bluuebunny/crossref_metadata_2025 huggingface dataset:
Uncertainty Analysis of Free Running Manoeuvring Model Test on a Modern Ferry, With Emphasis on Heel Angles
Authors: Kisjes Anton , Quadvlieg Frans , Ferrari Victor
Published: November 2019

This paper presents an uncertainty study on the manoeuvring behaviour of a twin screw ferry. In particular, we are interested in heel angles that this ship achieves while manoeuvring. Earlier published uncertainty analysis has focused on the uncertainty of overshoot angles and tactical diameters see [1] and [2]. The heel angles of these ships are not large. However, there is a class of ships that may encounter large heel angles due to steering. Ferries are such ships. The present paper quantifies also the uncertainty of the measured heel angles due to manoeuvres.

During the model test series, results are obtained for various values of the stability (GM), where large heel angles are observed. This provides a unique insight in the relation between the GM, approach speed, directional stability and the achieved heel angles. Because of the demonstrated large heel angles, it was important to make an uncertainty analysis of these tests. More publications have been written on the uncertainty of overshoot angles and dimensions of turning circle manoeuvres. However, the uncertainty of heel angles during manoeuvres hasn’t been published yet, which makes this a unique paper. The uncertainty analysis will be based on repeat tests for the zigzag 10°/10° and 35° turning circle manoeuvres. Repeat tests are carried out for these manoeuvres to verify the mean and the uncertainty of the experimentally obtained values. The methodology for estimating the uncertainty with 95% confidence bounds are derived by accounting for 1) uncertainty from measurement, 2) repeat tests and 3) the uncertainty from propagation of initial conditions and the error in check angle and rudder. The uncertainty results are compared with a previous study of uncertainty of manoeuvring characteristics of model tests with the KVLCC2 [1] and [2].
Vortex-Induced Vibration of a Flexible Cylinder Experiencing Oscillatory Flow With Different Aspect Ratios
Authors: Deng Di , Wu Lei , Wan Decheng
Published: November 2019

In deep sea oil exploitation, offshore platforms will move periodically in the water under the combined effects of waves, currents and winds. The relatively oscillatory flow is generated between the riser connected to the platform and the water. Vortex-induced Vibration (VIV) features of a single cylinder in the oscillatory flow are more complicated than that in the uniform flow. In this paper, numerical investigations on VIV of a flexible cylinder with different aspect ratios exposed to the oscillatory flow are carried out by the in-house CFD solver viv-FOAM-SJTU, which is developed based on the open source toolbox OpenFOAM. The flexible cylinder is forced to oscillate harmonically in the in-line direction in the still water and is allowed to freely vibrate in the cross-flow direction. Firstly, comparisons on referred experiments and simulations are conducted to verify the validity of the solver. Then, the modal decomposition analysis method and the Fast Fourier Transform (FFT) method are used to obtain the dominant vibration modes and frequencies of the cylinder in the following simulations.
A Broadband Array Signal Processing Method Based on Joint Sparsity of Signal Spatial Domain
Authors: He Daqian , Zhang Dahai , Wang Congying , Peng Xirui
Published: November 2019

Broadband underwater acoustic signal direction of arrival (DOA) estimation method is an important part of underwater array signal processing. The commonly used array signal DOA estimation algorithms due to the restriction of algorithm principles, are unable to process broadband array signal effectively, at the case of the arriving signals have strong correlation, small sampling snapshots or small arrival angle. Therefore, we need a new efficient algorithm to meet the increasing demand of broadband under water acoustic signal processing method. This paper makes use of the broadband acoustic signal similarity of joint sparsity in signal spatial domain received by underwater sonar arrays, establishes the whole space grid covering all broadband frequency domain slices. On the basis, the global sparsity of each frequency domain slice is combined with sparse element extraction class algorithm. By integrating the energy of signal on each slice, the spatial sparsity of each slice is obtained, from which we can get the directions of the arriving broadband wave signals. Through the simulation analysis and experimental verification on lake, we can be see that: The SDJS algorithm improves the performance and signal processing capability of the algorithm compared with the traditional algorithms. Therefore SDJS algorithm has a widely range of research value and application space.
New Material Development for Offshore Mooring Chains: High Manganese Steel
Authors: Verma Neerav , Wasson Andrew , Li Zhen , Sidhar Harpreet , He Haiping , Jin HyunWoo , Jun HyunJo , Ozekcin Adnan , Ling Shiun
Published: November 2019

Oil and gas industry experiences indicate that corrosion and wear of conventional mooring chain is an issue, which can result in costly pre-emptive chain replacement in an offshore environment. There is a need to develop new material technologies with improved performance over conventional carbon steel mooring chains to improve chain reliability.

This paper summarizes the development work on one such material — High Manganese Steel (HMS). A version of this steel has been utilized for superior wear performance at oil sands operation in Canada. This paper describes details on HMS chemistry optimization and lab testing for mooring chain application. In addition, Gleeble experiments were carried out to understand HMS hot deformation behavior and weldability.

