bluuebunny/crossref_metadata_2025 huggingface dataset:
Tracking of Buoyancy Flux of Underwater Plumes for Identification, Close Visual Inspection and Repair of Leaking Underwater Pipelines in Muddy Waters
Authors: Agbakwuru Jasper A. , Gudmestad Ove T. , Groenli John , Skjæveland Helge
Published: November 2013

The importance of close visual inspection of leaking sections of pipelines prior to repair activities cannot be over-emphasized. Underwater optical cameras are important gadgets for most underwater inspection vehicles and submarines. However, the optical camera ordinarily immersed in a gaseous or liquid plume in muddy water condition will see no reliable trace but nearly blank or the scatter of diffused light of illuminating lamps. The implication is that underwater water tools that could be used in clear water to inspect and identify point of leaks on pressure containment structures would not be useful when such structure is installed in muddy or poor underwater visibility conditions.

Recent developments have demonstrated a diver assisted technique of close visual inspection of leaking containments structures installed in muddy water using clean water injection. This present paper demonstrates a technique of tracking and identifying leaking points on pipelines installed in unclear/muddy water conditions using optical cameras installed in a novel manner. The method leads a remotely operated or hyperbaric system to the point of leak in muddy water conditions for close visual inspection and subsequent repair.

The tool performance is validated in a muddy water of Secchi measure of less than 1 cm and in a number of trials, the tool is found sitting at the leak point. Secchi measure is the visual depth into the water column.

Forces that could be found in the plume and the consequences of buoyancy loss to floating or submarine equipment are also examined. Some techniques using remotely operated vehicles and manned hyperbaric bells for leak identification, close visual inspection and repair of pipelines installed in muddy water using the benefit of this presented methodology are proposed and discussed.
Overturning Reliability Analysis of Jack-Up Platforms Using Spreadsheet
Authors: Yasseri Sirous F. , Mahani R. B.
Published: November 2013

In designing jack-ups to resist environmental loading, decisions are made under a great deal of uncertainty that may lead to a finite risk of exceeding limit states of the structures. In order to minimize the risks, conventional safety factors based on deterministic analyses, are commonly used. This paper present a simple spreadsheet based reliability analysis for assessing the safety of independent leg jack up platforms subjected to the overturning effect of environmental loads. The intention is to develop and illustrate procedures that can be used by structural engineers to assign conditional probabilities of failure to jack ups as a function of environmental loads.
Winterization Needs for Platforms Operating in Low Temperature Environment
Authors: Gudmestad Ove Tobias , Efimov Yaroslav , Kornishin Konstantin
Published: November 2013

According to a common belief which is shared by a lot of specialists from different areas petroleum business goes far into the offshore. One of the most promising regions of hydrocarbon production in the nearest future is stated to be Arctic region. But this surprisingly rich region is also happens to be one of the toughest and challenging areas to operate. Due to extremely cold air temperatures, strong winds, presence of ice and other harsh physical and climatic conditions safety requirements and technological demands call for new conceptual solutions for constructions that are panned to be used in Arctic. For exploration and production facilities in the cold climate the following aspects are essential: personnel and environment safety, as well as uninterrupted fail-safe technological process. In cold climates the main concern goes to low ambient air temperatures and presence/accretion of ice. Cold temperatures affect both personnel and equipment on the platform. To protect platform from influence of cold temperatures special heated covers can be used. This solution has certain concerns with heating, ventilation and air conditioning (HVAC) and energy supply systems. Yet another way here might be partial cover of equipment with special shelters and climatic modifications. Ice is considered to be the second limiting factor: sea ice that requires ICE CLASS vessels or specially designed platforms (not to mention ice bergs) and icing that endangers all unprotected systems on the platform. To deal with these threats different strategies might be used but there is still no one answer. Every case is some kind of unique when speaking about Arctic constructions. Ice-induced vibrations observed on platforms in the Bohay bay that haven’t been studied or even considered is a good example. Winterization for platforms is not fully developed yet and requires deeper research. The paper anticipates different codes and standards for offshore oil and gas facilities to be designed to operate in low temperature environments (American Bureau of Shipping ABS, Russian Maritime Register of Shipping, Det Norske Veritas DNV, International Organization for Standardization ISO, Canadian Standards Association CSA). These rules are compared against the experience of several major oil and gas operators and service companies gained during studies of conceptual design for Arctic exploration and production constructions at pre-FEED and FEED stages. The most important winterization concerns are highlighted and scrutinized.
Experimental Analysis and Modelling of the Thermomechanical Behaviour of Field Joint of Thermally Insulated Pipeline
Authors: Phan V. T. , Cognard J. Y. , Choqueuse D.
Published: November 2013

Ultra Deep offshore oil exploitation (down to 3000 meters depth) presents new challenges to offshore engineering and operating companies. Flow assurance and particularly the selection of insulation materials to be applied to pipe lines are of primary importance, and are the focus of much industry interest for deepwater applications.

