bluuebunny/crossref_metadata_2025 huggingface dataset:
Copula-Based Bivariate Modelling of Significant Wave Height and Wave Period and the Effects of Climate Change on the Joint Distribution
Authors: Vanem Erik
Published: October 2016

The joint distribution of several met-ocean variables is required for risk assessment and load and response calculations in marine engineering. For example, a joint description is needed to construct environmental contours for probabilistic structural reliability analyses. Typically, the joint distribution of significant wave height and wave period is required as a minimum.

This paper presents a study on various bivariate modelling techniques for the joint distribution of significant wave height and zero-crossing wave period, i.e. a conditional model, a bi-variate log-normal model and several meta-models based on parametric copulas. Each of the models is fitted to data generated from a numerical wave model for the current climate and for two future climates consistent with the RCP 4.5 and RCP 8.5 scenarios. Thus, the objective of this study is twofold. First, the joint models obtained by the various modelling techniques will be compared. Secondly, the potential effect of climate change on the simultaneous distribution of significant wave height and wave period will be explored.

The results indicate that straightforward application of many of the most common families of copulas fails to capture the dependence structure in the data, and that the conditional model performs better than these naive approaches. However, if more advanced copula construction techniques are applied, significant improvements can be achieved. The results also suggest that significant wave height and zero-crossing wave period tend to be more correlated in a future climate, at least in the extremes.
Evaluating Vibration Performance of a Subsea Pump Module by Full-Scale Testing and Numerical Modelling
Authors: van Beek Pieter J. G. , Pereboom Hajo P. , Slot Harmen J.
Published: October 2016

Prior to subsea installation, a subsea system has to be tested to verify whether it performs in accordance with specifications and component specific performance evaluation criteria. It is important to verify that the assembled components work in accordance with the assumptions and design criteria used in the detailed engineering. These criteria also cover the vibration performance. In the current study, the pump module within the Åsgard subsea compression station has been subjected to such system evaluation test, including its vibration performance. Vibrations may be caused by internal and external flow through a complex process that is affected by numerous factors such as the piping geometry, flow and operating conditions and also the fluid properties. When severe, mechanical vibrations can lead to fatigue failure of the equipment components.

One of the major parameters that affects the vibration response of the subsea piping is the surrounding water. It is generally known that surrounding water does participate in some vibration modes by adding mass to the total, dynamic mass participating in the vibration. Therefore, resonant frequencies of a piping system will have different values for non-submerged and submerged cases. In addition, the surrounding water can also lead to higher damping of the vibration modes.

In this paper the effect of submerging a pipe system in water is quantified, by analyzing the changes in damping coefficient and the characteristics of measured pipe vibration in-situ. This is achieved by analysis of full-scale frequency response tests performed on a subsea pipe system within the pump module in both non-submerged and submerged conditions. The results are used for validation of numerical techniques that are used to quantify pipe vibration in submerged conditions. Different modeling techniques for the submerged case are investigated.

It is shown that the effects from the surrounding water on pipe vibrations are different for small-bore piping than that for main piping. Furthermore the different modeling approaches and general observations and trends in damping coefficients are discussed and compared with the measurements.
Estimation of Stick-Slip Characteristics by Simulation With Actual Drilling Data
Authors: Inoue Tomoya , Katsui Tokihiro , Ishiwata Junya , Matsuo Miki Y. , Rheem C. K.
Published: October 2016

Stick-slip, a term for the spontaneous torsional vibrations occurring on a drill bit, is a critical problem in offshore drilling because of the damage it causes to the drill bits. Moreover, it leads to crushing or grinding of the sediment layer, which is a critical problem especially in scientific drilling where the purpose is to recover core samples from the layers.

To detect the occurrence of stick-slip and its fundamental characteristics onboard in real-time, we developed an analytical model is developed to express the torsional motions at the drill bit, leading to the stick-slip phenomenon. The analytical model includes the unknown frictional torque at the drill bit, which depends on the properties of the drilling layers and is a dominant factor for demonstrating the stick-slip phenomenon. This study proposes a method to determine the frictional torque by integrating the analytical model with the surface drilling data. It would be advantageous to identify the characteristics of stick-slip from the surface drilling data. The method is validated with the results of small-scale model experiments in a water tank to observe the stick-slip phenomenon.

Furthermore, we acquired the actual drilling data, which are surface drilling data and downhole data, during the drilling operation of the scientific drilling vessel Chikyu.

This study focuses on attempts to estimate the stick-slip characteristics by the simulation based on the established analytical model with actual surface drilling data acquired during the drilling operation of the scientific drilling vessel Chikyu, and a comparison with the downhole data including the bit motions.
Effects of Curvature on Slamming Loads
Authors: Weber John B. , Das Raj , Battley Mark
Published: October 2016

Much research has been directed at understanding and predicting water slamming loads for a range of geometries of varying rigidity and size. Analytical and numerical studies focused on slamming of cylindrical rigid bodies are present in literature but there are relatively few experimental studies useful for validation purposes, none of which methodically investigate a range of curvatures. Despite the current understanding of slamming loads and structural responses, high speed marine vehicles still experience slamming related failures in operation.

