Doubling up proves of benefit beyond safety

01 Nov 2002

Two is sounder than one. That was the philosophy behind the introduction of mandatory requirements for double skins on oil tankers and the same theory underlies arguments for legislation demanding double skins on bulk carriers. Yet the two-for-one call on bulk carriers is not purely one of safety.
The only clear advantage of a double skin tanker is safety; the decreased pollution risk in the event of the ship being involved in a collision or grounding. With a bulker there is more to gain from having a double skin, argue class societies and other industry experts.
Yes, a double hull on a bulk carrier will, as on tankers, increase safety. A study submitted to the IMO?s Maritime Safety Committee by the United Kingdom in March this year found that the major cause of recorded bulk carrier losses is side shell damage. The inner hull of a double skin bulk carrier will prevent either the cargo or the cargo handling equipment coming into contact with and damaging the outer side shell, thus increasing safety.
However with bulk carriers there are potential revenue-earning operational advantages that can accrue to the ship?s owner from having a double skin too. Ongoing maintenance and repair costs, for example, will potentially be lower.
The absence of hull frames and brackets protruding into the cargo holds, replaced by the smooth side of the double skinned bulk carrier?s inner hull, also create the potential of more efficient cargo handling, easier hold cleaning and a quicker port turnaround. Less time in port means more revenue. On small handysize ships (13,000 dwt) conducting shorter voyages German owner Orion Schiffahrts has noted that an extra voyage per year is possible because of the double hull bulk carrier?s increased cargo handling efficiency.
Yet larger ships are generally engaged on longer voyages and by this very nature spend a lesser percentage of their working life in port. The advantages of quicker port turnarounds are diluted to the extent that only a partial extra voyage might be possible. However this hasn?t restricted bulk carrier operators from specifying double skins on just the smaller ships, nor has it prevented naval architects from designing large double skin bulkers.

A consortium led by UK-based Graig Group, for example, has just signed a letter of intent to build four double skinned handymax bulk carriers at Bohai and other nominated CSIC shipyards in northern China. The global handymax fleet currently comprises about 1,000 vessels and fewer than 1.5% of these are double sided making this order, if realised, a significant departure from the norm.
The first of the four ships, built to a design called Diamond 53, jointly developed by Graig and Denmark?s Carl Bro, is scheduled for delivery in 2004. The design features cargo gear, wide hatches, five cargo holds and wing ballast tanks (see page 22).
Chris Williams, a director at Graig, admits that the grain capacity of the design is not completely comparable with a single skin ship but counters those who use this as an argument against double skins by saying that handymax ships actually transport very little grain. He adds that the carrying capacity of the ship is actually higher because the hoppers are smaller.
It would also appear that there are seeds of interest from charterers for the ships. "We are in detailed discussions for a long-term charter for a multiple of these vessels," says Williams. The success of these discussions may impact the conversion of the letter of intent and four further options into firm contracts.

If double sided handymax bulk carriers are rare then double sided Panamax bulkers are like gold dust. Less than 1% of the 1,000 or so Panamax bulkers trading are double sided. Hong Kong?s Top Glory Shipping accounts for about half of these with its four 73,941 dwt ships delivered from Japan?s Oshima Shipbuilding in 1999.
These vessels ? Top Leader, Top Knight, Top Beauty and Top Vigour ? have an overall length of 225m, a depth of 18.9m and a design draught of 13.9m. Oshima says the ships? cargo hold volume is as big as Panamax ships of single skin construction. It has achieved this through a design with inclined hatch coamings and minimal double skin breadth.
The ships have six rather than the standard seven holds of a single-skin Panamax bulk carrier, thanks to the ability to carry ballast in wing tanks, and this combined with the smooth sides of the holds has, says a Top Glory manager, made them efficient for the charterer in terms of loading and unloading. They also achieve comparable speed (14 knots laden; 14.5 knots ballast) and consumption (25t/day) to single skin ships, he adds.
The ships have been taken on charter by various companies, such as US-based Bunge. How have they met such companies? expectations? "Charterers have asked us if they can have the ships again," says the manager; a good gauge that they are happy with them. He adds that the charter revenue achieved for the ships thus far has possibly been marginally higher than comparably sized single-skin ships.

