bluuebunny/crossref_metadata_2025 huggingface dataset:
Investigation of Bottom Slamming on Ships in Irregular Waves
Authors: Wang Shan , Rajendran Suresh , Guedes Soares C.
Published: September 2018

The bottom slamming of a chemical tanker and a LNG carrier advancing in irregular waves is investigated numerically and compared with experiments. The probability of slamming occurrence on longitudinal locations and the slamming induced pressures on the bottom of two ships are discussed. It is considered that slamming occurrence at a point is dependent upon two conditions: the relative vertical motion at the same longitudinal position of the ship being larger than the vertical distance from the still water to the concerned position, and the entry velocity exceeding some threshold velocity.

Ship motions in irregular waves predicted by a time domain seakeeping code and measured from the model tests are used to calculate the slamming occurrence statistically based on the two conditions mentioned above. Only heave and pitch motions are considered in the calculations. The seakeeping code combines body linear radiation and diffraction forces with body nonlinear Froude-Krylov forces, hydrostatic forces and shipping of green water on the bow. The effects of body nonlinearity are considered by a simplified method: the memory functions, infinite frequency added masses and the radiation restoring coefficients are assessed at each time instant as function of the instantaneous wetted surface. A similar procedure is used to calculate the diffraction forces.

The experimental data of the wave-induced loads on these two vessels in different sea states are analyzed statistically. Probability of exceedance of entry velocities and pressure peaks for the sections at the bow and stern are computed for various irregular sea states. The results of the slamming occurrence on longitudinal locations and wave induced loads on these two types of ships are discussed.
Code-to-Code Verification of End-Anchored Floating Bridge Global Analysis
Authors: Viuff Thomas , Xiang Xu , Leira Bernt , Øiseth Ole
Published: September 2018

A code-to-code comparison of the dynamic characteristics of an end-anchored floating pontoon bridge is presented for two commercial software commonly used in the offshore industry, i.e. SIMO-RIFLEX and ORCAFLEX. Furthermore, a second comparison of the hydrodynamic properties of the floating pontoons are compared between two commercial Boundary Element Method software, i.e. AQWA and WADAM. The pontoons are modelled with an extended bottom to change the dynamic properties of the bridge structure. Viscous effects of this so-called heave plate is not considered in the study. The study shows good comparisons between the natural periods of the floating bridge structure and both software yield good convergence in the time domain. However, significant differences in the stochastic response has been found.
Instrumented FLIP Test and Static Pressure Influence on the Onset Velocity and Frequency on an Industrial Scale
Authors: Decuupere M. , Charliac D. , Roques JP. , Karnikian A. , Galeron G. , Mattei PO. , Amielh M.
Published: September 2018

Since the early 2000, Flow Induced Pulsations (FLIP) has been more and more encountered on platforms. This phenomenon generates high acoustic pressure pulsations that may cause noises up to one hundred and ten dB and significant fatigue stress levels in small piping either at topside or subsea equipment. The source of the phenomenon is inside of the flexible pipe but FLIP has no effect on it. Nevertheless, in case of FLIP experience companies may have to reduce their flow rate. Therefore, FLIP must be understood in order for the companies to avoid this constraint.

In this frame, a FLIP test was performed with protagonists who are involved in the understanding of this phenomenon. The test was done in 2016 at CESAME Poitiers (France) in an eighteen meter-long and six-inch flexible pipe on an air open loop. The prototype was fully instrumented and pressures up to forty bars were tested and mass flow rates up to 6 kg.s−1 to reproduce the FLIP phenomenon.

The test setup and signals analysis are presented in this paper. Moreover, FLIP onset velocities and frequencies are compared with TechnipFMC models. As a conclusion of this paper pressure influence for the six-inch tested on the FLIP initiation will be presented.
Application of Morison Equation in Irregular Wave Trains With High Frequency Waves
Authors: Trubat Pau , Molins Climent , Hufnagel Philipp , Alarcón Daniel , Campos Alexis
Published: September 2018

Most numerical models for the analysis of offshore wind platforms are based on one of two different approaches, depending on how waves forces are applied to the structure: 1) the potential flow theory, and 2) the Morison equation. Potential flow theory allows to compute the wave forces more accurately when diffraction is relevant. Otherwise, this kind of models assume a fixed position of the floating platform when computing the wave forces. Additionally, second-order effects, as the position and the spin of the structure relative to the incident wave can only be taken into account if second order potential flow is considered. On the other hand, Morison equation can apply the wave forces on a structure based on its spin and position which can be assessed at each time step, but is prone to overestimate the waves forces at the frequencies where diffraction is relevant.

