bluuebunny/crossref_metadata_2025 huggingface dataset:
Linearization of the Wave Spectrum: A Comparison of Methods
Authors: Barratt Dylan , Bingham Harry B. , Taylor Paul H. , van den Bremer Ton S. , Adcock Thomas A. A.
Published: December 2020

The relative contributions of free waves and bound waves to the formation of an extreme wave event remains a topic of interest in offshore engineering. A variety of methods have been proposed for identifying and removing the bound wave components. The method of “phase separation” or “phase manipulation” repeats simulations/experiments of a wave field with an offset in the initial phase of the wave components and relies upon summation of the resulting wave fields to isolate the bound harmonics, following from a Stokes expansion in steepness; the method has proven effective in isolating bound harmonics but requires that all cases be repeated. Alternatively, the bound harmonics can be removed using a three-dimensional fast Fourier transform (3D-FFT) of the wave field. However, the Fourier transform requires periodicity in the signal and assumes homogeneity in space and stationarity in time, producing spurious modes otherwise. We compare the phase separation and 3D-FFT approaches for a steep, focusing wave group in deep water using the numerical simulation tool, OceanWave3D, and discuss the effectiveness of both methods.
Micro-Testing While Drilling for Rate of Penetration Optimization
Authors: Nystad Magnus , Pavlov Alexey
Published: December 2020

The Rate of Penetration (ROP) is one of the key parameters related to the efficiency of the drilling process. Within the confines of operational limits, the drilling parameters affecting the ROP should be optimized to drill more efficiently and safely, to reduce the overall cost of constructing the well. In this study, a data-driven optimization method called Extremum Seeking is employed to automatically find and maintain the optimal Weight on Bit (WOB) which maximizes the ROP. To avoid violation of constraints, the algorithm is adjusted with a combination of a predictive and a reactive approach. This method of constraint handling is demonstrated for a maximal limit imposed on the surface torque, but the method is generic and can be applied on various drilling parameters. The proposed optimization scheme has been tested on a high-fidelity drilling simulator. The simulated scenarios show the method’s ability to steer the system to the optimum and to handle constraints and noisy data.
Laminar and Turbulent Flow Past a Hydrofoil Predicted by a Distributed Vorticity Method
Authors: Wu Chunlin , Kinnas Spyros A.
Published: December 2020

A distributed viscous vorticity equation (VISVE) method is presented in this work to simulate the laminar and turbulent flow past a hydrofoil. The current method is proved to be more computationally efficient and spatially compact than RANS (Reynolds-Averaged Navier-Stokes) methods since this method does not require unperturbed far-field boundary conditions, which leads to a small computational domain, a small number of mesh cells, and consequently much less simulation time. To model the turbulent flow, a synchronous coupling scheme is implemented so that the VISVE method can resolve the turbulent flow by considering the eddy viscosity in the vorticity transport equation, and the eddy viscosity is obtained by coupling VISVE with the existing turbulence model of OpenFOAM, via synchronous communication. The proposed VISVE method is applied to simulate both the laminar flow at moderate Reynolds numbers and turbulent flow at high Reynolds numbers past a hydrofoil. The velocity and vorticity calculated by the coupling method agree well with the results obtained by a RANS method.
Sub-Zero Temperature Fatigue Strength of Butt-Welded Normal and High-Strength Steel Joints for Ships and Offshore Structures in Arctic Regions
Authors: Braun Moritz , Milaković Aleksandar-Saša , Ehlers Sören , Kahl Adrian , Willems Tom , Seidel Marc , Fischer Claas
Published: December 2020

Ships and offshore structures operating in Artic regions face specific challenges such as ice loads and seasonal low temperatures. In order to meet these extreme environmental requirements, the effect of temperature on material behaviour needs to be considered. It is well known that static material properties (yield strength, fracture toughness etc.) undergo significant changes with temperature. In recent studies, significantly higher fatigue strength was observed in welded joints in comparison to estimates based on international standards. Fatigue strength increased even for temperatures far below the allowed service temperature based on fracture toughness results; however, studies on fatigue strength of structural steel at sub-zero temperatures are scarce. Moreover, material selection for ships and offshore structures is usually based on empirical Charpy and fracture toughness relations at the design temperature, minus a safety margin.

