bluuebunny/crossref_metadata_2025 huggingface dataset:
Development of Ice Management Tactics and Navigational Simulation of Ice Management Operations on a State-of-the-Art Training Complex
Authors: Kazantsev Mikhail , Karulina Marina , Karulin Evgeny , Proniashkin Aleksander
Published: October 2017

Ice management is one of the ways to achieve a more efficient operation of marine ice-resistant platforms in winter, to secure oil offloading operations and, if necessary, to support rescue and evacuation operations. Since 2012, Krylov State Research Centre, St. Petersburg, Russia, runs a training complex for navigational simulation of ship movement in ice conditions, developed with participation of TRANSAS, a global leader in the field of marine navigation simulators. The core functional elements of this training complex are the models of interaction between marine technology (ships and platforms) and ice in given water areas. One of the completed Russian Projects, Prirazlomnaya offshore ice-resistant platform, is taken as a case study to illustrate a stage-by-stage process of working out a tactic of ice management near the platform, including:

1) analysis of ice conditions in the area of Prirazlomnoye oil field, with consideration of environmental variability;

2) development of maneuvering procedures for the ice-breaking vessels to ensure safe oil offload to tankers;

3) polishing these maneuvers by means of a training complex;

4) analysis of the real maneuvers performed near the platform.

This approach allowed a deeper understanding of the developed maneuvering layouts for supply vessels, with respect to specific ice conditions and given operational circumstances.
Optimization of Floating Offshore Wind Turbine Platforms With a Self-Tuning Controller
Authors: Lemmer Frank , Müller Kolja , Yu Wei , Schlipf David , Cheng Po Wen
Published: October 2017

The dynamic response of floating offshore wind turbines is complex and requires numerous design iterations in order to converge at a cost-efficient hull shape with reduced responses to wind and waves. In this article, a framework is presented, which allows the optimization of design parameters with respect to user-defined criteria such as load reduction and material costs. The optimization uses a simplified nonlinear model of the floating wind turbine and a self-tuning model-based controller. The results are shown for a concrete three-column semi-submersible and a 10 MW wind turbine, for which a reduction of the fluctuating wind and wave loads is possible through the optimization. However, this happens at increased material costs for the platform due to voluminous heave plates or increased column spacing.
Full-Scale Validation of Axial Carcass Loads in Flexible Pipe Structure From Cyclic Pressure and Temperature
Authors: Kristensen Claus E. , Muren Jan , Gjendal Andreas , Hanssen Erik B. , Melve Bjørn , Sødahl Nils , Engh Bjørn , Søfferud Mario
Published: October 2017

With basis in field experiences from several carcass axial failures in HT/HP unbonded flexible risers a comprehensive program was initiated to determine the root cause and develop a theoretical framework for prediction of the loads and mechanisms leading to damage initiation and development. The physical understanding, theory, tools and methods were developed in a close cooperation between the field operator Statoil, 4Subsea and DNV GL.

Following the discovery of carcass tearing as a new failure mode in May 2011 substantial work has been put into small- and mid-scale testing together with the establishment of various analysis models to describe the carcass tearing phenomenon. This development and test work has been presented at OMAE-2013/14. Since 2011 substantial work has been put into small- and mid-scale testing together with various analysis models to describe the carcass pull-out phenomenon.

As a final full-scale validation, an onshore fully instrumented test was conducted to assess the accuracy of the carcass tearing models and evaluate variability and uncertainties related to use of the developed tools and methods for risk assessment of similar HT/HP pipes still in operation. The flexible riser used for the test was a full length 830m long pipe recovered after 15 years’ successful operation at a North Sea Field. The pipe integrity was not jeopardized and the pipe was fully functional at the time of recovery.

The pipe was recovered due to unfavorable risk assessment and plans for full-scale testing. The full-scale test gathers as many aspects as possible into a single test to verify the analytical solutions proposed during this work. The test pipe was used with the original end-fittings with the full pipe length intact. The end sections of length were unreeled, secured, protected and supported.

As preparation to the test, finite element analyses of the flexible riser were performed to predict the carcass axial load in the end-fitting. The analysis also served as valuable input to the test procedure and instrumentation plan. Along with analyses, a set of novel measurement and instrument solutions were developed. This includes, live video monitoring of carcass movements and strain measurements at carcass, with temperature and pressure well outside standard solutions. Bore temperature and pressure varied from 0–90 °C and 0–500 barg, respectively.

