bluuebunny/crossref_metadata_2025 huggingface dataset:
Evaluation of the Solutions for Calculating Misalignment-Induced Stress Concentration Factor at Girth Welds in Pipelines
Authors: Zhang Yan-Hui , Dore Matthew , Chukkan Jazeel R.
Published: December 2020

Pipelines are commonly joined by girth welds. A centreline offset at a girth weld would inevitably be introduced due to either a difference in wall thickness of the pipes at each side of the weld or due to misalignment. An additional bending stress can result when the pipeline containing the girth weld is subjected to axial loading. This partly contributes to fatigue crack initiation from the weld toe. Therefore, the misalignment-induced stress concentration factor, Km, must be considered in fatigue design of girth welded pipelines.

There are several solutions available to calculate Km including BS 7910, DNVGL-RP-C203 and Connelly and Zettlemoyer. In some cases, the difference in the stress concentration factors calculated using these solutions is significant. This paper reviews these and other solutions and verifies them by finite element results and experimental data obtained through strain gauge measurements. Based on the study, the most appropriate Km solution is recommended.
Time-Domain Simulation of Subsea Equipments Installation Using Hydrodynamic Derivatives
Authors: de Andrade Emerson M. , Sales Joel S. , Fernandes Antonio C. , Ribeiro Mario L. , Teixeira Pedro V.
Published: December 2020

The installation of a subsea equipment such as manifold needs careful planning and coordination. Studies on the behavior of the dynamic responses are crucial to guarantee safety.

Some important factors in these operations include the current profile, waves characteristics, winches motions at topside, and the elastic behavior of the cable (due to resonance effects).

Currently, most of the available commercial codes use simplified models for the hydrodynamic forces of submerged equipment. However, for cases with complex geometries and strong interactions with the environmental loads, those models fail to represent correctly the dynamics.

In this paper we present an initial method and a hydrodynamic model to include terms that allow the modelling of complex behavior of submerged complex geometries by using hydrodynamic derivatives extracted from model tests. To verify the procedure, tests were performed both at a flume tank and at a towing tank. The model was implemented in a commercial code by using a Simplified Buoy model, to which a python procedure that calculated the hydrodynamic forces was attached. The study was divided into two phases: the first one consisted of the verification of the effectiveness of the external routine. This was done for a manifold in 1DOF and then in 6 DOF. In the second phase, the dynamic maneuvering model using Hydrodynamic Derivatives was implemented as an external routine and, using the output from dynamic excitation experiments at small scale with a manifold, kinematical behavior results were compared.

Results showed good adherence, although some further investigations are still needed.
Using Machine Learning to Identify Important Parameters for Flow-Induced Vibration
Authors: Ma Leixin , Resvanis Themistocles L. , Vandiver J. Kim
Published: December 2020

Vortex-induced vibration (VIV) of long flexible cylinders in deep water involves a large number of physical variables, such as Strouhal number, Reynolds number, mass ratio, damping parameter etc. Among all the variables, it is essential to identify the most important parameters for robust VIV response prediction. In this paper, machine learning techniques were applied to iteratively reduce the dimension of VIV related parameters. The crossflow vibration amplitude was chosen as the prediction target. A neural network was used to build nonlinear mappings between a set of up to seventeen input parameters and the predicted crossflow vibration amplitude. The data used in this study came from 38-meter-long bare cylinders of 30 and 80 mm diameters, which were tested in uniform and sheared flows at Marintek in 2011. A baseline prediction using the full set of seventeen parameters gave a prediction error of 12%. The objective was then to determine the minimum number of input parameters that would yield approximately the same level of prediction accuracy as the baseline prediction. Feature selection techniques including both forward greedy feature selection and combinatorial search were implemented in a neural network model with two hidden layers. A prediction error of 13% was achieved using only six of the original seventeen input parameters. The results provide insight as to those parameters which are truly important in the prediction of the VIV of flexible cylinders. It was also shown that the coupling between inline and crossflow vibration has significant influence. It was also confirmed that Reynolds number and the damping parameter, c*, are important for predicting the crossflow response amplitude of long flexible cylinders. While shear parameter was not helpful for response amplitude prediction.
Informing Components Development Innovations for Floating Offshore Wind Through Applied FMEA Framework
Authors: Rinaldi Giovanni , Thies Philipp , Johanning Lars , McEvoy Paul , Georgallis Georgios , Moraiti Anastasia , Cortés Lahuerta Carlos , Vidmar Marijan
Published: December 2020

