Safer ships through changing dues

Ernst Vossnack has further developed his design for a chemical tanker (The Motor Ship, April 2001), which may prove cheaper to build than conventional chemical tankers and, he says, will offer operational advantages, a longer life and, last but not least, will enhance safety. However, its increased gross tonnage is likely to discourage demand for such a ship until such a time that ports change the way they claculate their dues. Vossnack`s design is based on cylindrical tanks which are placed vertically in the hold and protrude through the main deck. Although space is lost in the hold, this is compensated for by the part of the cylinders sticking through the deck. Thus, the vessel can have the same cargo capacity as a standard tanker. The penalty is a higher gross tonnage because the part of the tanks above the main deck will be included in volume calculations. The Vossnack Group is convinced that such a tanker is cheaper to build than a conventional chemical tanker with stainless steel tanks, because: n the cylindrical tanks can be constructed by a specialised manufacturer for installation during the outfitting stage, thus shortening the construction period n with the cargo tanks being subcontracted, many more shipyards will be able to build such vessels, thus increasing competition between shipyards n calculations indicate that the cylindrical tanks need only about 75% of the stainless steel that goes into a chemical tanker of the same capacity with corrugated bulkheads. The cylinders also offer operational advantages, claims Vossnack. It will be possible to have a greater variety of tank sizes. Moreover, tanks will be easy to clean and inspect. Heating coils can be avoided by applying outside heating. If desired, the owner can specify insulated or pressurised tanks. Ballast tanks may need less maintenance because there will be no coating damage caused by adjacent heated cargoes. With the cargo completely separated from the hull, a longer life can be anticipated, continues Vossnack. The stainless steel tanks will easily last 30 years, he says, and even then may still have a high resale value, e.g. for use on terminals. On the issue of safety, he says it will be enhanced considerably. The cylinders will form a third barrier against penetration after the double hull. Thus in case of a collision, a cylinder is more likely to be dented than teared. There is also no risk of dangerous cargoes entering the ballast tanks on account of small leaks in the bulkhead. Salvage experts could investigate whether it will be possible to retract the cylinders with the cargo intact from a vessel that has sunk in an upright position. High manifold Apart from the greater gross tonnage, the high position of the manifold above the tanks may cause problems at some terminals concedes Vossnack. Consideration, however, could be given to placing connections with the shore-based pipelines on the main deck above the double hull, he says. This would place the connections nearer to the vessel`s railing and thus make them more susceptible to green water damage. But if gross tonnage is no longer a problem, this can be solved by having an enclosed area above the ballast tanks to protect the connections. Large tankers The cylindrical tanker design can, says Vossnack, also be used for product tankers with tanks of mild steel. The construction and operational advantages will be somewhat less than on a stainless steel chemical tanker. There is also a limit to the size of the cylindrical tanker, because the sides and bottom of the tanks have to be strengthened as height increases. Preliminary calculations indicate that, on a large tank with a diameter of 18.5m, the thickness of the sides has to increase from 10mm for the first 10m to 28mm at the bottom of a 28m high tank. Currently, 22mm thickness is considered the practical limit for succesful welding. Vossnack says this problem can be circumvented partly by decreasing the diameter, but this will involve the installation of more tanks on the vessel, thus increasing building costs. These constraints should, however, not apply to a tanker up to about 35,000 dwt. New technology or new materials may in due course raise the limits of the design. Gross tonnage The main problem for introducing new designs, both for tankers and container vessels, remains the practice of ports and canals to calculate port dues on the basis of a vessel`s gross tonnage. Pieter Struijs of the Municipal Port Management of Rotterdam, maintains that safety at sea is not a port`s concern. If the gross tonnage rules promote the building of less safe vessels, IMO and the flag states should in his opinion tackle the problem by revising the rules. In contrast, some ports in France and Poland have already replaced gross tonnage-based fees with more transparent parameters such as length, breadth and draught. A vicious circle is completed by the IMO, which states it does not control the ports and is thus unable to disallow their use of gross tonnage. Some flag states study the problem, but so far they have refrained from publicly admitting that gross tonnage-based port fees present a problem. Germanische Lloyd and the Dutch shipowners` association KVNR have, however, publicly endorsed the ideas of the Vossnack Group on gross tonnage. Realising the difficulty of prohibiting ports` use of gross tonnage, the group even goes a step further by advocating that the gross tonnage measurement is abolished altogether. Dongedijk capsize Vossnack says the case against gross tonnage has been much strengthened by the group`s investigations covering the capsize of the container feeder Dongedijk off Port Said in August 2000. In September, the Dutch Shipping Council investigated the accident and asked Ernst Vossnack to appear as an expert witness. The government`s shipping inspector asked the Council to suspend the master and the first mate of the Dongedijk for a period of three weeks because they had left Port Said with insufficient stability (clearance of about two feet). However, the Council decided further study was required into the causes of the capsize. The design of the 2900g Dongedijk is common to several shipyards, with containers stowed two high in the hold and up to four high on deck. This minimises gross tonnage, while still giving compliance with the 1966 Load Lines rules. Vossnack demonstrated the impact of minimum gross tonnage by presenting two designs of a 381 TEU vessel. The first, with containers four high in the hold and two high on deck, has a tonnage of 5,230g. The second, with a three/three pattern, will measure less than 4,000g. Thus, apart from the saving on port dues, this vessel will be allowed to sail with three men less. Loaded more cargo At the Council hearings it was established that Dongedijk loaded more cargo than indicated by the port agent. A witness explained that the weight of the containers had not been included by the agent without informing the vessel thereof. On departure, the draught of the vessel did exceed the one calculated, but this was attributed to a software error which the master had experienced before and which he had already reported to the software makers. The vessel capsized after departure from Port Said when it left the deep, dredged channel, thus entering more shallow water, while making a 40 deg turn at the same time. Vossnack explained to the Council that the design may have been an important, contributing factor for the accident. With a list of 8.5 deg, water could already enter the gangboard and accumulate aft against the accommodation, he stated. With a steep stern wave and with the vessel having a trim of 1.6m by the stern, this will occur near the aft ship with a list of nearer 6 deg, he added. The mass of water against the deckhouse could freely move from port to starboard below the containers, because the hatch coamings do not continue up to the deckhouse, he said. Moreover, although most ballast tanks were said to be empty, some water will usually remain in the ballast tanks, aggravating the problem, he continued. Privately, Ernst Vossnack believes that an urgent review of the 1966 Load Lines Convention is required. At the time, vessels were of a completely different design with little thought given to high deckloads. Cargo ships could cope in the 1960s with lists of 30 to 40 deg. Nowadays, they are optimised with the lowest possible freeboard and the highest possible deckload whereby the free movement of say 80t of remaining ballast water and/or the accumulation of seawater on deck can already have serious consequences. It is well known that double bottom ballast tanks are not easily drained completely. Vossnack urges the inclusion of the `Rahola` criterium in the Load Line rules. Rahola was a Finnish professor who developed his ideas on stability in the 1930s, but they were disregarded by those who drafted the 1966 convention because they had little relevance to cargo ships at the time. When applying the Rahola criterium of minimum 8cm rad. on the Dongedijk, Vossnack says the ship would not comply, although it does comply with current rules.