In this study, nominally rigid, singly curved prismatic specimens of varying curvature are subjected to constant velocity water impacts relevant to those encountered by high performance offshore racing yachts and other high-speed craft.

Peak impact forces of 14 to 52 kN were recorded while testing specimens with radii ranging from 0.300 to 5.000 m. Experimental peak impact force and event impulse are found to be significantly lower than predicted by numerical and small scale empirically derived methods. A modification is introduced which improves the empirical model.
Strength Performance of Steel Catenary Riser Using Short Term and Long Term Analysis Methodologies
Authors: Wang Feng , Burke Roger , Sablok Anil , Aronsen Kristoffer H. , Dalane Oddgeir
Published: October 2016

Strength performance of a steel catenary riser tied back to a Spar is presented based on long term and short term analysis methodologies. The focus of the study is on response in the riser touch down zone, which is found to be the critical region based on short term analysis results. Short term riser response in design storms is computed based on multiple realizations of computed vessel motions with various return periods. Long term riser response is based on vessel motions for a set of 45,000 sea states, each lasting three hours. The metocean criteria for each sea state is computed based on fifty six years of hindcast wind and wave data. A randomly selected current profile is used in the long term riser analysis for each sea state. Weibull fitting is used to compute the extreme riser response from the response of the 45,000 sea states. Long term analysis results in the touch down zone, including maximum bending moment, minimum effective tension, and maximum utilization using DNV-OS-F201, are compared against those from the short term analysis. The comparison indicates that the short term analysis methodology normally followed in riser design is conservative compared to the more accurate, but computationally more expensive, long term analysis methods. The study also investigates the important role that current plays in the strength performance of the riser in the touch down zone.
Numerical Investigation of Vortex-Induced Vibration of a Circular Cylinder Close to a Plane Boundary Subject to Oscillatory Flow
Authors: Munir Adnan , Zhao Ming , Wu Helen
Published: October 2016

This paper presents a numerical study of flow around an elastically mounted circular cylinder in close proximity to a plane boundary vibrating in the transverse and inline directions in an oscillatory flow. The Reynolds-Averaged Navier-Stokes (RANS) equations and the SST k-ω turbulent equations are solved using the Arbitrary Langrangian-Eulerian (ALE) scheme and Petrov-Galerkin Finite Element Method for simulating the flow. The equation of motion is solved using the fourth-order Runge-Kutta method to find the displacements of the cylinder in the transverse and inline directions. The numerical model is validated against the previous results of vortex-induced vibration of an isolated circular cylinder in both cross-flow and inline directions. The flow model is further extended to study the vortex-induced vibration of a cylinder near a plane boundary with a very small gap ratio (e/D) of 0.01, with D and e being the diameter and the gap between the cylinder and the plane boundary, respectively. Simulations are carried out for two Keulegan-Carpenter (KC) numbers of 5 and 10 and a wide range of reduced velocities. It is observed that both the KC number and the reduced velocity affect the vibration of the cylinder significantly.
CFD Simulation of Roll Damping Characteristics of a Ship Mid-Section With Bilge Keel
Authors: Irkal Mohsin A. R. , Nallayarasu S. , Bhattacharyya S. K.
Published: October 2016

The prediction of nonlinear roll motion of ships depends highly on the accurate estimation of roll damping. The nonlinear nature of roll damping arises from the viscous flow and the associated phenomenon of flow separation around the ship hull. Roll damping changes noticeably with a slight change in the ship hull geometry and appendages. The estimation methods employed in industry are highly empirical in nature. These empirical methods were derived from combinations of model tests conducted in wave flumes and basins, and some selected formulae used in fluid dynamics. These methods have limitations and the roll damping prediction show large variation with change in the ship parameters. The advances made in Computational Fluid Dynamics (CFD) in recent times, and validation of the CFD results using experimental studies, can help in predicting roll motion and damping more accurately. The present work uses CFD as a tool to estimate roll damping of a ship mid-section with bilge keel with validation using published experimental results.
Coupled Simulation Between Fast and Hydro-Structural Code for a Flexible FOWT Considering Blade Pitch Control Malfunction
Authors: Srinivasamurthy Sharath , Iijima Kazuhiro , Nihei Yasunori , Hara Naoyuki
Published: October 2016