Future offshore wind technology solutions will be floating to facilitate deep water locations. The EUH2020 funded project FLOTANT (Innovative, low cost, low weight and safe floating wind technology optimized for deep water wind sites) aims to address the arising technical and economic challenges linked to this progress. In particular, innovative solutions in terms of mooring lines, power cable and floating platform, specifically designed for floating offshore wind devices, will be developed and tested, and the benefits provided by these components assessed. In this paper a purpose-built Failure Modes and Effect Analysis (FMEA) technique is presented, and applied to the novel floating offshore wind components. The aim is to determine the technology qualification, identify the key failure modes and assess the criticality of these components and their relative contributions to the reliability, availability and maintainability of the device. This will allow for the identification of suitable mitigation measures in the development lifecycle, as well as an assessment of potential cost savings and impacts of the specific innovations. The methodology takes into account inputs from the components developers and other project partners, as well as information extracted from existing literature and databases. Findings in terms of components innovations, their main criticalities and related mitigation measures, and impacts on preventive and corrective maintenance, will be presented in order to inform current and future developments for floating offshore wind devices.
Numerical Simulation of Hydrodynamics Around Net Meshes Using REEF3D
Authors: Wang Gang , Martin Tobias , Huang Liuyi , Bihs Hans
Published: December 2020

Hydrodynamics and turbulence around net meshes have drawn more and more attention because it is closely related to forces on the structures and safety issues of offshore fish farms. In terms of numerical modeling of forces on nets, Morison or screen force model is ordinarily adopted to account for its hydrodynamics. However, these methodologies mainly rely on empirical experimental or cylindrical hydrodynamic coefficients, neglecting flow interactions between adjacent cruciforms or net bars. In this study, REEF3D open-source hydrodynamic toolbox is adopted to analyze flow around net meshes explicitly and investigate the hydrodynamics related to forces on the structure. The simulation accuracy is in good agreement with flume experiments and previous research. Flow velocity and vorticity around net bars and knots are investigated. The results demonstrate that 2 × 2 or 3 × 3 mesh cases are more reliable when studying turbulence around net meshes, flow interactions around adjacent net bars, knots should be taken into consideration. Two patterns to control Sn, one is to change the diameter of net bars and the other is to control length, have different effects on the flow around meshes. This paper presents a first step in the aim to derive a new empirical formula for Cd depending on Sn, and Re, which are more related to the physics in offshore conditions.
Vibration Control of a Jacket Offshore Wind Turbine Under Earthquake Wind and Wave Loads by Tuned Mass Damper
Authors: Li Xin , Wang Wenhua , Pan Zuxing , Wang Bin
Published: December 2020

The fully coupled analysis model of a jacket offshore wind turbine (OWT) is established based on the governing equation of motion of the structure which is derived in accordance with the blade element momentum theory (BEM), Morison formula and theories of structural dynamics. The dynamic characteristics and structural responses of the jacket OWT under the different combined seismic cases are analyzed. It can be seen that the interactions of the wind and wave loads are non-negligible in the seismic analysis of an OWT, and the abundant dominant frequencies of the responses of the support system under the combined seismic cases are discovered. Meanwhile, the passive control method of tuned mass dampers (TMD) is applied to the support system of the OWT under the earthquakes, and the influence of the TMD parameters on the reduction of the responses of the support system are investigated. Furthermore, according to the reduction of the structural responses, the suggestions for the design of a TMD under the combined seismic cases for the bottom fixed OWT are summarized.
Experimental Investigation on Vortex-Induced Vibration of Dual Pipes With Unequal Diameters in Tandem
Authors: Zhang Mengmeng , Fu Shixiao , Zhang Zhiqi , Ren Haojie , Xu Yuwang
Published: December 2020

With the massive use of buddle risers and pipelines in deep-water oil production industries, the demand to focus on the research of interference effects of dual pipe has been greatly enhanced. This paper presents interference experiments of dual flexible pipes with unequal diameters under uniform flow with Reynolds numbers ranging from 1.8E3 to 1.1E4. The pipe with larger diameter was set to be the upstream pipe. Various tandem arrangements with wall surface-to-wall surface distances being 3D to 8D were tested, where D is the smaller pipe diameter. Fiber Bragg grating (FBG) strain sensors were used to measure both in-plane and out-of-plane strain responses. Modal superposition method was applied to reconstruct IL mean displacement. Significant interference effect was found under the condition that wall-to-wall gap is smaller than 8D, where CF and IL vibration frequency ratio of downstream pipe equals to 1.0 and IL mean displacement gets smaller compared to those of single pipe in isolation. Moreover, a special ‘capture’ phenomenon, that the dominant vibration frequency and mode of downstream pipe were as the same as that of the upstream pipe subjecting to the uniform flows, was found when wall-to-wall distances were 4D and 8D.
The Effect of Storm-Induced Precipitation on Flooding in Macau City
Authors: Yang Jie , Liu Jiandong , Kim Dong-Eon , Li Linlin , Mok Kai Meng , Liu Philip L.-F.
Published: December 2020

