Bulkcarriers Paper presented at DNV's bulkcarrier forum in London on 7 May 2003

The International Maritime Organisation [IMO] and The International Association of Classification Societies [IACS] have recently adopted, or are in the process of adopting, a number of new requirements to the design and equipment of new bulkcarriers [see separate text box for a summary of the new requirements].
While the objective has been to increase safety, future bulkcarriers will also be more robust and operational friendly than bulkcarriers have been in the past.

Among the new requirements it is in particular IACS' unified requirement S25 "Harmonised Notations and Corresponding Design Loading Conditions for Bulk Carriers" as well as IMO's planned amendments to SOLAS Chapter XII regarding double side skin and accidental flooding of cargo holds that will add to the robustness and operational flexibility of bulkcarriers.

The increased robustness and operational flexibility may, however, come at a cost in the form of increased hull steel weight, reduced deadweight and hold cubic capacity as discussed in this article.
Harmonised Notations and Corresponding Design Loading Conditions for Bulk Carriers [IACS UR S25]
For all bulkcarriers above 150 metres in length contracted for construction on or after 1st July 2003, the IACS classification societies will assign one of the harmonised class notations BC-A, BC-B or BC-C, based on a unified set of design loading conditions. The objective for IACS has been to:

• Ensure that bulkcarriers are designed for those cargo and ballast loading conditions which a standard bulkcarrier is likely to encounter during its life time,
  
• Make it transparent within the entire bulkcarrier industry what a bulkcarrier has been designed to do whilst in operation,
  
• Reduce the risk for damages/casualties as a result of wrong loading and operations.

The longitudinal loading conditions specified in UR S25 for use in determining the requirements to hull girder strength have been widely published and are generally well known. What is less well known and understood is that UR S25 also contains a set of local strength loading condition defining the maximum allowable and/or the minimum required cargo mass allowed in individual cargo holds as a function of draught. These conditions are important in determining the required strength of a.o. double bottom, side shell, transverse bulkheads and cross deck structures, and will ensure that standard bulkcarriers have the ability and flexibility to:

• allow effective loading and unloading sequenses [reduced number of pours]
• load multiple grades of cargo in varying proportions
• load and unload in multiple ports
• carry grain cargoes with slack holds
• have a safety margin for accidental overloading
• allow final trimming upon completion of loading sequences

Shipbuilders and designers, who are in the process of incorporating IACS UR S25 into their bulkcarrier designs, have indicated that the increase in hull steel weight will be in the range of 5 percent. Typical figures indicated by a number of shipbuilders in several countries falls within the following ranges:

Handymax: ~ 300-350 tonnes
Panamax: ~ 450-500 tonnes
Capesize: ~ 800-900 tonnes
Assumed to be based on current minimum designs, which would not normally be in full compliance with DNV rules

DNV has, by independent calculations, found that the increase in steel weight are within the following ranges:

Handymax: ~ 45-55 tonnes
Panamax: ~ 130-150 tonnes
Capesize: ~ 320-420 tonnes
Based on actual bulkcarriers built in full compliance with DNV rule requirements
Possible reasons for the difference between the indications from shipbuilders and the DNV study might be:

• The DNV study is based on bulkcarriers in full compliance with existing DNV rules, which already incorporate important parts of IACS UR S25 requirements. It is therefore to be expected that the additional steel weight for DNV class would be less than for some other class societies.
  
• The preliminary yard estimates of steel weight might be based upon the most optimised and competitive standard designs, which several shipowners' associations have expressed were not "fit for purpose" as general bulkcarriers.
  
• The preliminary yard estimates of steel weight might include a margin to cover uncertainties regarding remaining detailed technical requirements and remaining design development work.

SOLAS Chapter XII Amendment
The Maritime Safety Committee of IMO agreed at its meeting in December 2002, that double side skin should be mandatory for new bulkcarriers in the future, and that all bulkcarriers shall be designed so that they are able to withstand accidental flooding of individual cargo holds. The latter requirement presently applies to single side bulkcarriers only. Various IMO subcommittees and IACS working parties are presently discussing all the related detailed requirements to be included in an amendment to SOLAS Chapter XII, which is scheduled for approval by IMO in May/June 2004, in which case it will apply to new bulkcarriers keel-laid on or after the middle of 2006.

