ADVANCES IN TANKER PROPULSION
A major VLCC newbuilding program is foreseen by market analysts who estimate that as many as 300 ships of this size must be replaced up to the end of the decade and into the next century. It should be recalled that practically no new VLCCs and ULCCs were built from the mid-1970s until a couple of years ago and that the existing ships are coming up for their fourth and fifth surveys, when extensive renewal of steelwork is likely to be indicated. They will, moreover, have difficulty in meeting the stricter legislation and even more stringent OPA '90 requirements. Charterers are imposing more onerous conditions which may be hard to fulfill and it has been reported that the oil majors have a "black list" of owners/operators and their ships which are unacceptable. Today's basic requirement for propelling machinery best suited to a VLCC of around two million barrels (280,000 dwt) capacity is for an engine of 20 to 30MW (broadly 27,000 to 40,000bhp) according to the speed and margin required, but at very much lower revolutions than has hitherto been normal; this to take advantage of the higher propulsive efficiency to be gained by a well-designed, very large diameter and slow-running propeller.
The capital cost must be competitive, implying a low number of cylinders, and the fuel consumption curve flat and low over a wide range of speeds, typically 55 to 75 rpm. The main propelling machinery of tankers, as with that for most other types of ships today, can no longer be considered in isolation and choice must take into account the secondary demands for energy on board: for electric and hydraulic power generation, cargo pumping in port and ballast discharge in port and transfer at sea. This is among the main reasons why steam remained dominant in the tanker field for so long after it became possible and practical to install single diesel engines of appropriate power.* The main boilers also provided steam for turbo-generators, cargo and ballast pump turbines, cargo tank heating and also flue gases of low oxygen content for ullage volume inerting. Weighed against the steam turbine was a specific fuel consumption inherently inferior to that of the diesel engine; a characteristic aggravated during the 1970s and 1980s when periods of "slow steaming" became necessary while markets were found for the oil they carried.
The first fuel crisis of the early 1970s resulted in an immediate and total halt in large tanker newbuilding, and many cancellations. Only a handful, already too far advanced to be stopped, were delivered in 1976-77.
Except for oil/ore and OBO-carriers (which are less frequently ordered today anyway), tankers spend half their sea time sailing in ballast with the machinery developing relatively lower power. The development of modifications to be incorporated as standard has engaged the lead- ing manufacturers in recent years to enable the engines to provide near-optimum economy over a very wide range of powers. Previously only modest advantage could be obtained by fitting "slow steaming" fuel injection nozzles prior to such a passage. Now, the necessary alterations to injection timing can be performed without stopping the engine.
The successive fuel crises with their dramatic rise in ship-operating costs have had, ironically, a very beneficial effect on the development of the large crosshead diesel engine. For decades during which oil was cheap and a relatively low factor in operating costs, the improvement of fuel consumption had not been a pressing requirement, with only year-by-year minimal reductions.
That the latest models are now burning 25 percent less fuel than they did many years ago, is a measure of the intense and successful effort expended on the problem by designers, metallurgists, turbocharger and lubricant suppliers. Moreover this is a lasting benefit, but the law of diminishing returns makes such progress unlikely to be sustained at such a rate.
This has been achieved by employing much higher working pressures and temperatures, calling, in turn, for close attention to the fuel injection processes, controlled cooling of critical areas, and economical distribution of cylinder lubricant precisely when and where it is required. Long life of wearing parts is a factor closely considered today, as is simplicity and ease of maintenance, important in view of the much-reduced number of skilled and knowledgeable engineering staff carried at sea.
The thermal efficiencies of these latest designs of engine is so high that the heat content of the exhaust gas is insufficient, particularly at reduced powers, to make the capital cost of a waste heat boiler recoverable.
Hull Appendages The naval architects were not to be outdone in the general move to higher operating efficiency; not an easy task when dealing with the necessarily full-bodied hull form of a large tanker. Attention paid to the afterbody and water flow to and from the propeller led to the introduction of a number of appendages; full or partial ducts forward and aft of the propeller and fins in way of the bossing to correct straighten the flow of the water rising up in vortices on its approach to the propeller disc.
Others were radial fins fitted to the rudder post, and angled to recover wake energy. The free-running Grim Wheel, mounted on the propeller cap or the rudder post immediately aft of it, makes use of the aero engine fan-jet principle for the same purpose.
Some of these are not new, and can be seen in maritime museums as examples of past inventions which, although promising, did not show sufficient return for full-scale application be- cause fuel was then too cheap to make energy recovery worthwhile. One of the latest developments is a pair of co-axial contra-rotating propellers (CRP), now at sea on two Japanese VLCCs. The CRPs are reportedly returning a gain of over 14 percent; really worthwhile, but at some considerable extra first cost and mechanical complication. Late in 1992 the 258,000-dwt Cosmo Delphinus went to sea from the Mitsubishi Nagasaki shipyard with a 28,000-hp CRP installation driven by a Mitsubishi 7UEC75LSII engine through a Renk Tacke starsimple planetary gear set. The engine drives the after propeller directly and the forward propeller through an outer quill shaft which receives its drive from the toothed annulus of the planetary gear. Last year the Okonishima Maru, of the same capacity, came into service with a DU-Sulzer 7RTA84M engine of 27,220bhp and a propeller pair driven by a star-compound gear of IHI's own design and construction. Ultra-Long Strokes—The "T" Engines The longer-stroke versions of the engines offered by the three surviving crosshead engine design houses have proved very successful in large tankers, but clear indications that even slower propeller speeds would be beneficial for enhanced propulsive efficiency has led to even longerstroke versions for running at revolutions down to less than one per second being designed and offered by New Sulzer Diesel and MAN B&W. Mitsubishi has had such an engine, with a successful sales record, in their portfolio for some time.