Development testing work on HMS material has shown promising results in terms of corrosion and wear performance, relative to carbon steel mooring chains.
Hydrodynamic Forces on Near-Bed Small Diameter Cables and Pipelines in Currents, Waves and Combined Flow
Authors: Griffiths Terry , Teng Yunfei , Cheng Liang , An Hongwei , Draper Scott , Mohr Henning , Fogliani Antonino , Mariani Alessio , White David
Published: November 2019

The on-bottom stability design of subsea pipelines and cables is important to ensure safety and reliability but can be challenging to achieve, particularly for renewable energy projects which are preferentially located in high energy metocean environments. Often these conditions lead to the seabed being stripped of all loose sediment, leaving the cables to rest on exposed bedrock where roughness features can be similar in size to the cables. As offshore renewable energy projects progress from concept demonstration to commercial-scale developments, new approaches are needed to capture the relevant physics for small diameter cables on rocky seabeds to reduce the costs and risks of export power transmission and increase operational reliability. These same considerations also apply to the cables and small diameter pipes — such as umbilicals — required by oil and gas projects located on rocky seabeds.

Recent experimental testing using the University of Western Australia’s unique Large O-tube has enabled the experimental measurement of hydrodynamic forces on small diameter cables and pipes in proximity to smooth and rough beds. The tested conditions extend well beyond the existing published parameter range including much higher KC conditions together with seabed roughness which is comparable in size to the diameter. The results provide design data of great relevance to the ongoing development of marine renewable and conventional oil and gas projects, especially on rocky seabeds. This paper presents a summary of the existing knowledge on the subject as a preface to preliminary test results and gives tentative conclusions on the likely outcomes from this work.
Critical Review of Early Age Cycling Effects on the Capacity of Pile to Sleeve Grouted Connections As Treated in ISO 19902
Authors: Merxhani Andi , Jensen Jacob Fisker , Caetano Joao , Christiansen Casper Klintø
Published: November 2019

The treatment of early age cyclic loading (EAC) on pile-sleeve grouted connections is one of the challenging problems encountered in the design of offshore jacket structures. ISO 19902 appears to be the only offshore structural design code that quantifies the strength de-rating due to EAC. However, the mechanism of EAC considered in the ISO standard is little understood. Main provision is that the strength reduction due to EAC is considered to be linearly proportional to the compressive strength of the grout. This provision is conservatively applicable for medium strength grouts. However, it is over-conservative when modern high-performance grouts are used, and it can be further argued that it actually loses its physical meaning. Thus, a further investigation into the mechanics of the problem is deemed beneficial in order to understand better its underlying mechanism.

Using existing experimental evidence and a simple mechanical model, it is shown that the EAC mechanism considered in ISO 19902 triggers the grout matrix failure mechanism of the grouted connections. This behavior is characteristic of segregating grout materials. The benefit of the approach followed is that it links the interpretation of EAC to the grout material properties. The phenomenon is then understood using methods and experimental results that are well established in concrete mechanics. After a comparison with existing experimental evidence on grouted connections and concrete members, it is suggested that the strength drop for the specific shearing mechanism is independent of the uniaxial compressive strength of the grout material.
Dynamic Response of a Conceptual Designed Articulated Offshore Wind Turbine
Authors: Li Yan , Liu Zheng , Tang Yougang , Zhu Xiyang , Zhang Ruoyu
Published: November 2019

Focus on the 75-meter-depth offshore area, an articulated buoy is proposed for supporting NREL 5 MW offshore wind turbine. Based on the optimization, the main parameters are set for better hydro performance and less environmental loads. According to the quasi-static approach, the intact stability was examined. Then, an in-house code was programmed to simulate the dynamic response of the articulated offshore wind turbine. The aerodynamic load on rotating blades and the wind pressure load on tower are calculated based on the blade element momentum theory and the empirical formula, respectively. The hydrodynamic performance is simulated by the 3-D potential flow theory. The motions of platform, the loads on the articulated hinge and the power generation performance are documented in different cases. According to the simulations, the articulated offshore wind turbine shows good hydrodynamic performance under operation conditions.
An Analytical Approach for Predicting the Collapse Pressure of the Flexible Risers With Initial Ovalization and Gap
Authors: Li Xiao , Jiang Xiaoli , Hopman Hans
Published: November 2019

A flexible riser is a flexible pipe that transports materials between seafloor and topside structures. As oil and gas production heads to water depths greater than 3000 meters, huge hydrostatic pressure may cause the collapse failure of flexible risers. Generally, the collapse strength of a flexible riser is designed by considering the effects of initial imperfections, e.g., ovality of the carcass, and radial gap between the carcass/liner and pressure armor. These two imperfections may cause a significant reduction in the collapse strength of flexible risers under the flooded annulus condition. However, there are few analytical models available in the public literature that could take those factors into account. In this paper, an analytical approach is presented to predict the critical collapse pressure of the flexible risers with initial imperfections. The analytical results were compared with the numerical simulation, which showed reasonably good agreement.
Wear Amount of Steel Structure in Ice-Infested Sea by Sliding Wear Test
Authors: Takeuchi Takahiro , Kioka Shinji
Published: November 2019

