Wind turbines and renewable energy devices are important components for thefuture of energy sector. This is due to fast depleting sources of oil and gasall around the globe. Offshore wind turbines offer numerous advantages, due totheir remote location from land, thereby reducing noise, higher power output(due to large wind speeds) and utilization of existing offshore platforms fortheir installation and operation.
This paper presents a general analytical solution for calculation of dynamicbending stresses induced in a wind turbine tower as a result of rotatingimbalance in the rotor blades. The formulation is based on elastic beam theory,with inclusion of rotating imbalance forces. The formulation neglects the bladegyroscopic effects and models only the lateral tower vibrations.
The dynamic stresses are highly important for fatigue evaluation of the steelwind turbine towers. The formulation models the linear elastic response of thetower under the effect of rotor imbalance, and proceeds to compute the dynamicbending stresses as a result of lateral vibrations. The model also includes theeffect of linear viscous damping at the rotor-nacelle at the top of tower, andshows the beneficial effect of damping to suppress vibrations and reducebending stresses. The damping may arise from sources such as aerodynamic dragand internal steel structural dissipation. The motivation of this technicalpaper is to provide a simple analytical desktop tool for an engineer toevaluate the dynamic stresses for a given wind turbine size (tower height,diameter, wall thickness, blades and nacelle mass, rotating imbalance androtational speed), for an initial dynamic stress and fatigue screening, beforeproceeding to perform more comprehensive aero-structural finite element modelsof the wind turbines for simulating the dynamics of the wind turbines.
Wind turbines; dynamics; rotordynamics; bending stresses; dynamic stresses;vibrations; fatigue; critical speed
Currently, the so-called state-of-practice approaches are commonly used inriser VIV analysis. DNV RP F204 has indicated that riser axial stress fatiguedue to VIV is not considered due to the limitation of the state-of-practiceapproach. This paper gives a methodology for considering the top tensionedriser (TTR) axial stress fatigue due to VIV, using nonlinear coupledbeam-column modeling, and proposes a procedure, using Flexcom and Shear7, toconduct TTR axial stress fatigue damage analysis. Case studies are discussed,including TTR VIV in the Gulf of Mexico (GoM) and West of Africa (WoA).
On the other hand, API RP 2RD suggests that TTR with relatively stifftensioning systems may experience tension fluctuations that are significantrelative to the mean tension, leading to significant changes in the lateralstiffness. Further, it was found in field tests in Norway that if VIV frequencycoincides with half the TTR axial mode frequency, extreme axial stressesresult. This paper demonstrates that the phenomenon observed in the field testis due to the Mathieu effects. Using Mathieu theory on TTR axial stressresonance due to VIV is a novel idea and this paper provides a novelmethodology to assess Mathieu Instability (MI), specifically, stabilitydiagrams with damping effects in parameter plans are generated. These diagramsare intended to cover possible combinations of TTR properties, such aspre-tension, mass, damping, axial and bending stiffness etc. Finally, thispaper illustrates applications of new method in total VIV fatigue analysis,including axial fatigue, and MI engineering assessment.
The Ocean Current Monitoring from 500 - 1,000 meters study is part of theTechnology Assessment & Research (TA&R) program of the Bureau of OceanEnergy Management, Regulation and Enforcement (BOEMRE) of the U.S. Departmentof the Interior. The objectives of the study were:
• To assess the characteristics of Gulf of Mexico current, forcing in the 500to 1,000 meter water depth range and occurrence of elevated events; and
• To use those data to assess the importance of such currents on the fatigueand design of risers, moorings and TLP tendons.
The study was conducted as collaboration between MCS Kenny, Fugro GEOS, andPrinceton where the first objective of the study was addressed by Fugro GEOSand Princeton. The first objective is a topic for another paper and it will notbe discussed in detail in this paper. This paper addresses the second objectiveonly and was undertaken by MCS Kenny.
Fugro GEOS study describes how BOEMRE's NTL ADCP data, historical mooringcurrent data and Princeton University's PROFS model data were used to derivethe current profiles to be used in the riser/Tension Leg Platform (TLP) tendonVortex Induced Vibration (VIV) fatigue study described in this paper.