In this field, particular attention is now focussed on the long term behaviour of the assembly between coated field joints and parent coating material which appears to be critical for in-service durability. A field joint is the uncoated area that results when two pipe sections with coating cutbacks are assembled by welding.

The in-service environmental conditions of the structure are severe and include high hydrostatic pressure up to 30 MPa and large thermal gradients through the thickness of the material (inner temperature up to 100°C and external temperature 4°C). This study aims to establish a model allowing the thermo-mechanical behaviour of the assembly to be evaluated and to describe the experimental protocol required in order to identify the model parameters.

First, the mechanical behaviour of the parent coating (usually glass syntactic polypropylene material) and coated field joint material are determined using hydrostatic compression tests (static and creep) at different temperatures and Dynamic Mechanical Analysis (DMA).

Then, a thermo-mechanical model of the parent material allowing hydrostatic creep deformation is developed and implemented in Comsol Multiphysic ™ software. A global model of the structure under service conditions, from the manufacturing process to the in service conditions, is then proposed and allows the strain distribution in the structure to be predicted. This distribution highlights the critical area of the assembly which is the bond between the parent coating material and the field joint coating.
Challenging Design Confronted in a Deepwater/HPHT PiP Tie-Back Flowline System
Authors: Sun Jason , Jakl Sandra , Shi Han
Published: November 2013

A challenging problem that pipeline industry has to face in deepwater is the high energy reservoir with high pressure and high temperature. For piping, flowline, and riser, High Pressure (HP) leads to much thicker pipe wall that increases manufacturing and installation cost. High Temperature (HT) has even wider impact on design since the flowline system has to operate over a greater temperature range between non-producing situations such as installation and shut down, and the maximum production flow.

Subsea tie-back to the existing floating production facility, generally named as Brown Field Development, has many engineering and financial advantages. It becomes more popular in the Gulf of Mexico (GoM), North Sea, and West African due to the economical benefits. This paper presents some of the design challenges of a deepwater subsea tie-back project, which is composed of an 8″ by 12″ pipe-in-pipe (PiP) flowline loop from three (3) subsea fields to a semi-submersible platform located in the GoM at a water depth of 2,000m (∼6,600ft). Some of key efforts are worth to mention:

• Mitigation of thermal expansion and global buckling as facing very soft clay soil;

• Transition tie-in of PiP to structure piping - a valiant strength design to meet the deepwater installation loading;

• PiP inner pipe lock-in compressive load - effect of flowline (non-bonded) section length variation and locked-in stress;

• Tight installation target box for the separately installed structure mudmat and upper module.

This paper presents the solutions that Project team has generated to address these design/installation challenges. Lessons learned from the designs and installations are also presented. Advanced analysis tool — FEA are utilized through the entire design stage, from global 3-D flowline modeling to local component strength design.
Prediction of Sloshing Effects Using a Coupled Non-Linear Seakeeping and CFD Code
Authors: Thanyamanta Worakanok , Bass Don , Molyneux David
Published: November 2013

In this paper, a numerical approach for predicting sloshing or roll-stabilization effects is proposed. A 3D non-linear time domain seakeeping code, MOTSIM, was coupled with a commercial CFD code (Flow-3D) and used to predict roll stabilizing performance of an unconventional U-tube tank installed in an oceanographic vessel. The codes were fully coupled and thus provided coupled effects of the external flow field and the motion of the fluid with a free surface inside the anti-roll tank on the ship motion in six degrees of freedom. MOTSIM is a well validated code that has been proven to provide accurate motion prediction for various vessels. The CFD code allows for modeling of complex tank geometry as well as detailed investigation of locations in the tank where severe loads might be experienced. Comparisons of the simulation results with experimental data showed good agreement and significant effects of the anti-roll tank on decreasing the ship’s roll motion. This study also demonstrated the coupled code’s potential use for any type of sloshing problems including the design of roll-stabilization tanks and LNG carriers.
Dynamic Positioning Simulation of a Simplified FPSO in Waves
Authors: Zhan Dexin , Very Stephen , Bass Don , Millan James
Published: November 2013

This paper presents a numerical station-keeping simulation in waves (regular and irregular waves) for a simplified FPSO with two DP systems. The main difference of the simulation with these two DP systems was that the first DP system (called MOT DP in this paper) was included in a seakeeping simulation code (MOTSIM) while another DP system (called IOT DP, developed by NRC/IOT) was an external system and installed on another computer (which was the same as the one used to control the vessel in physical model experiments). In order to exchange command messages between the simulation code (MOTSIM) and the IOT DP system, a communication code executed in the same time step was developed by using Windows™ socket functions.

