bluuebunny/crossref_metadata_2025 huggingface dataset:
Autonomous Inflow Control Device Pilot Application for Extra Heavy Oil Field
Authors: Abdel Rafea Mahmoud , Criado Cristhian
Published: September 2018

Autonomous Inflow Control Device (AICD) completion was successfully designed and applied in a horizontal well drilled in a deep reservoir in an extra heavy oil field located in South America where the average total depth of the targeted reservoir is around ten thousands feet and the in-situ viscosity is 600 cps while API Gravity is ranged between 8.5–9.5.

Due to geological and petro-physical features in this area which turns into permeability variations and thick transition zone across the reservoir, a horizontal well of 2500 feet length was drilled and completed with a standalone screen along with Autonomous Inflow Control Device (AICD) to avoid sand production and delay water production. The initial design for the AICD considered the variation of permeability, rock quality, pressure differential across the horizontal length including the operational factors. Accordingly, multiple scenarios using reservoir simulation built-in model (Petrel-RE) and Netool for ICD selection, design and placement where the geological properties of the model were updated based on the run NMR and Caliper logs while geo-steering the well. Also, a fine grid sector model was generated to assess optimum well completion design.

The AICD completion was successfully deployed and resulted in extending the well life by delaying water production and it is expected to get its ultimate benefit whenever starting the implementation of a water flood project near that producer well.
The Effect of Yaw Error on the Mooring Systems of Floating Offshore Wind Turbines in Extreme Weather Conditions
Authors: Hunsberger Evelyn R. , Hallowell Spencer T. , Fontana Casey M. , Arwade Sanjay R.
Published: September 2018

As floating offshore wind turbines (FOWTs) become the most viable option for wind farms in deeper waters, it is important to investigate their dynamic response in inclement conditions when failures, such as yaw misalignment, are more likely to occur. This research uses hour-long simulations in FAST, software developed by The National Renewable Energy Lab (NREL), to analyze the effect of yaw error on anchor tensions and platform displacements in both a traditional single-line wind farm geometry, where each anchor is connected to one turbine, and an optimum multiline anchor geometry, where each anchor is connected to three turbines. NREL’s 5 MW reference turbine on a semi-submersible base is analyzed using six realizations of each combination of co-directional wind and waves, wind speed and yaw error; resulting in 2,484 simulations in total. The variability in platform displacements and mooring forces increases as wind speed increases, and as yaw errors approach critical values. The angle of incidence of the co-directional wind and waves dictates which anchor experiences the most tension for both the single-line and multiline concepts. In the multiline geometry, the greatest increases in anchor tension occurs when the downwind turbine has yaw error. Yaw error increases the maximum anchor tension by up to 43% in the single-line geometry and up to 37% in the multiline geometry. In the multiline geometry, yaw error causes the direction of the resultant anchor force to vary by up to 20°. These changes in anchor tension magnitudes and directions are governed by the platform displacements, and are a direct result of the differences in the tangential and normal coefficients of drag of the turbine blades. When designing floating offshore wind farms, the influence of yaw error on loading magnitudes and directions are to be considered when determining the necessary capacities and calculating the corresponding reliabilities for wind turbine components.
Algorithm for Auto-Piloting Efficient Seismic Line-Change Turning Maneuvers
Authors: Grindheim Jan Vidar , Revhaug Inge , Welker Ken , Solheim Peder , Pedersen Egil
Published: September 2018

Seismic surveys for petroleum exploration utilizing towed streamers are typically performed line by line. Between the lines, a 180° turn of the complete towed gear is necessary. This operation is time consuming and thus expensive. There is evidence in the literature that the turn maneuver can be made more efficient by taking advantage of favorable sea currents [1].

In the present study, an autopilot algorithm is developed for performing efficient line-change maneuvers, taking into account sea current, if present. The algorithm controls the vessel path and speed during the turn, as well as the streamer bird forces. The algorithm has been developed and validated utilizing a proprietary towed seismic streamer simulator, employing realistic seismic streamer spreads.

Simulation results for ranges of sea current speeds and bearings are presented for two different seismic spread configurations. Turning times are compared to estimated traditional turning time ranges.
Two-Phase Model for Simulating Current-Induced Scour Beneath Subsea Pipelines at Different Initial Elevations
Authors: Lee Jun Y. , McInerney Jasmin B. T. , Hardjanto Fauzi A. , Chai Shuhong , Cossu Remo , Leong Zhi Q. , Forrest Alexander L.
Published: September 2018