The extent and depth of coastal flooding caused by tropical cyclone can be underestimated without considering the contribution by storm-induced precipitation. In this study, the dynamic flooding processes in Macau are simulated accounting for the combined effect of storm surge and precipitation during Typhoon Mangkhut in 2018. The surface wind and atmospheric pressure during this typhoon event are generated by a parametric vortex model and are used to drive a surge–tide–wave model for producing storm-induced water levels. The precipitation rate from ERA5 reanalysis data is utilized in this modelling suite as an additional source of water mass. The numerical model is first thoroughly validated by comparing the numerical results against the measured water levels and significant wave heights at tidal gauge and wave buoy stations. By switching on/off the rainfall function in the mass conservation and momentum equations, we compare and analyze the differences of inundation depths and inundation extent induced by precipitation. The differences in the maximum inundation depths vary spatially and the increments owing to precipitation are less than 50 mm in most flooded regions, the associated contribution is less than 10% in areas with moderate and severe flooding. The effect of precipitation as additional rainfall-induced forces in the momentum equations plays a more important role than that in the mass conservation. The inundation depths induced by tide and surge are approximately two times of that induced by precipitation. Moreover, the compound effects by ocean waves and precipitation tend to decrease the increment of precipitation-induced inundation depths by precipitation alone. Finally, we also assess the performance of the overland flow model Itzï on Macau Peninsula for the same typhoon event. The present study helps identifying vulnerable areas subject to heavy precipitation in Macau and gaining more understanding of flooding mechanisms by different physical drivers.
Study on Safety Operation Period Model of Deep Water Typhoon Avoidance Well Control Considering Rheological Characteristics of Drilling Fluid
Authors: Lou Wenqiang , Wang Zhiyuan , Du Kai , Zhang Jianbo , Liao Youqiang , Pan Shaowei , Sun Baojiang
Published: December 2020

During the period of avoiding typhoon in deep-water drilling, the gas intrusion in the wellbore brings higher risk to the well opening operation. It is very important to improve the accuracy of typhoon avoidance cycle prediction to improve the efficiency of drilling operation and reduce the risk of well control. According to the yield stress characteristics of drilling fluid, the gas migration experiment in the yield stress fluid is carried out in this paper. Based on the experimental results and stress analysis, a prediction model of bubble rising velocity suitable for the yield drilling fluid is established. Considering the rheological properties of drilling fluid and pore characteristics of formation, a prediction model of initial bubble size is established. Combined with the velocity prediction model and bubble size prediction model, the calculation method of typhoon avoidance period is proposed. Through the calculation and analysis, it is found that under the condition of low invasion rate, the formation porosity has little influence on the initial size of bubbles, and the yield stress value of drilling fluid has great influence on the initial size of bubbles. At the same time, the model in this paper makes up for the problem that the prediction value of safe typhoon avoidance period calculated by the previous model is large, and reduces the risk of well opening operation. According to our calculation results, the measures of injecting drilling fluid with low yield stress at the bottom of the well and injecting drilling fluid with high yield stress at the top are put forward, which can effectively increase the safe typhoon avoidance period.
The Fair Evaluation of Wave Energy Converters
Authors: Tan Jian , Polinder Henk , Wellens Peter , Miedema Sape
Published: December 2020

In this paper, a fair evaluation method of WECs (Wave Energy Converters) is established based on frequency domain simulation. In this fair evaluation, size optimization and downsizing of PTO (Power Take-Off) capacity are included to minimize the Cost of Energy for the concerned wave location. Based on this fair evaluation, a techno-economic evaluation of a generic point absorber is conducted for a specific wave location, and two different control strategies of PTO are considered. The results show that this fair evaluation method can contribute to the improvement of techno-economic performance of WECs. Furthermore, a comparison among three different size optimization methods of WECs is performed.
EOR Techniques Tailored to Venezuelan Conventional and Unconventional Oils: Critical Review
Authors: Rodriguez M. Fernancelys
Published: December 2020

Venezuela has been ranked as a potential oil producer country thanks to its huge reserves of conventional and unconventional oils. Conventional reservoirs with complex fluid systems, located in the North of Monagas state, where it is possible to observe thick fluid columns with significant compositional gradients (showing changes from gas condensate to non-mobile oil-Tar mat). In these types of reservoirs EOR methods such as miscible gas flooding have been successfully applied to compensate pressure decline and avoid asphaltene deposition issues.

Production of unconventional oils, the largest highly-viscous oil reservoir of La Faja Petrolifera del Orinoco (La FPO), demands great challenges. Discovered in the 1930’s, the first rigorous evaluations of this reservoir started in the 1980s [1]; those huge deposits of highly viscous oils were considered technically and economically unattractive at that time. Due to production decline of conventional oil reservoirs, efforts are being done by the Venezuelan National Oil Company and collaborators to develop EOR projects to achieve increasing oil production in unconventional (heavy and extra-heavy) reservoirs, being the most promising options thermal and chemical EOR methods.

