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NAERINGSPH-Nærings-phd

Dynamic analysis in design and operation of large offshore wind turbine drivetrains

Alternative title: Dynamisk analyse i design og drift av drivverk i store havvindturbiner

Awarded: NOK 1.6 mill.

Project Number:

263819

Project Period:

2016 - 2021

Funding received from:

The project has resulted in a versatile simulation model including elements required to model rolling-element bearings as main bearings in large floating offshore wind turbines. Furthermore, the project has resulted in new knowledge on the dynamic behavior of a main bearing based on field measurements in a floating wind turbine. Larger turbines are the most important contribution to reducing cost of energy for installations on both floating substructures and fixed foundations. This requires lightweight and compact main bearing and drive train solutions and challenges established layouts, components selection and analysis methods. Floating wind turbines are necessary to access the full offshore wind power potential. Although pioneering floating wind farms exist, floating operation is currently not a design driver for the standardized parts of a turbine. It is simply a different set of conditions for the global structural analysis. Considering the solution spaces for drivetrains and floating substructures in combination with the limited experience with large turbines and floating operation, more knowledge on drivetrain load effects is needed. Reliability problems in wind turbine gearboxes have partly been attributed to limited knowledge on drivetrain dynamics. Numerous studies addressing design and analysis of gear-based drivetrains have followed. Despite increasing interest in floating turbines, only a few studies have assessed the effects of floating operation on the drivetrain. These generally point to the main bearings as the most critical components. Main bearings have received limited attention in literature, regardless of the design challenges. The main bearings are an integral part of the load-carrying structure, and replacement may require the removal of both the rotor and the nacelle. Consequently, the bearing design life should match the turbine design life. This is important with respect to life extension studies. Development of cost-efficient solutions for heavy maintenance is critical for floating turbines, and the value of avoiding bearing replacement is potentially higher than for bottom-fixed turbines. Modeling and simulation are essential for offshore wind power development. Global structural analyses and local drivetrain analyses are commonly performed separately, in what is termed the decoupled approach. The main bearings are effectively connecting these domains, and attention to modeling requirements is needed. The model presented by the project is developed through consistent use of the bond graph method, providing a clear model structure. Body-fixed equations of motion are derived using Lagrange's method and contained in field elements. An alternative cage model and an elastic outer ring are introduced, providing a more complete modeling basis. Modal representation is used for the elastic bodies, including a solution for moving loads on the outer ring. An example simulation of a generic rotor-bearing system during run-up with and without bearing damage is presented to demonstrate the capability and usefulness of the model for transient analyses. The simulation results illustrate that the behavior of the non-linear system during a transient is not intuitive or easily inferred from the characteristics of the individual parts. The project has also presented a first-tier experimental analysis based on field measurement data from a main bearing in a 6 MW turbine on a spar-type floating substructure. Optical Fiber Bragg Grating sensor arrays have been used to measure circumferential strain in multiple positions on the fixed bearing ring. Time domain and frequency domain analyses show that in-plane bending occurs in the fixed ring, largely driven by differential blade bending moments at 3P frequency caused by wind loads and with limited influence from floater motion. This is an important result for future design of floating turbines. Furthermore, sum and difference frequencies in the measured response as a result of the non-linear system indicate that the decoupled approach may be insufficient in design analyses. The results from the modeling and simulation contribute to advancing knowledge on rotordynamic system models suitable for studying rolling-element bearing dynamics in applications involving mechatronics and control, transient events, and damage. The results from the experimental analyses confirm the modeling basis with respect to elastic ring representation and thus indirectly with respect to the importance of a cage. Furthermore, it can be inferred from the experimental analysis that the significance and implications of the results depend on the actual main bearing design and the turbine size. Coupled analyses should be considered in some cases. Furthermore, it should be considered to include the hub consistently as an elastic element in modeling and full-scale testing of the drivetrain, thus moving the load interface from the main shaft to the blade attachment.

- Økt kunnskap om lastvirkninger og analysebehov for drivverk i flytende havvindturbiner. - Økt kunnskap om modeller, målinger og analysemetoder som kan anvendes i nye løsninger og praksiser. - Styrking av involverte kompetansemiljøer. - Dreining av kompetanse fra olje og gass til fornybar energi. - Sterkere tverrfaglig kobling. - Økt akademisk samarbeid.

Formålet med prosjektet er å øke Statoil, som utbygger og operatør innen havvind, sin kunnskap om dynamiske lastvirkninger på drivverk og hovedlagre i store vindturbiner, og finne frem til egnede analysemetoder for bruk i ulike faser i prosjekter. Utviklingen av havvind som en bærekraftig industri er sterkt knyttet til teknologiske fremskritt, og trenden mot stadig større turbiner som tilnærming for å bedre totaløkonomien i utbyggingsprosjektene forutsetter en svært grundig forståelse av de dynamiske egenskapene til hele vindturbinstrukturen, inklusiv drivverket, slik at en har kontroll over levetid og pålitelighet i alle detaljer, og kan minimere vekt og materialbruk. Det vil gjøre Statoil i stand til bedre å kunne vurdere og håndtere teknisk risiko og levetidsaspekter, og dermed være i stand til å ha som konkurransefortrinn å tidlig høste gevinstene av å ta i bruk ny teknologi i et stadig mer konkurranseutsatt og krevende marked for utbyggingslisenser. Sentrale utfordringer for forskningsprosjektet er 1) Å definere et representativt referansecase for hovedlagre og drivverk i 10MW-klassen som gir bredest mulig gyldighet av analyseresultatene, uten å kompromittere turbinleverandørenes rett til beskyttelse av eget design, 2) Nå relevant grad av «vertikal» kobling/integrering av analysemodeller innenfor akseptabel beregningstid, 3) Finne effektive analysemodeller på detaljert nivå utenom lagerleverandørenes proprietære verktøy, og 4) Få tilgang på relevante eksperimentelle data. Statoil sin industrielle posisjon er viktig for punktene 1) og 4) og NTNU sin ekspertise er viktig for 2) og 3). Prosjektresultatene har flere potensielle anvendelser, hvorav de viktigste er 1) Analyseverktøy og metoder som kan brukes systematisk og av flere personer i forbindelse med teknologikvalifisering, prosjektutvikling og drift i Statoil, og 2) Bygge kunnskap og vedlikeholde sårbare miljøer både hos Statoil og NTNU gjennom ny og oppdatert forskning på et område i rask utvikling.

Funding scheme:

NAERINGSPH-Nærings-phd