bluuebunny/crossref_metadata_2025 huggingface dataset:
Multi-Objective Optimization of Wire-Electric Discharge Machined Ultrathin Silicon Wafers Using Response Surface Methodology for Solar Cell Applications
Authors: Bhartiya Divyanshu , Rana Pinal , Singh Meinam Annebushan , Marla Deepak
Published: December 2020

Recent investigations on the fabrication of ultrathin silicon (Si) wafers using wire-electric discharge machining (wire-EDM) were observed to possess some inherent limitations. These include thermal damage (TD), kerf-loss (KL), and low slicing rate (SR), which constraints its industrial use. The extent of TD, KL, and SR largely depends on the process parameters such as open voltage (OV), servovoltage (SV), and pulse on-time (Ton). Therefore, optimizing the parameters that pertain to minimum TD and KL while maintaining a higher SR is the key to improvement in the fabrication of Si wafers using wire-EDM. Thus, this study is an effort to analyze and identify the optimal parameters that relate to the most effective Si slicing in wire-EDM. A central composite design (CCD)-based response surface methodology (RSM) was used for optimizing the process parameters. The capability to slice Si wafers in wire-EDM was observed to be influenced by the discharge energy, which significantly impacted the overall responses. The severities of TDs were observed to be mainly dominated by the variation in OV and Ton due to the diffusion of thermal energy into the workpiece, leading to melting and subsequent resolidification. For high productivity, the optimized parameters resulted in a SR of 0.72 mm/min, TD of 17.44 μm, and a kerf-loss of about 280 μm.
Prestall Disturbances and Stall Inception for an Eccentric Low-Speed Axial Compressor With Inlet Swirl
Authors: Wang Xuegao , Hu Jun , Guo Jin , Jiang Chao , Wang Zhiqiang
Published: December 2020

Owing to the manufacturing and assembly error or the fatigue of long-time operation, nonaxisymmetric tip clearance is actually a common phenomenon in compressors. It is also well accepted that tip leakage flow, associated with tip clearance size and loading, has a strong influence on the performance and stall inception of compressors. The work of this paper, based on an eccentric compressor with/without inlet swirl distortion, is aimed at strengthening the understanding of stall inception for a real geometry compressor experimentally. Results indicate that rotor tip blockage and flow unsteadiness vary evidently around the circumference. For this compressor, the maximum tip flow unsteadiness and blockage happens at the location near the minimum clearance under the condition of clean inlet flow. Before the occurrence of stall inception, disturbances arise and vanish intermittently within the region of high unsteadiness. However, it fails to rotate due to the inhibition of the low unsteadiness region. Once the most robust region is no longer able to suppress disturbances, stability finally breaks down and stall inception generates. After exerting inlet paired swirl, unsteadiness within the region of positive preswirl decreases significantly and the maximum unsteadiness location shifts, while the increase for the region with negative preswirl is nearly negligible. As a result, stall margin of the compressor is improved.
Scavenge Performance of an Optimized Shallow Sump at Various Flow Conditions
Authors: Chandra Budi , Johnson Kathy
Published: December 2020

Oil scavenge flow in aero-engine bearing chamber remains largely a challenging problem for many engine designers. Research campaign on scavenge flow has been conducted by G2TRC—Gas Turbine and Transmissions Research Centre (previously Rolls Royce University Technology Centre in Gas Turbine Transmission Systems) at the University of Nottingham. It was recognized that a deep sump performs better than shallower one due to its ability to “shield” the collected oil in the sump from the shaft windage, thus reducing the amount of oil being picked up by the bulk air rotation. However, such a deep sump design cannot be employed in some engines and especially at certain locations where space is limited. A parametric study combined with phenomenological approach on shallow sump geometry has been conducted and presented in the previous publication, where a certain optimized shallow sump variant was proposed depending on whether the flow in the chamber is wall film dominated or airborne droplets dominated. The parametric phenomenological approach was employed since it can be done relatively quicker than typical data gathering through an experiment. However, the approach relies on qualitative interpretation of the flow features, and its application in bearing chamber flow research has never been validated before. This paper presents the results of quantitative measurements of residence volumes of an optimized shallow sump variant identified in the parametric phenomenological study. Comparison was then made with the residence volumes of some existing engine sumps. It was found that the optimized shallow sump for wall film dominated flow has lower residence volumes compared to some existing engine sumps. In some cases, the residence volume can be reduced by up to 75%. An optimized shallow sump variant for airborne droplets dominated was also identified in the previous parametric phenomenological study, although the residence volume measurement is yet to be conducted. The optimized shallow sump for wall film dominated flow was also identified as a good sump regardless of the flow regime. However, when it was tested in airborne droplets dominated flow, its residence volumes are higher than some of the existing engine sumps. This highlights the importance of considering the flow regime in the bearing chamber in any attempt to optimize a sump geometry.
Fully Coupled Turbojet Engine Computational Fluid Dynamics Simulations and Cycle Analyses Along the Equilibrium Running Line
Authors: Briones Alejandro M. , Caswell Andrew W. , Rankin Brent A.
Published: December 2020