By using these two DP systems a station-keeping simulation for a 1:40 scale model FPSO with six azimuthing thrusters was conducted in regular and irregular waves and wind. The seakeeping RAO results in regular waves were compared between these two DP systems, and also with physical model experiments with external IOT DP system and analysis using a commercial linear, 3-dimensional panel code, HydroStar. In irregular waves a mean station keeping force Fx, Fy in horizontal plane was calculated and its total mean force was compared with results from the physical model experiments. Finally, a discussion between these two DP systems was provided.
Advanced Numerical and Analytical Tools for J-Tube Pull-In Through Multiple Bends
Authors: Fan Chengye , Wang James , Banneyake Ranil
Published: November 2013

J-tube pull-in is a conventional riser installation method used for connecting pipelines to offshore platforms. In general the J-tube pull-in analysis is performed either using simplified analytical models or finite element method. Finite element (FE) tools become more important for the scenarios with complicated J-tube design, because of its flexibility in adapting material properties, 3D geometry, and multiple bends.

Conventional FE models employ tube-to-tube contact elements (ITT elements in Abaqus) to model the contact between J-tube and riser, where both J-tube and riser are all modeled as beams. In this paper, a new FE model is developed that modeled the J-tube as a surface. The new model was benchmarked with the conventional ITT element model and shows a better performance in terms of convergence speed using a simple geometry. The new model was further applied to a 3D multiple bends J-tube application. The impacts from J-tube size, J-tube bend radius, number of bends, and material yield strength were also studied. The outcome of this analysis provides guidance for effective and reliable J-tube pull-in studies.
A Comparison of Two Wave Models and Their Influence on Fatigue Damage in Ship Structures
Authors: Mao Wengang , Prasetyo Fredhi Agung , Ringsberg Jonas W. , Osawa Naoki
Published: November 2013

In the maritime industry, fatigue failure is one of the most significant failure modes for ship structures. The fatigue damage in ship structures is mainly caused by the variation of wave loadings applied on ships, leading to variable structural stresses. Therefore, a reliable description of wave environments encountered during a ship’s service life is essential for accurate fatigue assessment of ship structures. Besides the wave scatter diagram provided by classification society rules, different statistical wave models have also been built up to model wave environments along arbitrary ship routes. The wave models could provide more specific wave environment for any chosen sailing routes of an individual ship. They may have the potential to be used for some practical applications, such as conceptual ship fatigue design, remaining fatigue life prediction when a ship plans to change its original trade region, and crack maintenance planning etc.

Since the development of these models may be based on different sources, e.g. satellite measurements, hindcast data, buoys, etc., the reliability and consistence of wave generations from various wave models must be validated by the measured wave environments in order to be used for those practical applications. In this paper, waves generated from two different wave models, one based on hindcast data and one mainly on satellite data, are compared with measured wave environments encountered by a 2800 TEU container vessel on the North Atlantic route. These wave models are used in the calculation of the fatigue damage in the vessel. The results obtained using waves generated from the two wave models are compared with the fatigue damage calculated based on strain measurements in the ship. Recommendations for future development of the wave models and further investigation to make the applications more realistic for ship fatigue assessment are also presented.
Stress Concentration Factors of Dented Rigid Risers
Authors: Pinheiro Bianca , Pasqualino Ilson , de Azevedo Nathália
Published: November 2013

Rigid risers are widely employed in the offshore industry nowadays, assuming the vertical (riser tower) or catenary (steel catenary riser, SCR) configurations. During operation, these structures undergo dynamic loads generated by the action of currents and waves. Rigid risers can also be subjected to collision from neighboring vessels or impact of heavy objects launched from them, resulting in the introduction of defects. The possibility of a fatigue failure must be addressed since these defects induce high localized stresses in the damaged section. The aim of this work is to evaluate the stress concentration induced by plain dents on rigid risers under combined dynamic bending and constant tension loads. A finite element model is developed to reproduce denting and spring back processes and estimate the stress concentration on the dented riser under combined bending and tension loads. The model is used in a parametric study to evaluate stress concentration factors (SCFs) for varying dimensions of dents and risers. Analytical formulae are developed considering the results from the parametric study to estimate SCFs of dented risers, which can be used in a theoretical fatigue life study, modifying standard S-N curves, and help to forewarn a fatigue failure.
Numerical Prediction of the Cooling of Burnt Cable for the Prevention of Re-Ignition
Authors: Lanin Dmitry G. , Balakin Boris V. , Sabodash Olga A. , Tsarichenko Sergey G.
Published: November 2013

Fires in the cable structures are a serious danger for the offshore facilities. Inflammations of the cables are mostly caused by cable overheating as a result of power loads in the net or by the shortcut circuits. Fire extinguishing comes, in the first place, to neutralization of the external flame on its surface. However, even when the external flame is prevented, the re-ignition of the cable’s insulation may happen in case its metal core has not been sufficiently cooled down to a safe temperature level. It is therefore important to clarify the dynamics of the core cooling under the influence of the flow of extinguishing agents in order to manage the temporal characteristics of the entire process. The current paper reports a numerical model of a hot cable cooling under the influence of external flow. The model resolves the temperature fields as in the cable as in the surrounding media in connection with the flow profiles of the cooling agent. Several cooling scenarios are considered for the problem: natural convection of the cooling gas around the cable, and forced heat utilization due to an induced gas-water flow directed towards the cable. The model is validated with the use of experimental data presenting a suitable agreement with empirics.
Modeling Suction Pile Lowering Through the Splash Zone
Authors: Gordon Robert B. , Grytøyr Guttorm , Dhaigude Mayuresh
Published: November 2013

