bluuebunny/crossref_metadata_2025_split huggingface dataset:
Comparison of Global Riser Analysis to Full Scale Measurements on the NCS
Authors: Grytøyr Guttorm , Hørte Torfinn , Russo Max , Gregersen Kathrine , Aronsen Kristoffer H. 
Published: October 2017

Fatigue of subsea wellhead systems due to wave-induced loads from riser and rig motions has been subjected to increased attention in recent years. It is expected that calculated riser loads are conservative, as both input parameters and methodology are associated with some uncertainty. However, it is difficult to quantify the degree of conservatism in analytical results unless reference to measurements can be made.
Statoil has conducted drilling riser load instrumentation campaigns in several locations around the world over the last few years in order to gather high quality data for accurate assessment of the fatigue loads imposed on the subsea wellheads (see e.g. ref. /1/, /2/, /3/, and /17/). Four (4) of these measurements campaigns have been studied in more detail, with the intention to quantify the degree of conservatism to be expected from drilling riser analysis. Three (3) of these cases have direct bending moment measurements from strain sensors at the BOP connector elevation, giving high confidence in the results. For one (1) of the cases, the bending moments at the WH have been established by use of indirect methods (ref. /2/). The campaigns have been anonymized; They are from the Norwegian Continental Shelf (NCS), with water depths ranging from 110 to 400m. This paper presents the findings from our comparison of measurements and analytically derived drilling riser loads from these 4 campaigns. The analysis models have not been “tuned” to match the measurements. The goal is rather to get a measure of how well riser analyses are able to predict the real world.
The conclusion in this paper is that the global drilling riser analyses accurately predict the cyclic loads on the subsea wellheads, provided that the input data are known with high degree of detail, including e.g. riser tension setting; drill pipe tension variation over time; and hydrodynamic loads. We found that scatter in the results is due to the uncertainty inherent to several of the input parameters.
It is also shown that the accumulated fatigue damage from a full drilling campaign, can be established with sufficient degree of accuracy with somewhat lower requirement to the level of detail of the input, like e.g. using unidirectional waves instead of short crested waves. Directionality and spreading of the wave field can be handled by use of factors on the damage rate. A directionality factor is proposed, to enable comparison of directionality of the measured response to the predictions from global analysis.
A Comparative Study of Wave Breaking Models in a High-Order Spectral Model
Authors: Seiffert Betsy R. , Ducrozet Guillaume 
Published: October 2017

We examine the implementation of two different wave breaking models into the nonlinear potential flow solver HOS-NWT. HOS-NWT is a computationally efficient, open source code that solves for surface elevation in a numerical wave tank using the High-Order Spectral (HOS) method [1]. The first model is a combination of a kinematic wave breaking onset criteria proposed by Barthelemey, et al. [2] and validated by Saket, et al. [3], and an energy dissipation mechanism proposed by Tian, et al. [4, 5]. The wave breaking onset parameter is based on the ratio of local energy flux velocity to the local crest velocity. Once breaking is initiated, an eddy viscosity parameter is estimated based on the pre-breaking local wave geometry, as described in [4, 5]. This eddy viscosity is then added as a diffusion term to the kinematic and dynamic free surface boundary conditions for the duration of wave breaking. Results implementing this wave breaking mechanism in HOS-NWT have shown that the model can successfully calculate the surface elevation and corresponding frequency spectra, as well as the energy dissipation associated with breaking waves [6–8].
The second model implemented to account for wave breaking in HOS-NWT is based on the method proposed by Chalikov, et al. [9–11]. This model defines wave breaking onset by the curvature of the water surface and defines the wave as broken if it exceeds a certain value. A diffusion term is added to the kinematic and dynamic free surface boundary conditions which dissipates energy based on the local curvature of the water surface, which is consequently not constant in space nor time.
Calculations made using the two models are compared with large scale experimental measurements conducted at the Hydrodynamics, Energetics and Atmospheric Environment Lab (LHEEA) at Ecole Centrale de Nantes. Comparison of calculations with measurements suggest that both models are successful at predicting wave breaking onset and energy dissipation. However, the model proposed by Barthelemy, et al. [2] and Tian, et al. [4] can be applied without knowing anything about the breaking waves a priori, whereas the model proposed by Chalikov [9] requires tuning to specific conditions.
Dynamic Response of Floating Wind Turbine Under Consideration of Dynamic Behavior of Catenary Mooring-Lines
Authors: Guo Shuangxi , Li Yilun , Li Min , Chen Weimin , Fu Yiqin 
Published: October 2017

