bluuebunny/crossref_metadata_2025 huggingface dataset:
Study on a Time Domain Prediction Method for Wave Force: Application for a Point-Absorber WEC
Authors: Li Qiao , Murai Motohiko
Published: September 2018

There are a lot of numerical analysis for solving hydrodynamic responses of a floating body in the time domain. Most of them can give a theoretical solution in given irregular waves. It means, however, that the solution can be obtained only if the accurate irregular waves represented by the wave spectrum should be given. As we consider the actual operation, we know it is difficult to detect the accurate irregular waves instantaneously as needed accuracy in the most of the time domain analysis for feed backing the control force to the system.

This paper proposes a new method to predict the practical wave force from the displacement of waves at a floating body in time domain analysis almost instantaneously. The method, that can apply to predict forces in wave energy converter with linear electric generator, helps us to choose the control force for convert more electric power in irregular waves. We confirm the algorithm and examine its effectiveness.
Carbon Dioxide and Water Mixture in Pipeline Flow Systems
Authors: Ben Rajeb Faraj , Odan Mohamed , Zhang Yan , Imtiaz Syed , Aborig Amer , Aziz Rahman Mohammad
Published: September 2018

Global warming is considered the most challenging issue facing humanity today, with many research studies now focusing on investigating the main cause of this problem. Studying the behaviour of carbon dioxide in its different phases can provide the key to resolving this critical issue. In the present study, pipe flows are used to investigate the behavior flow of water-CO2 mixtures at different pressures and temperatures. The flow rate and pressure of water and CO2 are changed by using a pump placed before the mixing point. Pressure and temperature levels are recorded by sensors and thermocouples affixed at points along the pipe loop. The flow rate of water and carbon dioxide is changed simultaneously, and the flow regime of two-phase water-CO2 flow is visualized through transparent tubes. After repeating experiments several times, we found that the mean pressure drop along the tube for water-CO2 system flow is about 4 kPa/m. We also predict that the flow regime of the flow is often intermittent.
New Generation Structural Steel Plate Metallurgy for Meeting Offshore and Arctic Application Challenges
Authors: Jansto Steven G.
Published: September 2018

The new generation of value-added low carbon-low manganese-niobium microalloyed structural steels for both low and high yield strength, energy absorption, fatigue and fracture resistant applications is under development for offshore and arctic materials engineering applications. These materials engineering considerations are shifting designers to consider new lower cost and more robust construction materials even for low yield strength applications require improved fatigue, fracture arrest and toughness performance. The civil engineering and end user community demand structural reinforcing bars, shapes, beams and plates with improved energy absorption and fatigue properties. With more severe climatic conditions evolving every day, demands also necessitate improved fire and seismic resistance, yield-to-tensile ratio consistency, improved bendability and weldability. These attributes are difficult to obtain from steel producers today with their current higher carbon microalloyed steel approaches and hot rolling practices. There is a global shift in motion to low C-Nb-Mn bearing construction steels displacing traditional materials. The technological and metallurgical advancements of value-added niobium (Nb) microalloyed thermo-mechanical controlled process (TMCP) plate steels have been further developed to meet more demanding fatigue, fracture and low temperature toughness end user requirements. Niobium enables achievement of substantial grain refinement and grain size uniformity when the plate is rolled with the proper reduction, thermal schedule and process metallurgical operational practices. The effects of microalloying elements on the continuous cooling transformation behavior must be carefully controlled during the process metallurgy of the reheating and rolling process to successfully achieve the desired mechanical properties. TMCP applications have been successfully developed in numerous product sectors with thickness exceeding 120 mm. Since the very fine grained microstructure improves toughness and increases the yield strength, this Nb-TMCP process enables the required tensile properties with the growing trend to leaner chemical composition designs (less than 0.10%C) and excellent toughness properties. From an operational cost perspective, in today’s very competitive market environment, there exists a huge opportunity for structural offshore and arctic plate producing steel mills to improve their profitability by thoroughly assessing a shift to lower carbon and manganese steels in their product mix. Through the adoption of these lower carbon Nb-containing structural materials, several design and/or manufacturing companies are initiating new offshore steel designs that will further provide improved overall lifetime and cost performance at reduced maintenance expense. These high strength plate steels offer the opportunity to manufacture complex heavy-lift and fatigue-critical components for larger offshore structures without increasing the weight of the platforms.
A Fundamental Study on Motion Characteristics and Cargo Handling Efficiency of the Large-Scale Floating Coal Transshipment Station
Authors: Eto Hiroaki , Shikita Yoh , Ikoma Tomoki , Masuda Koichi , Kihara Hiroaki
Published: September 2018