Several temperature and pressure cycles were done. Comparison between detailed finite element analysis and full-scale results shows excellent correlation. The riser and end-fittings have been dissected after testing. Key findings from dissection match the predicted behavior well.
Parametric Resonance of a Fishing Vessel With and Without Anti-Roll Tank: An Experimental and Numerical Study
Authors: Ghamari Isar , Faltinsen Odd M. , Greco Marilena , Lugni Claudio
Published: October 2017

Parametric resonance (PR) in roll is of concern for fishing vessels, especially in head-sea waves. Here the effect of passive anti-roll free-surface tank is investigated experimentally and numerically on realistic fishing-vessel geometry at zero forward speed. On the numerical side, the onboard tank is simulated using an open source computational fluid dynamic (CFD) development platform, OpenFOAM (Open Field Operation and Manipulation). The internal flow solver is coupled with the seakeeping solver, which is based on the weakly nonlinear method proposed in [1]. Experimentally, two different relevant scenarios were examined: 1) a 2D rectangular tank with shallow-water filling depth was forced to oscillate in roll and the loads induced on the tank were measured; 2) a fishing vessel was tested in a towing tank, prescribing regular head-sea waves. The vessel was examined both without and with anti-roll tanks and a mooring-line system was designed so to control the horizontal motions with limited effect on the parametric occurrence. The experiments on the fishing vessel are considered to assess the seakeeping solver. Both model tests and numerical results confirmed the effectiveness of an on-board tank in avoiding PR. For the examined cases with tank, the parametric resonance did not occur without forcing an initial roll. Moreover, the initial roll amplitude and roll phase relative to the heave motion matter for triggering the instability.
Effective Power and Speed Loss of Underwater Vehicles in Close Proximity to Regular Waves
Authors: Daum Stefan , Greve Martin , Skejic Renato
Published: October 2017

The present study is focused on performance issues of underwater vehicles near the free surface and gives insight into the analysis of a speed loss in regular deep water waves. Predictions of the speed loss are based on the evaluation of the total resistance and effective power in calm water and preselected regular wave fields w.r.t. the non-dimensional wave to body length ratio. It has been assumed that the water is sufficiently deep and that the vehicle is operating in a range of small to moderate Froude numbers by moving forward on a straight-line course with a defined encounter angle of incident regular waves.

A modified version of the Doctors & Days [1] method as presented in Skejic and Jullumstrø [2] is used for the determination of the total resistance and consequently the effective power. In particular, the wave-making resistance is estimated by using different approaches covering simplified methods, i.e. Michell’s thin ship theory with the inclusion of viscosity effects Tuck [3] and Lazauskas [4] as well as boundary element methods, i.e. 3D Rankine source calculations according to Hess and Smith [5]. These methods are based on the linear potential fluid flow and are compared to fully viscous finite volume methods for selected geometries.

The wave resistance models are verified and validated by published data of a prolate spheroid and one appropriate axisymmetric submarine model. Added resistance in regular deep water waves is obtained through evaluation of the surge mean second-order wave load. For this purpose, two different theoretical models based on potential flow theory are used: Loukakis and Sclavounos [6] and Salvesen et. al. [7]. The considered theories cover the whole range of important wavelengths for an underwater vehicle advancing in close proximity to the free surface. Comparisons between the outlined wave load theories and available theoretical and experimental data were carried out for a submerged submarine and a horizontal cylinder.

Finally, the effective power and speed loss are discussed from a submarine operational point of view where the mentioned parameters directly influence mission requirements in a seaway.

All presented results are carried out from the perspective of accuracy and efficiency within common engineering practice. By concluding current investigations in regular waves an outlook will be drawn to the application of advancing underwater vehicles in more realistic sea conditions.
On the Distribution of Wave Impact Loads on Offshore Structures
Authors: Johannessen Thomas B. , Lande Øystein , Hagen Øistein
Published: October 2017

For offshore structures in harsh environments, horizontal wave impact loads should be taken into account in design. Shafts on GBS structures, and columns on semisubmersibles and TLPs are exposed to impact loads. Furthermore, if the crest height exceeds the available freeboard, the deck may also be exposed to wave impact loads.

Horizontal loads due to waves impacting on the structure are difficult to quantify. The loads are highly intermittent, difficult to reproduce in model tests, have a very short duration and can be very large. It is difficult to calculate these loads accurately and the statistical challenges associated with estimating a value with a prescribed annual probability of occurrence are formidable.

Although the accurate calculation of crest elevation in front of the structure is a significant challenge, industry has considerable experience in handling this problem and the analysis results are usually in good agreement with model test results. The present paper presents a statistical model for the distribution of horizontal slamming pressures conditional on the incident crest height upwave of the structure.

The impact load distribution is found empirically from a large database of model test results where the wave impact load was measured simultaneously at a large number of panels together with the incident crest elevation. The model test was carried out on a circular surface piercing column using long simulations of longcrested, irregular waves with a variety of seastate parameters. By analyzing the physics of the process and using the measured crest elevation and the seastate parameters, the impact load distribution model is made seastate independent.

The impact model separates the wave impact problem in three parts:

– Given an incident crest in a specified seastate, calculate the probability of the crest giving a wave impact load above a threshold.

– Given a wave impact event above a threshold, calculate the distribution of the resulting peak load.

