bluuebunny/crossref_metadata_2025 huggingface dataset:
Data Mining for Estimating Fatigue Strength Based on Composition and Process Parameters
Authors: Keprate Arvind , Ratnayake R. M. Chandima
Published: November 2019

Accurately estimating the fatigue strength of steels is vital, due to the extremely high cost (and time) of fatigue testing and often fatal consequences of fatigue failures. The main objective of this manuscript is to perform data mining on the fatigue dataset for steel available from the National Institute of Material Science (NIMS) MatNavi. The cross-industry process for data mining (CRISP-DM) approach was followed in the paper, in order to gain meaningful insights from the dataset and to estimate the fatigue strength of carbon and low alloy steels, using composition and processing parameters. Of the six steps of the CRISP-DM approach, special emphasis has been placed on steps 2 to 5 (i.e. data understanding, data preparation, modeling and evaluation). In step 4 (i.e. modeling), a range of machine learning (parametric and non-parametric) is explored to predict the fatigue strength, based on the composition and process parameters. Various algorithms were trained and tested on the dataset and finally evaluated, using metrics such as root mean square error (RMSE), Mean Absolute Error (MAE), Coefficient of Determination (R2) and Explained Variance Score (EVS).
Impact of Simulation Duration for Offshore Floating Wind Turbine Analysis Using a Coupled FAST-OrcaFlex Model
Authors: Pillai Ajit C. , Thies Philipp R. , Johanning Lars
Published: November 2019

This paper uses a coupled FAST-OrcaFlex model in order to explore the impact of simulation duration on model convergence. The work analyses both operational and extreme cases, assessing the estimated fatigue and extreme loads experienced by a floating offshore wind turbine and its mooring system. Considering an OC4 semi-submersible deployed with the NREL 5 MW turbine, the case study performs a parametric sweep over a range of wind speeds, sea states, and simulation durations. Through this sweep, the paper establishes the impact of the simulation duration for this particular floating offshore wind turbine and characterizes the convergence properties of the loads and excursions as a function of the simulation duration. The results inform the selection of simulation durations to be used in coupled aero-hydro models and optimization frameworks for floating offshore wind applications and can be used to aid the development of guidance and standards for coupled floating offshore wind turbine models.
The Branched Riser Systems: Concept Development
Authors: Ogbeifun Achoyamen Michael , Oterkus Selda , Race Julia , Naik Harit , Decnop Eduardo , Dakshina Moorthy
Published: November 2019

A new riser system concept is being investigated for deep-water application. This is the Branched Riser Systems with its three types: the Branched Steel Catenary Riser (BSCR) System, the Branched Steel Lazy Wave Riser (BSLWR) System and the Branched Lazy Wave Hybrid Riser (BLWHR) Systems. The branched riser system consists of a large bore pipe, which is terminated at an optimized water depth considering the minimum wall thickness requirement from burst and collapse criteria. The larger bore riser pipe is extended from this cut off water depth to the seabed by two smaller riser pipes via a connecting component (connector). The concept draws benefits from the performance of smaller riser bore pipe in strength and fatigue performance besides the opportunities provided by larger bore pipe for maximum flow throughput and vessel top decongestion.

Preliminary results from ongoing investigation of type 1 (BSCR) of the branched riser systems, using a 12-inch pipe in combination with two 8-inch riser pipes in a water depth of 3,000m for a cut off water depth of 1500m, indicates a reduction in overall riser weight, stress utilization and fatigue damage at the riser hang off (HO) and touch down zone (TDZ). Results obtained indicate that possible benefits can be achieved through a combination of advantages of smaller and larger bore riser pipe and justifies further research interest on the novel riser concept.
A New Three-Layer Model for Gravel Packing Applications in Horizontal Wells
Authors: Sarraf Shirazi Alireza , Frigaard Ian
Published: November 2019

In [1] we have developed a modified three-layer model for solid-liquid flow in horizontal pipes, which overcomes the limitations of previous mechanistic models. The steady-state model predicts the pressure loss, critical velocity, concentration profile in the heterogeneous layer, mean heterogeneous layer and moving bed layer velocities, and bed layer heights for each set of parameters. The steady-state model predictions show very good agreement with experimentally measured results in the literature.

In this paper we extend the steady-state three-layer model to annular geometries and apply it to the design of open-hole gravel packing operations, in the typical parameter ranges of gravel packing operations for alpha wave placements. Alpha wave design is a key factor for successful gravel packing, and the models typically used are either based on small-scale experiments or are not specifically developed for gravel packing, e.g. cuttings transport models. In gravel packing the hydraulic configuration is slightly different. We explain how bed height is selected via coupling between the inner and outer annuli and from the outer annulus hydraulics. We investigate the effects of important parameters such as the slurry flow rate, mean solids concentration, wash pipe diameter, etc. on gravel packing operations.
Dynamic Response of a Large-Diameter Monopile Considering 35-Hour Storm Conditions
Authors: Bachynski Erin E. , Page Ana , Katsikogiannis George
Published: November 2019