This paper describes the motion characteristics and cargo handling efficiency of the Large-Scale Floating Coal Transshipment Station (LFTS). Indonesia is a main country supplying coal in the Asia-Pacific region, it is important to ensure a stable coal supply to Japan. Because the topography of the seabed near East Kalimantan Island, Indonesia’s main coal production area, is shallow, it is difficult for bulk carriers to reach the coast. Therefore, LFTS is proposed, which will be used as a relay base between coal-barging barges from land and bulk carriers offshore. By installing LFTS, improvement of coal transport efficiency is expected. In considering feasibility of the LFTS system, it is important to know the cargo handling operation rate in the target area, LFTS can load 500,000 tons of coal and the draft will fluctuate greatly depending on the loading condition of coal. Therefore, when the draft is shallow, the freeboard becomes large and resonates with long-term component of the wind load and when the draft is deep, the wave force and fluid force including the slowly varying wave drift force affect the fluctuation. Also, LFTS and bulk carrier are large-scale structures, the fluid forces acting on both affect each other, so the influence of hydrodynamic mutual interference between two floating bodies cannot be ignored.

In this study, fluid analysis in consideration of the hydrodynamic mutual interference of LFTS system is conducted. And, response analysis of LFTS and a bulk carrier in irregular wave which considered compound external forces such as wave load and slow varying wave drift force, wind load, tidal current was performed. As a result, it was confirmed that the motion response of LFTS was not upset because LFTS was large. Therefore, without considering the motion response of the LFTS, the cargo handling efficiency is calculated from the response analysis results of the bulk carrier and the oceanic condition of the setting sea area. As a result, the cargo handling efficiency is satisfied in the state where bulk carrier is installed leeward of LFTS, and it was confirmed that the LFTS system could be operated satisfactorily if the installation was appropriate.
Numerical Evaluation of Mechanical Property Change and Collapse Strength of ERW Pipes Considering Manufacturing Process
Authors: Han Seong-Wook , Kang Soo-Chang , Yi Jiwoon , Kim Ho-Kyung
Published: September 2018

Along with the development of the energy industry, demand for oil and gas pipelines has increased, and as the low oil price era has been prolonged, more economical pipe design and construction are required. Typical examples are ERW pipes used as OCTG or reel-lay pipeline. The ERW pipe is made by passing the plate through continuous rollers, where repetitive loading and unloading causes unintentional plastic deformation and changes in initial steel properties. So, this study focused on both the change of mechanical properties during manufacturing process and collapse strength of ERW pipe considering the Bauschinger effect in order to produce more economical and high performance steel pipe.

In this paper, the ERW manufacturing process was divided into three stages: forming station, sizing station, and flattening station. The ERW manufacturing process was simulated as 3D nonlinear finite element models using ABAQUS (6.14-1). Then, the change of mechanical properties at each process station was examined through finite element analysis and PEEQ, Alpha, and residual stress in each process station were evaluated for maintaining continuity of analysis. And flattening station where the reverse bending gives a large change in the mechanical properties was also performed. Finally, the collapse strength of the ERW pipe was evaluated in consideration of the change in compression strength during the manufacturing process. The ABAQUS analytical model was verified by showing analytical results to be identical with the outer diameter measured from the full-scale size pipes. Using the developed analytical model, it is possible to numerically predict the mechanical properties and collapse strength of ERW pipe.
Numerical Investigation Into the Effect of Stand-Off Distance on the Blast Performance of Metallic Corrugated-Core Sandwich Panels
Authors: Cai Sipei , Liu Jun , Cheng Yuansheng , Hao Weiwei , Zhang Pan
Published: September 2018

The ANSYS/AUTODYN software was employed to investigate the dynamic response of the metallic sandwich panels subjected to air blast loading. The sandwich panels were composed of two face sheets and a trapezoidal corrugated-core. To validate the numerical models, the simulation results were compared with experimental data reported previously. In the simulation works, the process of shock wave propagation and the structural dynamic response were analyzed. Meanwhile, the influences of the stand-off distance between the explosive charge and the front face sheet on the fluid-structure interaction effect, dynamic response and the energy absorption of sandwich panels were investigated. Numerical results demonstrated that the impulse intensity decreased dramatically with the increase of stand-off distance. The slapping between the front face sheet and the back face sheet could be observed at the stand-off distances of 50 mm and 100 mm, while the sandwich panel exhibited the “strong core” response mode under the stand-off distance of 150 mm. Investigations into energy absorption characteristic revealed that the total energy absorption reduced with the increase of stand-off distance. The front face and corrugated-core provided the most contribution on total energy absorption. Moreover, the energy absorption proportion of corrugated-core had a positive correlation with the stand-off distance.
Numerical Investigation of Ultimate Strength of Stiffened Plates With Various Cross-Section Forms
Authors: Ren Huilong , Liu Yifu , Li Chenfeng , Zhang Xin , Wu Zhaonian
Published: September 2018

