Stability calculationspaper or PC?

Oct 15 2013

When verifying stability compliance, which is best for your ship - paper or PC?*

Mariners are well aware that stability is a critical aspect of ship safety, and traditionally a stability book has been provided on board under Loadline Regulations[1] to give the Master information on how to load his ship safely.

In recent years these documents have been supplemented with computer-based versions, typically termed loading instruments or (strength and) stability loading calculation software (LCS).

In our modern day lives we cannot get away from computers – whether it’s accessing bank accounts, booking holidays, or checking out the half-time football score – and it makes good sense to use these tools where commercially of benefit.

But is a computerised approach always compatible with the way seafarers have carried out some of these tasks in the past – especially those which are safety critical? Are the associated limitations clearly understood? And what impact may imminent legislation have – particularly when it comes to verifying compliance with complex stability requirements?

Significant financial commitment from owners and operators on such tools is likely over the next couple of years and so ensuring the right approach is taken in good time may be critical both for ongoing operations and effective investment of time and finances.

A ship’s stability book is required to include the basic information needed by the Master to check stability compliance, following guidance from the IMO. In many cases, the information is simplified for ease of use and the contents are therefore conservative. Modern computerised LCS systems are, on the other hand, sophisticated tools and are able to assess pretty much any loading scenario and in much greater rigor than using the stability book for hand calculations.

As a result, a properly set up LCS can maximise the operability of a vessel, but it must be thoroughly verified to ensure that its methodology and functionality would nevercompromise the safety of the ship.

This is especially relevant to tankers, where the distribution of cargo has a significant influence on both intact and damage stability compliance. In October/November 2011 port state inspections under the Paris Memorandum of Understanding were undertaken on a variety of tankers in a concentrated inspection campaign (CIC).

The inspections found that a number of tankers were sailing without following the stability book – in other words without any formal verification of damage stability compliance - and a proportion were in fact planning to sail in a non-compliant load condition. Pat Dolby, co-ordinator of the CIC, commented that “The most significant finding from the campaign was that 16.2% of tankers that were inspected the Master could not demonstrate that the ship was normally loaded in accordance the SIB. This is a significant number of tankers that, during a ‘spot check’, could not show compliance with stability requirements and thus may pose a risk to the environment” [2].

The view that tankers are inherently safe with regard to stability is not correct. It is well understood that there can be multiple free surfaces present and this is accounted for thoroughly in the design approval process. But the content and scope of approval of stability documentation can vary greatly. For example, typically the damage stability approval might only cover the specific load conditions contained within the stability book and there is no guarantee that a new (or even very similar) load condition will also comply with the regulations (approval certification often includes a clause such as ‘proposed new cargo load conditions should be submitted to the administration for approval’).

The form and content of the stability book is often driven by the newbuild contract and understandably, to minimise cost these documents tend to contain the minimum information necessary to meet statutory requirements and practical for use on board. It could be assumed that an LCS is usually installed under class society strength requirements anyway, and so the Master could then examine and verify compliance for any intended loading condition.

But LCS performing strength calculations may not have been necessary and even if on board it may not have needed approval - or to cover relevant stability functions.

It is also worth noting that the fundamental requirement of an LCS approval under the 2008 Intact Stability Code [3] and IACS UR L5 [4] (for class approval) is that it is to be based upon the stability book.

If the approved stability documentation is very limited in scope and content, then the LCS will also be limited in its use. In fact for older vessels, it may be almost impossible to develop a useful LCS using the approved information to hand and significant additional work may be needed to avoid producing a tool, which would actually make operability even more restricted.

Approval of stability documentation also tends to be approached from the perspective of safety rather than precision. Independent checks are often carried out, rather than a time consuming and costly line by line review of every last number.

Consequently, where there are conservatisms built into the stability documentation, or results which are not explicitly as expected but are ‘on the safe side’– these are generally accepted without amendment if the included load conditions comply, the safety of the ship is not compromised and sufficient data is provided for checking other conditions safely.

If we then consider that the formal approach to approving an LCS for a specific ship is that derived results should match the stability book values within some small prescribed margins [3][4], it is clear that problems could arise in the approval process.

In fact, a high quality LCS, using data and/or methods, which are more thorough than that used in the stability book, could be regarded as non-compliant if the numerical checks are outside the tolerances. Approval can then become unnecessarily complicated and delayed.

A modern LCS tool may often not include some conservatism necessary in stability books and therefore trying to force an LCS to match a simplified stability book can be a backward step now that tools are available, which can do a better job – if properly set up.

Experience while working for a major UK-based classification society for the past 15 years has highlighted that much effort is often expended when approving modern LCS systems, as little if any groundwork is done to identify the most effective solution for each ship.

Where LCS systems cover damage stability aspects in particular, significant extra work is often necessary, as the approved paper documentation can never include everything, due to the sheer volume of data and calculations involved. Rarely does sufficient forethought go in to the process to properly specify the right, comprehensive tool that will both fit the regulatory bill and maximise operability and so profitability. For tankers this is commonly an issue.