Double side skin
Formal Safety Assessment studies performed as a result of the large number of bulkcarrier casualties/fatalities over the past 20 years have concluded that double side skin is one of the most cost effective risk control options available to improve bulkcarrier safety, and is therefore being mandated by SOLAS Ch XII for new bulkcarriers in the future. Although not finally decided, it is reasonable to assume that the requirement regarding width of the double side skin will primarily relate to the free internal space for easy access, where the required free space will be 600/800 mm for transversely and longitudinally framed structures respectively.
The increased safety of a double ship side is partly related to the fact that there will be 2 barriers, i.e. the outer and inner side shell, protecting against water ingress. The main increase in safety is, however, a result of the double side shell structure being a cell structure where a combination of vertical web frames and horizontal stringer will support the outer and inner side shell.

Preliminary indications from shipbuilders and designers as regards the difference in hull steel weight between a single side skin and double side skin bulkcarrier varies greatly, and fall within the following ranges:

Handymax: ~ 0-250 tonnes
Panamax: ~ 0-300 tonnes
Capesize: ~ 0-500 tonnes

DNV has not performed an independent study of the steel weight difference between a single side skin and a double side skin bulkcarrier, but have noted the significant variation in the estimates obtained from the yards. If a comparison is made between a single side skin bulkcarrier which is presently subject to accidental hold flooding conditions [eg. IACS URs S17, S18 and S20] and a double side skin bulkcarrier which is presently exempted from such accidental hold flooding conditions, it is DNV's opinion that those yards who estimate zero increase in steel weight probably have the most realistic estimate.

It should, however, be noted that a double side skin structure contains significantly increased number of structural elements, welding length and coating area, all matters which will also increase the cost of double side skin bulkcarrier construction as compared to a bulkcarrier with single side skin.

From an operational point of view, double side skin, represents a number of advantages, some of which are:

• Reduced unloading time [up to 25% for some sticky types of cargoes]
• Reduced time and cost for cleaning
• Reduced risk for mechanical damage during cargo handling [more than 80% reduction according to information from major unloading terminals]
• No need for permanent means of access for close up inspection of ship side structures [can be inspected from inside the double side skin space]

Accidental ingress of water
When SOLAS Chapter XII was introduced for new bulkcarriers being keel-laid on 1st July 1999 and later, it became a requirement that bulkcarriers should be designed so that they would be able to withstand accidental flooding of any single cargo hold. Detailed requirements are given in IACS UR S17, S18 and S20. However, bulkcarriers with double side skin were at that time exempted from these requirements. IMO has now decided that the hold flooding requirement shall apply also to new double side skin bulkcarriers in the future, and be part of the amendment of SOLAS Chapter XII now under development.

IACS is presently in the process of defining the actual hold flooding conditions to be applied during the design of future double side skin bulkcarriers. It is likely that several such hold flooding conditions will have to be considered in order to take into account that hold flooding may occur as a result of water ingress through hatches/deck openings, ship side etc.

While the detailed requirements remain to be finalised, preliminary estimates by shipyards regarding the increase in hull steel weight as a result of hold flooding conditions being required also for double side skin bulkcarriers falls within the following ranges:

Handymax: ~ 70-100 tonnes
Panamax: ~ 100-150 tonnes
Capesize: ~ 170-260 tonnes

Summary
Shipowners' organisations have made strong calls for more robust bulkcarriers which are "fit for purpose". The combined effect of the new requirements prescribed in IACS UR S25 and in the SOLAS Ch XII amendment [double side skin/accidental hold flooding] constitutes a significant reply to this call.
They will ensure that future bulkcarriers are being designed to be "fit for purpose", i.e. that they are designed for those cargo and ballast loading conditions and operational requirements which a standard bulkcarrier is likely to encounter during its life time.

They will also increase the robustness of future bulkcarriers, by adding 5 to 10% to the hull steel weight as compared to present standard designs. Preliminary estimates of the combined increase in hull steel weight of these new requirements fall within the following ranges:

Handymax: ~ 370-700 tonnes
Panamax: ~ 550-950 tonnes
Capesize: ~ 970-1670 tonnes

The deadweight capacity of future bulkcarriers will be reduced in the range 0.5-1.5%. For some types of bulkcarriers, the cargo hold cubic capacity will also be reduced, depending on the need for ballast tank capacity in order to meet new requirements to ballast loading conditions in IACS UR S25.
The additional hull steel weight will increase the construction cost of future bulkcarriers. It is, however, important to realize that the added steel weight is not the only factor which will increase the cost.
Such increased cost will apply to all new bulkcarriers, irrespective of class society, shipyard or owner. A level playing field among competitors within the bulkcarrier industry is therefore being maintained.