bluuebunny/crossref_metadata_2025 huggingface dataset:
A Conceptual Large Autonomous Subsea Freight-Glider for Liquid CO2 Transportation
Authors: Xing Yihan
Published: October 2021

The produced fluids in a subsea field development can be transported from the subsea well to a floating production unit using pipelines where they are thereafter offloaded to a tanker (surface ship). The flow direction is reversed in the case of CO2 injection into the subsea well. This CO2 offloading process is highly dependent on the weather conditions and it cannot be performed when the conditions are severe. Furthermore, subsea pipeline systems can be expensive to install and maintain. In the present study, a novel subsea freight-glider system is proposed as a suitable, cost-effective, energy-efficient alternative to tanker ships and pipelines. The proposed vehicle is autonomous, 50 m long, has a 1500 DWT displacement, and can carry approximately 800 tons of cargo. The subsea freight-glider uses variable-buoyancy propulsion instead of traditional propellers/thrusters. It changes ballast to provide positive and negative net buoyancy which allows it to glide subsea through the water using hydrodynamic wings. This is an extremely energy efficient way of transporting large amounts of cargo over medium/long distances. Since the subsea freight-glider operates underneath the sea surface, it is not affected by wind and waves and can operate in any weather condition. Furthermore, subsea fields that are not large enough to justify the installation of subsea pipelines can still be developed. Even though the subsea freight-glider is proposed as a vehicle for liquid CO2 transport, it can also transport different types of cargo such as hydrocarbons, injection fluids and gasses, and even carry electrical power using batteries.
Failure Mechanism and Limit Load Capacity of Subsea Pipelines With Dent Defects
Authors: Chen Yanfei , Zong You , Dong Shaohua , Yan Yufeng , He Guoyan , Hou Fuheng
Published: October 2021

With the development of oil and gas resources extending to the deep sea, more and more submarine pipelines are applied, and the importance of the structural and functional integrity of submarine pipelines is becoming more and more prominent. Dent has long been considered one of the potential factors affecting the integrity of submarine pipelines, which can be formed when pipelines are impacted by fallings objects from the sea surface. Due to the seabed subsidence, pipeline suspension and earthquake, mudslide activity and other factors, submarine pipelines may be under bending moment, seriously affecting the safe operation of submarine pipelines. The study of failure and limit load capacity of submarine pipelines under external pressure and bending moment is of great significance. In this paper, the nonlinear finite element method is used to study the collapse failure and local buckling failure of subsea pipelines under external pressure and bending moment respectively. The process of the pipeline being dented by the impact of objects is simulated, spherical denter and wedge-shaped denter are considered. The influence of D/t and dent depth on collapse capacity and bending capacity are investigated. The results can be referred in engineering practice.
Evaluation of Belleville Springs and Damping Through Static Cyclic Loading “Hysteresis” for pVARD Optimization: Part-I
Authors: Abugharara Abdelsalam , Butt Stephen
Published: October 2021

One unconventional application that researchers have been investigating for enhancing drilling performance, has been implemented through improving and stabilizing the most effective downhole drilling parameters including (i) increasing downhole dynamic weight on bit (DDWOB), (ii) stabilizing revolution per minutes (rpm), (iii) minimizing destructive downhole vibrations, among many others.

As one portion of a three-part-research that consists of a comprehensive data analysis and evaluation of a static compression hysteresis, dynamic compression hysteresis, and corresponding drilling tests, this research investigates through static cyclic loading “Hysteresis” of individual and combined springs and damping the functionality of the passive Vibration Assisted Rotary Drilling (pVARD) tool that could be utilized towards enhancing the drilling performance.

Tests are conducted on the two main pVARD tool sections that include (i) Belleville springs, which represent the elasticity portion and (ii) the damping section, which represents the viscous portion. Firstly, tests were conducted through static cyclic loading “Hysteresis” of (i) a mono elastic, (ii) a mono viscus, and (iii) dual elastic-viscus cyclic loading scenarios for the purpose of further examining pVARD functionality.

For performing static compression tests, a calibrated geomechanics loading frame was utilized, and various spring stacking of different durometer damping were tested to seek a wide-range data and to provide a multi-angle analysis.

Results involved analyzing loading and displacement relationships of individual and combined springs and damping are presented with detailed report of data analysis, discussion, and conclusions.
Agree – Disagree: Comparing Traffic Limitations in the Kara Sea
Authors: Kim Ekaterina , Panchi Nabil
Published: October 2021

