Hans van Rooij, president of the International Salvage Union, expressed views on co-operative approach for enhanced casualty response during recent seminars held in Madrid and Seoul. He stressed a number of important issues related to new technical challenges. Larger, more complex vessels require new response tactics from salvors, in his opinion. Ultra large containerships, for example, raise some challenging problems. The world container fleet shows the fastest growth of any ship type and today, the fleet of 2,600 fully cellular containerships totals 53 million GT and accounts for nearly 10% of world merchant tonnage. Around 70% of general cargo are already containerised and this figure is forecast to grow to more than 90% by 2010.
Excessive wave loads are one concern. Containerships are often built with considerable bow-flare, to maximise the number of deck-stowed containers. There have been many cases where the bow structure failed to withstand heavy impact loads from waves, resulting in considerable damage. It is often difficult to see the problem developing, as no green water comes on board and the deck containers block the view of the bow. Heavy rolling is another important issue. It is widely known that containerships, having fine lines, can experience heavy rolling in following seas (depending on wave conditions and ship dimensions). This phenomenon – parametric rolling – may, in extreme cases, capsize the vessel if the crew fails to take appropriate action. The traditional remedy is to turn the ship into the seas. Theoretical as well as experimental investigations now indicate, however, that parametric rolling can develop when a very large containership is heading into the seas.
Another ship type-specific risk is associated with container securing, fires, and removing containers prior to re-floating. There is always the concern that incorrect advice from shippers to shipowners and operators may put vessels and people in jeopardy. Containerships have been involved in a number of very severe fires and several vessels and their cargoes have been seriously damaged. Due to the substantial increase in vessel dimensions (with length of 380 m and breadth of 57 m), it is now even more difficult to remove containers, to lighten a casualty prior to re-floating.
There are also important questions surrounding double hull tankers in a salvage situation. IMO’s accident statistics suggest that spills occur in 25% of the collisions involving double hull vessels. In some 16% of collision cases, the upper deck above the cargo tank may be breached and a fire may start. Oil may flow down in the empty double side and the fire will spread. There is also the explosion risk. The breakdown of the tank coating can initiate corrosion and pitting in the bottom of cargo tanks. This results from a combination of factors. For example, sulphuric acid and biological matter may concentrate in the bottom water layer of certain crude oils. High temperatures inside the double hull can heat up the cargo. Other factors include elastic deformation of the inner bottom plate (up to 10 mm between stiffeners and floors) and crystalline modification of the steel in way of welding butts and seams.
The explosion risk presented by a double hull ballast space is ever present, as the space is not inerted. The problems of ventilation have prompted many debates. If there is an explosion in the double bottom, it is likely that it will expand forward and aft (rather than sideways and up the narrow double sides) so blowing into the pump room or engine room. There are also the stability risks. Many double hull tankers have lost stability during simultaneous cargo discharge/ballasting, with the sudden list tearing off the cargo hoses. Typically, the cause is an inadequate allowance made for the free surface effects of slack tanks in the double bottom. Similar stability risks are involved when changing ballast at sea.
There are alternatives to double hull tankers. They include the Marpol I/13F(4) Mid-deck tanker or the Coulombi Egg tanker design, the latter being IMO and European Commission approved. Salvors have responded to the new challenges with creativity. One example is the development of a new system to facilitate the emergency offloading or pump-out of oils and chemicals from casualties. The JLMD, developed by an ISU member in association with partners, is a pre-installed system providing ready access to the highest and lowest point of each tank, whatever the position or the angle of the casualty. Various owners have specified this system for new tonnage.