As a part of the assessment of foundation resistance for monopiles, several offshore wind standards prescribe symmetric 35-hour (or 42-hour) storm sequences in terms of wind speed and significant wave height. The temporal evolution of the peak period is not specified explicitly in the standards, despite the fact that large monopile wind turbines are sensitive to the wave period. In the present work, the storm sequences according to the standards are first compared to hindcast data for intermediate water depth locations in the North Sea. An alternative storm sequence is proposed based on the hindcast data, and possible values of the peak period evolution are proposed for the standard models. The responses of a 10 MW monopile wind turbine are then computed for both the standard and proposed sequences using a time domain aero-hydro-servo-elastic code coupled to a macro element model for the soil-structure interaction. The resulting mudline load cycles are then compared for the different storm sequences.
URANS Predictions of Low-Frequency Damping of a LNGC
Authors: Jaouën Frédérick , Koop Arjen , Vatinel Lucas
Published: November 2019

The horizontal motions of a moored offshore structure in waves are dominated by the resonance phenomena that occur at the natural frequencies of the system. Therefore, the maximum excursions of the structure depend on both the wave loads and the damping in the system. At present, potential flow calculations are employed for predicting the wave loads on offshore structures. However, such methods cannot predict hydrodynamic damping which is dominated by viscous effects. Therefore, the current practice in the industry is to obtain the low-frequency damping based on model testing. Nowadays, CFD simulations also have the potential to predict the low-frequency viscous damping of offshore structures in calm water. To obtain confidence in the accuracy of CFD simulations, a proper validation of the results of such CFD calculations is essential.

In this paper, the flow around a forced surging or swaying LNGC is calculated using the CFD code ReFRESCO. The objective is to assess the accuracy and applicability of CFD for predicting the low-frequency viscous damping. After a description of the code and the used numerical methods, the results are presented and compared with results from model tests. Both inertia and damping coefficients are analyzed from the calculated hydrodynamics loads. Extensive numerical studies have been carried out to determine the influence of grid resolution, time step and iterative convergence on the flow solution and on the calculated damping. The numerical uncertainty of the results are assessed for one combination of amplitude and period for the surge motion. The CFD results are compared to experimental results indicating that the calculated damping coefficients agree within 5% for both surge and sway motion.
Corrosion-Fatigue Crack Growth Performance of Titanium Grade 29 Welds in Tapered Stress Joints
Authors: Rombado Gabriel , Baker David A. , Haldorsen Lars M. , Craidy Pedro , Feiger Jim H. , Hudak Stephen J.
Published: November 2019

Design of steel catenary risers (SCRs) requires the use of connection hardware to decouple the large bending moments induced by the host floater at the hang-off location. Reliability of this connection hardware is imperative, especially in those applications involving high pressure and temperature fluids. One option for connection hardware is the metallic tapered stress joint. Because of its inherent density, strength and stiffness, steel is not well suited for these applications as it would result it excessive length and weight for deepwater applications. Titanium grade 29 (Ti 29) has been identified as an attractive material candidate for demanding stress joint applications due to its unique mechanical properties including greater flexibility, excellent fatigue performance, and high resistance to sour fluids. Industry has successfully used this technology in over 60 SCR applications.

Titanium stress joints (TSJs) for deep-water applications are typically not fabricated as a single piece due to titanium ingot/billet volume limitations, thus making an intermediate girth weld necessary to satisfy length requirements. Fracture and fatigue performance of these welds in the presence of cathodic potential in seawater and galvanic potentials in sour production fluids that may produce hydrogen embrittlement effects must be assessed to ensure long term weld integrity. This paper describes a joint industry project (JIP) performed to qualify titanium stress joints welds for ultra-deep water applications under harsh service and environmental conditions. Fatigue crack growth rate (FCGR) results for Ti 29 1G/PA gas tungsten arc welding (GTAW) specimens in air, seawater under cathodic potential and sour brine environments under galvanic potentials are presented and compared to vendor recommended design curves.
Experimental Study on Bubble Size Measurement for Development of Seafloor Massive Sulfides
Authors: Imai Seira , Nakajima Yasuharu , Murai Motohiko
Published: November 2019

Seafloor Massive Sulfides have been expected to be future mineral resources. To promote the development of Seafloor Massive Sulfides, Seafloor Mineral Processing, a method of extracting valuable minerals from the ores on deep seafloor using flotation to reduce the lifting cost of ores from the seafloor to the sea surface, was proposed. To apply flotation for the seafloor mineral processing, a measurement method of bubble size applicable to deep-sea conditions has been desired because it is necessary to generate fine air bubbles suitable to flotation under pressure conditions on deep seafloor. Then, the authors have studied on bubble size measurement by image analysis, which is expected to be applicable to deep-sea conditions. At the first phase of this study, photographic conditions suitable to image analysis of air bubbles were distinguished. Air bubbles were generated by using a porous nozzle at some air flow rates in a bubble column with a rectangular cross-section. Video images of air bubbles were taken by using both high-speed camera and video camera with a low frame rate. Bubble size was measured by binarizing the video images of bubbles. Under optimal photographic conditions, bubble size was obtained from not only the high-speed camera but also the video camera; and both size data agreed relatively well, which implies that bubble size measurement by using image analysis would be applicable to deep-sea conditions. At the second phase, experiments were carried out under high-pressure conditions up to 2.4 MPa. Single bubble generation by using a capillary nozzle in a small pressure chamber with a sight glass was observed by using a digital microscope. Bubble size measurement by image analysis was carried out by the procedure established at the first phase. While the process of bubble generation at the high pressures was similar to that at the atmospheric pressure, the bubble size was decreased as the pressure rose. The result implies there is a strong correlation between the pressure and the bubble size.
Numerical Simulation and Analysis of Phase Focused Breaking and Non-Breaking Wave Impact on Fixed Offshore Platform Deck
Authors: Moideen Rameeza , Behera Manasa Ranjan , Kamath Arun , Bihs Hans
Published: November 2019

