Direct damage stability for tankers discussed

Sep 02 2014


The IMO has adopted guidelines and applicable IMO Code amendments for the mandatory carriage of damage stability verification instruments on board new and existing tankers.*

The entry into force date is 1st January, 2016 with existing ships having to comply by the first renewal survey after this date and no later than 1st January, 2021.

In a White paper, Herbert-ABS discussed the options open to tanker owners/managers and operators.

In April this year, IMO/MEPC 66 adopted the guidelines for demonstrating compliance with the requirements for damage stability.

Amendments to MARPOL Annex I, BCH Code, IBC Code and to the Survey Guidelines under HSSC to mandate the provision of a computer program capable of calculating the applicable damage stability requirements, were agreed.

The approval generally applies to the software, but may include hardware, for example, when the instrument receives input from sensors for the contents of tanks. Similar revisions for gas tankers and the IGC Code were adopted by MSC 93 in May 2014.

All tankers on international voyages must meet the IMO requirements for damage stability. These regulations are contained in the MARPOL Convention for general purpose tankers, the IBC and BCH Codes for bulk chemical carriers and the GC and IGC for gas carriers.

In 2005, several port states, led primarily by the UK’s Maritime and Coast Guard Agency (MCA), recognised that many tankers had on board documentation to demonstrate compliance with these damage stability requirements only when the ships were loaded in accordance with the ships standard loading conditions in the approved Stability Booklet.

However, during actual operations many tankers are loaded to conditions, which significantly differ from these standard loading conditions. A survey by the MCA indicated that ‘more than 50% of vessels are operating to conditions, which are not in the approved Stability Information Booklet’.

It is generally understood that since most tankers use computer programs to evaluate stability and strength for any loading condition, there is no longer a practical incentive to stay with the standard loading conditions. It is also generally recognised that modern double hull tankers are generally more vulnerable to damage stability scenarios and the new regulations, including bottom raking damage, are more onerous then past damage stability regulations.

Compliance options

There are four possible options for operators to demonstrate compliance with the IMO requirements for damage stability:

Load the ship only in strict accordance with the standard approved loading conditions from the Stability Booklet, which have been approved for both intact and damage stability.

Obtain specific approval for a loading condition which has a significant variation from these standard loading conditions.

Load the ship in accordance with a limiting KG, or required GM, envelope curve (or curves), which have been developed in accordance with the damage stability requirements.

Use an approved computer program to verify that the non-standard loading condition complies with the damage stability requirements, as well as the intact stability requirement.

The administration should take into account the guidelines for the approval of stability instruments (MSC.1/Circ.1229) when reviewing stability instruments. An approved on board stability instrument would not replace the approved Stability Booklet. Stability software should be approved, but the same should not apply to the hardware which could be covered by national standards.

The intent is written to apply to all vessels with provisions for the administration to provide waivers to existing tankers with any of the following conditions:

·         Tankers with stability instruments already installed on board capable of verifying intact and damage stability.

·         Tankers operating on a dedicated service with a limited number of loading permutations.

·         Tankers where stability verification is made remotely by means approved by the administration.

·         Tankers loaded within an approved range of loading conditions.

·         Tankers provided with approved limited KG/GM curves that verify compliance with all applicable intact and damage stability requirements.

It should be noted that the UK MCA defines significant variation as ‘a deviation in mass in cargo or ballast tanks exceeding 1%, or a deviation in the centre of gravity exceeding 0.02 m’.

The author of this paper commented on the compliance options set out above, thus - Option 1 – meets the current regulations, but it is not a practical operational restriction for many, if not most, tankers.

Option 2 – meets the current regulations, but the practical reliance on gaining these voyage specific approvals on a timely basis may be a burden to both the operator and to national administration and may limit operational flexibility.

Option 3 – meets the current regulations and many ships are currently operating effectively and safely using this method. For this type of system, the limiting KG (or required GM) curves versus draft are pre-developed and preapproved and typically would be added to both the Stability Booklet and the loading computer.

This would insure compliance with both the damage stability and intact stability requirements.

However, in practice these curves are complicated and expensive to produce and also have other application and enforcement concerns as noted in MSC 82/18/2, ‘because of the need to consider all possible loading and damage combinations and any associated limiting provisions such as tank filling ratios.

The resulting stability books may be complex and not easily applied by ships’ officers and port state control inspectors’.

For these reasons Herbert-ABS said that in general, it did not recommend this approach. Option 4 - The only practical solution is to fit an approved damage stability computer program on all tankers.

Herbert-ABS said that it agreed and believed that Option 4 provided a solution that will make it easy to demonstrate compliance with the damage stability requirements to the Port State authorities for any cargo, or ballast distribution.

With Option 4, the use of an approved computer program to verify that the nonstandard loading condition, complies with the damage stability requirements, can be readily applied to new ship loading computers, or implemented as an upgrade to existing loading computer programs.

Loading computer programs with this feature are generally referred to as ‘IACS Type 3 Loading Instruments’, as specified in IACS URL 5 (applicable for newbuildings since July 2005), which define Type 3 as ‘software calculating intact stability and damage stability by direct application of preprogrammed damage cases for each loading condition’.

Herbert-ABS’s CargoMax loading computer with the direct damage stability (DDS) module fully meets the requirements of IACS URL 5, Type 3, for any type of tanker. It can demonstrate compliance with the damage stability requirements for any of the relevant regulations from IMO and national administration for any type of loading, or ballast loading. And it can also be used to demonstrate this compliance to Port State inspectors or vetting surveyors.

The company has had approved CargoMax systems with the DDS option fitted on board ships since 1996. This feature has been approved by class societies ABS, DNV, LR, GL, NK and BV.

Herbert-ABS claimed to have to the first ABS Class approved IACS Type 3 system, the first LR Class type approval for a IACS Type 3 System and a type approval from DNV GL.

*This article was taken from a White paper on direct damage stability, published by Herbert- ABS Software Solutions.



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