This study aims at introducing an S-N curve database for welded joints that can be used to verify the fatigue design approaches for ships and offshore structures subject to sub-zero temperatures. Therefore, the effect of temperature on the fatigue strength of butt-welded normal and high strength steel structures is analysed experimentally for sub-zero temperatures. For this purpose, fatigue test results of SAW and MAG welded joints for temperatures down to −50 °C are analysed and the potential for changes regarding material selection for ships and offshore structures are discussed.
Revisiting the FPSO São Mateus Accident From a Human Reliability Perspective Using Phoenix HRA Methodology
Authors: Ramos Marilia A. , Almeida Alex , Martins Marcelo R.
Published: December 2020

Several incidents in the offshore oil and gas industry have human errors among core events in incident sequence. Nonetheless, human error probabilities are frequently neglected by offshore risk estimation. Human Reliability Analysis (HRA) allows human failures to be assessed both qualitatively and quantitatively. In the petroleum industry, HRA is usually applied using generic methods developed for other types of operation. Yet, those may not sufficiently represent the particularities of the oil and gas industry.

Phoenix is a model-based HRA method, designed to address limitations of other HRA methods. Its qualitative framework consists of three layers of analysis composed by a Crew Response Tree, a human response model, and a causal model.

This paper applies a version of Phoenix, the Phoenix for Petroleum Refining Operations (Phoenix-PRO), to perform a qualitative assessment of human errors in the CDSM explosion. The CDSM was a FPSO designed to produce natural gas and oil to Petrobras in Brazil. On 2015 an explosion occurred leading to nine fatalities. Analyses of this accident have indicated a strong contribution of human errors. In addition to the application of the method, this paper discusses its suitability for offshore operations HRA analyses.
Integrated Probabilistic-Mechanistic Deterioration Modeling for Preventive Maintenance of Aging Fixed Offshore Jacket-Type Platforms
Authors: Srikanth Ishwarya , Arockiasamy M.
Published: December 2020

Most of the offshore platforms in the world are designed as steel jacket-type platforms for oil and gas exploration and production. A significant amount of research has been carried out over the past two decades to understand ageing and the associated degradation of offshore structural members. The paper proposes an approach for prioritization and preventive maintenance of jacket-type platform. Offshore platform maintenance systems are dependent on deterioration models to predict the future condition of structural elements. The input for these models consists of condition states of different jacket elements and their evolution as a function of time. One of the challenges includes lack of availability of condition state records for the aging platforms. This paper proposes a methodology for developing deterioration models when there is a scarcity of inspection records.

This study focusses on developing a methodology for i) the impact ranking of different bracing member groups of the jacket platform and ii) deterioration models using integrated probabilistic-mechanistic modeling. Firstly, a typical jacket-type offshore platform under the North Sea conditions is modeled and analyzed under wave and current loads. Then, nonlinear static (pushover) analyses are performed for the undamaged jacket platform. The criticality of the different groups of bracing elements is obtained in terms of Residual Resistance Factor by carrying-out redundancy analyses. Soil-structure interaction effect is considered in the analyses and the results are compared with the hypothetical fixed-support end condition. The member groups are then ranked based on their impact on the ultimate strength of the jacket platform. Secondly, deterioration models for jacket legs and bracings in splash and immersion zones are obtained based on integrated mechanistic-probabilistic approach. An idealized uniform corrosion model is used in generating the member condition state probabilities. Condition state probabilities for jacket legs and bracings are then generated using Monte Carlo simulations. The residual service life of the jacket elements is predicted from the deterioration models which could be utilized for preventive maintenance of the jacket platform.
A Fracture Characterization Framework for Large-Scale Marine Metal Structures
Authors: Londono Juan G. , Mobasher Mostafa E. , Woelke Pawel
Published: December 2020

Prediction of large-scale marine structures under extreme loading conditions (e.g. impact, fire) requires the use of computational tools that are both accurate and efficient. To resolve models at this scale, engineers normally use shell elements in which two of the (in-plane) dimensions are much larger than the third (thickness) one. Herein, we propose a consistent characterization framework for shell-element models commonly used in metal marine structures. The underlying model is based on a three stress-invariant plasticity model that accounts for the effects of stress triaxiality and shear-dominated stress states and accounts for the micromechanical void growth and coalescence that leads to fracture. This framework serves as both a constitutive model as well as a back-end calibration engine which outputs material cards that can be readily used with commercial finite element solvers like LS-DYNA and Abaqus. The calibration framework supports data-driven constitutive models that require engineers to prescribe the Fracture Locus (FL) and Instability Curves (IC) from experimental data, such as the Johnson-Cook model. Moreover, the calibration framework uses the underlying physics based model to computationally generate additional surrogate data that compliments the experimental testing. Thus, it allows the analyst to calibrate a material model based on limited experimental data, hence, reducing the costs associated with data acquisition and processing. A standalone calibration tool (VistaCal) that highlights the benefits of this characterization framework is also presented.
Jump Bifurcation Phenomenon During Varying Wind Speeds in Floating Offshore Wind Turbines
Authors: Jose Alwin , Falzarano Jeffrey
Published: December 2020

Floating Offshore Wind Turbines (FOWTs) are susceptible to an instability which has come to be called “negative damping”. Conventional land based wind turbine controllers when used with FOWTs may cause large amplitude platform pitch oscillations. Most controllers have since been improved to reduce motions due to this phenomenon. In this paper, the motions induced using one of the original controllers is studied.

