Saturday, July 17, 2010

The stability of the fishing vessel Trident

The Court hearings into the loss of the FV Trident have now concluded and we have been advised that the Sheriff Principal has retired to write his report. 
Although the control of the technical information, relevant to these hearings, has been unprecedented for a public inquiry, secretive even, the Advocate General’s views on what the preferred outcome should be have been frequently aired in the press:

She said: There is no reliable evidence to support a finding that the loss of the Trident was caused by deficiencies in her design stability, in particular non-compliance with the recommended IMCO intact stability criteria, or by capsizing in different circumstances.
The most probable cause of the loss of Trident was a sudden and catastrophic capsize in heavy seas, which most likely occurred within two or three seconds and was followed by rapid sinking.

Notwithstanding these points, one piece of factual information about Trident, which has recently managed to break through into the public domain as a result of a freedom of information request to the DfT, is a 34-year-old technical report on the tank testing that was carried out on a scale model of the Trident by the National Maritime Institute in 1976.

Although the NMI report is very carefully worded, it concludes that the Trident’s stability reserves were insufficient to prevent her from capsizing in sea conditions that were relatively moderate, ie in conditions similar to those that were recorded on the day she was lost. The report also shows that on her last voyage, Trident’s stability was deficient when compared to the IMCO minimum stability standards. It is unfortunate that for many years the Department of Trade have been unwilling to share these important conclusions with the relatives of those who were lost.

Apart from its conclusions, the NMI report also contains some information that is of real interest to Naval Architects: it contains a scaled body plan and loading data. What the release of this information actually means is that the curious amongst us can now check out the stability reserves of the Trident for ourselves – we no longer have to accept the official, sanitised, line that has been consistently promulgated by the DfT over the years and which has now also been adopted by the OAG.

Stability assessment

We constructed a computer model of the Trident’s hull and carried out an assessment of Trident’s stability reserves against the minimum standards that are laid down by IMCO for fishing vessels:

We have thus discovered that Trident did not meet the IMCO minimum stability criteria in any of the four principal loading conditions. 

We were also able to confirm that the Trident did not meet the IMCO stability criteria on the day she was lost and, furthermore, even if 10 tonnes of steel ballast had been added to her keel, she would still have been unable to meet the IMCO stability criteria in all of the four standard sailing conditions.

The following image contains the summary results of our stability assessment, the data that is highlighted in orange shows IMCO non-compliances in each of the six sailing conditions examined. Alternatively, this file [link] contains a copy of the stability assessment in a pdf format.

The IMCO stability standards are minima, which, when met, should prevent a vessel from capsizing in all but the most severe of weather conditions. They come as a package and they need to be complied with in their entirety.
Since 1975, all UK fishing vessels of the Tridents type and size have been obliged to meet the IMCO stability standards in full. Any vessel that did not meet the IMCO stability standard would not have been issued with a UK fishing vessel safety certificate by the MCA and would, therefore, have been unable to fish.

The Trident did not meet IMCO’s minimum stability standards and unfortunately the nature of her non-compliance was such that the mere addition of ballast would not have resolved this problem. For Trident, as in the case of her sister vessel, the Silver Lining, significant structural modifications would have been necessary to bring her stability reserves up to the required standard.

A ship’s propensity to capsize and its inherent stability are inextricably interrelated; the very fact that Trident capsized is conclusive evidence that Trident had insufficient stability for the sea conditions on the day she was lost.
Additionally, the fact that Trident did not meet IMCO’s minimum stability standards (i.e. her stability was deficient) would certainly have increased her propensity to capsize.

ADDENDUM (19 July 2010)

Stability model comparison

The output from our stability model was compared against the results obtained for the Trident’s loss condition from the DOT’s SIKOB program in 1976 (the SIKOB results are contained both in the final report of the original Formal Investigation and in the 1976 NMI model test report). The results from the 2010 computer program and the 1976 SIKOB program were found to be virtually identical (only 3mm difference in floating draught and 7mm difference in trim over 22 metres):

However, it was noted that the original 1976 calculations contained a small input error and, when the input to the 2010 program was modified to rectify this anomaly, the stability results were reduced slightly:


RAJ said...

