBM) for waves [1–4], available publicly as HAWASSI-VBM software [5], is based on the Hamiltonian structure of surface gravity waves. The model has tailor-made dispersive properties, which can be set to be sufficiently accurate for simulating a desired wave field. In this paper, we extend the nonlinear property of the HAWASSI-VBM from weakly nonlinear to be fully nonlinear. To show the improvement in nonlinearity, simulations of the model with a Finite Element implementation is tested against laboratory experiments, of regular and irregular waves propagating above a submerged bar and the dam-break problem.