Editor’s Note: A collaboration involving APL, Hyundai Heavy Industries and DNV has resulted in an innovative new hull design expected to make APL’s 10 new 13,800 TEU containerships some 20 percent more fuel efficient per TEU compared with existing designs in the same class. The first of the new ships is expected to be delivered next year by Hyundai Heavy Industries (HHI) where they will serve APL’s Far East to Europe route. For APL, the ships cannot come soon enough, considering the resulting fuel savings are estimated to be worth about 3 million USD per ship per year to APL. Here’s a look at a presentation about the new hull form presented jointly by APL, HHI, and DNV at last week’s SMM conference in Hamburg, Germany.
By Jost Bergmann
When APL decided to order Ultra Large Container Ships in 2011, it was a clear goal right from the start that these ships were to be the most efficient in the world once they were taken into operation. Inspired by DNV’s QUANTUM study, published in 2010, in which the opportunities to design for operating conditions were outlined for the first time in the industry, APL invited tenders from seven shipyards in both Korea and Japan. The invitations to tender requested not only a normal-specification ship design but also the expected performance data of the offered design, covering a possible speed and draft range. This allowed APL to select a design not only from a technical and commercial point of view but also based on the ship’s future performance under operating conditions.
In addition, APL agreed with the shipyard, HHI, which was finally selected for this newbuilding project, that further design-optimisation measures had to be taken even after the contract was signed. One of the key areas looked into in this respect was the adaptation of the hull lines to the future operating profile.
PARADIGM CHANGE IN CONTAINERSHIP DESIGN
In the past, container ships were normally designed and built for relatively high speeds – larger container ships normally for 25kn or more. Designers and owners paid full attention to what the performance would be at contract conditions, which were normally design speed and design draft. The design condition determined key features like the hull form, rudder and propeller design as well as size of the main engine, including capacities and the layout of auxiliary systems. Very little attention was usually paid to the ship’s performance at “off-design” conditions at reduced speed and/or reduced draft.
CHALLENGES OF OPERATING AT “OFFDESIGN” CONDITIONS
However, in the present and expected future market conditions, container ships are mostly operating at these “off-design” conditions, not least due to slow steaming and the varying utilisation of the ships’ deadweight due to dynamics in the trades.
What are the typical consequences of operating container vessels at off-design conditions?
1. Hull – increased resistance due to:
- An emerging bulbous bow, creating adverse wave systems or even breaking waves
- Flow separation, creating additional drag
2. Propeller – reduced efficiency due to:
- Worse inflow to the propeller creating uneven loading, cavitation and other losses
- Sub-optimal loading of propeller and blades
- Underperforming wake field creating additional drag
3. Main engine – reduced efficiency due to:
- An over-dimensioned engine operating at sub-optimal conditions leading to higher specific and total fuel oil consumption
- Low load operation may lead to insufficient performance of turbo chargers so that auxiliary blowers may have to be used – consuming additional fuel and creating additional maintenance
4. Auxiliary systems – reduced efficiency due to:
- Over-dimensioned cooling and lube oil system may have to be throttled
- Auxiliary boiler, shaft generator, waste heat recovery system, etc, may not be fully utilised due to too low load
In order to avoid these disadvantages, future container ship newbuildings need to be optimised for a range of speed/draft conditions which are most likely to occur during later operations.
In a joint workshop, APL and DNV analysed the past and current trading conditions of the present APL ships on the Far East to North Europe trade. Based on this analysis as well as expert interviews within the APL organisation, the possible future operating profile for 13,800 TEU container ships was predicted. The expected speed is in the range of 15kn to 19.5kn and the expected draft is in the range of 11 m to 14.5 m. The top speed for these ships will be about 23kn. Finally, nine speed/draft combinations were identified and time ratios were estimated with regard to how often these conditions will appear.
The purpose of hull-line optimisation is to find a hull form with minimum resistance that requires the least propulsion for the future operating conditions established in the operating profile.
The hull-line optimisation was carried out by experienced engineers using advanced Computational Fluid Dynamics (CFD) tools. In order to cater for both linear and non-linear effects like breaking waves, it was decided to deploy fully 3D RANSE (Reynolds-Averaged Navier-Stokes Equations) methodology solvers. HHI and DNV used the “WAVIS” and “Star-CCM+” software tools for this purpose.
As a starting point, an electronic 3D geometric model based on the initial hull form needs to be generated. The model’s performance is then analysed by conducting “virtual tank tests” at the speed and draft conditions stated in the operating profile. By evaluating the required propulsion power in combination with the virtual wave pattern and the pressure distribution along the hull areas, measures can be identified and used to reduce the resistance for each of the conditions. Modifications of the hull shape need to be considered in a holistic way because what is beneficial for one condition may be adverse to another one.
After the identified modifications have been implemented by changing the shape of the 3D geometric model, the testing needs to be repeated in order to compare whether and by how much the performance has improved.
Based on an initial hull form suggested by HHI, the yard and DNV experts each proposed how the lines could be modified in order to improve efficiency with a focus on the targeted operating profile. Both the pros and cons of the two options were discussed at a joint workshop with a representative of APL. Finally, the owners opted for a version giving an extra improvement in propulsion efficiency despite of a little less container intake.
The main modifications have been to the design of the bulbous bow and hull lines in the forward part of the ship.
16% FUEL SAVING
For a speed range of between 15kn and 18kn, the fuel efficiency per TEU of the future APL design will be 20% to 30% better than that of existing designs. For the nine operating profile conditions, the required propulsion power could be reduced by about 16% on average. This corresponds to annual fuel oil savings of about USD 3 million per ship, assuming USD 700/t fuel oil costs and 280 sailing days.
The results of the CFD analysis were verified by model tests carried out in the Hyundai Maritime Research Institute model basin during September 2011. The maximum difference identified between the calculated and tested speed/draft conditions was in average less than 2%.
The final verification will be carried out during sea trials to be held in 2013 when the first vessel is delivered.
Based on the reduced power demand for the optimised hull form, the main engine output and propeller design as well as the performance of the auxiliary systems could be optimised, further improving the energy efficiency of the newbuildings.
NEW WAY OF COOPERATION
The design optimisation process for this series of ship represents a new way of cooperation between the owners, yard and class society and is expected to be the “new norm” in the future.
These newbuildings will support APL’s strategy of being a commercially successful and environmentally sustainable company, demonstrate HHI’s capacities as the world’s largest shipbuilder and show the extra value that class society DNV can bring to its customers.