FV Trident inquiry – confusion and instability

The Trident was a typical example of the Scottish trawlers that were built in the late 60’s and early 70’s of just less than 24.4m (80 ft) in registered length. Outwardly it exhibited no obvious characteristics or features that would set it apart from the other similar vessels built at that time.

This particular size and type of trawler had a proven reputation for being seaworthy in all weather conditions, and in this respect we would hope that, ultimately, the Court of inquiry will be able to identify those critical differences on Trident which set her apart from the rest of the Scottish fleet and which caused her to capsize and founder in relatively moderate sea conditions.
The Trident was only 18 months old at the time of her loss.

Stability

Judging by the latest press reports on the debate about Trident’s stability, it seems that currently, there is some confusion within the Court as to what ‘stability’ actually means in the context of a fishing trawler and on what stability standards should normally apply. There also seems to be some confusion as to how a fishing vessel’s stability is actually measured and assessed, and, additionally, the terms ‘static’ and ‘dynamic’ stability appear to have the Court’s official experts and Counsel talking at cross purposes.

In recent days it has been reported:

“Sheriff Principal Sir Stephen Young, who is overseeing the inquest, ordered him [the counsel for the families] to compile a second document restating his case.
The first order was served on Monday, when the court ruled that Mr Anderson’s arguments on static stability, dynamic stability and stability curves – all of which must be in check for a boat to remain upright – were not clear.” (Aberdeen Press and Journal 18 November 2009)

“The inquiry heard yesterday that an incline test on the Trident would not have revealed if she was at risk of capsizing.
Richard Anderson, representing some of the families, said it is their belief that the test, which is used to measure the stability of a boat in calm conditions, would have uncovered problems with the Trident’s stability.
William Boyd, a director of TMC Marine Consultants, told the inquiry the test “has no relevance” when a boat is out at sea. […] “An incline test is a necessary and useful test, but in predicting what external forces are going to arise at sea it has no relevance.” (Aberdeen Press and Journal 17 November 2009)

A MARINE expert insisted a test of a Peterhead-registered trawler which sank would not have proven whether it was sea-worthy. […] Mr Boyd said a test on the Trident would have been “non applicable” because it would have been carried out in calm waters. (Aberdeen Evening Express 17 November 2009)

“Master mariner Graeme Bowles said a static test on the boat would not have correctly assessed her stability when at sea, and that a dynamic stability test was usually done to check this. […] The inquiry had previously heard that an inclining test, usually done when the boat is static, had not been carried out. It examines the vertical centre of gravity and its effect on a vessel’s stability. […] When asked by Ailsa Wilson, QC for the advocate general, to explain the difference between static and dynamic tests, Mr Bowles said: 'Dynamic takes into account everything to do with the ship’s behaviour when she is at sea.' The test takes into account the risk of capsizing and the threat posed by violent winds and waves”. (Aberdeen Press and Journal 28 October 2009)

Perhaps we should consider the possibility that the personnel making up this ‘expert panel’ may not be wholly impartial, and that their ‘expert pronouncements’ and arguments, although developed at taxpayer’s expense, may be influenced, to some degree, by the specific interests of their clients.

Mr Bowles and Mr Boyd’s assertions, which have been quoted above, unless taken out of context, are incorrect and misleading; they don’t reflect the stability standards that are applied either on current UK fishing vessels or on those built in 1973. The two marine experts also play down the critical importance that an ‘inclining test’ has in determining a vessel’s stability.
Their implication that the International Maritime Organization’s mandatory requirements for inclining experiments and stability [1] were developed for purposes other than vessels operating at sea is really quite surprising.

Currently, inclining tests are an essential part of the statutory processes that ensure UK fishing vessels have adequate stability while operating at sea. (ref. Merchant Shipping Notice 1770 – contains mandatory static and dynamical stability criteria for contemporary fishing vessels of a type and size similar to Trident).

It may be useful, perhaps, to provide some clarification on the types of ‘stability’ that have been discussed during this inquiry:

All vessels have an inbuilt or inherent level of stability/resistance to capsize; however, this remains an ‘unknown quantity’ until an inclining test has been carried out. The inclining test enables the weight of the vessel and the position of its centre of gravity to be determined. It is only when these values are known that the elements of a vessel’s static and dynamical stability can be calculated and compared against the standards that are required to ensure safety at sea.