This paper describes a direct time domain modeling procedure for suction pile lowering through the splash zone. The paper specifically addresses pile lowering from a relatively small installation vessel with the pile oriented with its long axis nearly parallel to the water surface. The model consists of time domain simulation of the crane/wire/sling/pile system. This system is subjected to vessel motions at the crane tip, hydrodynamic loading on the pile, and pile flooding.

The paper also describes the sensitivity of winch wire tension to seastate, lowering velocity and hydrodynamic model coefficients, and suction pile characteristics.

Finally, a brief comparison is made between this model and others that have been proposed within the industry, some of which have been incorporated in design standards and guidelines such as DNV-RP-H103.
Analytical Hierarchy Process (AHP) for Selecting and Designing a Mini LNG Plant: A Case Study of Batam Indonesia
Authors: Artana Ketut Buda , Putranta Dinariyana Dwi , Arief Irfan Syarief , MadeAriana I
Published: November 2013

Increase in demand for clean energy is one of the strategic issues in Indonesia nowadays, considering the significant economic growth of the country. A conventional LNG supply chain is not the best solution taking into consideration its high investment. The possibility of using a small scale LNG supply chain concept (Mini LNG) is recently sought by the government and private sectors in Indonesia. It is even more promising when we consider the amounts and number of stranded gas fields in the country. One of the main obstacles to the development plan is the geographical position of Indonesia as an archipelagic country.

This paper presents a case study of LNG supply chain model of 10 mmscfd Gas Sales Agreement (GSA) in Batam and its design of LNG transportation model from Batam to Siantan-West Kalimantan [1]. The distance between Batam and Siantan is approximately 392 nautical miles. Two main objectives are covered in this paper. The first one is an implementation of the Analytical Hierarchy Process (AHP) to select the best location for mini LNG plant, and the second one is to design the LNG supply chain model based on optimization approach.

The AHP model uses a pairwise comparison of 4 (four) qualitative attributes and 14 (fourteen) sub-attributes. 3 alternatives of location for mini LNG plant are evaluated, namely: Tanjung Uncang, Pemping Island and Janda Berhias Island. A sensitivity analysis by varying the weight of some critical attributes is also conducted to ensure that preferred location is sensitively selected with minimum error.

The optimization of the LNG supply chain model is carried out by means of Gradually Reduced Gradient (GRG) methods. The Objective is to attain one design that will minimize investment (cost). Decision variables of the model are LNG plant capacity, storage tank capacity in loading and receiving terminal, vessel size, number of round trip, number of operating vessels, regasification capacity at the receiving terminal, and others.
The Extremal Dependence of Storm Severity, Wind Speed and Surface Level Pressure in the Northern North Sea
Authors: Towe Ross , Eastoe Emma , Tawn Jonathan , Wu Yanyun , Jonathan Philip
Published: November 2013

Characterising the joint distribution of extremes of significant wave height and wind speed is critical for reliable design and assessment of marine structures. The extremal dependence of pairs of oceanographic variables can be characterised using one of a number of summary statistics, which describe the two different types of extremal dependence. Quantifying the type of extremal dependence is an essential pre-requisite to joint or spatial extreme value modelling, and ensures that appropriate model forms are employed.

We estimate extremal dependence between storm peak significant wave height and storm peak wind speed (Hs, WS) for locations in a region of the northern North Sea. However, since the extremal dependence itself may vary with storm direction, we introduce new covariate-dependent forms of the extremal dependence measures that account for the direction of the storm.

We discuss the implications of all of the estimates for marine design, including specification of joint design criteria for extended spatial domains, and statistical downscaling to incorporate the effects of climate change on design specification.
An Analysis of Structural Performances for Bottom Longitudinal Girder and Attached Stiffeners During Shoal Grounding Accident
Authors: Yu Zhaolong , Hu Zhiqiang , Wang Ge , Liu Kun
Published: November 2013

Simplified analytical methods have been considered fairly efficient for assessing the structural performances during collision and grounding of ships. This paper is centered on modeling of bottom girders and the attached stiffeners during shoal grounding accidents. The published researches on the behavior of bottom girders during shoal groundings were reviewed. Numerical analyses using the code LS_DYNA were conducted for understanding the behavior of bottom girders and the attached stiffeners. A parametric study of the influencing factors was conducted. Simulation included a wide range of sloping angles and indentations. The grounding responses of stiffeners were also presented. The results and conclusions from this paper are preparations for a subsequent establishment of new refined simplified models of bottom longitudinal girders and the attached stiffeners during shoal grounding accidents.
Simulating the Financial Cost of a Deepwater Loss of Well Control
Authors: Black Jared L. , Hinrichsen Eckhard , Zenteno Alfredo Tello
Published: November 2013

An international oil company developed a deep water exploratory drilling program scheduled to start in 2011. The company’s safety department required a financial risk analysis of the drilling program to aid in establishing insurance coverage levels. The purpose was to establish a probability cost distribution associated with a lost well control (LWC) incident. A numerical probabilistic simulation approach was deemed necessary since historical LWC cost data were insufficient for statistical analysis.