This work presents fully coupled computational fluid dynamics (CFD) simulations and thermodynamic cycle analyses of a small-scale turbojet engine at several conditions along the equilibrium running line. The CFD simulations use a single mesh for the entire engine, from the intake to the exhaust, allowing information to travel in all directions. The CFD simulations are performed along the equilibrium running line by using the iterative Secant method to compute the fuel flow rate required to match the compressor and turbine power. The freestream pressure and temperature and shaft angular speed are the only inputs needed for the CFD simulations. To evaluate the consistency of the CFD results with thermodynamic cycle results, outputs from the CFD simulations are prescribed as inputs to the cycle model. This approach enables on-design and off-design cycle calculations to be performed without requiring turbomachinery performance maps. In contrast, traditional off-design cycle analyses require either scaling, calculating, or measuring compressor and turbine maps with boundary condition assumptions. In addition, the CFD simulations and the cycle analyses are compared with measurements of the turbojet engine. The CFD simulations, thermodynamic cycle analyses, and measurements agree in terms of total temperature and pressure at the diffuser–combustor interface, air and fuel mass flow rate, equivalence ratio, and thrust. The developed methods to perform CFD simulations from the intake to the exhaust of the turbojet engine are expected to be useful for guiding the design and development of future small-scale gas turbine engines.
A Review of Mooring Analysis Methodologies for Permanent Offshore Installations
Authors: Pahos Spiro J. , Maldonado Georgina , Westlake Paul C.
Published: December 2020

Traditionally mooring line strength assessment is based on a deterministic approach, where the mooring system is evaluated for a design environment defined by a return period. The mooring system response is then checked against the mooring strength to ensure a required factor of safety. Some codes adopt a deterministic approach [1], [2], [3]. Other codes like [4] adopt a partial safety factor format where uncertainties are addressed through load factors for load components and material factors for line strength. Industry practices give guidance on mooring analysis methodology together with analysis options like coupled, de-coupled, time domain, frequency domain and the associated line tension safety factors. Prior work has demonstrated that discrepancies in mooring line tensions are observed when different analytical approaches are used [5]. Namely, the mooring line tensions of a semi-submersible unit in a coupled time domain analysis, were found to be non-compliant, whereas those calculated using a decoupled time domain analysis returned compliant tensions.

This work focuses on a coupled dynamic analysis where all inertial, hydrodynamic and mechanical forces are assessed to determine the subsequent motions. Despite being considered the most accurate to capture the true dynamic response, a coupled analysis is also the least efficient in terms of the required computer resources and engineering effort [1].

This paper presents further discussion on the above observation in mooring tensions and also considers differences in the installation’s excursion. All responses are evaluated in the time domain where the nonlinear dynamic behavior of the mooring lines, slowly varying wave drift forces and coupling effects are captured.

Agreement is found in the present computations, carried out with two renowned hydrodynamic codes, which validate former results and reiterate the need to distinguish between time domain methods and recommended appropriate safety factors accordingly.
Experiment on the Effect of Superficial Gas-Liquid Velocities on Slug Flow-Induced Vibration in an Inclined Sagged Riser
Authors: Ma Bowen , Srinil Narakorn , Zhu Hongjun , Gao Yue
Published: December 2020

Subsea liquid-gas flows conveyed through a flexible riser or pipeline may develop into various flow patterns including slug flow. In this study, the slug flow-induced vibration (SIV) of an inclined sagged riser conveying upward air-water flows is experimentally investigated. A small-scale experiment is carried out in an air-water test loop with a section of a free-hanging catenary tube made of silica gel. Attention is placed on the effect of superficial gas and liquid velocities on SIV responses. Both pipe motions and flow patterns are recorded using non-intrusive high-speed cameras. Pressure variations are also measured at the pipe inlet and outlet by two pressure transducers. The SIV system is tested by employing different ratios of the superficial gas-liquid velocities. Occurrence of unstable slug flows is captured at the relatively high gas-to-liquid velocity ratios, leading to a large-amplitude SIV. Experimental results of the space-time varying riser responses and oscillation frequencies are reported together with the associated slug flow features. Depending on the gas-liquid superficial velocities, slug flow characteristics are observed to vary significantly. These entail an intermittent SIV with modulated amplitudes and frequencies along riser span, signalling a potential dynamic stress and fatigue-related concern. In all experimental cases, the riser responses are found to be multi-modal and dominated by the fundamental planar mode whereas an out-of-plane vibration is negligible. Experimental observations suggest the key interrelationships of the two-fluid flow conditions, the slug characteristics and the pipe dynamics. This finding is meaningful for a practical design of riser transporting internal multiphase flows.
1300 Tons Structural Support to Secure the Integrity of an Offshore Platform
Authors: Becerro Francisco Javier , Alonso Beatriz , De Burgh Daly Roland , Vabø Petter
Published: December 2020

