Page 38: of Maritime Reporter Magazine (December 15, 1980)

Enlargement Of Suez Canal Requires

Special And Unique Tugs

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

Since opening in 1869, enlarge- ment 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 re- opening on June 5, 1975. Through- out this time, plans were being made by the Suez Canal Author- ity (SCA) for the eventual re- opening with account being taken of the changing pattern of world shipping, particularly tankers, in order to plan the future devel- opment of the waterway.

As soon as the Canal was re- opened, work commenced on en- largement of the cross section to 3,300 square meters to accom- modate vessels of 16.2 meters draft. Since the capital invest- ment required for such a scheme is enormous, SCA sought the as- sistance 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 after- ward by a French team led by

Sogreah. It is noteworthy that both teams reached similar con- clusions independently and that these largely agreed with the program envisaged by SCA. Fol- lowing the first phase of develop- ment, 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 per- cent 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 pro- peller not fully submerged. Once the first phase is open, the Canal will be able to accommodate laden 120-150,000 dwt tankers and UL-

CCs in ballast of up to 375,000 dwt. The intention of the second phase is that it would accommo- date laden 260,000-dwt VLCCs.

Canal capacity is strongly de- pendent upon the gap left be- tween ships in a convoy, partic- ularly VLCCs. Accordingly, as part of the feasibility study some 1:40-scale trials were made with a model VLCC, and it was con- cluded that tug assistance would be required if convoy intervals were to be safely kept within rea- sonable 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 at- tached except when towing a pon- toon or a "dead" ship.

With the advent to the Canal of much larger tankers having a greatly reduced power to dis- placement ratio, it is not un- reasonable 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 mat- ters associated with the stopping and mooring of large ships in the two phases of Canal development.

This study was awarded to a con- sortium of British consulting en- gineers comprising Maunsell Con- sultants Ltd., and Rendel, Palmer & Tritton.

The scope of this work in- volved 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 var- ious trails. Study of mooring pro- cedures and line handling was the main responsibility of Captain

Colin McMullen & Associates, who also assisted in the field and sim- ulator trials. Simulator modeling and operation were undertaken at the Swedish Maritime Research

Center (SSPA). Other mathemat- ical and laboratory modeling of vessel interaction and mooring- line forces were performed by the British National Maritime

Institute (NMI). The trials tank- er was provided by Shell Inter- national 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 duck- peller 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 pres- sure, care must be taken to main- tain the alignment of the ship parallel to the axis of the Canal; 2. One tug aft on a bridle pro- vided the most compact and ef- fective 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 un- der 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 rep- resent the major cause of diffi- culty 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 simu- lator 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 bol- lard pull of about 60 tons.

Tug Performance

One of the main objectives of the simulator study was the eval- uation of the comparative merits of alternative arrangements for tugs. The task of these tugs will be to control the ship from swing- ing during a stopping maneuver rather than to reduce the stop- ping distance. When the ship has

Figure 1—Tugs 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 spe- cial 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 rud- der-propellers. Figure 1 illus- trates a Voith Water Tractor with two vertical-axis Voith

Schnieder propellers and duck- peller tug with two ducted thrust- ers astern. In view of the expe- rience obtained, the simulator tugs were assumed to be fitted with rotatable rudder-propellers.

Needless to say the maneuver- ing qualities of omni-directional tugs are further improved by the twin-screw arrangement; a com- mon joystick control input actu- ates a certain thrust for each of the propellers to produce the pull or push, or the turning moment desired. The thrust vector dia- gram 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- continued on page 46) *Mr. Ashburner, Maunsell Consultants

Limited, and Mr. Norrbin, Swedish

Maritime Research Center, presented the paper abstracted here before the recent International Symposium on

Ocean Engineering Ship Handling 1980 held in Gothenburg, Sweden. 44 Maritime Reporter/Engineering News