According to Germischer Lloyd (GL), their new "BEST-plus" Aframax crude oil tanker design shows that opportunities exist to push green shipping concepts through the use of advanced design technologies.
Before the introduction of the BEST-plus design, little attention had been paid to Aframax tankers in terms of their performance over their lifecycle, in particular with respect to fuel-efficiency, said GL Although oil tankers are considered to be among the most energy efficient vessels today, their fuel efficiency had not improved in the last 20 years despite general improvements in ship systems.
The recently developed Energy Efficiency Design Index (EEDI), which is expected to become a mandatory standard for shipbuilding in the future, is a simple measure of a vessel's inherent fuel efficiency and compares CO2 emissions to transport work. A reference EEDI is determined by IMO for each type of ship.
A particular ship's EEDI can then be measured against this reference. Future newbuild ships would then have to meet, at a minimum, this reference EEDI. This reference line would then be tightened, over time, as determined by the IMO. An individual vessel's EEDI can also be benchmarked against a competitor's vessel or against vessels within the operator's own fleet.
The Aframax tanker design approach used an advanced optimisation environment by integrating software tools to predict required propulsion power, stability, oil outflow index, cargo capacity and hull structural scantlings according to IACS Common Structural Rules. This was achieved by linking the Friendship-Framework with ShipFlow, NAPA and Poseidon, and by using parametric models for the hull form, layout and structure. Related design parameters were systematically varied and approximately 2,500 design variants were generated and assessed.
Compared to the reference design, an existing pre-CSR tanker, a seven percent improvement in cost of transport is claimed to have been realised due to the better hull form for the best design variant. Determining the "best" variant for the final design depends on the weighting of the different optimisation targets. Depending upon the choice of the designer, a design optimised for oil outflow, EEDI, or for cost-of-transport might be selected.
Optimised hull increases speed, improves cargo capacity
The optimisation of the hull form resulted in a speed of 15.6 knots at design draught with a 95 percent confidence interval. The speed at ballast draught of 7.4 metres is 16.8 knots. This represents a favourable speed increase when compared with recently built vessels of the same size. Cargo capacity was also improved to "best in class" levels. With a standard main engine for Aframax oil tankers, a MAN 6S60MC-C, the fuel consumption of this vessel is comparable to similar vessels.
This combination of high speed and large cargo capacity means that the vessel would easily meet future expected EEDI requirements. The attained EEDI value is 84 percent of the latest reference value for ships of this size, most likely putting the design in compliance even with the IMO's first set of reductions to the EEDI reference line.
Double hull improvements improve safety, reduces environmental impacts
To reduce the environmental impact of accidents, the double hull side width was set to 2.65 metres, reducing oil outflow. The inner bottom of the cargo oil tank 1 was raised from 2.10 metres to 2.75 metres to further reduce cargo tank penetration in grounding events. To ensure structural continuity, an inclined inner bottom is proposed between two frames. The global strength of the hull structure in the cargo hold area was evaluated with finite element analysis according to IACS CSR.
Long term competitiveness – outlook for the design
Although the EEDI will not apply to vessels commissioned before its formal introduction, older vessels will have to compete with newer builds in terms of energy efficiency. According to GL, ships built using the BEST-plus design concept, which exceed EEDI benchmarks, have the ability to remain competitive for longer than non-optimised designs.
