Utmatting som følge av OPB i fortøyningsliner av kjetting : FEM analyse av topp-kjedet utsatt for OPB i henhold til retningslinjer av Bureau Veritas : Beregning og evaluering av utmattingslevetid for kjettingen, der effekten av en innfestningsarm med et lavfriksjonslager i enden mot den flytende konstruksjonen undersøkes
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Offshore structures are exposed to harshenvironments where there are many kinds of loads acting on the structure. These loads lead to complicated conditions with many possible load combinations. In addition, they are changing in both time and spacesince the intensity and orientation varies with time.Mooring lines in floating structures consists of mooring system where chains are widely used in what is called catenary and semi taut mooring. The mooring line is a key part in the station-keeping system thatforms a connection between the structure on the surfaceto the seabed.Here it will keep floating structures stable with the least possible movementover a point of interest. This means that the response from the loads on the structure must be absorbed by the mooring lines.Since the timespan in which the offshore structure is to be stationed often will be around 20 years, the mooring lines are significantly subjected to fatigue damage. This is because of the dynamic motion of the structures where the varying displacements induces tension cycles in the mooring system.In the 20 first chain links from the connection point at the floating structure, we will have a combined stress state with tension and bending stress because of tension-tension load, in plane bending and out of plane bending. This phenomenon occurs because of the angular differences between connection point on the floating structureand chain, friction between the links and changes in geometry from proof load that will help resist rolling and the sliding effect between the links. This results inbending moments to the first links of the top chain.To reduce the effect of OPB in the top chain, a “hang off” configuration can be implemented in the system. This configuration consists of anattachment arm which are free to rotatehorizontally and vertically throughlow-friction-bearings.The rod at the coupling point will relieve the chain by curving and unburdensrotations and stressesfrom the chain. The tensions and curvature here will be related to moments that occur when theyare belowor equal tothe sliding boundary of the bearing configuration linkage. The sliding boundary for the low friction bearing will also lie at a lower level relative to the momentthan the sliding boundary in the chain. This means that when using such an arm with rotational properties, the potential for moment is reduced just byimplementation.To obtain stresses due to theloads,stress concentrations, OPB interlinkmoments and IPB interlink moments are to be evaluatedthrough FEM calculations.The rod is also evaluated through FEM analysis.The fatigue damage accumulated in a mooring component due to cyclic loads will be the sum of the fatigue damage from occurring cycles.In this report one uses a wave period T of 6.3 seconds, thismeans that in a year you will have 5005714.286 cycles.This is further used with S-N curves given through standards from Bureau Veritas and DNVto calculate the lifetime of the top chain and attachment arm.The attachment arms influence on the mooring line has shown to have animposing effect in terms of OPB. Friction coefficient, pretension and length are all factors that is decisive in this matterandfor the arm itself in terms of fatigue.It can also be seen that the fatigue life of the attachment arm will be lower in relation to the fatiguelife of the chain with corresponding parameters. This means that when designing mooring lines based onthis report, the fatigue properties of the attachment arm will bedecisive for the choice of parameters.
Masteroppgave bygg BYG508 - Universitetet i Agder 2019