The current study is performed using the coupled time domain program FAST-SIMDYN that was developed in Marine Dynamics Laboratory (MDL) at Texas A&M University. It is capable of studying large amplitude motions of Floating Offshore Wind Turbines.

FOWTs use various controller algorithms of operation based on the available wind speed depending on various power output objectives i.e., to either maximize or level out power absorption. It is observed that the transition region for controllers is often chaotic. So most studies focus on operations away from the transition region i.e., both below and above the transition wind speeds. Here we study the transition region using the theoretical insight of non-linear motion response of structures. This study reveals the presence of a very interesting and potentially hazardous nonlinear phenomenon, bifurcation. This finding could help explain the chaotic motion response that is observed in the transition region of controllers. Understanding the nature and cause of bifurcation could prove very useful for future design of FOWT controllers.
Active Absorption of Random Waves in Wave Flume Using Artificial Neural Networks
Authors: Ramos Áureo I. W. , Fernandes Antonio C. , Thomaz Vanessa M.
Published: December 2020

A wave flume is primarily intended to reproduce actual sea conditions in order to provide a reliable means of testing for small scale models. The realization of scaled tests is extremely important for the validation of a project in real scale, since, through the laws of similitude, such tests make it possible to predict the behavior of structures in the ocean as well as their performance during operation. This research aims to develop, test and validate an active control algorithm for wave absorption in a 2D wave channel — that is, when the waves propagate in only one direction — based on artificial neural networks (ANN). The ANN control algorithm relies on the linear wave theory and the principle of time reversal of wave propagation, i.e. the phenomenon of wave absorption corresponds to the wave generation when observed in the reverse direction of time. Through this principle, data from wave generation experiments, after proper manipulation, are used to train an ANN capable of generating the control signal used to move the wave generator device, this time as a wave absorber.
Experimental Identification of an Advanced Spar’s Low Frequency Drag Damping in Waves
Authors: Wright Christopher , Yoshimoto Haruki , Wada Ryota , Takagi Ken
Published: December 2020

Climate change and increasing population growth are accelerating the need for new clean renewable energy generation. One such proposed type of renewable energy is offshore floating wind, which has yet to reach a convergence as to the optimum type of floating platform to support a wind turbine. To date there has been numerous proposals of novel floating platforms with unique hull characteristics. One such type is the advanced spar, which has a large area of sharp edges and therefore considerably different hydrodynamic viscous effects than typical cylindrical platforms.

Prediction of a floating body’s low frequency drag damping is crucial to successfully predicting the horizontal motions. Calculation of the viscous effects have been seen to have the most uncertainty. Published literature shows that the viscous drag effects of floating bodies either increase or decrease with increasing wave severity as compared to still water decay tests.

In this paper a combined experimental and numerical method for identifying low frequency viscous damping effects on the hull of a moored platform is introduced. An initial offset is applied to the platform, which is then released in still water and regular sinusoidal waves. The experimental results will then be compared to a weakly nonlinear time domain model in order to identify how the drag coefficients vary with wave conditions. Discussions on Keulegan–Carpenter (KC) number dependent damping are given. Finally simulations using results from these experiments are compared against a full scale deployed floating platform in multi-directional waves, current and wind.
Wear-Factor Prediction Based on Data-Driven Inversion Technique for Casing Wear Estimation
Authors: Mittal Manish K. , Samuel Robello , Gonzales Aldofo
Published: December 2020

Wear factor is an important parameter for estimating casing wear, yet the industry lacks a sufficient data-driven wear-factor prediction model based on previous data. Inversion technique is a data-driven method for evaluating model parameters for a setting wherein the input and output values for the physical model/equation are known. For this case, the physical equation to calculate wear volume has wear factor, side force, RPM, tool-joint diameter, and time for a particular operation (i.e., rotating on bottom, rotating off bottom, sliding, back reaming, etc.) as inputs. Except for wear factor, these values are either available or can be calculated using another physical model (wear-volume output is available from the drilling log). Wear factor is considered the model parameter and is estimated using the inversion technique method. The preceding analysis was performed using soft-string and stiff-string models for side-force calculations and by considering linear and nonlinear wear-factor models. An iterative approach was necessary for the nonlinear wear-factor model because of its complexity. Log data provide the remaining thickness of the casing, which was converted into wear volume using standard geometric calculations.