Extreme wave impact due to tsunamis and storm surge create large wave heights causing destruction to coastal and offshore structures. These extreme waves are represented by focused waves in the present study and the impact on offshore deck is studied. Numerical wave tank used is modelled using open-source software REE3D, where the level set method is used to capture the air-water interface. Vertical impact force on offshore deck is computed and compared with the experimental results to validate the numerical model. Focused wave is generated by phase focusing a group of waves at a particular position and time. The nonlinearity of focused wave and its effect on the vertical impact force is quantified for different airgap and increasing wave heights. The steepness of this focused wave is increased to initiate phase focused breaking in the numerical wave tank, which is validated with experimental results of Ghadirian et al., 2016. The main purpose of this paper is to examine breaking focused wave group loads on the offshore deck and to study the impact on deck at different breaking locations. The positioning of the deck with respect to breaker location have shown that the maximum horizontal impact force due to breaking wave occurs when the plunging crest hits the deck side.
Model Experiments of Floating Side-by-Side Barges
Authors: Chua Kie Hian , de Mello Pedro Cardozo , Nishimoto Kazuo , Choo Yoo Sang
Published: November 2019

The interaction between two floating vessels has been a subject of much study in recent years due to its relevance to floating liquefied natural gas (FLNG) developments. The safety and operability of these facilities are directly influenced by the wave elevation in the gap between the two vessels as well as the relative motions between the vessels. In the industry, it is common practice to use potential flow models to calculate free-surface responses under various wave conditions. Given that these numerical models are inviscid, calibration of additional damping terms are usually carried out using model tests to in order to account for the viscous dissipation on the gap hydrodynamics. However, it is known that the dissipative effects of viscosity may be nonlinear and thus, model test data obtained using one set of wave conditions may not be suitable for use in another scenario. In this paper, model experiments of two identical side-by-side barges of 280m (length) × 46m (breadth) × 16.5m (draught) under various wave excitation are described. The experiments considered a range of parameters such as gap width, wave heights, periods and wave directions. The results obtained for each set of these parameters are discussed and compared between the two types of incident waves (regular and irregular).
Experimental Study of the Effect of the Pontoon Presence on the Flow-Induced Motion of a Semi-Submersible Platform With Four Square Columns
Authors: Gonçalves Rodolfo T. , Suzuki Hideyuki , Cenci Fredi , Fujarra André L. C. , Hirabayashi Shinichiro
Published: November 2019

The Flow-Induced Motions (FIM) is an essential topic on multi-column platforms due to the effect on the mooring line fatigue life. Vortex-Induced Motions (VIM) or galloping behavior can be observed for an array of four columns with square sections. The presence of pontoons showed to be important for changing the flow around the array and promote different amplitude behavior of the motions in the transverse direction mainly. This article aims to understand the effect of the presence of two pontoons on the FIM of a semi-submersible platform (SS) with four columns and square sections. Model tests of a floating system supported elastically utilizing four springs were performed in a towing tank. Five different pontoon ratios were tested, namely P/L = 0, 0.25, 0.50, 0.75 and 1.00; where P is the pontoon height, and L is the length of the square column face. The draft condition was kept constant as H/L = 1.5; where H is the draft of the platform. The spacing ratio of the columns was S/L = 4; where S is the distance between column centers. Three incidence angles of the current were carried out, i.e., 0-deg incidence represents the condition in which the two pontoons are aligned to the current, 45-deg incidence represents the non-symmetric condition in which the pontoons are 45 degrees positioned to the current, and 90-deg incidence represents the condition in which the two pontoons are perpendicular to the current. The transverse amplitudes decreased with increasing the pontoon ratio for 0 and 45-deg incidences. On the other hand, the transverse amplitudes increased with increasing the pontoon ratio for 90-deg incidence. The pontoon presence needs to be well investigated to choose the best condition to avoid raising the FIM.
Study on Pipe Wear Evaluation Based on Large Scale Experiment for Deep Sea Mining
Authors: Takano Satoru , Sato Hirotaka , Terao Takashi , Masanobu Sotaro , Kawano Seiya
Published: November 2019

Japan Oil, Gas and Metals National Corporation (JOGMEC) has been conducting R&D of production system for Seafloor Massive Sulfides (SMS) mining. In order to reach commercialization stage in SMS development, the pipe wear due to slurry flow is one of many technical issues to be solved.

