Special And Unique Tugs

R.H. le F. Ashburner and N.H. Norrbin*

Since opening in 1869, enlargement of the cross section of the Suez Canal has been carried out at frequent intervals such that the original cross section of 300 square meters for ships of up to 6.7 meters draft has increased to 1,800 square meters by 1963 when it could accommodate ships of up to 11.3 meters draft. By 1967, traffic in the Canal had reached a level not far short of capacity when acts of aggression led to closure of the Canal for exactly eight years until the reopening on June 5, 1975. Throughout this time, plans were being made by the Suez Canal Authority (SCA) for the eventual reopening with account being taken of the changing pattern of world shipping, particularly tankers, in order to plan the future development of the waterway.

As soon as the Canal was reopened, work commenced on enlargement of the cross section to 3,300 square meters to accommodate vessels of 16.2 meters draft. Since the capital investment required for such a scheme is enormous, SCA sought the assistance of consultants to check its strategy. Two separate but similar studies were made: the first by a British team led by Maunsell Consultants Ltd., and the other starting shortly afterward by a French team led by Sogreah. It is noteworthy that both teams reached similar conclusions independently and that these largely agreed with the program envisaged by SCA. Following the first phase of development, this program included a second phase to accommodate vessels of 20.4 meter draft and having a cross section area of 5,400 square meters.

It is interesting to note that since reopening in 1975, Canal traffic increased rapidly to an average of 54 vessels per day in 1977, and now has increased to about 57 vessels per day. The non-oil traffic accounts for about 85 percent and has considerably exceeded the 1967 level. Oil trade has reached only about 20 percent of the preagression level in terms of tonnage though it has included regular transits, about an average of 25 per month, of tankers in excess of 200,000 dwt passing southwards in ballast at very shallow draft with the propeller not fully submerged. Once the first phase is open, the Canal will be able to accommodate laden 120-150,000 dwt tankers and ULCCs in ballast of up to 375,000 dwt. The intention of the second phase is that it would accommodate laden 260,000-dwt VLCCs.

Canal capacity is strongly dependent upon the gap left between ships in a convoy, particularly VLCCs. Accordingly, as part of the feasibility study some 1:40-scale trials were made with a model VLCC, and it was concluded that tug assistance would be required if convoy intervals were to be safely kept within reasonable limits. The use of tugs to escort large ships transiting the Suez Canal is not new and the present regulations require up to two tugs to be in attendance on large laden tankers or those in ballast. These tugs are free running ahead or astern of the vessel ready to assist if required; they are never permanently attached except when towing a pontoon or a "dead" ship.

With the advent to the Canal of much larger tankers having a greatly reduced power to displacement ratio, it is not unreasonable to expect that past practice in respect to tugs might need to be modified. It was with this in mind that in 1978 the SCA issued terms of reference and invited submissions for the detailed study and design of matters associated with the stopping and mooring of large ships in the two phases of Canal development.

This study was awarded to a consortium of British consulting engineers comprising Maunsell Consultants Ltd., and Rendel, Palmer & Tritton.

The scope of this work involved full-scale stopping and mooring trials in the Suez Canal as well as simulator studies both for these trials and for future enlargement of the Canal. It was therefore appropriate for the Maunsell-Rendel consortium to enlist the help of others. Marine consultants Cleghorn, Wilton and Associates advised and assisted in the tanker stopping trials, both in the field and on the simulator, and also provided the necessary pilots and tugmasters for the various trails. Study of mooring procedures and line handling was the main responsibility of Captain Colin McMullen & Associates, who also assisted in the field and simulator trials. Simulator modeling and operation were undertaken at the Swedish Maritime Research Center (SSPA). Other mathematical and laboratory modeling of vessel interaction and mooringline forces were performed by the British National Maritime Institute (NMI). The trials tanker was provided by Shell International Marine Ltd.

Tugs used in the trials would ideally have been of between 10 and 20 tons bollard pull, but the only ones available from the Suez Canal fleet were rated at about twice this power. Those used were one Voith Schneider tug with a bollard pull of 35 tons (two units of 1,500 bhp each) and two duckpeller tugs, each with a bollard pull of 45 tons (two units of 1,600 bhp each).

Principal observations made during the trials were: 1. Although currents can be of assistance in providing bank pressure, care must be taken to maintain the alignment of the ship parallel to the axis of the Canal; 2. One tug aft on a bridle provided the most compact and effective means of controlling the stopping maneuver though the extra power provided by two tugs on the other rigs did reduce the stopping distance; 3. A tug running free some 200 meters ahead ready to come under the bow and take a line or push once the ship's speed had decreased to three knots was found to be the most acceptable procedure; 4. Although no strong adverse winds wTere encountered, such conditions would undoubtedly represent the major cause of difficulty in the stopping maneuver.

5. A duckpeller tug aft on a 30 meter bridle was able to sit in the vessel's wake with engines idling in neutral. On quarter wires the outboard engine was kept at minimum revolutions ahead holding the tug alongside and against the pressure of the ship's wake.

On the basis of earlier simulator work, desk studies and the field trials, it was decided that an appropriate size of tug for handling laden VLCCs and ULCCs in ballast would be one with a bollard pull of about 60 tons.

Tug Performance One of the main objectives of the simulator study was the evaluation of the comparative merits of alternative arrangements for tugs. The task of these tugs will be to control the ship from swinging during a stopping maneuver rather than to reduce the stopping distance. When the ship has Figure 1 — T u g s with Omni-directional propulsion devices for the Suez Canal.

been brought to a rest, the tugs may be used for garing-up along the side bank.