When a subsea pipeline is laid on an uneven seabed, the pipeline can have an initial elevation, potentially compromising its on-bottom stability; scouring due to flow conditions around the pipe can further exacerbate the problem. We assess the capability of the two-phase Eulerian-Eulerian OpenFOAM solver, twoPhaseEulerFoam, in terms of predicting the equilibrium scour depth beneath a pipe at different initial elevations under a steady current for the live bed condition. The predictions were found to be in good agreement with published experimental and numerical results; however, similar to a recent study involving another two-phase Eulerian-Eulerian model, the scour time scale was under-predicted. The predicted equilibrium scour depth was seen to decrease with an increase in the initial pipe elevation. The numerical results were also compared to predictions that were made using previous empirical equations. The most comprehensive equation to date showed a good agreement with the present numerical results. We conclude that this open-source solver, twoPhaseEulerFoam, can be used to predict the equilibrium scour depth beneath subsea pipelines, with short computation times and negligible mesh dependency.
Development of Rapid Prediction Method for Residual Strength of Oil Tankers Subjected to Ship-Ship Collision
Authors: Baek Seung Jun , Sohn Jung Min , Paik Jeom Kee , Kim Sang Jin
Published: September 2018

Collision and grounding accidents account for more than half of all accident cases in most cases. Such marine accidents cause severe structural damage to the ship and lead to marine pollution as well as life and financial loss. For preventing the loss of property and pollution, and preparing a countermeasure, it is needed to predict a residual hull girder strength after accident regardless of someone’s special skill. The aims of this study are to i) investigate the residual hull girder strength by quantitative approach with collision location (height and penetration), ii) develop an empirical formula for calculating a residual hull girder strength which whoever can calculate in association with collision locations. In this study, three kinds of ships such as very large crude oil carrier, Suezmax, and Aframax class double hull oil tankers are selected as target struck vessels. And, the Intelligent Supersize Finite Element Method (ISFEM) is applied to assess the residual hull girder strength of damaged structures after collisions. Based on the ISFEM results, an empirical formula for calculation of residual hull girder strength is developed as a function of the collision depth and penetration. The developed formula in this study can be applied by anyone, and rapidly calculate its strength for preventing sequential events (collapse, fuel spill, etc.) after collision.
Numerical and Experimental Study on the Seakeeping Behavior of Floating Closed Rigid Fish Cages
Authors: Kristiansen David , Lader Pål , Endresen Per C. , Aksnes Vegard
Published: September 2018

The present paper addresses the seakeeping behavior of a rigid type of floating closed fish cages, with focus on effects of sloshing on the coupled motions and mooring loads. Closed cages have gained much attention recently as a strategy to avoid sea-lice infections in farming of Atlantic salmon. However, closed cages are novel structures and more knowledge is needed on the wave induced motions and coupling effects with sloshing for development of reliable closed cage structures to reduce risk for failure and possible escape of fish. In this paper, dedicated scaled model tests of closed cages in waves are presented and compared with numerical simulations using linear potential theory in frequency domain. The results shows that the influence of sloshing on the rigid body motion is significant. A nonlinear effect of sloshing was observed for a small region of excitation frequencies, where the surge amplitude increased with increasing wave steepness. Mean wave loads were also affected by sloshing. Hence, coupled motions with sloshing is important to consider in design of floating closed rigid fish cages and their mooring system.
Steady-State Cuttings Transport Simulation in Horizontal Borehole Annulus
Authors: Ignatenko Yaroslav , Bocharov Oleg , Gavrilov Andrey , May Roland
Published: September 2018

The paper presents the results of modeling the steady-state flow of drilling fluid with cuttings in an annulus for the flow regimes typical for horizontal drilling. The studied parameters include effects like fluid rheology, drillstring rotation and eccentricity on flow regime, pressure drop and cuttings bed.

It has been demonstrated that increasing the drilling fluid’s effective viscosity increases the pressure drop, but it decreases the cuttings bed area, while drillstring rotation significantly changes the flow structure, improving cuttings transport and reducing the pressure drop. The considered flow structure can change abruptly due to changed drill string positioning and rheological fluid properties. Such structural changes are followed by abrupt changes in the pressure drop and cuttings bed area.
On the Importance of the Exact Nonlinear Interactions in the Spectral Characterization of Rogue Waves
Authors: Ponce de León Sonia , Osborne Alfred R. , Guedes Soares Carlos
Published: September 2018

This work is focused on the analysis of the wave action equation with full 4-wave interactions (Snl4). For this purpose, we have applied a state-of-the-art spectral wave model (Wave Watch III), using an exact method for the calculation of the full nonlinear Boltzmann interactions in the evolution of the wave spectrum. We emphasize the use of the exact WRT method [Van Vledder, 2006] for the computation of the Snl4 interactions instead of the approximate DIA method. The WRT algorithm includes the full Boltzmann integrations. We discuss how the WRT method is important in any assessment of rogue waves in the ocean and discuss how the enhanced spectral peak assists the formation of rogue waves packets. We demonstrate how the most nonlinear part of the peak of the spectrum is reduced in amplitude when the nonlinear interactions are instead computed using the DIA interactions. These results suggest that a clear understanding of the physics of nonlinear interactions and of rogue wave dynamics requires the use of the full Boltzmann interactions. Future work would include faster WRT computations so that practical forecasting/hindcasting can become possible using the full four-wave interactions.
The Effect of Uniaxial Ratcheting on Piezomagnetic Response of X80 Pipeline Steel Considering Loading History
Authors: Bao Sheng , Lou Huangjie , Gu Yibin
Published: September 2018