Some authors agree that in the FPO, only 40–65% (depending on the site) of the oil-bearing formations is suitable for thermal EOR methods. Recent works have been showing the potential of chemical EOR for extra-heavy oils in La FPO [2, 3, 4, 5, 6, 7, 8, 9], mostly for mobility control and mobilization of residual oil.

This work presents a literature review of the EOR projects in Venezuela for conventional and highly viscous oils, based on both lab and field experiences, and the perspectives for applications to increase Venezuelan oil production.
Research on Fixed Route Speed Optimization Based on Deep Neural Network and Genetic Algorithm
Authors: Wang Ziming , Chen Shunhuai , Luo Liang
Published: December 2020

In the downturn of the shipping industry, optimizing the speed of ships sailing on fixed routes has important practical significance for reducing operating costs. Based on the ship-engine-propeller matching relationship, this paper uses BP neural network to build main engine power model, and correction factors are introduced into the main engine power model to reflect the influence of wind and wave. The Kalman filter algorithm is used to filter the data collected by a river-sea direct ship during the voyage from Zhoushan to Zhangjiagang. The filtered data and the meteorological data obtained from the European Medium-Range Weather Forecast Center are used as the data set of the BP neural network to predict the main engine power. Based on the main engine power model, a multi-objective optimization model of ship speed under the influence of actual wind and waves was established to solve the conflicting goals of reducing sailing time and reducing main engine fuel consumption. This multi-objective model is solved by a non-dominated fast sorting multi-objective genetic algorithm to obtain the Pareto optimal solution set, thereby obtaining the optimal speed optimization scheme. Compared with the original navigation scheme, the navigation time is reduced by 8.83%, and the fuel consumption of the main engine is reduced by 12.95%. The results show that the optimization model can effectively reduce the fuel consumption and control the sailing time, which verifies the effectiveness of the algorithm.
Numerical Investigation on the Global Behavior of Jack-Up Unit Next to Footprint With Rectangular-Shaped Spudcan
Authors: Yuan Ye , Duan Menglan
Published: December 2020

Footprints were left on the seabed due to previous operations, and the problem of the spudcan-footprint interactions gradually becomes one of the major concerns of jack-up unit installation. The spudcan is subjected to eccentric loading conditions, which can lead to structural failure and the overturn of the jack-up unit. This paper reports the global behavior of the three-legged jack-up unit with a novel rectangular-shaped spudcan when it needs to reinstall near an existing footprint. The coupled Eulerian-Lagrangian (CEL) approach was used. A simplified three-legged jack-up unit model was developed and the flexible stiffness between the hull and legs was considered. The free-rotation heading and force-controlled loading conditions were also adopted. The bending moment distribution along the legs, horizontal force and the overturn of the jack-up unit are discussed to assess the potential of the jack-up unit sliding toward the footprint center. The critical offset distance was found at 0.3D and an offset distance of at least 1 times of the spudcan diameter is able to diminish influence of the footprint. The ability of the novel rectangular large spudcan with a flat base is shown to be effective at easing spudcan-footprint interactions, compared with generic circular spudcan.
Effect of Pre-Strain on Bending Strain Capacity of Mechanically Lined Pipe
Authors: Tkaczyk Tomasz , Vasilikis Daniil , Pepin Aurelien
Published: December 2020

The high demand for subsea transportation of corrosive wellhead produced fluids has created interest in economical mechanically lined pipes (MLP) made of external carbon steel and a thin internal layer of corrosion resistant alloy (CRA). The bending strain capacity of an MLP, where a CRA liner is adhered to a carbon steel host pipe by means of an interference fit, is often governed by the liner wrinkling limit state. Although the strain capacity of the MLP with a typical 3 mm thick liner is enough to withstand bending to strains encountered during installation with the S-lay or J-lay method, the liner is at risk of wrinkling when the MLP is subjected to higher bending strains during reel-lay. To allow reeled installation, the liner strain capacity is enhanced by either increasing the liner thickness or pressurizing the MLP during installation.

In the former approach, the required liner thickness is proportional to the pipe diameter. For larger diameter MLPs, it is therefore often more economical to select a 3 mm thick liner and flood and pressurize an MLP to ensure liner stability during reeling. However, the MLP may need to be depressurized and partially drained during installation to allow welding a structure, performing reel-to-reel connection or pipeline recovery which impose bending strain on a plastically pre-strained and depressurized pipeline. Furthermore, reeled pipelines may be depressurized subsea while subjected to bending loads from operation.