In this paper, a modification of the implementation of the Morison equation is presented. This modification allows to reduce the forces in the diffraction frequency range based on the real response from MacCamy and Fuchs’s diffraction theory for cylinders. The implementation can be applied using a frequency-dependent coefficient of added mass, or modifying the amplitudes of the incident waves in the diffraction frequency range in a way that the accelerations derived from the regular wave theory used for the Froude-Krylov wave force computation in Morison equation are equivalent to those computed in the diffraction theory. The implementation is tested in the FloawDyn code, developed at the UPC, and FAST from NREL.
The True Digital Twin Concept for Fatigue Re-Assessment of Marine Structures
Authors: Tygesen Ulf T. , Jepsen Michael S. , Vestermark Jonas , Dollerup Niels , Pedersen Anne
Published: September 2018

The paper presents a true digital twin concept, which is a general and novel methodology that significantly improves the fatigue prediction models of existing marine structures. The actual structural condition of existing marine platforms can often change after several years in operation due to degradation mechanisms and/or other structural changes. It is within this context, the true digital twin concept has been developed and the general idea is to create a coupling between the digital twin and measurements. The measurements are performed by Structural Health Monitoring Systems (SHMS). This coupling facilitates a direct performance evaluation of the digital twin against measurements and most importantly creates the basis for improving the performance of the digital twin to accurately capture the actual condition of the structure, and thus become a true digital twin. The full concept of creating a true digital twin encompass novel advanced analysis methods ranging from linear system identification, expansion processes, Bayesian FE model updating, wave load calibration, quantification of uncertainties from measured data, and Risk- and Reliability Based Inspection Planning (RBI) analysis, [1]. This paper presents the first 3 levels for establishing a true digital twin. The levels are illustrated by 3 case stories.
Investigation of Higher-Harmonic Wave Forces and Ringing Using CFD Simulations
Authors: Kamath Arun , Bihs Hans , Pakozdi Csaba
Published: September 2018

Typical offshore structures are designed as tension-leg platforms or gravity based structures with cylindrical substructures. The interaction of waves with the vertical cylinders in high sea states can result in a resonant response called ringing. Here, the frequency of the structural response is close to the natural frequency of the structure itself and leads to large amplitude motions. This is a case of extreme wave loading in high sea states. This understanding of higher-order wave forces in extreme sea states is an essential parameter for obtaining a safe, reliable and economical design of an offshore structure. The study of such higher-order effects needs detailed near-field modelling of the wave-structure interaction and the associated flow phenomena. In such cases, a Computational Fluid Dynamics (CFD) model that can accurately represent the free surface and further the wave-structure interaction problem can provide important insights into the wave hydrodynamics and the structural response. In this paper, the open source CFD model REEF3D is used to simulate wave interaction with a vertical cylinder and the wave forces on the cylinder are calculated. The harmonic components of the wave force are analysed. The model employs higher-order discretisation schemes such as a fifth-order WENO scheme for convection discretisation and a third-order Runge-Kutta scheme for time advancement on a staggered Cartesian grid. The level set method is used to obtain the free surface, providing a sharp interface between air and water. The relaxation method is used to generate and absorb the waves at the two ends of the numerical wave tank. This method provides good quality wave generation and also the wave reflected from the cylinder are absorbed at the wave generation zone. In this way, the generated waves are not affected by the wave interaction process in the numerical wave tank. This is very essential in the studies of higher-order wave interaction problems which are very sensitive to the incident wave characteristics. The numerical results are compared to experimental results for higher-order forces on a vertical cylinder to validate the numerical model.
Relocatable Tide Prediction and Storm Surge Forecasting
Authors: Prime Thomas
Published: September 2018

The marine environment represents a large and important resource for communities around the world. However, the marine environment increasingly presents hazards that can have a large negative impact. One important marine hazard results from storms and their accompanying surges. This can lead to coastal flooding, particularly when surge and astronomical high tides align, with resultant impacts such as destruction of property, saline degradation of agricultural land and coastal erosion.

Where tide and storm surge information are provided and accessed in a timely, accurate and understandable way, the data can provide:

1. Evidence for planning: Statistics of past conditions such as the probability of extreme event occurrence can be used to help plan improvements to coastal infrastructure that are able to withstand and mitigate the hazard from a given extreme event.

2. Early warning systems: Short term forecasts of storm surge allow provide early warnings to coastal communities enabling them to take actions to allow them to withstand extreme events, e.g. deploy flood prevention measures or mobilise emergency response measures.

