Easy loading for world’s largest ore carriers
This year’s unfolding of the record-breaking programme of ore carrier construction instigated by Brazilian mining group Vale pushes back the bounds as regards bulk cargo transport economics and scale efficiencies, writes David Tinsley.
The progression encapsulated in the 400,000dwt generation is a factor of the huge shipment capacity and application of the latest two-stroke propulsion engine technology, which in combination change the established order as regards fuel consumption per tonne of cargo exported to distant markets such as China. It is also a measure of the heightened productivity that the new VLOCs(very large ore carriers) introduce to ore export logistics, arising not only from pure scale but also from the technical propensity for optimised cargo handling procedures.
In this regard, the design concept reflects vital input from the classification sector, having been developed in accordance with DNV’s latest Easy Loading notation EL-2.
Vale Brasil assumed the mantle as the world‘s largest bulk carrier. She is the first of seven 400,000dwt behemoths contracted from Daewoo Shipbuilding & Marine Engineering, and made her debut earlier this year. The service entry of sistership Vale Rio de Janeiro is imminent. Vale China, the first of the 12 ships of identical size awarded to Jiangsu Rongsheng Heavy Industries, is set for handover before the end of 2011. All are being constructed to DNV class requirements.
As embodied in Vale Brasil, the cargo carrying configuration is based on seven holds, separated by corrugated, transverse bulkheads, each with an upper and lower stool. Access is through hatchways served by electro-hydraulically driven, side-rolling hatch covers. The wing tanks are used as water ballast tanks, fuel oil tanks, or void spaces.
From the outset, and for the entire 19-vessel programme, Vale laid down the highest standards for safety and efficiency. Adoption of DNV’s CSA-2 notation, relating to fatigue assessment, as well as the EL-2 criteria, underpinned the owner’s technical expectations.
Ore loading terminals in the major exporting countries have been pressing for vessels to undertake cargo loading at nominal rates up to 16,000t per hour and with as few loading steps as possible, in an attempt to raise terminal efficiency and counteract port congestion. Waiting times at Brazilian iron ore ports were an average of 6-9 days when Vale ordered its VLOCs, and have since risen, standing at around 10-11 days on average in July 2011.
Very high loading rates for such a dense cargo as ore, and delivered from a position high above the hold, had given rise to safety concerns within the shipping industry. The International Association for Classification Societies(IACS) investigated the issue and concluded that the problem could not be directly attributable to loading rates, and instead highlighted increases in longitudinal stresses due to different loading and de-ballasting rates. It therefore recommended monitoring and synchronising loading and de-ballasting operations to obviate local and longitudinal stresses which, over time, could harm the structural integrity of the hull.
Ensuing IMO and class initiatives led to IMO’s ‘Code of Practice for the Safe Loading and Unloading of Bulk Cargoes‘, the so-called BLU Code, and extra SOLAS requirements.
Loading flexibility was increased in 2003 through the IACS’ unified requirement URS25, relating to harmonised class notations and loading conditions. DNV was the first class society to introduce an enhanced level of loading flexibility, compared to URS25, through the 2007 launch of the EL voluntary notation. This provided for more automated control of the critical vessel design parameters and allows for easier and more flexible loading sequences.
The subsequent revision of the EL notation extended its scope and divided it into two categories. EL-1 specifies that a vessel be designed in such a way that a minimum 50% of maximum permissible cargo intake can be safely loaded in one pour. EL-2 mandates a design for loading 100% of the maximum allowed intake of each cargo hold in a single pass, without compromising the vessel’s strength and fatigue life. In this way, loading times for a VLOC could potentially be cut from 26 to 12 hours.
The acceptable loading rate, and thus loading time, is dependent on vessel strength and de-ballasting arrangements. If the de-ballasting rate is not properly taken into account, this can lead to hull girder stresses approaching their allowable limits in port. The shear forces in way of transverse bulkheads are a factor of the differences in upward and downward forces acting on adjacent holds, or between the aftermost hold and the engine room area, and control and monitoring of these is pivotal to safe and flexible loading.
In the new 400,000-tonners for Vale, ballast pumps and piping dimensions are appropriately sized to handle the workload to allow synchronised de-ballasting and loading or unloading operations. The design incorporates additional longitudinal and local strength, and the ships are fitted with a remote tank sounding and draught reading system with an on-line interface into the onboard loading computer.
Another feature of the so-called Valemax generation is the use of separate ejector pumps and ballast stripping system, to ensure that vessels may perform simultaneous de-ballasting by pumps or by gravity stripping of near-empty tanks.
It is essential that all key parameters bearing on the application of the EL-2 notation, not least envisaged operating and loading patterns, are established and taken into account at the initial design stage, as has been the case in the Vale VLOC construction projects at the Daewoo and Jiangsu Rongsheng yards.
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