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HANSA 05-2022

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Kran- und Hebetechnik | Danelec | Compit | Korrosionsschutz | HullPIC | HANSA & WISTA Germany | Norwegens Reeder und die Börse | 175 Jahre Hapag-Lloyd | MPP-Schifffahrt


HÄFEN | PORTS Crane Cracks and Fatigue Lately it seems that ports have been putting off crane purchases and want to use their existing cranes beyond their design life. There is a trend to push cranes harder for longer, which can lead to cracks and other fatigue-related issues, though Although steel can have an infinite design life if the stresses are low enough, designing cranes to have infinite life would increase the cost to the point where it is not competitive. Ports compete for shipping lines, so if a port pays a much higher cost for a container crane, the box cost will be higher to repay the investment. The lines will go to a port with lower operational costs unless there is some other hourly operational benefit. Even if crane structures were designed to have infinite life, they would still eventually become obsolete due to the increasing size of container ships. Container cranes 20 years ago did not service ships as large as they do today, so it was unheard of to have a crane with an outreach over 200 ft. (approx. 61 m) because container ships weren’t that big. Back then, the largest cranes could pick up a single 40 ft. or twin 20 ft. containers, whereas today the largest cranes can lift tandem 40 ft. containers or four 20 ft. containers. In addition, the largest cranes have a second trolley to help sort the containers when the primary trolley places them on the dock, which speeds up the yard operations. Today, cranes with outreaches that extend beyond 230 ft. (70 m) are common on big ship-to-shore cranes, made to service the largest container ships with beams of 202 ft. (61.5 m) with some room to spare for the future growth of container ships. All that said, crack and fatigue issues may be present in cranes of all sizes. Steel fatigue Many people think of fatigue as »wearing out« of the steel but this is not quite right. Fatigue failure occurs in components subjected to a high number of fluctuating stresses. Under these conditions it is possible for failure to occur at a stress level that is significantly less than the tensile or yield strength for a static load. If the fluctuating stresses are low enough and in an ideal environment, steel has infinite fatigue life. Cracks can initiate from many sources, such as high cycle fatigue, poor manufacturing, corrosion, or overload events such as snag, earthquake, or storm winds. The initial flaws may be microscopic or macroscopic. Growth rate increases with crack size so a flaw that has grown from microscopic to a detectable size is well along toward reaching critical size. However, if the steel lacks reasonable notch toughness, the critical crack length will be significantly smaller than a steel with excellent notch toughness. It is very unlikely that someone without training would be able to detect the first signs of cracking or fatigue. It is also unlikely that the operator would notice a change in the performance of the crane, even if they are very familiar with it. Further, not all cracks are equal – it’s all about risk management. If failure of a structural beam would cause catastrophic failure, it is considered a fracture critical member. If a fracture critical member has a crack, it is not worth the risk of a catastrophic failure to continue operating until the crane can be taken out of service and the crack is repaired. Fatigue and corrosion failure of container cranes have been rare, but there has been at least one incidence of total collapse and several close calls where imminent failure was avoided because cracks were discovered just in time. Numerous fatigue failures of individual members and connections have occurred but usually loads shift to an alternate load path, avoiding total collapse. Therefore, alternate load paths are an important fatigue design consideration. Fatigue issues are not always a byproduct of overuse. Sometimes the crane design is not robust enough for its specified duty class, while other times there are manufacturing defects that were not caught by the quality assurance / quality control (QA/QC) programs. Also, accidental overload cases such as collision, stall, snag loads as well as high wind events and earthquakes can reduce a crane’s fatigue life. From our perspective, fatigue problems appear to be increasing due to the record amounts of container traffic. © CPA Subsurface cracks are only detectable with non-destructive tests, such as ultrasonic or radiography testing. This forestay crack (see photo on the right) was discovered just in time No formal body Unlike other industries, the container crane industry has no formal body for investigation, documenting and reporting 74 HANSA – International Maritime Journal 05 | 2022