Data regarding sea level height can be provided from various in-situ observations such as tide gauges and remote observations such as satellite altimetry. However, to provide a forecast at high spatial and temporal resolution a dynamic ocean model is used. Over recent decades the National Oceanography Centre has been a world leading in developing coastal ocean models. This paper will present our progress on a current project to develop an information system for the Madagascan Met Office. The project, C-RISC, being executed in partnership with Sea Level Research Ltd, is translating the current modelling capability of NOC in storm surge forecasting and tidal prediction into a system that will provide information that can be easily transferred to other regions and is scalable to include other hazard types The outcome, an operational high-resolution storm surge warning system that is easy to relocate, will directly benefit coastal communities, giving them information they need to make effective decisions before and during extreme storm surge events.
Analysis of Wellbore Instability in an Offshore Field: A Case Study
Authors: González Molano Nubia Aurora , Alvarellos Iglesias José , Vargas Mendoza Pablo Enrique , Lakshmikantha M. R.
Published: September 2018

Several wellbore instability problems have been encountered during drilling a shale formation in an offshore field, leading to the collapse of the main borehole and resulting in several sidetracks. In this study, an integrated 1D & 3D Geomechanical model was built for the field in order to investigate the major factors that control the instability problems from a Geomechanical point of view and to design an optimum mud window for planned wells in the field. Effect of bedding on wellbore stability was the most important factor to explain the observed drilling events.

Optimized well paths for planned wells were proposed based on results of a sensitivity analysis of the effect of bedding orientation on wellbore stability. It has been observed that bedding exposed depends not only on well inclination but also on dip of the formation, attack angle, and azimuth.
A Modified Engineering Critical Assessment Approach for Offshore Pipelines
Authors: Holtam Colum , Saraswat Rajil , Thodla Ramgopal , Gui Feng
Published: September 2018

Environmentally assisted sub-critical static crack growth can occur in offshore pipelines exposed to aggressive production environments. Recent advances in fracture mechanics testing methods have shown that slow static crack growth rates can be reliably measured in sweet and sour environments under constant stress intensity factor (K) conditions. This has potential implications for the engineering critical assessment (ECA) of pipe girth welds subject to low cycle fatigue loading with long periods of operation under constant static load between cycles, e.g. lateral buckling. This paper demonstrates the influence of including static (i.e. time dependent) crack growth as well as fatigue crack growth in a modified pipeline ECA approach.
Optimal Online Configuration and Load-Sharing in a Redundant Electric Power System for an Offshore Vessel Using Mixed Integer Linear Programming
Authors: Thorat Laxminarayan , Skjetne Roger
Published: September 2018

This paper proposes several schemes for optimal online configuration and load sharing for a shipboard power system of a typical offshore vessel, having a number of varying capacity gensets. Different methods are presented for optimal online scheduling and minimization of specific fuel oil consumption using mixed integer linear programming. The simulations for optimization of specific fuel oil consumption for three different scheduling methods demonstrate their properties.
Active Load Mitigation to Counter the Fatigue Damage Contributions From Unavailability in Offshore Wind Turbines
Authors: Sørum Stian H. , Smilden Emil , Amdahl Jørgen , Sørensen Asgeir J.
Published: September 2018

The offshore wind industry continues to grow, but there is still a need for more economical designs. As unavailability conditions can be critical for the fatigue damage in support structures, design standards use conservative values for availability. This leads to most turbines having an unused fatigue capacity at the end of the lifetime. This paper investigates the potential for reducing this unused capacity in order to reduce the capital expenses. The proposed strategy is to design the turbines for a higher availability, closer to the expected value. For individual turbines that experience lower availability than the design value, active load mitigation is imposed to reduce the fatigue damage. The potential of this methods is explored, together with its limitations. It is found that the effect of faults occurring early in the turbines lifetime can be reduced. This is not the case for faults occurring towards the end of the lifetime.
Comparison of the Long-Term Trends of the Largest Waves in the Ice-Free Arctic Waters From Different Reanalysis Products
Authors: Waseda Takuji , Nose Takehiko , Webb Adrean
Published: September 2018

The long-term trends of the expected largest waves in the ice-free Arctic waters from Laptev to Beaufort Seas was studied analyzing the ERA-interim reanalysis from 1979 to 2016. The analysis showed that the positive trend is largest in October and increased almost 70 cm in 38 years. For ships navigating the Northern Ship Route, it is important to know what the possible largest waves to expect during its cruise. In view of conducting the extreme value analysis, the uncertainty of the largest wave needs to be validated. However, the observation in the Arctic Ocean is limited. We, therefore, rely on the reanalysis wave products in the Arctic Ocean, whose uncertainty is yet to be determined. ERA-Interim and ERA-5 are compared in the Laptev, the East Siberian, Chukchi and Beaufort Seas. The comparison is relevant as the two products differ in its horizontal grid resolution and availability of the satellite altimeter significant wave height data assimilation. During 2010–2016 when the ERA5 is available, only a small difference from ERA-Interim was detected in the mean. However, the expected largest waves in the domain tended to be large for the ERA-5, 8% normalized bias. The tendency was quite similar with a high correlation of 0.98.
Maximum Dissimilarity-Based Algorithm for Discretization of Metocean Data Into Clusters of Arbitrary Size and Dimension
Authors: Kanner Samuel , Aubault Alexia , Peiffer Antoine , Yu Bingbin
Published: September 2018