In this research, a coupled numerical simulation method for a Floating Offshore Wind Turbine (FOWT) is developed. Flexibility of the platform and blade pitch control malfunction can be accounted for by the proposed method. The numerical method is validated qualitatively against a series of scaled model experiment and further simulations are carried out to predict the structural load due to the abrupt failure of blade pitch control system. The influence of blade pitch malfunction for a FOWT is confirmed by utilizing a SPAR and a semi-submersible type floaters and compared against onshore wind turbine case. The behavior and tendency for combined effect of wind and wave is compared and the tool developed is validated. It is found that the abrupt change of the rotor thrust induces the tower flexible modes for the onshore case while almost rigid body motions are the dominant for the floater cases with almost no excitation of the flexible vibration mode. The maximum bending moment after malfunction is almost comparable among the onshore and floating cases however it is observed that the time duration during which the vertical bending moment takes the largest value is different.
Optimal Energy Harvesting From Vortex-Induced Vibrations of Cables
Authors: Antoine Guillaume O. , Michelin Sébastien , de Langre Emmanuel
Published: October 2016

We propose to use flexible cables instead of rigid rods in devices extracting energy from Vortex-Induced Vibrations (VIVs). We use a linear equation of motion for the structure coupled with a nonlinear wake oscillator and numerical tools to simulate the VIVs of hanging strings/cables. While extracting energy from the VIVs of a straight cable with a harvester attached to one of its ends is as efficient as extracting energy from the VIVs of rigid structures, we find that the former is much more robust to flow fluctuations than the latter. We finally show that those results carry over to more complex geometries (e.g. catenary).
Periodic and Fixed Boundary Conditions for Multi-Scale Finite Element Analysis of Flexible Risers
Authors: Rahmati M. T. , Alfano G. , Bahai H.
Published: October 2016

In this paper the implementation of two types of boundaries, periodic and fixed in-plane boundaries, for a detailed finite-element model of flexible risers is discussed. By using three-dimensional elements, all layer components are individually modelled and a surface-to-surface frictional contact model is used to simulate their interaction. The approach is applied on several riser models with various lengths and layers. It is shown that the model with periodic boundaries can be effectively employed in a fully-nested (FE2) multiscale analysis based on computational homogenization. In fact, in this model only a small fraction of a flexible pipe is needed for a detailed nonlinear finite-element analysis at the small scale. The advantage of applying periodic boundary conditions in capturing the detailed nonlinear effects and the efficiencies in terms of significant CPU time saving are demonstrated.
Equilibrium Scour Prediction for Uniform and Non-Uniform Cylindrical Structures Under Clear Water Conditions
Authors: Tavouktsoglou Nicholas S. , Harris John M. , Simons Richard R. , Whitehouse Richard J. S.
Published: October 2016

Offshore Gravity Base Foundations (GBFs) are often designed with non-uniform cylindrical geometries. Such structures interact with the local hydrodynamics which amplify the adverse dynamic pressure gradient, which is responsible for all flow and scour phenomena including the bed shear stress amplification. In this study a method for predicting the effect non-uniform cylindrical structure geometries have on local scour around offshore structures under the forcing of a unidirectional current is presented. The interaction of the flow field with the sediment around these complex structures is described in terms of non-dimensional parameters that characterize the similitude of water-sediment movement. The paper presents insights in the influence a form of the Euler number has on the equilibrium scour around uniform and non-uniform cylindrical structures. Here the Euler number is defined as the depth averaged pressure gradient (calculated using potential flow theory) divided by the product of the square of mean flow velocity and the fluid density. The insights are confirmed through a series of experiments where the equilibrium scour was monitored for different types of structures and flow conditions.

The results of this study show that the Euler number is a more appropriate parameter for describing the scour potential of a structure compared to using the equivalent pile diameter. The experimental data show that an increasing Euler number yields an increase in the non-dimensional equilibrium scour. The results of this study also suggest that an increase in the water depth yields a decrease in the equilibrium scour depth for the conical, cylindrical base structures and truncated cylinders and an increase in the equilibrium scour depth for the uniform cylinders which can also be explained in terms of changes in the Euler number. Finally, the Buckingham π theorem in conjunction with the experimental data was used to derive a simple shape correction factor that could be used to determine the scour depth of a non-uniform cylindrical structure based on the equilibrium scour produced for the same flow conditions by a uniform cylinder.
The State of the Art and Challenges in Geomechanical Modeling of Injector Wells: A Review Paper
Authors: Bautista J. F. , Dahi Taleghani A.
Published: October 2016

