bluuebunny/crossref_metadata_2025_split huggingface dataset:
On the Adequacy of Existing Foundation Schemes for Offshore Wind Turbines Subjected to Extreme Loading
Authors: Kourkoulis Rallis , Gelagoti Fani , Georgiou Irene , Karamanos Spyros , Gazetas George 
Published: October 2017

Dictated by the world’s escalating energy demands, offshore infrastructure is moving beyond the immediate continental shelf into deeper waters. Although the monopile solution has proven its reliability for many years, its feasibility in larger depths is questionable, or even limited, and multi-pod foundations, such as jacket structures, could be regarded as viable alternatives. Their main advantage, compared to the monopile alternative, is that they are able to sustain large lateral loads through axial stressing rather than bending at their supports (usually materialized using piles or suction caissons).
Recognizing this reality, the present study attempts to compare the performance of a conventional monopile system with that of a jacket foundation when taking into consideration extreme earthquake loading. Although safety fuses do exist to isolate the mechanical equipment from the direct effects of such loading, our focus in this study is on the irrecoverable deformation at the foundation level which, under circumstances, may render the turbine inoperable.
To this end, two foundation alternatives supporting an offshore wind turbine in the Mediterranean Sea are comparatively discussed: the conventional large diameter monopile and a jacket foundation supported by smaller piles or suction caissons. Results show that under expected operational loads the performance of the two systems is practically equivalent. However, extreme loading conditions may significantly alter the response and may, in some cases question the common practice.
On Maintainability of Winterised Plants Operating in Arctic Regions
Authors: Naseri Masoud 
Published: October 2017

In Arctic regions, oil and gas (O&G) operations are adversely affected by harsh weather conditions and severe meteorological phenomena such as icing storms and, in certain regions, polar low pressures. Potential solutions, such as implementing winterisation concepts, are explored in the design and even operation phases in order to overcome such obstacles. Simply, the main aim of winterisation is to provide the crew and equipment units with a range of normal environmental and working conditions through, for instance, insulating equipment units, installing heat tracers, enclosing working areas, providing the crew with adequate clothing, etc.
There are, however, some concerns about the efficiency of such winterisation measures and potential changes in operation risk level, of which the changes in plant downtime, production loss, and plant maintainability are the focus of present study. The issue of complex effects of winterisation measures on maintainability analysis of O&G plants operating in the Arctic offshore has gained little attention in the literature. In this study, different aspects of winterisation from the viewpoint of equipment maintainability are discussed. Further, a mathematical framework for maintainability analysis of equipment units subjected to winterisation measures is proposed. The impact of winterisation-related downtimes on plant downtime is analysed as well by employing a Monte Carlo system simulation technique. The application of the proposed framework is illustrated by a case study. The results are further compared with those for a non-winterised system designed for normal-climate regions.
Structural Reliability Analysis for Offshore Drilling Riser Deployment Operability
Authors: Zhang Xiaodong , Yang Hezhen , Adaikalaraj Peter Francis Bernad , Low Ying Min , Koh Chan Ghee 
Published: October 2017

Drilling riser system provides a short-term connection between subsea oil well and drilling vessel or platform. The analysis of different operability envelopes are required for drilling riser analysis, for example, connected drilling, connected non-drilling and hang-off analysis. The operability envelope analysis could provide operators statistical information for riser operational management.
The current practice to calculate the drilling operability envelope is to use deterministic approach. However, deterministic approach could not take the randomness from environmental loadings and structures into consideration. Structural reliability method is an analysis tool to quantify probability of failure of components or systems accounting for uncertainties in environmental conditions and system parameters. It is particularly useful in cases where limited experience exists or a risk-based evaluation of design is required. It is gaining increasing popularity in the offshore and marine industry to predict failure probability.
In this paper, structural reliability analysis is adopted to analyze the offshore drilling riser deployment. The uncertainties are mainly from wave and current loadings. The significant wave height HS is modeled by a Weibull probability density function, the zero-crossing wave period TZ conditional on HS is modeled by a lognormal distribution, and the surface current speed is modeled as Weibull distribution. Multiple simulations are performed by software Flexcom [1] and efficient structural reliability methods are adopted to get the failure probabilities. The deployment operability will be calculated based on structural reliability methods and the results will be compared with those calculated based on deterministic approach.
A 3D Wave-Structure-Seabed Interaction Analysis of a Gravity-Based Wind Turbine Foundation
Authors: Li Yuzhu , Tang Tian , Ong Muk Chen 
Published: October 2017