In order to accurately estimate the fatigue life a floating structure, it is necessary to have a large set of discrete environmental conditions. If the damage to a structure largely stems from wave-induced forces, then the creation of a set of environmental conditions or ‘bins’ is trivial. However, when considering a floating platform supporting a wind turbine, it is necessary to consider not only the wave conditions, but also the wind conditions (and perhaps current, if possible). Thus, it is common to have greater than 5 dimensions in the timeseries (e.g., significant wave height, wave period, wave direction, wind speed, wind direction, etc). The creation of bins in two dimensions is quite easily solved by creating an arbitrary grid and taking the mean of all the observations which fall in a specific cell. In higher dimensions, a p-dimensional cell is not easily visualized and so the resulting set of bins cannot easily be graphically represented. In this paper, an iterative algorithm is developed to convert N observations, each with p-dimensions, into a set with M discrete bins, where M << N.

The algorithm presented borrows heavily from the maximum dissimilarity algorithm used in a wide array of fields. The benefit of using this algorithm is that there is no ‘bias’ introduced by an initial grid from the user. That is, given a desired final number of clusters and a certain distance tolerance, a unique set of cluster exists for a given data set. Inherently, the algorithm selects a diverse array of observations, usually including extreme events or outliers, which may have undue impact on the fatigue life of a structure. Although the algorithm is computationally expensive O(N2M), reductions in computational cost are possible. Most importantly, the algorithm can be written in such a way that memory constraints are not an issue even for N = O(105).

The clustering algorithm is described in both graphical and logical terms. A case study is presented, using publicly available data from the Netherlands Enterprise Agency. The data is visualized in two dimensions with the final number of bins equaling approximately 50, 100, 200, 500, 1000, and 2000 bins. These bins are compared with a previous algorithm from these authors. Various measures are presented to assess the fidelity of a set of bins with respect to the initial observations. Each set of bins are analyzed and it is clear the MDA-based algorithm outperforms the previous algorithm.
Experimental Investigation on Resistance Reduction by Means of Air-Bubbling Technique
Authors: Ravina Enrico , Guidomei Sofia
Published: September 2018

The paper refers on a research activity, focused at DREAMS Lab of the University of Genoa (Italy) and still under development, oriented to experimental application of air-bubbling techniques on flat plates and hull models. In this study the reduction in the frictional resistance by air bubbling generated by customized pneumatic circuits is tested, both on the lower surfaces of flat plates characterized of different geometries of holes and on a hull model tested in towing tank. The effective shape of air bubbles is observed, and changes in the local frictional drag are measured, using flexible and low cost thin sensors at different levels of flow rate and pressure of injected air. In towing tank tests the experiments compare hull without and with holes on the bottom, modifying the characteristics of speed, pressure, flow rate and areas interested to the air injection.

Systematic tests campaign has been developed, using also actuation pneumatic workbenches expressly designed for the experiments.
Modelling Changes to Submarine Pipeline Embedment and Stability due to Pipeline Scour
Authors: Draper Scott , Griffiths Terry , Cheng Liang , White David , An Hongwei
Published: September 2018

In this paper a beam bending model is combined with existing predictive formulas for pipeline scour to study changes to pipeline stability during scour and lowering. The model is introduced and demonstrated for a range of simplified conditions, including scour-induced lowering of a pipeline resulting from multiple uniformly spaced scour initiation points. The model is then used with a synthetic seabed generated with a variety of length scales. In this simulation the pipeline is ‘laid’ onto the seabed, leading to the formation of ‘natural’ initiation points for scour. The distribution and spacing of the initiation points (which are a function of the pipeline bending stiffness, tension and seabed roughness) lead to different rates of pipeline lowering and stability. The resulting model may be used within a probabilistic framework to estimate changes to pipeline stability resulting from sediment mobility and scour.
Time-Dependent Computational Fluid Dynamics (CFD) Simulations of a Closing Blowout Preventer
Authors: Barreca Daniel , Franchek Matthew , Tyagi Mayank
Published: September 2018