Fluid injection is a common practice in the Oil and Gas industry found in many applications such as waterflooding and disposal of produced fluids. Maintaining high injection rates is crucial to guarantee the economic success of these projects; however, there are geomechanical risks and difficulties involved in this process that may threat the viability of fluid injection projects. Near wellbore reduction of permeability due to pore plugging, formation failure, out of zone injection, sand production, and local compaction are challenging the effectiveness of the injection process. Due to these complications, modeling and simulation has been used as an effective tool to assess injectors’ performance, however, different problems have yet be addressed. In this paper, we review some of these challenges and the solutions that have been proposed as a primary step to understand mechanisms affecting well performance.
Effect of Acidizing Treatments on the Fatigue Crack Growth Behavior of C-Mn Line Pipe Steels
Authors: Batra Apurva , Bowman Jonathan , Yu Weiwei , Thodla Ramgopal , Holtam Colum , Gerst Brandon
Published: October 2016

Acidizing treatments are typically performed intermittently during the life of a well. However, more recently there has been a desire to perform an increased number of acidizing treatments in order to improve production. The acidizing treatments typically involve highly corrosive acids, such as hydrofluoric (HF), hydrochloric (HCl) and acetic acid, which are known to cause significant corrosion, but could also lead to environmentally assisted fatigue and fracture. A study was performed to evaluate the effect of acidizing treatments on the fatigue behavior of welded C-Mn line pipe steels. This paper describes the results of fatigue crack growth rate (FCGR) tests on as-welded (i.e. unstrained) pipe.

FCGR tests were conducted at room temperature (RT) in three different acid conditions: fresh acid with corrosion inhibitor, spent acid with corrosion inhibitor and spent acid without corrosion inhibitor. Frequency scan FCGR tests were performed on compact tension (CT) specimens notched in the parent pipe (PP), heat affected zone (HAZ) and weld centerline (WCL). The FCGRs in all three environments were higher than in air and exhibited a frequency dependence. Tests in fresh acid with inhibitor exhibited plateau FCGR values around 20–30 times higher than in air. Tests in spent acid with inhibitor exhibited a strong frequency dependence with plateau FCGR values approximately 80–100 times higher than in air. In spent acid without inhibitor, the plateau FCGR was around 20 times higher than in air, however at the lowest frequencies the FCGR decreased, most likely due to crack closure/blunting effects. This behavior is consistent with the higher corrosion rate in this uninhibited environment.

The role of corrosion products in causing crack closure/blunting was further evidenced in tests performed at elevated temperature (165°F / 74°C), where the FCGR at 1Hz was significantly higher than at RT. The plateau FCGR in fresh acid and spent acid with inhibitor was approximately 40–50 times higher than in air, but the FCGR decreased at lower frequency. This is similarly believed to be due to the higher corrosion rates at elevated temperature causing crack closure/blunting. The FCGR in spent acid without inhibitor at 165°F (74°C) was high initially at 1Hz but then decreased sharply, which is consistent with the highest corrosion rates expected at elevated temperature and in the absence of corrosion inhibitor.

Paris curve FCGR tests were subsequently conducted at 0.1Hz. Tests were performed in the worst case combinations of microstructure/environment/temperature identified from the frequency scan tests.
Effect of Sour Acidizing Treatments on the Fatigue Crack Growth and Fracture Toughness Behavior of C-Mn Line Pipe Steels
Authors: Yu Weiwei , Bowman Jonathan , Batra Apurva , Thodla Ramgopal , Holtam Colum , Gerst Brandon
Published: October 2016

Acidizing treatments are typically performed intermittently during the life of a well. However, more recently there has been a desire to perform an increased number of acidizing treatments in order to improve production. The acidizing treatments typically involve highly corrosive acids, such as hydrofluoric (HF), hydrochloric (HCl) and acetic acid, which are known to cause significant corrosion. In the presence of hydrogen sulfide (H2S), these acidizing treatments could cause environmentally assisted fatigue and fracture (i.e. increased fatigue crack growth rates and reduced fracture toughness).

A test program is underway to evaluate and quantify the effect of sour acidizing treatments on the fatigue and fracture behavior of welded C-Mn line pipe steels. This paper describes the preliminary findings from fatigue crack growth rate (FCGR) and fracture toughness (FT) tests on as-welded (i.e. unstrained) pipe.

All tests were conducted at room temperature (RT) using compact tension (CT) specimens notched in the parent pipe (PP). Frequency scan FCGR tests were performed in the following sour acid conditions: simulated production environment (PE), spent acid without inhibitor and spent acid with residual corrosion inhibitor. The PE consisted of a simulated brine with pH = 4.5 and partial pressure of H2S (pH2S) = 0.21psia. FCGRs in the sour PE were of the order of 20 times faster than in air. The pH2S was the same for the tests in spent acid environments, but the pH was lower (approximately 3.5). As would be expected, the FCGRs were much higher in the low pH environment. The highest FCGRs were observed in the inhibited sour spent acid environment and were up to 100 times faster than in air.

Sour FT tests were also conducted in the PE and in spent acid with and without inhibitor. In all cases, the measured FT values were significantly lower than in air. The test in PE exhibited higher FT than in the sour acidizing environment. The lowest FT values were observed in spent acid with inhibitor.