In particular, based on the analysis of current characteristics, ocean currentkinetic energy distribution and observational data availability; fourrepresentative zones (1-4) for Gulf of Mexico were indentified. Zone 1 and zone3 represented the area dominated by Loop Current/Loop Current Eddies andhurricane generated currents. Zone 2 represented an area with relatively strongcurrents found near the continental rise and slope in the northern Gulfespecially where isobaths converge are narrow. Zone 4 represented the new frontarea corresponding to the high kinetic energy at 500 m water depth. For each ofthe zones, both long term and short term full water column current profilecharacterizations (represent current profiles and associated probabilities)were derived and provided for the use in the riser VIV fatigue study, by FugroGEOS.
These current profiles were then used as a part of the study performed by MCSKenny, to address the second objective. In particular, the current profilesprovided by Fugro GEOS were used in determining whether monitoring oceancurrent between 500 m and 1,000 m water depth range should be required and usedin assessing VIV fatigue damage of risers (Drilling, SCR, TTR, and Hybrid) andTLP tendons in the GoM.
As a part of the methodology to achieve this objective was to compare the VIVfatigue damage calculated based on the following current characteristicsbetween 500 m and 1,000 m:
• Existing Current Profile: Current profile typically assumed by designers,with the lack of actual recorded profiles, which is generally an extension ofthe current velocity at 500, dubbed as linear current profile;
• New Current Profile: Current profiles, as monitored and provided by FugroGEOS.
Depending on the relative values of the VIV damage resulting from eithercurrent profile, a conclusion can be drawn as to the importance of monitoringand using current data between 500 m and 1,000 m in assessing VIV damage ofrisers and tendons in the GoM.
As part of the Bureau of Ocean Energy Management, Regulation and Enforcement(BOEMRE) sponsored study for ocean current monitoring from 500 - 1,000 metersin the Gulf of Mexico, this paper describes the methodology and results ofassessing the characteristics of the forcing and occurrence of elevated currentevents.
BOEMRE NTL ADCP data, historical mooring current data and three types ofPrinceton Regional Ocean Forecasting System (PROFS) model data were used forthis study. Based on the analysis of current characteristics, ocean currentkinetic energy distribution and observational data availability, fourrepresentative zones are indentified. For each zone, the long term and eventcurrent profile characterizations and subsurface elevated event screening areundertaken, and long- and short-term full water column current profilecharacterizations (representative current profiles and associatedprobabilities) are derived from selected quality controlled NTL ADCP data,historical mooring data and PROFS model data. These current profiles andspecific riser models for each type (Drilling, SCR, TTR, Hybrid Risers, and TLPTendons) are used to assess the VIV damage, which is documented in theaccompanying paper by Ozturk et al. (OTC 23570).
This study will evaluate the significance of ocean currents in water depthsbetween 500 and 1000 meters on riser and mooring systems, as well as toidentify whether any significant events have resulted. The study results willhelp BOEMRE to define whether monitoring below 500 meters is justifiable inregards to fatigue analysis of riser or mooring systems.
Safe and efficient exploration and development of deepwater offshore oil andgas fields require the development of a comprehensive knowledge of the localoceanographic current regime. The study of ocean current monitoring from 500 -1,000 meters in the Gulf of Mexico was sponsored by the Bureau of Ocean EnergyManagement, Regulation and Enforcement (BOEMRE) of the U.S. Department of theInterior. The purpose of this study was three-fold:
1) Assess the characteristics of Gulf of Mexico current forcing in the 500 to1,000 meter range and occurrence of elevated events;
2) To evaluate the significance of ocean currents in water depths between 500and 1000 meters on the fatigue and design of risers, moorings and TLPtendons;
3) Based on the findings, to form a recommendation on whether monitoring below500 meters is justifiable in regards to fatigue analysis of riser or mooringsystems.
This paper describes the activities associated with the first purpose of thestudy. The accompanying paper by Ozturk et al. (OTC 23570) addresses theremaining goals.