Reliability of blowout preventers (BOP) is central for the safety of both rig workers and the surrounding environment. Analysis of dynamic fluid conditions within the wellbore and BOP can provide quantitative data related to this reliability. In cases of a hard shut in, it is suspected that the sudden closure of rams can cause a water hammer effect, creating pressure vibrations within the wellbore. Additionally, as the blowout preventer reaches a fully closed state, fluid velocity can drastically increase. This results in increased erosion rates within the blowout preventer. To investigate fluid movement and pressure vibrations during a well shut-in, CFD simulations will be conducted. Dynamic meshing techniques within ANSYS® FLUENT can be utilized to simulate closing blowout preventer configurations for both 2-D and 3-D geometries. These simulations would deliver information that could lead to a better understanding of certain performance issues during well shut-ins. Such information includes flow velocity magnitude within the BOP and maximum pressure pulse values within the wellbore.
AMBEMAR-DSS: A Decision Support System for the Environmental Impact Assessment of Marine Renewable Energies
Authors: Guinda Xabier , Puente Araceli , Juanes José A. , Royano Francisco , Fernández Felipe , Vega Marco A. , García Andrés , García Javier , Aragón Germán , Abascal Ana J. , Otero César , Manchado Cristina , Gómez-Jauregui Valentín , López Joaquín , Monteoliva Agustín
Published: September 2018

The high energy demand and the threat of climate change have led to a remarkable development of renewable energies, initially through technologies applied to the terrestrial environment and, recently, through the awakening of marine renewable energies. However, the development of these types of projects is often hampered by failure to pass the corresponding environmental impact assessment process.

The complexity of working in the marine environment and the uncertainties associated with assessing the impacts of such projects make it difficult to carry out objective and precise environmental impact assessments.

AMBEMAR-DSS seeks to establish a basis for understanding and agreement between the different stakeholders (project developers, public administrations, environmental organizations and the public in general), in order to find solutions that allow the development of marine renewable energies, minimizing their environmental cost. For this purpose, a DSS is proposed which, based on cartographic information and using objective and quantifiable criteria, allows comparative assessments and analyses between different project alternatives. The analytical procedures used by the system include, among others, hydrodynamic modeling tools and visual impact simulators. In addition, impacts on marine species are assessed taking into account intrinsic ecological and biological aspects. The magnitude of the impacts is quantified by means of fuzzy logic operations and the integration of all the elements is carried out by an interactive multi-criteria analysis. The results are shown in tables, graphs and figures of easy interpretation and can be also visualized geographically by means of a cartographic viewer.

The system identifies the main impacts generated in the different phases of the project and allows establishing adequate mitigation measures in search of optimized solutions. The establishment of the assessment criteria has been based on the abundant, but dispersed, scientific literature on the various elements of the system and having the opinion of experts in the various fields. Nevertheless, the DSS developed constitutes a preliminary basis on which to build and improve a system with the input of researchers, promoters and experts from different disciplines.
Flexible Flowline Walking Tendency on Seabed With Longitudinal Undulations and Transverse Gradients
Authors: Roberts Graeme , Sriskandarajah T. , Colonnelli Gianluca , Roux Arnaud , Roy Alan , Ragupathy P.
Published: September 2018

A method of carrying out a combined axial walking and lateral buckling assessment for a flexible flowline has been developed using finite element analysis. The method overcomes limitations of screening assessments which could be inconclusive when applied either to a flexible flowline on an undulating seabed with transverse gradients or to one that buckles during hydrotest.

Flexible flowlines that were to be surface-laid on a seabed with longitudinal undulations and transverse gradients were assessed using the method. The flexible flowlines were simulated in their as-laid state, and the simulation incorporated hydrotest pressure and the pressure & temperature gradients and transients associated with multiple start-ups. The objective was to quantify the axial walking and lateral slip tendency of the flexible flowlines and the impact that walking might have on the connected end structures. The lateral buckle locations predicted by finite element analysis were compared to a post-hydrotest survey and the radius of curvature from analysis was compared to the minimum bend radius of the flexible.
Experimental Investigation on the Hydrodynamic Responses and Coefficients of Straked Flexible Pipe in Oscillatory Flow
Authors: Ren Haojie , Cheng Jingyun , Zhang Mengmeng , Cao Peimin , Fu Shixiao
Published: September 2018

The helical strakes are now widely being used in offshore riser design for the suppression of vortex induced vibrations (VIV). The purpose of this paper is to investigate the responses and suppression effectiveness of a straked pipe in a more real working conditions of the risers which will endure a kind of “oscillatory flow” due to the relative motions between the fluid around and the risers induced by the top platform motions. Experiments are performed on a flexible straked pipe with pitch length/height of 15D/0.25D in ocean basin. The pipes are forced to harmonically oscillate in various combinations of amplitude and period with Keulegan-Carpenter (KC) number varying between 5 and 165, and maximum reduced velocities from 4 to 12. Responses in both in direction are firstly investigated. Inverse analysis method and the Least Square method are adopted to identify drag coefficients and added mass coefficients. The results show that strakes can reduce the higher frequency responses in both CF and IL direction. Suppression efficiencies of the strakes and are not ideal as expected in oscillatory flow. Moreover, the hydrodynamic coefficients change dramatically under the small KC number and stabilize under the large KC number. The drag coefficients obviously magnify at KC ∼ 20.
Strain-Based Fatigue Reliability Analysis of a Load-Carrying Fillet Welded Cruciform Joint
Authors: Dong Yan , Garbatov Yordan , Guedes Soares C.
Published: September 2018