The largest of the bulk carriers, the Capesizes, are better served with double sided vessels, although the percentage remains small. About 2.5% of the 550-600 ships in the world fleet are adorned with double sides.
Taiwan?s China Steel Express operates two double skinned Capesize bulk carriers, the 154,600 dwt China Steel Trader and China Steel Investor. These two ships, built in 1997 by the CSBC Shipyard in Taiwan, are principally employed by China Steel Express?s parent China Steel Corporation carrying two of the most aggressive bulk cargoes ? iron ore and coal ? from Australia to Taiwan.
ABS has calculated that the additional steel cost on such a ship is about $484,000 while reduced maintenance and cleaning costs over 20 years contribute to a saving of $9,700 per year. Assuming an interest rate of 6%, the net present value of this accrued operating cost saving is $111,250. The total additional cost of building a double-skinned Capesize bulker of this size, as opposed to a single skin ship, is therefore $372,750 by its reckoning.
Both the Top Glory and the China Steel Ships were built to and are operated to ABS class. ABS says that survey experience to date indicates that the process is routine.
Norwegian shipbroker OJ Libaek has recently unveiled an even larger double skin design. Its Optimum 2000 design bulk carrier, like the China Steel Express ships, ranks as a Capesize. Yet it is intended for open charter and its larger deadweight ? 171,000 dwt ? reflects the dimensions needed to carry dry bulk cargoes worldwide.
The principal difference between the Optimum 2000 design and other double hull bulk carriers is that it has a centreline bulkhead. This, claims Libaek, enables large hatch openings, which combined with single piece piggy-back-type hatch covers creates an open hatch effect that allows faster cargo handling (see page 22).

Ultra Capesize
For sheer size though, the Optimum 2000 pales into insignificance in comparison to a double skin bulk carrier design Hong Kong-based naval architect Peter Cheng is developing for the newly opened Shanghai Waigaoqiao yard in China. His design is for a 230,000 dwt ?Super Green? Capesize. Aside from a double skin it has features such as an absence of fuel oil tanks in the double bottom and wing ballast tanks in the void spaces.
"It is a little more expensive to build but not that much more if you design the vessel at the beginning with a double skin," he explains. In this way weight is saved and use of space optimised, critical on such a large bulk carrier which is subject to very high shear forces, especially in its different loading conditions.
"Shear force can be taken up by the double skin," says Cheng. This allows a constant plate thickness of the outer shell and therefore relatively simple welding procedures in comparison to a single skin ship, which requires varying shell plate thicknesses to take up the shear forces. "Its less [the increase in price] than people think," says Cheng.

Mandatory overnight
Overall double skin bulk carriers currently account for about 2% of the fleet. Increased newbuilding price is the number one obstacle as far as many owners are concerned preventing this percentage being higher. "If the price is OK they [bulk carrier operators] would consider to build double hulls," says the manager from Top Glory.
Other owners, particularly in Greece and Japan, are anti double hull because they believe good seamanship contributes more to safety than any structural measures. They fear a repetition of the tanker market, should legislation for double skins be introduced, where all single-skin ships, regardless of quality, are bracketed together by charterers.
Substandard single skin bulk carriers and substandard practices are in the minority. But however many good ships there are, while there remain owners and charterers that are prepared to deal with single-skin bulk carriers whose integrity has been compromised ? by the rigours of cargo handling operations or carrying corrosive cargo ? as there were with single skin tankers, the likelihood is that the next major incident involving loss of life on a bulk carrier crewed or flagged by one of the more influential maritime nations or in well-watched waters will spur legislators into action and heighten the call for mandatory double skins.
"We do not believe that the evidence in support of the double hull configuration for bulk carriers is sufficient to warrant the mandating of such designs at this time," says an ABS spokesman. "However, we do strongly recommend that shipowners who are considering ordering new bulk carriers should evaluate the relative merits of the double hull configuration when selecting the design."
"All we need is one accident in European waters and it [a double skin] will become mandatory overnight," warns Williams.

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