– Given a peak load, calculate the distribution of slamming pressures at one spatial location.

The development of the statistical model is described and it is shown that the model is appropriate for fixed and floating structures and for wave impact with both columns and the deck box.
The Role of a Structural Mode Shape Based Interaction Law to Suppress Added Mass Instabilities in Partitioned Strongly Coupled Elastic Structure-Fluid Systems
Authors: Hosseini Zahraei S. M. , Veldman A. E. P. , Wellens P. R. , Akkerman I. , Huijsmans R. H. M.
Published: October 2017

The physical level of interaction between fluid and structure can be either one-way or two-way depending on the direction of information exchange at the interface of fluid and solid. The former can be solved by a partitioned approach and weak coupling. In problems involving two-way fluid-structure interaction, using a partitioned approach and strong coupling, sometimes stability restriction is encountered. This is an artificial added mass effect, which is independent of the numerical time step. Unfortunately an accurate and efficient method to deal with all the different levels of interaction is scarce. Conventionally, relaxation is applied to remedy this problem. The computational cost is directly related to number of sub-iterations between fluid and structural solver at each time step. In this study, the source of this instability is investigated. A discrete representation of a basic added mass operator is given and instability conditions are assessed. A new method is proposed to relax this restriction, the idea essentially is to remove the instability source from the structure and move it to the fluid and solve it monolithically with the fluid. We call this an interaction law. An estimate of the structural response is derived from structural mode shapes. As a test case, a 2D dam break problem interacting with an elastic vertical flexible beam is selected. The interaction of fluid with the beam undergoes several stages. The breaking waves on the beam can increase the added mass drastically, therefore the added mass ratio increases as well. In such a cases, the asset of interaction law is better elaborated, while the stability condition requires very high relaxation without interaction law, but the relaxation can be lowered by only using first five beam mode shapes. As a consequence, the number of sub-iterations reduces by one order. The numerical observations confirm the reduction in computational time due to utilization of the interaction law.
A Semi-Analytical Model to Calculate Forces Exerted on Horizontal Cylinder by a Solitary Wave
Authors: Gurnari Luana , Filianoti Pasquale
Published: October 2017

Several authors have studied the solitary wave load on a submerged horizontal cylinder. In the present work, a semi analytical expression of the horizontal force exerted by a solitary wave on a horizontal cylinder is derived. The formula is based on the speed drop factor ƒr, that is the ratio (greater than one) between the time needed by the wave pressure to cross the solid body and the travel time across a transparent cylinder. The ƒr, is calculated numerically by means of the Boundary Element Method on assuming that a solitary wave and a periodic wave having the same wavelength undergoes the same slowing down. (Clearly the wavelength for the solitary wave is estimated approximately.) Abaci for the ƒr, as a function of the ratio between the diameter and the wavelength, for assigned A/d (= amplitude of the solitary wave / bottom depth) have obtained. In order to check the appropriateness of BEM, we carried out an experiment in a numerical wave flume, using the Computational Fluid Dynamics (CFD) technique.
Implementation of Circular Wave Measurements and Multiple Drilling Parameter Analysis in Rock Anisotropy Evaluation
Authors: Abugharara Abdelsalam N. , Hurich Charles A. , Molgaard John , Butt Stephen D.
Published: October 2017

A laboratory procedure has been developed to evaluate the anisotropy of Rock Like Material (RLM), granite, red shale, and green shale. This procedure involves detailed anisotropy evaluation steps through implementing circular ultrasonic wave velocity measurements, representing physical measurement and multiple drilling parameters (MDP), representing drilling performance. The physical tests involved circular pattern measurements of compressional and shear wave velocities, VP and VS, respectively. The drilling tests involved drilling samples of each rock in different a 25.4 mm Diamond Coring bit. The MDP included the study of the variations of Rate of Penetration (ROP), bit cutter Depth of Cut (DOC), Revolution Per Minute (RPM), and Torque (TRQ). The MPD were studied as function of orientations under atmospheric pressure. In addition to the physical and drilling evaluation, mechanical tests, such as Oriented Unconfined Compressive Strength (OUCS) were also used in rock anisotropy evaluation. Concrete with fine aggregate and Portland cement is used as RLM for much of the laboratory work. This material was cast into cylinders measuring 101.6 mm by 152.4 mm and 203.2 mm by 203.2 mm, from which NQ; 47.6mm core samples were taken. Coring was performed in three main orientations including 0°, 45°, and 90°. Characterization tests were performed on the RLM cores as they were conducted on the natural rock that included granite and red shale as isotropic and vertical transverse isotropic rocks, respectively. A fully instrumented lab-scale rotary drilling rig was used in conducting the drilling experiments. Details on the strategy for the tests on the anisotropy evaluation with results from laboratory work on natural rocks and RLM are reported. Result of the effect of shale anisotropy orientation on the drilling parameters that influence ROP as means of anisotropy evaluation are also, reported.
Dynamic Behavior of a Seawater Intake Riser
Authors: Sakugawa Patrícia Mika , Morooka Celso Kazuyuki
Published: October 2017

