COLUMN | Convert with care [Grey Power]

Featured Stellar Daisy in 2013 Image: MarineTraffic.com/Yeong Ryong Kim Stellar Daisy in 2013

It seemed a good idea at the time to get hold of some very large crude carriers when values of these ships were low, and convert them speedily into ore carriers, for this was what the market clearly required.

Tankers, after all, are ships of proven structural strength, with their longitudinal framing and the two useful longitudinal bulkheads that can be simply reinforced to form the new ore-carrying holds, with hatches cut in the deck above. Thus, in a far shorter time than it would take to negotiate, order and construct a newbuilding ultra-large bulk carrier, a ship is available to take advantage of the promising market.

That, at any rate was the theory behind the rash of tanker to bulk carrier conversions that were to take place around the middle of the first decade of this century. One of these ships was the Stellar Daisy, built initially in Japan in 1993 and bought by South Korean owners after her second Special Survey, to be converted in China for the carriage of heavy ore cargoes, in 2008. She was to be lost on March 31, 2017.

The fate of this huge ship is well known, sinking in minutes with 22 of her 24 man crew in the South Atlantic with a full cargo of Brazilian ore bound for the east. The two survivors told a confused tale of the ship assuming a steep port list in minutes, and a desperate attempt to escape from the doomed vessel before she submerged. Apart from the two shocked survivors, there was otherwise little trace of one of the world’s biggest bulk carriers. It brought back memories of that dreadful period in the 1980s and 1990s, when predominantly elderly large, fully laden bulk carriers were sinking, mostly with their crews, at an alarming rate.

It has taken more than two years for the Marshall Islands, the flag state of the ship, to publish its report on the investigation of this modern shipping mystery of the sea. It has been arguably hindered by the jurisdictional and legal complexities of dealing with a ship owned in one country, managed by another, built in one country and converted in another, classed in a different country and manned from elsewhere.

But this is shipping today. It is also worth noting that it was pressure from the relatives who lost their lives in this ship that “encouraged” an expedition earlier this year to find the ship and photograph the wreck site in the depths of the South Atlantic. Even the vessel’s VDR has been recovered, although what it has revealed is still being analysed.

The report attributes the loss of this huge ship to catastrophic structural failure, with the hull giving way in the region of No.2 ballast tank, and then rapid and progressive flooding following the breach, with resultant loss of buoyancy. It will be recalled that there were questions about this series of conversions following the discovery of structural cracking on sister ships. The report cites damage discovered on the Stellar Daisy during earlier drydockings.

Naval architects and marine professionals will probably be unsurprised at these findings, when they consider the construction and conversion of these and other tankers. It might be suggested that trying to convert a tanker into a bulk carrier (while not new) is like making an apple into an orange, as they are very different ships. Consider the operation of a single-hulled tanker. If laden, the water pressure outside the hull will be equally counteracted by the pressure of the oil in the tanks. With the ship in a ballast condition, she will be “flying light” and air on both sides of the shell plating for all above the waterline.

But with the vessel converted, hatches cut above the reinforced centre tanks, and fully laden with ore in these centre holds, the ship is deep in the water, but with nothing in the huge wing tanks to port and starboard, so there is no countering pressure from inside the ship against the pressure of the sea outside. There is also a complete change in the structural stresses acting on the hull – very different to those acting upon the ship in her earlier life as a tanker. There remain to many unknowns about these forces on ageing structures.

It is also worth thinking of the state of these vast caverns, which would have been uncoated from their oil-carrying days, but regularly filled with salt water when the ship is empty of ore on her ballast passages. What sort of state will these huge tanks have been in as the corrosion ate their steel away?

Outwardly, these ships presented an excellent appearance, but what was the real structural state below decks. There had, we know, been cracking identified but not regarded as something that would threaten the ship. But perhaps it is not difficult, in retrospect, to imagine the propagation of one of these cracks, with the ship underway, into a serious breach that would fill the gigantic wing tanks, throw the ship on her beam ends, and hasten a complete structural collapse.

If there is any major lesson to be learned from this tragedy, it must be that you should always convert with caution, bearing in mind the locked-in stresses that are established in a huge steel structure through usage of one regular kind, especially if a completely new operational life is contemplated. That, and the importance of regular and thorough inspection, especially of those parts of a ship that are less easy to get at.

Sign up to the FREE Work Boat World newsletter