The objective of this research is to investigate the effect of uniaxial ratcheting on the piezomagnetic field variations of X80 pipeline steel where the loading history is a major consideration. The piezomagnetic field variations of initially undemagnetized X80 pipeline steel specimen were recorded simultaneously by an APS 428D fluxgate magnetometer during the whole asymmetrical cyclic stressing process. The variations of the ratcheting strain and its rate with the changing mean stress were obtained. The influence of the loading history on the uniaxial ratcheting behavior was investigated. It was found that the prior loading step with higher mean stress greatly restrains the ratcheting behavior of subsequent loading step with lower mean stress. On the contrary, the prior loading step with lower mean stress does not play a significant role on the subsequent ratcheting behavior. The corresponding variations of the piezomagnetic hysteresis curve with the uniaxial ratcheting proceeding were systematically studied in terms of its shape and position, and the underlying reasons were discussed based on the magnetic domain theory and the dislocation theory.
Enhanced Kinematic Hardening Model for Load-Dependent Stiffness and Damping of Jack-Up Foundations
Authors: Hoogeveen Maas , Hofstede Hugo , Kaynia Amir M.
Published: September 2018

Dynamic analysis of jack-up platforms is generally carried out using approximated linear foundation springs and equivalent viscous damping. Advanced geotechnical analysis of foundations of jack-up platforms results in load-dependent stiffness and damping. Such analyses are often based on the finite element method as used for detailed site specific analyses with proper nonlinear soil models to generate nonlinear response curves, the so-called backbone curve, for the relevant loading conditions. The same FE model can be used to compute the strain energy in the soil elements and assign the corresponding energy losses in the elements based on lab tests or literature data, and integrate over the domain to compute the foundation hysteretic damping as function of loading. The state of the art method of using the backbone curve together with a kinematic hardening model to account for the hysteretic foundation response does not provide a good match between the simulated and computed damping. The hysteresis model proposed in this paper is a kinematic hardening model enhanced with a non-linear spring. It is an engineering solution to implement both a given load-dependent stiffness and load-dependent damping of a complex element subject to an irregular loading signal for purposes of time domain simulation. This model combines a kinematic hardening model which provides the required hysteresis with a non-linear elastic spring which provides the required stiffness. This model is suitable for time domain simulation of irregular loads and yields a propeller-like shape in the load-displacement plane. This paper introduces the problem of load-dependent stiffness and damping through a case study considering time domain simulation of the dynamic behavior of a jack-up platform. The paper presents a validation of the proposed model and a comparison between the common practice model and the enhanced kinematic hardening model.
Performance of WAVEWATCH-III and SWAN Models in the North Sea
Authors: Ponce de León Sonia , Bettencourt J. , Van Vledder G. Ph. , Doohan P. , Higgins C. , Guedes Soares Carlos , Dias Frederic
Published: September 2018

This paper presents the hindcast of the winter of 2013 in the North Sea using two wave models: WAVEWATCH-III (WW3) and SWAN. The performance of the WW3 and SWAN models was assessed for this winter, when successive storms hit the hindcast area in a short time period, and examined in terms of the averaged wave parameters (Hs, peak period and wave direction) and the power wave spectrum. The assessment was made from an operational point of view. Possible effects of the accuracy of the wind fields, the physics chosen in each model and numerical settings are discussed. We elaborate on efficiency, accuracy and grid issues for both models, aiming to provide guidelines for ocean engineering wave forecasts.
Effects of Hull Flexibility on the Structural Dynamics of a TLP Floating Wind Turbine
Authors: Silva de Souza Carlos Eduardo , Bachynski Erin E.
Published: September 2018

Structural analysis of floating wind turbines is normally carried out with the hull considered as a rigid body. This paper explores the consequences of modeling the pontoons of a tension leg platform (TLP) wind turbine as flexible structures. The analysis is based on numerical simulations of free decays, structural response to wave excitation and short-term fatigue damage accumulation at chosen points of the platform. In addition, the importance of considering hydroelasticity effects is evaluated. It is observed that pontoon flexibility can change the platform natural periods significantly, as well as the intensity and peak frequencies of internal structural loads. The adoption of a fully rigid-body is shown to be non-conservative for the fatigue damage analysis. Loads due to hydroelasticity have order of magnitude comparable to those related to rigid-body motions, but still lower enough to be considered of secondary importance.
Modelling Turbulent Flow in Deformable Highly Porous Seabed and Structures
Authors: Elsafti Hisham , Oumeraci Hocine
Published: September 2018