A ten branch fault tree with up to 38 elements (events) per branch was selected to represent the range of damage conditions that could follow a LWC incident. Probability density functions representing each event’s cost range were selected using a best fit procedure. The simulation procedure randomly selected a tree branch and then passed through the branch selecting costs randomly from each event’s cost-probability distribution. Event costs were summed to provide a total LWC cost for that simulation run. The simulation was repeated multiple times providing a suite of LWC total cost values. These data were then evaluated statistically to establish a LWC cost probability distribution. These results were used by the company’s risk division as an aid in determining the level of insurance to acquire.
Safety Factors Calibration for Wall Thickness Design of Ultra Deepwater Pipelines
Authors: Oazen Eduardo , Antunes Bruno R. , Cardoso Carlos O. , Solano Rafael F.
Published: November 2013

Wall thickness often presents a considerable influence in offshore pipeline capital expenditure (CAPEX). This influence is enhanced in design of ultra deepwater trunk lines of large diameter, where any wall thickness increase provides a huge impact on project costs. In ultra deepwater scenarios, thicker pipelines may eventually implicate not only in higher costs, but may also compromise the project feasibility due to installation load constraints related to laying vessels availability.

One potential way to reduce the pipeline wall thickness is to calibrate fitness-for-purpose safety factors through application of structural reliability methods, instead of utilizing the standardized safety factors presented in international codes. Since mid-nineties, several offshore pipeline design codes have been allowing the calibration of safety factors by structural reliability analysis. The purpose of such an allowance is that structural reliability methods would eliminate some eventual conservatism presented in the safety factors proposed by codes. Although this enables the achievement of optimized safety factors, more than fifteen years have passed and only few pipeline projects have taken advantage of the benefits of safety factor calibration.

This paper evaluates which potential benefits are available through safety factor calibration, particularly for wall thickness reduction purposes in ultra deepwater pipeline design. Calibrated safety factors are presented for some scenarios related to ultra deepwater export pipelines, considering “system collapse criteria” limit state. The calibrated safety factors are compared with the standardized safety factors presented by international pipeline design codes. The potential for safety factor reduction by the utilization of linepipes with more stringent manufacturing tolerances and the consideration of the thermal ageing imposed by coating application are also discussed.
Quantitative Calculation of Resonant Modes in Harbors Induced by Solitary Waves
Authors: Gao Junliang , Dong Guohai , Ma Xiaozhou , Ma Yuxiang
Published: November 2013

The response amplitudes of resonant modes in harbors with different bathymetry induced by solitary waves with different wave heights are calculated using the Normal Mode Decomposition (NMD) method. Wave conditions inside harbors are simulated with a set of fully nonlinear Boussinesq equations. It is found that the resonant energy is dominated by the first few modes and the higher modes only possess a very small proportion of the total wave energy when the initial wave heights of incident solitary waves are relatively small. However, when the incident wave heights increase, the relative energy distribution of different resonant modes inside harbors becomes uniform and the proportion of energy in the higher modes increases.
Safety Factor for Use in the Fatigue Crack Growth Assessment of Pipeline Weld Integrity and Engineering Critical Assessments
Authors: Sriskandarajah T. , Zhou Daowu , Hobbs George , Roberts Graeme
Published: November 2013

Engineering critical assessments (ECA) are routinely used to provide defect acceptance criteria for pipeline welds. The fatigue crack growth evaluation as part of ECA involves the use of a safety factor applied to the number of cycles of a given stress spectrum. It has been routinely presumed that a safety factor value of half of the corresponding safety factor used for S-N fatigue should be used in the ECA. For example, this was suggested in DNV OS F101 earlier than the 2012 edition.

This paper presents a rationale and example calculations as to why a safety factor of 1.0 for fatigue crack growth is wholly appropriate for design stage calculations (for re-assessment when the various parameters are well defined then it could be appropriate to use a higher safety factor for the cycle counts). A probabilistic approach accounting for the statistical coincidence of misalignment, defect size, weld under maximum loading, etc is described in this paper. The probabilistic approach (based on fatigue crack growth) was compared with the S-N fatigue calculation and the deterministic ECA result. The results demonstrate that the probability of occurrence of fatigue failure using the probabilistic ECA approach with safety factor of 1 is far less than the DNV-OS-F101 requirement of 25 × 10−4 for medium safety class.