In the present paper, the authors conducted the large scale experiment using full scale pipes to obtain the various data on pipe wear, and investigated the pipe wear amount in commercial operation. In the experiment, SUS 304 and UHPE were used for the inner materials of test pieces. In addition, the test pieces were installed in the vertical, horizontal and inclined section. Then the authors estimated the thickness reduction of pipe due to slurry flow based on the experimental results.
Magnetoelastic Characteristics of Pipeline Steel Under Tensile Stress
Authors: Bao Sheng , Jin Pengfei , Mustapha Ashri , Zhao Zhengye
Published: November 2019

This research is to investigate the stress-induced magnetic fields surrounding initially undemagnetized pipeline steel specimen. A series of tensile tests were carried out. The variations of magnetic fields at different positions of the specimen were measured. Analysis of the results indicates that the specimen tends to be magnetized as a magnet with a N pole and S pole at two ends with the increase of load. The possible reasons underlying the experimental results are discussed on the basis of the theory of the interaction between dislocation and domain.
Submerged Production Unit: Design and Method for Launch and Tow to Field
Authors: Arumugam Elumalai Venkatesan , Daasvatn Sigbjørn , Karunakaran Daniel , Larsen Kjell , Leira Bernt Johan
Published: November 2019

The requirement for fossil fuels expedites for an advancement in the existing subsea technology. The developments evolved as the search for hydrocarbons moved from onshore to offshore, followed by a transition from shallow to deep and ultra-deep waters. Another huge milestone was achieved, when production systems made a transition from topsides to subsea units for efficiency. Currently, there is an enormous drive to minimize the operational costs involved in the processing of hydro-carbons. Researches are underway towards what would be yet another significant feat in the oil and gas industry, which is by moving the processing systems to subsea. One such impressive concept, which is being developed, is the Submerged Production Unit (SPU). This study is an initial attempt to investigate the challenges associated with the SPU focusing on the factors influencing design, launching and towing. A design concept that goes back and forth from performance and design spaces was used in modelling the SPU, solving the complexity that revolved around assembling the hollow Glass Reinforced Plastic (GRP) beams with subsea buoyancy materials. Submerged Tow Method (STM), an adaptation of Controlled Depth Tow Method (CDTM) was used instead of the conventional way of lifting the equipment using cranes of heavy lift vessels or construction vessels on site during deployment considering cost and safety. OrcaFlex software was used for towing analysis. End force in global X direction on towline, obtained from static analysis was used to identify the Bollard Pull (BP) required for towing the SPU. Dynamic analysis was performed for different environmental conditions to identify the maximum effective tension on the towline. BP requirement of 100t was estimated from the towing analysis. This study was carried out by author as a master’s thesis [1].
Sensitivity Analysis of a Bottom Fixed Offshore Wind Turbine Using the Environmental Contour Method
Authors: Barreto David , Moghtadaei Abdolmajid , Karimirad Madjid , Ortega Arturo
Published: November 2019

In the field of stochastic dynamics of marine structures, environmental conditions play a vital role. Considering wind and waves as random processes, determining the environmental parameters which correspond to an annual exceedance probability for a certain structural concept is of vital importance for the respective assessment of the loads and their effects. The accuracy in predicting the conditions, especially those corresponding to the sea, is of a great relevance when a probabilistic design is performed in order to ensure the structural integrity of an offshore wind turbine. In particular, models are not always completely perfect and accurate data is not always available. The Environmental Contour Method (ECM), which is based on the IFORM methodology, is one of the most popular methods in the offshore industry when determining the environmental conditions, for a given annual exceedance probability, is required. The ECM allows analysing proper sea states for operational and extreme conditions with lower computational efforts than the most accurate method (Full Long-Term Analysis). In the present study, effects of progressive variations (uncertainties) of the sea states parameters (i.e. significant wave height, spectral peak period) on the dynamic response of a Monopile Wind Turbine (NREL 5MW) are analysed. Two operative conditions are considered: rated wind and cut-out wind speed. In each case, the 50-year environmental contour (EC) is plotted for a site located in the North Sea. Some sea states are selected from the EC (base cases) and then derived cases with percentage variations are generated. All the cases are simulated in FAST (NREL) and the standard deviations of the time series are compared with its respective values of base cases. The results for the dynamic responses at mudline (e.g. overturning moments and shear forces) are presented as the most important parameters governing the design of the monopile. In this analysis, the wave height shows more influence on the response variation percentage than the peak period. This work shows the importance of accurately setting up the input parameters and their impact on the calculation of the dynamic responses.
Experimental Evaluation of Wave Impact Loads on Semi-Submersible Structure According to Trim Angle
Authors: Seo Min-Guk , Ha Yoon-Jin , Kim Nam-Woo , Nam Bo Woo , Lee Kang-Su
Published: November 2019