There is an increasing interest in the lightweight design of ship and offshore structures, more specifically, choosing aluminum alloys or other lightweight high-performance materials to build structure components and ship equipments. Due to its better mechanical properties and easy assembly nature, extruded aluminum alloy stiffened plates are widely used in hull structures. When the load on the hull reaches a certain level during sailing, partial or overall instability of stiffened plate makes significant contribution in an event of collapse of the hull structure. It is very necessary to investigate the ultimate strength of aluminum alloy stiffened plate to ensure the ultimate bearing capacity of large aluminum alloy hull structure. Most of studies of the ultimate strength of stiffened plates deal with stiffened plates with T–shaped stiffeners. Stiffeners of other shapes have seldom been explored. In this research, the ultimate strength of six different cross–section aluminum alloy stiffened plates and one steel stiffened plate was studied based on the non–linear finite element analysis (FEA). Taking into account stiffness, weight and other issues, the new cross–section aluminum stiffener has finally been concluded for replacing the original steel stiffener in upper deck of a warship.
Model Experiment of a Controllable Depth Cage and its Mooring System
Authors: Yu Shuai , Yoshida Takero , Han Jialin , Mizukami Yoichi , Kitazawa Daisuke , Liu Lili
Published: September 2018

A wide variety of submergible cages has been developed mainly for aquaculture in an exposed sea. However, a submergible cage generally positions only at the surface or certain depth. The vertical position of the cage cannot be controlled. Hence, a fish cage installed at various depths (hereafter called controllable depth cage; CDC) is one of the promising cages to cope with environmental variations such as algal blooming, hypoxic water, high waves, etc. CDC consists of a cage, floats and anchors, which are connected by ropes in this order. The vertical position of the cage is controllable by adjusting the buoyancy of floats. In the present study, the effects of waves on motion of CDC were tested in smaller and larger water tanks. A cage model with the scale ratio of 1/100 was made and installed with its mooring system. From the video image of this experiment, the motion of the cage was reduced when the position of the cage was deeper in the same wave condition. The model with the scale ratio of 1/25 was made and installed with the mooring system. The motion of CDC and the tension on the mooring ropes were measured by a motion capture system and load cells, respectively. The range of motion and the tension on the mooring ropes of CDC increased with increasing wave height. When the wave length was around the two times of the size of CDC, the tension of the mooring ropes increased. Under the same wave condition, the displacement and the inclination of the cage, and the tension of the mooring ropes decreased by about half when the cage was installed at 0.6 m below the water surface. However, the effects of wave height and wave period on the motion of CDC and the tension of the mooring ropes could not be analyzed enough, so further analysis will be required in the future.
Reproduction of Freak Waves Using Variational Data Assimilation and Observation
Authors: Fujimoto Wataru , Waseda Takuji
Published: September 2018

This study proposes ideas to reproduce freak waves from observational data. The reproduced data will apply to investigations on freak wave impact to offshore structures. Four-dimensional variational method (4DVAR) was used for the freak wave reproduction. Under a dynamical constraint, 4DVAR minimizes the squared error between observation and model prediction by adjusting the initial condition iteratively. This study utilizes the Higher Order Spectral Method (HOSM) to predict the nonlinear wave evolution, which is essential for freak wave generation. Information on wave spectrum estimated beforehand by a wave model is also employed to stabilize the reproduction. To increase convergence speed with fewer efforts of coding, a type of ensemble-based variational method (a4dVar) was adopted. The a4dVar performs perturbed ensemble simulations to evaluate the gradient of the squared error and is easy to parallelize and implement. This paper conducted twin experiments of HOSM+a4dVar data assimilation. HOSM model generated the true state of the uni-directional wave field, and the spatiotemporal wave field was reconstructed from time series of one virtual wave gauge located in the model. It is assumed that the virtual wave gauge detected a freak wave. The estimation accuracy of linear estimation and HOSM estimation were compared.
Numerical Prediction of Bow-Flared Slamming on ULCS in Oblique Waves
Authors: Xie Hang , Ren Huilong , Li Hui , Tao Kaidong
Published: September 2018

Bow-flared slamming loads of ULCS in oblique waves were studied systematically. Firstly, relative motion features between ship and wave were analyzed by seakeeping theory. The result shows the transverse and roll motion cannot be ignored in oblique waves. Secondly, a long-term and short-term analysis was applied to obtain the parameters of equivalent design wave. Then slamming pressure of two bow-flared sections with three kinds of motions was predicted by using the Computational Fluid Dynamics (CFD) method in commercial code FLUENT. Pressure characteristics in different wave directions were discussed and the results show slamming loads in some oblique wave cases are larger than that in the head sea. And the torque moment and transverse force caused by asymmetrical pressure distribution on the two sides should be drawn more attention.
A Reduced Draft Spar Concept for Large Offshore Wind Turbines
Authors: de Guzmán Santiago , Marón Daniel , Bueno Pedro , Taboada Miguel , Moreu Manuel
Published: September 2018

This paper describes a new floater type: the Reduced Draft Spar (RDS). The RDS is in essence a spar, and so stability in operation is achieved by having the center of gravity below the center of buoyancy. Spars thus need a relevant draft and some ballast at their bottom. The RDS instead, and compared to classic spars, increases the mass below the center of buoyancy to substantially reduce the draft. This counter-intuitive approach considerably increases the overall mass of the solution. But fortunately this additional mass can be provided by cost-effective solid ballast. In the same way gravity-based structures weigh much more than jackets or monopiles yet they can still be economically feasible, the RDS is considerably heavier than classic spars. Thereby, the RDS can have the benefits of reduced draft solutions like semis while keeping the inherent simplicity of spars.