The Yme oilfield is located approximately 100 km from the Norwegian coastline in the Egersund basin. The original field development in 2008 consisted on a jack-up type platform (MOPU) and a subsea storage tank at 93 m depth. Both are interconnected through a caisson containing all risers, piping and umbilicals which also supports a wellhead platform (WHM). That original MOPU was removed in 2016 [1] and a redevelopment of the field is currently under execution making use of some of the remaining facilities. In order to re-use the existing tank and caisson, advanced structural fatigue analyses including fracture mechanics, probabilistic assessment, motion monitoring and regular NDT inspection concluded with the requirement for a support structure to extend the service life of the old structure, which fatigue life was limited.

A screening process that covered different alternative solutions lead to a design (called Caisson Permanent Support or CPS) with overall dimensions (L.B.H) of approx. 70m × 50m × 40m and a dry-weight of 1300 t (figure 9). It consists on a beam structure composed of steel tubular braces with diameters ranging from 1,1 to 2,7 m that supports a clamp (6 m height and 5,15m diameter) connected to the caisson. In an iterative process between structural, fabrication and installation designers the optimal final concept was selected after detailed analyses to demonstrate that is capable of meeting the requirement of 20 additional years of structural service life for Yme facilities.

The CPS was successfully installed in June 2019 ensuring the integrity of the offshore structures and enabling the further re-development of the Yme field. Given its dimensions and its particular design purpose, the CPS is regarded as a unique project in the industry. This paper describes the challenges and the engineering work required for the design of the CPS.
Environmental Contours Based on a Direct Sampling Approach and the IFORM Approach: Contribution to a Benchmark Study
Authors: Vanem Erik , Huseby Arne Bang
Published: December 2020

Environmental contours are often applied in probabilistic structural reliability analysis to identify extreme environmental conditions that may give rise to extreme loads and responses. It represents an approximate method for performing long-term extreme response analyses in cases where full long-term analyses are not feasible due to computationally heavy and time-demanding response calculations. There are various methods for deriving environmental contours given a set of metocean data. These relate to different approaches for modelling the joint behaviour of the metocean variables, i.e., a joint distribution function fitted to the data, but also different ways of establishing the environmental contour given a joint distribution for the environmental variables. In light of this, a benchmark exercise was announced at OMAE 2019 [1], asking for contributions from different practitioners involved with environmental contours. Various bivariate datasets are provided and two exercises are specified for which different solutions are elicited. The first part of the exercise concerns the estimation of the actual contours, whereas the second part relates to the uncertainty characterization of the contours in light of sampling variability. This paper is a response to this announcement and provides one contribution to these benchmark exercises; environmental contours based on a direct sampling approach as well as contours based on the IFORM approach will be presented. Both sets of contours are based on the same models for the joint distribution of the environmental variables, i.e., a conditional model where the joint distribution is modelled as a product of a marginal model for one variable and a conditional model for the other. Both the joint modelling of the environmental variables and the different approaches to estimate environmental contours are described in this paper and the results for the provided datasets are shown.
Gas Suspension Effects in Riser Unloading and Appropriate Modelling Approaches
Authors: Gomes Dalila , Fjelde Kjell Kåre , Bjørkevoll Knut Steinar , Frøyen Johnny
Published: December 2020

A kick entering a drilling riser and expanding upwards uncontrolled can lead to severe consequences such as riser unloading and riser collapse, and in the worst case a blowout scenario may evolve. If the riser is filled with water-based mud, the kick will normally migrate on its own to surface, but it has been observed both in small scale experiments and in field tests that small amounts of gas are trapped by the mud during the kick migration. In some cases, the kick is not able to reach the surface without additional circulation. Hence, a certain kick size may or may not lead to an unloading scenario depending on the effect of gas suspension in the drilling fluid.

In this paper, two different modelling approaches for describing the unloading scenario will be compared and the differences will be highlighted. In the first approach, the single bubble model will be considered. Here the gas bubble is assumed to occupy the whole cross-sectional area, and it is fully separated from the mud regions. This will be solved by two different calculations methods, one that is taken from literature and one that is based on a shooting technique. The second and recommended approach is to use a transient drift flux model, which includes friction, acceleration terms, and gas slip. For the gas slippage model, different flow patterns will be accounted for, as will the suspension effect that causes small amounts of gas to be trapped by the mud. The drift flux model will be solved numerically using the explicit AUSMV scheme.

The impact of gas suspension will be studied by varying the onset for gas suspension to determine from simulations whether a riser will be unloaded or the kick become fully trapped in the riser. In addition, a sensitivity analysis will be presented where kick size, riser ID and riser length are varied to determine when the riser will be unloaded. The different simulations presented solves physical equations of the unloading scenario to calculate pressure at BOP, displaced mud volume (pit gain), liquid mass in well, surface rates, riser friction, and depth profiles of the gas distribution at a certain time. the tables provide a comprehensive overview of which combinations of parameters lead to a trapped gas scenario or and which lead to unloading the riser.