This study investigates variability of ships’ operational limitations for ice conditions in view of national rules and international recommendations (the Rules for Navigation in the Water Area of the Northern Sea Route and the International Maritime Organization Recommendations). As an example, the Kara Sea region is considered for the period of 2017–2019. The ship data from two datasets were considered: (1) the Northern Sea Route Administration data and (2) the Automatic Identification System data. For each ship’s ice class, the ice information (concentration and type) was used to derive operational limitations based on the Polar Operational Limit Assessment Risk Indexing System (POLARIS), and the results were compared with the newly updated Northern Sea Route access criteria. Preliminary results indicate that national rules are generally stricter than international recommendations, however on some occasions, the national rules impose lesser restrictions on operations as ships with ice class PC7/Arc4 are allowed to operate when the international recommendations suggest an elevated probability of an accidental event.
Comparison of Different Modeling Techniques for Solid-Fluid Interface for Wave Loading
Authors: Saladin Tara , Kwon Young W. , Klamo Joseph T.
Published: October 2021

Computational fluid dynamics (CFD) has been used to estimate the wave loading applied to a fully submerged body near the surface. The Navier-Stokes equations were used for the present study. In terms of modeling the fluid-solid interface, two different techniques are available in ANSYS CFX. One is the Rigid Body Method (RBM) and the other is the Immersed Solid Method (ISM). This paper compares the two modeling techniques in terms of accuracy and modeling flexibility. For this study, a CFD model of the NPS tow tank with wave generation and a submerged body was created to investigate different methods of solid body modeling. A comparison of the RBM and ISM was performed modeling a submerged rectangular body at different depths. The models produced similar results when the body was lower beneath the wave surface with limited fluid-solid interaction. As the amount of fluid-solid interaction increased, the RBM showed increased amounts of wave energy dissipation as compared to the ISM. This disruption of the wave energy resulted in the RBM showing smaller body forces and moments when compared to the ISM solid model. The increased wave energy dissipation in the RBM is likely caused by the different mechanism for modeling body-solid interaction. The numerical results were also compared to the experimental data.
Enhancing Machine Learning Models With Prior Physical Knowledge to Aid in VIV Response Prediction
Authors: Ma Leixin , Resvanis Themistocles L. , Vandiver J. Kim
Published: October 2021

Practical engineering prediction models for flow-induced vibration are needed in the design of structures in the ocean. Research has shown that structural vibration response may be influenced by a large number of physical input parameters, such as damping and Reynolds number. Practical response prediction tools used in design are inevitably a compromise between complexity and simplicity of use. Modern machine learning tools may be used to identify which input parameters are most important. Standard machine learning techniques enable the researcher to compile a list of the most important input parameters, ranked or ordered by the effect of each on the prediction error of the model. When all inputs are treated as equals, blind application of machine learning may lead to predictions that are inconsistent with prior physical knowledge. To address this problem, we conducted a parameter selection process using a prior knowledge-based, trend-informed neural network architecture. This approach was used to identify features important to the prediction of the cross-flow vibration response amplitude of long flexible cylinders, given the known prior effect of Reynolds number and damping. The model balances the usual goal of minimizing the model prediction error, but doing so in a manner that closely follows the extensive knowledge we have of the influence of Reynolds number and damping parameter on response. The resulting neural network model was able to reveal additional insights, including the role of mode number shifting, mode dominance and travelling waves in the regulation of VIV response amplitude.
Hydrodynamic Coupling of Viscous and Non-Viscous Numerical Wave Solutions Within the Open-Source Hydrodynamics Framework REEF3D
Authors: Wang Weizhi , Pákozdi Csaba , Kamath Arun , Martin Tobias , Bihs Hans
Published: October 2021

A comprehensive understanding of the marine environment in the offshore area requires phase-resolved wave information. For the far-field wave propagation, computational efficiency is crucial, as large spatial and temporal scales are involved. For the near-field extreme wave events and wave impacts, high resolution is required to resolve the flow details and turbulence. The combined use of a computationally efficient large-scale model and a high-resolution local-scale solver provides a solution the combines accuracy and efficiency. This article introduces a coupling strategy between the efficient fully nonlinear potential flow (FNPF) solver REEF3D::FNPF and the high-fidelity computational fluid dynamics (CFD) model REEF3D::CFD within in the open-source hydrodynamics framework REEF3D. REEF3D::FNPF solves the Laplace equation together with the boundary conditions on a sigma-coordinate. The free surface boundary conditions are discretised using high-order finite difference methods. The Laplace equation for the velocity potential is solved with a conjugated gradient solver preconditioned with geometric multi-grid provided by the open-source library hypre. The model is fully parallelised following the domain decomposition strategy and the MPI protocol. The waves calculated with the FNPF solver are used as wave generation boundary condition for the CFD based numerical wave tank REEF3D::CFD. The CFD model employs an interface capturing two-phase flow approach that can resolve complex wave structure interaction, including breaking wave kinematics and turbulent effects. The presented hydrodynamic coupling strategy is tested for various wave conditions and the accuracy is fully assessed.
Effect of the Installation Process on the Pullout Capacity of Helical Anchors
Authors: El Haj Abdul Kader , Soubra Abdul-Hamid
Published: October 2021