The RDS concept replaces the main cylinder of classic spars by a shorter one, which is in turn held by a large caisson at the bottom of the floater. This allows the assembly of the Wind Turbine (WT) onshore and gives to the RDS enough floating stability to perform the Transport and Installation (T&I) marine operations with a significant reduction of auxiliary means. The proposed floater is made of concrete. It supports an 8 MW turbine in a generic North Sea offshore location. Besides and like in some semis, the unit is fitted with an Active Ballast System (ABS) used to compensate the environmental mean loads (mainly the WT mean thrust).

In the paper, a parametric design process is used to obtain the platform main dimensions. The intact stability, both in operation and during all marine operations phases, is checked taking into consideration reasonable design margins. The dynamic response of the RDS to extreme wind, waves and currents is also analyzed. A state-of-the-art seakeeping program coupled with a simplified aerodynamic load model accounts for the effect produced by the wind dynamics on the unit response. The performance of the platform in operation is similar to that of classic spars. Therefore, the paper focuses on the study of the survival conditions. Since the platform cross section is high, survival current loads become differentiating. The dynamic loads at the mooring lines are thus analyzed to assess their feasibility in severe storm environmental conditions, which rule over the mooring design.
Numerical Simulation on Residual Responses of Adjacent Piles After Spudcan Penetration and Extraction
Authors: Wang Jianhua , Fan Yifei , Ji Haibo
Published: September 2018

The effect of the mobile jack-up spudcan penetration and extraction on adjacent platform piles is an important issue in ocean engineering. Residual moments along piles will exist due to strata plastic deformation after spudcan extraction. If the residual response is large, the combination of the residual load and extreme environmental load may become the controlling load case for the piled structure. In order to understand the variation of the pile responses during spudcan penetration and after extraction, adjacent pile responses are calculated using the Coupled Eulerian Lagrangian (CEL) finite element method. Two kinds of typical seabed, clay and fine sand, are considered during calculation. The effects of the spudcan penetration depth, the spudcan-pile clearance, the shear strength of strata and the pile head constraint on adjacent pile responses are analyzed during spudcan penetration and after extraction. Calculated results show that residual responses of adjacent piles depend on the penetration depth, the clearance, the shear strength and the pile head constraint. The residual response of piles in soft clays is different from that in sands. For piles in soft clays, the residual response of adjacent piles will increase with decrease of the shear strength and the maximum residual pile shaft moment is larger than that during spudcan penetration. For piles in sands, the maximum residual pile shaft moment is about 70–80% of that during spudcan penetration. Therefore, the residual response of piles should be considered when the effect of spudcan penetration on adjacent platform piles is evaluated.
Towards a Method Evaluating Control Actions in STPA-Based Model of Ship-Ship Collision Avoidance Process
Authors: Wróbel Krzysztof , Gil Mateusz , Montewka Jakub
Published: September 2018

As system-theoretic methods of safety assessment are developed, certain aspects of them remain under-explored, including the question of prioritizing one control action’s adequacy over another. This paper investigates this feature based on an analysis of a process of merchant vessels’ collision avoidance. We first elicit experts’ views on the structure of the process and importance of particular interactions occurring therein. Thence, we analyse a number of incident reports to examine the percentage of cases in which failure to ensure the particular control action contributed to the loss. By comparing the results, we strive to determine whether any of the approaches might be beneficial in determining safety control actions’ priorities.
Surface Facilities for Thermal EOR Projects: Extra Heavy Oil
Authors: Peña Villegas Gilberto , Echeverrías María del Carmen
Published: September 2018

A Giant Heavy Oil Field requires extending and maintaining the production plateau during a continuous period of more than 30 years. In order to increase the revenues of the global project, the construction of Upgrader plant is always considered.

Cold production conditions are first, the project has estimated between 10-8% of the STOIIP obtained through cold production but it would not be enough to extend the production plateau. Therefore, later on it will be necessary to apply thermal EOR techniques, as:

• Steam Injection: 1-CSS + Steam Flooding, 2-SAGD or HASD

• In-situ combustion

Aim for this integrated study was to visualize the new facilities design and modification on the exiting cold production facilities to manage the hot production (investments) in function of reservoir/production requirements. The benefit of this integrated study is to value the additional investments in surface facilities required under thermal EOR production to get the global integrated project evaluation.
Kick Management Study on Automated Well Control for Managed Pressure Drilling in Long Wells
Authors: Leulseged Amare , Nepal Sima A. , Sui Dan , Geekiyanage Suranga C. H.
Published: September 2018

In drilling operations, the downhole pressure (BHP) requires to be closely monitored and precisely managed to avoid potential drilling events harmful to personnel and environment. If the BHP is lower than the pore pressure, kick (amount of influx) from formation will enter the wellbore, which might result in (underground) blowout. If not properly managed, this could be more costly than surface blowouts [1]. Well control aims to stop and remove the influx and re-establish primary barriers.