It is shown that a fully transient drift flux model can cover a vast range of different situations e.g. gas becomes fully trapped in the riser, the riser becomes fully unloaded, and situations where only a very small part of the kick reaches the surface. The simulations show how the dynamics of the scenarios are quite different. A single bubble model will not have this capability.
Uncertainty Evaluation of Wellbore Stability Using Comparative Rock Strength Criterions
Authors: Khosravanian Rasool , Aadnøy Bernt Sigve
Published: December 2020

The requirement of uncertainty analysis has shifted the transformation of sensitivity analysis from the deterministic area to the stochastic area.Geomechanical wellbore integrity problems during drilling operation can occur due to wellbore shear failure or tensile failure. To guarantee wellbore integrity, breakout and fracture geomechanical analysis is essential to estimate the Safe Mud Weight Window (SMWW). Wellbore stability problems causes many challenges in a drilling operation, such as pipe sticking, wellbore collapse, fluid loss and poor cement jobs. A drilling engineer must minimize the risk of these problems, however, there is a considerable uncertainty of different parameters such as geomechanical rock properties of drilled formation, and, data and parameters gathering are often incomplete. This uncertainty of main parameters have impact on the resulting SMWW.This paper perform an uncertainty evaluation of wellbore stability and its effect on the optimum interval of SMWW. The SMWW Uncertainty Evaluation of Wellbore Stability assessment for two failure criteria are compared, Mohr-Coulomb and Modified Lade criterion. We apply Monte Carlo simulations to investigate the uncertainty of the models and we do a sensitivity analysis and confidence level analysis. The paper will show the advantage of including uncertainty evaluation when determining the optimum SMWW window, as opposed to classical deterministic analysis. A case study is presented to draw a perfect understanding of the foundation of the MCS approach with practical and good results. It confirmed the capability of the proposed approach in solving such a strong-nonlinear, complex real problem.
Study on the Parametric Rolling of Medium-Sized Containership Based on Nonlinear Time Domain Analysis
Authors: Yang Seung Ho
Published: December 2020

The numerical analysis of parametric rolling of medium-sized containership has been carried out. Target containership was modeled by using two different numerical models, which are nonlinear numerical model and simplified dynamic mathematical model respectively. The simulations were performed in full-loaded operating condition for regular and irregular waves. For regular waves, the analysis was conducted with a wide range of wave periods including the vicinity of the wave period expected to cause parametric rolling of the target containership. On the other hand, regarding irregular waves, the wave period range that is highly likely to occur according to significant wave height was selected and used as input values of wave spectrum for nonlinear time domain analysis.

The analysis results are summarized as wave height versus wave period diagrams with the occurrences of parametric rolling motions for each speed. And also, time series based on time domain analysis are represented and compared between nonlinear numerical model and simplified dynamic mathematical model. In addition, the sensitivity of key parameters such as vessel speed, wave period, and roll damping to parametric rolling was investigated and estimated under operating condition. Finally, when the parametric rolling occurred, the characteristics of heave, pitch, and roll motions were analyzed. This study could be used as the basic data for determining the operational conditions for safe operation as well as initial design of the medium-sized containership.
Dynamic Modelling of Deep-Water Riser With Slug Flow Based on ALE-ANCF
Authors: Liu Depeng , Ai Shangmao , Sun Liping
Published: December 2020

The internal flow in gas-liquid mixing riser often displays a flow pattern known as slug flow, in which gas and liquid are alternately distributed. Dynamic effects due to slug flow is normally most obvious in areas along the riser with high curvature, which is caused by the centrifugal load component. The global riser response to this excitation can be predicted by nonlinear time domain analysis using the load model as described for slug flow conditions. In this study, the riser with internal slug flow is modeled under the framework of Arbitrary-Lagrange-Euler (ALE) description by using the Absolute Node Coordinate Formula (ANCF). The riser is discretized into ANCF cable element based on the Euler-Bernoulli beam assumption, while one-dimensional moving medium modeling method with time-varying density is used to model slug element. Compared with other FEA models of riser subjected to internal flow, the ALE-ANCF model allow easily modeling of complex mass flow and has the advantages of high speed and high precision in handling large deformation of riser, especially for the compliant riser configurations. Numerical simulations of two simplified models are carried out to validate the developed model, then the dynamic response such as displacement, tension force and bending moment of the riser conveying slug flow are analyzed.
Experimental Study of Hydrodynamic Damping for Water Intake Risers
Authors: Gao Zhenguo , Efthymiou Mike , Zhao Wenhua , Cheng Liang , Zhou Tongming
Published: December 2020

