Over the past years we have seen a renaissance of the ore carriers, these large cargo mass transporters. Although ore carriers are widely considered as sturdy workhorses, their safe construction and efficient operation provides challenges which should not be ignored.
The massive demand for raw material over the past years, especially iron ores, from China, Japan and Europe, has advocated the transportation of a single commodity in big parcels on long haul trades yielding scale of economy. Still, today’s world fleet of ore carriers is relatively small, comprising only 38 vessels of more than 200,000 dwt as by August 2008. According to LRFairplay database the order book shows 107 ore carriers with a capacity ranging from 200,000 to 400,000 dwt, the majority of which will be delivered in 2011 and 2012.
DNV is the leading classification society for ore carriers, with close to 40 per cent market share of the existing ore carrier fleet, and has gained unique experience and competence on challenges related to design and operation of these giants of the sea. Based on this experience, DNV has developed dedicated services and software tools to ensure sound design and construction as well as safe and efficient operation of modern ore carriers.
Main characteristics of ore carriers
Ore carriers are specially designed dedicated ships for the transportation of ores. The design of a pure ore carrier is characterised as a seagoing single deck ship with two longitudinal bulkheads bordering socalled ‘volume low’ centre cargo holds and a double bottom throughout the cargo area. The side tanks are used as ballast water tanks or void spaces, the double bottom is usually used as void space.
Ore carriers are in general large cargo mass transporters with the exclusive objective to carry iron ores of high density, typically more than three tonnes per cubic metre, on long haul trades. Since they do not carry any cargo on the back haul, ore carriers spend close to 50 per cent of their lifetime in ballast.
Unlike the general bulk carriers, which adopt an alternate loading condition when carrying iron ores, ore carriers are designed for homogeneous cargo distribution.
Loading flexibility and de-ballasting capacity
As a general rule, when contracting a new vessel, the envisaged operating patterns should be fixed as early in the initial design stage as possible in order to incorporate possible multi-port conditions in the specification.
To allow for a minimum set of multiport condition the optional DNV class notation BC-B can be assigned. This notation accounts for a maximum draught of 83 per cent of assigned summer load water line with any cargo hold being empty while at sea. Further on, it permits full laden cargo holds at a minimum draught of 67 per cent of assigned summer load waterline. Although the notation BC-B permits for a 15 per cent increase of mass at same draught when in harbour, loading sequences are to be carefully reflected.
The appropriate size of main ballast lines and capacity of ballast pumps is also closely related to the ship’s overall loading and operating flexibility. Installed ballast pump capacity provided for a new ore carrier should be scaled according to the loading rate of the ore loading terminals intended to call. At several of these terminals, the peak loading rate is more than 16,000 tonnes per hour; it is expected that other terminals under construction or being upgraded will use this figure as a benchmark.
An installed pumping capacity of three times 3,000 m³ per hour at 0.3 MPa for a 360,000 dwt ore carrier may be considered as a minimum standard. Beside the main ballast pumps a set of sufficient ejector pumps is to be provided for stripping of the last ballast water in general, and for the cargo hold bilges in particular. Also with respect to the exchange of ballast water using the sequential method an appropriate ballast system is essential to allow serving the ballast tanks individually. More efficient loading time of ore carriers can be obtained if any hold can be loaded in as few steps as possible, as shifting of shiploaders and reading marks is time consuming. To accommodate one- or two-pour loading in a safe manner, sufficient hull girder and cargo hold structural strength is a prerequisite, and need to be analysed by modern software tools applying the Finite Element Method (FEM).
In addition, a corresponding ballast water system must be installed. This can be obtained for new vessels if taken into consideration at the early design stage. Furthermore, an automatic draught reading system should be installed and linked to the loading computer in order to better control the critical design parameters. DNV was the first class society to introduce such an enhanced level of loading flexibility when we launched the voluntary notation EL – Easy Loading last year. In a revision of the EL notation this year, the scope of the notation is extended further, and it has been divided into two notations; EL-1 and EL-2. Designing an ore carrier according to the EL class notations results in:
- Safer loading operations
- More efficient loading operations
- A reduction in the loading time
Maintenance and vetting
Although structural integrity is verified based on the newbuilding condition, a constant maintenance and inspection plan needs to be implemented by the operator to keep ore carriers’ structural safety and operational capabilities. Today we recognise an increased focus on hull structural integrity by the shipping industry, both for safety reasons and to obtain cost efficiency. DNV’s Hull Integrity Management (HIM) is a practical concept developed for that purpose. It supports owners and operators to keep their ships’ hull fit for purpose through continuous control of the hull condition.
HIM comprises four service elements:
- Knowledge – Hull Competence courses
- Systematics – Ship-specific Hull Inspection Manuals
- Software tools – Nauticus Hull Integrity
- Support – Hull Advisory Services.