Managed Pressure Drilling (MPD) is an advanced drilling technology capable of precisely controlling annular pressure profile throughout the wellbore. In this study, a high fidelity transient flow model is used for simulating dynamic well control procedure in MPD to properly manage annular pressure during kick circulation after the kick is detected. In this work, an automated well control in MPD is simulated, where PID control algorithm is implemented by manipulating choke valve opening to dynamically regulate the BHP during kick circulation. The main aim is to investigate dynamic kick management with the use of different type of muds, water based mud (WBM) and oil based mud (OBM). For different mud systems, the well control performances for long extended reach wells are evaluated and compared. From simulations, it shows that the OBM is able to hide the influx to a large extent, than the WBM due to the much higher gas solubility of the OBM. In HPHT wells, the OBM is superior to the WBM with proper automatic surface pressure control in MPD operations. Using complicated dynamic flow model can provide more precisely surface pressure control for realtime dynamic kick management.
Uncertainty Quantification in Numerical Simulations of Parametric Roll
Authors: Düz Bülent , Ypma Egbert
Published: September 2018

Parametric roll is a nonlinear phenomenon that can rapidly create large roll angles coupled with significant pitch motions. Such motions can cause large loads especially in fore and aft parts of the ship structure, severe stresses on containers and their securing systems, and can be a threat for the crew of the ship. The large roll angles may increase to such values that can even cause capsizing. Even though there are many studies focusing on the parametric roll phenomenon using theoretical, numerical and experimental approaches, topics such as uncertainty quantification and sensitivity analysis in parametric roll have not attracted enough attention. In this paper, results from a study on uncertainty quantification in numerical simulation of parametric roll are presented. Numerical simulations were carried out in irregular seas using the Korean Container Ship (KCS). Motion of the ship was simulated with a time-domain program. Several parameters were considered as sources of uncertainty such as the GM, the length of the simulation time window, the phase, and the number of components of the irregular wave. To propagate these uncertainties in the computational model, Latin hypercube sampling (LHS) and Polynomial chaos expansion (PCE) methods were used, and their performances were compared. The paper ends with a summary of conclusions and recommendations.
Modelling Randomness in the Simulation of Ice-Induced Vibrations
Authors: Chen Yingying , Yu Bowei , Low Ying Min , Yap Kim Thow
Published: September 2018

Sea ice forms in a dynamic environment that affects its morphology. This results in the inhomogeneity of ice reflected for example in its thickness variation, flaws in terms of brine channels or cracks, presence of snow cover, and its various deformed states undergoing freeze-thaw cycles. This sea ice inhomogeneity introduces important effects observable in the response of a structure when it is interacting with sea ice. Failure to account for the randomness introduced by the sea ice inhomogeneity would risk producing unrealistic simulations not seen in the actual world. In this paper the proposed approach to model randomness in numerical simulation of ice-induced vibrations is presented. This is achieved by accounting for randomness in the ice crushing force. The study is carried out using a purpose-developed numerical model that simulates the ice-induced vibrations of structures. The model adopts a phenomenological basis that aims to capture the important processes during the dynamic interaction between ice and the structure. Full-scale measurement data is used for comparison in this study.
Experimental and Numerical Study on Holding Power of Rectangular-Shaped Anchors
Authors: Toh Kimihiro , Fukumoto Yusuke , Yoshikawa Takao
Published: September 2018

This paper discusses the experimental and numerical investigations for the holding power of rectangular-shaped anchors. As the offshore developments are promoted, the mooring systems are often used as the station keeping systems of the marine floating structures. From a viewpoint of the energy consumption, the mechanical mooring systems with anchors are better than the dynamic mooring systems with thrusters. Up to now, however, the research and development regarding the mooring systems with the high holding anchors in the deep sea area, especially more than 500 m in depth, have hardly been carried out in Japan.

In most cases, the conventional anchor shapes have experimentally and/or empirically been decided. In addition, only a few studies which relate the numerical analysis to the experimental test have been performed for the holding power. In order to obtain the optimal shape of anchors theoretically, therefore, the purpose of this study is to develop the estimation method for the holding power and to clarify the penetration mechanism of anchors in soil.