In this study, the fully-coupled and fully-dynamic Biot governing equations in the open-source geotechFoam solver are extended to account for pore fluid viscous stresses. Additionally, turbulent pore fluid flow in deformable porous media is modeled by means of the conventional eddy viscosity concept without the need to resolve all turbulence scales. A new approach is presented to account for porous media resistance to flow (solid-to-fluid coupling) by means of an effective viscosity, which accounts for tortuosity, grain shape and local turbulences induced by flow through porous media. The new model is compared to an implemented extended Darcy-Forchheimer model in the Navier-Stokes equations, which accounts for laminar, transitional, turbulent and transient flow regimes. Further, to account for skeleton deformation, the porosity and other model parameters are updated with regard to strain of geomaterials. The presented model is calibrated by means of available results of physical experiments of unidirectional and oscillatory flows.
Unsteady Viscous CFD Simulations of KCS Behaviour and Performance in Head Seas
Authors: Guo LiXiang , Yu JiaWei , Chen JiaJun , Jiang KaiJun , Feng DaKui
Published: September 2018

It is critical to be able to estimate a ship’s response to waves, since the added resistance and loss of speed may cause delays or course alterations, with consequent financial repercussions. Traditional methods for the study of ship motions are based on potential flow theory without viscous effects. Results of scaling model are used to predict full-scale of response to waves. Scale effect results in differences between the full-scale prediction and reality. The key objective of this study is to perform a fully nonlinear unsteady RANS simulation to predict the ship motions and added resistance of a full-scale KRISO Container Ship. The analyses are performed at design speeds in head waves, using in house computational fluid dynamics (CFD) to solve RANS equation coupled with two degrees of freedom (2DOF) solid body motion equations including heave and pitch. RANS equations are solved by finite difference method and PISO arithmetic. Computations have used structured grid with overset technology. Simulation results show that the total resistance coefficient in calm water at service speed is predicted by 4 .68% error compared to the related towing tank results. The ship motions demonstrated that the current in house CFD model predicts the heave and pitch transfer functions within a reasonable range of the EFD data, respectively.
Multibody Dynamics Simulation on Ice Resistance of Ship in Pack Ice Environment
Authors: Kim Jongyoung , Park Jongchun , Kim Sungpill , Kim Hyunsoo , Cho Yongjin
Published: September 2018

In this study, we used multi-body dynamic (MBD) technique to numerically calculate the ice load of the icebreaker in pack ice conditions. Because MBD is a useful method for analyzing the dynamic behavior of a rigid body using the contact mechanism, MBD analyzes the interaction between each ice and icebreaker. Each ice and icebreaker also interact with the fluid. Simultaneously analyzing the interaction between fluids and float is difficult and complex, so Computational Fluid Dynamics (CFD) is used to simulate fluid-ice interaction and apply it to ice modeling. The simulation was performed in the time domain and compared to the average ice impact load obtained from experiments in an ice traction tank.
Vortex Induced Vibrations and Fatigue Assessment of Pipe-in-Pipe Systems
Authors: Yang Zhengmao , Thorkildsen Fartein , Norland Kristian
Published: September 2018

The high thermal insulation potential of a pipe-in-pipe system makes it the preferred solution for challenging flow assurance conditions. Due to the higher bending stiffness of a pipe-in-pipe system, longer free spans would be expected for pipelines resting on uneven seabed. However, there are no clearly defined standard formulae for the calculation of structural response of free spanning pipe-in-pipe system exposed to vortex induced vibration (VIV) and the resulting fatigue damage. If the same method as for a single wall pipe was applied, the combined equivalent pipe properties would be assumed and the VIV response and stresses of the equivalent pipe could be obtained. However, the longitudinal stresses in the inner and outer pipe to be used for the fatigue assessment of the girth welds would not be easily obtained, especially for sliding pipe-in-pipe systems.

Based on previous experience and development work for pipe-in-pipe systems, a numerical model for VIV assessment of sliding pipe-in-pipe systems is proposed giving improved interpretation of individual pipe characteristics. Modal analyses of sliding pipe-in-pipe systems are performed by using this numerical model. The natural frequencies and mode shapes are extracted. According to the numerical analysis results, the longitudinal stress ranges due to VIV are obtained and fatigue assessment of the pipeline girth welds for the inner and outer pipes are performed. In order to understand the interaction between the outer and inner pipe, the effect of friction and initial gap between the centralizers and outer pipe surface are studied.
Numerical Study for the Progressive Collapse of Scale Model for ULCS Hull Girder Under Longitudinal Bending
Authors: Wang Chonglei , Wang Deyu
Published: September 2018

For evaluating reliable load carrying capacity of actual hull girder, experiments for similar scale model are necessary. The object of the present paper is to figure out a reliable FE analysis method in the similar scale model experiment regarding hull girder ultimate strength. The compared results between the true hull girder of a typical ultra large container ship (ULCS) and the scale experimental model created by the similarity criterion proposed in this paper assessed by finite element method (FEM) under longitudinal bending moment considering the effects of initial deflections are obtained guaranteeing the similarity in both elastic and inelastic range during the progressive collapse of plates, stiffened panels and hull girder. Finally, a series of elastic-plastic large deflection analyses is conducted to ensure the failure mode of hull girder is consistent with the actual ship.
Fatigue Analysis of Metallic Strips Flexible Pipe
Authors: Sun Guowei , Han Peihua , Xu Yuxin , Bai Yong , Hameed Hamad
Published: September 2018

