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design load cases developed for conver- sion analysis. Also, new dent-speci? c

DLA cases have been created to maxi- mize the global load components in way of each of the damage locations. The full global hull FEA model has been solved with these new load cases. The local ? ne mesh models have been loaded with the resulting pressures and enforced dis- placements from the global model. Ad- ditionally, new damage heel cases were developed to investigate the effect of the additional hydrostatic pressure on the

About the Authors dents. These speci? c cases have been

Fritz Waldorf (pictured) is Director selected as most likely to produce the of Sales and Marketing for Viking greatest hydrostatic pressure at each dent

Systems International and based in location due to vessel pitch and heel. their Houston of? ce. Viking assists

Conducting fatigue life assessment shipyards, ship designers and owner/ consists of the calculation of fatigue operators with the ef? cient implemen- damage from the past wave induced tation of advanced analysis tools for stress ranges resulting from the vessel ? oating vessels and structures.

history prior to conversion, the damage accrued along the transit route to the

Arnold Balster is an Engineering Man- operating site, and the expected fatigue ager for Viking Systems International. damage from desired design life in its

Arnold is based in Viking’s Houston operating environment, as well as dam- of? ce and has a naval architecture age resulting from the on and off load- and hull structural background and ing of the vessel’s operating purpose (if together with his team is responsible applicable). As previously mentioned, it for conducting the various engineering is typical to use site conditions that are and design tasks.

all corroded to the end-of-life scantlings - ensuring the most conservative results. tion frequency by a speci? c period of time to ensure continued vessel safety

The local ? ne mesh model of the dam- and avoid any downtime.

aged side shell, in their respective loca-

To approximate the effect of the resid- tions, were analyzed for fatigue using the element centroidal stress results from the ual stresses remaining in the dented steel elements in way of the areas of interest from the original impact, a nonlinear im- pact analysis was performed. To the best and the detail speci? c S-N curves. ability, a similar shape of the projectile

Within Viking’s structural assess- ment software suite, SAGA, there is a was modeled and the impact was recreat- crack growth program integrated that ed using the same mass, moving at a ve- performs a fracture mechanics analysis locity calculated to deform the side shell on the most highly stressed locations of plate to approximate the observed dent the damaged structure to determine the depth. It is not feasible to exactly match likelihood a crack will grow to critical the dent depth observed in the original length. The crack propagation analysis 3-D laser reports, due to the size and was performed on two locations within shape of the projectile chosen for these each of the areas of interest, chosen as tests. Failures begin to occur in the side the worst location for fatigue in the side shell plating as the corners of the coarse- meshed projectile contact the side shell. shell and side shell longitudinal in way of

The determined factors allow for inter- the dents, and the worst location for fa- tigue at the transverse web frame cutouts pretation of the stress locked in the struc- ture, which will work its way out over in way of the dents. The crack propaga- time – relieving itself of the pressure can tion analysis showed that the probabil- ity of a crack in the side shell plating or also help give an indication of how fast a side shell long growing to critical length crack could grow if one were to appear.

Experience in such advanced forensic is low. However, a crack developing in the web frame plating in way of the long analysis cases is key to prevent future stiffener cutout at the dented area may pain and expense for owner and op- become critical within the design life of erators of maritime assets. An approach both quantitative and qualitative, while the vessel. As a service provider for fo- rensic analysis, this conclusion leads to acting as doctors for vessels, leads to the most accurate diagnosis. the recommendation to increase inspec- www.marinelink.com 15

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