HÄFEN | PORTS structural failures. These failures are often a source of embarrassment, liability, or litigation and therefore remain confidential. If an accident occurs, disputes are usually settled through private litigation. This is unfortunate because the entire industry would benefit from sharing such information. Best practice is to prevent accidents, so it is always a good idea to have a qualified engineer review the crane manufacturer’s design before construction, as well as having a good maintenance program once the crane is in service. High cycle fatigue, corrosion Typically, cranes are designed only considering high cycle fatigue. Overload events are checked for strength but are not typically included in the fatigue analysis. Several moving load locations are considered and an equivalent lifted load for fatigue is decided based on the crane classification or specified by the purchaser. The more realistically the moving load paths model how the crane is used, the more accurate the results of the analysis will be. However, even the most thorough calculations have their limitations. They have many built-in assumptions that may not be accurate. For instance, if a weld has poor fusion or porosity, it may not be detected by a surface inspection such as a visual, dye penetrant, or magnetic particle testing. This weld can have subsurface cracks or defects that can grow to the surface much faster than calculations predict. Design to a fatigue criterion is no guarantee that fatigue will not occur. On Finite element analysis model of boom brace connection on a container crane. Two cracks were discovered in this location a statistical basis, fatigue design does provide reasonable fatigue protection. Another vital point to consider is corrosion. Of course, container cranes exist in a relatively hostile environment surrounded by saline and acid laden air. Corrosion is an ever-present enemy that is no mystery to any maintenance department or owner. Techniques and materials for preventing corrosion are well known and the failure to maintain a corrosionfree crane is tantamount to accepting a reduced life for the affected crane components. With advancing age, poor structural maintenance programs become obvious at an exponential rate. For some container cranes it is too late, and they should be retired. In other cases, it is possible to increase the life of cranes well beyond the original purchase specifications. If the design of a structural component is controlled by strength and not fatigue, its design life may well exceed the required minimum fatigue life. Life extension In this current economic environment, there is a lot of uncertainty surrounding steel fabrication. Steel prices, component availability, labor prices, workforce availability, etc. are all extremely volatile. Crane owners want to protect themselves by procuring cranes on a firm fixed fee contract. Such a contract includes all costs associated with crane procurement including delivery and commissioning for a single price (you’ll forgive me for not publishing figures). Likewise, crane manufacturers protect themselves by building in a lot of costs due to the uncertainty surrounding steel fabrication. This has further increased the number of crane owners taking interest in extending the life of cranes. A crane may have been designed for two million cycles, but the crane owners want to study the business case for upgrading the crane structurally and mechanically to extend the life up to three or even four million cycles. As cranes are used beyond their original design life structural strengthening is a common method to extend the useful life. After the initial investment, strengthening will lower the stresses in critical areas, which in turn decreases both downtime and inspection costs. If a critical area has been properly strengthened, it will be less likely to develop cracks and will require less frequent Managing cracks Just how critical is the problem when cracks appear? The answer depends on how much damage the area can safely withstand and the consequences of its failure. Damage due to a crack is directly related to how fast the crack can grow. The study of crack propagation is called »fracture mechanics«. Fracture mechanics combines analytical methods with experimental research to quantify a crack’s grow potential. The consequences of a member or joint’s failure also plays a part in determining the criticality of a crack. A member or joint that has no alternate load paths and whose failure would cause a crane to collapse is called fracture critical. Within fracture mechanics, one methodology to manage cracks and other defects is using »damage tolerance«. Pioneered by the aerospace industry, the idea behind damage tolerance is that the engineer assumes there is a crack of the smallest size with a given inspection method. From there, the engineer can calculate the crack growth rate that will occur during normal use. This analysis is then used to set the appropriate inspection intervals based on the criticality of the member or joint. By calculating the crack damage tolerance of a crane and implementing the resulting inspection program, chances are much better that repair work can be identified and scheduled to minimize operational down time. Much like your car, the oil needs to be changed more often than the timing belts. The same is true about cranes; some areas need more attention than others. It is typically dependent on both the duty cycle the cranes were designed for and how the cranes are operated. A damage tolerance program integrates these parameters to provide a rational basis for effective inspection intervals. HANSA – International Maritime Journal 05 | 2022 75

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