The objective of this work is to perform a fatigue reliability analysis of a load-carrying fillet welded cruciform joint based on the local strain approach. The effective notch stresses of the weld root and toe of the cruciform joint, where the fatigue cracks are usually initiated, are estimated by the finite element method and the fatigue notch factors of these two locations as a function of the weld leg and slit lengths are explicitly represented by response surface models. Within the context of the local strain approach, a critical fatigue notch factor that can exactly trigger fatigue failure is proposed. The statistical descriptors of the critical fatigue notch factor are determined by using the Monte Carlo simulation method, in which the nominal stress range, material properties and fatigue damage at failure are treated as random variables. The limit state functions of the weld root and toe are formulated based on the response surface models and critical fatigue notch factors. The first order reliability method is applied to evaluate the reliability against the fatigue damage. Finally, the cruciform joint system, composed by the two fatigue-prone locations, is evaluated as a series system of components.
Numerical Study on the Hydrodynamic Performance of Integrated Interceptor-Flap Fitted to the Transom of a Planing Vessel
Authors: Jangam Suneela , Krishnankutty Parameswaran , Subramanian V. Anantha
Published: September 2018

Depending on the type of support, vessels are classified as displacement, semi-displacement and planing. But all types of vessels are in the displacement mode when they operate at low speed. In planing, due to the supportive hydrodynamic pressure, the hull wetted surface area reduces leading to low frictional resistance and consequent increase in speed for the same power input. Planing vessels are used for different purposes such as for fast patrol, sport activities, service, ambulance, rescue and recreation. The use of stern flaps, both fixed or controllable, interceptors and integrated interceptor-flap in high speed boats has become an acceptable option to control the running trim of the vessel to enhance its speed and powering performance. The interceptor-flap changes the pressure distribution underneath the hull which in turn causes reduced resistance acting on ships aftbody. The integrated stern interceptor-flap effect on planing craft performance depends on its parameters and also on those of the craft. So, an in depth study on the hydrodynamic behaviour of integrated interceptor-flap is essential, before it is adapted to a vessel, to get the best performance during the craft operation. In recent years, the computational fluid dynamics (CFD) technique has proved to be accurate and robust for hydrodynamic calculation of high-speed planing hulls. The aim of this paper is to study numerically on the performance of planing hull fitted with integrated stern interceptor-flap configuration. These studies help in understanding the flow field and other parameters on resistance of planing hulls with different flap angles. The study shows that the interceptor-flap performs well compared to bare hull. The guidelines that could be derived from these studies help in improving the interceptor-flap design for a high speed planing craft.
Cutting of Fluid Saturated Rock: A Tentative Explanation of Dilation vs. Compaction
Authors: Helmons Rudy L. J. , van Rhee Cees , Detournay Emmanuel , Alvarez Grima Mario
Published: September 2018

Traditionally, rock cutting models for land-based and shallow water applications assume that the rock is cut under atmospheric and dry conditions. For deep sea applications this assumption is not valid, especially because the hydrostatic pressure can be of the same order of magnitude as the unconfined compressive strength of the rock. Therefore it is necessary that both the effect of hydrostatic pressure and the presence of a pore fluid are included in the rock cutting models as well.

This paper focuses on extension of the existing phenomenological rock cutting theory for deep sea applications. The suggested improvements of the model are based on observations from both experiments and simulations. The ideas presented can be helpful for the design of equipment for the drilling, deep sea mining and subsea trenching industries.
Impact of Connection Properties on Dynamic Response of Modular Floating Structures
Authors: Zhang Yuming , Liu Haixiao
Published: September 2018

A geometric model is established based on pre-existing model tests, and the rigid module flexible connector (RMFC) concept is adopted to perform hydrodynamic analysis of the modular floating structure. To obtain knowledge of the impact of tri-axial stiffness combinations, a large number of cases are designed and the dynamic performances are plotted. In order to reduce the extreme response induced by the structural resonance, damping components are designed and integrated with elastic connectors. Cases of tri-axial combinations with different damping ratios are also put into dynamic analysis. Finally, qualitative knowledge of the impact of connection properties on the dynamic response of modular floating structures is obtained and advices on the design and optimization of connecting structures are proposed.
Escort Tug Hydrodynamic Forces Estimation in a Design Framework: From Model Test to Manoeuvrability Simulation
Authors: Piaggio Benedetto , Viviani Michele , Martelli Michele
Published: September 2018