Stability (in ships) - is a measure of a ships ability to return to its upright position after being heeled through some angle to port or to starboard. The tendency of a ship to ‘right itself’ is caused by the horizontal separation of the ships weight and buoyancy forces when it is heeled. The term ‘stability’ has a distinct meaning for commercial seagoing vessels and its values may be calculated accurately for different sailing conditions. The principal stability standards that are applied in the UK today are those laid down by the International Maritime Organization in the form of static and dynamical stability criteria, all of which a vessel must meet before it can put to sea.
While the IMO criteria have been developed from ‘static’ rather than ‘dynamic’ considerations and do not explicitly take ship motions and sea conditions into account, they have been found, after many years of experience and feedback from the world’s seagoing fleets, to provide a base stability standard that will prevent a vessel from capsizing in all but the most severe of weather conditions.

Inclining experiment - An inclining experiment neither measures nor tests a vessel’s stability. The purpose of an inclining experiment is to provide data that will enable a vessel’s displacement (weight) and the position of its centre of gravity to be determined. The inclining test is ‘static’ in nature and must be carried out in flat, calm conditions with the vessel in equilibrium in order to obtain accurate results. The results from an inclining experiment are essential for the accurate determination of a vessel’s stability characteristics.

Static stability (righting moment) – For a ship, the static stability at any given heel angle is the product of the horizontal separation (called GZ) between the vertical lines of action of the ship’s buoyancy force and of its weight multiplied with its displacement (note these two lines of action pass through the ship’s centre of buoyancy and centre of gravity respectively). The value of GZ varies with the angle of heel, and, if this variation is plotted from 0 degrees to (say) 90 degrees, something called a curve of statical stability is obtained.

Dynamical stability – If the area under the curve of statical stability is calculated up to any particular angle or between two inclined angles then this is known as the dynamical stability for the vessel (for the range of inclinations considered). It is a measure of the work required to be done or energy expended when forcing the vessel to heel to that angle.

Dynamic stability – This is a term that currently has different meanings for different people within the maritime industry. Traditionally it has been used instead of the term ‘Dynamical stability’ and additionally it has been used to describe a vessel’s ‘directional stability’ (ref Rawson & Tupper – Basic Ship theory) but, nowadays, more often than not, it is used (or misused) in a generic sense to describe the various properties that a ship may exhibit when in motion in a seaway.
Recently, as a result of concerns on stability fluctuations on large vessels such as Container or passenger ships the IMO has decided to examine ‘Dynamic stability phenomena in waves’ with a view to the eventual development of agreed mandatory criteria. However, this is a complex matter and it will be a number of years before any new stability criteria emerge.

******

It is obvious from the above that there is some scope for confusion between the terms ‘dynamical stability’ and ‘dynamic stability’ and, just as has happened in maritime circles, the Court may also have fallen victim to this misunderstanding.
Perhaps the differences between the two opposing camps and their views on stability could be briefly summarized as follows:

• The Counsel for the families would very much like the investigation to focus upon the types of trawler ‘stability’ that can be accurately calculated following an inclining experiment and for which there are International and National standards laid down (criteria for static and dynamical stability) i.e. something which is tangible.

• It would appear that Counsel for the other parties (including the Advocate General) might prefer the investigation to consider ‘dynamic stability’, for which no industry standards have been yet agreed either Internationally or Nationally and which has different meanings for different people: i.e. something which, at this moment in time, is not tangible.

In its latest revision to the International Code on Intact Stability, 2008 the International Maritime Organisation had this to say regarding the stability of ships in a seaway:

“The safety of a ship in a seaway involves complex hydrodynamic phenomena which up to now have not been fully investigated and understood. Motion of ships in a seaway should be treated as a dynamical system and relationships between ship and environmental conditions like wave and wind excitations are recognized as extremely important elements. Based on hydrodynamic aspects and stability analysis of a ship in a seaway, stability criteria development poses complex problems that require further research.”

It is suggested that while ‘Dynamic Stability’ may currently be of great interest to researchers, designers and operators of large container and cruise vessels, it is inappropriate for this developing field of applied science, on which there is no consensus, to be used as a basis for legal argument in a court of inquiry into the loss of a small trawler.
----------------------------------------------------------
[1] IMO - International Code on Intact Stability