Document Actions
Efficient coastal tankers
The design objectives for coastal tankers are demanding and sometimes contradictory, maximum volume in minimum dimensions. Operation in coastal service means frequent harbour calls, often through very restricted waterways having high currents and winds. Good manoeuvring capabilities are thus also required and, of course, high system availability to avoid incidents and accidents in case of system malfunction.
How to manage to put all the above in a tanker configuration with good earning capability and at minimum cost?
Main dimensions are normally restricted by the harbours of call, at least length, draft and air draft. If we look at the typical tanker size of 5.000 deadweight tons, about 5500 m3 in tank volume, the main dimensions are: length below 100 m, draft about 6.5 m, air draft 21 m and beam about 16 to 17 m. Beam can be restricted by some extreme locks, but for most of the operational areas beam is not restricted. The main task is to maximise volume in the given length. With a single screw propulsion and rudder arrangement it is possible to go to certain limit values concerning length-beam-ratio and block coefficient, fullness of the hull. Typical values are 5.3–5.5 and 0.78–0.80. Lower length-beam-ratio means problems in course stability and course keeping ability and higher block coefficient will evidently have similar influence as well as propeller included vibrations and noise.
With a single screw configuration it is difficult to exceed these limits, even though the best type of hull form with barge type buttocks is applied with a slender skeg (condola) together with high lift rudder with large rudder area and flap.
Twin-screw arrangement offers an opportunity for a wider vessel within the same overall length. Length-beam-ratio could be even below 5 with a proper twin-screw configuration.
There are basically three alternative twin-screw arrangements available: two open shaftlines with two rudders, twin skeg (condola) configuration with two rudders and two azimuthing thrusters.
Open shaftline arrangement is hardly worth for trying but twin skeg and twin azimuthing thrusters have been built and tested successfully. Both arrangements allow for a higher beam up to 20 m in a coastal tanker of below 100 m in length. Three to three and a half meters wider vessel is very attractive for additional cargo volume, close to 20 % increase. Manoeuvrability and especially course stability and course keeping capability become an issue. Can they be settled with a successful hull form design? It is obvious that turning and steering capabilities and overall vessel handling capabilities are better with the twin azimuth configuration compared to twin skeg or single screw arrangement.
Course stability requires high lateral stabilizing area both in bow and especially in stern; a centre skeg is required. This is an additional third skeg for the twin skeg arrangement and introduces additional resistance, but is absolutely necessary for the low length-beam-ratio configuration. The two skegs are not giving the required course stability. The length of a skeg is also limited to about the position of rudders.
However, the twin azimuthing thruster configuration requires in any case a docking centre skeg, which can be easily extended up to the transom, if so required for adequate course stability. Even though the skeg would extend between the two thrusters required high manoeuvrability can still be reached. There are several road ferries built with this type of skeg arrangement.
Assuming the twin azimuthing thruster arrangement as the most promising and attractive one for overall cargo volume and operational capability other design objectives should be looked at as well.
Machinery arrangement can be made with three basic options: direct diesel driven, diesel-electric, or combination. Direct diesel driven arrangement requires controllable pitch propellers whereas the two latter ones can be fitted with fixed pitch propellers with somewhat better efficiency than with cpp´s.
High redundancy is attained with the twin azimuthing arrangement combined with two to three main engines/generators. Main engines can be built into separated machinery spaces and can be placed further aft than with a conventional single screw arrangement. Additional cargo space is thus gained.
Deltamarin has studied twin azimuthing thruster arrangements for tankers since the early nineties and the most extensive study was carried out in 1994 for Stolt for their Stolt Shearwater class. Three alternatives were compared: single screw mechanical, single screw diesel-electric and twin azimuthing thrusters with diesel-electric machinery.
This study was carried out with same main dimensions, so the advantage and possibility for higher beam was not utilized for this comparison.
Following conclusions could be drawn. Increase of cargo volume by 7 %, the alternative of twin azimuthing thrusters giving the highest volume and conventional single screw the lowest.
Following other areas were compared and differences were found in: investment cost, installation cost, fuel and lubricating oil cost, maintenance and repair and manoeuvrability. All in all the azimuthing configuration was always getting the best overall economy when calculated over the lifetime of the vessel. Several sensitivity studies were carried out with different fuels (MDO, HFO), value of the additional cargo space, value of the better manoeuvrability, value of easier installation and different building method and so on.
Even with all these sensitivity studies the twin azimuthing configuration was found to be the most promising alternative.
However, at the end the decision was made by the owner to go for the single screw diesel-electric arrangement, Stolt Shearwater class built in Italy at INMA Shipyard.
Lengthy discussions and meetings were held and probably the only reason why the twin azimuthing configuration was not selected was purely the lack of references at that moment, in 1994.
Today several tankers are built with different type of machinery and hull configurations: twin skeg, twin azimuthing with mechanical and diesel-electric machinery. Simple barge type hull forms, easy to build are applied.
The wide beam concept is being built by Stena for several large size tanker classes. The wide beam twin azimuthing configuration for coastal tankers is still to be proven. It is obvious that for safety and for environmental reasons the twin azimuthing configuration is very attractive. It is also simple and easy to build and to operate, however, requiring a different approach in her handling, perhaps a typical tug master approach.