Surface rust layers of corroded steel of coastal structures is removed by sea ice action in winter, leaving bare mild steel. This phenomenon is repeated every winter, promoting wear due to increasing corrosion. Serious damage to training levees comprising steel-sheet-pile seawalls on the Sea of Okhotsk, JAPAN occurred earlier than expected, since large wear rates were not considered in their design. This paper proposes a simple method for estimating wear amount (thickness) per year based on wear rate in a steady (mild) wear region obtained from sliding wear tests. This wear thickness can thus be taken as an additional thickness for the expected lifetime of a structure, as a rational countermeasure to corrosion as a sacrificial layer.
The Hydrodynamic Analysis of Dolphin Fluke Motion With a Flexible Tail
Authors: Chen Xi , Yang Wenjing , Yu Jiawei , Feng Dakui , Wu Yongfeng
Published: November 2019

This paper studies the effect of the the caudal fin superimposed chordwise deformation on the propulsion performance. The calculation results show that the appropriate chord deformation of the caudal fin can effectively increase the propulsion efficiency. It describes the shape change of dolphin tail by mathematical formula, and studies the propulsion performance of dolphin tail by numerical method combined with morphing mesh technology. The quasi-propulsive efficiency is used to evaluate the propulsive efficiency of self-propelled objects. The results show that the active deformation of the caudal fin can effectively increase the propulsive efficiency and provide a theoretical basis for the design of the flapping bionic propeller.
Analysis of the Momentum Transfer Operated by the Breaking in Modulated Wave Trains in Wind and No-Wind Conditions
Authors: Iafrati Alessandro , Falchi Massimo
Published: November 2019

An analysis of the flow and of the vertical transfer of the horizontal momentum induced by the breaking of modulated wave trains in wind and no-wind conditions is presented. The study is based on the results of two-dimensional numerical simulation of the Navier-Stokes equations for two-phase flow. The open source Gerris flow solver has been used, which employs a Volume of Fluid technique to capture the air-water interface.

The breaking is induced through the Benjamin-Feir instability mechanism. The numerical simulations cover the entire range from the initial development of the instability, the breaking phase and the post-breaking evolution. In order to investigate the role played by the wind, a uniform wind profile, twice the phase speed, is initialized in the air phase and it is left to evolve while interacting with the wave system.

Results in terms of averaged horizontal velocity and vertical flux of horizontal momentum are presented. It is shown that in the wind case the backward stresses induced at the wave troughs as a consequence of the flow separation at the crest influence significantly the flow in the upper water layer, particularly in the pre-breaking phase. No substantial differences are found between the wind and no-wind solutions in terms of the vertical transfer of horizontal momentum in the lower water layer. The vertical flux of horizontal momentum in air is consistent with the velocity reduction occurring in the wind case in the early stage.
Ice Model Tests for Semi-Submersible Platforms in Pack Ice Conditions
Authors: Luping Liu , Xin Li , Xiao Wu , Bo Wu
Published: November 2019

As development of the Arctic grows in intensity, semi-submersible platforms are one of promising type of offshore structures used for arctic oil exploitation. Generally a good ice management is equipped by a moored floating platform to reduce ice loads to manageable levels, thus the most common scenario for a polar operating semi-submersible platform is pack ice conditions. The resistance test of a 4-columns structure is performed in a normal towing tank in China using synthetic non-refrigerated material with similar density to model sea ice. Three component load cells on top of each column and a batch of single component load cells embedded in the surface of the columns near the waterline are used to measure indirect and direct ice loads on the structures. The effects of a series of parameters such as column shapes, orientations, column spacing ratios, ice floe shapes, ice drift speeds and ice concentrations are analyzed.
Mitigated Riser Response During Submerged BOP Move Through Use of Drillstring Active Heave Compensation System
Authors: Sevillano Lucas Cantinelli , Morooka Celso Kazuyuki , Sangesland Sigbjørn
Published: November 2019

Rig costs account for a substantial part of the total cost of an offshore field development project. Systems and methods that can reduce the non-productive time are therefore of interest to operators. In deep and ultra-deep waters, the duration of drilling campaigns with multiple, closely located wells may potentially be reduced by optimizing equipment-retrieving procedures between moving from one drilling location to another. This may be achieved by not retrieving the whole drilling riser and the BOP to the surface. Instead, retrieving just a few joints and navigating the drilling rig with the drilling riser and BOP suspended may save several days of rig time.