In this paper, a series of experiments utilizing the small-sized anchor model is conducted. Here, the fluke shape of specimen is modeled by the rectangular flat plate for simplicity. From several experiments varying the geometric characteristics of the anchor model, the experimental results, e.g., the history of the holding power, the penetration depth, and the fluke surface angle at the maximum holding power, are obtained. Furthermore, the numerical simulation to evaluate the holding power is also carried out using the dynamic explicit non-linear finite element analysis (NLFEA) code, LS-DYNA, as well as the in-house distinct element method (DEM) code. From the comparison between the numerical results and the experimental results, the calculation accuracy is verified.
Laboratory Study on the Turbulent Boundary Layers Over Wind-Waves Roughness
Authors: Bhirawa Tunggul , Kevin  , Lee Jung H. , Monty Jason P.
Published: September 2018

A laboratory study of turbulent boundary layers over wind-generated waves using Particle Image Velocimetry (PIV) in a wind-wave flume at the University of Melbourne is presented. The experiments are taken at two different wind speeds of 5.5 and 8.5 m/s at a fetch length of 3.5 m. Two types of multi-camera measurement are specifically tailored to capture the flow behaviours.

The first is a measurement with high spatial resolution, with aims of characterizing the mean velocity, surface drag and Reynolds stresses over the non-stationary surface. The second type is a large field-of-view measurement, designed to capture the large-scale turbulent motions which are directly associated with the surface-wave topography. Although the turbulent flow is developed over a non-stationary surface (i.e. wind-generated waves), it embodies similarities in both integral parameters and Reynolds stress behaviours to the turbulent flows over stationary rough surfaces. This observation could open a possibility to develop an important turbulence model as well as drag prediction over wind-generated waves, which could be closely related to stationary rough-wall boundary layers.
A Study of Ultimate Strengths of Typical Longitudinal Girders With Openings in Container Ships
Authors: Cui Jinju , Wang Deyu
Published: September 2018

Container ships are widely used and their strength has always been the concern of designers and engineers. Additionally, container ships for a given size are of similar arrangements and structures because the containers are of same standards worldwide. So investigation into the strength of typical structures is meaningful. This paper focuses on the ultimate strength of typical longitudinal girders with openings locating between double bottoms which are subjected to longitudinal compression and have not been extensively studied yet. On one hand, the ultimate strengths of longitudinal girders with openings are calculated by Nonlinear Finite Element Analysis (NFEA), and effects of opening sizes, opening locations, boundary conditions and initial deflections are studied, and prediction expressions of ultimate strengths are proposed by varying opening sizes based on benchmarks of intact structures. On the other hand, uniform corrosion is introduced into these structures, and its influences on ultimate strength are also investigated. The results are representative and can be of reference values for similar structures.
Removal of Topside Units in a Single Lift: The Repsol Yme Field Case Study
Authors: Alonso Castro Beatriz , Birkenes Terje , Oosterveld Huib
Published: September 2018

Decommissioning is an emerging sector in the UK and Norway, accounting for 2% of total industry expenditure in 2010 increasing to 8% in 2017. In accordance with existing regulations in the North Sea (OSPAR), dumping, and leaving wholly or partly in place disused offshore installations within the maritime area is prohibited. Over the next eight years, 200 platforms are expected to be removed in the North Sea.

There are a number of methods to remove offshore installations: Piece small, Reverse installation and Single lift. In the Single lift approach the jacket or the topside is removed in one piece, minimizing significantly the time offshore and therefore the safety and health incidents. But the Piece Small and Reverse Installation are the most common methods and are established and secure although are time consuming and labor intensive [1].

Several potential candidates for single lift technology at varying levels of technical readiness were considered in the past. The majority of the concepts remained on the drawing board, while some were awaiting project commitment. The only that was matured further than this is the Pioneering Spirit. Yme, its first commercial lift, gave this concept the “proven” status.

The Yme MOPU, owned by Repsol, was a jack-up type platform standing on three steel legs of 3.5 m diameter. The dry weight of the MOPU was approximately 13,500 t. The Yme MOPU was a challenging unit to remove mainly for three reasons: The platform motions due to the lack of stiffness in the leg support, its position in contact with the caisson wellhead platform, and the fact that the legs could not be pre-cut before the operation. The method selected to remove the platform was Single lift, using the dynamically positioned platform installation and removal vessel Pioneering Spirit.

The lifting arrangement consisted of 12 lift beams combined and connected in pairs to yokes. Five specifically designed yokes were installed. The yokes connect the TLS with the MOPU. The structural integrity of each interface was assessed with FE analysis. The Ballast system was used to provide additional clearance. Pioneering Spirit has a total of eighty-seven ballast water tanks, including four so called ‘Quick Drop Ballast Water Tanks’. The removal of the MOPU was performed successfully the 22nd August 2016, after two days work offshore.
Towards a Novel Type of Ice for Model Tests With Vertically Sided Structures
Authors: Ziemer Gesa
Published: September 2018