In this paper, an advanced numerical method called Coupled Eulerian-Lagrangian (CEL) method is used for the prediction of the behavior of helical anchors in sandy soil under ultimate limit state ULS including the effect of anchor installation process. The CEL analysis allows one to overcome the drawback of the classical finite element FE method in the case of large deformation problems as it takes the advantages of both Lagrangian and Eulerian methodologies. Results have shown that the CEL analysis is relevant for the computation of the helical anchor pullout capacity. Indeed, the CEL analysis was able to rigorously determine the ultimate capacity of the anchor contrary to the classical FE method; the calculation via the CEL approach has been carried out for relatively large displacement values without encountering any problem of convergence. Furthermore, CEL analysis was able to simulate the installation process of the anchor and thus enables one to consider the effect of the soil disturbance induced by the installation process on the computed pullout capacity. The numerical simulations have shown that the pullout capacity of the helical anchor may be significantly decreased when considering the anchor installation effect.
Determination of the Vertical Location of the Axis of Rotation of the Roll Motion From Full-Scale Measurements
Authors: Johnsen Lars , Krüger Stefan
Published: October 2021

The behavior of a floating structure results from the mechanics of its, more or less, rigid body and the hydrostatic and hydrodynamic forces acting on it. Particularly for ships, as long and slender bodies, the axis of roll and its vertical position is of special importance. It is around this axis that the lowest lateral accelerations in roll motion occur, which is not only weakly damped but also easily stimulated due to the relatively low mass moment of inertia around the ship’s longitudinal axis. With the intention of clarifying some widespread misconceptions about the location of this axis and to investigate its relation to the natural roll period, full scale measurements have been carried out using a set of two mobile Inertial-Measurement-Units. The Inertial-Measurement-Units were placed on different heights, one above and one below the assumed location of the axis of rotation. Based on the measured accelerations and angular velocities, the average vertical location of the axis of the roll motion for small angles is determined.
Using Computer Simulations to Aid the Analysis of Fluid Contaminations During Cement Plug Placement
Authors: Wang Shuai , Dai Hu , He Liang
Published: October 2021

Setting a cement plug on the target zone either creates a solid seal to stop fluid movement or provides a kick-off point for sidetrack drilling operations. Successfully placing cement plugs is one of most critical steps to ensure trouble free completion, reduce the risk of loss of circulation, isolate pressure zones and enhance wellbore integrity.

The traditional method is to pump all the fluids until each fluid level is equal to that inside the string. The limitation to this method is that the fluid could be contaminated once the string is pulled out of the hole, due to variable fluid densities, as well as wellbore and work string sizes. Thus, the volume of spacers pumped ahead and behind the cement and the volume of displacement are critical to the quality of the cement plug.

A computer program is developed to model the displacement hydraulics of fluids and simulate fluids contamination during pulling pipe out of hole. The computer modeling aids in optimizing the pumping schedule to ensure balanced slurry and spacer levels after POOH (Pull out of the hole), minimizing contamination within the cement slurry and spacer, ultimately, reducing the risk of loss of circulation and enhancing wellbore integrity.
Effects of Design Parameter on Occurrence of Snap Load and Wear of Mooring Line for Spar-Buoy With Ring-Fin Motion Stabilizer for Shallow Sea
Authors: Yamamoto Yusuke , Morimoto Taishi , Katayama Toru , Goto Masahiro , Kodama Ryo
Published: October 2021

The purpose of this study is to propose an optimal design method of the spar-buoy with ring-fin motion stabilizer for shallow sea and its mooring system, in order to avoid the occurrence of snap load. The mechanism of occurrence of snap load is investigated by model tests, and design parameters for avoiding the occurrence of snap load are investigated. From the observation of model’s motion, it has been realized that the snap load is caused by the tension of mooring line to stop the buoy’s horizontal motion, which relaxes the mooring line. Moreover, it has been confirmed that the horizontal motion is caused by the horizontal forces acting on the center pipe and float of the buoy, which relates to the acceleration component of wave excitation forces.

In this paper, the effects of changing of design of the buoy (: diameter of center pipe and float, size of stabilizer, density of the buoy, length of mooring line) on avoiding or reducing snap load are investigated by using a numerical simulation (OrcaFlexver.11.0b), and the wear amounts are also estimated by using an empirical method (Takeuchi et al., 2019). From the results, it is confirmed that changing the buoy’s motion mode by shortening mooring line is effective to avoiding the occurrence of snap load, and to reduce the wear amount of the mooring line.
Numerical Simulation of a Jumper Conveying Slurry for Deep-Sea Mining
Authors: Yamamoto Marcio , Fujiwara Tomo , Yamamoto Joji , Masanobu Sotaro
Published: October 2021

One key technology for Deep-Sea Mining is the riser system. The riser is already a field-proven technology in the Petroleum Industry. However, several differences exist between a petroleum production riser and a riser for Deep-Sea Mining, mainly related to the internal flow. The ore-slurry has a larger density than the hydrocarbons and shall be pumped with a much higher flowrate.