Previous works on this topic have investigated the riser response during the move and delineated under which environmental conditions (waves and current) this operation may be safely executed. The axial displacement of the riser column may be of concern, and it has been presented that the hang-off condition may substantially influence the response of the riser. Instead of looking at the riser hanging either from the spider-gimbal or from the tensioners, this work proposes to use the active heave compensation system of the drillstring to more efficiently decouple motions of the rig from the riser during the move. Thus, mitigating the riser response and enabling the navigation with the suspended riser and BOP to take place under broader environmental conditions.

A commercial program for computational simulations of the riser dynamic responses was used in the study presented in this paper. A representation of the active heave compensation drilling system and the tool used to achieve this purpose were added to the model. The responses of a typical riser used in deep water were determined under several environmental conditions, and the responses of different hanging systems were compared. Results indicate that the proposed approach for suspending the riser may decrease the dynamic forces and stresses during the rig move. The ability of the modeled rig equipment to withstand the move was also investigated.
Improvement on Toughness of Weld Heat Affected Zone of Cu-Containing Low Alloy Steel of Long Scale Forging for Offshore Applications by Optimizing Chemical Composition
Authors: Honma Yuta , Sasaki Gen , Hashi Kunihiko , Minami Fumiyoshi
Published: November 2019

Copper-containing low alloy steel based on ASTM A707 5L grade is widely used for structural parts of offshore wells. Applications of the steel for Ultra-deepwater development require excellent low temperature toughness from the viewpoint of marine accident prevention. However it is difficult to stably obtain good weld joint toughness because the welding condition is inevitably scattering. With those backgrounds, this paper focuses on metallurgical factors controlling the HAZ toughness of A707 modified steel. Potential factors considered are the grain size, M-A and precipitates. A challenge is demonstrated to improve the HAZ toughness by optimizing the Cu and Mn contents.

In this study, we investigated mechanical properties including crack tip opening displacement (CTOD) and we observed microstructure using welding tests or various weld heat cycle specimens. The weld heat affected zone (HAZ) of a conventional material had good toughness for the low heat input condition. However it was remarkably decreased for the high heat input condition due to the precipitating martensite-austenite constituent (M-A) in local brittle zones (LBZ).

The weld test results indicated the importance of suppressing the formation of M-A in order to improve toughness in the HAZ of the steel. Thereby, we challenged the optimization of chemical composition for HAZ toughness improvement. Cu had no bad influence on the HAZ toughness. It was demonstrated that the HAZ toughness is recovered by good use of Cu precipitates in SC cycle. Moreover the area fraction of M-A is decreased in keeping with Mn content, which leads to the improvement of the ICCG HAZ toughness. Based on our study, the recommended amounts of Cu and Mn are more than 1.0 mass% and less than 0.6 mass%, respectively, to ensure the HAZ toughness, especially ICCG HAZ toughness.
Multi-Focused Wave Groups in Wave Flume
Authors: Fang Qinghe , Zhao Cunbao , Guo Anxin
Published: November 2019

People can simulate extreme hydrodynamic conditions in a laboratory facility by interfering a numbers of regular waves at a certain point in space and time, which is focused wave. It is obviously higher and steeper than any other wave, e.g. regular or irregular waves, within the propagating wave group. The focused wave occurs at a designed point both in space and time. It represents an event with a large return period which would take a long time to reproduce within a random wave sequence. The focused wave, representing of a large wave occurring in a random sea, is quite frequently used to investigate wave loading on marine or coastal structures. However, most research only employ one single focused wave group. Taking the randomness of the wave-structure interaction, repeated tests would be suggested by some textbooks or codes to eliminate the odd results. However, it would take more time to conduct those tests no matter in the laboratory or in the numerical simulations.

In our present work, we use a novel method to experimentally generate several focused wave group with different focus time but same focus point at the same time to obtain multi-focused wave groups. The wave elevation and water particle kinematics are measured. The influence of peak frequency, frequency resolution and period of focused wave group are checked and discussed. The results show that present method can generate stable and repeatable focused wave groups in the wave flume.
Analysis of Heat Transfer Performance for Deepwater Phase Change Material Sandwich Pipes
Authors: An Chen , Wang Hui , Duan Menglan
Published: November 2019

As the exploitation of oil and gas gradually enters the deep sea, the low-temperature and high-pressure deep-sea environment poses a huge challenge to the flow protection of pipelines (2014a). In this paper, the phase change material sandwich pipeline which uses phase change heat storage and exothermic to maintain the pipeline temperature is taken as the research object, the heat transfer characteristics of the deep-water phase change material sandwich pipe are studied through the combination of theoretical analysis and numerical simulation (2014b). The main contents include: Firstly, through the establishment of two-dimensional and three-dimensional pipe models, analyzed the temperature distribution along the pipeline and the radial temperature distribution of the pipeline under steady oil flow conditions. Secondly, by using transient heat transfer, the effects of phase change material parameters, the proportion of phase change material in the insulation layer, and the difference in the ratio of phase change materials in the insulation layer on the insulation performance are analyzed to obtain the best results. Insulation material and optimal insulation layer layout; finally, the thermal storage and the phase change conditions of the phase-change material sandwich pipe is studied under the re-starting condition. The results show that the effective holding time of the phase change material insulation layer is close to 1.4 times that the non-phase change material insulation layer, and the melting point size has little effect on the insulation material. The closer the phase change material is to the inner tube, the better the insulation effect. This study provide guidance for the design and utilization of phase change material sandwich pipe.
Effect of the Wind Drag Estimation Methods on Numerical Storm Surge Modeling
Authors: Shankar C. Gowri , Behera Manasa Ranjan
Published: November 2019