The main purpose of this work is to study a two dimensional coupled analysis, in vertical and lateral directions, of the dynamic behavior of a seawater intake riser (SWIR). The dynamic solution is performed in time domain, through the time integration Newmark-beta method, and the use of the Newton Raphson method to solve the nonlinearity problem. A finite element computational method is deployed to have a better understanding of the vertical motion contribution in the overall system, taking into account the environmental loads (wave) and the heave motion of the FLNG vessel.
Fatigue Assessment of Damaged Pipelines After Glass Fiber and Epoxy Matrix Laminate Repairs
Authors: Regalla Sabrina , Pinheiro Bianca , Pasqualino Ilson , Lana Luiz Daniel , Perrut Valber
Published: October 2017

The aim of this work is to evaluate the residual fatigue life enhancement of damaged pipelines after the execution of composite material repairs made of laminates of epoxy matrix reinforced with glass fibers. In view of structural performance and cost concerns, the more suitable repair thickness should be proposed. The work comprises a numerical and experimental study on the stress concentration of damaged pipes under internal pressure before and after repair. A numerical model is developed, based on the finite element method, to provide stress concentration factors of damaged pipes (plain dent defect), under cyclic internal pressure, before and after applying glass fiber and epoxy matrix laminate repairs with varying thicknesses. Small-scale steel pipe samples are submitted to denting and the resulting stress concentration in the damaged region is estimated under cyclic internal pressure, before and after repair execution. From correlation between numerical and experimental results, the finite element model is calibrated and validated. A parametric study is carried out to evaluate stress concentration factors of dented pipes repaired with varying laminate thickness. Stress concentration factors of dented pipes under internal pressure after repair can be used in a fatigue assessment methodology from correction of S-N curves. The effect of repair thickness on the reduction of stress concentration factors is evaluated in view of the residual fatigue life enhancement of damaged pipes, beside repair procedure costs. Based on results of the parametric study, recommendations about the repair procedure using laminates of epoxy matrix reinforced with glass fibers will be proposed, comprising indications of the more suitable repair thickness, as a function of pipe and damage dimensions, in view of fatigue performance and cost concerns.
On the Long-Term Reliability Analysis of a Point Absorber Wave Energy Converter
Authors: Canning Jarred , Nguyen Phong , Manuel Lance , Coe Ryan G.
Published: October 2017

Of interest in this study is the long-term response and performance of a two-body wave point absorber (“Reference Model 3”), which serves as a wave energy converter (WEC). In a previous study, the short-term uncertainty in this device’s response was studied for an extreme sea state. We now focus on the assessment of the long-term response of the device where we consider all possible sea states at a site of interest. We demonstrate how simulation tools may be used to evaluate the long-term response and consider key performance parameters of the WEC device, which are the heave and surge forces on the power take-off system and the power take-off extension. We employ environmental data at a designated deployment site in Northern California. Metocean information is generated using approximately 15 years of data from this site (National Data Buoy Center site no. 46022). For various sea states, a selected significant wave height and peak period are chosen to describe representative conditions. Then, using a public-domain simulation tool (Wave Energy Converter Simulator or WEC-Sim), we generate various short-term time-domain response measure for these sea states. Distribution fits to extreme response statistics are generated, for each bin that represents a cluster of sea states, using the open-source toolbox, WDRT (WEC Design Response Toolbox). Long-term distributions for each response variable of interest are estimated by weighting short-term distributions by the likelihood of the sea states; from these distributions, the 50-year response can be derived. The 50-year response is also estimated using an approximate but more efficient inverse reliability approach. Comparisons are made between the two approaches.
Interactive Flow-Induced Vibrations of Two Staggered, Low Mass-Ratio Cylinders in the TrSL3 Flow Regime (2.5×104<Re<1.2×105): Smooth Cylinders
Authors: Ji Chunning , Xu Wanhai , Sun Hai , Bernitsas Michael M.
Published: October 2017

Flow-induced vibrations of two elastically mounted circular cylinders in staggered arrangement were experimentally investigated. The Reynolds number range for all experiments (2.5×104<Re<1.2×105) was in the TrSL3 flow regime. The oscillator parameters selected were: mass ratio m* = 1.343, spring stiffness K = 250N/m, and damping ratio ζ = 0.02. The experiments were conducted in the Low Turbulence Free Surface Water (LTFSW) Channel in the MRELab of the University of Michigan. A closed-loop, virtual spring-damper system (Vck) was used to facilitate quick and accurate parameter setting. Based on the characteristics of the displacement response, five vibration patterns were identified and their corresponding regions in the parametric plane of the in-flow spacing (1.57<L/D<4.57) and transverse cylinder-spacing (0<T/D<2) were defined. The hydrodynamic forces and frequency characteristics of the vibration response are discussed as well.
Property Evaluation of Q345 Welded Steel by Tangential Residual Magnetic Field
Authors: Bao Sheng , Bai Shuzhuang , Mustapha Ashri , Lou Huangjie , Fu Meili
Published: October 2017

