Since its inception, progress in the container crane industry has been remarkable in many ways. Considering the huge investment costs, one of the most amazing changes has been the increase in the size of equipment and facilities, with no obvious limit in sight. The only things that limit the size of the ships are the depth of the water and the possible bridges between the port and the open sea. Container cranes have followed the size of the ships by increasing their reach, lifting height and capacity lifting. It is obvious that the race to lower costs has prompted shipping companies to acquire larger and more efficient vessels.
As ships get wider, they are more stable. This added stability allows for higher container stacks above and below the ship’s deck. The economy of scale is an important factor leading to the continued growth of container ships. As vessel size increases, the shipping cost per container decreases. As older ships are retired, they are replaced by newer and larger ships. Taller cranes with longer reach are needed to service these larger vessels.
Recent events have also placed additional demands on the supply chain, causing port owners and operators to seek faster ways to increase container throughput. Using existing infrastructure is one way to achieve this. By installing new cranes on an existing quay, owners and operators avoid the expense and delays of building a new facility and benefit from maintaining larger vessels. Anyway, when bigger ships want to call at the port, the port needs the infrastructure to service the containers on the bigger ships. These larger cranes weigh more and place larger loads on the dock. Thus, the docks must have the greatest resistance necessary or be leveled to support greater loads.
A cantilevered nature
Designing cranes with longer spans and greater lifting heights while minimizing wheel loads is not easy to achieve given the cantilever nature of berth container cranes. A cantilever is structurally inefficient due to the need to balance its own weight. Cantilever structures seem to be the most efficient ship-to-shore container crane design. By the way, research is being done on other arrangements, so it’s possible that a better solution exists, but maybe that’s an article for another day. For cantilever cranes, waterside wheel loads are significantly increased as spans and lifting heights increase to keep up with vessel size. Simply put, a wheel load is the amount of load transferred from a crane wheel onto the dock support structure. This includes the crane’s own weight, the effects of the load being lifted, wind loads, etc.
One physical dimension that has remained constant is the 40ft hatch opening on almost all container ships. Terminal productivity requires industry standard practice to work adjacent cranes on every other hatch. This means that the overall width of the crane measured bumper to bumper along the quay should not exceed the center to center distance between alternate hatches. This limits crane widths to approx. 88.5 ft (27 m) regardless of crane reach and height. The 40 footer. hatch opening is the industry standard. Trains, trucks, yard cranes and of course shipping lines are all designed around this standard. It takes a lot of investment from multiple parties to adopt a new standard.
Dock designers should be concerned about this 88.5 foot crane width limit, as this dimension limits the effective length of dock that can be used to support each crane. This limitation, coupled with the inefficiency of the overhang, longer spans, increased load capacity, increased lift height and higher storm wind loads, resulted in an increase in loading docks from around 70 t/m at the turn of the century to 130 t/m – and growing. This is an increase of 185% over the past two decades.
Understand the strength of the dock
In an ideal world, a dock owner would have sufficient documentation, including as-built drawings, calculations, soil reports, and material certifications, to know how solid or structurally sound it is. . And these findings could reveal that the dock is not strong enough for larger cranes.
The usual practice is to use a larger rail and reinforce the rail support structure. Typically, new piles are installed for rails above water. For onshore rails, increasing the gantry beam or adding new pilings may be an option. However, this is not the only thing to consider. Often larger cranes can be safely installed on an existing dock by following a sound engineering approach that takes full advantage of the strength of the as-built dock. Sometimes dock designers make conservative decisions during the design and construction process, which can give the dock some reserve strength. In addition, concrete can become stronger over time, which also leads to a reserve of strength. The published dock strength, supported by good documentation, can be considered a reliable minimum; it is almost always possible to find additional capacity from such an existing design.
The original design safety factors may also be unnecessarily conservative. When selecting a safety factor, the design engineer should consider the likely accuracy of the applied loads. It is rational to assign a lower safety factor when the applied load is known with a high degree of certainty. Conversely, higher safety factors are justified when the loads are less well defined. Many civil engineers use high safety factors for crane loads because they design the dock based on crane load estimates. However, the load of the crane can be determined with a very high degree of certainty if the dead weight of the crane is physically measured. Often a lower dock safety factor than originally assumed can be rationally justified.
The crane design engineer also has an important role. By skillfully taking care to minimize weight during the design phase, they can minimize wheel loads. It takes engineering expertise to achieve minimum structural weight, but that extra effort can often pay for itself with reduced steel costs and lower demands on the dock.
Another way to reduce wheel loads in high seismic areas is to specify a base isolation system such as the Crane Base Seismic Isolation System (BASIS) developed by CPA. BASIS’ damper system, based on Nonlinear Time Analysis (NLTHA), slips when the applied lateral load exceeds the design lateral load, isolating the crane from the ground motion of an earthquake.
The damping system reduces wheel load in two ways. By limiting the amount of seismic energy transferred to the crane, it reduces the strength demand, which means the crane structure can be lighter. A lighter crane structure means reduced wheel loads. A damper system limits the lateral loads transferred to the crane during an earthquake. When lateral loads are applied to the structure of the crane, the wheel loads on one side increase and on the other side decrease. Limiting lateral loads during an earthquake limits this effect, which means a reduction in the wheel load on the quay. Another attractive factor of the BASIS system is being able to quickly get the cranes back into service after a major earthquake.
Establish safe wheel loads for new cranes
The first step in establishing safe wheel loads for an existing dock is to specify a realistic allowable wheel load based on the loads, reach and other physical requirements needed for the new crane. Too often, the owner of an understrength dock tries to transfer the problem to the crane manufacturer without understanding the physical limitations that can make compliance impossible. This often results in crane manufacturers refusing to bid or a delivered crane failing to meet wheel load requirements. A crane consultant can estimate realistic wheel loads for different crane sizes, wind speeds, and other environmental loading conditions to help the port specify the largest crane possible.
If a dock is found to have marginal strength, sometimes the most economical solution is to reinforce the dock rather than optimize the weight of the crane. If the decision is made to reinforce the dock, keep in mind mind the potential for a future crane climb or boom extension to increase the useful life of the cranes. Furthermore, it is very likely that future crane orders will be for even larger cranes and will require higher quay wheel load capacity.
If an owner needs to limit wheel loads, the manufacturer should be required to prove compliance by measuring the actual wheel loads after the crane is completed. The owner shall retain the right to cancel the order if the manufacturer has not fulfilled its contractual obligation. A good crane consultant can be an asset to an owner in avoiding contentious issues. Additionally, a good consultant will work with an owner to ensure that new cranes are the right size and will not prematurely become obsolete due to increased vessel sizes.
While companies like ours support terminal operators and crane owners who generally work with shipping companies, we generally do not have direct interaction with shipbuilders unless the shipping company and operator of the terminal are the same company. It is therefore important and appropriate that we continue to engage with operators and terminal owners who are likely to show a continued need to increase the size of their berth container cranes.
Phillips & Associates Inc. (CPA) offers a wide variety of services including procurement, specification, design, manufacturing review, modification, and accident investigation.