The current software tools for riser’s dynamic analysis may include the internal fluid hydrostatic pressure and the centrifugal and Coriolis forces imposed by the bent pipe’s internal flow. However, the internal pressure drop is not calculated. The internal pressure alters the pipe’s effective tension and can alter the pipe’s bending moment changing its mechanical behavior.

This article describes a computational script’s development to run embedded in a commercial software for riser’s dynamic analysis. Our script calculates the internal viscous pressure drop along with the jumper. This pressure is then converted into wall axial tension (buckling) and imposed on each node of the jumper’s numerical model.

Each simulation case was calculated twice with and without the internal flow viscous pressure drop. The comparison with experimental data revealed that the jumper’s average position has a good agreement among all cases. However, the amplitude caused by the top oscillation showed some discrepancies. Experimental data has the highest amplitude in the horizontal direction, while the simulation without viscous pressure calculation had the smallest. The simulation with our embedded script had intermediary amplitude in the horizontal direction. The vertical direction amplitudes have the same behavior for all cases, but the experimental data showed the highest amplitude.
Tangguh Project: Multidisciplinary and Challenging Design of a Novel Concept of Buckle Initiator
Authors: Federico Formentini , Foschi Luigi , Guidi Filippo , Iannucci Ester , Marchionni Lorenzo , Zulkarnain Yansa
Published: October 2021

This paper is based on the experience made during the design and installation of an offshore pipeline recently completed in Indonesia, where a 24” subsea production pipeline (16km long in 70m water depth) was found susceptible during design to lateral buckling. To limit the development of excessive deformation within the acceptance criteria, a mitigation strategy based on interacting planned buckles has been adopted installing three Buckle Initiators (BI) along the pipeline route.

Buckling is a well understood phenomenon. However, this project was characterized by major uncertainties mainly driven by soil characterization, soil-pipe interaction, seabed mobility and soil liquefaction. These uncertainties have played a key role in the in-service buckling design. A lot of engineering efforts have been spent to go through the screening between alternative concepts, the validation of the chosen solution and its detailed engineering phase. This paper discusses the main contributing factors and how the uncertainties have been tackled.

The Buckle Initiators are quite large and heavy structures with two main bars: the first ramp has an inclination equal to 30° and the pipeline has been laid on it; a second horizontal ramp was used as sleeper to accommodate the development of the lateral buckle during the operating life. A rotating arm was also used to restrict the pipeline lay corridor on the inclined ramp guaranteeing a combined horizontal and vertical out-of-straightness in the as-laid configuration. The rotating arm has been released as soon as the pipeline passed the BI permitting the pipeline to slide freely over the two BI ramps. The foundation of the Buckle Initiator has a footprint surface of about 60m2 guaranteeing its stability for different soil types characterizing the three installation areas. This more complex solution was preferred with respect to a typical sleeper to increase the robustness of the system in terms of buckle mobilization.

The design of the Buckle Initiator was a multidisciplinary activity where many novel concepts were developed and many issues were faced (i.e. pipeline laying on an inclined sleeper, anti-scouring system, foundation design, etc.). The Buckle Initiator design was focused on structural calculations against design loads expected during temporary and operating conditions, geotechnical verifications, installation analysis, pipeline configuration and fatigue assessment.

This paper presents all main engineering aspects faced during design and first feedbacks from field after the pipeline installation.
Results From Fatigue Testing of Subsea Wellhead Connector Segments
Authors: Horn Agnes Marie , Barros Pedro , Wormsen Anders , Osen Per , Macdonald Kenneth A.
Published: October 2021

For subsea well drilling, the drilling rig is connected to the subsea well by a marine riser and subsea BOP equipped with a remotely controlled wellhead connector latched onto the subsea wellhead profile.

The level of cyclic loading on subsea wellheads is steadily increasing due to use of increasingly larger drilling rigs with larger BOPs, the drilling of wells in harsher environments characterized by high waves.

The remotely controlled wellhead connector forces a series of locking dogs into an externally machined profile on the wellhead. This external profile is generally referred to as a wellhead profile. The fatigue resistance of this safety-critical connector is typically estimated by finite element analyses (FEA). Due to the large size of the equipment, and high cost of testing, very limited fatigue testing, if any, has been carried out. A test method has therefore been developed, where a special test fixture is used to apply realistic boundary conditions and variable tensile loads to a small sector or segment of the wellhead connector. A primary objective is to generate fatigue-critical stress fields in the segments under tensile test load that closely replicates the stress fields in the full-scale wellhead connector. A secondary objective is to evaluate the possibility of using segment testing to determine the fatigue capacity of the full-scale connector. The testing of narrow sector segments allows the use of readily available test apparatus. It is thereby envisaged that the total cost of testing (specimens and test laboratory costs) can be substantially reduced in comparison with full-scale connector fatigue testing.