A paper [1] was presented in OMC 2019 discussing a method for bridging the gap. A study was conducted in this paper for a real well based on the new method, and successful results were discussed. The current paper extends that study to another real well casing wear prediction with this novel approach. Some methods discussed are already included in the mentioned paper.
A Sensitivity Study of Vessel Hydrodynamic Model Parameters
Authors: Han Xu , Sævik Svein , Leira Bernt Johan
Published: December 2020

In the recent decade, maritime and energy industries have realized the potential of using operational data in combination with a virtual replication of the real physical asset, termed the digital twin. The digital twin then serves as a platform for data management, asset monitoring, and inspection and maintenance management, featuring an improved basis for cost effective operations and future decision making in terms of e.g. life extension. The present paper deals with application of the digital twin concept in marine operations where it is essential to handle the inherent uncertainties of vessel performance by applying a model that can adapt to the real operating conditions. In this paper a case study is presented for identifying the most sensitive parameters in the vessel hydrodynamic model w.r.t. the vessel motion RAOs. The study also shows that the parametric sensitivity depends on the interesting vessel response parameter, wave direction and loading condition. A digital twin adaptive to various operational conditions may require parametric tuning of the numerical model. It is important to identify the correct parameter(s) for modification. A simplified and idealized case study is also carried out to test the requirements to a successful parameter identification for model tuning.
Structural Integrity Management (SIM) via a Digitalized Structural Integrity Compliance System
Authors: Ng Sok Mooi , Isnadi Biramarta , Lee Luong Ann , Omar Syahnaz , Saminal Siti Nurshamshinazzatulbalqish , Wan Shafie Wan Hariz Fadli , Khan Riaz
Published: December 2020

The objective of this paper is to present the digitalization of Structural Integrity Management for PETRONAS, via a web-based Structural Integrity Compliance System (SICS). Developed initially as a software application to manage the vast database for integrity management of offshore fleet, the system covers the substructure, topside structures, onshore civil and structural assets and ship-shaped floating structure modules.

The system is developed in line with API RP 2SIM, taking the SIM process of Data-Evaluation-Strategy-Program. The Data captured in the system inclusive of the design, construction, characteristics, inspection, foundation and metocean data required for evaluation of the likelihood of failure. While the life safety, environmental pollution & business loss data is assigned to evaluate the consequence of failure. After successful implementation of substructure module, the system has been expanded to capture data and evaluate risk of other types of assets, namely the topside structures, onshore civil and structural assets and at last, a module developed for a ship shaped floating structure.

A centralized database to manage the integrity a large aging fleet provides good visibility to the management in order to prioritize resources (budget, manpower and logistic) for inspection and maintenance, at the same time reducing the risk of operation disruption due to non-compliance. The system has created much cost saving through Risk-Based Underwater Inspection (RBUI) and risk-based anomalies repair.

SICS consists of the risk ranking sub-module and also other decision making tools including Strengthening-Modification and Repair (SMR) Toolkit, and Technical Limits Weight Control (TLWC) Tool. These toolkits are codified to provide quick decision making to management to evaluate the feasibility of SMR scheme or modification involving additional topside weights.
Thruster Performance of an Azimuth Stern Drive Tug
Authors: Yiew Lucas J. , Jin Yuting , Zheng Yingying , Magee Allan R.
Published: December 2020

The development of an accurate digital performance twin of a tug requires a complete understanding of its propulsive capacity and hull-thruster interactions. In this study, the propulsion characteristics of an Azimuth Stern Drive (ASD) tug is investigated using model-scale Reynolds-averaged Navier-Stokes (RANS) simulations. The propulsion plant consists of two counter-rotating thruster units, with each having a Ka4-70 series propeller and 19A duct profile. Comparisons in propulsive performances using the steady-state moving reference frame (MRF) approach and the transient rigid body motion (RBM) models are shown, and validated against data from openwater experiments. The MRF method gives sufficiently accurate predictions of thrust and torque in forward flow and moderate angles-of-attack, while the RBM method performs better at larger inflow angles. The effects of thruster-hull and thruster-thruster interactions on wake characteristics and propulsion performance are also investigated over a range of advance and inflow/azimuth angles. Convergence and mesh independence studies are conducted to determine the optimal spatial and temporal simulation parameters. Results from this study identify flow regimes where hull and thruster interactions are significant.
Numerical Simulation of Motion Response of Multi-Floating Body System Considering Different Gap Between TLP and TAD
Authors: Zhang Zhigang , Bao Shanjun , Ding Zhaogang , Wei Zhiyuan , Sui Haibo , Kang Zhuang
Published: December 2020