These engines have been developed fairly rapidly as they are based on well-tried existing models. Interim research and development has led to their incorporating the latest technology, including electronic control of essential operating functions: timing and duration of fuel injection, timing of exhaust valve operation and of lubricant application to the cylinder liners. This development has been aided by the existence of full-size research engines at the licensor's headquarters, on which long-term tests can establish satisfactory new techniques. The "relaxed" regime of such slow-running engines, all of which are uniflow-scavenged with hydraulically- operated exhaust valves, has enabled the designers to optimize the combustion process to a degree never possible in the past.
New Sulzer Diesel's RTA84T engine, the first example of which, built by Diesel United (DU) of Aioi, their most energetic licensee, will shortly go to sea in a NYK VLCC built in DU's associated major shipyard at Kure, incorporates some of the most significant advances ever incorporated in a diesel engine, large or small (see Fig. 1, page 28). This has not been lost on other operators, and it is a measure of the importance of this step that, at the time of writing, 16 more engines of this type have been ordered for large tankers, building in Korea, Japan and Taiwan; all but one with seven cylinders, and totaling 623,040bhp (457,840 kW).
The brake specific fuel consumption (SFC) of the bare engine (i.e., without a power recovery gas turbine), is 125g/bhp-h (170g/kW-h).
Special attention has been paid to the improvement of part-load fuel consumption, for the reasons noted earlier. VLCCs are engaged on very long-haul, fully-loaded voyages, generally followed by a return voyage in ballast at relatively light engine load, and the prudent owner specifies generous continuous service margins.
RTA84T engines are equipped with a combination of variable exhaust valve closing (VEC) and variable fuel injection timing (VIT) which, together, enable SFCs down to 121g/bhp-h(165g/kW-h), and 118g and 160g, respectively, with an exhaust gas recovery turbine, to be achieved at 70 percent engine load. Load-dependent cylinder liner cooling and lubrication can be expected to extend even more the life of these expensive, but eventually consumable items.
Although the piston stroke is some 8.5 percent greater, the engine is no taller, indeed even shorter than the RTA84M, by the use of shorter connecting rods. Engine height, in any case, is seldom a problem in large tankers.
Three injection valves per cylinder head have shown to be an advantage in engines having S/B ratios approaching 4.0.
A noticeable change in appearance results from raising the camshaft to mid-cylinder height, calling for an extra idler wheel in the gear drive, but with advantages in performance of the fuel injection and exhaust valve actuation events. MAN B&W's S80MC model has proved a very popular engine for VLCCs with speeds of up to about 15 knots, in which the seven-cylinder version of34,650bhp (25,480 kW) at 79 rpm MCR is well suited. The Arosa, first double-hulled VLCC to be built in Japan, is an example of such an installation. The S90MC-T is being introduced for applications where a higher speed is required. This can be accommodated by a sixcylinder engine, or an economyrated seven-cylinder model.
Electric Transmission Electric transmission has returned as a significant medium for powering tankers, specifically those employed on special duties such as relatively short-haul shuttle runs between the loading buoys of offshore oil fields and mainland terminals or transshipment stations. This is not for reasons of expediency, as was the case during WWII when the many hundreds of T2 tankers and troopships built in U.S. yards were fitted with steam turbo-electric machinery because the nation's heavy gear-cutting facilities were totally committed to naval construction.
Electric transmission confers a number of advantages as it can be applied to secondary, but important non-propulsion purposes.
It solves at one step the problem in motor tankers of providing additional sources of power for the vital duties of cargo and ballast pumping, using variable-speed electric motors supplied from the main propulsion generators.
Dynamic positioning and keeping station when loading from an offshore terminal calls for high power being constantly available for applying short bursts of thrust by the main propeller and bow and stern thrusters.
This is technology, on a larger scale, which has been used for some time in research ships and minehunters. It relieves the installation from what has been described as the "tyranny of the shaft-line," enabling a much shorter engine room, as the prime movers and generators can be installed above the propelling motor (see Fig. 2, page 28).
Chevron has had five 40,000-dwt tankers in service for some 18 years with this arrangement of plant, but with GE heavy-duty gas turbines as prime movers.
A number of these ships presently on order have broadly similar machinery: a "power station" consisting of multiple constant-speed diesel-generators which provide energy through a frequency converter to a variable-speed electric motor coupled to the main propeller (which may or may not be of controllablepitch type).
Current is led from the HT busbars to further frequency converters, the output of which is applied to the variable-speed motors driving the cargo pumps and thrusters, respectively, under control from the cargo management space in harbor, and the wheelhouse through a "joystick" controller or satellite position reference when attending an offshore buoy.
A technically more attractive and potentially less expensive proposition is to use multiple (two or four) smaller propulsion motors geared to the single screw.
This would enable much simpler frequency converters to be used and, moreover, the same ones could be applied to the pumping and thruster supplies.
This system would also be attractive for conventional main haul tankers for which dynamic positioning is not a requirement, but high efficiency maintained down to low powers for longish periods could be an advantage.
ABB Energy of Helsinki, Finland is supplying the electrical transmission plant for all of the North Sea shuttle tankers of the next generation, three to be built in Spain and one in Korea.