Although there is a history of research into the limit loads and failure modes of MLPs, there is still no comprehensive guidance on determining the risk of liner wrinkling in plastically pre-strained MLPs. In this paper, an approach is proposed for evaluating the strain capacity and assessing the risk of liner wrinkling after an MLP, subjected to plastic bending during reeled installation at elevated pressure, is depressurized and subjected to installation loads during offshore intervention or operational loading in service. The combined effect of strain history at elevated pressure, reeling-induced ovality, bending direction after depressurization, differential pressure, temperature and residual strain is discussed. The recommendations for further work are also given.
Estimating Design Loads for Floating Structures Using Environmental Contours
Authors: Zhao Yuliang , Dong Sheng , Yang Zihao , Manuel Lance
Published: December 2020

To ensure acceptable operation and/or survival of floating structures in extreme conditions, nonlinear time-domain simulations are often used to predict the structural response at the design stage. An environmental contour (EC) is commonly employed to identify critical sea states that serve as input for numerical simulations to assess the safety and performance of marine structures. In many studies, marginal and conditional distributions are defined to construct bivariate joint probability distributions for variables such as significant wave height and zero-crossing period; then, environmental contours can be constructed using the inverse first-order reliability method (IFORM). This study adopts alternative models to describe the generalized dependence structure between the environmental variables using copulas; the Nataf transformation is also discussed as a special case. Environmental contours are constructed, making use of measured wave data from moored buoys. Derived design loads are applied on a semi-submersible platform to assess possible differences. In addition, the long-term extremes of the tension of the mooring lines are estimated, considering uncertainties in the structural response using a 3D model (that includes response variability, ignored with the EC approach) to help establish more accurate design loads using Monte Carlo simulation. Results offer a clear indication of the extreme response of the floating structure based on the different models.
Dynamic Behaviour of a Free Hanging Vertical Pipe Forced to Oscillate at the Top
Authors: Sales L. P. A. , Trigo C. C. O. , Sevillano L. C. , Morooka C. K.
Published: December 2020

Vertical pipe configuration happens in subsea well re-entry operations for wellbore drilling, subsea equipment installations, deepwater intake risers, among other scenarios, usually by hanging the marine riser at the top from a platform or ship. The simple operational configuration justifies its wide application; however, there are some drawbacks such as stress concentration, fatigue, among other issues. The purpose of this study is to contribute for the overcoming of those problems by improving the understanding of hanging risers through investigations of a vertical pipe forced to oscillate horizontally at the top.

For this purpose, laboratory experiments with small diameter pipe model have been developed with oscillatory motions applied at the top termination through a mechanical device. Displacements over time and along the pipe length were measured by an optical measurement system. In the present study, a numerical scheme has been implemented for simulations to support evaluations of the experimental results.

A vertical pipe response depends on several parameters, such as pipe overall length, frequency and amplitude of oscillation itself, among others. Due to existing relative velocity between the oscillating pipe and the water surrounding it, vortex shedding occurs along the pipe length which influences riser motion behaviour.

The results from experiment and numerical simulations bring important insights for describing and understanding oscillatory vertical pipe behaviour, in the plane and the out of plane of the forced top oscillation. The obtained results provide a further step toward the modelling of Vortex-Induced Vibration (VIV) in marine risers, aiming to contribute to relevant technological advancements.
Modeling of a Shared Mooring System for a Dual-Spar Configuration
Authors: Liang Guodong , Merz Karl , Jiang Zhiyu
Published: December 2020

As floating wind turbines become more technically mature, the development of floating wind farms is under way. Cost-effective solutions are desired to reduce the mooring costs. The concept of a shared mooring system has been proposed for this purpose. This work presents a method to model the shared mooring system for a dual-spar configuration. By applying the theory in elastic catenary of cable structures, a shared line can be modelled. To verify the method, a dual-spar system is modelled in a multibody simulation tool, in which two floating wind turbines are connected via a shared line. Static analyses are performed by using the present method and the simulation tool. Further, a sensitivity study is applied to the shared line properties. Different mooring line properties have been investigated. The influence of the shared line properties is shown on the mooring restoring properties of the dual-spar system. The present modeling method can be applied in the preliminary design stage of shared mooring systems.
Methodology for Assessing Wellhead Integrity During its Life Cycle
Authors: Jaculli Marcelo A. , Mendes José Ricardo P. , Miura Kazuo , Colombo Danilo
Published: December 2020

The wellhead is an important component in the context of well integrity and operational safety, as it serves as a link between well and riser in offshore oil wells. Wellheads, combined with the BOP and/or the Christmas Tree, act as the final barrier element preventing leakage of oil from the well into the environment. The wellhead must sustain loads from the casings, which are hung into it through the casing hangers, as well as sustain loads from the riser, which is connected at the top of the BOP (right above the wellhead) through the lower flex joint. In this work, we propose a methodology for analyzing the wellhead, considering a mechanical system that couples the vessel, the riser, the wellhead itself, the conductor and surface casings, and their interaction with the soil. The simulation of this coupled system provides the loads that are transmitted to the wellhead due to its coupling with riser, vessel, casing, and soil. Then, these loads are converted into stresses and a criterion, such as yield, is applied to verify if they will cause wellhead failure. The objective of this work is to assess wellhead failure due to environmental loads (waves, currents) acting on the riser, by varying parameters such as significant wave height, peak period and current speed, as well as assess wellhead integrity during its lifecycle for operational and survival conditions. Results show that the wellhead may fail under certain environmental conditions, which would compromise well integrity and cause leakage from the well. The analysis performed here, which is deterministic, also provides a basis for a subsequent probabilistic analysis from which the wellhead reliability can be found, based on uncertainties such as the sea state parameters.
Simulating the Acceleration to Take-Off Phase of a WIG-Craft Using Results of a Constrained Experiment
Authors: Pereowei Garrick Ombor , Hu Zhiqiang , Song Lei
Published: December 2020