Tropical cyclones have always proved the extent of its catastrophe on several occurrences over the years. In particular, the Bay of Bengal (BoB) basin in the Northern Indian Ocean has produced such historic devastating events, thereby mandating accurate real-time predictions. Numerical modeling of storm surge has always been an arduous task, as it is integrated with various uncertain factors. Among those, the major governing component being the wind forcing or the wind stress — that signifies, the computational accuracy of simulated surge and wave parameters. The present study is aimed at analysing the most suited wind drag evaluation method for real-time predictions of storm surge along the BoB. Cyclone Phailin (2013) was considered for the numerical simulations. To evaluate the wind drag coefficient, three most extensively used linear empirical relations along with the enhanced Wave Boundary Layer Model (e_WBLM) were used. The surge was subsequently simulated (using the coupled hydrodynamic circulation and wave model: ADCIRC and SWAN, respectively), individually for each of the above wind stress methods to obtain the corresponding storm surge (residual) and the storm wave features. The modeled values were further validated with the in-situ data obtained from tide gauge station and buoys respectively. It was quite intuitively observed that, e_WBLM based results correlated well with the in-situ values than its linear counterparts since, the former pragmatically includes the effects of air-sea interaction at high wind speeds in the model. The e_WBLM-based computation of significant wave heights (Hs) in deep as well as shallow water, nevertheless enabled efficient and reasonably-reliable estimations of the peak incidents.
Power Spectrum for Surface Description of Corroded Ship Structure From Laser Scan
Authors: Neumann Karoline M. , Ehlers Sören
Published: November 2019

Strength of corroded marine structures is in the industry traditionally assessed using average thickness reduction together with the assumption of a uniform surface. In academia it is however realised that the surface texture will influence the strength where the effect of the irregular surface is considered through various parameters such as reduced material properties, roughness, geometrical ratios and thickness distributions. In this paper, the objective is to present a power spectrum which describes the corroded surface topography. Therefore, power spectra of surface and thickness are produced from 3D laser scan of real corroded mild steel specimens from a 30 year old ship using Fast Fourier Transform (FFT). The frequency distribution describes the spatial variability, wherefrom the autocorrelation function (ACF) is deduced. A log-log fit is made to the power spectrum, where the interception with the ordinate axis describes the corrosion severity. This way of describing the surface pattern of the corrosion deterioration may serve as input to novel ways of calculating corroded strength and fatigue initiation as well as simulation of corroded surfaces.
Fatigue Life Estimation for HFMI Treated Weldments Considering Weld Toe Magnification Factor
Authors: Kim Dong Yub , Kim Myung Hyun
Published: November 2019

HFMI (High-Frequency Mechanical Impact) post-treatment is a statistically proven method to significantly improve fatigue life of welded structures. Positive effects of improved fatigue life by the HFMI treatment are attributed to the weld toe geometry and the compressive residual stress. This study investigates the effect of geometric and mechanical effects by HFMI treatment on fatigue life with an explicit consideration of the weld magnification factor Mk.

The conventional fatigue life estimation method to assess the effect of HFMI treatment by an experiment and analysis involves significant expense and time. In this regard, this study suggests an improved method by using Mk weld magnification factor and the stress ratio to assess the effect of HFMI post-treatment in more efficient manner. First step of this method is to calculate Mk weld magnification factor in HFMI treated condition to consider the geometrical effect by HFMI treatment at weld toe region. Second, the stress ratio model is introduced to consider the compressive residual stress by HFMI treatment based on Paris equation. The result is obtained by Paris-Erdogan equation for a S355 material. The result from this study is validated by a comparison with a previous fatigue test of HFMI treated welded specimen.
A Basic Study on Influence of Airchamber Volume on OWC Models to Power Conversion Performance
Authors: Ikoma Tomoki , Kihara Yoshiyuki , Hirai Shota , Aida Yasuhiro , Masuda Koichi , Eto Hiroaki
Published: November 2019