Metallic strips flexible pipe (MSFP) is widely regarded as an alternative for submarine pipelines. This paper presents a methodology for calculating the fatigue life of MSFP. Firstly, given a specific working condition of MSFP, the dynamic responses of MSFP are calculated through OrcaFlex. The obtained results from the global analysis are then implemented into a finite element model in ABAQUS to determine the stress-history curves of each steel strips layer. The estimated fatigue life is calculated by rainflow counting algorithms, S-N curve and Miner’s rule which are coded in MATLAB. Additional study about average stress correction is carried out, which might be useful for its marine engineering applications.
Survey and Analysis on Safety of Ship Mooring Operations in Japanese Ports Facing Open Seas
Authors: Sasa Kenji , Mitsui Masao , Tamura Masahiko
Published: September 2018

Ships have two major roles, namely navigation in oceans and cargo handling in ports. Various factors must be considered to maintain the safety of ships in all situations, from oceans to ports. The case of ferry ports was already presented by the authors. However, some important factors are still unknown, and thus, a nationwide survey is conducted to clarify them. The first survey is a questionnaire, in which responders point out various problems that prevent the maintenance of safety in ports. In particular, there are many ports with insufficient facilities because of the financial deficit. There are also differences in awareness on how to improve the current situation between the shipping companies (users) and local governments (port managers). The second survey, interviews during visits, is conducted in three ports. Detailed situations regarding mooring difficulties are detected. Those points are summarized with the aim to improve safety considering the standards of port planning under a financial deficit situation.
Refinement of Approaches to Estimation of Ship Ice Resistance in Ice Channel Based on Data From Physical Model Experiments
Authors: Dobrodeev Aleksei
Published: September 2018

The most urgent task in tapping of the Arctic resources today is organization of year-round navigation through the Northern Sea Route. The efficiency of any transport system strongly depends on the deadweight and speed of cargo carriers. In this connection it is required to thoroughly examine the issues related to propulsion of large-size cargo ships in ice, while developing the tactical planning and operation control technologies for icebreakers and ice-going ships. It is found that large-size cargo ships are not able to navigate independently in ice at economically efficient speeds even if their hullform is optimized for these operating conditions. Thus, it is required to ensure their effective operation in ice channels made by icebreakers. For appraising the efficiency of a marine transportation system it is necessary to have data on ship resistance versus a range of factors including physical and mechanical properties of ice, ice concentrations, ship speed, etc. This paper presents the results of studies undertaken to examine various methods of icebreaker-assisted shipping operations and to develop a method for calculation of ship resistance in ice channel based on model test data. The research was supported by RSF (project No.17-79-20162).
Challenges of Speedy Icebreaker-Assisted Operation of Heavy-Tonnage Vessels in Ice
Authors: Dobrodeev Aleksei , Sazonov Kirill
Published: September 2018

The Arctic plays a key role in the economic development of Russia. This region is fabulously rich with hydrocarbons and biological resources. One of the strategic goals in commercialization of the Arctic region is setting up of efficient transportation systems for year round navigation via the Northern Sea Route (NSR) to support shipment of various cargoes [1]. According to the latest studies, cargo vessels should be able to travel through NSR at speeds reaching 12 knots to make it a commercially competitively route. Highspeed moving can allow NSR to become competitive route as compared to the southbound route via Suez. It should be noted that ice conditions on this route are quite severe. Navigation in NSR of even ice-capable cargo carriers with icebreaker assistance will enable to increase the effectiveness of this shortcut shipping itinerary between Europe and the Pacific coast. For this purpose a novel nuclear Leader icebreaker has been designed, which, according to model tests in the Krylov Centre ice basin, will be able to sail in 2-meter continuous ice at 12 knots. The investigations of ship performance in ice at fast speeds are quite new and should be conducted very carefully. This paper focuses on some specific features of dynamic behaviour predicted for an icebreaker and a large-size vessel led by this icebreaker during speedy sailing in ice. It also discusses other important issues related to minimum power level requirement for vessels operating under these conditions as well as due account of the hydrodynamic resistance component in ice performance predictions.
Mechanical Behaviors of Metallic Strip Flexible Pipe Under Axisymmetric Loads
Authors: Jiang Kaien , Liu Ting , Yuan Shuai , Bai Yong
Published: September 2018

