The pure ore carrier is characterized as a seagoing single deck ship with two longitudinal bulkheads and a double bottom in between throughout the cargo region, intended for the carriage of ore cargoes in the centre holds only. The cargo hold cubic has commonly been based on a stowage factor of 19.5 to 21.5 cbf/ts at summer draught. These specially designed dedicated vessels spend much of their voyage time in ballast since they do not carry any cargo on the back haul.
The combined Ore/Oil carrier was introduced to reduce the ballast ratio.
This type of vessel is similar in construction to an ore carrier but also features side tanks for the carriage of oil. These ships do not carry ore and oil simultaneously. Although the Ore/Oil carrier (OO) is more expensive to build and normally has higher operating costs, it can carry cargo on both the front and back hauls by changing from dry to wet cargo and vice versa. Furthermore, this combination concept also offers the shipowner the advantage of trading for a certain time in one trade, utilizing seasonal high charter rates in the tanker market.
The combination carrier is particularly favoured by Scandinavian owners, who ordered a small combined carrier with a deadweight of 5 538 ts as far back as in 1957, whilst Japanese owners tended to prefer the pure ore carrier operating primarily in a shuttle service on the rather short-haul Australian leg.
In the mid-1970s, a number of up to 280 000 dwt Ore/Oil carriers were built, but seldom as a standard design. Few shipyards built more than two or three such vessels for owners on demand, although Uljanic yard managed to receive order for seven. Since then, only a few Ore/Oil carriers have been built, ranging from 286 000 to 310 000 dwt.
Today the majority of these are engaged in trading dry cargo only as they are no longer certified for the carriage of oil.
According to Fairplay’s database, there is now a total of 24 pure ore carriers of more than 200 000 dwt, including those former Oil/Ore carriers that are now exclusively trading dry cargoes.
Few ore carriers have been built in recent years, but those few have been amongst the largest carriers sailing, and include the 365 000 dwt Berge Stahl, which is still the largest ore carrier in the world . This vessel, which was commissioned at the end of 1986, was purpose built to transport iron ore from Punta de Madeira in Brazil to Rotterdam under a long-term contract with European steel mills. It utilises the maximum sailing draft of 23.0 m at both ports. Ore carriers normally built for a shuttle service between two ports only have few cargo holds since multi-port operation is not considered relevant. The Berge Stahl has five cargo holds, whilst the 136 000 dwt ex Amagisan Maru, now scrapped, which was built in 1976, had only three cargo holds, the largest of which was 86.7 m in length, and was served by three hatches. However, a four-hold configuration, like that adopted for the 199 000 dwt Onga Maru or the 233 000 dwt Onoe Maru, is more common.
For ore carriers intended for multi-port loading or unloading, such as:
1. loading at a Western Australian ore terminal to a maximum draught of 18.2 m,
2. topping up at Saldanha Bay, South Africa
3. partial unloading at Rotterdam
4. final discharge in Hamburg at a very restricted draught, more cargo holds are needed to accommodate the charterers’ requirement of “untouched” holds. To enhance operational flexibility for this mode of operation, a six- or seven-hold configuration is normally specified.
China’s huge and growing demand for iron ore and Japanese owners’ need to replace elderly smaller tonnage have resulted in recent orders for a rather significant number of ore carriers ranging from 200 000 to 323 000 dwt. Interesting to note is the development of a 363 000 dwt ore carrier design with a draught of 24.0 m and featuring eight cargo holds. One of the bottlenecks is the present limited availability of ports utilizing full cargo-carrying capacity for those large carriers that trade mainly from Australia and Brazil to Japan and China. In China, Dalian is the only port which can presently accommodate ore carriers up to 300 000 dwt.
Ore carriers can normally get assigned a reduced freeboard, B-60 or B-100, yielding substantial cargo-carrying capacity irrespective of the number of cargo holds, provided their longitudinal bulkheads are fitted at a distance at least B/5 from the side and are able to withstand one or two compartments flooding by utilising the side ballast tanks or voids respectively as stipulated in reg. 27 of the International Load Line Convention.
Like tankers whose side shell longitudinals are predominantly composed of high-tensile inverted angle bars, the ore carriers suffered from similar fatigue cracking prior to the introduction of special calculation procedures by the classification
societies in the early 1990s.
It was observed that for a particular trading area (mainly the North Atlantic) the service fatigue life of local structures exposed to dynamic loads was significantly reduced compared with ships trading worldwide. Wave-induced hull vibration owing to springing and in particular whipping during the west bound North Atlantic ballast voyage also contributed to premature fatigue-induced structural failures. Cracks in longitudinal hull members were observed before two years in service on a 1997-built ore carrier carrying primarily iron ore from St. Lawrence to Western Europe.
On a few large ore carriers, the transverse deck area between hatches did not withstand the compressive forces in heavy weather generated by passing huge waves in a following sea state with the vessel fully loaded. The cross deck strips and transverse hatch coamings partly collapsed,
suffering from severe buckling.
Amendments to the structural rule requirements will take care to prevent such events taking place, even under extreme weather conditions.
Date: 08 February 2008