Oscillating water column (OWC) types of wave energy converters have still several issues to be studied because system including wave-air-turbine interaction is complex. Besides, scale effects between small scale models and full scale models have not been definite. Although the effect in initial design of OWC devices might been able to be ignored, it is important to understand its characteristics exactly. The present study carried out forced oscillation tests in a water tank corresponding to the radiation problem. Three models whose scale are 1/1, 1/2 and 1/4 were used. In the forced oscillation test, we measured mean water elevation in an airchamber to predict the air flow rate and air pressure inside of the airchamber. From the mean water elevation and the air pressure, the primary power conversion was calculated. As a result, the pressure and the elevation were affected by not only the scale reduction but also the difference of airchamber volume. Variation of the volume of an airchamber affected on hydrodynamic performance sensitively.
The Design and Simulation of Hull Segmentation Docking and Correcting Hydraulic System
Authors: Zeng Yuhao , Yu Zuyao , Xu Tang
Published: November 2019

A hydraulic system for large cylindrical hull segmentation docking and correcting is developed. This system measures the roundness error of the hull before the welding phase and then corrects it by cold-pressed deformation and hydraulically locked until the end of the welding. The automation of the hull docking and correcting is implemented, the work efficiency and the accuracy of the hull manufacturing are improved. The design of the hydraulic system for docking and correcting and the simulation of it in AMESim are mainly introduced in this paper.
An Improved Methodology to Assess Weldability of Line Pipe Steels
Authors: Alleva Laura , Monti Mauro , Paravicini Bagliani Emanuele , Paggi Alessandro , Darcis Philippe
Published: November 2019

The exploration of oil & gas fields towards more demanding environments require the continuous development of line pipe steels capable to withstand severe working conditions (high pressure, high temperature, sour environment, large deformations) maintaining good weldability in terms of hardness control in the Heat Affected Zone (HAZ). Therefore, the response to welding thermal cycles needs to be taken into account since the first steps of the product development. This paper describes a methodology to ascertain the weldability of new line pipe steels.

A matrix of welding trials, based on different welding technologies, was designed; the manufactured welds were assessed in terms of hardness and microstructure using standard and non-standard approach.

Gas Tungsten Arc Welding (GTAW) cold wire technique with anthropomorphic robot was selected as principal process, being extremely robust, accurate and repeatable and able to reproduce with good accuracy the thermal field of different arc welding operations. Furthermore, GTAW process permits to manage separately the so called “Arc Energy” (Volt*Ampere) and wire feeding: this additional degree of freedom gives the possibility to produce different combinations of Heat Inputs and Diluting Ratios. Single and multi-pass bead on plates (WT∼40mm) with no pre-heating were used to promote high restrained configurations and high residual stresses to reproduce the worst welding condition. Furthermore bead on plate have been performed by Nd:YAG laser to generate fastest cooling rates and narrowest welding pools.

The investigation activity was performed according to the API RP 2Z for what concerns the characterization of the main Heat Affected Zone both for single and multi-pass. For the individuation and investigation of the HAZ sub-zones a thermal model has been selected, validated by dedicated instrumented welding activity and used to predict the thermal cycle in selected points/zones of the HAZ (e.g. CGHAZ, FGHAZ...).

In depth metallographic/hardness analysis was performed on the welded samples using non-standard methods: low, medium and high magnification optical microscope analysis to generate a complete morphological map, suitable hardness paths with low and standard loads along the main direction of heat propagation in the bead and “tint etching” to better individuate the phase fractions of the different sub-zones of the HAZ.

Finally, proprietary and commercial predictive tools, using different input parameters (e.g. heat input, cooling rate, composition...) were used to calculate hardness and phase fractions under the experimental operative conditions. Results were compared to those obtained by the experimental activity.
A Hybrid Power Generation Platform Combining Floating Wind Turbine and Oscillating Water Column Wave Energy Converters
Authors: Chen Zheng , Zeng Weijian , Tan Ming , Zhang Dahai , Si Yulin
Published: November 2019

Recent years have seen rapid development in offshore wind technology. Particularly, floating offshore wind turbines possess great potential in deep water coastal places around the world, though they are now still in the demonstration phase. At the same time, the unused wave energy is also abundant at the sites of offshore wind farms, especially those in deep sea regions. Collecting wave energy in offshore wind farms might benefit both total energy production and reduce maintenance cost. Therefore, integrating offshore wind turbine with wave energy conversion devices could be a good idea to achieve higher efficiency and lower cost.

In this paper, we report a combined wind and wave energy power generation concept called WindOWC, which constits of a 5MW wind turbine and three oscillating-water-column (OWC) wave energy converters (WECs). The wind turbine is mounted on a semi-submersible floating platform, which is similar to OC4-semibsubmersible, and the OWCs are located in its three offset columns. In this design, the wind turbine and WECs share the same supporting platform and the power transmission system, thus is expected to reduce the cost of energy. Also, it is possible the OWCs may improve the platform dynamic performance by providing positive damping through controlling the air turbine rotational speed. In this work, we describe the geometry properties of the proposed WindOWC concept and conduct preliminary hydrodynamic analysis using potential flow theory. The ANSYS AQWA is used to obtain the system dynamic responses in frequency and time domain, respectively. The OWC dynamics and expected positive damping from them will be investigated in the future.
Fatigue Performance of High Strength and Large Diameter Mooring Chain in Seawater
Authors: Zhang Yan-Hui , Smedley Philip
Published: November 2019