This research aims to investigate the relationship between the metal magnetic memory (MMM) signals of Q345 welded steel and its mechanical characteristics including the range of stress concentration zone and the tensile stress. A series of tensile experiments were carried out to measure the tangential residual magnetic field (RMF) on the surface of Q345 steel welded specimens under the action of stress. The variation of tangential RMF and the characteristic parameters (the peak-peak width and the peak-peak amplitude) of tangential RMF gradient curve were investigated. It was found that the tangential magnetic field curve of welded specimen has abnormal magnetic changes in the weld joint area. An analysis of the tangential RMF gradient curve shows that the range of stress concentration zone of welded specimen can be evaluated by the peak-peak width which is nearly constant under different tensile stress. Furthermore, the peak-peak amplitude of the tangential RMF gradient curve has quartic polynomial relationship with tensile stress and it is a potentially useful indictor of the tensile stress in welded steel.
Power Optimization of Model-Scale Floating Wind Turbines Using Real-Time Hybrid Testing With Autonomous Actuation and Control
Authors: Kanner Samuel , Koukina Elena , Yeung Ronald W.
Published: October 2017

In this research, real-time hybrid testing of floating wind turbines is carried out at model-scale. The semi-submersible, triangular platform is built to support two vertical-axis wind turbines (VAWTs). On account of incongruous scaling issues between the aerodynamics and the hydrodynamics, the wind turbines are not constructed at the same scale. Instead, remote-controlled (RC) plane motors and propellers are used as actuators to mimic only the tangential forces on the wind turbine blades, which are attached to the physical model. On a VAWT, the tangential force is proportional to the torque on the turbine, which contributes to the power production. A control algorithm is implemented using the wind turbine generators to optimize the platform heading and hence, the theoretical power absorbed by the wind turbines. The computer fluid dynamics simulations of two-dimensional counter-rotating turbines is briefly discussed. The results from the simulations are discussed in the context of existing onshore experimental data. This experimental approach only seeks to recreate the aerodynamic force which contributes to the power production. In doing so, the generator control algorithm can be validated. The advantages and drawbacks of this technique are discussed, including the need for low inertia actuators, which can quickly respond to control signals.
A Fundamental Research on Countermeasure of Disaster Mitigation and Impact Force to Cause Drifting Ship
Authors: Murata Kazuki , Masuda Koichi , Ikoma Tomoki , Eto Hiroaki , Aida Yasuhiro , Matsuoka Akihiro
Published: October 2017

The damage investigations that followed the Great East Japan Earthquake revealed that automobiles, shipping containers, and even ships themselves had been caught up in the resulting tsunami before being hurled into various port facilities, causing immense damage. The damage characteristics of such collisions must be an integral part of disaster mitigation measures aimed at reducing damage due to drifting ships. When considering the impact force of massive vessels on port facilities, it is necessary to take into account the changes in the drift velocity of the vessels due to waves reflected from the facilities and other coastal structures. In previous studies, the moving particle semi-implicit (MPS) method has been adopted to examine cascading tsunami flotsam damage by means of numerical simulations. In the present study, we use the MPS method to examine the drifting behavior and impact force of ships moored to an actual harbor, taking into account waves reflected from coastal area structures. Based on the results, we discuss the applicability of this method to disaster mitigation measures.
Pipeline Girth Weld Inspection and Flaw Acceptance Criteria for Sour Service Applications
Authors: Kumar Shashi Bhushan , Mohan Kapil , Zhang Shaodong , Tronskar Jens P.
Published: October 2017

To establish flaw acceptance criteria for carbon steel pipeline girth welds that are intended to transport sour crude, wet sour gas and condensate it is important to assess the effect of operating environment and strain levels by performing the fracture toughness/ resistance testing as per DNV-OS-F101: 2013 in a representative simulated service environment or under more severe test conditions. None the less many oil and gas field operators still believe that using workmanship criteria and radiographic inspection will be adequate to ensure a safe future operation of the pipeline meeting the design life requirements under sour or severely sour operating environments. Unfortunately, experience shows that this is a dangerous practice as radiography tends to miss out in detecting the most severe planar defects such a lack of fusion, hydrogen induced cracking and weld root centerline cracks, this is specifically so for narrow J-bevel welds. Hence, DNV GL based on experience from many projects advocates inspection of all sour service pipelines using inspection methods such as AUT with a high probability of detection for planar flaws. Further, the AUT acceptance criteria shall be chosen appropriately with due considerations as workmanship type of acceptance criteria without proper verification may result in non-conservatism in the pipeline girth weld inspection and weld sentencing. This present paper presents some recent project experiences from typical sour service subsea pipeline projects and provide advices representing what is considered current best practice for testing and qualification of AUT systems for sour service projects.
Spray Cloud Formation Over Marine Vessels by a Water Breakup Model
Authors: Dehghani Saeed R. , Naterer Greg F. , Muzychka Yuri S.
Published: October 2017

