bluuebunny/crossref_metadata_2025_split huggingface dataset:
Coupled Analysis of FPSO and CALM Buoy Offloading System in West Africa
Authors: Kang Youwei , Sun Liping , Kang Zhuang , Chai Shuhong 
Published: September 2014

FPSO and CALM (Catenary Anchor Leg Mooring) buoy offloading systems are being extensively employed in water field of West Africa. In this paper we did some research on the response of FPSO and CALM offloading systems under special environment conditions in West Africa. It is found that Swell dominate sea state has much larger effects on wave frequency motions of FPSO than responding results introduced by Wind Sea dominate sea state, and squall and current will introduce large horizontal motions of FPSO, such as surge and sway. As for CALM, swell and wind sea mainly affect its heave and roll motions, respectively. Due to its little wind area, CALM does not show great response to Squall which is the extreme environment condition for FPSO though.
By comparing the results from OOL (Oil Offloading Line) dynamic analysis and quasi-static analysis, it could be that OOL dynamic effect has little influence on motions of FPSO. But it can bring large changes of responses of CALM because of its small water displacement which indicates coupled analysis of the whole system is necessary.
Besides, investigations on OOL fatigue have also been done, and the results show that CALM motion plays an important role on fatigue damage and life of OOL under different sea states.
Measurement System for Wave Energy Converter: Design and Implementation
Authors: Lindblad Liselotte , Baudoin Antoine , Leijon Mats 
Published: September 2014

A Wave Energy Converter (WEC) measurement system has been constructed and installed with the purpose to measure, log and evaluate the WEC’s performance during operation at sea. The WEC is to be deployed at Uppsala University’s wave power research site in Lysekil on the west coast of Sweden.
In designing such a system the key research objectives has been (1) to study the risk of overheating due to high currents in the stator windings, (2) to evaluate how the WEC’s outer structure withstands drag and bending forces from the buoy line and (3) to construct a detection system which indicates if water leaks into the generator. The measurement system was designed to collect data essential to study these key objectives.
Transducers were used to measure: buoy line force, translator position, phase currents, bending and tensile strain on the generator hull, water level inside generator and the temperature at multiple places inside the generator. The measurement system has been installed and calibrated in the WEC. Furthermore, the design has been evaluated in lab experiments in order to verify the function and accuracy of the different measurements.
This paper presents the underlying research objectives for developing the WEC generator measurement system, together with a description of the technical implementation.
Numerical Assessment on Primary Wave Energy Conversion of Oscillating Water Columns
Authors: Sheng Wanan , Alcorn Ray , Lewis Tony 
Published: September 2014

Oscillating water column (OWC) wave energy converters (WECs) are probably the simplest and most promising wave energy converters due to their good feasibility, reliability and survivability in practical wave energy conversions and also regarded as the most studied and developed when compared to other types of the wave energy converters.
This research aims to develop a reliable numerical tool to assess the performance of the OWC wave energy converters, particularly in the primary wave energy conversion. In the numerical assessment tool, the hydrodynamics of the device and thermodynamics of the air chamber can be studied separately. However, for the complete dynamic system when a power takeoff (PTO) system is applied, these two dynamic systems are fully coupled in time-domain, in which the PTO can have a simple mathematical expression as the relation between the pressure difference across the PTO (the chamber pressure) and its flowrate through the PTO. And the application of a simple PTO pressure-flowrate relation very much simplifies the complicated aerodynamics and thermodynamics in the air turbine system so the whole dynamic system can be simplified.
The methodology has been applied to a generic OWC device and the simulation results have been compared to the experimental data. It is shown that the developed numerical method is reliable in and capable of assessing the primary wave energy conversion of oscillating water columns.
Hydrodynamic Coefficients for Straked Risers
Authors: Nestegård Arne , Voie Per Erlend , Sødahl Nils 
Published: September 2014

A considerable challenge in deep water field developments is the possibility of Vortex Induced Vibrations (VIV) of flexible risers due to presence of high currents. The most common remedy is the use of VIV suppression devices such as helical strakes.
The background for this study is the lack of guidelines for estimation of hydrodynamic force coefficients for straked risers. In particular there seems to be no publicly available experimental data on tangential force coefficients for straked risers. Analyses indicate that tangential drag may be important for compliant steel risers, e.g. SCRs and steel lazy-waves risers on high-motion platforms in harsh environments. For large vertical motion of the platform there will be a substantial tangential relative velocity along the riser. Adding strakes to the riser will then effectively increase the tangential drag.
Simplified formulations for hydrodynamic coefficients for straked risers have been derived, including added mass and drag coefficients for both normal and tangential flow. The simplified formulations are derived from basic hydrodynamic theory without experimental calibration except for the drag coefficients for normal flow where the proposed drag coefficients are empirical based on experiments reported in the literature.
An example of a global analysis of straked risers in deep water is presented to demonstrate the effect of tangential drag. Derived formulas for hydrodynamic coefficients to be used in global analysis of straked risers will be implemented in DNV GL Recommended Practices.
Wave Absorption Improvements Inside the Moonpool of a Monocolumn Platform With an Internal Dock
Authors: de Oliveira Allan C. , Malta Edgard B. , Vilameá Eduardo M. 
Published: September 2014