Fatigue design recommendations provided by API RP 2SK, ISO 19901-7 and DNVGL-OS-E301 for studless chain links are based on data of steel grades R3 and R4 and mainly of link diameter of 76mm. Mooring systems utilising larger diameter links and higher strength steels (e.g. grade R5) are now in operation. Consequently, industry expressed a need for fatigue test data in seawater of higher steel grade and larger diameter chain to confirm whether the existing fatigue design guidance is applicable. A joint industry project (JIP) was launched by TWI to investigate fatigue performance of high strength and large diameter mooring chain in free corrosion seawater. A test rig was designed and manufactured which was capable of testing studless mooring chain links up to 127mm link diameter under tension-tension loading. Twenty-three full-scale fatigue tests were conducted on high strength steel grades (R4 and R5) and larger diameter chains (76mm and 127mm) generating 72 link failures. Magnetic particle inspections (MPI) were carried out to characterise the location of cracking, crack size and crack growth rate. This paper describes the results obtained in the JIP.
Effect of Weld Porosity on Super Duplex Stainless Steel Umbilical Tubes Under Hydrogen Induced Stress Cracking Exposure Condition
Authors: Socariceanu M. , An X.
Published: November 2019

Super Duplex Stainless Steel (SDSS) is the preferred material utilized in steel tube umbilicals, due to its excellent corrosion resistance and high mechanical strength. SDSS is susceptible to Hydrogen Induced Stress Cracking (HISC) under the influence of a Cathodic Protection (CP) system, especially the welded zone. Porosity (relevant rounded indication) is one of the most common weld flaws that impacts the SDSS weld reject rate based on historical acceptance criteria.

A study has been carried out investigating the influence of weld porosity on 25% Cr (SDSS) butt welded tube, under the effect of CP potential by using a Slow Strain Rate Testing (SSRT) technique. The study focused on the correlation between the level of porosity and the material mechanical performance when exposed to a simulated CP system in seawater at an elevated temperature.

The impact of HISC was evaluated through an assessment of various tube welds including welds with different levels of porosity quantified by number, size and location of pores, when exposed directly to natural CP system in seawater for 4 and 8 weeks. Post hydrogen pre-charging, the samples were subjected immediately to a modified Slow Strain Rate Test (SSRT). After hydrogen charging and SSRT the resulting fracture morphology was examined using a scanning electron microscope (SEM), and the failure mode was assessed in relation to porosity locations.

Experimental results indicate that all hydrogen pre-charged samples with or without pores decrease in ductility after exposure to the CP system which was further confirmed by the fractography analysis. It was observed that susceptibility of SDSS to HISC is broadly similar for welds with porosity or without porosity and brittle phenomena is more pronounced at the exposed surfaces of the samples.

The study has demonstrated that the butt weld of SDSS umbilical tube with a controlled porosity level in line with ASME design codes does not show an increased susceptibility to HISC under CP system when compared to welds with no porosity.
Coupled Motion and Sloshing Analysis of a Rigid Cylindrical Closed Fish Cage in Regular Waves
Authors: Tan Yuelin , Shao Yanlin , Read Robert
Published: November 2019

In this paper, a coupled numerical model in the time domain has been developed to study the interaction between interior liquid sloshing and the motion of a cylindrical closed fish cage when the cage is exposed to regular waves. The single-dominant nonlinear multimodal theory for sloshing in a cylindrical cage presented in [1] was implemented to simulate the liquid responses in the cage. A time-domain simulator based on the Cummins formulation of the equations of motion [2] is used to solve for the cage motion, while WAMIT is used to provide all required frequency-domain hydrodynamic coefficients for the external diffraction/radiation problems. Details of the coupling between cage motion and sloshing will be presented.

The coupled solver is verified against the linear frequency-domain solution from WAMIT for the very small wave steepness, where linear theory is valid. The results show that the sloshing effect is a vital factor in the coupling process, which means that the liquid in the closed cage cannot be treated as a solid mass. This is particularly true close to the resonant frequencies of the liquid in the tank. Furthermore, the importance of nonlinearity due to sloshing responses is investigated by applying incident waves with different steepness. When the cage is exposed to regular waves, if certain criteria are met, nonlinear swirling waves are observed in the closed cage. The nonlinear swirling waves are due to the interactions between different sloshing modes, which can only be explained by a proper nonlinear theory, such as the multimodal theory applied in this study. The influence of the swirling waves on the cage motions will also be discussed in the paper. How this effect will impact the design of a closed fish cage and its mooring system can only be answered by studying the cage responses in irregular waves, which is the subject of ongoing research.
Bearing Capacities of Shallow Skirted Foundations After the Action of Multi-Directional Cyclic Displacements Considering Soil Degradation
Authors: Xiao Zhong , Zhang Donghai , Liu Haixiao , Liu Ying
Published: November 2019