A typical example is for compliance with MARPOL Regulation 28 deterministic damage, where regulations require both ‘full extent’ and ‘lesser combinations’ of damage to be examined. Often the approved stability documentation only shows the full extent damages (which might number 20, or less) because these may be the most critical damage cases for the handful of standard loading conditions included in the stability book.

However, this may not be the case for other loading patterns and so to provide a valid confirmation of compliance for any load condition, the LCS would need to include and assess all the required damage combinations – which could number 300 or more cases. It is understandable why these would not be stated in the original approved stability documentation if they were not critical, but clearly additional work may be required to define the ‘missing’ cases for inclusion in the LCS.

Similar problems arise in defining all openings to be used in such calculations, as often only the worst are reported. Saying this, in cases where this information can be compiled, the resulting LCS provides a tool, which would ensure compliance along with much wider flexibility in vessel loading than the stability book had offered. However, it might only be worth the investment if the added flexibility can be exploited. As you can see, the best solution may not be immediately clear.

As a tanker owner, or operator, you may well be wondering what is the immediate relevance to ongoing operation of your vessels?

First, from a risk management perspective, awareness of the limitations and scope of the existing tools such as stability booklets and LCS systems is essential - approvals are rarely carte blanche. In conjunction with this, it should also be acknowledged that a conservative approach is likely to have been taken in some respects for those tools and may well be limiting operability of your ship. Review of existing documentation and possibly provision of an approved LCS system, may well provide the opportunity to mitigate this and ultimately increase revenue.

Second and the more pressing issue, is the relevance of imminent statutory legislation. In February, 2013 the IMO MSC sub-committee on stability, loadlines and fishing vessels agreed proposed amendments to MARPOL, IBC and IGC Conventions/Codes [5,6] at its 55th session which were summarised as follows : ‘The sub-committee agreed mandatory carriage requirements for stability instruments on board tankers. The draft amendments to MARPOL Annex 1, regulation 28 (subdivision and damage stability) add a new paragraph to require oil tankers to be fitted with a stability instrument, capable of verifying compliance with intact and damage stability requirements.’

Consequently, all tankers, irrespective of age, will be required to have their stability verification process reviewed at the very least and in most cases this is likely to require updates to existing documentation and/or provision, or update of an on board LCS system.

For each ship, the cost-effective solution will be based upon the operational profile of the vessel and the current scope of approved documentation and any existing LCS. There is the risk that hasty provision of a means to comply with the requirements might in fact have a negative impact and impose significant and unnecessary operational restrictions on the ship.

But procurement of a brand new, fully specified and approved LCS may not be the only solution. Options, such as approval of additional loading conditions, enhanced basic ship data and/or limit curves in the stability book, or update of an existing approved LCS with the necessary functionality, may suffice.

The cost for each option must be weighed against the potential operational benefits. More modern vessels may already have a stability LCS on board – possibly even an approved system – but it may be the case that the functionality, or approved scope, does not cover all the required stability aspects.

Most older vessels may have no stability LCS on board and given the limited information often contained within older stability books, there may be significant effort needed to develop such a tool.

It is also worth noting that where a vessel has been assigned a new role, any previous loading provisions in the stability book, or LCS, may now be irrelevant, or at least need to be supplemented.

It would therefore be advisable for owners and operators to undertake a prompt and thorough technical review of the operational profiles of their ships, the available approved information and systems and the resulting options for compliance with the amendments.

There are a limited number of providers of suitable LCS systems – particularly those with the full ‘Type 3’ direct damage capability (which will often be the best option for a tanker) – and the number of vessels to which these requirements will apply is significant.

While implementation dates have yet to be set, even if a generous two to three year time scale is agreed, it will be challenging for all ships to be appropriately equipped in good time.

Owners and operators should address this as a priority issue - but with due consideration that the right solution may not be the most obvious and with appropriate forethought it is likely that operational gains may be possible at the same time as just 'meeting the new requirements'.

*This article was written by Philip J Royal, CEng MRI_A, independent consultant at With extensive experience claimed in this field, the company offers consultancy services to assist in this process.



1) Load Lines, 1966/1988 - International Convention on Load Lines, 1966, as Amended by the Protocol of 1988 - Annex I - Regulations for Determining Load Lines - Chapter II - Conditions of Assignment of Freeboard - Regulation 10 - Information to be supplied to the master.
3) International Code on Intact Stability ,2008 Resolution MSC.267(85) Resolution MSC.319(89) - Part B – Recommendation for Certain Types of Ships and Additional Guidelines - Chapter 4 – Stability calculations performed by stability instruments - 4.1 Stability instruments.
4) International Association of Classification Societies, Unified Requirement L5 ‘Onboard Computers for Stability Calculations’ Corr 1. Nov 2006.
5) International Maritime Organisation SLF 55/6/1 Development of Mandatory Carriage Requirements for Stability Instruments Onboard Tankers.


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