## Adequate linearization scheme for a jack-up in order to obtain sufficiently accurate fatigue assessments using a linear stochastic fatigue analyses

##### Master thesis

##### Permanent lenke

http://hdl.handle.net/11250/2455252##### Utgivelsesdato

2017##### Metadata

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- Institutt for marin teknikk [2472]

##### Sammendrag

Different techniques for linearizing the response of drag dominated structures is inthis work inspected in terms of fatigue damage. To establish reference responsecharacteristics, time domain simulations are carried out. Time domain simulationsmust be considered as state of the art methods for estimating fatigue damage., butdo however demand huge computational capabilities.Linear potential theory is used to calculate wave kinematics. To compensate for thedeviation to higher order wave kinematics, adjusted drag coefficients are used. TheJONSWAP spectrum is used to generate stochastic surface elevation and forces,which is realized using both randomness in phase and amplitude. This insures thata signal is never repeated within a short term sea state.Stress concentration factors are used to generate stress processes from beam reac-tions. To calculate both cycle ranges, and amount of cycles, the rainflow algorithmis utilized, which result in stress range records that are used as input to SN curvesand miner summation.Ground conditions are selected to give a highest natural period of 7.67s, whichis within energetic areas of the scatter diagram. Large dynamical amplificationscontribute to move most important fatigue damage sea state down to a spectralpeak period of 8.5s.The linearization consists of evaluating the response of the structure to differentharmonic input components with different frequencies in order to create transferfunctions. In this regard, the question is how the wave heights used as input to theseharmonic components is selected. Three schemes of selecting these are inspected.The two most promising are achieved by keeping the steepness or the ratio betweenheight and period constant. They overestimate the total accumulated damageduring 56 years by 20 % and 100% respectively. The steepness or the constantheight-period ratio is calibrated by matching a spectrally calculated wave actionwith a deterministic calculated wave action. This calibration process is workingwell, and gives reasonable calibrated values. Both methods tend to be efficient andgives reasonable results. Whether the constant steepness approach is conservativemight be questioned especially at higher frequencies. The constant height-periodratio however insures conservatism also at larger frequencies.It might also be possible to switch the drag term to a linear term and replace thedrag coefficient by a linear drag coefficient. This might open up