Shallow skirted foundations have been applied widely in ocean engineering. Under the action of external excitation, the shallow skirted foundations on soft soil undergo cyclic displacements during service state. Under the action of cyclic displacements, the foundations drive the surrounding soft soil to produce a continuous accumulation of absolute plastic shear strain, which weakens the shear strength of the soft soil around the foundations. Therefore, the bearing capacities of shallow skirted foundations reduce after the action of cyclic displacements considering soil degradation. In order to study the evolutions of bearing capacities of shallow skirted foundations after the action of multi-directional cyclic displacements considering soil degradation, the elastoplastic finite element models of shallow skirted foundations with different embedment ratio are established. Cyclic displacements are applied along different displacement loading paths, and the evolutions of soil shear strength and bearing capacities of shallow skirted foundations after the action of cyclic displacements are analyzed.

The results show that the soil softening zone gradually expands from the stress concentration zone of the soft soil to the surrounds with increasing number of loading cycles. Due to the enlargement and weakening of the soil softening zone, the failure envelopes of shallow skirted foundations gradually shrink, but the shrinkage trend gradually converges with increasing number of loading cycles. The shapes of the failure envelopes are relatively less affected by the cyclic number of displacements. The size of the failure envelopes is greatly affected by the loading paths while the shape of the failure envelopes is relatively less affected.
An Enhanced Interface Model for Friction Fatigue Problems of Axially Loaded Piles
Authors: Kullolli Borana , Baeßler Matthias , Cuéllar Pablo , Rica Shilton , Rackwitz Frank
Published: November 2019

The shaft bearing capacity often plays a dominant role for the overall structural behaviour of axially loaded piles in offshore deep foundations. Under cyclic loading, a narrow zone of soil at the pile-soil interface is subject to cyclic shearing solicitations. Thereby, the soil may densify and lead to a decrease of confining stress around the pile due to micro-phenomena such as particle crushing, migration and rearrangement. This reduction of radial stress has a direct impact on the shaft capacity, potentially leading in extreme cases to pile failure.

An adequate interface model is needed in order to model this behaviour numerically. Different authors have proposed models that take typical interface phenomena in account such as densification, grain breakage, normal pressure effect and roughness.

However, as the models become more complex, a great number of material parameters need to be defined and calibrated. This paper proposes the adoption and transformation of an existing soil bulk model (Pastor-Zienkiewicz) into an interface model. To calibrate the new interface model, the results of an experimental campaign with the ring shear device under cyclic loading conditions are here presented. The constitutive model shows a good capability to reproduce typical features of sand behaviour such as cyclic compaction and dilatancy, which in saturated partially-drained conditions may lead to liquefaction and cyclic mobility phenomena.
Rheology of Brine-Based Fuzzy-Ball Drilling Fluids in Deepwater Drilling
Authors: Li Zhaochuan , Zheng Lihui , Wei Panfeng , Dai Xiaojuan , Huang Weian
Published: November 2019

In deepwater drilling, the rheology of traditional drilling fluid is uncontrollable since the fluid usually mixes with brine and encounters low temperature. A solution may be to use the newly designed brine-based fuzzy-ball drilling fluids (BFDFs) since these have a well-adapted rheology under high salinity and low temperature condition. This has the potential to make drilling safer and more efficient.

In this experiment, the rheological properties of BFDFs under test conditions were characterized with a rheometer by varying salinity (2 to 20 mass%) and temperature (4 to 80 °C). The rheological parameters considered are apparent viscosity (AV), plastic viscosity (PV), yield point (YP), and θ6 reading. To characterize the magnitudes of changes of the rheological parameters and their low temperature dependence, their ratios at 4 and 25 °C, and 4 and 80 °C were calculated.

The results showed that the apparent viscosity (AV), the plastic viscosity (PV), the yield point (YP), and θ6 reading of BFDFs increased slightly with the decrease of salinity and temperature. The ratios of rheological parameters at 4 and 25 °C were close to unity, while the ratios at 4 and 80 °C were about two. The flow behavior of BFDFs under high salinity and low temperature condition was stable. Therefore, brine could be used as the base fluid for BFDFs. Theoretically, the flow behavior of BFDFs under low temperature condition seems to follow the Herschel-Bulkley model. Practically, the tests indicated that the BFDFs possess a strong tolerance to sandstone cuttings and Cabentonite, an excellent inhibitive property to shaly cuttings, weak corrosive characteristics against N80 casing steel, excellent lubricity properties, and remarkable biodegradability.

In summary, the empirical results showed that the newly designed fuzzy-ball working fluid can use brine instead of fresh water as based fluid and maintain remarkable properties under high salinity and low temperature condition. Properties of BFDFs could basically satisfy the requirement of deepwater drilling work.