Water Intake Riser (WIR), conveying cooling water from the sea, is key to liquefaction of natural gas in the Floating Liquefied Natural Gas (FLNG) facility. Due to the wave-induced vessel motion, WIRs may experience resonant vibrations, which influence its fatigue life. In such situations, the estimate of hydrodynamic damping is critical to the prediction of fatigue life. Due to its small motion amplitudes compared to the diameter of WIR, the Keulegan–Carpenter (KC) for motion-induced flow around WIR is normally small (e.g. KC < 5). For small KC values, the effect of steady current on the hydrodynamic damping is not well understood and the current practice of using the relative velocity Morison model for predicting the hydrodynamic damping with in-line steady current is challenged by guidelines such as DNVGL-RP-C205 and ISO-19902. In this study, the hydrodynamic damping of a smooth WIR oscillating in still water or in steady currents is measured with a series of experiments at KC < 5 and the Reynolds number (Re) in the range of 103 ∼ 105. The effect of in-line or cross steady currents on the in-line hydrodynamic damping is investigated and the performance of the relative velocity Morison model for predicting the hydrodynamic damping at low KC is examined. Experiments are also conducted for a WIR with helical strakes in in-line or cross currents. Based on these experimental results, recommendations are made for predicting hydrodynamic damping in the WIR design.
A Method for the Frequency Domain Seakeeping Analysis of Offshore Structures in the Early Design Stage
Authors: Liebert Maximilian
Published: December 2020

The motion analysis of floating offshore structures is a major design aspect which has to be considered in the early design stage. The existing design environment E4 is an open software framework, which is being developed by the Institute of Ship Design and Ship Safety, comprising various methods for design and analysis of mainly ship-type structures. In context of the development to enhance the design environment E4 for offshore applications this paper presents a method to calculate the response motions of semi-submersibles in regular waves. The linearised equations of motion are set up in frequency domain in six degrees of freedom and the seakeeping behaviour is calculated in terms of the amplitudes of the harmonic responses. The hydrodynamic forces onto the slender elements of the semi-submersible are accounted for by a Morison approach. As the drag and damping forces depend quadratically on the amplitudes, these forces are linearised by an energy-equivalence principle. The resulting response amplitude operators of the semi-submersible are validated by comparison with model tests. The method represents a fast computational tool for the analysis of the seakeeping behaviour of floating offshore structures consisting of slender elements with circular cross sections in the early design stage.
On the Necessity for Minimizing Risk Based Technology Qualification Variability: An Application to Offshore Floating Wind Turbines
Authors: Samarakoon S. M. Samindi M. K. , Ratnayake R. M. Chandima
Published: December 2020

Technology qualification (TQ) has been employed to perform assessments to verify whether a new technology performs within pre-specified functional limits after an application. If a best available technology (BAT) is used in a new environment, it is considered as a new technology. The TQ is vital in the implementation of best available technology (BAT) in a new environment. Risk based technology qualification provides an optimal approach for performing TQ of a BAT when it is necessary to implement in a new environment. This manuscript first demonstrates the standard TQ process. Secondly, it presents development of a risk matrix for failure mode identification and consequence risk ranking (FMI&CRR). Thirdly, it demonstrates the use of FMI&CRR in a risk-based technology qualification process. Finally, it presents use of the risk matrix to perform TQ on moorings solutions that have been selected as a BAT for a floating wind turbine sub-system. Fuzzy inference system has been used to assess the risk rank to minimize the variability that causes due to experts’ performance variability. Illustrative risk based TQ assessment has been performed and presented. The developed risk based TQ process (TQP), fuzzy inference system supported risk rank estimation, and illustrative risk based TQ recommendation are significantly important for practitioners while performing FMI&CRR in larger scale offshore floating wind turbines’ TQ projects.
Numerical Prototyping of Floating Offshore Wind Turbines: Virtual Operation in Real Environmental Conditions
Authors: Milano Daniel , Peyrard Christophe , Capaldo Matteo
Published: December 2020

The numerical fatigue analysis of floating offshore wind turbines (FOWTs) must account for the environmental loading over a typical design life of 25 years, and the stochastic nature of wind and waves is represented by design load cases (DLCs). In this statistical approach, combinations of wind speeds and directions are associated with different sea states, commonly defined via simplified wave spectra (Pierson-Moskowitz, JONSWAP), and their probability of occurrence is identified based on past observations. However, little is known about the difference between discretizing the wind/wave direction bins into (e.g.) 10deg bins rather than 30deg bins, and the impact it has on FOWT analyses. In addition, there is an interest in identifying the parameters that best represent real sea states (significant wave height, peak period) and wind fields (profile, turbulence) in lumped load cases.