Tender-assisted drilling system meets the strict requirements of deck space of the drilling platform, and provides a relatively safe and comfortable working environment for the staff, which has been widely used in drilling operations in recent years. The significant difference between multi-floating body and single offshore platform is that there may be risk of collision between the floating bodies under extreme metocean conditions or in emergency of mooring system failure. In order to prevent the collision during drilling operations, the initial gap between floating bodies should be designed carefully and provide a reasonable scheme to ensure the safety of the drilling system and the feasibility of drilling operations. Therefore, based on the three dimensional potential flow theory, frequency domain and time domain numerical simulation of the motion response of TLP and TAD is carried out according to the marine environment of West Africa with extreme metocean conditions, the effect of different initial gap on the motion performance of floating bodies is explored and the mechanical characteristics of the mooring system are analyzed. Thus, the reasonable initial gap between TLP and TAD is determined by comparing the simulation results. In general, the numerical simulation results of tender-assisted drilling system may provide reference for engineering practice to some extent.
A New Higher-Order Euler-Bernoulli Beam Element of Absolute Nodal Coordinate Formulation
Authors: Zhang Peng , Ma Jianmin , Duan Menglan
Published: December 2020

In this study, a new higher-order Euler-Bernoulli beam element of Absolute Nodal Coordinate Formulation (ANCF) is developed for geometrically nonlinear analysis of planar structures. The strain energy of the beam element is derived by applying the definition of the Green–Lagrange strain tensor in continuum mechanics. The first contribution of this research is to realize the accurate calculation of curvature on the beam element node by additionally considering the second derivative of the position vector obtained by quintic Hermite interpolation function. Furthermore, in traditional theory, the independent variable of finite formulation is arc-length coordinate s, while in this work, a correction is come up with and proven that it is actually an equivalent parameter. Some benchmark problems of straight beams are solved by the proposed element and accurate results are obtained by just fewer elements when compared with the other works including the traditional ANCF element and B23 element of ABAQUS. What leads to this accuracy result is that the precise calculation of nodal curvature is obtained from higher order interpolation scheme. The correctness and accuracy of the proposed element are validated in this work and it can be further developed for tackling large deformation and large rotation problems of spatial curved beams.
Development of a Fluid-Structure Interaction Model of an Oscillating Wave Surge Converter Using OpenFOAM
Authors: Winter Andrew O. , Motley Michael R.
Published: December 2020

Wave energy converters (WEC), particularly those of the oscillating paddle type, are one of the least utilized categories of marine energy harvesting devices. As such, the goal of this research is to develop improved WEC technology through numerical modeling that will inform future experimental testing, which is being carried out by the Pacific Marine Energy Center. A novel scheme using mesh distortion and re-meshing to simulate the rotation of a wave-driven oscillating wave surge converter is presented herein using only the default functionality of the OpenFOAM computational fluid dynamics software package as a framework to implement the scheme as a convenient tool for engineers to study OWSC performance. This scheme relies on the use of finite volume mesh quality parameters, namely mesh cell non-orthogonality and skewness, to control when re-meshing operations are undertaken, minimizing the number of re-meshing operations needed to accommodate OWSC motions while maintaining solution stability and accuracy.
Numerical and Experimental Comparison of the Wave Response of a Very Light Floating Offshore Wind Turbine With Guy Wires
Authors: Shiohara Hiroki , Gonçalves Rodolfo T. , Houtani Hidetaka , Suzuki Hideyuki , Schnepf Anja , Hirabayashi Shinichiro , Carmo Lucas H. S. , Nihei Yasunori
Published: December 2020

A floating offshore wind turbine (FOWT) concept with a guy wire-supported tower was investigated to obtain results of motion in waves considering its elastic model characteristics. The FOWT concept aims to reduce construction costs by using a light-weight structure tensioned with guy wires and a downwind turbine concept type.

A wave tank experiment of an elastically similar segmented backbone model in the 1/60th scale was conducted to clarify the dynamic elastic response features of the structure. The results were compared with numerical simulations obtained with software NK-UTWind (in house software developed by the University of Tokyo) and WAMIT code.

It was clarified that the bending moment for tower and pontoons had two peak values when the response for each wave period was examined. The peak in the short-wave period was due to sagging when the wavelength matched the floater length. The other peak was due to the largest tower top acceleration, which caused a large bending moment at the tower base and pontoon to support the inertia force.
Attempt of a Real-Time Drilling State Identification With Machine Learning
Authors: Inoue Tomoya , Ishiwata Junya , Wada Ryota , Tahara Junichiro
Published: December 2020

Japan Agency for Marine-Earth Science and Technology (JAMSTEC) operates the scientific drillship Chikyu for scientific research. However, Chikyu has experienced problems with respect to drill pipe failure. This may be due to the limitation of indication of anomalies such as drill pipe failure in conventional drilling data monitoring.

As one of primary aims of scientific drilling is to recover core samples from sediment layers under the seabed, improving core recovery rate is very important because it can enhance the operation efficiency. Obtaining the information of lithology of drilling layer is also helpful for both scientific and operational aspects. However, there is no direct information regarding the core recovery rate and lithology. The recovery rate and lithology can be determined after retrieving a coring tool. Therefore, this study applies a machine learning technique to identify the drilling states, which includes anomaly detection of the drilling torque assuming the drill pipe failure, the prediction of core recovery rate as well as lithology.