Future work will investigate the effect of reeling on the fatigue and FT performance of pipe girth welds in sour acidizing environments.
The Effect of Current on the Dynamic Responses of Truss Spar in Malaysia Water
Authors: Tuhaijan S. N. A. , Ng C. Y. , Kurian V. J.
Published: October 2016

In South East Asia, Malaysia is one of the leading countries in the oil and gas industry. Today, Malaysia has expanded the explorations into the deeper water region. Before the installation of the Malaysia first deepwater platform, the Kikeh Spar, spar platforms can only be found in the Gulf of Mexico. Malaysian offshore regions are subjected to significant water current. From the literature review carried out, it was found that the current would change the behavior of the sea waves. This is contributing significantly to the environmental loading and affect the dynamic responses of the offshore structures. Hence, the study that focused on the effects of the current together with the wave on the structural dynamic response is necessary. In this study, the effect of the current coexisting with the wave on the dynamic responses of a truss spar model was experimentally investigated and quantified. The model tests were performed in the wave tank of the Offshore Laboratory in Universiti Teknologi PETRONAS with a scaling factor of 1:100. Two sets of environmental conditions were considered in the model tests i.e. wave only and wave-current condition. The dynamic responses of the truss spar model subjected to these conditions were measured. In order to quantify the effect of current, the measured results for the condition with and without current were compared among and presented here. From this investigation, it was found that the existence of the current in the water body has increased the truss spar motions, whereby the higher current velocity, give the higher response.
Velocity Effects on Wide Sloping Structure Ice Loads
Authors: Wang Yihe , Poh Leong Hien
Published: October 2016

Sloping structures are widely used in ice-infested waters because of their ability to reduce the ice loads by changing the ice sheet failure mode from crushing to bending. Model test data showed significant velocity effects on breaking component of sloping structure ice loads (Matskevitch, 2002), which are induced by both the dynamic effect of the ice sheet and the hydrodynamic effect of the sea water beneath the ice sheet. However, existing design codes and most models idealize the underlying sea water as a Winkler-type elastic foundation, without taking into account the velocity effects in the calculation of ice loading. The added mass concept has been utilized by researchers to incorporate the hydrodynamic effect (Sørensen, 1978; Sodhi, 1987), though the potential theory was reported to be more adequate in capturing the (additional) forces from the fluid foundation because the added mass varies with time and space (Zhao and Dempsey, 1996; Lubbad et al, 2008). In general, however, there is limited work done on the incorporation of velocity effects into the computation of ice breaking loads on sloping structures. In this paper, we study the velocity effects on ice breaking load through a two-dimensional problem. The ice-structure interaction problem is studied numerically by incorporating the dynamic effect of the ice sheet and the hydrodynamic effect of the sea water beneath the ice sheet. The ice-fluid interaction is captured by adopting the Euler-Bernoulli beam theory for the ice sheet and the potential theory for the fluid foundation, leading to a set of two governing equations with two loading boundary conditions. For ease of computation, we consider sub-problems with the same set of governing equations, each with modified loading boundary conditions. The numerical models are first validated against available analytical solutions for a simple problem before solving for the sub-problems. Finally, the solution to the original set of governing equations defining the ice-structure interaction is obtained from the superposition of the solutions to two sub-problems. Initial results show that the velocity effects can have a significant influence on ice breaking loads for wide sloping structures.
The Computation of Higher Order Derivatives of Velocity Potential Based on B Spline Function
Authors: Li Hui , Lizhu Hao , Ren Huilong , Chen Xiao-bo , Li Fang
Published: October 2016

When solving the forward speed hydrodynamic problem in frequency domain adopting the matching method with a meshless cylinder surface as the control surface, the simple Green function is used in the interior domain. To tackle the integration containing the first and second order derivatives of velocity potential on free surface about x, a method in which the velocity potential on the free surface and its derivatives are fitted by the cubic B spline is given, and the regular wave is chosen as the incident wave, and the theory solutions of its velocity potential and the first and second order derivatives about x are compared to the numerical solutions get from the cubic B spline.
Bilge Keel Induced Roll Damping of an FPSO With Sponsons
Authors: Ommani Babak , Fonseca Nuno , Kristiansen Trygve , Hutchison Christopher , Bakksjø Hanne
Published: October 2016

