On the loss of the Herald of Free Enterprise
R. E. D. Bishop* C B E , FEng, FRS and W. G. P r i c e * * FEng suggest that the interpretation placed by the Court of Enquiry on the ship's observed behaviour is not complete.
Herald of Free Enterprise capsized outside Zeebrugge harbour. What were the dynamic reasons for her toss?(Photograph by Smit Tak International Ocean Towage and Salvage Co).
F O L L O W I N G the loss of the ro-ro passenger and vehicle ferry Herald of Free
Enterprise on 6 March 1987, a formal
in-vestigation was held. The Court reported*" in September 1987. With due deference to the Court, the writers o f the present paper suggest that the interpretation placed by the Court on the.ship's observed behaviour is not complete. Moreover, the aspect that seems to have been overlooked is potentially serious.
The ferry Herald of Free Enterprise sail-ed from Zeebrugge at 18.05 hours. The ship was manned by a crew of 80, and was laden with 81 cars, 47 freight vehicles and 3 other vehicles; she had also embarked about 459 passengers for the voyage to Dover. The weather was good, and there was a light easteriy breeze, with very little sea or swell. The vessel passed the outer mole at 18.24 hours and capsized about 4 minutes later. During her fmal moments, the/feraWmmed rapidly to starboard and rolled to port. (Regrettably the Report contains no details of the ship's curved path.)
The vessel came to rest on her beam ends with her port side bn the ground in shallow water. The result was that the hull rapidly filled with water below the surface level, and about 150 passengers and 38 members ofthe crew lost their lives. Many others were injured.
The Herald was a triple screw vessel, built in 1980, with details as follows:
Lengtii, o.a 131 -9 m Length, b.p 121 • 1 m Breadtti moulded 2 2 - 7 m Weight (gross) 7,951 • 4 4 tonnes
(net register) 3 , 4 3 9 - 0 5 tonnes Maximum service speed 22 knots
Understandably, the exact operating con-ditions of the ship are not known with great precision. The Court was
'satisfied that at departure the Herald had a mean draught of between 5 • 68 m and 5 • 85 m with a trim by the head of about 0-8 m. The value of G M was about 2-04 m . '
The speed of the ship when she commenc-ed to veer away from her intendcommenc-ed course is not known exactly but is thought to have been approximately 17 knots (and ac-celerating). The depth of water, too, is not specified accurately but calculations were made for the Court using representative depths of 16-5 m and 12-2 m.
One remaining operating condition is of crucial importance. The ship had her bow doors open.
The Court's findings
It is common knowledge among naval ar-chitects that free water on an open deck within a ship is potentially dangerous. It is destabilising in the hydrostatic sense. This can be explained in a highly simplified way by consideration of static conditions.
Brief-ly, any departare from the upright condition will cause the free water to move athwart-ships. This will shift the centre of gravity sideways, and so reduce the net righting mo-m e n t . T h i s danger, inherent in ro-ro ships, is spelled out in characteristically forthright terms by Barnaby'''*.
It is known that water entered the hull of the Herald of Free Enterprise. In the Court's view the ship scooped up water as it moved forward and it was this water which brought about the capsize. Having entered the hull, it suddenly moved across the ship to port and gave the ship a pronounced loll (about 30°). With an unsymmetrical water plane and underwater hull form, and with her port propeller more deeply immersed than the other two, the ship began to turn to star-board. Although the explanation seems to raise interesting questions as regards the tum to starboard, this, in essence, is the Court's diagnosis of what happened to the Herald. It is held that the open bow doors admitted water which subsequently brought about the catastrophe by reason of hydrostatic in-stability. The Report is much taken up with questions of loading and compliance with rules concerning ship stability. Rightly, the fact that the bow doors were not closed is strongly condemned.
The Report thus reaches an entirely understandable—indeed, almost predictable—conclusion. While at first sight its explanation of what happened seems irreproachable, however, it nevertheless seems to us that there is room for serious doubt.
Sources of doubt
Tucked away in Appendix IV there are two statements whose significance seem.s not to have been recognised. They are:
' . . . a number of passengers and crew
. . . were convinced that the vessel started to list slowly and then suddenly lurched to about 3 0 ° , throwing people from the starboard side of the ship to port with observed violence.'
As we shall see, this convicdon on the part of some passengers and crew is a signifi-cant piece of evidence; if no-one had observ-ed this behaviour then some serious questions would have to be asked. Secondly,
'The helmsman had warned the Master in the early stages of the final capsize that he could not control the vessel's turn to starboard, even with full port helm. This was the first intimation that Captain Lewry had of anything untoward.' The helmsman's loss of control obviously occurred before the 30° lurch to port.