This study aims to achieve real-time drilling state identification. Accordingly, a drilling data acquisition and distribution system was developed. The drilling data distributed from the system is read by data analysis systems in several languages for real-time analysis.

The drilling state identification models created in Python include the anomaly detection model and the prediction models of the core recovery and lithology. The models were installed in the real-time drilling data analyzing system, and real-time drilling state identification was attempted during the operation to confirm the health of the real-time drilling data analyzing system and to demonstrate identification with machine learning.
Nonlinear Mechanical FE Analysis of Thin-Plate Complex Structures Using the Shell-Solid Mixed Method
Authors: Ikushima Kazuki
Published: December 2020

In the construction of ships and ocean structures, thin steel plates are welded in order to join parts. Due to the welding, deformations and residual stresses may occur. These deformations and residual stresses can cause problems in the assembly process. Therefore, the prediction of welding deformations and residual stresses is necessary in advance of production. Welding deformations and residual stresses can be predicted using thermal elastic plastic (TEP) finite element analysis (FEA). However, solid elements are used in the conventional analysis method of TEP-FEA. The modeling of the thin-plate structures using solid elements is very complicated and difficult. In addition, the number of elements increases when using solid elements compared to shell elements. This leads to an increase in the required computing resources. Therefore, an efficient modeling method is necessary for thin-plate structures.

In the present research, in order to realize an efficient welding mechanics analysis method for thin-plate structures, the authors proposed an efficient FEA method for the welding mechanics problem for thin-plate complex structures using the proposed shell-solid mixed analysis method. For the shell-solid mixed analysis method, the multipoint constraint (MPC) technique was used in the finite element analysis to connect shell elements and solid elements. In order to compare the analysis accuracy with the conventional analysis, which uses solid elements, the proposed shell-solid mixed analysis method was applied to the fundamental welding mechanics problem of thin-plate structures. The results revealed that the proposed method has approximately the same analysis accuracy as the conventional method. These results indicated that the proposed method can effectively analyze the welding deformations and residual stresses in thin-plate complex structures.
Development of a Pile Mooring System for Large Scale FSRUs
Authors: Park Taeyoon , Jeon Junhwan , Kim Jung , Jeon Sangbae , Kim Bongjae , Lee Dongyeon
Published: December 2020

In this paper, a pile mooring system is introduced as an alternative mooring solution for FSRU. Also, the methodologies of mooring analysis and structural analysis to verify a design of pile mooring system are introduced. The mooring performance of pile mooring system can be assessed by coupled mooring analysis considering stiffness of pile, resistance of soil and hull interface mechanism. The structural integrity of pile, foundation and hull interface can be assessed by non-linear contact finite element analysis. Using these methods, the basic design of pile mooring system for 160,000-CBM large scale FSRU is developed considering practical environmental conditions.
Low-Height Lifting System for Offshore Wind Turbine Installation: Modelling and Hydrodynamic Response Analysis Using the Commercial Simulation Tool SIMA
Authors: Vågnes David , Monteiro Thiago Gabriel , Halse Karl Henning , Hildre Hans Petter
Published: December 2020

With the increasing demand for renewable energy sources in the past years, the interest in expanding the use of wind energy has grown. The next frontier in this expansion process is the use of floating wind turbines offshore. One of the main factors dictating the economic feasibility of such wind turbines is the complexity of their installation process. The dimensions of modern offshore wind turbines, the distance from the installation sites to the coast and demanding environmental factors all contribute to the difficult of developing an efficient installation concept for this kind of structures. In this work, we present a new concept for a catamaran vessel capable of handling the deployment of offshore wind turbines on floating spar platforms using a low-height lifting system that connects to the lower end of the wind turbine. The low-height lifting system is controlled by an active heave compensation system and constant tension tugger wires attached to the turbine mid-section are used to ensure the balance of the tower during the installation process. We conducted a series of hydrodynamic analysis using the software suit SIMA to study the dynamic response of the proposed system under different weather conditions and different operational layouts. This preliminary concept was proven feasible from a hydrodynamic point of view and can now be pushed forward for further studies regarding other aspects of the operation, such as impact and structural loads and mechanical design of components.
Floating Hose Behavior During Different Scenarios of an Offloading Operation
Authors: Paruolo Nathalia , Mello Thalita , Brandão Leonardo
Published: December 2020

Considering the deep water oil exploitation in remote fields far away from the coast, the use of shuttle tankers connected to FPSOs (Floating, Production, Storage and Offloading) for offloading and oil transporting shows to be a preferred alternative compared to constructing oil pipelines considering technic and economic matters, even if offshore offloading operation is still considered one of the riskiest operations in the offshore environment.