Monocolumn platforms are usually considered an alternative concept for drilling and offshore production unit designs. This concept could provide high stability, small motions and also storage capacity. More recently, it was considered for a logistic hub platform design. The use of this concept with an opening for boat docking seems to provide a feasible solution for the people transfer in harsh environments, especially in areas like pre-salt Brazilian offshore fields, which are also very far from the coast. Although the helicopter technology has been improved in recent years to attend transport requirements from shore to the platforms, the use of a hub platform at sea still shows some economical and operational advantages, related to the conventional operations.
Although the sheltered area provided by the interior of the monocolumn platform seems to solve the transfer problem in harsh environments, the hydrodynamic behavior of the moonpool became a problem to be solved during the feasibility studies of the concept, since the moonpool water column elevations tends to be resonant in special cases. To deal with this problem, some attempts were done to absorb the wave energy inside the moonpool and eliminate its resonance, achieving the desired level of absorption. This work shows the attempts and the results obtained, which has proven the feasibility of the concept.
Comparative Study on the Collapse Response of Flexible Pipe Using Finite Element Methods
Authors: Cooke Nathan , Kenny Shawn 
Published: September 2014

Although the scope and use of flexible pipe systems in deepwater developments is expanding, the mechanical behavior for these environments is not fully understood. This is due to the complex response and interaction between multiple layers within the pipe system that introduces significant difficulties and constraints into the engineering analysis. As future developments look to extend the use of this technology to greater water depths and harsher operating conditions there is a need to develop advanced numerical tools that can evaluate the mechanical integrity of these complex hybrid pipe systems. Availability of increasingly advanced computational packages has enabled substantial improvements to be made in the complexity of simulation tools for combined loading, external pressure collapse and fretting.
This study establishes a foundation for the development of advanced numerical modeling procedures to assess the collapse failure of composite flexible pipe systems for deepwater applications. Here, a continuum finite element model is constructed using the software package ABAQUS/Standard, and studied using non-linear (arc length) methods. The carcass, pressure armor and corresponding polymer layers are represented in detail and modeled with three dimensional solid brick elements in order to examine the interlayer relationships influencing collapse initiation. In many recent studies, an initial geometric imperfection in the form of general ovality is explored as the predominant bifurcation mode. A similar approach is adopted here, coupled with case studies chosen such as to facilitate validation against existing analytical and numerical data. The importance of element selection, contact mechanics, interface properties and initial imperfections on the system mechanical response and performance is presented and compared to the available literature.
Fatigue Design Methodology for Large Mooring Chains Subjected to Out-of-Plane Bending
Authors: Lassen Tom , Aarsnes Jan , Glomnes Einar 
Published: September 2014

The present paper presents a fatigue life prediction method for chains subjected to tension and Out-of-Plane-Bending (OPB). The investigation was carried out on a high strength mooring chain segment with a diameter of 165 mm and a steel quality R5. Such chains are used in mooring systems for large Floating Production Storage and Offloading (FPSO) units in harsh weather condition. Due to the fact that the mooring chains are pre-tensioned the wave induced displacements will introduce secondary bending effects about the weak axis in the chain links, particularly in the bilge hang-off area.
In such chains both conventional tension fatigue and fatigue damage accumulation due to Out-of-Plane-Bending (OPB) have to be analyzed. Results from full scale behavior tests, Finite Element Analyses and a case study with simulation of in-service loading are included in the present study. Finally, fatigue life predictions and an operational strategy are presented for a case study for a floater in the Barents Sea. With a target service life of 30 years, the endurance with respect to fatigue damage is a major design criterion for the mooring chain in this case.
Wave Instability in Finite Depths
Authors: Babanin Alexander V. , Ewans Kevin 
Published: September 2014