This conclusion presumes that in the standard ECA, a worst onerous selection is made from the range of input parameters, e.g. wall thickness, misalignment, defect location etc.
Fully-Nonlinear Wave-Current-Body Interaction Analysis by a Harmonic Polynomial Cell (HPC) Method
Authors: Shao Yan-Lin , Faltinsen Odd M.
Published: November 2013

In the Ronald W. Yeung Honoring Symposium on Offshore and Ship Hydrodynamics in OMAE2012 hold in Rio de Janeiro, Shao & Faltinsen [1] have proposed a new numerical 2D cell method based on representing the velocity potential in each cell by harmonic polynomials. The method was named the Harmonic Polynomial cell (HPC) method. The method was later extended to 3D to study potential-flow problems in marine hydrodynamics [2]. With the considered number of unknowns that are typical in marine hydrodynamics, the comparisons with some existing boundary element based methods including the Fast Multipole Accelerated Boundary Element Methods showed that the HPC method is very competitive in terms of both accuracy and efficiency. The HPC method has also been applied to study fully-nonlinear wave-body interactions [1, 2], for example, sloshing in tanks, nonlinear waves over different sea-bottom topographies and nonlinear wave diffraction by a bottom-mounted vertical circular cylinder. However, no current effects were considered.

In this paper, we study the fully-nonlinear time-domain wave-body interaction considering the current effects. In order to validate and verify the method, a bottom-mounted vertical circular cylinder which has been studied extensively in the literature will first be examined. Comparisons are made with published numerical results and experimental results. As a further application, the HPC method will be used to study multiple bottom-mounted cylinders. An example of the wave diffraction of two bottom-mounted cylinders is also presented.
Dual Stiffness Approach for Polyester Mooring Line Analysis in Time Domain: Semisubmersible Case
Authors: Tahar Arcandra , Sidarta Djoni
Published: November 2013

This paper is a continuation of a series of investigation for the dual stiffness approach for polyester mooring lines. Tahar et. al. (2012) has presented the global performance comparison between the dual stiffness method and the traditional method for the Spar platform. As shown in that study, there are appreciable differences between the former and the later methods especially in lateral motions, which, however, result in little difference in SCR strength response. Is it because the Spar has better motion characteristics than other wet tree floating platforms such as the semisubmersible and FPSO? This paper will investigate the effect of the dual stiffness method and the traditional method to SCR response for a Semisubmersible platform. The fully coupled dynamic analysis tool CHARM3D has been modified to incorporate the dual stiffness approach. Two axial stiffnesses (EA) of polyester line, post installation (static) stiffness and storm (dynamic) stiffness have been convoluted into a dual stiffness to represent the total response of the floating platform in a single run. In the traditional method, the analyses are done twice, one run for each stiffness. Then, the extremes from each run are used as governing values for design. The SCR will be modeled and analyzed using ABAQUS software.
High-Order Simulation of the Flow Instability Over a Non-Circular Structure
Authors: Minguez Matthieu , Meliga Philippe , Serre Eric
Published: November 2013

Bluff bodies can be prone to well-known self-excited instability such as galloping. Opposite to classical and well documented Vortex Induced Vibrations (VIV), oscillation amplitude is not bounded in such a way galloping-like instability can alter the integrity of the structure on short term. Although stability criteria exist in the literature [1], there is a real lack considering the cause and the way to inhibit such instability.

It is consequently proposed in this paper to investigate the flow over the classical ‘non-circular’ structure, i.e. the square box body, by means of advanced Spectral Large Eddy Simulation (LES) solver, [16], [17] & [18]. The main objectives of this numerical analysis will be to provide an accurate solution to underline the possible mechanisms that trigger the instability as well as a base solution for future wake stability & control analysis, [6].
Fatigue Study of Conductors at the Conductor Guides’ Level on North Sea Platform
Authors: Mosalam Bita Jalali , Gudmestad Ove Tobias
Published: November 2013

This paper presents the method and results from a fatigue study of conductors at the conductor guides’ levels as well as a static and dynamic hydrodynamic analysis of a 20″ conductor installed on a North Sea jacket platform using the OrcaFlex 9.5 computer program. This study presents only the results of the analyses and does not comment on the program’s efficiency or ability to model the various physical effects. Due to the conductor guides’ situation with gaps between the conductor and its guides, which was reported by the offshore survey, there was great demand for a fatigue life study of the conductors. The hydrodynamic analysis was performed with Finite Element Software, the OrcaFlex 9.5, for regular waves for 1-, 10- and 100-year return period sea states. Afterwards, fatigue calculations were carried out according to the output from the hydrodynamic analysis. The main achievements from this study are:

• Some elements have been identified as key factors, which have a great influence on the fatigue life of the conductors at the guides’ levels: bending moment, wall tension of the conductor, hydrodynamic & structural damping factors (which are seen to be main sources of uncertainty) as well as the clearances (gaps) between the conductor and its guides at the different guide decks.