Recently, wind turbine has been developed from onshore area to offshore area because of more powerful available wind energy in ocean area and more distant and less harmful noise coming from turbine. As it is approaching toward deeper water depth, the dynamic response of the large floating wind turbine experiencing various environmental loads becomes more challenge. For examples, as the structural size gets larger, the dynamic interaction between the flexible bodies such as blades, tower and catenary mooring-lines become more profound, and the dynamic behaviors such as structural inertia and hydrodynamic force of the mooring-line get more obvious. In this paper, the dynamic response of a 5MW floating wind turbine undergoing different ocean waves is examined by our FEM approach in which the dynamic behaviors of the catenary mooring-line are involved and the integrated system including flexible multi-bodies such as blades, tower, spar platform and catenaries can be considered.
Firstly, the nonlinear dynamic model of the integrated wind turbine is developed. Different from the traditional static restoring force, the dynamic restoring force is analyzed based on our 3d curved flexible beam approach where the structural curvature changes with its spatial position and the time in terms of vector equations. And, the modified finite element simulation is used to model a flexible and moving catenary of which the hydrodynamic load depending on the mooring-line’s motion is considered. Then, the nonlinear dynamic governing equations is numerically solved by using Newmark-Beta method.
Based on our numerical simulations, the influences of the dynamic behaviors of the catenary mooring-line on its restoring performance are presented. The dynamic responses of the floating wind turbine, e.g. the displacement of the spar and top tower and the dynamic tension of the catenary, undergoing various ocean waves, are examined. The dynamic coupling between different spar motions, i.e. surge and pitch, are discussed too. Our numerical results show: the dynamic behaviors of mooring-line may significantly increase the top tension, particularly, the peak-trough tension gap of snap tension may be more than 9 times larger than the quasi-static result. When the wave frequency is much higher than the system, the dynamic effects of the mooring system will accelerate the decay of transient items of the dynamic response; when the wave frequency and the system frequency are close to each other, the displacement of the spar significantly reduces by around 26%. Under regular wave condition, the coupling between the surge and pitch motions are not obvious; but under extreme condition, pitch motion may get about 20% smaller than that without consideration of the coupling between the surge and pitch motions.
Wellhead Fatigue Analysis: How Conservative Is Conservative Enough?
Authors: Macke Michael , Hørte Torfinn , Aronsen Kristoffer Høyem , Grytøyr Guttorm 
Published: October 2017

In wellhead fatigue analysis, results are quite often sensitive to input parameters that are associated with considerable uncertainty. Such parameters may, e.g., be the cement level in the annulus between surface casing and conductor, the soil and template support, friction and down weight on the landing shoulder, etc. A common approach in deterministic design to deal with such problem is to use worst case assumptions on sensitive parameters. However, this may result in unreasonable short fatigue lifetime estimates.
In this paper, it is shown how existing fatigue design equations using the concept of design fatigue factors can be augmented with characteristic design values for additional uncertain variables such that the overall safety level obtained is comparable to the recommended target levels for typical uncertainties in load effect calculations. Results from sensitivity analysis and SRA are compared to investigate to what extent sensitivity analysis results can be utilized as part of the design analysis. The intention of the proposed approach is to be reasonably conservative but to avoid being overly conservative.
An Integrated Environment for Design and Analysis of Umbilical Cables
Authors: Provasi Rodrigo , Garcez Leonardo Riccioppo , Cardoso Meirelles Christiano Odir , de Arruda Martins Clovis , Freitas Barbosa Andre , Otte Filho Olaf Oswaldo 
Published: October 2017

The design of an umbilical cable begins with the definition of the operational functions it must implement and the environmental conditions to that it will be subjected to. Those functions suggest the components that it must have, usually chosen from a pre-defined set. Also, structural elements must be added, based on project and manufacture requirements, so the cable can withstand the environmental conditions of use. All the components must be geometrically arranged and the cross section of the cable must be defined, usually with the help of CAD software.
But the structural behavior of the designed cable must be analyzed under several environmental conditions, using numerical and analytical tools. If this behavior does not fulfill the desired structural requirements, the cable must be redesigned: structural and functional components must be changed and the cross section of the umbilical must be rearranged in an iterative process.
This article presents an environment that integrates a CAD tool dedicated to the design of the cross section of an umbilical cable with structural analysis tools, both analytical (Utilflex) and numerical (UFLEX2D). The CAD tool architecture is based on a component-instance model that enables both drawing and reusing of components. It also can export the designed cross-section to AutoCAD using AutoLISP language. Finally, it automates the generation of the data-sheet of the designed umbilical both in AutoLISP and Microsoft Word, including the basic structural properties calculated by means of analytical formulae.
Modeling, Parameter Identification and Thruster-Assisted Position Mooring of C/S Inocean Cat I Drillship
Authors: Bjørnø Jon , Heyn Hans-Martin , Skjetne Roger , Dahl Andreas R. , Frederich Preben 
Published: October 2017