Metallic Strip Flexible Pipe (MSFP) is regarded as a promising alternative for submarine pipelines. During the operation and service phases, MSFP will inevitably experience significant tension as well as internal and external pressures. In order to ensure the security and reliability of MSFP in the application, the mechanical responses of MSFP subjected to axisymmetric loads should be carefully estimated. In this paper, a theoretical model estimating the tension stiffness under axisymmetric loads is established, which takes the material plasticity into consideration. Additionally, a finite element model is developed by ABAQUS to verify the feasibility of the proposed theoretical model. The results from full-scale tensile tests of MSFP are also adopted to verify the proposed method’s validity in calculating its tension behavior. The coincidence between the results from different methods illustrates the accuracy and reliability of the proposed theoretical model. Based on the parametric study, some interesting conclusions have been obtained which may be useful for the practical application of MSFP.
Estimating Storm Surge Design Parameter for Oil and Gas Exploitation in the Blocks of Liaodong Bay
Authors: Dong Sheng , Zhai Jinjin , Huang Weinan , Wang Zhifeng , Zhang Ri
Published: September 2018

Liaodong Bay is located at the northern part of the Bohai Sea. There are four blocks where oil and gas are found. In order to exploit these resources in these connected four blocks, extreme value statistics is used to estimate the storm-surge design parameters. Long-term series of storm surge are hindcast by numerical models. Using data from series of extreme storm surge caused by tropical cyclones in the Liaodong Bay, statistical analysis of extreme storm surge is carried out with Poisson-generalized extreme value distribution by considering the occurrence frequencies of tropical cyclones. The 100-year return storm surge is 195cm and the 50-year storm surge is 164cm. Both of them occur in Block 1. Due to the influence of topography of the total four blocks, the intensity trends of storm surges in Blocks 1, 2 and 3 are similar, whereas the intensity of storm surge in Block 4 is relatively small. These results are reference for the structural design in these blocks.
A Study on the Dynamic Ultimate Strength of Global Hull Girder of Container Ships Subjected to Hogging Moment
Authors: Yamada Yasuhira , Kameya Kyoko
Published: September 2018

The purpose of the present study is to investigate dynamic ultimate strength of global hull girder of container ships using large scale non-linear finite element analysis. A series of time domain non-linear FE-simulation is carried out using large scale FE models of a 8000 TEU container ship where a hogging moment is applied to the midship section. 5 types of finite element models (three full models, a half hold model, a 1 transverse model) are used. These models adopt elasto-plastic material model which includes strain rate effect.

The hogging moment which is modeled by sinusoidal impulse is applied to these models, and collapse mechanism as well as dynamic hull girder ultimate strength is investigated by varying the load time duration. Moreover effects of load time duration, mass inertia, strain rate and analysis models are investigated in detail. It is found from the present study that ultimate strength as well as collapse mode are significantly dependent on load time duration of hogging moment.
Fracture Mechanics Based Mooring Chain Fatigue Analysis for a Semi-Submersible Subjected to Tension and Torque
Authors: Xue Xutian , Chen Nian-Zhong
Published: September 2018

Mechanism of torque induced by chain twist and its effects on the predicting fatigue lives of mooring chains of a semi-submersible operated in OWA (Offshore Western Africa) are investigated in this paper. The fatigue lives of mooring chains are estimated based on a fracture mechanics analysis. Stress ranges on the mooring chains induced by tension ranges acting on the twisted chains are achieved by a finite element analysis. The low frequency (LF) and wave frequency (WF) tension processes induced by the motions of LF and WF are regarded as two random loading processes and the combined tension process of LF and WF is predicted by a dual narrow-band method. The mooring chains are treated as round bars and initial surface cracks are assumed to propagate at the surface of mooring chains. The influence of the chain twist on the fatigue lives of mooring chains is investigated and the results show that when the twist angle is less than 20 degree, fatigue lives of mooring chains are increased with the increase of the twist angle. However, when the twist angle approaches lock-up angle of 30 degree, fatigue lives of mooring chains are significantly decreased due to the torque effects induced by the chain twist.
Efficient Calculation of Low Frequency Motions for Air-Gap Prediction
Authors: Pan Zhiyuan , Vada Torgeir Kirkhorn , Nestegård Arne
Published: September 2018

The importance of the low frequency motions to air-gap estimation is evaluated on a column-based unit by using a computer program based on the linear and second-order potential theory. First and second order upwells are combined statistically and examined for different short term sea state conditions. To be able to speed up the analysis process, the potential flow code used in this study had been optimized in efficiency by introducing the multithreaded computation for the iterations of second order analysis.
Dynamic Responses of a Floating Tidal Turbine Considering Prescribed Wave-Frequency Motion
Authors: Guo Xiaoxian , Yang Jianmin , Li Xin , Lu Wenyue , Peng Tao , Li Jun
Published: September 2018

A numerical model based on blade element momentum theory was developed to investigate the effects of floater motion on the dynamic responses of a floating tidal turbine in this paper. SJTU-FTT, a floating tidal turbine system, was introduced. A simplified numerical approach was proposed, in which the 6-DOF motion of the floater was calculated in frequency domain in advance and then was transferred to the unsteady BEM model as inputs. The hydrodynamic coefficients including (added mass, damping, and RAOs) of the floater were presented. The response spectra of the thrust, torque and blade bending moments on the tidal turbine were discussed. Two peaks in the response spectra can be clearly identified, caused by surge and pitch motion of the floater. It was seen that the dynamic loads considering 6-DOF floater motion were much larger than the results from wave only cases.
Strouhal Number for VIV Excitation of Long Slender Structures
Authors: Potts Andrew E. , Potts Douglas A. , Marcollo Hayden , Jayasinghe Kanishka
Published: September 2018