This paper describes the fatigue testing of wellhead connector segments, and the test results in terms of cycles to failure and the failure modes, i.e. crack initiation point, and final crack geometry. The test scope consists of nine segments tested in-air at ambient temperature (nominal 20 °C), at a frequency of approximately 2 Hz under axial load of R = 0.1. At the time of writing this paper, six out of these nine tests have been performed. These six fatigue tests are presented in this paper.

The test results are compared with estimates achieved by FEA of the test assembly and relevant S-N curves for the materials. It will be determined if the test results can be accurately predicted by the fatigue analysis methodology in Section 5.4 of DNVGL-RP-C203 (C203), including use of the new series of S-N curves for high strength materials in Appendix D.2 of C203. This design approach assumes that other failure modes (e.g. fretting or other local effects in the interface between components) do not govern the fatigue life, as this cannot be predicted by the fatigue analysis method applied here.

The fatigue test set-up and the finite element analysis of the segment test is presented in the OMAE2020-18652 paper.
Tangguh Project: In-Service Buckling Design of Offshore Pipelines With Major Uncertainties on Soil Characterization and Seabed Mobility
Authors: Bughi Sabrina , Foschi Luigi , Marchionni Lorenzo , Vichi Roberta , Zulkarnain Yansa
Published: October 2021

This paper is based on the experience made during the design and installation of an offshore pipeline recently completed in Indonesia, where a 24” subsea production pipeline (16km long in 70m water depth) was found susceptible during design to lateral buckling.

Buckling is a well understood phenomenon. However, this project was characterized by major uncertainties mainly driven by soil characterization, soil zonation, soil-pipe interaction, seabed mobility and seabed liquefaction. These uncertainties have played a key role in the in-service buckling design. In particular, extreme pipeline embedment scenarios ranging from fully exposed to fully covered (due to natural sand transportation) were accounted with a significant impact on soil-pipe interaction.

To limit the development of excessive strain within the acceptance criteria, a mitigation strategy based on interacting planned buckles has been adopted installing three Buckle Initiators (BI) along the pipeline route. During design great efforts have been spent with the aim to demonstrate the robustness of the proposed solution.

3-D FEM simulations with ABAQUS have been performed taking into account the pipeline route including route curves and the sea bottom profile and the buckle initiators with their main geometries. All uncertainties have been considered following a deterministic approach. The impact of environmental and accidental loads due to a potential trawl-gear interaction were assessed as well.

The pipeline susceptibility to lateral and/or upheaval buckling along the sandwave areas has been analyzed as well in order to evaluate the need of mitigation measures suitable to freeze the pipeline configuration during the operating life.

Finally, once the lateral buckling design philosophy was established, the cyclic expansion and walking behavior of the pipeline were assessed to verify the pipeline structural integrity at buckles, route curve pull-out and the accumulative pipeline expansion at spools.

This paper presents all main engineering aspects faced during design and first feedbacks from field after the pipeline installation.
Coupling of a Boundary Element Method With a Boundary Layer Method for Accurate Rudder Force Calculation Within the Early Design Stage
Authors: Carstensen Björn , Krüger Stefan
Published: October 2021

During the early design stage, it is essential to produce quick and reliable results. Regarding the calculation of forces acting on a rudder in the propeller slipstream, potential flow solvers are often used for this purpose due to their low computational effort. The drawback of these solvers is that viscous effects as drag or flow separation cannot be calculated. To overcome this drawback, the coupling of an inviscid three dimensional boundary element method with a viscous two dimensional boundary layer method is presented. The inflow from the propeller and the propeller rudder interaction is calculated using a lifting line approach. The inviscid calculation of the rudder forces is done with a boundary element method and the viscous rudder forces are calculated with the boundary layer method. Two different approaches are presented for the coupling. The results calculated with the implemented methods are compared to reverse open water model tests.
A Study on Performance Evaluation of Non-Reflective Boundary for Progressive Waves Reproduction Introduced in Numerical Wave Tank With MPS Method
Authors: Aida Yasuhiro , Takeo Tomotaka , Ikoma Tomoki , Masuda Koichi
Published: October 2021

Numerical simulation based on the moving particle semi-implicit (MPS) method is effective for the analysis of floating motion in stormy waves in both coastal and offshore areas. However, when the outer circumference of the calculation area is composed of wall boundaries, superimposed waves are generated by the reflected waves, which makes it difficult to reproduce wave fields in offshore areas. Therefore, in this study, we developed two types of non-reflective boundary that can be applied to a numerical wave tank with the MPS method. One type is an attenuation zone in which a high-viscosity region with a finite width is set from the end of the water tank. The other type is a wave absorption control boundary that detects the amount of water surface fluctuation in front of the boundary and prevents reflection via position control. Regular and irregular waves were created in a numerical wave tank with these boundaries and the wave dissipation performance was quantitatively evaluated by comparing the estimates for incident and reflected waves, the time-series waveform, and the wave spectrum.
Hydrodynamic Response of Buoy Form Spar With Heave Plate Near Free Surface
Authors: Hegde Pooja , Nallayarasu S.
Published: October 2021