In order to prevent the future risk of soil and structural failures, it is essential to evaluate the dynamic seabed soil behaviors in the vicinity of the offshore foundations under dynamic wave loadings. Three-dimensional (3D) numerical analysis is conducted on the interaction between waves, seabed soil and a gravity-based wind turbine foundation. An OpenFOAM based numerical code developed by Tang [1]for wave-structure-seabed interaction is applied. The nonlinear waves are modeled by solving the Navier-Stokes equations for incompressible flow. The dynamic structural response of the foundation is computed using a linear elasticity solver. The transient responses of the seabed are solved by an anisotropic poro-elastic soil solver. The dynamic interaction between different physical domains is implemented by boundary condition coupling and updating in the integrated FVM based framework.
The dynamic wave pressure on the structure and the seabed, the elastic responses of the structure and the changes of the pore pressure, shear stress and seepage flow structure in the seabed are investigated. Highest wave-induced shear stress along the foundation is predicted by solving the deformable structure model. For the seabed soil in the vicinity of the foundation, it is found that the presence of the foundation affects the soil responses by amplifying the wave induced shearing effect on the underlying seabed. Vertical distributions of the pore pressure in the seabed beneath the foundation are investigated with different angles relative to the wave propagation direction. A parametric study of isotropic and anisotropic soil permeability is performed and demonstrates that for the simulated soil in this work, the consideration of the anisotropic permeability is suggested.
Model Test and Simulation Comparison for an Inclined-Leg TLP Dedicated to Floating Wind
Authors: Caillé François , Bozonnet Pauline , Perdrizet Timothée , Poirette Yann , Melis Cécile 
Published: October 2017

A new floating foundation for multi-MW wind turbine is being developed within a collaboration between SBM Offshore and IFP Energies nouvelles. This inclined leg TLP, is made up of four immersed buoys and a bracing structure, making the floater transparent to wave excitation. The particular mooring arrangement gives the floater interesting motion properties since it creates a fixed point close to the nacelle, strongly reducing the motion at this elevation.
In order to validate the concept and the simulation strategy, a model test campaign has been carried out during three weeks in 2015 at MARIN’s offshore basin. The downscaling is performed according to the Froude law of similitude to maintain the hydrodynamic loadings and behavior. The tower bending natural period, the mooring stiffness, and the turbine rotation speed are also maintained in order to reproduce the relevant structural modes and check that no unexpected phenomena occur in the system during production or parked conditions.
The scale 1/50 was initially selected so that the MARIN Stock Wind Turbine (MSWT) can be used. This model wind turbine was designed by MARIN with low Reynolds blade airfoils to mimic the NREL 5 MW wind turbine, especially the thrust force. However because of mass distribution issues, the scale has to be changed from 1/50 to 1/40, at this scale only the thrust force and the rotation speed can be replicated.
First, a set of calibration tests are performed in the basin and simulated with Orcaflex™ and DeepLinesWind™ for a better understanding of the system and to validate independently the various components of the numerical models. Secondly, design parked and operational cases are conducted with wind, wave and current loadings for two floater orientations and two water depths.
The objective of this campaign is to validate the concept behavior as well as the simulation tools and methodologies. Hydrodynamic and structural models are very similar in both software and are checked with the calibration tests from the basin, whereas two strategies are implemented to model the aerodynamic contribution. The Simplified Coupled Simulations (SCS), performed with Orcaflex, use the aerodynamic forces recorded during the model tests to be imposed at tower top; the Fully Coupled Simulations (FCS), run with DeepLinesWind, use the aerodynamic loading computed with the BEM theory from the measured wind.
Improvement on Mechanical Properties of Cu-Containing Low Alloy Steel of Long Part Forging for Offshore Applications by Manufacturing Process
Authors: Honma Yuta , Sasaki Gen , Hashi Kunihiko 
Published: October 2017

Effects by intercritical quenching, which is quenching from dual-phase of ferrite (α) and austenite (γ) region from 953 to 1068 K, on mechanical properties and microstructures of Cu-containing low alloy steel based on ASTM A707 5L grade (hereafter called A707 modified steel) were investigated using 50 kg test ingots. The mechanical properties of the A707 modified steel, i.e. strength at room temperature and fracture toughness at low temperature, were significantly improved by intercritical quenching. This is probably because its effective grain size decreased by intercritical quenching. Then, the optimum temperature of intercritical quenching for A707 modified steel was 1068 K near the AC3 point.
Based on the experimental results of the test ingot, we applied intercritical quenching to a trial full-size forging production of about 20,000 mm in length, and researched the tensile, Charpy impact, crack tip opening displacement (CTOD) and drop weight test (DWT) properties across whole length of the trial production. It was found that the trial production has good mechanical properties across whole length. From the present work, an appropriate intercritical quenching is considered to apply for improvement method of the mechanical properties in A707 modified steel forgings.
Variability of Extreme Riser Responses due to Wave Frequency Motions of a Weather-Vaning FPSO
Authors: Armstrong C. , Drobyshevski Y. , Chin C. , Penesis I. 
Published: October 2017