This study considers the wave impact loads on the semi-submersible structure. To evaluate wave impact loads on the semi-submersible structure, a series of experiments are conducted in a 2D wave flume. In the experimental test, the semi-submersible half model is used, and 11 uniaxial force sensors are installed in deck side, column side, and deck bottom. Wave probes are, also, attached in the test model to measure the relative wave elevation. To generate horizontal and bottom wave impact on the test model, focusing wave is applied. The test model is fixed without any motion during each test, while the trim angle of the test model is changed to examine the effect of trim angle on wave impact load. Through this, the characteristics of the wave impact force at each position were investigated.
Introduction of Virtual Structural Boundary for Collision Force Analysis of Tsunami Drifting Objects in Particle Method
Authors: Aida Yasuhiro , Masuda Koichi , Ikoma Tomoki , Eto Hiroaki
Published: November 2019

One of the reasonable methods to analyze the collision force of a tsunami drifting object against a structure is a particle method. However, when both the structure and the drifting body are composed of particles, there are various problems such as particles of the collision object slipping through particles of the structure. Therefore, the authors have constructed a particle method - analytical solution hybrid method which can analyze the collision force of a tsunami drifting object to an elastic member by constructing a structure as a boundary condition acting on a drifting object. However, since this boundary was introduced as a virtual boundary that acts only on drifting particles, the collision force of the tsunami drifting object to the structure can be analyzed, but the fluid force can’t be analyzed. Therefore, in this study, in addition to the boundary condition as the structure, we further reconstructed the collision force and the fluid force as a method that can analyze the collision force and the fluid force simultaneously by setting the mirror particle boundary condition for the fluid particle. By developing this method, it became possible to calculate the collision force in a situation where a stagnation point occurs like a flow field at the front of the wall type structure, and the drifting object is decelerating.
RANS Based Calm Water Resistance Prediction for Tumblehome Hull With Different Bow Appendages
Authors: Sun Shuzheng , Zhao Xin
Published: November 2019

The calm water resistance of the models a bare tumblehome hull and with 3 different bow appendages (triangle fins, rectangular foil, and semi-submerged body) are predicted based on RANS and k-ε turbulence model using STAR CCM+ software. VOF method is used for the 2-phases simulation. The resistance calculation results of the bare hull are validated against model test data. The verification and convergence studies are carried out on the grid size and time step. The validation studies show that the relative changes of the resistance calculation results compared with the model test data for Fr = 0.294 are 5–6%. The results of resistance prediction for the hulls with 3 bow appendages show that the bow fin configuration shows the best performance at Fr = 0.220, and the semi-submerged body configuration shows the best performance at Fr = 0.367. The details of flow field of different hulls will be analysed in this paper.
Spring-Dashpot Simulations of Polyester Ropes: Validation of the Syrope Model
Authors: Falkenberg Erik , Yang Limin , Åhjem Vidar
Published: November 2019

The way polyester ropes change their length in response to tension is often represented using a static-dynamic model with parameters for dual linear stiffness, based on testing according to defined procedures. This may not be sufficient for the mooring analysis since the stretch response to tension of fiber ropes is nonlinear. In the Syrope Joint Industry Project, a model to be used in mooring analyses was developed together with a test procedure to obtain the parameters for this model. Based on these parameters and experience from the testing, an equivalent non-linear spring-dashpot model was developed. This model can predict the response in the rope also for other test procedures than those used for establishing the parameters in the analysis model. This demonstrates the validity of this Syrope analysis model.
Integrity Assessment of Subsea Pipeline Dent / Buckle Using ILI Data
Authors: Kagita Gurumurthy , Achary Gudimella G. S. , Addala Mahesh B. , Srinivasan Balaji , Pottem Penchala S. K. , Gupta Deepak , Sripada Subramanyam V. R.
Published: November 2019

Mechanical damage in subsea pipelines in the form of local dents / buckles due to excessive bending deformation may severely threaten their structural integrity. A dent / buckle has two significant effects on the pipeline integrity. Notably, residual stresses are set up as result of the plastic deformation and stress concentrations are created due to change in pipe geometry caused by the denting / buckling process. To assess the criticality of a dent / buckle, which often can be associated with strain induced flaws in the highly deformed metal, integrity assessment is required. The objective of this paper is to evaluate the severity of dent / buckle in a 48” subsea pipeline and to make the rerate, repair or replacement decision. This paper presents a Level 3 integrity assessment of a subsea pipeline dent / buckle with metal loss, reported in in-line inspection (ILI), in accordance with Fitness-For-Service Standard API 579-1/ASME FFS-1. In this paper, the deformation process that caused the damage (i.e. dent / buckle) with metal loss is numerically simulated using ILI data in order to determine the magnitude of permanent plastic strain developed and to evaluate the protection against potential failure modes. For numerical simulation, elastic-plastic finite element analyses (FEA) are performed considering the material as well as geometric non-linearity using general purpose finite element software ABAQUS/CAE 2017. Based on the numerical simulation results, the integrity assessment of dented / buckled subsea pipeline segment with metal loss has been performed to assess the fitness-for-service at the operating loads.
Dynamic Positioning System: Systematic Weight Assignment for DP Sub-Systems Using Multi-Criteria Evaluation Technique Analytic Hierarchy Process and Validation Using DP-RI Tool With Deep Learning Algorithm
Authors: Fernandez Charles , Dev Arun Kr. , Norman Rose , Woo Wai Lok , Kumar Shashi Bhushan
Published: November 2019

