14 April 2015
Jeroen De Haas and Simon Burnay from the BMT Group discuss the issues and challenges associated with mega container ships and the actions that can be taken to mitigate the risks associated with designing, building and operating this new class of vessel.
At the time of writing the largest container ships in service are around 157,000 DWT with a cargo capacity of approximately 14,770 twenty-foot equivalent units (TEU). While going larger has potential benefits, it certainly presents challenges operationally and from the infrastructure and naval architecture perspectives.
The exceptional size of the hull and its inherent flexibility could ultimately prove to be limiting factors for the mega container ship. Issues such as 'springing' or 'whipping' are still to be fully understood and more research and full-scale measurements are required to ensure adequate structural capacity over the ship's lifetime. Relocating the accommodation structure from the stern, to amidships can help reduce the longitudinal bending moments but the naval architect must still resolve related issues such as shaft alignment and manage the deflections that will occur during operation - the larger the ship, the longer the shaft and the greater challenge to ensuring satisfactory shaft alignment. The ultimate hull girder strength could still be limited by the thickness of the steel used as practically, it is very difficult to manufacture mild steel plate much thicker than around 100mm. Areas such as hatch coaming tables, can be very sensitive to excessive forces, especially in bending and hence the natural tendency would be to look at utilising high tensile steel, but the significant increase in material costs could have a negative impact on the profitability of operating the vessel. The design of larger vessels must also take account of hydrodynamic effects such as parametric rolling which is affected by the relationship between ship length and wave length.
The main appeal of larger container ships is the economy of scale. Certainly a larger ship carrying twice as much cargo as two smaller ones will be more efficient in terms of fuel consumption per TEU transported. But the propulsion plant required to transport a 22,000TEU vessel presents significant challenges to the designer. Current container ships are almost exclusively single screw with slow speed diesel propulsion but as ship size increases (especially if operating at higher speeds of circa 25 knots) then propeller loadings are higher and we start to see diminishing returns in terms of propulsive efficiency as well as greater challenges with cavitation and erosion.
Assuming that the traditional service speeds of large containerships will be required again in the future, the only viable option is for a twin-screw design. This will inevitably lead to a higher build cost and specific design requirements including a beam of around 50m due to draft limitations. A twin-screw design would have a greater wetted surface area, higher frictional resistance and a less efficient wake field but can have improved propeller efficiency due to lighter propeller loadings (due to the reduced power per shaft to be transferred to the propeller) which may overcome deficiencies in hull form and is therefore a more efficient design on a relative basis.
Another more innovative option would be the contra-rotating concept with an azimuthing pod mounted behind a single shaft propeller. It is a relatively untried idea with greater mechanical complexity and the potential for increased cavitation due to the 'pod' operating in the highly unsteady flow behind the main propeller and the constant turning of the pod for steerage.
However if the current move towards slower steaming is maintained as many industry analysts predict, it is possible that a single screw design could now be achievable for ships of this size. It is unlikely that engines much larger than those currently available would be used despite the increase in hull size due to the practical difficulties in transferring that power to the propeller which is limited to circa 9m diameter by the ships draft and cavitation. This could therefore give a viable solution of using tried and tested propulsion arrangements that could be optimised for the slower service speed of the ultra large containership providing a more efficient vessel and mitigating the risks associated with running these engines at slow speeds that they were not originally designed for.
Factors that will affect a mega containership are not only limited to the vessel's hull and machinery. In order to accomplish the desired cargo capacity in excess of 15000TEU, current proposals are to stack containers up to ten high rather than the maximum of nine that is currently used. The updated design criteria have ensured that all new containers fabricated post-2005 are suitable for stacking ten high. However there are still millions of containers in service that were built pre-2005 which do not meet these criteria and are therefore unusable within the proposed scheme. How these unsuitable units would be segregated and barred from transit within ten-high stacks is a minor but important issue that would need to be resolved.
Even if all the containers comply with the 2005 standard, stacking ten high will exacerbate existing stacking and lashing issues. BMT De Beer's work with container casualties has indicated that some of the current methodology might underestimate the forces acting on a container stack. Current design criteria are based on a 25 year return period using data from 1980. However hind-cast research carried out by BMT ARGOSS using met-ocean data as part of casualty investigations has indicated that actual wave heights are significantly higher and occur far more frequently than the design data suggests. If this is correct, then a fundamental rethink on how a ten-high container stack might be lashed and secured will be required. However it is worth noting that since the first launch in 2006, none of the new generation of 14,000+TEU container ships has been involved in a weather related incident. While this can be attributed to a wide range of mitigation measures, it is important to highlight the investment in weather-routing technology made by many of the major container lines. With in-house weather centres now providing accurate up to date information to ships at sea it is far easier for Ship's Masters to adjust their routing to avoid heavy weather. Rather than purely routing on the basis of the shortest course with the least bunker costs, safety is the dominant factor.
Larger ships with larger volumes of cargo don't necessarily equate to a higher level of risk. However the consequences of a loss are far greater. There is a real concern within the salvage community that in the event of a large container ship running aground, suitable marine cranage just isn't available to offload the cargo in a limited timeframe. Without the means to lighten these big ships and reduce the draft, refloating a vessel before widespread damage is inflicted might not be possible. This is a major risk for the operators and insurers and something where there is not currently a viable solution. Another major concern is with the cargo itself. BMT De Beer has been involved in the aftermath of two recent casualties where a dangerous cargo was not properly prepared and the containers caught fire. Fire in a container stack is notoriously difficult to control and the scale of the implications increase with the size of ship and volume of cargo. The major shipping lines have started to implement schemes to determine exactly what is inside each container but it is still far too easy to declare that there are benign cargos in the container, when it might be a highly flammable chemical such as calcium hypochlorite.
One potential downside of the mega container ship is that existing infrastructure will not always be able to support the new vessels. Because of their scale, only 20-25 ports around the world can currently accommodate these very big container ships because of the manoeuvring space required, draft limitations, locks, tidal restrictions and the need for shore-side facilities such as gantry cranes with sufficient reach to load/un-load containers from a single side of the ship. It is likely that any port wishing to accept such vessels would have to invest in new infrastructure that might include capital and maintenance dredging as well as purpose built dockside facilities including specialist long-reach container cranes.
The whole concept of the mega container ships arose in an era when economic buoyancy was never in question. The current economic situation however, does not lend itself to long term investment in either mega container ships or port infrastructure. Furthermore, challenging trading conditions have lead to companies becoming strongly risk-averse. There is a school of thought that suggests that any investment in new ships should focus on the 6,000 to 7,000 TEU size of vessel, which allows greater flexibility to the operator as these vessels can be sent to a far wider spectrum of ports and can be easily re-routed when the need requires. Certainly a larger ship will be more efficient in terms of fuel consumption per TEU transported but it is more likely that other factors will probably dominate the decision. The argument at the end of the day is an economic one, so these leviathans might be stillborn, not because of an insurmountable engineering or technical problem, but by the downturn in the global economy. The desire to go bigger and push the design envelope reflects very well on the naval architecture community; however, we will have to wait and see whether economic conditions will improve sufficiently to shepherd in a new age of mega container ship.
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