Hydrodynamic response of spar with appendages such as heave plate has been investigated in the past, mostly attached at the bottom of the spar. The effect of geometry and appendages on the hydrodynamic response of spar has been investigated in this article. A curved neck form with a heave plate near the free surface is proposed as an energy dissipation device for both heave and pitch responses. Numerical simulation using Computational Fluid Dynamics (CFD) is used for capturing the flow around the curved neck with heave plate and corresponding damping characteristics. CFD free decay simulations have been carried out to obtain heave and pitch damping and were noted to be higher than the conventional spar with heave plate at the bottom. Comparison of the proposed geometry and heave plate at the free surface with a conventional heave plate at the bottom of the spar has been made, and significant changes to the response and hydrodynamic characteristics have been noted. It is observed that the buoy form spar combined with the heave plate located near the surface (within 10% of the draft) helps dissipate energy and thus reduce the heave response.
Neural Network Optimization for Lifetime Structural Adaptation to Evolving Natural Events
Authors: Wang Xiaowei , Heo YeongAe
Published: October 2021

Recent advances in data analytics, numerical modeling, and structural health monitoring (SHM) boost the application of machine learning methods in the field of structural engineering. Among them, the multi-layer neural network (MLNN) is one of the most popular ones. However, mathematical details of MLNN have yet to be well understood for structural problems. This study aims to identify optimal MLNN parameters for regression modeling of structural response estimates. SHM data-validated finite element models considering stochastic uncertainties in the natural events and structural properties are used to prepare a large dataset for regression modeling. The efficacy and accuracy of regression modeling are optimized by extensive sensitivity analyses for key MLNN parameters (e.g., numbers of hidden layers and neurons, activation functions and learning rates) via a k-fold cross-validation process. The optimized regression modeling is incorporated into a conceptual smart framework for lifetime structural performance assessment adapting to evolving natural events. The presented optimization process and smart framework is applicable to marine and offshore structures by characterizing the offshore hazards and structural responses.
Development and Verification of Modeling Practice for CFD Decay Calculations to Obtain Roll Damping of FPSO
Authors: Koop Arjen , Crepier Pierre , Loubeyre Sebastien , Dobral Corentin , Yu Kai , Xu Haihua , Zheng Yingying , Kim Jang , Huang Jerry
Published: October 2021

Estimates for roll damping are important input parameters for simulation studies on vessels operating at sea, e.g. FPSO mooring in waves, wind and current, workability and operability investigations, Dynamic Position studies, ship-to-ship operations and safety studies of vessels. To accurately predict the motions of vessels this quantity should be determined with confidence in the values. Traditionally, model experiments in water basins using so-called decay tests are carried out to determine the roll damping. With recent advancements in CFD modelling, the offshore industry has started using CFD as an alternative tool to compute the roll damping of FPSO’s. In order to help adopt CFD as a widely accepted tool, there is a need to develop confidence in CFD predictions. Therefore, a practical CFD modelling practice is developed within the Reproducible CFD JIP for roll decay CFD simulations. The Modelling Practice describes the geometry modelling, computational mesh, model set-up and post-processing for these type of CFD calculations. This modelling practice is verified and validated by three independent verifiers against available model test data.

This paper provides an overview of the developed modelling practice and the calculated CFD results from the verifiers. The CFD modelling practice is benchmarked against available model test results for a tanker-shaped FPSO. By following this modelling practice, the CFD predictions for the equivalent linear damping coefficient and natural period of the roll motions are within 10% for all verifiers and within 10% from the model test results. Therefore, we conclude that when following the developed modelling practice for roll decay simulations, reliable, accurate and reproducible results can be obtained for the roll damping of tanker-shaped FPSOs.
Effect of the Simplified Superstructure and Soil-Structure Interaction Models on the Natural Frequencies of an Offshore Wind Turbine
Authors: Alkhoury Philip , Soubra Abdul-Hamid , Rey Valentine , Aït-Ahmed Mourad
Published: October 2021

Monopile-supported offshore wind turbines (OWTs) are dynamically sensitive structures, in which their design is principally based on a dimensioning criterion related to its fundamental frequencies. Therefore, an accurate estimation of the natural frequency is essential to assess the working lifetime of the OWT.