The Dynamic Positioning (DP) System of a vessel involves complex interactions between a large number of sub-systems. Each sub-system plays a unique role in the continuous overall DP function for safe and reliable operation of the vessel. Rating the significance or assigning weightings to the DP sub-systems in different operating conditions is a complex task that requires input from many stakeholders. The weighting assignment is a critical step in determining the reliability of the DP system during complex marine and offshore operations. Thus, an accurate weighting assignment is crucial as it, in turn, influences the decision-making of the operator concerning the DP system functionality execution. Often DP operators prefer to rely on intuition in assigning the weightings. However, it introduces an inherent uncertainty and level of inconsistency in the decision making. The systematic assignment of weightings requires a clear definition of criteria and objectives and data collection with the DP system operating continuously in different environmental conditions. The sub-systems of the overall DP system are characterized by multi-attributes resulting in a high number of comparisons thereby making weighting distribution complicated. If the weighting distribution was performed by simplifying the attributes, making the decision by excluding part of them or compromising the cognitive efforts, then this could lead to inaccurate decision making.

Multi-Criteria Decision Making (MCDM) methods have evolved over several decades and have been used in various applications within the Maritime and Oil and Gas industries. DP, being a complex system, naturally lends itself to the implementation of MCDM techniques to assign weight distribution among its sub-systems. In this paper, the Analytic Hierarchy Process (AHP) methodology is used for weight assignment among the DP sub-systems. An AHP model is effective in obtaining the domain knowledge from numerous experts and representing knowledge-guided indexing. The approach involved examination of several criteria in terms of both quantitative and qualitative variables. A state-of-the-art advisory decision-making tool, Dynamic Positioning Reliability Index (DP-RI), is used to validate the results from AHP. The weighting assignments from AHP are close to the reality and verified using the tool through real-life scenarios.
Evaluation of an Equivalent Design Wave Method to Define Lifetime Combined Loading Scenarios for Trimarans
Authors: Seyffert H. C. , Kana A. A.
Published: November 2019

The evaluation of marine system reliability requires the definition of the most extreme loading that a vessel is expected to experience. To quantify the risk levels associated with a full ship lifetime, some decisions must be made about which vessel scenarios are worth analyzing, i.e. the worst-case vessel scenarios. The problem is further complicated when considering extreme combined loading on a novel hull form such as a trimaran. To evaluate structural compliance with given acceptance criteria many classification societies suggest defining simultaneous load combination cases by an Equivalent Design Wave (EDW) method for use in a finite element model. This paper examines whether the EDW approach defines realistic lifetime combined loading scenarios specifically for a novel hull form like a trimaran.
A Three-Dimensional FE Approach for the Stress Analysis of Tensile Armors Inside End Fittings
Authors: Miyazaki Marcelo N. R. , de Sousa José Renato M. , Ellwanger Gilberto B.
Published: November 2019

In this work, a solid three-dimensional finite element (FE) approach is proposed to investigate the stress distribution along the tensile armors of flexible pipes inside their end fittings (EFs). This approach employs two different models. The first model consists of a single tensile armor, which is meshed with solid FEs, and a rigid surface that represents the EF. A toroidal template is also considered and the EF mounting process is simulated. In the second model, the deformed armor and the stress state from the first model are considered and the voids between the armor and the EF are filled with resin, which is also modeled with solid FEs. Geometric and material nonlinearities are addressed and the interaction between the different components are ensured with contact elements. A case study is conducted to observe the stress distribution along the tensile armor considering typical operational loads. The results obtained are compared to those calculated with a previously proposed analytical model showing good agreement, but also evidencing the need to adequately simulate the EF mounting process and the pre-operational tests. Moreover, intense stress concentrations at the entrance of the EF are observed thus indicating that this region can be critical in fatigue analyses.
A Symbolic Regression Formulation to Estimate the Lateral Buckling Resistance of Tensile Armors in Flexible Pipes
Authors: Gonzalez Gabriel M. , de Sousa José Renato M. , Sagrilo Luis V. S. , Martins Ricardo R. , Rocha Djalene M.
Published: November 2019

In this work, a previously proposed finite element is applied in conjunction with a modal approach to predict the lateral buckling resistance of the tensile armors in flexible pipes. The finite element represents the mechanical behavior of tensile armors settled on elastic foundations, which model the frictional interaction between these armors and the surrounding layers. This FE modal approach is used to evaluate the buckling response of 44 different tensile armors considering 15 different friction coefficients between layers. The responses obtained formed a dataset employed in symbolic regression analyses that led to an analytical formulation capable of adequately reproducing the numerical results with minimum computational effort. The results obtained with this analytical formulation are compared to those from other numerical models and experimental measurements showing good agreement and evidencing the potential of the proposed formulation.
Prediction of Liner Wrinkling During High Strain Bending of Mechanically Lined Pipe
Authors: Pépin Aurélien , Tkaczyk Tomasz , Martinez Michael , O’Dowd Noel , Nikbin Kamran
Published: November 2019

