Propagation of damage in ship structure caused by collision or grounding accident

Abstract

U dosadašnjim istraživanjima iz područja analize sigurnosti brodskog trupa oštećenog sudarom ili nasukavanjem, uglavnom se pretpostavlja da oštećenje ne propagira za vrijeme spašavanja oštećenoga broda te da ne utječe na njegovu preostalu uzdužnu čvrstoću. Ovaj rad se temelji na pretpostavci da fluktuirajuća valna opterećenja za vrijeme tegljenja oštećenoga broda mogu generirati visoka ciklička naprezanja koja bi doprinijela propagaciji oštećenja kao zamorne pukotine. S obzirom na vremenski period tegljenja, generirani broj ciklusa valnog opterećenja iznosom je na granici između niskocikličkog i visokocikličkog zamora te je potrebno analizirati obje pojave. Niskociklički zamor se određuje metodom lokalnog naprezanja i deformacija i tretira se kao posebno granično stanje koje ugrožava uzdužnu čvrstoću broda. Visokociklički zamor, odnosno propagacija pukotine, analiziran je metodom mehanike loma i primjenom dijagrama procjene loma te se također razmatra kao granično stanje oštećene brodske konstrukcije opasno po uzdužnu čvrstoću. Uspoređene su dostupne metode za računanje koeficijenta intenzivnosti naprezanja, kao temeljnog parametra analize propagacije pukotine. Odabrana je metoda pomaka, koja zadovoljava kriterije točnosti, robusnosti, brzine i dostupnosti. Dan je pregled mogućih načina modeliranja oštećenja prilikom sudara, kao i pregled metoda proračuna valnoga opterećenja oštećenoga broda. Na primjeru „pojednostavljeno realno“ oštećenoga tankera za prijevoz nafte, napravljen je proračun niskocikličkog i visokocikličkog zamora. Zaključeno je da su vjerojatnosti pojave niskocikličkog zamora materijala te propagacije pukotine tijekom tegljenja vrlo niske. Nešto je veća vjerojatnost loma prema kriteriju dijagrama procjene loma, ukoliko je lomna žilavost materijala niska te ukoliko se uslijed sudarnog oštećenja brodskoga trupa stvore inicijalne zamorne pukotine. Predložen je jednostavan model pomoću kojeg bi se u slučaju pojave veće pukotine na oštećenom brodu na točan i brz način mogla odrediti preostala granična uzdužna čvrstoća brodskoga trupa, uz uključen utjecaj rotacije neutralne osi.The number of ships in the global fleet increases every day resulting in higher risk of accidents, such as collision and grounding. Studies show that in the case of oil tankers, collision is the main cause among all types of tanker accidents, closely followed by grounding. The outcome of an oil tanker collision can be a large oil spill with devastating economic and environmental consequences. After collision, damaged ship needs to be towed to the salvage harbour as fast as possible. Almost all classification societies (ABS, DNVGL, BV, etc.) provide emergency response services and software tools which enable quick damage stability and residual longitudinal strength calculation. It is often assumed in the emergency response procedures that the damage is time invariant during towing period, which could last from one day to couple of weeks. Propagation of the initial damage during the ship salvage due to the fluctuating wave loads, is generally ignored. However, large and irregularly shaped damage caused by collision or grounding could increase fluctuating stress level and as a result fatigue cracks could appear and propagate. Fatigue crack propagation in deck or bottom region is especially dangerous when considering longitudinal structural capacity of a hull-girder, since deck and bottom panels are the largest contributors to the ship hull-girder sectional modulus. During towing period damaged ship can encounter 105 or more wave cycles which is borderline condition between low-cycle fatigue (LCF) and high-cycle fatigue (HCF). For that reason, both failure modes are investigated in the thesis. LCF occurs when ship is exposed to loadings with very large amplitude (e.g. large waves) which can in some parts of the structure generate high cyclic plasticity. Classification societies prescribe that the LCF should be considered as principal failure mode, associated with ultimate limit state (ULS) or accidental limit state (ALS). HCF is calculated as crack propagation using linear elastic fracture mechanics and Paris Law. Principal governing factor during crack propagation is the stress intensity factor (SIF). Available methods for calculation of SIF were analysed and compared. Displacement method, chosen for further work, met the requirements of speed, availability, and accuracy. Application of LCF and HCF are compared with available experimental data on the case of the fatigue life of welded structural detail. The possibility of fatigue failure of damaged oil tanker during salvage period was investigated. Modelling options of collision damage and methods for computation of wave loads on damaged ship are reviewed. The effect of damage shape and size was examined through 50 “almost realistic” collision damage scenarios modelled by the finite element method (FEM). Such approach enables calculation of stress concentration factors (SCF) around damage opening using very fine mesh of finite elements. Fluctuating wave–induced stresses during relatively short salvage period were assumed to be induced by vertical wave bending moments (VWBM), distributed according to the Weibull two-parameter probability function. Parameters of the distribution were calculated by the seakeeping analysis in the typical sea environment where collision events usually occur and for the North Atlantic. Individual stress amplitudes were obtained using Monte Carlo simulation based on the Weibull distribution. As the damage accumulated by individual wave cycle is non-linear, total damage accumulation highly depends on the ordering of random stress amplitudes. Therefore, it was necessary to repeat large number of such analyses, using different sets of random wave amplitudes, all originating from the same initial Weibull distribution, but using different “seeds”. Parametric analysis was then performed to investigate the influence of towing duration and uncertain input parameters on LCF damage accumulation and crack propagation. It is found that LCF could not generate severe damage case while parametric study showed that the sea environment and towing period have some influence on accumulated LCF damage. Overall results are quite sensitive to mean zero crossing periods and heading angle. Failure criteria for crack propagation analysis was based on Failure Assessment Diagram (FAD). The results show that the low value of the fracture toughness of material and the existence of the initial crack size could increase fatigue failure probability during salvage period. In this particular case of damaged Aframax oil tanker, it was concluded that propagation of large fatigue crack could influence longitudinal strength of the ship during salvage period, what is quite unlikely. If unexpected large fatigue crack would appear on damaged ship structure, the ultimate longitudinal strength of ship could be reduced. In this work a procedure was proposed for calculation of the residual ultimate vertical bending capacity of damaged ship with presence of fatigue crack, taking into the account important effect of the rotation of the neutral axis