The acceleration to take-off (in calm water and rough seas) is a short duration but very important motion regime of a WIG-craft. It determines the transport efficiency of a WIG-craft as a viable alternative to high speed marine vehicles or low speed aircrafts used in conveying workers to and offshore oil and gas fields. The development of a simulation model based on results from constant speed captive model tests for a WIG boat is imperative, when in the absence of appropriate experimental test rig, there is the need to investigate the attitude of the WIG-craft during its acceleration phase. Theoretical tools for investigating the characteristics of the acceleration phase of a WIG boat are uncommon and where they exist, they are almost unreliable, not been experimentally validated. Moreover, the cost associated with conducting acceleration tests is huge. The test facilities are not readily available in most maritime engineering research institutions. This study is concerned with the development of a simulation model with input from results of captive model tests to investigate the running attitude, forces and moments acting on a WIG-boat accelerating to take-off in calm water. A constrained model tests at constant speed levels were conducted for a range of model draughts and trim angles. Multivariate multiple regression method was used to develop model equations that fits the measured aero-hydro dynamic lift, drag and moment data as a function of draught, speed and trim angle. The hydrostatic and aero-hydrodynamic steady state forces and moments where combined into a state-space form which are solved in MATLAB. The state variables and the first and second derivatives of the states of the boat as well as the forces and moments acting on it are generated as output from the simulation model. Though the simulation model proved successful in predicting the attitude of the WIG-boat including crashing and excessive acceleration and power requirement during take-off, the results from the model still need to be verified with CFD analysis, a full WIG-boat trial tests or with the expensive high speed towing tank capable of carrying out acceleration runs. The method has the potential to be improved to account for the unsteady forces and moments that exists when the WIG-boat accelerates from offshore environment.
Dynamic Behavior of a Novel Heave-Compensated Floating Platform for Well Interventions
Authors: Jaculli Marcelo A. , Leira Bernt J. , Sangesland Sigbjørn , Morooka Celso K. , Mendes José Ricardo P.
Published: December 2020

A new type of floating platform design has been investigated. It consists of a relatively small semi-submersible floating structure with an air chamber that aims to keep a constant buoyancy, thus effectively reducing heave motion and enabling its use under harsh environmental conditions such as in the North Sea. It aims to provide an alternative solution compared to large floating structures, such as drillships and semi-submersible platforms, in terms of time availability, drilling costs and operational flexibility. One recent focus has been on the application of this platform for reducing well intervention costs. A small diameter (workover) riser may be used for installing the well control stack on the wet Christmas tree and for performing well intervention through the riser using a wireline cable. Alternatively, the operation can take place without a riser; this operation is termed riserless well intervention (RLWI). In this work, we investigate the dynamic behavior of this system, which is attached either to a wireline — for RLWI — or to a small-sized riser for well service through the riser. By modeling this system — which acts similarly to a passive heave compensation system — we have verified that this new platform indeed experiences smaller displacements when compared to conventional platform. The reduction observed varies depending on the platform design; in some cases, it reduces the displacement by a factor of two. A relatively heavier platform with a small cross sectional water plane area is found to be the best design option, but a lighter platform might be preferable for increased flexibility, as long as its dynamic behavior is satisfactory for safe operations.
Leakages Through Radial Cracks in Cement Sheaths: Effect of Geometry, Viscosity and Aperture
Authors: Skorpa Ragnhild , Vrålstad Torbjørn
Published: December 2020

Annular cement sheath is considered to be one of the most important barrier elements in the well, both during production and after well abandonment. It is however well-known that mechanical damage to the cement sheath might result in leakage pathways, such as microannuli and radial cracks, and thus loss of zonal isolation. In this paper we have studied the effect of geometry, aperture and viscosity on the resulting pressure driven flow through real radial cracks in cement sheaths using Computational Fluid Dynamics (CFD) simulations.

Real radial cracks were created by downscaled laboratory pressure cycling experiments and the resulting geometries were mapped by X-ray Computed Tomography (CT). This gave a unique 3D volume of the degraded cement sheaths which provides detailed information about the morphology, such as the irregular apertures and roughness, as well as locations of the radial cracks.