The bilge keel induced roll damping of an FPSO with sponsons is investigated numerically and experimentally. The influence of the bilge keel size, on the roll damping is studied. Free decay tests of a three-dimensional ship model, for three different bilge keel sizes are used to determine roll damping coefficients. The dependency of the quadratic roll damping coefficient to the bilge keel height and the vertical location of the rotation center is studied using CFD. A Navier-Stokes solver based on the Finite Volume Method is adopted for solving the laminar flow of incompressible water around a section of the FPSO undergoing forced roll oscillations in two-dimensions. The free-surface condition is linearized by neglecting the nonlinear free-surface terms and the influence of viscous stresses in the free surface zone, while the body-boundary condition is exact. An averaged center of rotation is estimated by comparing the results of the numerical calculations and the free decay tests. The obtained two-dimensional damping coefficients are extrapolated to 3D by use of strip theory argumentations and compared with the experimental results. It is shown that this simplified approach can be used for evaluating the bilge keel induced roll damping with efficiency, considering unconventional ship shapes and free-surface proximity effects.
Corrosion Fatigue Behavior of Mooring Chain Steel in Seawater
Authors: Arredondo Alberto , Fernández Jonathan , Silveira Elena , Arana José Luís
Published: October 2016

With a majority of the reported chain failures related to fatigue, this phenomenon is one of the main topics to be studied as part of Mooring Integrity Management. Present fatigue design is mainly based on fatigue curves for chains under tension-tension loads in seawater. However, the applicability of these curves for different loading modes and specific environments remains unclear.

This paper studies the fatigue behavior of the material used on chains as it builds the baseline for the performance of these mooring components. It includes uniaxial fatigue tests that were undertaken on R4 and R5 steel grades obtained from actual chains after all their manufacturing steps. Samples were not only tested in air and in synthetic seawater but different corrosion related parameters were also studied: frequency, temperature and cathodic protection.

From the results of these tests, separated SN curves were obtained. Subsequently, these curves were analyzed and compared against present recommended design curves for material. Fractographic examination was undertaken to assess the effect of corrosion and cathodic protection and comparison between material and component response was also addressed. Results showed the strong synergy between corrosion and fatigue. Also, the improvement from fatigue design curves to actual response of the materials was quantified.
Probability Analysis of Critical Load of Sphere-Cylinder Combined Shell Structures
Authors: Xu Weijun , Feng Yan , Wang Xiaotian
Published: October 2016

The critical load of submersible structures is normally analyzed in deterministic method. However, some factors including structural sizes, material properties and the location of the components, etc., have random characteristics due to smelting and construction process. In other words, the critical load of submersible structures has uncertainty, so these parameters should be considered as random variables. In terms of sphere-cylinder combined shell structures, the critical loads on the cylindrical shell and spherical structures should be considered together to avoid the influence of boundary conditions on the analysis results. This paper focuses on the probability analysis of sphere-cylinder combined shell structures under critical load. According to the GJB calculation method of submarine design, the security calibration ranges of ring-stiffened cylindrical shell and spherical shell are determined, and seven kinds of failure modes of sphere-cylinder combined shell structure are supposed. The corresponding probability models of sphere-cylinder combined shell structure have been established, and the reliability calculations of each failure mode are conducted in FOSM method, JC method and Monte Carlo simulation method. Considering the correlation between different failure modes, reliability series model of sphere-cylinder combined shell structure is applied in the probability analysis. It is verified by comparison of ring-stiffened cylindrical shell and sphere-cylinder combined shell, with the increase of the structure components, the reliability of structure system shows a decline tendency.
Model of Transit Transport in Arctic Based on Graph Algorithms
Authors: Zvyagin Petr , Voitkunskaia Anna
Published: October 2016

Transportation in Arctic seas is connected with variability of routes. Availability of those routes is depending on current environmental conditions. In the paper the algorithm for navigation system, which is intended to advice captain the most economically effective and less risky route with the presence of ice chart, is proposed.

Smart navigation system for Arctic seas has specific: shortest ways can occur to be impassable or too risky. More long routes through free waters can finally take less fuel comparing to shorter, but covered with ice. Thus, economical profitability of operating in Arctic seas depends on effectiveness of route choosing.

To make estimations about most effective route and its length, the method based on graph algorithms is presented in this paper. The ice chart is covered by the graph, which can have form of grid, with neighbor nodes connected by edges. In general, multiple parameters can be assigned to each edge — length, maximal ice thickness on the way, risk, etc. In this paper two separate cost functions are considered: first is responsible for travel expenses, and the second is responsible for ice passability on the route.

To find most economically efficient route with minimal possible ice thickness on the way the method with graph modification and Dijkstra algorithm was used. This route provides Pareto-optimal solution for reduced version of the problem.