In this context, the aim of this work is to better understand the uncertainties associated to wind/wave direction bin size and to the use of metocean parameters as opposed to real wind and sea state conditions. A computational model was developed in order to couple offshore wind turbine models with realistic numerical metocean models, referred to as numerical prototype due to the highly realistic wind/wave conditions in which it operates. This method allows the virtual installation of FOWTs anywhere within a considered spatial domain (e.g. the Mediterranean Sea or the North Sea) and their behaviour to be evaluated in measured wind and modelled wave conditions. The work presented in this paper compares the long-term dynamic behaviour of a tension-leg platform (TLP) FOWT design subject to the numerical prototype and to lumped load cases with different direction bin sizes. Different approaches to representing the wind filed are also investigated, and the modelling choices that have the greatest impact on the fidelity of lumped load cases are identified. The fatigue analysis suggests that 30deg direction bins are sufficient to reliably represent long-term wind/wave conditions, while the use of a constant surface roughness length (as suggested by the IEC standards) seems to significantly overestimate the cumulated damage on the tower of the FOWT.
Prediction of Short-Term Extreme Response of Floating Offshore Wind Turbine Under Storm Condition
Authors: Zong Zhi , Hu Guanqing , Jiang Yichen , Zou Li
Published: December 2020

To predict the short-term motion responses of floating offshore wind turbine under extreme wind-wave excitation, a numerical model based on the two-phase flow finite volume method was developed. In this paper, uni-directional irregular waves composed of 100 cosine waves with equal frequency interval were simulated by the wave forcing technique, resulting in the measured spectrum in accordance with the target spectrum. Then, a 100-seconds wave segment containing the maximum wave height was selected for fully coupled dynamic analysis of the OC4-DeepCwind system in CFD, and the results were compared with those of FAST under the same wind and wave sea state. It was found that the motion responses of heave and pitch motion responses predicted by two methods agree well. The second-order slow drift force generated in CFD led to the difference in surge motion. The predicted sway, roll, and yaw motions by these two methods were also compared. In addition, significant differences between two methods’ predictions on mooring tension were found.
How the Hydrodynamic Response of PA-WECs’ Array Under Maximizing the Power Generation by its Arrangement and Control Force Change?
Authors: Murai Motohiko , Funada Junki , Li Qiao
Published: December 2020

Marine renewable energy is expected as an alternative energy source to fossil fuel. Wave energy is one kind of the marine renewable energy. The subject of this paper is an array of Point Absorber Wave Energy Converters (PA-WECs) and their motion in maximizing the power generation. The PA-WEC generates electricity by using its heaving motion. And the assumed PA-WEC has a linear PTO system in which the generator also can provide arbitrary additional force able to change the damping and the restoring effect of its motion. This means that by choosing appropriate force as a control force we can maximize the power generation of PA-WEC. One of the feature points to make an array of PA-WECs is taking positively the hydrodynamic interaction among the array like trapping waves among the array to maximize the power generation.

As for the maximization of the power generation of the array, the authors have proposed the arrangement concept of the array increasing the power generation potential and the way of the determination of the individual control force on each WEC for maximizing the power generation of the array. In the research, we also examined the sensitivity of the arrangement and control force. As a result, there is a best interval among the WECs for each number of the array of WECs and wave frequency. On the other hand, we also confirmed that the sensitivity of the arrangement is not so high around the best arrangement, when we consider the expected power generation in a certain wave spectrum. However, except for the heaving motion on WECs which directly affects the power generation the relation between the other motions and maximizing the power generation was not investigated in detail yet.

In this paper, we examine how to change the whole hydrodynamic motion which is 6 times of the number of arrayed WECs by the hydrodynamic interaction among them under controlled for maximizing the arrayed WECs power generation.
Effects of Empirical Orthogonal Functions in Simplification of Structural Response of a Deep-Water Offshore System Under Current Profile Loading
Authors: Lubis Michael Binsar , Kimiaei Mehrdad , An Hongwei , Azarhoush Reza
Published: December 2020

Typical recommended current profiles for marine operations can be found in offshore engineering guidelines and standards. However, for some offshore components (e.g. risers, umbilicals, risers) typical simplified current profiles can easily lead to unrealistic and conservative results. Due to recent developments in current measuring technology, current speed for deep water location can be easily acquired. However, the current speeds are usually recorded for long periods and in many measurement points along the water column. Hence, finding the extreme current profile based on the recorded time-history data is not an easy task since it needs excessive computational efforts.

To determine the overall response of an offshore system, various methods have been developed to minimize the required computational efforts in working with big number of irregular current profiles. Mode-based analysis using empirical orthogonal functions is one of these methods. Total number of the utilized modes plays an important role in the numerical complexity of the problem as well as the accuracy of the results. In this study, for a given deep water location, the effects of the reduced number of modes are investigated through response analysis of a simple vertical fixed slender structure under thousands of current profiles. It is found that the reduced-mode profile can produce a good representation of the measured current profile, however it tends to underestimate the structural response.
Real Time Monitoring of Subsea Well Foundation Integrity
Authors: Bolger David , Kilbourn Stuart , Crombie Jeff , Kidd Steven
Published: December 2020

Production wells have been constructed in an area offshore which is known for having localised deposits of very soft soil close to mudline. During well planning, it was identified that two other operators had attempted to drill in the region but had to suspend or abandon the well due to excessive degradation of the soil supporting the well foundation. The objective of this work was to provide real-time wellhead integrity data to support the well management team during operations.