• The bending moment could be reduced by decreasing the gap for the conductor in the guides.

• Selecting a DFF (Dynamic Fatigue Factor) of 2 means that the fatigue life of the conductors at the guide levels will not match the requirement for conductor fatigue service life, according to our analysis; and the conductors need inspection and modification.
The Dynamic Response Induced by the Random Heave of Platform and Vortex for TTRs
Authors: Tang Yougang , Wang Liyuan , Li Yangqing , Liu Zhongbai
Published: November 2013

The dynamic responses of TTRs in the deep water were studied considering coupled the random heave of platform and VIV. The random parametric excitation is set up considering the random motion of platform. The vortex lift force is calculated with the time-varying lift coefficient based on the Van der Pol drafting oscillator model. The equation of coupled VIV and parametric excitation vibration (PEV) is established and the program of solving coupled PEV and VIV is developed. The coupled dynamic response induced by random parametrical excitation and VIV is calculated for the TTR of Truss Spar, and the dynamic characteristics are investigated. The results show that the random parametrical excitation can magnify the dynamic response of TTRs, and it will change the main frequency of TTRs vibration.
Structural Integrity of a Spar in Collision With a Large Supply Vessel
Authors: Ning Xinguo , Zhang Bob L. , Tallavajhula Sudhakar
Published: November 2013

The objectives of this study are to establish numerical approaches to evaluate the structural integrity of a generic Spar hull in collision with a large supply vessel and to reveal its progressive collision damage characteristics.

Dynamic and nonlinear finite element analysis is implemented using ABAQUS/Explicit module [1] respectively for two collision scenarios. One is a realistic simulation where the impact kinetic energy governed by an initial impact speed and total mass of a ship is gradually depleted during the collision. The other is a simplified analytical method where the impact speed of a ship bow throughout the collision is constant or the total impact energy is unlimited.

With a combination of calibrated material progressive damage models and Mises plasticity, progressive collision damages of the hull structures are accurately captured for structural integrity assessment. The collision energy absorption characteristics, the impact force-deformation curves, the progressive damage modes and the correlation between the impact force, kinetic energy and damages are revealed. Based on numerical investigation, the two analytical scenarios are compared and the implication for the design analysis is elucidated. As a complementary to the ABS code [2], the alternative collision damage criterion in ABS MODU [3] applicable to column-stabilized units is justified to be applicable to a Spar subjected to high-energy impact.
Dynamic Analysis for a Spar-Type Wind Turbine Under Different Sea States
Authors: Ma Yu , Hu Zhiqiang
Published: November 2013

Due to the energy crisis and environmental issues like pollution and global warming, the seeking for renewable and clean energy is becoming crucial nowadays. The offshore floating wind turbines draw much attention recently as a means to capture the steadier and stronger wind resources available in deep water seas. This paper mainly deals with a spar-type wind turbine which is known as the OC3-Hywind concept and potentially set to be applied in South China Sea. Based on the specific environmental conditions in South China Sea, the dynamic responses of the turbine are evaluated. A steady wind direction and a series of different wave directions are set to estimate the effect of coupled wind-wave induced loads. Response characters of motions, mooring loads and several other critical parameters of the system are summarized and discussed. The calculations are carried out with the numerical simulation code FAST in time domain and frequency analysis are obtained using FFT method.
Effect of Initial Distortions on the Ultimate Capacity of Pitting Corroded Plates
Authors: Jiang Xiaoli , Guedes Soares C.
Published: November 2013

The aim of the present paper is to investigate the effects of the initial deflection shape on the ultimate capacity of pitting corroded plates under biaxial compression. A series of nonlinear FEM analysis of plates with partial depth pits are carried out, changing the geometrical attributes of both pits and plates, i.e., the radius, depth and location of the pits and the slenderness of the plates. A realistic thin-horse shape initial deflection and a simplified buckling shape initial deflection are assumed and compared. It is shown that the initial deflection shape has a tremendous effect on the ultimate strength of pitted mild steel plates. The ultimate strength of a pitted plate with thin-horse shaped initial deflection is generally lower than that of a plate with buckling shaped initial deflection provided that other influential factors are same. Their deviation is sensitive to plate slenderness and pit intensity. The simplified buckling shaped initial deflection could be sufficient to predict the ultimate strength of the plate under longitudinal uniaxial compression. However, in case of less predominant biaxial compression, the buckling shaped initial deflection may overestimate the prediction and give non-conservative results. In this respect, it is preferable to use the initial deflection measurements as input data for the analysis of the plate collapse behavior.
Study on Ship Structural Hydroelasticity and Fatigue Assessment in Irregular Seaways
Authors: Kim Jung-Hyun , Kim Yonghwan , Kim Yooil , Kim Kyong-Hwan
Published: November 2013