The software, which implements the method, was built. The example of searching for least expensive and Pareto-optimal route is provided. Results are discussed.
Study on the Bubble Dynamics and the Exciting Force in a Bended Pipe Based on BEM
Authors: Liu Y. L. , Tian Z. L.
Published: October 2016

Nonlinear bubble dynamics in a pipeline and its exciting force are investigated by a numerical model based on BEM. The bubble motion is one of the main causes that the pipeline vibrates and generates noise in modern ships. The numerical bubble dynamics model is established under the incompressible potential theory. Bubble motion with different incoming flow in a bended pipe is simulated. We found that the bubble develops jet when it passes by the bend, and adjoin to the pipe surface in the side of the fillet center. The pulsation and the direction change of the bubble apply an exciting force on the pipe which has a positive correlation with the incoming flow speed and may lead vibration and noise.
Parameterization and Visualization of Marine Crane Concept Design
Authors: Chu Yingguang , Deng Yuxiang , Pedersen Birger Skogeng , Zhang Houxiang
Published: October 2016

The paper presents a flexible and lightweight fast prototyping approach for marine crane concept design based on WebGL. The goal is to develop a user-friendly tool that facilitates the communication between designers, system engineers and customers during the early product and system design stage. In this study, we focus on the design of the mechanical parts of the crane in addition with the actuation systems, typically the hydraulic power systems. The degrees of freedom, main geometric dimensions of the crane links and joints, and the configurations of the actuators can be evaluated according to the requirements of workspace, load capacity, etc. The implementation and user interface of the designer tool are developed based on the application of WebGL. In this way, it also enables remote communications between departments in different locations over the web. The development of the parameterization and visualization tool will bring in the customer directly into the product and system design process, thus to improve the work efficacy and reduce the overall time and cost. The results from the designer tool also provide primary information and data for later design processes and simulations in the virtual prototyping environment.
Hierarchically Decomposed Multi-Level Optimization for Ship Structural Design
Authors: Ma Ming , Paik Jeom Kee , McNatt Tobin
Published: October 2016

As the demand for modern ships grows to meet greater reliability, fuel efficiency, and economy, the interest in reducing structural weight while enhancing safety has also increased. Ship structural optimization is usually a mixed discrete-continuous design problem constrained by buckling and material strength, and involves the optimization of a large number of variables such as (continuous/discrete) plate thickness, scantlings of stiffeners and frames, and the (discrete) number of stiffeners and frames. Further complication arises when the structure is constrained by buckling and strength under compression and subject to practical design rules. Although 3D finite element analysis are widely used for full ship structure stresses and limit state analysis, their application for structural optimization is still prohibitive due to the computational cost. In this paper a multi-level heuristic based multi-objective optimization is formulated and used to optimize a large and complex thin-wall structure on the basis of weight, and safety. The optimization procedure has three levels: (1) For each stiffened panel, a heuristic based multi-objective optimization method is used to minimize its weight and maximize its safety, and a Pareto front is obtained. The ALPS/ULSAP ultimate limit state criteria, which is parametric formulated, mesh free, computational efficient, and is able to predict six different failure modes for a stiffened panel, are used to formulate the safety objective function. (2) Use the Pareto front of each panel as a gene pool to optimize the hull girder global properties, such as hull girder cross section area moment of inertia and vertical center of gravity (VCG). (3) Minimize the difference between the design variables from the local optimization and the global optimization by iterating 3D finite element analysis of the full ship. An example of optimizing the longitudinal structures of a full cargo hold of a 200,000 ton oil tanker is presented. The numerical results show that the proposed method is very useful to perform ultimate strength based ship structural optimization with multi-objectives, namely minimization of the structural weight and maximization of structural safety.
Comparison of Software for Analysis of Risers Tied Back With Pull Tubes
Authors: Wang Feng , Burke Roger , Yu Alan
Published: October 2016

Dynamic analysis of steel catenary risers (SCRs) tied back to a Spar through pull tubes has most commonly been performed with the general purpose finite element analysis program ABAQUS, which is generally considered to be most suitable for this type of configuration involving pipe-in-pipe contact between the riser and the pull tube. Version 9.6a and subsequent versions of the riser specific software package OrcaFlex include the capability to handle pipe-in-pipe contact through a line contact model. This model is similar to the contact model in ABAQUS in that it supports relative axial motion and axial friction between the contact lines. A benchmark study is performed to compare riser strength and fatigue performance of the two software packages. The focus of the study is on the pull tube and the riser inside the pull tube, where pipe-in-pipe contact occurs. Bending moment and von Mises stress from the two programs in design storms are found to agree closely. Governing loads on the pull tube guides computed with ABAQUS agree well with the loads computed with OrcaFlex. The two software packages compute similar vessel-motion induced fatigue damage of the bottom of the pull tube (pull tube stress joint). Agreement is not as close for the pull tube above the pull tube stress joint or for the riser pipe. Dynamic run times are comparable between the two programs. Results of the study can assist in the selection of the most suitable software for pull tube riser design and analysis, and in understanding the differences in results from the two software packages.
Dynamic and Static Sagging Characterization and Performances of Four Oil Based Muds
Authors: Belayneh Mesfin , Aadnøy Bernt S. , Thomas Sharman
Published: October 2016

This paper presents the barite sagging phenomenon of four OBM systems having the same density, but different rheology properties. The investigations of barite sagging is based on dynamic sagging and viscoelasticity testing. The viscoelastic properties related to gel formation of the drilling fluids were investigated under amplitude and frequency sweeps.