The real-time monitoring system was deployed on a subsea BOP stack and riser during drilling and completion operations to monitor motions. From the recorded signals, wellhead integrity was inferred by determining the BOP stack characteristic frequency and effective fixity depth. These integrity parameters were relayed to the vessel via a hydroacoustic link and then delivered onshore. An automatic alarm system was setup based on pre-defined limits which would be triggered during operations if the integrity of the well foundation became at risk of permanent damage. Additionally, the BOP stack motion data allowed the accumulated fatigue damage to be tracked.

The alarm system provided a regular 15-minute check on the integrity of the wellhead and conductor system. If advisory limits were triggered, the well management team were able to assess weather forecasts and make an informed decision to continue drilling or to disconnect from the wellhead to avoid a period of rough weather. This would prevent excessive degradation of the soil and protect the integrity of the well. When increased BOP motions were observed by the drilling team, operations were able to continue with confidence because the integrity data remained within acceptable limits. This avoided unnecessary downtime, providing significant savings in both cost and rig-time as well as improving the overall safety profile of the operation.
Characterization of Rock and Fluid Properties for Low-Salinity Water Flooding of Highly Paraffinic Oil in a Deep Low-Permeability High-Pressure High-Temperature Offshore Carbonate Reservoir
Authors: Singh N. , Gopani P. H. , Sarma H. K. , Mattey P. S. , Negi D. S.
Published: December 2020

Characterization of the rock and fluids is an essential step in screening a reservoir for Low-Salinity Water Flooding (LSWF). A detailed characterization of rock and fluid properties using appropriate methods is being presented for LSWF in a low-permeability deep carbonate reservoir together with a critical analysis of findings. The techniques used are assessed against other possible alternative methods, with inferences drawn on advantages and disadvantages of each to better interpret and apply data so gathered. In so doing, discussions on their key features as to how they can be used effectively and efficiently to screen a reservoir for LSWF are also provided. Such integration of results with other available reservoir and production data should result in a comprehensive description of the target reservoir, and it will help interpret the mechanisms and process dynamics more reliably during a low-salinity waterflood. This integration should allow us not only to gain confidence on the experimental studies but could also help optimize the key parameters responsible for formulating a more robust, reliable and representative regime for tests relevant to the LSWF prior to its eventual implementation in the field. To authors’ knowledge, such integration of experimental studies has not yet been reported in the literature, particularly for the tight carbonate reservoirs with highly paraffinic oil.
Ship-Wave Impact Generated Sea Spray: Part 1 — Formulating Liquid Water Content and Spray Cloud Duration
Authors: Mintu Shafiul A. , Molyneux David , Colbourne Bruce
Published: December 2020

When a wave impacts a ship, a cloud of water spray may form. This spray water, in cold climates, significantly contributes to the deposition of icing on the ship. Estimation of the spray flux is a first step towards predicting the marine icing. The amount of spray water, termed as liquid water content (LWC), the time of ship exposure to the spray cloud in a spray event known as spray duration, and the frequency at which the spray is generated are all important parameters required to define the spray flux. Most of the spray flux formulas found in the literature are based on field observations of small fishing vessels. Moreover, they consider meteorological and oceanographic parameters only and ignore the characteristic behaviors of the vessel. These formulas are therefore not applicable to any size and type of vessel. This paper develops methods to quantify the spray properties in terms that can be applied to vessels of any size.

Formulas to estimate two crucial spray properties, LWC and spray duration, are derived based on the energy conservation principles and by non-dimensional analysis. The formulas take into account the ship’s principal particulars, its operating conditions, and the environmental parameters. The formulas are validated against full-scale field measurement from a Russian fishing trawler, MFV Narva, and a medium-size US coast guard vessel, USCGC Midgett. Reasonable agreements are found in both cases.
Design and CFD Self-Propulsion Analysis of a Ducted Propeller for a DARPA SUBOFF Hull Autonomous Underwater Vehicle
Authors: Sbragio Ricardo , Moura Alceu Jose dos Santos , Silva Rubens Cavalcante da
Published: December 2020