For further information please contact:
Markku Kanerva
Director, Marketing and Sales
Email: firstname.surname@deltamarin.com
Main dimensions are normally restricted by the harbours of call, at least length, draft and air draft. If we look at the typical tanker size of 5.000 deadweight tons, about 5500 m3 in tank volume, the main dimensions are: length below 100 m, draft about 6.5 m, air draft 21 m and beam about 16 to 17 m. Beam can be restricted by some extreme locks, but for most of the operational areas beam is not restricted. The main task is to maximise volume in the given length. With a single screw propulsion and rudder arrangement it is possible to go to certain limit values concerning length-beam-ratio and block coefficient, fullness of the hull. Typical values are 5.3–5.5 and 0.78–0.80. Lower length-beam-ratio means problems in course stability and course keeping ability and higher block coefficient will evidently have similar influence as well as propeller included vibrations and noise.
With a single screw configuration it is difficult to exceed these limits, even though the best type of hull form with barge type buttocks is applied with a slender skeg (condola) together with high lift rudder with large rudder area and flap.
Twin-screw arrangement offers an opportunity for a wider vessel within the same overall length. Length-beam-ratio could be even below 5 with a proper twin-screw configuration.
There are basically three alternative twin-screw arrangements available: two open shaftlines with two rudders, twin skeg (condola) configuration with two rudders and two azimuthing thrusters.
Open shaftline arrangement is hardly worth for trying but twin skeg and twin azimuthing thrusters have been built and tested successfully. Both arrangements allow for a higher beam up to 20 m in a coastal tanker of below 100 m in length. Three to three and a half meters wider vessel is very attractive for additional cargo volume, close to 20 % increase. Manoeuvrability and especially course stability and course keeping capability become an issue. Can they be settled with a successful hull form design? It is obvious that turning and steering capabilities and overall vessel handling capabilities are better with the twin azimuth configuration compared to twin skeg or single screw arrangement.
Course stability requires high lateral stabilizing area both in bow and especially in stern; a centre skeg is required. This is an additional third skeg for the twin skeg arrangement and introduces additional resistance, but is absolutely necessary for the low length-beam-ratio configuration. The two skegs are not giving the required course stability. The length of a skeg is also limited to about the position of rudders.
However, the twin azimuthing thruster configuration requires in any case a docking centre skeg, which can be easily extended up to the transom, if so required for adequate course stability. Even though the skeg would extend between the two thrusters required high manoeuvrability can still be reached. There are several road ferries built with this type of skeg arrangement.
Assuming the twin azimuthing thruster arrangement as the most promising and attractive one for overall cargo volume and operational capability other design objectives should be looked at as well.
Machinery arrangement can be made with three basic options: direct diesel driven, diesel-electric, or combination. Direct diesel driven arrangement requires controllable pitch propellers whereas the two latter ones can be fitted with fixed pitch propellers with somewhat better efficiency than with cpp´s.
High redundancy is attained with the twin azimuthing arrangement combined with two to three main engines/generators. Main engines can be built into separated machinery spaces and can be placed further aft than with a conventional single screw arrangement. Additional cargo space is thus gained.
Deltamarin has studied twin azimuthing thruster arrangements for tankers since the early nineties and the most extensive study was carried out in 1994 for Stolt for their Stolt Shearwater class. Three alternatives were compared: single screw mechanical, single screw diesel-electric and twin azimuthing thrusters with diesel-electric machinery.
This study was carried out with same main dimensions, so the advantage and possibility for higher beam was not utilized for this comparison.
Following conclusions could be drawn. Increase of cargo volume by 7 %, the alternative of twin azimuthing thrusters giving the highest volume and conventional single screw the lowest.
Following other areas were compared and differences were found in: investment cost, installation cost, fuel and lubricating oil cost, maintenance and repair and manoeuvrability. All in all the azimuthing configuration was always getting the best overall economy when calculated over the lifetime of the vessel. Several sensitivity studies were carried out with different fuels (MDO, HFO), value of the additional cargo space, value of the better manoeuvrability, value of easier installation and different building method and so on.
Even with all these sensitivity studies the twin azimuthing configuration was found to be the most promising alternative.
However, at the end the decision was made by the owner to go for the single screw diesel-electric arrangement, Stolt Shearwater class built in Italy at INMA Shipyard.
Lengthy discussions and meetings were held and probably the only reason why the twin azimuthing configuration was not selected was purely the lack of references at that moment, in 1994.
Today several tankers are built with different type of machinery and hull configurations: twin skeg, twin azimuthing with mechanical and diesel-electric machinery. Simple barge type hull forms, easy to build are applied.
The wide beam concept is being built by Stena for several large size tanker classes. The wide beam twin azimuthing configuration for coastal tankers is still to be proven. It is obvious that for safety and for environmental reasons the twin azimuthing configuration is very attractive. It is also simple and easy to build and to operate, however, requiring a different approach in her handling, perhaps a typical tug master approach.
For further information please contact:
Markku Kanerva
Director, Marketing and Sales
Email: firstname.surname@deltamarin.com
Deltamarin opens an office in Poland