Waves in finite-depth and shallow water represent an important and challenging research topic, both from practical and academic perspective. Most of practical applications related to the ocean are conducted in coastal areas, but most of research attention has been dedicated to the surface waves in deep water. Shallow areas are effectively a different physical environment, where wave kinematics change, waves become steeper and consequently more nonlinear but less or non-dispersive, and as a result their nonlinear behaviors change dramatically, waves directly interact with the bottom, in a number of different ways, and most importantly release their energy and momentum through intensive and extensive breaking.
In the paper, modulational instability in finite depths will be discussed. This mechanism is regarded responsible for rogue waves, and hence extreme wave heights in deep water. Extreme wave heights are a key design criterion for the ocean industry, including shallow environments too. It is believed, however, that wave breaking rather than wave instabilities is the primary mechanism that controls the maximum possible wave height in depth-limited environments. Here, data of the LoWish JIP will be used in order to identify the criteria for transition from modulational instability being active in the deep-water environments to being suppressed in the shallow waters.
Centre of Pressure of 6 Degree of Freedom Loads, Applied to Impact Measurements on a Propeller Blade
Authors: Brouwer Joris , Hagesteijn Gerco 
Published: September 2014

Load measurements on various structures and objects are nowadays common in technical industries and research fields. The same applies to numerical computations where fluid forces are integrated over surfaces to come to resultant forces and moments. When forces and moments on a rigid body in 3D are considered, the total load is a 6 degree of freedom system. These are often denoted as the force in x-, y- and z-direction and the moments around the x-, y- and z-axis. A question that often arises in case of these 6 degree of freedom loads, is to find the position of the centre of pressure. This is the position with zero resulting moment when the loads are translated to it. In 2D this is straightforward. However for 3D systems there is no straightforward solution. Extending the mathematics from 2D to 3D one might find a system of equations with a determinant that is zero. Which means there is no solution or there are an infinite number of solutions.
Studying this system of equations, a very convenient alternative can be posed. This alternative solution has been applied to find the centre of pressure on a propeller blade originating from open water loads, artificial ice impacts and combined ventilation and cavitation impacts. These measurements are all performed at model scale using 5 or 6 degree of freedom load sensors. The alternative method proved to deliver meaningful results which are backed up by high speed video footage of the impacting events.
Development of a Time Domain Boundary Element Method for the Seakeeping Behavior of a Cruise Ship Under Combined Strong Wind and Extreme Irregular Beam Seas
Authors: Gkikas G. D. , van Walree F. 
Published: September 2014

A computational method for the seakeeping behavior of a cruise ship at zero speed and under severe wind and oblique wave loads is presented. The proposed methodology is a time-domain panel method where the transient Green functions used for the estimation and implementation of the free surface effects on the vessel’s motions are estimated assuming constant low lateral speed, instead of the common practice zero speed influence functions.
For the evaluation of the overall hydrodynamic forces, the so called “blended approach” is followed in the sense that the induced hydrodynamic pressures due to the scattering and radiation phenomena are calculated over the linearized position of the body, ignoring any displacements with respect to its mean position, while the hydrostatic and non-linear Froude-Krylov forces are considered at the actual body location and taking into account the free surface elevation at each time step.
For the validation of the proposed methodology, heave and roll motions, the drift velocity as well as lateral accelerations of the vessel were investigated for two cases of severe beam seas combined with a constant strong wind load and the results were compared against experimental model tests. The model tests were performed to investigate the vessel’s behavior under extreme weather conditions.
The low lateral speed Green functions were estimated for a speed similar to the one that the vessel was expected to drift, an estimation based on the model tests, as well as for the case where the input speed corresponded to the half of the expected speed. Good agreement was presented for both cases, showing that accurate and computationally efficient numerical simulations of the vessel’s motions under severe wind and wave excitations can be obtained by using low lateral speed transient Green functions.
An Experimental Study on Progressive and Dynamic Damage Stability Scenarios
Authors: Lorkowski Oliver , Dankowski Hendrik , Kluwe Florian 
Published: September 2014