This paper introduces an analysis of ship hydroelasticity for a fatigue assessment of ship structural design. In this study, the hydroelastic analysis for springing and whipping is carried out by using a fully coupled three-dimensional BEM-FEM approach with two-dimensional slamming theoreis, and a sequential fatigue assessment is performed. The fatigue damage is decomposed to wave frequency and high frequency components. Furthermore, the high frequency component is again decomposed to 1st harmonic springing, super harmonic springing and whipping contributions. This decomposition is achieved using a modal superposition principle. Amount of the contributions are compared in irregular sea states.
Experimental Investigation of Two-Degree-of-Freedom VIV of Circular Cylinder With Low Equivalent Mass and Variable Natural Frequency Ratio
Authors: Srinil Narakorn , Zanganeh Hossein , Day Alexander
Published: November 2013

This paper presents an experimental investigation and validation of numerical prediction model for a 2-DOF VIV of a flexibly mounted circular cylinder by also accounting for the effect of geometrically nonlinear displacement coupling. A mechanical spring-cylinder system, achieving a low equivalent mass ratio in both in-line and cross-flow directions, is tested in a water towing tank and subject to a uniform steady flow in a sub-critical Reynolds number range of about 2000–50000. A generalized numerical model is based on double Duffing-van der Pol (structure-wake) oscillators which can capture the structural geometrical coupling and fluid-structure interaction effects through system cubic and quadratic nonlinearities. Experimental results are compared with numerical predictions in terms of response amplitudes, lock-in ranges and time-varying trajectories of cross-flow/in-line motions. Some good qualitative and quantitative agreements are found which encourage the use of the proposed numerical model subject to calibration and tuning of empirical coefficients. Various features of figure-of-eight orbital motions due to dual resonances are observed experimentally as well as numerically, depending on the natural frequency ratio of the oscillating cylinder.
Two-Dimensional Coupled Vortex-Induced Vibration of Circular Cylinder: Prediction and Extraction of Hydrodynamics Properties
Authors: Zanganeh Hossein , Srinil Narakorn
Published: November 2013

An advanced model for predicting a two-dimensional coupled cross-flow and in-line vortex-induced vibration (VIV) of a flexibly-mounted circular cylinder in a uniform flow is proposed and investigated. Attention is placed on a systematic extraction of variable hydrodynamics properties associated with a bi-directional fluid-structure interaction system. The governing equations of motion are based on double Duffing-van der Pol (structural-wake) oscillators with the two structural equations containing cubic and quadratic nonlinear terms. The cubic nonlinearities capture the geometrical coupling of cross-flow/in-line displacements excited by hydrodynamic lift/drag forces whereas the quadratic nonlinearities allow fluid-structure interactions. The combined analytical and numerical solutions of the proposed model are established. By varying flow velocities in numerical simulations, the derived low-order model qualitatively captures several key VIV characteristics of coupled in-line/cross-flow oscillations. By making use of a newly-derived empirical formula, the predicted maximum cross-flow/in-line VIV amplitudes and associated lock-in ranges compare well with several experimental results for cylinders with low/high mass or damping ratios. Moreover, such important hydrodynamic properties as VIV-induced mean drag, added mass, excitation and damping terms can be systematically determined via the proposed model and compared well with some experimental results in the literature.
Energy Harvesting by Vortex-Induced Vibrations in Slender Structures
Authors: Grouthier Clement , Michelin Sebastien , de Langre Emmanuel
Published: November 2013

Vortex-induced vibrations (VIV) are self-sustained oscillations of an immerged bluff body strongly coupled with its fluctuating wake. In this paper, energy harvesting by VIV of slender structures is investigated using a wake-oscillator model. The generic case of an infinitely long tensioned cable with periodically distributed harvesters is first investigated. The linear stability analysis of the model is a good tool to understand the strong influence of the different harvesting parameters on the efficiency. As the achieved efficiency is rather high, VIV seem promising as a mechanism for energy harvesting. The more realistic case of a hanging string with a single energy harvesting device at its upper extremity is thus considered. The optimal efficiency is still rather high, travelling waves develop towards the harvester when parameters are in the efficient region of the parameter space, which is identified thanks to a forced analytical calculation.
Current Situation and Difficulty of Wave Forecast From Viewpoint of Ship Management
Authors: Sasa Kenji , Terada Daisuke , Shiotani Shigeaki , Wakabayashi Nobukazu , Ohsawa Teruo
Published: November 2013

Numerical forecasts of weather and oceanography are increasingly common in the field of ship operations due to advances in computer science. However, in some situations, the accuracy of forecasts is too unreliable to ensure safe operations. In the current study, a nationwide questionnaire is used to determine the effectiveness of wave forecasts in enhancing navigation safety. Data analysis is also shown for two cases of failed forecasts in low-pressure weather systems near Japan in the winter. Finally, recommendations are made for improving wave forecasts from the viewpoint of ship operations.