A thruster-assisted position mooring (TAPM) system includes different control functions for stationkeeping and motion damping for a moored offshore vessel with assist from thrusters. It consists of a conventional mooring system and a dynamic positioning (DP) system. The thrusters are used to provide damping and some restoring to the vessel motion and compensate if line breakage occurs. The mooring system absorbs the main loads to keep the vessel in place. This paper presents a complete modeling, parameter identification, and control design for a 1:90 scaled TAPM model vessel. The numerical values for the different model parameters are identified from towing tests.
State-of-the-art TAPM control algorithms have been tested on the vessel in the Marine Control Laboratory (MC Lab), to see the behavior resulting from the different control algorithms. The presented experiments focus on the setpoint chasing algorithm, where the position setpoint slowly moves to the equilibrium position where the environmental loads are balanced by the mooring loads. This avoids conflicts between the mooring system and the control actions. If the environmental loads are too large so that the setpoint exceeds a user-defined safety radius, the setpoint is set to this radius and thruster forces grow to support the mooring system in counteracting the environmental loads to avoid line breakage. The experiments show that the vessel and setpoint chasing control algorithm behaves as expected, minimizing thruster usage and maximizing utilization of mooring system.
Validation of a Quasi-Linear Numerical Model of a Pitching Wave Energy Converter in Close Proximity to a Fixed Structure
Authors: Heras Pilar , Thomas Sarah , Kramer Morten 
Published: October 2017

Although linear theory is often used to analyse wave energy devices, it is in many cases too simplistic. Many wave energy converters (WECs) exceed the key linear theory assumption of small amplitudes of motion, and require the inclusion of non-linear forces. A common approach is to use a hybrid frequency-time domain model based on the Cummins equation with hydro-dynamic inputs coming from linear wave theory (Ref. [1]). Published experimental data is sparse (Ref. [2]) and the suitability for the broad variety of WEC technologies has yet to be proven.
This paper focuses on the challenges faced when attempting to validate a numerical model of a WEC using a variety of scaled physical tests in a waveflume. The technology used as a case study in this paper is a pitching WEC in close proximity to a fixed structure. Challenges are presented relating to waveflume effects and obtaining accurate physical input parameters to the numerical model.
Validation Exercises for the Calculation of the Flow Around a Squared Column With Rounded Corners at High Reynolds Numbers With the RANS Equations
Authors: Eça Luís , Vaz Guilherme , Koop Arjen , Pereira Filipe , Abreu Hugo 
Published: October 2017

This paper presents Validation studies, i.e. evaluation of the modelling error, for the ensemble averaged Navier-Stokes (RANS) equations supplemented by eddy-viscosity models (SST k-ω and Spalart & Allmaras). The selected test case is the flow around a squared column with rounded corners at Reynolds number ranging from 105 to 107. Selected flow quantities include time-averaged quantities of drag and lift coefficients, base pressure coefficient, Strouhal number and the standard deviation of the lift coefficient. For this latter quantity, ensemble (phase) averaging must also be applied to the experimental data to obtain quantities with the same physical meaning, as illustrated in this work.
Even for the simple assumption of two-dimensional flow of the present simulations, the level of grid refinement and iterative convergence criteria required to obtain acceptable numerical uncertainties is more demanding than those typically observed in the open literature. However, the most important result obtained in this study is that the use of turbulence models developed for statistically steady flows may lead to flow fields that physically are at least questionable. From the three Reynolds numbers tested using three eddy-viscosity models, statistically periodic solutions with zero lift were only obtained with the SST k-ω at Re = 105. However, in that case, it is possible to numerically (including statistical, iterative and discretization errors) converge two different flow fields for the same space boundary conditions, but different solution strategies (start up of the vortex shedding).
The Application of Nonlinear Fourier Analysis to Soliton Quantification for Offshore Engineering
Authors: Jeans Gus , Xiao Wenting , Osborne Alfred R. , Jackson Christopher R. , Mitchell Douglas A. 
Published: October 2017

