bluuebunny/crossref_metadata_2025_split huggingface dataset:
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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 &lt; 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 &lt; 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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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.