The manoeuvring capabilities of an escort tug are essential key-features with a view to an all-around design approach devoted to an optimum oriented framework. From the hull geometry settlement and propulsive solution definition, the ability to predict thoroughly the operative life in terms of handling, effectiveness, and safety becomes fundamental, even more with the perspective of developing smart control logics supporting the masters. With such an aim, the availability of a reliable manoeuvrability model is of fundamental importance. In this context, it is important to define an optimum set of captive manoeuvrability tests, whether experimental or numerical, in order to correctly identify the vessel escort performances in simulation; this, always keeping in mind the necessity to limit the effort in view of an application at an early-design stage. Starting from a wide experimental matrix, purposely designed and realized, the present investigation focuses on the quality and robustness of the different regression manoeuvrability models by letting vary the subsets of tests, in order to determine, finally, which is the most suitable hull model to be adopted and which is the minimum set to be performed.
Internal Surface Crack Growth in Offshore Rigid Pipes Reinforced With CFRP
Authors: Li Zongchen , Jiang Xiaoli , Liu Zhiping , Hopman Hans
Published: September 2018

Rigid pipelines have been widely applied in offshore oil & gas operation and transmission industries on account of their structural simplicity, cost-effectiveness, ease of installation and maintenance. However, surface cracks frequently appear in the internal surface of rigid pipes due to dynamic loads or hydrogen embrittlement, etc. Under dynamic fatigue loads, surface cracks may continue to propagate and finally develop into penetrated cracks, which may cause leakage and serious accidents.

Fiber-reinforced polymer (FRP) strengthening technology is already a reliable technique for structure maintenance in onshore pipelines and penetrated cracks in load-bearing circular hollow sections (CHS). Nevertheless, there are very limited systematic investigations of surface crack in rigid pipes reinforced with FRP, which has a remarkable significance for offshore rigid pipes.

This paper aims to understand the mechanism of semi-elliptical surface crack growth in the internal surface of rigid pipes under fatigue bending moment reinforced with FRP. Stress intensity factors along the crack front are computed through finite-element (FE) models, which are validated by experimental data from references. The influence of wrapping orientation of CFRP are discussed as well. The numerical results show that under CFRP reinforcement, surface crack growth rate decreases significantly which ensures the safety use of rigid pipes in offshore industry.
Generation of Regular and Irregular Waves in Navier-Stokes CFD Solvers by Matching With the Nonlinear Potential Wave Solution at the Boundaries
Authors: Choi Youngmyung , Bouscasse Benjamin , Seng Sopheak , Ducrozet Guillaume , Gentaz Lionel , Ferrant Pierre
Published: September 2018

The capability of wave generation and absorption in a viscous flow solver becomes important for achieving realistic simulations in naval and offshore fields. This study presents an efficient generation of nonlinear wave fields in the viscous flow solver by using a nonlinear potential solver called higher-order spectral method (HOS). The advantages of using a fully nonlinear potential solver for the generation of irregular waves are discussed. In particular, it is shown that the proposed method allows the CFD simulation to start at the time and over the space of interest, retrieved from the potential flow solution. The viscous flow solver is based on the open source library OpenFOAM. The potential solvers used to generate waves are the open source solvers HOS-Ocean and HOS-NWT (Numerical Wave Tank). Several simulation parameters in the CFD solver are investigated in the present study. A HOS wrapper program is newly developed to regenerate wave fields in the viscous flow solver. The wrapper program is validated with OpenFOAM for 2D and 3D regular and irregular waves using relaxation zones. Finally, the extreme waves corresponding to the 1000 year return period condition in the Gulf of Mexico are simulated with the viscous flow solver and the wave elevation is compared with the experiments.
Instantaneous Center of Rotation in Pitch Response of a FPSO Submitted to Head Waves
Authors: Costa Daniel de Oliveira , Fernandes Antonio Carlos , Sales Junior Joel Sena , Asgari Peyman
Published: September 2018

When under influence of an incident wave system, any floating body presents a general motion with all six degrees of freedom, unless it presents some kind of restrains on it. For a free moving body, the center of rotation will depend on the force distribution and might not coincide with its center of gravity. For long and slender floating structures, such as FPSO platforms, a small change in the center of Pitch rotation would result in significant change in the overall motions in its fore and aft regions. Therefore, it is of high importance to obtain a better understating of the instantaneous position of the body center of rotation in Heave and Pitch response.