These two statements conform exactly to the behaviour that would be experienced i f the ship lost directional stability. One might almost say that they are 'textbook' statements. The essential features of direc-tional instability are;
(a) a speed is reached at which the rudder ceases to be able to control direction. (b) the vessel sheers o f f to port or to
starboard.
(c) the yawing motion is accompanied by heeling (and for that matter, sway). (d) there is no requirement for there to be
waves on the surface, but i f there are, the parasitic motion of the ship in the waves would be superimposed upon the unstable forward motion.
Directional instability is promoted by high speed, the possession of a forward skeg and trim by the bow (particularly in shallow water). It is seldom possible to find an easy justification for a dynamic instability but an explanation of sorts can be seen for this one by imagining a really exaggerated trim by
Herald of Free Enterprise is righted by the salvage team. Could a new research programme prevent such tragedies recurring or do we already have the answers?
(Photograph by Smit Tak International Ocean Towage and Salvage Co).
the bow; with it, the vessel would attempt to pivot about its forefoot on a sort of con-ical motion.
It would seem that, by the time the helmsman reported loss of control, the
Herald was well and truly unstable
direc-tionally but was not unstable in the hydrostatic sense. In this condition the slow listing noted by some of those on board could be relied upon not merely to grow but to speed up exponentially.
There is no suggestion in the Report that anything of this sort occurred. No calcula-tions were made which shed light on possi-ble directional instability. How, for instance, was the initial speed at which directional stability would impend related to angle of trim? There being no reference in the Report to the possibility of directional instability, it is not noted even that the presence of a bow rudder would have a directionally destabilising effect.
Implications of model test results
Unfortunately it is necessary to guess the answer to a most important question. Had water already commenced to enter the hull or not at the onset of directional instabili-ty? The Report is not clear on this point, though there are two relevant pieces of evidence suggested by model tests and refer-red to in Appendix I V :
'. . . there was a critical speed below which water did not enter the open bow door. Below the critical speed, water was thrown forwards and sideways by the spade and the bow. As soon as the critical speed was reached, a sharp tran-sition took place and water began to flow aft into the vehicle deck . . . . This critical speed was sensitive to trim at any given mean draught.'
Moreover
'The critical speed was found to be sen-sitive to water depth.'
Onbalance, it appears that water/!ö<i started to enter the bow doors before directional in-stability set in (though the text of the Report is not clear on this point). Let us, then,
assume that this was the case. With the bows trimmed down already, the water that enter-ed remainenter-ed in the forward part of the ship and depressed the bows still further. With trim by the bow gradually increasing, the ship would become directionally unstable i f it was not already or more so if it was. Fur-thermore, Appendix I V tells us that, as water entered the ship, the trim would in-crease, and so the tendency to ship water would also increase.
Following the onset of directional in-stability, one might reasonably conjecture that water continued to pour into the ship, depressing the bow ever more deeply, un-til the process described in the Report took over. That is to say, there came a point at which a significant amount of water within the vessel was thrown sideways—the ship was already following a curved path—and the 30° loll was assumed. The conditions referred to by the Court might then very well be those that actually prevailed, though the turn to starboard is far more easily ac-counted for by the directional instability than by the process described in Appendix I V of the Report, we think.
Full scale tests
Full scale trials were carried out on the Pride
of Free Enterprise at Zeebmgge.
Understan-dably, these were performed with the bow doors closed, but Appendix I V states that ' . . . the trim by the head was less than at the time of the casualty, being under 0-3 m, whereas at the casualty it was probably over 0-6 m . '
In other words, the full scale trials had lit-de bearing on the process we have sug-gested.
A l l in all, it is hard to see what possible purpose the full scale trial was expected to serve. It was cleariy not intended to study hydrostatic behaviour so one assumes that directional instability was the object of in-vestigation. But without any knowledge of how sensitively the condition of directional instability depends on speed, bow trim or water depth this would appear to be
fool-hardy. What provision was made, one wonders, to monitor approaching in-stability?
Discussion
There are three distinct types of objection to this line of speculation. First of all ex-perimental evidence is adduced in the Report which apparendy conflicts with the sugges-tion that we have made. In Appendix IV it states that
'The model test showed that the ship had adequate power to maintain at least 18 knots in water of 16 • 5 m depth, and that
trirrt by the head did not give rise to any handling problems.'