The prediction of Vortex-Induced Vibration (VIV) of cylinders under fluid flow conditions depends upon the eddy shedding frequency, conventionally described by the Strouhal Number. The most commonly cited relationship between Strouhal Number and Reynolds Number for circular cylinders was developed by Lienhard [1], whereby the Strouhal Number exhibits a consistent narrow band of about 0.2 (conventional across the sub-critical Re range), with a pronounced hump peaking at about 0.5 within the critical flow regime. The source data underlying this relationship is re-examined, wherein it was found to be predominantly associated with eddy shedding frequency about fixed or stationary cylinders. The pronounced hump appears to be an artefact of the measurement techniques employed by various investigators to detect eddy-shedding frequency in the wake of the cylinder.

A variety of contemporary test data for elastically mounted cylinders, with freedom to oscillate under one degree of freedom (i.e. cross flow) and two degrees of freedom (i.e. cross flow and in-line) were evaluated and compared against the conventional Strouhal Number relationship. It is well established for VIV that the eddy shedding frequency will synchronise with the near resonant motions of a dynamically oscillating cylinder, such that the resultant bandwidth of lock-in exhibits a wider range of effective Strouhal Numbers than that reflected in the narrow-banded relationship about a mean of 0.2. However, whilst cylinders oscillating under one degree of freedom exhibit a mean Strouhal Number of 0.2 consistent with fixed/stationary cylinders, cylinders with two degrees of freedom exhibit a much lower mean Strouhal Number of around 0.14–0.15.

Data supports the relationship that Strouhal Number does slightly diminish with increasing Reynolds Number. For oscillating cylinders, the bandwidth about the mean Strouhal Number value appears to remain largely consistent. For many practical structures in the marine environment subject to VIV excitation, such as long span, slender risers, mooring lines, pipeline spans, towed array sonar strings, and alike, the long flexible cylinders will respond in two degrees of freedom, where the identified difference in Strouhal Number is a significant aspect to be accounted for in the modelling of its dynamic behaviour.
Dynamic Response Analysis on the Interaction Between Flexible Bodies of Large-Sized Wind Turbine Under Random Wind Loads
Authors: Li Yilun , Guo Shuangxi , Li Min , Chen Weimin , Kong Yue
Published: September 2018

As the output power of wind turbine increasingly gets larger, the structural flexibility of elastic bodies, such as rotor blades and tower, gets more significant owing to larger structural size. In that case, the dynamic interaction between these flexible bodies become more profound and may significantly impact the dynamic response of the whole wind turbine. In this study, the integrated model of a 5-MW wind turbine is developed based on the finite element simulations so as to carry out dynamic response analysis under random wind load, in terms of both time history and frequency spectrum, considering the interactions between the flexible bodies. And, the load evolution along its transmitting route and mechanical energy distribution during the dynamic response are examined. And, the influence of the stiffness and motion of the supporting tower on the integrated system is discussed.

The basic dynamic characteristics and responses of 3 models, i.e. the integrated wind turbine model, a simplified turbine model (blades, hub and nacelle are simplified as lumped masses) and a rigid supported blade, are examined, and their results are compared in both time and frequency domains. Based on our numerical simulations, the dynamic coupling mechanism are explained in terms of the load transmission and energy consumption. It is found that the dynamic interaction between flexible bodies is profound for wind turbine with large structural size, e.g. the load and displacement of the tower top gets around 15% larger mainly due to the elastic deformation and dynamic behaviors (called inertial-elastic effect here) of the flexible blade; On the other hand, the elastic deformation may additionally consume around 10% energy (called energy-consuming effect) coming from external wind load and consequently decreases the displacement of the tower. In other words, there is a competition between the energy-consuming effect and inertial-elastic effect of the flexible blade on the overall dynamic response of the wind turbine. And similarly, the displacement of the blade gets up to 20% larger because the elastic-dynamic behaviors of the tower principally provides a elastic and moving support which can significantly change the natural mode shape of the integrated wind turbine and decrease the natural frequency of the rotor blade.
Effects of Correlation Between Waves and Currents on Pipeline Free Span VIV Fatigue: A Case Study
Authors: Besedin Dmitry , Peek Ralf , Ang Sze Yu , Vedeld Knut , Fyrileiv Olav , Gulyaev Alexey
Published: September 2018

In shallow waters, subsea pipelines can suffer fatigue damage from Vortex Induced Vibrations (VIV) by the combined effects of waves and currents.