In this study, we have used five experimentally created geometries, varying from barely connected to fully connected and almost uniform cracks. Additionally, theoretical uniform models with homogeneous aperture and a smooth surface were created for comparison. The simulations were performed by importing the experimentally created leak paths into a CFD simulation software, making it possible to determine the actual flowrate as a function of pressure drop. Methane gas, water and oil was used as model fluids.

The simulation results show that fluid flow through real cracks in cement sheath is complex with torturous paths, especially around bottlenecks and narrow sections. Additionally, the results show that flow of both methane gas- and water are not linear and hence does not follow Darcy’s law. This illustrates that simple models are not able to fully describe fluid flow through such complex geometries.
Structural Reliability Analysis Method for Assessing the Fatigue Capacity of Subsea Wellhead Connectors
Authors: Hørte Torfinn , Reinås Lorents , Wormsen Anders , Aardal Andreas , Gustafsson Per
Published: December 2020

Subsea Wellheads are the male part of an 18 3/4” bore connector used for connecting subsea components such as drilling BOP, XT or Workover systems equipped with a female counterpart — a wellhead connector. Subsea wellheads have an external locking profile for engaging a preloaded wellhead connector with matching internal profile. As such connection is made subsea, a metal-to-metal sealing is obtained, and a structural conduit is formed. The details of the subsea wellhead profile are specified by the wellhead user and the standardized H4 hub has a widespread use.

In terms of well integrity, the wellhead connector is a barrier element during both well construction (drilling) activities and life of field (production). Due to the nature of subsea drilling operations, a wellhead connector will be subjected to external loads. Fatigue and plastic collapse due to overload are therefore two potential failure modes. These two failure modes are due to the cyclic nature of the loads and the potential for accidental and extreme single loads respectively. The safe load the wellhead connector can sustain without failure can be established by deterministic structural capacity methods.

This paper outlines how a generic and probabilistic engineering method; Structural Reliability Analysis, can be applied to a subsea wellhead connector to estimate the probability of fatigue failure (PoF). As the wellhead connector is a mechanism consisting of a plurality of parts the load effect from cyclic external loads is influenced by uncertainty in friction, geometry and pre-load. Further, there is a inter dependence between these parameters that complicates the problem. In addition to these uncertainties, uncertainties in the fatigue loading itself (from rig and riser) is also accounted for.

This paper presents results from applications of Structural Reliability Analysis (SRA) to a wellhead connector and provides experiences and learnings from this case work.
Numerical Investigation on Wave Transmission by a Submerged Reef Using MIKE 3 WAVE FM
Authors: Raju Rahul Dev , Arockiasamy Madasamy , Sundaravadivelu Ranganathan , Selvaraj Sherlin Prem Nishold
Published: December 2020

Wave transmission of a 1:10 gradient slope submerged reef is studied using MIKE 3 WAVE FM software. The study focuses on the effect of wave steepness and relative submergence depth over the reef on the wave transmission for three water levels. The results from the numerical analysis are compared with published literature on wave transmission of submerged structures. The results from the numerical analysis using MIKE 3 WAVE FM software show that the wave steepness and relative submergence depth over the reef have strong dependence on the wave transmission of submerged reef. The wave transmission coefficient shows a decreasing trend with increasing wave steepness.
Calculation of Environmental Load for a Large FPSO Undocking
Authors: Yuchun Du , Zhimei Chen , Hankun Yang , Bo Zhou , Chunhui Li
Published: December 2020

Ship undocking is an essential process in the construction of large ship. Nowadays, the traditional empirical method is usually used to calculate the environmental load during large ship undocking of large ship and to determine the power and number of assistant tugboats in the world shipbuilding industry. Firstly, the OCIMF with traditional empirical formula mathematical model for solving the environmental load problem is established in this paper. According to the location of the dry dock and the meteorological and hydrological conditions while undocking, the environmental load of a large FPSO with different wind, wave, and current combination from 0°to 90°are is calculated by using the OCIMF with traditional empirical formula method, and then the influence of the wind, wave and current load on large FPSO are is analyzed. Secondly, the environmental load is calculated by using numerical simulation calculation method based on the same environment conditions. Through comparing the analysis results between the two calculation methods, the reliability of using the OCIMF method is testified reliable to large barge-type FPSO, and the maximum environmental load is determined. Lastly, as a result the undocking plan and the power and number of assistant tugboats are finally determined. The actual undocking project shows that the OCIMF with traditional empirical formula method is reliable for calculating the environmental load of a large barge-type FPSO.
Surface Waves Under the Sea Ice in the Western Arctic During 2019 R/V Mirai Cruise
Authors: Kodaira Tsubasa , Waseda Takuji , Nose Takehiko , Inoue Jun , Sato Kazutoshi , Voermans Joey , Babanin Alexander
Published: December 2020