This paper presents the design and the CFD self-propulsion analysis of a ducted propeller to operate in a DARPA SUBOFF hull Autonomous Underwater Vehicle (AUV). The ducted propeller is of the pump jet type with 9 blades at the rotor and 11 blades at the stator. The interactive process of design and optimization uses the potential lifting line theory and CFD RANS analysis for obtaining the self-propulsion point, with the propeller placed behind the AUV hull. During the lifting line design, the rotor diameter, hub diameter, design rotation, blade section chord and length of the duct are modeled by a Kriging Metamodel technique and optimized through random sampling in order to maximize the quasi propulsive coefficient. The optimized configuration from the lifting line and Kriging Metamodel is analyzed using Ansys Fluent 2019 solver. The CFD analysis behind the hull allows including wake effects, thrust deduction factor and viscous effects directly into the model. The lifting line and CFD processes are used interactively to optimize the pitch, the circulation and the camber until the required thrust is achieved.
The Development of an Analytical Wake Model for the Flow-Field Assessment of Offshore Wind Farms
Authors: Nelson Bryan , Lin Tsung-yueh
Published: December 2020

There are several analytical wake models in current use which have, over the years, been shown to adequately predict the flow-fields through large wind farms. However, many of these wake models are based on momentum theory, which assumes no frictional drag and a non-rotating wake, the effects of which may not be neglected at non-optimum operating conditions. To address these shortcomings, we are developing an analytical wake model, derived from unsteady, 3D, full-rotor CFD simulations of the flow field behind a single wind turbine for its full range of operating conditions. The proposed model will be validated against site data from the literature, as well as against data predicted using commercial tools. This study is part of the development of a framework for the real-time assessment of the wind farm power generation, together with the wind/wave loads acting on the individual wind turbines and their support structures, within a large offshore wind farm.
Hydrodynamic Assessment of a Biofouled Wave Buoy in Coastal Zone
Authors: Islam Hafizul , Campos Ricardo M. , Ferreira Tobias R. S. , Guedes Soares C.
Published: December 2020

Accumulation of biological growth or biofouling can significantly influence the measurement accuracy of a buoy, unless it is cleaned on frequent intervals. This paper addresses the hydrodynamic impact of biofouling on met-ocean buoys moored in coastal zones. The analyses are mostly based on a Brazilian Navy buoy on the coast of Rio de Janeiro – Brazil. After only four months moored in the warm waters of the Guanabara Bay, the same buoy gained 25% in weight due to fouling, with additional important modifications on the shape and drag below the waterline. Initially, historical data measured by the buoy was analyzed to observe changes in its response over time. A motion response factor (lateral motion/vertical motion) was calculated from the data that showed a significant impact of biofouling for particular wave conditions. Next, OpenFOAM is used to perform RANS simulations for the buoy, at three different wave scenarios, in both clean and fouled condition to observe the difference in hydrodynamic response. The results indicated that the response amplitude operator varies notably for low and high-frequency cases between cleaned and fouled buoy. Our paper suggests that standard guidelines related to buoy cleaning intervals for biofouling and uncertainty applied in their measured readings may not be equal for all conditions. Buoys placed near the coast with warmer waters are more prone to biofouling and might have higher uncertainty in their measured data.
Fire Phenomenon of Natural Gas Leak Accidents on the LNG-Fueled Ship Using Computational Fluid Dynamic
Authors: Nubli Haris , Prabowo Aditya Rio , Sohn Jung Min
Published: December 2020

The emission of greenhouse gases has tightened up in the shipping industries, since the International Maritime Organization established the Emission Control Area (ECA), to reduce the emission of NOx and SOx. It results in an increase of Liquefied Natural Gas (LNG) as an alternative fuel for merchant vessels. However, the LNG-fueled ship should be verified with respect to reliability and its safety due to the high risk of natural gas.

This work aims to establish the scenario of gas leak accident, and the critical zone due to fire spreading. The variables of leak size, leak location, wind direction and wind speed were included into the scenario. The probabilistic approach also applied to determine the scenario. This study provides the release duration which is associated to the ignition probability. The Latin Hypercube Sampling was used to generate the credible scenario.

Finally, Computational Fluid Dynamics (CFD) analysis of fire release scenarios was performed by using a commercial package software, Kameleon FireEx. The numerical simulation results show the thermal radiations which consisted of several levels of heat intensity. The size of critical zone can be determined from the contour of thermal radiation.
Unsupervised Trajectory Anomaly Detection for Situation Awareness in Maritime Navigation
Authors: Murray Brian , Perera Lokukaluge P.
Published: December 2020

Situation awareness is essential in conducting effective collision avoidance in potential ship encounter situations. It has been shown that data driven trajectory prediction techniques, utilizing historical AIS data, have the potential to aid in providing such awareness. However, such data driven techniques will not perform well for unusual ship behavior, i.e. anomalous trajectories. Additionally, such anomalies in the dataset can corrupt the predictions. In this study, an unsupervised approach to anomaly detection is presented to aid such trajectory predictions. Gaussian Mixture Models are used to cluster trajectories, such that clusters of both normal and anomalous trajectories are discovered. Further, anomalies are discovered within clusters of normal behavior. Novel trajectories can then also be evaluated based on a parametric description of the historical ship traffic. The approach is shown to be effective in detecting anomalies relevant in such a trajectory prediction scheme.