As current attention of the offshore industry is drawn by developing pilot farms of Floating Wind Turbines (FWTs) in shallow-water between 50m and 100m, the application of nylon as a mooring component can provide a more cost-effective design. Indeed, nylon is a preferred candidate over polyester for FWT mooring mainly because of its lower stiffness and a corresponding capacity of reducing maximum tensions in the mooring system. However, the nonlinear behaviors of nylon ropes (e.g. load-elongation properties, fatigue characteristics, etc.) complicate the design and modeling of such structures. Although previous studies on the mechanical properties and modeling of polyester may be very good references, those can not be applied directly for nylon both on testing and modeling methods. In this study, first, an empirical expression to determine the dynamic stiffness of a nylon rope is drawn from the testing data in the literature. Secondly, a practical modeling procedure is suggested by the authors in order to cope with the numerical mooring analysis for a semi-submersible type FWT taking into account the dynamic axial stiffness of nylon ropes. Both the experimental and numerical results show that the tension amplitude has an important impact on the dynamic stiffness of nylon ropes and, as a consequence, the tension responses of mooring lines. This effect can be captured by the present modeling procedure. Finally, time domain mooring analysis for both Ultimate Limit State (ULS) and Fatigue Limit State (FLS) is performed to illustrate the advantages and conservativeness of the present approach for nylon mooring modeling.
In the present paper, a new fully coupled simulator based on DeepLines™ software is described in order to address floating wind turbines dynamic simulation. It allows its user to take into account either separately or together the hydrodynamic and aerodynamic effects on one or several floating wind turbines. This simulator includes a non linear beam finite elements formulation to model the structural components — blades, tower, drivetrain, mooring lines and umbilicals — for both HAWT and VAWT layouts and advanced hydrodynamic capabilities to define all kinds of floating units and complex environmental loadings. The floating supports are defined with complete hydrodynamic databases computed with a seakeeping program. The aerodynamic loads acting on the turbine rotor are dynamically computed by an external aerodynamic library, which first release includes BEM (blade element moment for HAWTs) and SSM (single streamtube method for VAWTs) methods. The integration in time is performed with an implicit Newmark integration scheme.
After a few weeks, underwater components of offshore structures are colonized by marine species and after few years this marine growth can be significant. It has been shown that it affects the hydrodynamic loading of cylinder components such as legs and braces for jackets, risers and mooring lines for floating units. Over a decade, the development of Floating Offshore Wind Turbines highlighted specific effects due to the smaller size of their components. The effect of the roughness of hard marine growth on cylinders with smaller diameter increased and the shape should be representative of a real pattern. This paper first describes the two realistic shapes of a mature colonization by mussels and then presents the tests of these roughnesses in a hydrodynamic tank where three conditions are analyzed: current, wave and current with wave. Results are compared to the literature with a similar roughness and other shapes. The results highlight the fact that, for these realistic roughnesses, the behavior of the rough cylinders is mainly governed by the flow and not by their motions.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractPrincipia R.D. is the leader of the CALM BUOY JIP, which purposeis to study the behavior of deepwater CALM Buoys systems. The final objective is to provide guidelines for the analysis of deepwater offloading systems covering the main aspects related to the OOL/mooring lines fatigue analysis. Most of the attention is focused on the buoy hydrodynamics using both experimental campaigns and numerical investigations (full and model scales). Captive tests and forced oscillations tests at scale of 1/25 were performed with different skirt dimensions and drafts characteristic of a typical buoy.A new methodology and formulation is derived for damping estimation which is a key point of the analysis. The various damping formulations are implementedand tested in a fully coupled buoy / lines finite element solver. The results are validated against experimental measurememts in regular and irregular waves. Recommendations are deduced on the "model of model" approach classicaly used for model tests.Finally, use of CFD calculations to replace forced oscillations tests is also explored through comparisons with model tests. CFD appears to be a reliable tool to assess scale effects and to compare buoy geometries.
Bundles arrangements are currently used in the design of riser towers or oilexport lines. Some of them are characterized by a non-circular cross section[6] and therefore may be prone to plunge instability, so-called galloping orplunge instability when exposed to strong current. It is important to be ableto assess, at conceptual design stage, their likelihood of being subject tothis phenomenon. Galloping is taking place in the low frequency range compared to VIV, but withlarger amplitude, up to several diameters, which could be critical in term ofglobal motion. Galloping occurrence is related to the dissymmetry of the crosssection and then there is a risk for non-circular geometries, such as riserbundles, buoyancy tanks and floater columns. Instability can also occur intorsion or rotation by a coupling effect between transverse oscillations. Riser Vortex-Induced-Vibrations have been studied for decades, and numerousexperiments have been performed both in-situ and in model test facilities tounderstand and predict the response of a slender cylindrical structure in acurrent. The main reason is the influence of VIV on riser fatigue life. If galloping and fluttering are well known in aerodynamics [10], no largespecific experiment/study exists for hydrodynamic flows [1], [9]. So it is notevident to assess whether or not galloping may occur for a given riser bundledesign, and, in case of expected galloping, whether there is a potential riskof damage to the individual pipes in the bundle. Until recently, only theBlevins criteria [1] are available to predict the risk of instability but thereare limitations. Based on recent examples of riser tower, experimental and numericalinvestigations have been done within the CITEPH Gallopan project, with the goalto propose guidelines to help designing a bundle cross section in a way toavoid or reduce the risk of galloping. Two cross section shapes supported the investigations, the academic squarecross section, for which previous studies have been done [1], and a tokenbundle cross section expected to be subject to galloping. Model tests have beenperformed in two steps:Captive tests and transverse forced oscillation tests in steady current toderive hydrodynamic coefficients (using a multi-DoF motions generator), to beused to check the Blevins instability criteria.Free oscillations in steady current to identify the instability domain inrelation to the reduced velocity and to estimate galloping amplitudes. Aspecific experimental arrangement, based on a vertical pendulum system, hasbeen designed and set-up for this step. A methodology has been proposed to assess the risk and consequence of gallopinginstability using standard riser numerical tools in which hydrodynamiccoefficients are issued from model tests. This paper presents the main resultsof the Gallopan project in term of methodology based on model tests to analysegalloping occurrence and response for non-circular slender geometries. Usingthese results, it is now possible to develop a galloping-free riser bundledesign.
The purpose of this work is to use in situ measurements during service to adjust the estimation of the cumulated damage, and achieve an increased accuracy in predicting the remaining life span of the power cable. Direct monitoring of fatigue damage is not feasible, nor is it possible to completely measure the power cable response along all its length. Measurements can though be obtained concerning environmental conditions and the floater motion, as well as the cable response in a few points. However numerical simulations are still required to compute the state of stress of the cable. Therefore the intended monitoring methodology aims at calibrating several parameters from the numerical model with respect to available measurements, in order to reduce the uncertainty on these parameters, and thus the uncertainty on the model output. Selection of these parameters must therefore be based on their influence on damage and on their a priori uncertainty. In this respect marine growth influence on cable characteristics is of particular interest.
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