The study also tries to correlate the results obtained from dynamic sag with the dynamic viscoelastic properties of the drilling fluid and standard API rheology parameters.

The results show that as the oil water ratio increases the drilling fluid rheology parameters such as lower shear yield stress (LSYS), yield stress (YS) and plastic viscosity (PV) parameters also increases. In addition, the viscoelastic loss and storage modulus decrease. From the viscoelasticity study, except for 90:10 OWR, it is observed that as the oil water ratio increase, the yield stress and the flow point also increases. The 90:10 OBM shows no viscoelasticity behavior.

Comparing the extreme 60:40 and 90:10 OBMs (i.e. as OWR increase), the experimental result shows that the sagging index increases by 9%. The dynamic sagging factor decreases as the ratio of storage modulus to loss modulus increases (i.e. as OWR decrease).

Except for high viscosity and hydraulics, the overall analysis of drilling fluids shows that the 60:40 OWR is the better in terms of sagging, filtrate loss and hole cleaning performance.
Comparison Between Two Forecast Systems Implemented With WAM and WaveWatch 3 for the North Atlantic
Authors: Bento A. Rute , Gonçalves Marta , Campos Ricardo , Guedes Soares C.
Published: October 2016

The performance of two operational forecasting systems implemented for the North Atlantic area is compared. One forecast system runs the WAM model and the other using the WaveWatchIII model, both driven by GFS wind fields and with the same resolutions for time and space.

The GFS wind fields used to drive the models have a spatial resolution of 0.5° by 0.5° and a time resolution of 3h. Regarding the geographical grids implemented, they cover the North Atlantic from the longitude of 90° West to 33° East and from the latitude of 80° North to 2° South, with a fine grid resolution of 0.5 by 0.5 degrees. The wave spectrum is discretized using 30 frequencies, being the lowest frequency equal to 0.0418 Hz, and 36 directions. A 4-days warm up was implemented and a 4-days forecast was calculated.

In general both models performed well, as forecasting systems, presenting the same behavior and magnitude of values, when compared to the in-situ measurements.
Efficient Multiline Anchor Systems for Floating Offshore Wind Turbines
Authors: Fontana Casey M. , Arwade Sanjay R. , DeGroot Don J. , Myers Andrew T. , Landon Melissa , Aubeny Charles
Published: October 2016

A mooring and anchoring concept for floating offshore wind turbines is introduced in which each anchor moors multiple floating platforms. Several possible geometries are identified and it is shown that the number of anchors for a wind farm can be reduced by factors of at least 3. Dynamic simulation of turbine dynamics for one of the candidate geometries and for two directions of wind and wave loading allows estimation of multiline anchor forces the preview the types of loads that a multiline anchor will need to resist. Preliminary findings indicate that the peak demand on the anchor may be reduced by as much as 30% but that anchors used in such a system will need to be able to resist multi-directional loading.
Numerical Simulation of Green Water and the Safety Analysis Research on Structures and Equipments
Authors: Wang Xu , Ren Huilong , Lu Xiaolong , Feng Guoqing
Published: October 2016

With the high speed development of ship building and the widely use of the deep-sea oil platforms in severe sea conditions, green water have attracted more and more attention. Based on the relative motion of ship and wave, a numerical simulation of the distribution of green water on the wet deck of ship is achieved according to the dam break model and flood wave model, and the strength of local structure is calculated. In addition, the calculation model of the impact force on the objects of wet deck is got through the momentum theorem in hydrodynamics, which is used to analyze the safety of the equipment on the wet deck. The research is significant to the safety of the ship and the platform in rough sea.
Net Bottom Sediment Transport Pattern Related to Residual Currents in the Pearl River Estuary, South China
Authors: Wu Yao , Zhang Wei , Guan Mingkai , Hu Huanghao
Published: October 2016

In this study, the net bottom sediment transport pattern is measured and compared with flow pattern in the Pearl River Estuary (PRE). Based on 106 bottom sediment samples taken from the PRE, the spatial distribution of bottom sediment of mean grain size μ (mm), sorting coefficient σ, and skewness SK is calculated. Then the grain size trend analysis (GSTA) was used to detect the net bottom sediment transport characteristics. The bottom sediment is transported southward and southeastward in the upper part of the estuary and northward in the lower part, while a clockwise rotation trend occurs in the central part. Furthermore, a numerical flow model, based on Princeton Ocean Model (POM), is used to modulate the hydrodynamic conditions in the PRE. The simulated long-term bottom residual currents correlate well with the GSTA results, implying that the net bottom sediment transport is mainly controlled by the residual currents.