The variability of extreme responses of a flexible riser due to wave frequency motions of weather-vaning FPSO is investigated numerically. The objective of this study is to examine such variability in isolation from that caused by the low frequency (slow drift) vessel motions and vessel offsets.
Investigation of the extreme value distributions of flexible risers provides the statistical foundation for flexible riser Response Based Analysis (RBA) for use in system design; the determination of the statistical properties of extreme flexible riser responses provides a method for the prediction of extreme responses of offshore systems in cyclonic conditions.
A case study conducted in OrcaFlex included an FPSO vessel with a Lazy-S configured riser system. Five riser responses were selected in critical locations including tension, heave, and curvature responses. The environmental cases included two cyclonic storms consisting of multiple half-hour intervals. For each interval, time domain simulations included 40 wave realizations in order to provide a dataset for robust fitting of the extreme value distributions in the Gumbel format.
Once the short term interval distributions were established, response distributions in a storm were generated by multiplying the short term distributions and the most probable maximum (MPM) response in a storm computed. A comparison of maximum interval, storm and 3-hour MPMs is presented, which indicates to what extent the MPM response in a storm exceeds the corresponding maximum interval response. Differences between the tension and heave responses are compared with those observed in the curvature responses.
This study was limited to riser excitation by waves, current and wave frequency motions of a turret moored FPSO and it is expected that further inclusion of low frequency motions would contribute to the response variability. The inclusion of such variability will ultimately enable the storm-based statistical approach to be used for the development of long-term distribution of the riser responses.
On Long Term Statistics of Ocean Storms Starting From Partitioned Sea States
Authors: Laface Valentina , Arena Felice , Maisondieu Christophe , Romolo Alessandra 
Published: October 2017

The paper proposes an analysis of ocean storms carried out starting from significant wave height time series of HOMERE sea-states hindcast database based on WAVEWATCH III model. Considering that wave spectra often exhibit multiple peaks due to the coexistence of wind waves and swells, here sea states are described by partitioned sea states that can be interpreted physically as representing independent wave systems. The analysis presented here in the paper deals with the contribution of swells to the storm peaks and on how they influence the long term statistics. The sensitivity of return values of significant wave height to swell contribution is investigated via an application of the Equivalent Triangular Storm Model (ETS).
The ETS model provides analytical solution for the calculation of the return period R(Hs>h) of a sea storm whose maximum significant wave height exceeds a given threshold h. The approach of ETS consists in substituting each actual storm with an ETS described by two parameters: the storm intensity, that is the triangle height and it is equal to the maximum significant wave height during the actual storm; the storm duration, that is achieved imposing the equality between the maximum expected wave height of actual and equivalent storms. It has been experimentally proved that the actual storm and associated ETS are statistically equivalent because they have the same maximum significant wave height and the same probability P(Hmax>H) that the maximum wave height exceeds a given threshold H. The sequence of ETSs obtained in this way represents the equivalent sea, while the sequence of actual storms is the actual sea. The equivalent and actual seas present the same wave risk because they are characterized by the same number of storm events, each of them with the same intensity and the same P(Hmax>H).
For the proposed analysis a set of four points from open sea to the coast is considered in area of the Gulf of Biscay (France). The results show that the contribution of swells is more significant for the storms of small and medium intensity and decreases for increasing storm intensities. Further return values variability neglecting swell is less than 7% at any point for return periods up to 100 years.
Effect of the Beam Element Geometric Formulation on the Wind Turbine Performance and Structural Dynamics
Authors: Delhaye Virgile , Karimirad Madjid , Berthelsen Petter Andreas 
Published: October 2017

In this paper, the original double symmetric cross section beam formulation in RIFLEX used to model the blades is compared against a newly implemented generalised beam formulation, allowing for eccentric mass, shear and elastic centres. The generalised beam formulation is first evaluated against an equivalent ABAQUS beam model (Using the generalised beam formulation implemented in ABAQUS) which consists of DTU 10MW RWT (reference wind turbine) blade in static conditions. A static displacement is applied to the tip, which is close to an operating load. The results appear very similar and ensure that the implementation is correct.
The extended beam formulation is afterwards used on the Land-based 10MW turbine from DTU with external controller. This case study aims at evaluating the effect of the newly implemented formulation on realistic, flexible structure. During the study, the blades were discretised using both the old and new formulation, and dynamic simulations were performed. The effect of the beam formulation was evaluated using several wind conditions that are thought to be characteristic of operating conditions. Results show slight difference between two formulations but could be more significant for next generation flexible blades.
Sensitivity Study of Calculated Jacket Fatigue Damage due to Long Term Distribution of Wave Heights
Authors: Bang Hege Halseth , Smedsrud Siri Hoel , Hagen Øistein , Nybø Terje 
Published: October 2017