This paper investigates the position of the Instantaneous Center of Rotation in Pitch Response of a scaled down model of a FPSO platform under different regular wave conditions. The investigation uses basic kinematics equations for rigid body, defining the 6 degrees of freedom of the rigid body motion from a finite number of markers installed in the model. A high quality tracking system captures the markers positions in order to define the rigid body at each instant of time. For an initial approach, the study considers the response due to head waves seas with experimental validation.
Instrumented Wellhead Load Relief System for Shallow Water Arctic Conditions: Paper 1 — System Design, Installation and Preliminary Results
Authors: Benson Scott , Russo Massimiliano , Rasten Eivind , Avery Ward , LeGrow Paul , Ash Michael
Published: September 2018

In recent years, lower oil prices have forced many oil companies to reduce capex costs by revitalizing brown fields, rather than developing new green fields. At the same time, the offshore drilling rig market has seen many old rigs, typically used for shallow water operations, being scrapped, leaving new generation, deep and ultra-deep water MODUs as the only viable option for new drilling campaigns. Based on the above, wellhead fatigue on older assets, especially in harsh, shallow water environments, has started to gain a central role during the planning phases of workover and intervention operations. In recent years, Suncor Energy began investigating an extension to its Terra Nova field, which began production in 2002. The field uses subsea wells tied back to an FPSO which is moored in 95m of water off Canada’s eastern Grand Banks, an area frequented by icebergs. Drilling operations for the field extension were planned to commence in summer 2017, and continue with a year-round drilling campaign using a Cat 6 MODU. Since the extension would involve sidetracks and interventions from existing wellheads, a series of wellhead fatigue studies were undertaken using a variety of industry recognized methodologies [1] to understand the levels of fatigue accumulation. Although there has been no evidence of wellhead fatigue damage, Suncor chose to take a very prudent and proactive approach, aimed at minimizing fatigue, and maintaining fatigue life for potential future drilling operations. An Instrumented Wellhead Load Relief (iWLR) system was installed, which is designed to restrain BOP motions, thereby reducing the wellhead loads considerably. The load reduction system virtually eliminates additional fatigue accumulation for the planned operations. Additionally, the instrumentation system enables the precise monitoring and tracking of loads applied at the wellhead for future analysis. This paper describes the engineering challenges needed to develop and install the iWLR system in a harsh, shallow water, arctic environment. This area is characterized by very stiff soils pitted with iceberg scours, where subsea equipment must be protected within 10m deep excavated drill centers to prevent iceberg collisions in the relatively shallow water. Additionally, the paper describes how the instrumentation system was integrated with the BOP MUX cable communication system, for the first time, to enable real time monitoring of BOP motions using high accuracy gyroscopes and load cells which monitor dynamic iWLR tether forces. A topside data gathering and processing system was developed to present wellhead loads based on the indirect method, with new algorithms established to account for the tether forces. Finally, the paper presents some preliminary high-level results, showing the efficiency of the system based on measured data.
Large-Scale Experiments of Wave-Overtopping Loads on Walls: Layer Thicknesses and Velocities
Authors: Cappietti Lorenzo , Simonetti Irene , Esposito Andrea , Streicher Maximilian , Kortenhaus Andreas , Scheres Babette , Schuettrumpf Holger , Hirt Matthias , Hofland Bas , Chen Xuexue
Published: September 2018

Wave-Overtopping loads on vertical walls, such as those located on top of a dike, have been investigated in several small-scale experiments in the past. A large-scale validation for a mild foreshore situation is still missing. Hence the WALOWA (WAve LOads on WAlls) experimental campaign was carried out to address this topic. This paper, first presents a description of the large-scale model, the measurement set-up and the experimental methodologies, then it focuses on the layer thicknesses and velocities of the flows created on the promenade by the wave overtopping. A set of resistive wave gauges, ultrasonic distance sensors and velocimeters have been used to conduct these measurements. Preliminary data analysis and results, related to a 1000 irregular waves long test, are discussed. The momentum flux of these flows is studied and its implications, for the wave-overtopping loads acting on the vertical walls, are highlighted.
Wave Energy Extraction by the End of the Century: Impact of the North Atlantic Oscillation
Authors: Janjić Jelena , Gallagher Sarah , Gleeson Emily , Dias Frédéric
Published: September 2018

Using wind speeds and sea ice fields from the EC-Earth global climate model to run the WAVEWATCH III model, we investigate the changes in the wave climate of the northeast Atlantic by the end of the 21st century. Changes in wave climate parameters are related to changes in wind forcing both locally and remotely. In particular, we are interested in the behavior of large-scale atmospheric oscillations and their influence on the wave climate of the North Atlantic Ocean. Knowing that the North Atlantic Oscillation (NAO) is related to large-scale atmospheric circulation, we carried out a correlation analysis of the NAO pattern using an ensemble of EC-Earth global climate simulations. These simulations include historical periods (1980–2009) and projected changes (2070–2099) by the end of the century under the RCP4.5 and RCP8.5 Representative Concentration Pathway (RCP) forcing scenarios with three members in each RCP wave model ensemble. In addition, we analysed the correlations between the NAO and a range of wave parameters that describe the wave climate from EC-Earth driven WAVEWATCH III model simulation over the North Atlantic basin, focusing on a high resolution two-way nested grid over the northeast Atlantic. The results show a distinct decrease by the end of the century and a strong positive correlation with the NAO for all wave parameters observed.