A high demand for transport of corrosive fluids subsea has generated interest in solid corrosion resistant alloy (CRA) and bi-metal pipes. Bi-metal pipes, including hot-roll bonded (HRB) clad and mechanically lined pipes (MLP), are made of a carbon steel (CS) pipe lined with a CRA layer. Mechanically lined pipes, where the CRA liner is held inside the host pipe by means of an interference fit, offer shorter lead times and are considerably more economical than equivalent solid CRA and HRB clad pipes with a metallurgical bond between CS and CRA layers.

Reel-lay is a cost-effective method for installing subsea pipelines up to 18” (457.2 mm) in diameter. However, plastic straining associated with reeling may trigger wrinkling of the CRA liner. Two approaches for safe installation of reeled MLPs have therefore been proposed: pressurised and non-pressurised reeling.

This paper focuses on reel-lay installation at atmospheric pressure. Nevertheless, the numerical analysis framework presented is also applicable to MLPs installed at elevated pressure in a scenario where they are subjected to bending after being depressurised. Small-scale mechanical tests were carried out to assess the effect of manufacturing and cyclic plastic bending on the tensile behaviour of the CRA liner. After full-scale bending trials had been undertaken, they were simulated numerically to demonstrate the suitability of the proposed numerical approach for predicting liner separation from the host pipe and subsequent wrinkling during high strain bending. To improve ovality prediction, which governs liner separation and wrinkling, the authors developed an advanced metal plasticity model.
Two-Phase MPS Method for Dam-Break Flows
Authors: Wen Xiao , Wan Decheng
Published: November 2019

In this paper, the dam-break problem is numerically simulated using two-phase moving particle semi-implicit (MPS) method. Firstly, the two-phase MPS method is extended from single-phase MPS method. The present method treats the multiphase system as a multi-density and multi-viscosity fluid, and a single set of equations could be used for the whole system. The interaction between particles of different phases is considered through the applications of inter-particle viscosity and density smoothing technique. The two-phase MPS method is then successfully applied to violent dam-break problem. The numerical results obtained by single-phase and two-phase method are both compared with experimental results in the open literature. The characteristics of air cavity are analyzed, including the pressure inside the cavity and the cavity shape deformation.
Numerical Study on Aero-Hydrodynamics With Inter-Turbine Spacing Variation for Two Floating Offshore Wind Turbines
Authors: Huang Yang , Wan Decheng
Published: November 2019

To investigate the influence of the inter-turbine spacing on the performance of the floating offshore wind turbine (FOWT) in the floating wind farm, coupled aero-hydrodynamic simulations of two spar-type FOWT models with inter-turbine spacing variation under shear wind and regular wave conditions are performed in the present work. An unsteady actuator line model (UALM) is embedded into in-house code naoe-FOAM-SJTU to establish a fully coupled CFD analysis tool for numerical simulations of FOWTs. From the simulation results, the unsteady aerodynamic power and thrust are obtained, and the hydrodynamic responses including the six-degree-of-freedom motions and mooring tensions are available. Detailed flow visualizations of wake velocity profiles and vortex structures are also illustrated. The coupled performance of floating offshore wind turbines with inter-turbine spacing variation are analyzed, and the influences of inter-turbine spacing on aero-hydrodynamic characteristics of coupled wind-wave flow field are discussed. It is found that the power output of downstream wind turbine increases with inter-turbine spacing. Coupled aero-hydrodynamic characteristics of flow filed are significantly affected by inter-turbine spacing.
Developing Sensitivity Response Curves to Evaluate Mechanical Integrity of Set Cement
Authors: Patel Harshkumar , Salehi Saeed , Teodoriu Catalin
Published: November 2019

Cement sheath is an important barrier element for maintaining well integrity. Poor quality cement has often been cited as one of the major causes for loss of well control events. This paper presents a useful approach for quick assessment of fitness for service and mechanical integrity of cement sheath.

For this study, three dimensional finite element models consisting of concentric casings and annular cement sheath were created. Different wellbore and annular pressure loads were applied. Performance of cement sheath was assessed by analyzing radial, hoop, axial, and maximum shear stresses for mechanical failure. The FEA model was validated by analytical calculation of the stresses.

Parametric study was performed by individually varying influencing factors such as — cement sheath material properties, its dimensions, and wellbore pressure loads. Sensitivity response curve was developed by normalizing influential parameters and representing them against a mechanical stress on the same plot. The plot can be used to estimate whether cement will structurally fail due to various operational wellbore pressure changes, or variation in material properties caused by aging or temperature changes. The plot can also be utilized to identify the most critical factors influencing cement’s performance.