Marine structures like jacket structures are often highly utilized structures operating in an environment dominated by dynamic loading. The fatigue limit state is of main concern and is to a large extent governing the structural dimensions and the amount of resources utilized in inspection and maintenance of members and joints.
There is a considerable degree of uncertainty related to the parameters determining the fatigue damage. The models applied, both for describing the fatigue driving mechanisms e.g. the wave-description and load modeling and the deterioration mechanism, are always compromises between the ability to accurately describe the nature and computationally efficiency.
The main focus in this paper is to show how sensitive the calculated fatigue damage of a jacket is to different models for the short term variability of wave heights. To obtain consistent basis for comparison a deterministic fatigue analysis is considered and a potential structural dynamic amplification is not included in the comparison study. Sensitivity to selection of wave spectra will not be addressed.
In a deterministic approach the long term distribution of individual wave heights is used to calculate the stress ranges occurring in the joints and butt welds. Typically, the long term variability of sea state conditions is given by a scatter diagram of significant wave height (Hs) and the peak period (Tp). When converting the scatter diagram of sea states to the long term distribution of wave heights, it is common to assume that the individual waves in the sea states are Rayleigh distributed. Later developments indicate that a Forristall distribution may be a more accurate assumption.
The following cases have been considered:
1. Assuming that the individual waves in each sea state are Rayleigh distributed.
2. Assuming that the individual waves in each sea state follows a Forristall distribution.
3. Calculating the long term wave height distribution from time domain simulations.
In the third method, second order wave theory was used to simulate all sea states in the Hs/Tp scatter diagram. I.e. extensive time domain simulations were carried out to cover the complete scatter diagram of possible sea states.
The study is performed for an 8-legged jacket. The analyses are performed for a typical North Sea wave environment for water depth about 110 m.
The objective of this study is to investigate the robustness in the current design practice for jacket structures where the individual waves in the sea states are Rayleigh distributed. The paper documents the calculated fatigue lives for main joints along the height of the jacket for the three wave height distributions. Further, the paper gives advice on application of wave distribution models for design of new structures and reassessment of existing structures.
Subsea Structural Monitoring With Machine Vision
Authors: Authén Kristian 
Published: October 2017

Structural monitoring of subsea structures is widely used subsea in the oil and gas industry. Monitoring may be performed to keep track on position, movements, loads and fatigue accumulation. BOPs may have motions sensors installed to monitor the lower flex joint angle and the movements of the BOP itself. Installing subsea equipment like templates or conductors may also require monitoring to ensure the installation is within the set requirements.
Motion sensors containing accelerometers and gyroscopes are commonly used for these applications. These sensors are well suited for accurately determining inclination, but the sensors estimate displacements poorly.
An alternative approach for such measurements is to use machine vision. The machine vision tracks selected features from a video feed and calculates position, displacement and inclination directly. A benefit of this monitoring technique is that it does not require any hardware on the actual structure, only a camera and a tracking software.
Two video tracking algorithms are proposed for tracking. One marker based algorithm that require a pre-installed visual marker, and a markerless algorithm that uses natural features on the structure. The article presents the accuracy and project examples of in-field use of these algorithms.
Dynamic Behavior of Flexible vs Rigid Spools During Seismic Loading Events
Authors: Kaurin Christian , Panjaitan Hans , Dybwad Jacob , Damsleth Per 
Published: October 2017

In regions of high seismic activity, the most severe loading experienced by subsea production systems during earthquake events can be significantly greater than normal static design loads. The major components of such systems comprising pipelines, manifolds and wells linked by tie-in spools and jumpers all respond differently when excited by seabed accelerations. Individual components may resonate or move out of phase with each other. It is of vital importance that the design of the system accounts for the dynamic loading at interfaces such as flanges, connectors and pipe supports. High dynamic loads due to a severe earthquake may damage the connection and cause leakage of hydrocarbons to the marine environment.
Tie-in spools and well jumpers provide flexibility in the system but layouts and physical properties that satisfy static loads may not be as suitable in a dynamic environment. Where earthquake loads are severe, the dynamic response of the entire system needs to be considered, including the structure and connected spool response together as they interact individually with the soil movement.
A comparative study of the seismic responses of flexible and rigid spools has been performed using 3D non-linear finite element simulation in the time domain. The system analyzed consists of a flexible or a rigid spool tied-in to a subsea manifold via a mechanical connector. The FE analysis model of the spool configurations includes the free-span at the structure and seabed contact with both static and dynamic pipe-soil interaction. The effects of spool metrology and fabrication tolerances are included as well as complete static load histories due to displacements from installation to operation phases as the starting point for the seismic simulations.
The flexible spool was found to provide significantly lower of dynamic interface stresses compared to a rigid spool with similar geometry. The load stress variance was also found to be reduced, indicating that flexibles may also offer more predictable behavior when subject to the highly variable spectral energy of extreme earthquakes.