For the calculation of the OWT natural frequency, several studies exist but few of them simultaneously consider both the real geometrical configuration of the OWT superstructure (blades, nacelle, tower, and transition piece) and the three-dimensional (3D) soil domain and its interaction with the foundation. The aim of this paper is to investigate the suitability of (i) the different simplifying assumptions used in literature for the superstructure and (ii) the recently proposed PISA soil-foundation interaction model, when calculating the natural frequency. This is performed by comparing the results obtained using a rigorous three-dimensional (3D) finite element-based model recently developed by the authors of this paper with those obtained based on these simplifications. The comparison results have shown that both the simplified superstructure models and the newly developed soil reaction curves of the PISA project provide very good results of the OWT natural frequency.
Numerical Simulation of 5th-Order Stokes Wave in Finite Water Depth Based on Momentum Source Method
Authors: Wang Wenjie , Gao Zhiliang
Published: October 2021

For numerical simulation of structure-wave interaction, the wave generation with high accuracy is prime to analyze the wave loads and motions of the structure. Based on the fifth-order Stokes theory, a two-dimensional viscous wave flume, which was modeled using the commercial CFD solver ANSYS-FLUENT, was applied to the generation and propagation of regular waves in finite water depth. With the user-defined function provided by the solver, the momentum source term and boundary condition, which are used for the wave generation and dissipation, were developed to ensure the accuracy of wave simulation with large steepness. In addition, the wave flume was separated into two regions, which are governed by the laminar model and turbulent model, respectively. The separation of laminar and turbulent regions can alleviate the side effect of turbulence on the accuracy of wave generation. In order to validate the present method, the regular wave propagating with different steepness in finite water depth were simulated. The numerical results were in good agreement with the theoretical ones. The study showed that the present method was effective for the simulation of Stokes wave in finite water depth, especially effective to improve the numerical accuracy in case of large wave steepness.
Response of Multiple Bend System to Multi Phase Flow
Authors: Pereboom H. , Belfroid S. P. C. , Gonzalez-Diez N. , Reijtenbagh J.
Published: October 2021

Multiphase flow can induce high amplitude vibrations in piping systems. Several experimental campaigns focused on the force spectrum on a single bend. To evaluate the evolution of the forces from bend to bend, experiments have been done on an air-water, one inch pipe system consisting of six bends at near-atmospheric conditions. In a first phase, all individual bends were clamped to measure the phase relation and correlation of the flow-induced forces on the subsequent bends. In a second phase, all clamps were removed to measure the free vibrations. In this paper which focusses on the phase 2 results, the vibration measurements were compared to the calculated vibrations. For the excitation forces and phase relations, the measured force spectra from phase 1 are used. Damping values are based on experimental results from phase 2. The results show a good match between modeled and measured vibrations levels. Including the measured correlation between forces at multiple bends, improves the modeled results for slug flow cases. It is possible to directly use extract damping values from the measured signals, however, robustness of the damping estimation needs to be improved. Using average damping values currently leads to the best match.
Evaluation Study on Integration of Comfort and Energy Efficiency Models in Cruise Ship
Authors: Cai Wei , Liu Wushuai , Wan Shuqiao , Zeng Qingsong
Published: October 2021

In the design of cruise ships, it is necessary to make them have an excellent level of energy efficiency and at the same time provide tourists with a good comfort experience. Therefore, it is urgent to explore the balance between cruise comfort and energy efficiency. Based on Maslow’s hierarchy of needs theory, a cruise comfort evaluation system was established combined with the “Cruise Leisure Design Index” issued by China Classification Society; according to the characteristics of cruise ships, a specific “Energy Efficiency Design Index” (EEDI) for cruise ships was proposed. From the perspective of a life cycle, the “Life Cycle Assessment” (LCA) was applied to the cruise design. Meanwhile, the EEDI-LCA cruise ship energy efficiency model was established, and the baseline fitting and comparative analysis were carried out. The combination weighting method combines comfort index and energy efficiency index to establish a cruise ship comfort and energy efficiency evaluation system. The model was verified with a 60,000-ton MSC Sinfonia cruise ship. The results show that the model can find a comparatively good balance between comfort and energy efficiency so that the cruise ship’s energy can be reduced without sacrificing comfort. The balance is a good guide on the selection of cruise design parameters.
Void Fraction, Pressure Drop, and Flow Pattern Behavior for Two-Phase Non-Newtonian Slug Flow
Authors: Ben Rajeb Faraj , Imtiaz Syed , Zhang Yan , Aborig Amer , Awad Mohamed M. , Rahman Mohammad Azizur
Published: October 2021

Slug flow is one of the most common flow patterns in non-Newtonian two-phase flow in pipes. It is a very common occurrence in gas-liquid two-phase flow in the pipe. Usually, it is an unfavorable flow pattern due to its unsteady nature, intermittency as well as high pressure drop. The differences between slug flow and elongated bubble flow are not clear because usually these two types of flow combined under one flow category. In general, these two-phase flow regimes are commonly defined as intermittent flow. In the present study, pressure gradient, and wave behavior in slug flow have been investigated depending on experimental work. In addition, void fraction has been estimated regarding available superficial liquid and gas velocities. The experimental records of superficial velocities of gas and liquid for slug flow and other flow patterns is used to create flow regime map for the gas non-Newtonian flow system. The effect of investigated flow regime velocities for non-Newtonian/gas flow on pressure drop and void fraction is reported. Pressure drop has been discovered to be reduced in slug flow more than other flow patterns due to high shear thinning behavior.
Advancing the Performance of Wave Forecast Models Under Low Wind Conditions
Authors: Yasrab Muhammad , Babanin Alexander V.
Published: October 2021