This is of critical importance but may only indicate that insufficient trim was used in the tests, for it is not at all difficult to show that a model with adequate trim by the bow is quite impossible to control by means of the rudder—and not merely because its rudder or propeller leaves the water. It is a pity that the magnitude of the trims that were used in the model tests are not quoted.
It might be asked whether or not the model of the Herald displayed the behaviour that has been suggested. Again, we must refer to Appendix I V :
"To prevent the model sinking when flooding took place through the open bow doors, a bulkhead was fitted across the vessel between the entry longitudinal bulkheads at station 8 • 5 approximately at the position of the inner watertight door.' In other words, the model was constructed in such a way as to preclude the possibility that we have suggested.
A quite different sort of objection to the present hypothesis is that it is technically obscure. It lies outside common knowledge of ships and is now produced like a rabbit out of a hat. We have to admit that we know of no textbook of naval architecture which deals with the effect of trim on directional stability (or, indeed, with any aspect of directional stability really adequately for that matter). Even so
• the behaviour referred to will hardly come as a surprise to a dynamicist, • a recent paper''*, prepared under the
SAFESHIP Project, contained the statement:
'Trim by the stern enhances stability, whereas a vessel with bow trim is more prone to instability and therefore to per-form large motions (possibly leading to capsize).'
• the possibility of this sort of behaviour was drawn to the Court's attention but the deposition was ignored.
• the behaviour is very easy to demonstrate with a toy model of a ship.
The third sort of objection is obvious. At first sight it may seem that the suggestion of directional instability is merely of theorefical importance. After all, the initial heeling to port was slight. I f it required a large amount of water in the forward end of the ship to depress the bow sufficiently
Continued on page E29
The final stage of the research programme was to install a system on a ship. This has been done, and results are expected early this new year. On the basis of the field tests and the fact that existing copper and chlorine anfifouling technologies have proved effective, favourable results are expected.
The basic shipboard system comprises copper anodes, plafinised dtanium anodes and a control box. A circuit is set up in which copper ions are produced from the copper anode and chlorine is produced from the platinised titanium anode. The ship's hull is used as the cathode.
Electrodes can either be placed in a sea-chest or a sea strainer, though it is preferable to have the platinised titanium electrodes in the strainers. This is to target the chlorine produced as close to the heat exchange sur-faces as possible and to allow any magnesium and calcium hydroxide scale produced by the electrolysis process to be removed easily during routine strainer clean-ing. I f a seachest is used for installation of
the copper anodes then the vessel needs to be drydocl<ed so that a hole can be cut in the seachest, to which a flange is welded. The anode is then lowered through the hole and bolted to the flange. As an extra security device, a top plate is bolted to the flange on top of the anode.
The anode assembly is designed to be easy to replace when the copper has been dissi-pated, but difficult to remove accidentally. The anode seals are tested to 100 bar prior to dispatch. The section of the anode assembly passing through the seachest is made of a high density plastic with a low dielectric constant so that the copper and platinised titanium are effectively insulated from the ship's steel, preventing a galvanic cell being set up.
The platinised titanium anode is con-structed and installed as for the copper anode, with a hole being cut in the strainer lid and a flange being welded in place. A copper anode can be made to last between two and five years depending on require-ment, or a series óf copper anodes can be
installed. The plafinised titanium anode will last 20-30 years. I f the copper is installed with the platinised titanium in the strainer box, as may happen i f the ship does not ex-perience fouling before the strainer box, then smaller copper anodes can be used and changed when required whilst the strainer is undergoing roiifine cleaning.
In a seachest installation the typical dimensions of a copper anode are between 0-6 and 0-9 m long with a diameter of 0-076 m. A plafinised titanium electrode might be 1 m long and 0 • 025 m in diameter. The control box, built to M o D standards, is bulkhead or plinth-mounted with the option of an on/off switch on the bridge.
Though the technology is the same, pre-cise installation details will vary for each ship because designs and operational pro-files are different. The BFCC philosophy is therefore to dicuss and examine the prob-lem with the client and recommend a solu-tion, usually the BFCC system, since this represents the 'state of the art' in
anti-biofouling control. @
On the loss of the Herald of Free Enterprise
—Contmued from page E I3 .to render her directionally unstable, surely closing the bow doors would remove all possibility of a catastrophe. Is all this speculation not just argument for argument's sake? Let us consider whether this must be classed as cantankerousness or not. (a) That the inifial list to port was 'slight'
is misleading. It could be relied on not only to grow but to do so at an increas-ing rate.