Model tests in ice are well-established to assess ice loads acting on ship-shaped structures. Model basins worldwide have elaborated different types of model ice over the past decades and gained confidence in e.g. predicting ship’s resistance which is validated by full scale data. Driven by industrial needs, the model ice was invented and modified with emphasis on the failure observed on ship hulls: Mostly flexural failure with only limited influence of crushing, i.e. against a vertical stem contour. Nowadays, the same model ice is occasionally used for structures exposed to ice action which are far from being ship-shaped, such as vertical sided monopiles or artificial islands. This approach is often questioned as the currently used model ice is usually insufficiently brittle, overstates out-of-plane deformation and flexural failure of the ice sheet, and transfers most of the ice load by a hard top layer rather than creating a wedge-shaped ice edge with a line-like contact approximately at middle height of the ice thickness as observed in full scale indentation tests. Therefore, results from tests with vertical structures in model ice have to be treated cautiously and not all observations are directly scalable. In an attempt to overcome the most severe issues with HSVA’s model ice in tests with vertically sided structures, a new way of initiating the formation of a model ice sheet was tested as a part of the IVOS project. Instead of spraying a top layer, the water in the basin was kept in motion by a wave maker while crystals formed. When the waves were stopped, an ice sheet with larger crystals in the top layer grew. A compliant vertical structure was pushed through this ice sheet and global and local loads were measured. The measurements were compared to tests with the same structure in conventional columnar model ice. Various ice properties were measured throughout the tests.

This paper introduces an alternative way to create a model ice sheet for tests with vertically sided structures utilizing a wavemaker, and discusses the findings from first model tests.
The Application of Reliability Based Methods in the Optimisation of Reeled Rigid Pipeline Wall Thickness Requirements
Authors: Smith Daniel , Peters Craig , Lahiri Subhajit
Published: September 2018

Reel-lay is a fast and cost-effective means for the installation of subsea flowlines and Pipe-In-Pipe systems with outer diameters up to 18”. Pipelines installed by the reel lay method are plastically deformed during installation. The most critical step in the installation of a reeled pipeline occurs at the spool base, when the assembled pipeline is spooled on to the hub of the reel.

The nominal level of deformation is dictated by the vessel equipment geometry, applied back tension, and pipe dimensions. Localised increases in deformation are caused by mismatches in bending stiffness between adjacent pipes. The mismatch potential is dictated by the natural variation of yield strength and by dimensional variation that is inherent to linepipe manufacturing processes. Reliability based assessments are commonly applied in the assessment of minimum acceptable wall thickness for reeling. These assessments enable the minimum acceptable wall thickness to be determined with a defined target reliability level, assessing mismatches based upon distributions of wall thickness and yield strength.

The mismatch parameter calculation method and the definition of appropriate acceptance criteria are the two most important factors in reeling assessments. Neither of these two factors has been specified in a pipeline design code or a recommended practice available in the public domain. However, there is an increasing level of familiarity in industry; mismatch calculation methods and strain or ovality based acceptance criteria, defined by installation contractors are gaining widespread acceptance.

This paper presents a review of the application of reliability based methods currently under use, focusing on mismatch calculation methods, acceptance criteria, and probability of failure calculation methods. Minimisation of costs is of particular importance in the current oil and gas industry climate. Because of this, the ability to specify an optimum wall thickness enables installation contractors to provide more cost effective reeled rigid pipeline solutions.

After reviewing the subject matter and existing body of work this paper looks in detail at the deformation responses and failure modes for a range of sizes of reeled pipelines with mismatches. The assessment of deformation responses demonstrates a significant level of conservatism in recently proposed acceptance criteria that is based upon averaged axial strain levels. This conservatism is quantified by probability of failure calculations and provides a strong justification for further optimisation of the minimum wall thickness for reeling. Finally, the beneficial effect of increased reeling tension is quantified in terms of its effect upon probability of failure.
Development of a Tool to Identify Potential Zones for Offshore Aquaculture: A Global Case Study for Greater Amberjack
Authors: Weiss Carlos V. C. , Castellanos Omar , Ondiviela Bárbara , Juanes José A. , Guanche Garcia Raúl
Published: September 2018

This paper presents an innovative tool to recognize potential zones for the development of offshore aquaculture activities from an expanded spatial perspective. The methodology — developed in the framework of the TEN-SHORES project (Development of new technologies in oceanic aquaculture RTC-2014-2485-5) — aims to identify optimal conditions for the growth of fish species and for cage resistance. The first step is based on the Delphi method and consists of the selection of variables according to their relevance to fish species and to the cage location. The selected variables were acquired from reanalysis models and remote sensing data (time series of 20–30 years). In the second step, an evaluation system was developed to estimate the percentage of time (on a 0-1 scale) that the selected variables remain in optimum conditions, for the fish and the cage, in the whole data series (grid of 0.25°). Suitability maps were generated according to the conditions for the fish species growth and to house a generic cage. The integration of these maps allows for the identification of potential zones (>0.7) for farming the Greater amberjack (Seriola dumerili), highlighting unexploited offshore zones in South and North America, Oceania and in the Africa continent. The developed tool can be applied to large-scale studies of different aquaculture species, as well as to other types of marine uses.
Low Frequency Excitation and Damping of Four MODUs in Severe Seastates With Current
Authors: Fonseca Nuno , Stansberg Carl Trygve , Larsen Kjell , Bjørkli Rune , Vigesdal Tjerand , Dalane Oddgeir
Published: September 2018