This paper describes the first application of Nonlinear Fourier Analysis to the quantification of internal soliton current speeds in offshore engineering design. Large amplitude solitary internal waves produce strong, rapidly varying currents that may cause hazards to offshore operations in several regions of the world. These phenomena are commonly referred to in Industry as “solitons.” Soliton quantification was undertaken using the latest methodologies and software available from the Nonlinear Fourier Analysis Spectral Tools (NFAST) Joint Industry Project.
Solitons require rapidly sampled in-situ data for reliable quantification. Such measurements are typically of very short duration compared to the time scales needed for engineering quantification. Similarly, numerical models capable of representing solitons are computationally expensive, and thus have limited capabilities for efficiently developing the long-term simulations required to supplement in-situ data. NFAST aims to address these issues by enabling new Hyperfast Nonlinear Fourier Analysis computational techniques.
Interface displacements, derived from temperature measurements, were the primary input to soliton quantification. Associated current speeds were estimated from relevant theory and validated with available measured current data. In this particular case, the temperature measurements are considered to be more reliable than using the measured current data directly.
Application of NFAST codes produced a synthetic dataset of soliton amplitudes and speeds with an effective duration of approximately 100 years, a period that is considerably greater than the duration of available measured data. This provided extreme values consistent with extrapolation of the measured soliton data, but with a considerable reduction in uncertainty.
A Novel Measure to Reduce Ship Resistance in Waves
Authors: Guo Bingjie , Vartdal Bjørn-Johan , Steen Sverre 
Published: October 2017

Ships travel in waves most of the time. The horsepower increase or speed loss in waves can become large in adverse sea. Speed loss needs to be compensated for by increasing the power. Moreover, the associated speed-loss can possibly not be compensated due to the limit of installed power. This will cause delays, and may even lead to safety concerns in maneuvering. Thus, there is a need for minimizing added resistance due to both economical and safety reasons. Ships with better performance in waves even with reduced power are desired. Latest researches on advanced ship optimization are taking added resistance into account. Ship bow optimization has been the main measure to reduce added resistance in waves.
Based on analyzing the working principles and potential benefit of the different kinds of novel ship bows, a novel measure is proposed to reduce the ship resistance in waves is proposed. A novel measure to reduce the ship resistance in waves of existing ships by installing a simple structure at the ship bow. The structure is designed to sharpen the ship bow and therefore reduce added resistance due to wave reflection. Thus, it is suitable for the large tankers and bulk carriers, which have blunt bows. The volume above free surface was expected to reduce ship motions and the added resistance due to ship motions consequently. This measure does not change the original ship design. It can be retrofitted on existing ships fairly easily at low cost. In order to verify the benefit due to the retrofitted structure, CFD simulations are performed in both head and oblique waves, which can take 3D effect into account. Four different retrofitted structures are designed and the numerical simulations are performed with the same numerical and mesh settings in each wave condition. The CFD simulation results confirm that the novel measure can reduce ship added resistance efficiently and it also has benefits in oblique waves.
Second Order Wave Loads on TLP-TAD Multi-Body System
Authors: Ramirez Miguel A. M. , Fernandes Antonio Carlos 
Published: October 2017

The motions of floating moored structures are affected by first order wave loads which are proportional to the wave amplitude and associated with the wave frequency. On the other hand, second order wave loads are proportional to the square of the wave amplitude and related to the sum or difference of a pair of frequencies of the irregular sea. Although the second order loads are usually smaller compared to the first order loads, these loads can excite resonance motions in frequencies that the system has very low damping. Therefore, second order wave loads have particular importance in the design of mooring systems.
The multi-body system composed by Tension Leg Platform (TLP) and Tender Assisted Drilling (TAD) is particularly susceptible to the second order effects, due to the very low natural frequencies of their horizontal modes and the very high natural frequencies of the vertical modes of the TLP.
This work presents a numerical study of second order wave loads on the TLP-TAD multi-body system. An extensive hydrodynamic analysis focus on the hydrodynamic interactions between the floaters and how these effects modify the second order loads on the platforms was performed. The second order quadratic transform functions (QTFs) were evaluated using the indirect and the direct method. Moreover, the importance of the free surface integral was checked. Finally, the accuracy of Newman approximation for the low-frequency QTF was evaluated.