The relatively high convoy speed in the Canal as well as the presence of the sloping side banks of the narrow channel pose special handling problems for which tugs with omni-directional thrust capabilities are likely to be best fitted. Typical examples are tugs with cycloidal (and variable pitch) propellers or rotatable rudder- propellers. Figure 1 illustrates a Voith Water Tractor with two vertical-axis Voith Schnieder propellers and duckpeller tug with two ducted thrusters astern. In view of the experience obtained, the simulator tugs were assumed to be fitted with rotatable rudder-propellers.

Needless to say the maneuvering qualities of omni-directional tugs are further improved by the twin-screw arrangement; a common joystick control input actuates a certain thrust for each of the propellers to produce the pull or push, or the turning moment desired. The thrust vector diagram is mainly eliptic, the astern and sideway bollard pull being some 75 or 80 percent of the ahead value. Typical turning rates at zero forward speed are in the order of 180° in 10 seconds.

Recommendations The main recommendations to be made by the consultants as a result of the field and simulator trials were the requirement in respect of tugs, the details of such tugs and the manner in which these should be deployed.

Although it is obviously import- ant that the tugs be designed to fulfill their role in assisting in the stopping maneuver, it is expected their use in such a role will be infrequent and they must, therefore, function equally efficiently while being towed behind, or escorting ahead of a VLCC.

Similarly it would obviously assist in operational planning if a common design could be derived to fulfill each role associated with escort duties, which might at times involve the need to perform a conventional tow.

Tugs with a bollard pull of 60 tons would be required to handle safely either 260,000-dwt laden VLCCs in phase 2 or 375,000-dwt ULCCs in ballast in phase 1. By analogy, the existing 40-ton tugs should be adequate to escort 150,- 000-dwt laden tankers in phase 1.

Normal Canal transit speed for large vessels is up to 16 km/hr so that in order to be effective the escort tugs should have a free running speed of 20 km/hr.

Propulsion systems should be of the multi-directional type and in view of the stable behavior of the duckpeller tugs, the use of twin rotatable propellers located aft was incorporated in the consultant's design. The stern tugs need not be of the multi-directional type but such a tug would seem advisable to give maximum operational flexibility.

Deck equipment should include a forward towing hook, cruciform bollard and rope/wire handling winches. Towing equipment also should be provided aft, and some tugs should be equipped with firefighting equipment.

Tug Attachment The main purpose of the stern tug during the stopping maneuver is to assist in the directional coursing of the ship which itself provides most of the stopping power. The bridle rig, Figure 2, is considered to be more effective than the American rig, Figure 3, in performing this role, and in addition would: 1. Interfere less with navigation buoys due to its more compact nature; 2. Not be sensitive to the backwash from the ship's propeller during the latter most critical stages of stopping; 3. Involve handling fewer lines when being made fast. In addition, all line handling onboard ship would be carried out on the poop deck where there always is an ample number of winches, etc.

This is not usually the case on the main deck of a VLCC, which is the area where the American rig would need to be attached; 4. Avoid the possibility of a tug becoming trapped between the ship and the Canal bank.

5. Give rise to less bank erosion from tug propeller wash, and 6. Be less expensive to the ship operator in terms of transit dues.

The bridle legs on either large vessel would be about 30 meters in length, and in rigging the bri- die the leads on the ship should be chosen to provide the widest possible angle for the bridle and the bridle lines should be attached to bitts on the poop deck.

A bow tug would be necessary with either rig, and a "snatchwire" should be rigged from the bow of the ship ready for use by the tug if required.

Due to the long towing distance and r e l a t i v e l y high speed, the stern tug should run in the same sailing attitude as the ship.

Steel-wire towing lines should be used. Although artificial fiber lines could perform satisfactorily, the use of these must be carefully monitored to ensure that they are replaced both regularly and whenever severe usage could have led to thermal damage.

Tug Deployment The stern tug should be attached throughout the Canal transit so as to be ready to act at immediate notice and not be subject to uncertainty or delay, as might be the case in poor weather, at night or even at all times now that tanker crews are continually being reduced in number.

The bow tug should run free some 200 meters ahead of the ship, ready to be called in to assist as the bow falls off during the latter stages of stopping.

Transit of large vessels in 2.5 knot head or stern currents should be avoided whenever possible, and transit of laden VLCCs should not be permitted under wind speeds in excess of 25 knots.

For ULCCs in ballast, a limit of 20 knots should apply.

Stopping procedure would normally involve reducing the ship's speed without the use of tugs until a speed of about 11 km/hr is reached. At this stage, the stern tug could safely be brought into use. To avoid severe interactive effects the bow tug should not be used until the ship's speed has reduced to about 7 km/hr, at which time it could push or take up the snatch line and be able to pull or push.

Once stopped, the tugs can assist the ship over to the bank, the bow tug by pushing on the forward shoulder and the stern tug, still attached, by pushing against the outboard leg of the bridle which should be bearing against the vessel's stern.

From an analysis of the results of the trials, it was possible to give an indication of the gap which should be left ahead of large ships in a convoy. The separation between ships is defined in the Suez Canal Rules of Navigation in terms of a minimum time interval to be maintained ahead of various classes of ships.

The basic criterion for ship separation is obviously a distance, but in practice a time interval is easier to apply and check, though it is only relevant at one speed, conveniently the normal convoy speed.

Maritime Reporter Magazine, page 44,  Dec 15, 1980 PORT AUTHORITIES Directors

Read Special And Unique Tugs in Pdf, Flash or Html5 edition of December 15, 1980 Maritime Reporter

Other stories from December 15, 1980 issue

Content

Maritime Reporter

Maritime Reporter & Engineering News is the largest audited circulation publication serving the global maritime community.