Model tests have been performed with four mobile offshore drilling units (MODUs) with the aim of identifying wave drift forces and low frequency damping. The MODUs configuration is different, namely on the number and diameter of columns, therefore the sample is representative of many of the existing concepts. The model scale is the same as well as the wave and current conditions. The experimental program includes irregular waves with systematic variations of the significant wave height, wave peak period, current velocity and vessel heading.

The test data is post-processed to identify the surge and sway quadratic transfer functions (QTFs) of the slowly varying excitation, together with the linearized low frequency damping. The post-processing applies a nonlinear data analysis technique known as “cross-bi-spectral analysis” to estimate characteristics of second-order (quadratic) responses from the measured motions and undisturbed incident wave elevation. The empirical QTFs are then compared with numerical predictions to conclude on the role of viscous drift and the applicability of Newman’s approximation for calculation of drift forces in irregular waves. Finally, the empirical drift forces, empirical low frequency damping coefficients and low frequency motions statistics are compared for the three MODUs to conclude on the relation between the Semi configuration and the low frequency responses.
Casing Removal Tests in Laboratory Setup
Authors: Taghipour Ali , Ytrehus Jan David , Stroisz Anna
Published: September 2018

Through the life-time of a production field in the offshore petroleum industry it is normal to drill several new wells for both production and injection purposes. The initial well template, either at the platform or at a subsea installation, has space for a fixed total number of wells. When this limit is reached an old well needs to be plugged and the well slot reused to allow new wells to be drilled.

In order to re-use well slots and benefit from full diameter when constructing the new well, it is required to remove the tubulars in the upper part of the plugged well. The outside of these tubulars are normally in contact with cement or settled particles from shut-in drilling fluids. Removing the tubular through the cement or settled particle is always challenging and there is need for using new techniques.

In order to address the dominating effects in these operations, down-scaled laboratory tests are performed. The experiments reported here are performed by pulling steel pipes out of a cemented annulus. The pipes used in the tests are down-scaled from typical casing sizes. They are either normal pipes, grooved pipes or pipes with and without collars. Two setups with different geometries are used. The first is selected to study the de-bonding effect from the cemented annulus and the mechanical friction that must be overcome to remove the pipe. The other setup is designed to show the effect of collars when pulling out the tubulars. Since most tubulars in wells have collars between each stand with extended diameter, this effect is important to consider when comparing laboratory results to field operations.

Results show that the loosening force (de-bonding) and pulling force can be significantly reduced by manipulating the pipes with grooves prior to pulling them out. Further, the results show that the most significant resistance when pulling the tubulars are caused by the collars outside the pipe. It is also observed that the effect of collar is significantly reduced when the pipe is grooved between the collars. In total these results provide improved understanding on the dominating effects when pulling pipes from packed wellbore annulus.
A Segmented Ship Model: Experimental Research on Hydroelastic Effect of a Large Ship in Wave
Authors: Tang Ying , Sun Shili , Jin Kai , Yang Zheng , Ren Huilong
Published: September 2018

In this paper, rigid ship hull and elastic ship hull were respectively designed. For the former, the hull is divided into two segments, while for the latter, into five segments. And a series of experiments of ship model navigating in regular wave were carried out. Time histories of motion and vertical bending moment of the two models were recorded and analyzed. By comparing results of the two models, the impact of elasticity of hull grider on motion and vertical bending moment can be analyzed.
Applications of Wavelet Transform in Ice-Induced Vibrations of Structures
Authors: Yu Bowei , Chen Yingying , Low Yingmin , Yap Kim Thow
Published: September 2018

Spectral methods such as Fourier Transform are used to analyze the response of structures to reveal characteristics of time series in frequency domain, as is typically done in the study of ice-induced vibrations. The limitations of these methods lie in that they assume the signal to be stationary and decompose the signal as a whole. Wavelet Transform has been developed for several decades to overcome these limitations. In this paper, the technique of Wavelet Transform is applied to ice force and structural response signals that provides a means to simultaneously analyze the data in both time and frequency domains. The results show that the applied method provides useful visualization of signals, allowing fast identification of vibration events of interest from long time series. A frequency locked-in event can be identified at intervals with high energy concentration at the fundamental frequency of the structure and relatively small variations in phase lag between the load and response. A random vibration is found that energy at the fundamental frequency of the structure occurs intermittently in time domain and almost all frequencies are excited showing a wide band spectrum. Simulation results obtained from a purpose-developed numerical model show that the same characteristics can be observed compared to full-scale measurements from the Norstromsgrund lighthouse.