From the concerns about hazards of an offloading operation, one is related to hose failure during oil transferring and the consequences on the environment. Different aspects of FPSO and shuttle tanker influence floating hose behavior during offloading operation as environmental condition, ships loading conditions, ships distance, heading, FPSO offloading station position and number of hose sections. The different aspects may be combined in several scenarios, in which integrity of floating hoses in terms of tension and bending strength shall be guaranteed.

In the present paper, global numerical analyses of offloading operation considering a spread moored FPSO and a typical dynamic positioning shuttle tanker (DPST) moored in tandem are carried out. The analyses are based on a coupled numerical model of FPSO, DPST and floating hose [2]. Covering a wide combination of the scenarios considering the aspects listed above, full time domain simulation is used to estimate hose sections force and moments during the operation. Numerical model results are validated for a couple of scenarios against field data.

With focus on floating hose integrity, the main objective is to define minimum requirements for a safe offloading operation, as environmental criteria, maximum and minimum distances between FPSO and DPST, limit operating sectors and minimum number of hose sections. The results can be used as reference to offloading procedures and manuals of owners and operators in combination with recommendations of international organizations.
Design, Analysis, and Development of a Wave-Current Laboratory
Authors: Nguyen Nhu , Davis Jacob , Alshuwaykh Ahmed , Sharman Krish Thiagarajan
Published: December 2020

In real ocean environments, offshore structures are exposed to a combination of wave and current loading conditions. This scenario presents the need to study fluid-structure interactions in the presence of both conditions, achievable through experimentation in a recirculating flume coupled with a wavemaker. The Ocean Resources and Renewable Energy (ORRE) group set out to design a recirculating wave-current flume at the University of Massachusetts Amherst to enable the study of technologies such as scale floating platforms and marine energy converters. In this paper, we present the methods used to arrive at an optimal flume design under strict spatial constraints posed by the available lab space. Limitations on the length, width, and height of flume are overcome via innovative flow designs and compact structures. The final design is approximately 11.5 m (37.7 ft) in length and 1.2 m (3.9 ft) wide with a nominal water depth of 1 m (3.3 ft). The 2 m long test section begins 6 m beyond the inlet of the flume to maximize flow uniformity. A 24” thruster driven by 75 hp electric motor maintains a current velocity of 0.5 m/s throughout the section while a wedge-shape plunger is implemented at the inlet to generate 0.6–2.8 s period waves with a maximum height of 0.2 m. During the design process, 2D computational fluid dynamics (CFD) simulations are employed to maximize flow uniformity over a range of inlet angles and guide vane configurations. In the optimal scenario, a flow nonuniformity of 8.7 % was obtained across a 0.7 m water column measured from the free surface. Results from the 3D simulation around the tight corner section showed significant increase in flow nonuniformity. The implementation of the screens along the flow path might be necessary in the future.
Nonlinear and Machine-Learning-Based Station-Keeping Control of an Unmanned Surface Vehicle
Authors: Sinisterra Armando J. , Barker Alexandrea , Verma Siddhartha , Dhanak Manhar R.
Published: December 2020

This study is part of ongoing work on situational awareness and autonomy of a 16’ WAM-V USV. The objective of this work is to determine the potential and merits of application of two different station-keeping controllers for a fixed-pose motion control of the USV. The assessment includes performance and power consumption metrics tested under harsh environmental disturbances to evaluate the robustness of the control methods. The first is a nonlinear trajectory-tracking control method based on the sliding-mode control technique, while the second method uses a machine-learning approach based on Deep Reinforcement Learning. Results from both the approaches are compared for various case studies.
Improved Empirical Formulation for Seabed Trench Profile at Touchdown Zone and its Effects on Fatigue of Steel Catenary Risers
Authors: Zhuang S. , Shi C. , Lou M. , Liu Y. , Bao X. , Li H.
Published: December 2020

Steel catenary risers (SCRs) offer a cost-effective solution to deep water deployments. Hanging from a floater, an SCR is commonly subjected to large tension at hangoff location and large bending moments at touchdown zone (TDZ) which lead to fatigue damage. Field observations showed that the depth of a seabed trench might reach to a depth of four to five times of the diameter of a riser, however, a flat seabed was often assumed for the modeling of SCRs which surely affects simulations of fatigue behavior of the SCR at the touchdown zone. Studies on the effects of seabed trench on TDZ fatigue conducted by different researchers led to contradictory conclusions, i.e., some studies suggested that considering seabed trench reduced fatigue damage at TDZ of SCRs, while, others drew opposing conclusions. The contradiction may be explained by factors including inappropriate trench profiles and different sea states assumed in the analysis model. An iterative procedure initially developed by Wang and Low and further improved in the present work was used to estimate the position and the length of a seabed trench beneath an SCR and, then, an improved empirical formulation was generated to approximate the profile of the seabed trench. Additionally, dynamic simulations were conducted to study the effects of seabed trench on fatigue behaviors of SCRs encountering variant directional waves.
A Principal Strain Data-Driven Method for Damage Identification of an Offshore Pile Structure
Authors: Bao Xingxian , Yang Guanlan , Li Hongwei , Shi Chen
Published: December 2020