Water breakup affects the variety of droplet sizes and velocities in a cloud of spray resulting from a sea wave striking a vessel bow. The Weber and Reynolds numbers of droplets are the main parameters for water breakup phenomena. “Stripping breakup” is a faster phenomenon than “bag breakup” and occurs at higher velocities and with larger diameters of droplets. A water breakup model employs droplet trajectories to develop a predictive model for the extent of spray cloud. The governing equations of breakup and trajectories of droplets are solved numerically. Stripping breakup is found as the major phenomenon in the process of the formation of wave-impact sea spray. Bag breakup acts as a complementary phenomenon to the stripping breakup. The extent of the spray as well as wet heights, for a Mediumsized Fishing Vessel (MFV), are obtained by numerical solutions. The results show that bag breakup occurs at higher heights. In addition, there is no breakup when droplets move over the deck.
Internal Fluid Effect Inside a Floating Structure: From Frequency Domain Solution to Time Domain Solution
Authors: Jin Jingzhe , Magee Allan Ross , Han Mengmeng , Hoff Jan Roger , Hermundstad Elin Marita , Hellan Øyvind , Wang Chien Ming
Published: October 2017

Liquid inside a floating structure influences both hydrostatic and hydrodynamic response of the floating structure. Some examples of floating structure with internal liquid are: vessels with roll damping tanks, floating hydrocarbon storage facility, LNG tankers and etc. A floating oil storage tank is considered in this study, which has a cuboid-shaped external wall, cylinder-shaped internal wall and simple structure configurations for its roof and bottom. Influence of internal liquid on the hydrostatic response of floating structures is well established and must be taken into consideration. The internal liquid reduces the stability of floating structures. The focus of this study is the influence of internal liquid on the hydrodynamic response of floating tank. Frequency domain analysis is performed with WAMIT, for partially filled tank with both solid mass and liquid mass. By comparing the different cases, the force induced by the internal liquid on the floating tank is illustrated. Based on the WAMIT calculated radiation damping force for the external flow, Impulse Response Function (IRF) connecting frequency domain and time domain solution is constructed and the force and moment induced by internal liquid is considered as an excitation force. By assuming linear tank motion, the internal liquid induced force is related to the incoming wave by a set of force transfer functions and it is moved to the right hand side of the tank motion equation. In practice, this set of force transfer function due to internal liquid has to be combined with the wave excitation force transfer function and it is the total force transfer function (internal liquid plus wave excitation) to be imported to SIMO. SIMO time domain simulation is performed in regular waves. Motion transfer functions from WAMIT frequency domain and SIMO time domain calculations are compared and reasonable agreement is achieved.
Fatigue and Strength Performance of Underwater Fillet Welds and Broco® Underwater Cutting Edges
Authors: Vargas Pedro M. , Altstadt Steven , Lewis Max
Published: October 2017

This paper presents the results of fatigue and strength tests for: 1) cruciform joints with the fillet welds performed underwater, and 2) test coupons with one sided cut by Broco® underwater cut. The laboratory testing was conducted in air. The results show that the under-water fillet welds can have root cracking. For large root cracks this results in fatigue performance that is lower than the equivalent in-air welds without root cracks. In the absence of root cracks the DNV [3] approach for cruciform joints is shown to be applicable. The Broco® underwater cut edge cuts performed above the DNV category F3 [3] fatigue performance for the in-air environment, but below the F1 category. Hardness testing on the cut edge heat affected zoned surface showed values as high as 38 HRC. This is typically a concern for hydrogen stress cracking in offshore environments, and thus the using any category above the F3 curve likely is not justified. For in air testing the strength of the underwater welds and the Broco® underwater cut edges showed no appreciable strength reduction vs. what would be expected from conventional flame cut edges.
Simulation of Normal Differentiable Stochastic Process of Ice Loads
Authors: Zvyagin Petr
Published: October 2017

Ice loads time series should be treated in the other way than separate independent ice loads observations. The stochastic process approach can provide information about such important characteristic as mean length of signal’s outcome beyond some critical level and expected number of such outcomes.

The paper considers global ice loads registered in an ice tank experiment with a cylindrical indenter of 100 mm width. The autocorrelation function is fitted in a manner that the observed load process is differentiable. The study conducted in the paper demonstrates that characteristics, such as the number outcomes beyond some critical level and the time spent off this level, are governed in the same way as parameters of a stationary differentiable normal process.