Current damage stability rules for ships are based on the evaluation of a ship’s residual stability in the final flooding stage. The consideration of the dynamic propagation of water within the inner subdivision as well as intermediate flooding stages and their influence on the resulting stability is very limited in the current damage stability regulations.
The investigation of accidents like the one of the Estonia or the European Gateway reveals that intermediate stages of flooding and the dynamic flooding sequence result in significant fluid shifting moments which have a major influence on the time-dependent stability of damaged ships. Consequently, the critical intermediate stages should be considered when evaluating designs with large cargo decks like RoRo vessels, RoPax vessels and car carriers. Also the safety of large passenger ships with respect to damage stability is affected by the aforementioned effects.
In this context a new numerical flooding simulation tool has been developed which allows an evaluation of a ship’s time-dependent damage stability including all intermediate stages of flooding. The simulation model is based on a quasi-static approach in the time domain with a hydraulic model for the fluxes to ease the computation and allow for fast and efficient evaluation within the early design stage of the vessel. This allows studying multiple damage scenarios within a short period.
For the further validation of this numerical simulation method a series of model tests has been particularly set up to analyse the time-dependent damage stability of a floating body. The test-body has been designed specifically to reflect the most typical internal subdivision layouts of ships affected by the effects mentioned above.
The experimental study covers a static model test series as well a dynamic one. The static model test series has been set up with the aim to analyse the progressive flooding of selected compartments in calm water. Within the dynamic model test series, the model is excited by a roll motion oscillator to evaluate the influence of the ship motion on the water propagation and the associated damage stability.
The model tests presented in this paper comprise side leaks in typical compartments which are used for a basic validation of the simulation toll and the measurement devices. Particular attention has been drawn on damage scenarios with critical intermediate flooding stages in consequence of restricted water propagation. The presented results enable a further validation of the numerical flooding simulation and give an insight view on the chosen experimental setup.
North Sea Mooring Systems: How Reliable Are They?
Authors: Brindley Will , Comley Andrew P. 
Published: September 2014

In recent years a number of high profile mooring failures have emphasised the high risk nature of this element of a floating structure. Semi-submersible Mobile Offshore Drilling Units (MODUs) operating in the harsh North Sea environment have experienced approximately 3 mooring failures every 2 years, based on an average population of 34 units. In recognition of the high mooring failure rates, the HSE has introduced recommendations for more stringent mooring strength requirements for units operating on the UK Continental Shelf (UKCS) [17]. Although strength requirements are useful to assess the suitability of a mooring design, they do not provide an insight into the question: what is the reliability of the mooring system?
This paper aims to answer this question by evaluating failure statistics over the most recent decade of available data. Mooring failure rates are compared between the Norwegian Continental Shelf (NCS), the UKCS, and with industry code targets to understand how overall reliability is related to the strength capacity of a mooring system.
The failure statistics suggest that a typical MODU operating in the UKCS would experience a mooring line failure in heavy weather approximately every 20 operating years. This failure rate appears to be several orders of magnitude greater than industry targets used to calibrate mooring codes. Despite the increased strength requirements for the NCS, failure rates do not appear to be lower than the UKCS. This suggests that reliability does not correlate well with mooring system strength. As a result, designing to meet the more rigorous HSE requirements, which would require extensive upgrades to existing units, may not significantly increase mooring system reliability. This conclusion needs to be supported with further investigation of failure statistics in both the UKCS and NCS. In general, work remains to find practical ways to further understand past failures and so improve overall reliability.
Comparison of Second-Order Loads on a Semisubmersible Floating Wind Turbine
Authors: Gueydon Sébastien , Duarte Tiago , Jonkman Jason 
Published: September 2014

As offshore wind projects move to deeper waters, floating platforms become the most feasible solution for supporting the turbines. The oil and gas industry has gained experience with floating platforms that can be applied to offshore wind projects. This paper focuses on the analysis of second-order wave loading on semisubmersible platforms. Semisubmersibles, which are being chosen for different floating offshore wind concepts, are particularly prone to slow-drift motions. The slack catenary moorings usually result in large natural periods for surge and sway motions (more than 100 s), which are in the range of the second-order difference-frequency excitation force.
Modeling these complex structures requires coupled design codes. Codes have been developed that include turbine aerodynamics, hydrodynamic forces on the platform, restoring forces from the mooring lines, flexibility of the turbine, and the influence of the turbine control system. In this paper two different codes are employed: FAST, which was developed by the National Renewable Energy Laboratory, and aNySIM, which was developed by the Maritime Research Institute Netherlands. The hydrodynamic loads are based on potential-flow theory, up to the second order. Hydrodynamic coefficients for wave excitation, radiation, and hydrostatic forces are obtained with two different panel codes, WAMIT (developed by the Massachusetts Institute of Technology) and DIFFRAC (developed by MARIN).
The semisubmersible platform, developed for the International Energy Agency Wind Task 30 Offshore Code Comparison Collaboration Continuation project is used as a reference platform. Irregular waves are used to compare the behavior of this platform under slow-drift excitation loads. The results from this paper highlight the effects of these loads on semisubmersible-type platforms, which represent a promising solution for the commercial development of the offshore deepwater wind resource.