(b) Judging by para. 9.2 o f t h e Report"* it is by no means uncommon for a ro-ro ship to proceed at sea while trimmed by the bow. This being so, it is surely sen-sible to try to discover what limits (if any) should be placed on this practice. Appendix I V of the Report makes it plain that the practice is more dangerous in shallow water than in the deep ocean whether the bow doors are open or closed.
(c) It is not easy to take speed o f f a ship quickly so that, i f directional instabili-ty occurs with its attendant roll, it is almost inescapable that a large angle of heel will be reached. It seems to be by no means impossible that then the vehicles within a ro-ro ship could behave like free water, and topple sideways. It is not an essenfial require-ment that water should have entered the hull (with attendant hydrostafic instabili-ty) for a large angle of heel to be assumed by the hull.
(d) Nor does the possibility of trim by the bow arise only in the context of ro-ro ships. It is possible for an ocean-going vessel to sustain damage near the bow by reason of fatigue cracking or as a direct consequence of a violent slam'". I f serious flooding occurs for-ward, remote from the inhabited part of a large OBO carrier for instance, would it be possible for a trim o f , 2 ° , say, to
J A N U A R Y 1 9 8 8
be assumed without the knowledge of the Master? In this condition, could ac-celeration of the ship lead to capsize? This is one of the possibilities - though admittedly not a prime suspect - to which we pointed in a report'" on the loss of the Derbyshire.
Simple experiments with a toy model strongly suggest that the directional instabili-ty may be a fairly 'hard' one. For what it is worth, a model Liberty Ship was severe-ly directionalsevere-ly unstable when given a trim by the bow of about 1-12°, whereas no sign of directional instability could be produced when the trim was about 0 - 8 7 ° . This, it must be emphasised, was found with a toy model on a swimming pool!
Conclusions
1. It appears that the Herald first suf-fered direcfional instability. This was followed by a massive hydrostatic instability and it is only this latter that is referred to in the Report.
2. Had the bow doors been closed, direc-tional instability alone might have occurred. Although the Report is unclear on this point it seems likely that it would not actually have done so, but normal operafional practice would bring the ship at least fairly close to a condition in which directional instability
would impend with the bow doors closed.
3. A massive direcfional instability would be almost impossible to control and could conceivably be as catastrophic as the hydrostafic instability was.
4. The phenomenon of directional in-stability is well established in the literature, but its dependence on bow trim is probably not widely understood. While this latter has been referred to in published work, it is hardly the smff of day to day naval architec-ture and it stands in need of further research.
5. For these reasons, the Investigation Report on the Herald of Free Enterprise disaster should not be regarded as the last word. The improvement of, and compliance with rules relating to hydrostatic stability that it so strongly advocates are all very well, but those rules are highly empirical. Worse, work along the lines suggested in the Report can do nothing to increase our understanding of directional instability. 6. It is no more than common prudence to investigate the possible occurrence of direc-tional instability of ships more generally, notably in the operation of trawlers. In par-ticular the effects of trim by the bow should be closely examined. This recommendation has in fact been made before.
7. Full scale trials with a ship like the Pride
of Free Enterprise should only be
perform-ed with great care and should be closely supervised by an experienced dynamicist.
' Vice-chancellor Brunei University, Uxbridge, Middlesex.
* * Professor of Applied Mechanics, Brunei University, Uxbridge, Middlesex.
References
1. MV Herald of Free Enterprise: Formal In-vestigation. Report of Court No. 8074, Department of Transport, HMSO, 1987. 2. K. J. Rawson and E. C. Tupper. 'Basic Ship
Tiieory', Longman, London, Ed.2, vol. 1, 1976.
3. B. R Clayton and R. E. D. Bisliop. 'Meciianics of Marine Veiiicles', Spon, London, 1982. 4. K. C. Barnaby. 'Some Ship Disasters and their
Causes', Hutchinson, London, 1968. 5. R. E. D. Bishop, W. G. Price and P. Temarel.
'General Linear Antisymmetric Motions of a Rigid Ship', Proc. Internat. Conf. on the SAFESHIP Project: Ship Stability and Safety, RINA,^aper 5, 1986.
6. R. E. D. Bishop, W. G. Price and P. Temarel. 'A Hypothesis Concerning the Disastrous Failure of the OnomichiMaru', Trans. RINA, vol. 127, 1 6 9 - 1 8 6 , 1986.
7. R. E. D. Bishop, W. G. Price and P. Temarel. 'The Derbyshire —A Design Review', Report to the Department of Transport, 1984.