Normal stationary model of ice loads process allows its simulation, if autocorrelation function is given. In the paper, such simulation is performed.
An Experimental Method for Model Propeller-Ice Interaction in Air: Concept and First Results
Authors: Bach Constantin , Myland Daniela
Published: October 2017

One of the aims of the German national funded research project ProEis is to develop a methodology for determination of ice loads on model propellers which will then aid in improving and developing software tools for their prediction. For this purpose, a prototype device has been designed at HSVA within this project which is used to guide defined ice floes with one degree of freedom into a model propeller where they are milled and the resulting forces and moments are measured. This paper focuses on the description of the ice feeding device and presents some first results with respect to the physical process as well as measured load characteristics. For now, experiments are conducted in absence of water in order to exclude all hydrodynamic effects. A podded propeller is used which allows measuring of shaft torque and shaft thrust. A high speed video camera is also employed to record the propeller-ice interaction process at 5000 frames per second, allowing to observe the milling process in detail. Two basic impact patterns are observed with respect to their relative load levels. When the greater part of the leading edge of the propeller strikes the ice from the top, i.e. it crushes the granular layer and cuts off a relatively large chunk of ice, the maximum shaft thrust is approximately 1.5 times as high as when the leading edge of the propeller makes contact at the front face of a floe, scraping off a thin layer of ice. Shaft torque is mostly unaffected by the type of impact. The performance of the ice feeding device and the findings of the first tests presented here are reviewed and discussed critically. Recommendations for the planning of such tests and possible improvements of the device are given.
Design, Construction, and Installation of Off-Shore Wind Turbine With Tripod Suction Bucket Foundation
Authors: Ryu M. , Lee J. , Kwag D. , Bang S.
Published: October 2017

As part of the national off-shore wind power development project by the government of Korea, the very first off-shore wind turbine utilizing tri-pod suction buckets for its sub-surface foundation has been successfully completed. This off-shore wind turbine with a capacity of 3MW has been designed, constructed, and installed in late 2016. It is located approximately 200 meters offshore with a water depth of approximately 10 meters. Sub-surface soil consists of interbedded clay and sand layers. Details of the design, construction, and installation of this wind turbine with tripod suction bucket foundation system are described and discussed.
Application of Confidence Regions to Ice Ridge Keel Data Statistical Assessment
Authors: Zvyagin Petr , Heinonen Jaakko
Published: October 2017

Sets of measurements of underwater ridge parts usually contain a limited amount of data. Outcomes need to be made while relying on small sample sizes. In this event, the chance of making inaccurate estimations increases.

This paper proposes to use stochastic confidence regions in the estimation of the unknown parameters of keel depths. A model for a random variable with a lognormal distribution for keel depths is assumed. Regions for the mean and standard deviation of keel depths are obtained from Mood’s and minimum-area confidence regions for parameters of the normally distributed random variable. Conservative safety probability of non-exceeding the critical keel depth in one random interaction of the ridge with structure is estimated.

An algorithm for statistically assessment of ice ridge keel data by means of confidence region building is here offered. The assessment of a set of ridge keel depths for the Gulf of Bothnia (Baltic Sea) is performed.
Analysis of Offshore Wind Turbine Foundations With Soil Damping Models
Authors: Hemmati Arash , Khorasanchi Mahdi , Barltrop Nigel
Published: October 2017

Soil damping is usually modeled as a contribution to global proportional or Rayleigh damping in industry and academia. However, this soil damping model does not represent the physical behavior. In this paper, soil damping for offshore wind turbine foundation is modeled by means of dashpots with damping parameters associated with the soil stiffness. The case study used in this paper is a 5MW NREL wind turbine resting on a jacket foundation as the effect of soil damping in this type of foundation is more significant due to its structural nature and number of piles. A number of dashpots for each pile are used with six degrees of freedom to represent soil damping.

First, free decay vibrations are assesed for different soil damping levels. Next, the aerodynamic loading is applied at the interface level and the integrated analysis is performed in dynamic mode for stochastic loading. Displacements at top of the foundation, base shear and base moment forces for a number of design load cases are evaluated to assess the effect of quantifying soil damping with dashpots. Also, fatigue damages of different hotspots are assessed and compared with the damages from models with no soil damping. It can be concluded that soil damping modeled by dashpots may provide some additional damping and it is more realistic. It leads to load reductions especially mudline moment, with the maximum of 20% for the peak values. Therefore, this type of soil damping modeling can be used to optimize the design of offshore wind turbine especially for the critical hotspots near the mudline.
Reliability Prediction for Offshore Renewable Energy: Data Driven Insights
Authors: Ewing Fraser J. , Thies Philipp R. , Waldron Benson , Shek Jonathan , Wilkinson Michael
Published: October 2017