A full characterization of the joint probability distribution of waves and currents involves at least 5 variables, 2 for currents (magnitude and direction), and 3 for waves (significant wave height, mean wave direction, and wave period at the peak of the spectrum). In lieu of sufficient data to adequately characterize the associated joint probability density function, DNV GL in their “Recommended Practice” propose an approximation: Assume that the direction of current and wave effects is always the same, but for a given direction, waves and currents are assumed statistically independent.

In this paper 28 years of hindcast data are used to test the accuracy of this colinearity approximation in the Sea of Okhotsk. Rather than attempting to estimate a joint probability density function in 5 variables, the span fatigue assessment is simply performed for the entire 28 years of the hindcast database to obtain an average rate of fatigue damage. It is found that this history-based approach can lead to fatigue damage rates that are much higher than those derived from the colinearity assumption.

This non-conservatism of the colinearity assumption, arises for pipeline orientations for which both waves and currents can contribute strongly to the VIV response without being exactly colinear.

It is concluded that caution is needed in using the colinearity assumption, but an update of the span assessment procedure should also address issues for which current assessment procedures are conservative, such as seabed proximity and trench effects, nonlinearity in the pipe-soil interaction, and the change in axial forces due to transverse displacements of the pipe, which are expected under extreme combined current and wave conditions, as envisioned on on-bottom stability design criteria.
Prediction Error Statistics in Deterministic Linear Ship Motion Forecasting
Authors: Fucile Fabio , Bulian Gabriele , Lugni Claudio
Published: September 2018

Deterministic ship motions predictions methodologies represent a promising emerging approach, which could be embedded in decision support systems for certain types of operation. The typically envisioned prediction chain starts from the remote sensing of the wave elevation through wave radar technology. An estimated wave field is then fitted to the data, it is propagated in space and time, and it is finally fed to a ship motion prediction model. Prediction time horizons, typically, are practically limited to the order of minutes. Deterministic predictions are, however, inevitably associated with prediction uncertainty which is seldom quantified. This paper, therefore, presents a semi-analytical methodology for the estimation of ship motion prediction error statistics in ensemble domain as function of the forecasting time, assuming linear Gaussian irregular waves and stationary linear ship motions. This information can be used, for instance, to supplement deterministic forecasting with corresponding confidence intervals. The paper describes the theoretical background of the developed methodology and reports some numerical application examples.
Research on Hydrostatic Stability Calculation Method for Mobile Offshore Units
Authors: Ding Yong , Cao Shunli , Lan Linxin , Chen Mo , Pan Liming
Published: September 2018

Since most offshore platforms have comparable length and breadth, the transverse stability is not sufficient to estimate safety of floating platform. The current solution is to follow conventional transverse stability methodology to any orientation, then take the weakest one as a key consideration. However, the analysis ignores trim in all process of heeling. Longitudinal balance at each heel angle reduce the righting arm of platform, which is more reasonable.

In this study, the Euler angles are used as floating parameters of platform, and three kinds of longitudinal balance cases are divided. The oblique free trim method and the free twist method, two of the three cases mentioned, are chosen to establish floating equations of platform. The potential energy surface and the longitudinal coefficient of stiffness are applied to the analysis from standpoints of energy balance and moment balance separately, the former is used to get the most critical axis for a given heel angle, and the latter can be used as the stability criterion of floating condition. Following this idea, a numerical tool has been developed.

This paper describes the method used to estimate the stability of offshore platform, and validation work is presented for simple geometries. Then, results from two operational stability studies are discussed. Finally, possible further work is discussed.
Benchmark of Erosion Criteria in a Deepwater Development
Authors: Gómez-Álvarez Susana , García Ruíz Fernando , Merino-García Daniel
Published: September 2018

Ultra-Deep water projects worldwide and particularly in Brazil are characterized by high capital investment costs. This fact generated the need to explore opportunities to improve project value, even though the project was still during visualization phase.

One of the identified opportunities was related to API 14E erosion standard application, as it is considered to be conservative for estimating the erosional limits in oil and gas applications. There are additional industry practices that are widely used in industry that provide more accurate estimations of erosional velocity limits. If these values are increased, there is an opportunity to enhance project value as it would allow a reduction of the required number of wells and production line / riser diameters. If the API erosional limit is considered in the project, the wells would produce at lower rate than reservoir potential, which would lead to an increase in the number of development wells. Based on the significant perforation cost of deepwater wells, the increase on this number would be reflected in the total development Capex, eroding the project value.

This paper describes the work performed in order to better understand the possible erosion phenomena in the production system due to the high fluid velocities in a gas condensate field and to determine the maximum allowable gas rate with different erosional models (API 14E and Tulsa SPPS).

The proposed methodology provides the opportunity to enhance project value by predicting erosional rate limits via SPPS, as the current applied standard was considered to be conservative in erosional rate prediction. If confirmed as feasible, significant cost reduction could be achieved in the project as the individual gas well production could be increased by the reduction of the number of wells; estimated well cost reduction is around 400 MMUSD per well, plus associated subsea production system.