A sincere thanks for your support over this RFI your latest effort is I think excellent.

From your table of conditions I see that with the tons of ballast the vessel still failed the IMCO criteria.From the marin modelling it would seem that this condition allowed for better probability of survival is this due totally to the increase in AVS?

Once again I would like to extend my thanks and hopefully will be in a better position to contribute to the blog in due course


gadfly said...


Thanks for your comment.

Certainly, the increased range of stability – where the AOVS is increased to 58° (from 48° without ballast) is significant in resisting capsize.
However, the immediate effect of the ballast is to lower the VCG and this increases the amount of righting energy available to combat any 'upsetting' forces from the sea.
i.e. With extra ballast it would be more difficult to heel the Trident and, additionally, once heeled, she had stability to allow a further 10° of heel before capsize became inevitable.

The following Youtube video provides an excellent illustration on how the survivability of a small boat is enhanced by being blessed with an AOVS of circa 90°:

Best regards

RAJ said...

Having read this blog in its entirety now and the NMI report I must admit to being rather puzzled with the statement from OAG
"She said: There is no reliable evidence to support a finding that the loss of the Trident was caused by deficiencies in her design stability, in particular non-compliance with the recommended IMCO intact stability criteria, or by capsizing in different circumstances."
Am I correct in suggesting that the NMI tests indicated otherwise as did the tests from Marin with the vessel modelled with an additional 7.5 tons of ballast both of these would suggest to me to be evidence to the contrary. I have also failed to read any evidence to substanciate her statement possibly due to the fact that the testing carried by Marin as specified by the JPE? did not investigate that particular avenue of behavior and IMCO compliance unlike the NMI experiments which did.
If the physical model at Marin capsized in 4.5m seas equivalent and was said to be an optomistic picture it would I suggest stand to reason that something was wrong surely?

Best Regards RAJ

gadfly said...


We think that the points that you make are well founded as the NMI tank tests did investigate the model's probability of capsize in the context of IMCO criteria compliance.

It is suggested that the OAG's statement you quote is factually incorrect. The OAG did not really look for evidence of that nature and, if any looked like turning up, it was quickly suppressed or rubbished.

The NMI tests were carried out on a correctly scaled model of Trident with all major parameters correctly represented. The NMI test facility was considered to be cutting- edge and at the forefront in its day (not so long ago). The test house staff, particularly Dr Morrall, were internationally renowned. The waves were realistic, correctly sized and the model capsizes did really happen (photos and video records) – this would appear to us to be reliable evidence, but then we’re not solicitors!

As for the MARIN tests – the JPE did not specify any tests that would shed light on the IMCO issue, this appears to have been something of a 'no go' area as far as the RFI was concerned. Hence any conclusions drawn from the MARIN ballast condition regarding IMCO compliance can only be inferred.

One very important fact that the NMI tests did reveal was something they called a 'limiting condition for survival' In essence they found that if they increased the GM i.e. lowered the VCG from 3.197m above keel to 3.0m above keel (a reduction of 197mm), the vessel would not capsize (it just so happens that this 3.0m figure also corresponds with the 0.89D ratio that is indicated in the Seafish Stanhope III paper. It follows therefore that other designers of Trident sized fishing vessels must have regularly arrived at VCG/D ratios of 0.89 (without ballast) on vessels that did not subsequently capsize, so there is a possible element of design there).

We have also tested the NMI's lightship 'survival' VCG in all of the standard IMCO sailing/loading conditions as well as in the JPE's standard conditions and found that it gave full IMCO compliance in virtually all sailing conditions except for a couple where it failed the Max GZ>25ยบ criteria.

Thus, in our view, if the Trident's lightship VCG had been only 3.0m above the keel, then it could realistically be said that Trident's stability was in substantial (but not full) compliance with IMCO minimum stability standards.

Best regards,