Vibration-based damage identification technique, categorized as parametric and nonparametric methods, plays an important role in guaranteeing the success of offshore operations and the integrity management of marine structures. In the parametric approaches, modal parameters and their derivative are usually applied to obtain the damage-sensitive features. Nevertheless, there are still some challenging problems for the parametric method in application. Nonparametric (data-driven) approaches are aim to extract damage sensitive features directly from the measured data. For the data-driven approaches, it is vital to find which indicator derived from the response signal is sensitive to structural damage. Considering the derivatives of strain are more sensitive to small structural damage than that of displacement, the strain-related damage detection methods have currently gotten more attention. In this study, a principal strain data-driven method is proposed, which is derived from strain energy dispersion theory and directly employs the time history data of three principal strains of an intact and damaged structure to construct the damage identification indicator. The efficiency of the method is validated by a numerical offshore pile structure with several damage cases considering noise scenarios. Due to the difficulty of determine the correct three principal strains in the real experimental measurements, the approximation damage indicators are constructed based on one, two or three kinds of strain response among the axial, radial and hoop strains response, instead of the three principal strains response. The results show that satisfying damage identification results can still be gotten for both the single- and multiple-damage scenarios using only axial or hoop strain response even under noise conditions. The proposed principal strain data-driven damage identification method can be used as a viable and effective technique for damage localization and severity estimation of marine structures.
Study on Acid Fracturing Technology for Carbonate Reservoirs in Ordos Basin
Authors: Li Mu , Zhang Lufeng , Li Minghui
Published: December 2020

Acid fracturing, a key stimulation technology, has been widely applied in carbonate formations, which have the complex reservoir space and natural fracture. However, the conventional acid fracturing technology has limitations in target reservoir due to serious acid leakage and rapid acid-rock reaction speed. Therefore, a deep-penetration acid fracturing technology was proposed to enhance the stimulation effect. The reservoir basic physical properties, such as permeability and porosity, microstructure characteristic of pore throats, mineral composition, and rock mechanics characteristics, were systemically investigated by a series of laboratory experiments. The acid system was also optimized by evaluating rheological properties and residual acid damage. Furthermore, the most important two parameters of acid fracturing, conductivity and effective acid-etched fracture length, were obtained by numerical simulation with commercial software Fracpro PT. Based on the laboratory studies, the acid fracturing technology was applied in the field, and the production of well increased by 1.7 times after stimulation.
Nonlinear Optimal Control of a Deep Borehole Trajectory Design
Authors: Gulyayev Valery , Glazunov Sergii , Andrusenko Olena
Published: December 2020

In modern oil/gas producing industry, vertical, 2D and 3D directed, and multilateral (branched) boreholes are drilled. Their trajectories are designed depending on the petroleum deposit depth and structure, properties of mining rocks, their hardness, heterogeneity, fracturing anisotropy, permeability, and so on. Therefore, the borehole cost and its productivity are determined by the length, smoothness, and configuration of its trajectory. To enhance efficiency of a borehole and to reduce cost of its drivage, to enlarge rate and volume of the reservoir depletion, it is proposed to use methods of optimal control for the best tracking of its trajectory. Through application of the differential geometry correlations, the mathematic model of the borehole outline in the form of nonlinear ordinary differential equations system is elaborated. Different objective functions, representing total integral curvature of the borehole axis line, its length, and cost of its drivage, are selected; additional constraints, separating allowed and forbidden zones of passing, are chosen. The functions of the trajectory curvature and torsion are used as controlling variables.

The continuous correlations of the model are discretized and, further, the techniques of nonlinear programming and optimal control are employed.

On the basis of the method of objective function gradient (antigradient) projection on the linearized constraint planes, the step-by-step algorithm of approaching to the optimal trajectory is elaborated. To correct the spoilt constraints, at every step of calculations, the Newton method is used. The elaborated approach is applied to optimization of deep curvilinear borehole outlines. The results of numerical analysis are discussed. It is shown that smoothing the hole trajectory permits also to diminish the contact and frictional interaction between the drill string and bore-hole wall and, by this, to decrease the resistance forces acting on the string during tripping in/out operations performing and to diminish energy expenditures for these operations fulfillment; to decrease the rate of the drill string tube wear; and to reduce the drill string sticking occurrence probability.