Accurately quantifying and assessing the reliability of Offshore Renewable Energy (ORE) devices is critical for the successful commercialisation of the industry. At present, due to the nascent stage of the industry and commercial sensitivities there is very little available reliability field data. This presents an issue: how can the reliability of ORE’s be accurately assessed and predicted with a lack of specific reliability data? ORE devices largely rely on the assessment of surrogate data sources for their reliability assessment. To date there are very few published studies that empirically assess the failure rates of offshore renewable energy devices [1]. The applicability of surrogate data sources to the ORE environment is critical and needs to be more thoroughly evaluated for a robust ORE device reliability assessment. This paper tests two commonly held assumptions used in the reliability assessment of ORE devices. Firstly, the constant failure rate assumption that underpins ORE component failure rate estimations is addressed. Secondly, a model that is often used to assess the reliability of onshore wind components, the Non-Homogeneous Poisson Power Law Process (PLP) model is empirically assessed and trend tested to determine its suitability for use in ORE reliability prediction. This paper suggests that pitch systems, generators and frequency converters cannot be considered to have constant failure rates when analysed via nonrepairable methods. Thus, when performing a reliability assessment of an ORE device using non-repairable surrogate data it cannot always be assumed that these components will exhibit random failures. Secondly, this paper suggests when using repairable system methods, the PLP model is not always accurate at describing the failure behaviour of onshore wind pitch systems, generators and frequency converters whether they are assessed as groups of turbines or individually. Thus, when performing a reliability assessment of an ORE device using repairable surrogate data both model choice and assumptions should be carefully considered.
Design Parameters for Increased Operability of Offshore Crane Vessels
Authors: Gutsch Martin , Sprenger Florian , Steen Sverre
Published: October 2017

Marine subsea operations are performed by highly specialized ships, referred to as Inspection, Maintenance, and Repair (IMR) and Offshore Construction Vessels (OCV). Although the ships and their on-board equipment are designed to operate in harsh environmental conditions, the current practice often is to terminate operations when a rigid and conservative weather limitation is reached, often specified in terms of the significant wave height as the exclusive criterion. Such general limitations do not account for vessel specific motion behavior. Since the offshore industry is aiming for all-year-round safe subsea operations, there is a strong interest amongst ship designers, owners and operators to establish vessel and task specific criteria. The project Vessel Performance within the Norwegian Centre for Research-based Innovation on Marine Operations (SFI MOVE), is developing response-based procedures, that are leading to case-specific operational ranges. This approach enables the full exploitation of vessel performance capabilities for safe and efficient offshore operations. Two methods with different complexity levels are proposed. Firstly, on the higher level, detailed operability analysis for a fleet and sea area of interest are performed by means of numerical tools.

This level can be used to obtain detailed results for existing ships, but the procedure can also be applied as guidance in the design stage. Secondly, on the lower level, generic diagrams can be used to estimate and compare the operational performance of different vessels based on fact sheet parameters. This is especially relevant for decision making processes where a detailed study cannot be performed.
Numerical Investigation Into Uncertainty of Wave-Induced Vibration of Large Container Ships due to Ship Operation
Authors: Iijima Kazuhiro , Ueda Rika , Fujikubo Masahiko
Published: October 2017

A series of seakeeping simulations accounting for the wave-induced vibration is performed on three large container ships with different sizes. Time series of bodily motions, accelerations and stress due to vertical bending moment are calculated for the three ships navigating in a short-term sea state. Ship forward speed is varied from 0 knot to 20knots to investigate the sensitivity of the hydroelastic response to the change of the speed. Statistical analysis is made over the time series results, and the results are compared in terms of significant value. The uncertainty of the wave-induced vibration with respect to the ship speed is evaluated for the respective ships. It is found out that the increase rate of pitch motion, accelerations and stress to the increase of the ships’ forward speed is different from each other. It is further observed that the acceleration and vertical bending moment increase is less prominent for the largest ship.
A Constitutive Model for Soft Mud Considering the Initial State of Effective Stress
Authors: Xu Chunyang , Chen Yongping , Yu Lili
Published: October 2017

Consolidation is one of the most important processes that are needed to be accounted for in the model of cohesive sediment transport. Previous studies have shown that the consolidation paths for soil elements are not unique at low effective stress (here low effective stress means the effective stress is smaller than a threshold effective stress, which is short as TES and denoted as σb) In this study, a new theoretical constitutive equation for the effective stress of soft mud is derived to describe the non-unique consolidation paths at low effective stress.

Firstly, based on the concept of fractal dimension, the mud flocs are treated as self-similar fractal entities and the relationship between mud floc size and sediment volume fraction is established. Due to the fact that the fractal dimension decreases as the floc size increases, the variation of fractal dimension is accounted for. Based on the self-similar model, the theoretical constitutive equation for the effective stress of soft mud is derived.

The constitutive equation is validated using two data sets obtained from consolidation tests in a settling column for soft clay. At low effective stress, the model results curves follow the data trends and the non-uniqueness of consolidation paths are captured. The model results are also compared with existing numerical results and better agreement is presented especially when η is less than 0.5.