This paper presents a simple and effective approach of developing sensitivity response curves for a set cement sheath. Industry standards or guidelines can be improved by adding sensitivity response curves for standard cement recipes. This can help design engineers, and regulators alike in quickly evaluating short-term or long-term “fitness for service” of cement sheath from structural integrity perspective.
Vortex Shedding and Roll Damping for Hulls With Rounded Bilges
Authors: Milne Ian A. , Tong Feifei , Graham J. Michael R.
Published: November 2019

Vortex shedding and eddy generation can provide a substantial contribution to the hydrodynamic roll damping of vessels. While sharp-edge bilge geometries generally result in greater damping, a more rounded bilge may be beneficial for the other ship motions and mooring design. To this end, the effect of the bilge rounding on the vortex shedding dynamics and the hydrodynamic damping of hulls is investigated numerically. A series of new computations of the response of hulls with differing bilge radii and subject to forced oscillatory motion are reported. The severity of the vortex shedding and the magnitude of the damping is shown to depend strongly on the bilge geometry. Computations using an efficient isolated edge discrete vortex model are also found to be generally in qualitative agreement with the CFD results. The results extend the relatively scarce literature on the effect of bilge rounding and can assist in informing hull design and concept selection.
Optimization of Temporary Plugging Parameters Under Rough Fractures
Authors: Yuan Lishan , Zhou Fujian , Li Ben , Gao Junjie , Wang Caizhong , Tan Yanxin , Huang Jianbo
Published: November 2019

Temporary-plugging-and-diverting (TPD) fracturing technology is widely used in the development of the unconventional reservoir. The operational procedure of temporary plugging and the size and combination of diverters are very much concerned by field engineers. This study compares different pumping procedure of diverters and optimizes the combination and pumping rate of diverters under the different width of the fracture. The experimental method is based on a simulated fracture apparatus, which is manufactured by the 3D printing technology. The surface morphology of the fracture is obtained through a 3D scanning of a fracture. The experimental procedure is pumping the carrier liquid and diverter mixtures into the fracture while recording pumping pressure and the outlet volume of carrier fluid. The fracture plugging efficiency was evaluated through the recorded parameters. The diverter concentration and composition were optimized at a wide range of fracture width (1 mm to 4 mm). Low the diverter concentration could help to reduce the operational risk of the diversion. Under the low concentration of the diverters, the plugging mechanism is that the large particle diverters bridge in the fracture due to the fracture tortuosity and roughness; the smaller particle and fiber diverters then fill the voids of the large particles and form a strong and low permeable diverter pack. The results indicate that pumping the mixture particle and fiber diverters are more beneficial to plug the fracture than pumping them separately and sequentially. High particle concentration has a two-sided effect, which leads to the existence of an optimal fiber-to-particle ratio. The concentration of diverters could decrease when the size of diverters is increased. At a constant fracture width, higher pumping rates can help to temporarily plug the fracture more efficiently.
Efficiency of Carbon Fibre Buckle Arrestors for Subsea Pipelines
Authors: Karampour Hassan , Alrsai Mahmoud , Hall Wayne
Published: November 2019

This paper experimentally investigates the feasibility and efficiency of using Carbon Fiber Reinforced Polymer (CFRP) buckle arrestors in controlling the buckle propagation failure of subsea pipelines. Hyperbaric chamber tests are conducted on 1.6m Steel pipe with D/t = 28 and using CFRP buckle arrestors with different thickness, fiber orientation and spacing. Using an external pressure gauge and a high-pressure camera inserted inside the hyperbaric chamber, the pressure magnitude, rate and shape of collapse and its propagation in the vicinity of the arrestors are measured. The dynamics of buckle propagation and efficiency of different arrestor configurations are reported.

It is observed that in the vicinity of the CFRP arrestors wrapped in the hoop direction, the well-known dog-bone buckle shape changes into a U-shape and the pressure level upsurges significantly. The optimum results were obtained with CFRP as thick as the pipeline wall-thickness and wrapped in the hoop direction of the pipeline. The results show that at similar arrestor efficiency, the CFRP arrestors can be much thinner than the existing steel slip-on arrestors. Also, the spacing between the CFRP arrestor can be larger than that of the steel slip-on arrestor.
Finite Element Analysis of Seafloor-SCR Interaction in Touchdown Zone
Authors: Wei Zhang , Peng Peng
Published: November 2019

The touch down zone (TDZ) of steel catenary riser (SCR) is subject to fatigue damage. As a significant fatigue indicator, the variation of the stress range largely depends on the instantaneous changing bending moment. In this paper, a non-linear interaction model between seafloor and SCR is developed to consider the formation of trench at TDZ and its effect on the variation of bending moment. The seafloor soil is modeled by the nonlinear springs, whose stiffness is calculated according to nonlinear load-deflection relationship, such that the effect of cyclic loading can be simulated. The modeling of trench evolution and bending moment variation near the touchdown point (TDP) under cyclic loading are further conducted using the finite element software ABAQUS. The results show that the trench shaping is related to the uplift displacement of SCR and its number of cycles. The dependence of the bending moment on the displacement cycles near TDP is also investigated.