Ocean surface is complex and difficult to predict accurately due to its random nature. Ocean surface waves in strong wind conditions have been widely studied for last few decades. Almost half of world’s winds are below 7.5 m/s and the physics of such winds contains a lot of uncertainties. The simulation of ocean waves is largely dependent on the driving winds force accuracy and source term parameterizations. However, low winds are often ignored on the perception of their lesser effect on overall results of existing models. It is important to understand the relative strength/ weaknesses of wave forecast models under low wind conditions from scientific perspective which should lead to improved wave forecast and wave-ocean-weather coupling capabilities. There are many critical thresholds involved in the initial generation and growth of wind waves whereas current parameterizations of wave models are mostly based on moderate – high wind conditions. Wave model’s performance, although not very prominent, contains bias under low winds conditions and these thresholds need to be embedded in current physics of wave forecast models for more accurate simulations. In this study, WAVEWATCH III (v6.07) wave forecast model with observation based source terms parameterizations (ST6 package) is used to simulate waves on a global scale. The model’s output is analyzed with a globally calibrated and cross validated global dataset of 13 altimeters to analyze its performance under low wind conditions. A relative error of −1 to 6 is observed in global significant wave heights simulated by WAVEWATCH III model compared to altimeter’s measured wave heights for wind speeds less than 5ms−1.
Numerical Simulation of Microscopic Formation of Carbon Dioxide Hydrate in Two-Phase Flow
Authors: Yamaguchi Alan Junji , Kobayashi Kaito , Sato Toru , Tobase Takaomi
Published: October 2021

The global warming is an important environmental concern and the carbon capture and storage (CCS) emerges as a very promising technology. Captured carbon dioxide (CO2) can be stored onshore or offshore in the aquifers. There is, however, a risk that stored CO2 will leak due to natural disasters. One possible solution to this is the natural formation of CO2 hydrates. Gas hydrate has an ice-like structure in which small gas molecules are trapped within cages of water molecules. Hydrate formation occurs under high pressure and low temperature conditions. Its stability under these conditions acts like a cap rock to prevent CO2 leaks. The main objective of this study is to understand how hydrate formation affects the permeability of leaked CO2 flows. The phase field method was used to simulate microscopic hydrate growth within the pore space of sand grains, while the lattice Boltzmann method was used to simulate two-phase flow. The results showed that the hydrate morphology within the pore space changes with the flow, and the permeability is significantly reduced as compared with the case without the flow.
Study on Dynamic Stability of Single Floating Pier Under Waves
Authors: He Yikuan , Han Bing , Ji Wenyu
Published: October 2021

Considering the upper structure restraint effect of the floating bridge, the diffraction effect and radiation effect of linear monochromatic waves, the dynamic response equation of floating pier is derived and the factors affecting the dynamic stability of the floating pier are analyzed in this paper. Based on the theory of potential flow, the calculation domain is divided into the interior region and the exterior region. The wave diffraction and radiation problems are solved by the matched eigenfunction expansion method (MEEM). After obtaining the wave excitation force, additional mass and radiation damping coefficient, considering the restraint effect of the upper structure of the floating bridge, the motion differential equation of the floating pier is established, and the response amplitude operator (RAOs) of the floating pier is obtained. The effects of span, mass and stiffness of upper structure, as well as the draft depth, size and net height of floating pier on dynamic stability of floating pier under wave are analyzed. The results show that the increase in the span of upper structure will significantly increase the peak RAOs of sway and heave, and the increase in stiffness is helpful to reduce the peak RAOs of sway and heave. The increase of the floating pier radius can reduce the heave RAO, and the net height on the water surface of the floating pier increases the heave and roll.
A Case Study on Consensus Building With Fisheries for Offshore Wind-Power Generation in Japan
Authors: Suda Sayaka , Kularathna A. H. T. Shyam , Tabeta Shigeru , Takagi Ken
Published: October 2021

Local fishery unions have a significant authority over the use of marine space in Japan. Building consensus with them on marine space use have been identified as significant barriers to development of offshore wind projects. Creating non-monetary benefits and adding value to the local fisheries is a required strategy for getting fisheries consensus for future offshore wind projects. Previous studies have proposed and evaluated several such benefit creation options. However, there still remains uncertainty of the effectiveness of such benefit creation options. This study aims to compare the local residents’ and fisheries’ preference of the benefit creation options by using questionnaire surveys and stakeholder interviews in three areas. Those valid responses have been selected for the preference analysis by using the DS-AHP multi criteria decision making model. This study shows that project location and other regional differences are significant factors of general residents’ attitudes of offshore wind projects. On the other hand, fishers, the most important stakeholders, preferred added values related to their fishing industry. Additionally, regional location and fishing method is found to have an impact on their preference.