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PETROMAKS2-Stort program petroleum

Fatigue damage from dynamic ice action

Alternative title: Utmatting på konstruksjoner utsatt for dynamiske iskrefter

Awarded: NOK 6.3 mill.

In the FATICE project we have addressed the fatigue damage on fixed offshore structures exposed to drifting ice. This is an important challenge in the development of energy production from offshore wind in the Baltic and involves at least five elements: a) define ice statistics, b) predict the structural response (ice-structure interaction simulations), c) estimate the fatigue damage and d) carry out scale-model tests. We have used the Copernicus database and simple analytical equations to define the large-scale ice statistics and studied down-scaling to structural scale by comparing with ice load data on the Norströmsgrund lighthouse (LOLEIF and STRICE data). A statistical description of 30 years of ice and selected met-ocean parameters has been developed. In the areas with annual sea ice cover the Copernicus data gives reasonable statistics, but in the areas with rare and infrequent ice conditions the statistics becomes uncertain and the data from Copernicus needs to be supplemented with modelling of physical processes. A comparison between the locally measured parameters on the Norströmsgrund lighthouse and the Copernicus data shows that meteorological conditions give a close to perfect match. The ice parameters (velocity, thickness and concentration) have much larger spatial and temporal variability and do not match that well. In particular the local ice thickness is difficult to predict with only the large-scale data at hand. The data from the Norströmsgrund lighthouse was also used together with met-ocean-ice data and an algorithm predicting the days of occurrence of ice-induced vibrations for a structure in these ice conditions. The VANILLA model allows for ice-structure interaction simulations and has been validated against the full-scale LOLEIF and STRICE data and against the model-scale ice in HSVA. The fully coupled and the traditional methods are compared. The main result is that the traditional method predicts a higher Damage Equivalent Load (DEL) than the fully coupled model. It may overestimate the ice action and make structures too expensive. In the fatigue estimations studies the assumption of linear damage accumulation is challenged and load combinations from wave, wind and ice studied by assessing simulated time-series of the different loads. The main result is that sea ice causes the higher loads than wind and waves do, but the cumulative frequency of ice loads is much smaller than for wind and waves. The traditional model-scale ice tends to be too soft and/or too viscous so that a realistic breaking pattern combined with realistic force-time series are not obtained for large aspect ratios. HSVA has overcome this challenge and developed a crushing model ice (ICMI) in which the ice crystals are larger and the texture more uniform and with which both realistic failure mechanisms and force-times series are modelled. FATICE has with this contributed to a reduction of uncertainties with respect to dynamic ice actions on fixed offshore structures and contributed to more efficient and cost-saving design without compromising safety.

De viktigeste effektene er: 1. Vitenskapelige partnerne går ut av prosjektet som verdensledende innen estimering av utmatting fra dynamiske islaster på bunnfaste konstruksjoner for Havvind. 2. Et styrket samarbeid mellom europisk industri og akademiske institusjoner 3. Utdannelse av MSc og Phd kandidater til europeiske industri og samfunn 4. Vi fortsetter et digitalt forum for videreutvikling av fagfeltet

FATICE deals with assessment of fatigue damage on fixed offshore structures exposed to drifting sea ice. Offshore structures are important for European industry and society with respect to exploitation of oil and gas, other mineral resources, aids for navigation and Development of offshore wind. Fatigue is one of the main challenges for these structures and the present guidelines include large uncertainties that may give very heavy and expensive structures. This has three main reasons: a) Ice conditions are poorly defined, b) ice loads and structural response are estimated independently and c) the assumption of linear (or independent) damage accumulations found in fatigue estimation. We will combine the two fields of Ice-induced Vibrations (IIV) and Fatigue assessment to a) define time-series of load cycles and structural response (occurrence of IIV + coupled ice-structure interaction model) and b) develop a new method to assess fatigue life through Variable Amplitude Loading (VAL). This allows for the first time for the design of reliable and thus cost-efficient fixed offshore structures exposed to drifting sea ice and consequently for recommendations for novel guidelines. The project will be led by Norwegian University of Science and Technology (NTNU), the other partners are Hamburg Ship Model basin (HSVA), Technical University of Hamburg (TUHH), DIMB Engineering and Siemens Wind Power (SWP). Additionally, TÜV Nord will contribute to fatigue assessment without being a full project partner. Together the partners have a worldwide unique competence to address assessment of ice induced fatigue damage on fixed offshore structures. The seven work packages starts with analysis of available full-scale data (WP1) providing data for WPs 2 -5. WPs 2-4 deal with IIV, response curve in WP2, modelling in WP3 and model basin testing in WP4. WP5 deal with fatigue assessment. In WP6, the recommendations for new guidelines are developed. Finally, WP7 will carry out cases studies.

Funding scheme:

PETROMAKS2-Stort program petroleum