Surface ocean waves are one of the potential processes that influence on the Arctic sea-ice extent. A better understanding of the generation, propagation, and attenuation of ocean waves under the sea ice is necessary to discuss the future Arctic climate change. We deployed two drifting wave buoys in the marginal ice zone in the western Arctic. Since the surface wave observation in the marginal ice zone is rare, the obtained data are useful for validation of the numerical modeling of the surface waves under the sea ice. The first buoy was deployed in the pancake-ice covered area and the second one in the open ocean. The distance between the two buoys at the deployment was about 40km, and the second buoy was deployed approximately 5 hours after the first deployment. The comparison of the wave bulk statistic measured by the two buoys shows the wave transformation under the sea ice. In general, the significant wave height decreases, and the mean wave periods increase by the presence of the sea ice.
Dynamic Response of a Wave Energy Converter With Resonant U-Tank
Authors: Afonja Adetoso Justus , Brizzolara Stefano
Published: December 2020

This paper describes the concept design and preliminary dynamic analysis of a pitching wave energy converter (WEC) device, based on a pitching resonant floater, a pitch resonance tuning tank system and Wells turbines in regular head waves. The device has a bow/stern symmetry, which gives an advantage of the U-tank been strongly coupled with the floater in the pitch degree of freedom and both chambers will have their separate pneumatic turbines. The integrated dynamic model coupling the U-tank system as given with the motion of the floating body in regular waves and the power take off (PTO) device is physically and mathematically defined. This coupling effectively creates a multi-body dynamic system and thus alters the motion response amplitude operator of the device in waves creating multiple resonance peaks. The integrated dynamic model is solved in time domain to account for non-linearities. Excitation, radiation and diffraction forces are calculated in frequency domain from a 3D boundary element method (BEM) and corrected by Cummins equation (convolution integral) for memory effects to be used in the time domain solution. The time dependent motion of the free surface creates a pressure difference inside the chamber with respect to the atmosphere which is used by the PTO turbine. The dynamic model of the integrated system is used to predict the maximum extracted power for a given incident wave power. A systematic study, considering a change in PTO damping is performed to search for the maximum extracted power in any given regular wave condition.
Reinforcement Learning From Pixels: Waterflooding Optimization
Authors: Miftakhov Ruslan , Efremov Igor , Al-Qasim Abdulaziz S.
Published: December 2020

The application of Artificial Intelligence (AI) methods in the petroleum industry gain traction in recent years. In this paper, Deep Reinforcement Learning (RL) is used to maximize the Net Present Value (NPV) of waterflooding by changing the water injection rate. This research is the first step towards showing that the use of pixel information for reinforcement learning provides many advantages, such as a fundamental understanding of reservoir physics by controlling changes in pressure and saturation without directly accounting for the reservoir petrophysical properties and wells.

The optimization routine based on RL by pixel data is tested on the 2D model, which is a vertical section of the SPE 10 model. It has been shown that RL can optimize waterflooding in a 2D compressible reservoir with the 2-phase flow (oil-water). The proposed optimization method is an iterative process. In the first few thousands of updates, NPV remains in the baseline since it takes more time to converge from raw pixel data than to use classical well production/injection rate information.

RL optimization resulted in improving the NPV by 15 percent, where the optimum scenario shows less watercut values and more stable production in contrast to baseline optimization. Additionally, we evaluated the impact of selecting the different action set for optimization and examined two cases where water injection well can change injection pressure with a step of 200 psi and 600 psi. The results show that in the second case, RL optimization is exploiting the limitation of the reservoir simulation engine and tries to imitate a cycled injection regime, which results in a 7% higher NPV than the first case.
Sensitivity Study About Ultimate Strength and Postbuckling Behavior of Stiffened Box Deck Girder Under Compression
Authors: Zhou Chunqi , Song Dan
Published: December 2020

The proper determination of this paper is to study the ultimate strength and postbuckling behavior for complicated stiffened deck girders of offshore platforms. Normally, offshore platforms are designed with reinforcing stiffeners on box deck girder.

The box deck of column stabilized platform will be simulated with FEM software ABAQUS. Linear eigenvalue analysis is firstly applied to introduce the initial imperfection into the structure. Then the non-linear postbuckling analysis for the box deck girder will illustrate ultimate strength and the failure mode under axial load after collapse. The ways how the stiffeners, eigenmode and imperfections influence the ultimate capacity will also be analyzed.

Comparing variable elements affecting the structural strength, the work shows the offshore box deck girder section from the engineering project in this paper is quite robust to the axial compression. For this kind of robust design, such as the box deck girder, the eigenmode selection and amplitude of imperfection will exert slight effects on the ultimate strength, but the severe imperfections will accelerate the local buckling. Stiffeners design will exert dramatic influence on the ultimate strength and postbuckling behaviors. And the work to find the optimum balance among structural strength variable load and allowable imperfections seems to be very promising in engineering. The conclusions provide a more clear insight into the further optimization of the offshore square structure design.