Computational Fluid Dynamic (CFD) tools can be used in combination with model tests to minimize hull resistance, improve propeller efficiency and interaction with the hull, reduce the required power, improve fuel efficiency and ultimately mitigate the environmental impact from shipping.
The challenge
The shipping industry is focusing more and more these days on mitigating the environmental impact of the world fleet, and bulk carriers are no exception. The Marine Environment Protection Committee (MPEC) of the International Maritime Organization (IMO) has been working around the clock to establish new regulations with respect to several marine pollution topics such as accidental spills, exhaust gases, waste delivery ashore, hazardous materials, ship recycling, ballast water, bilge water, anti-fouling paints and sewage. The significant increase in the world fleet over the past five years combined with the proximity of ship traffic in coastal areas and the general public awareness of global warming has resulted in emissions to air being the single most important item on shipping’s regulatory agenda.
New technology New technologies with respect to cleaner energy sources are being developed very rapidly, but it will still take some time before kites, sails, solar panels and fuel cells become standardized equipment on ships. However, today’s designs can be further improved by using existing technology and proven analysis methods. One way to minimize the emissions to air of new ship designs is to optimize the hull shape, propeller, rudder and the interaction between all of them to improve the ship’s overall fuel efficiency
DNV’s response to the chal-lenge
The Ship Hydrodynamics and Noise and Vibrations sections in DNV Maritime Technical Consultancy are working together to develop new services in the field of ship resistance and propulsion in order to help shipyards, designers and owners improve the performance of new and existing bulk carriers.
CFD tools can be used to numerically simulate the ship’s behaviour in calm waters so as to predict the overall hull resistance. Wave-making patterns can be estimated relatively quickly using today’s computers, which allow for the screening of several conceptual hulls shapes and identification of key areas for improvement in existing designs. The performance trends and possible efficiency gains observed in the CFD analyses can be verified later on using model tests.
Wave patterns and wetted surfaces are very specific to each ship type, so the focus areas for design improvement will also vary according to ship type.
Bulk carriers are characterized by blunt hull shapes with high block coefficients to maximize the cargo area. They typically operate at medium speeds of around 14.5 knots. The combination of shape and speed results in the contribution from frictional resistance (drag effects from the wetted surface) dominating over the wave making resistance. For some loading conditions, the viscous effects from the hull can be as much as 80% to 90% of the overall resistance.
Typical potential areas for design improvements to bulk carriers to minimize the fuel consumption are:
Bow design and loading condition optimization
Minimized hull friction
Aft ship, propeller and rudder efficiency improvement
Fuel efficiency improvements in the hull and propeller can range from 2-3% up to 8-10% but it should be noted that these may vary according to the bulk carrier design as they are directly linked to the maturity of the design and the ship’s actual trade.
Hull frictional resistance
Several research projects are currently under way to produce solutions to minimize the hull frictional resistance in water, such as air lubrication systems and advanced coating systems. While a lot of promising results are coming out of these research areas when applied to some test case ships, the technology is not yet ready to be rolled out to the whole bulk carrier fleet.
Fore ship design When it comes to the fore ship design, the future operating profile should be considered. While it may not be possible to design all bulk carriers for a specific trade, certain aspects should be kept in mind:
Ensure an optimum bow design for different loading conditions – study the combined effect including a shallow draft ballast condition, rather than a single optimized design draft
Avoid unfavourable drafts and trim levels for long voyages
Aft ship design and propulsive efficiency
The greatest potential for improvement in bulk carriers these days lies in the aft ship design and, in particular, in the propulsive efficiency. Because of the high block coefficients, bulk carriers will typically generate quite a non-homogenous wake at the propeller disk which could lead to unnecessary energy losses.
Numerical simulations using CFD, including viscous effects, can accurately predict the wake distribution for each specific hull design, including the water particle velocities and rotations at the propeller intake. This can be used for:
Designing the aft hull shape and appendages (fins, vortex generators) to improve the wake pattern and increase the propeller efficiency
Estimating the hull and propeller interaction effects to improve the propeller efficiency and optimize the propeller clearance
Studying the propeller and rudder interaction to reduce energy losses and drag as well as the selection of energy recovering systems aft of the propeller
The future – and today
In the next few decades, rather radical changes to hull designs and new propulsive systems may be expected in order to achieve the ambitious emission reduction levels proposed for the year 2040 and beyond. But in the meantime something needs to be done about new designs using existing technology. Even today, numerical simulations using CFD tools make it possible to improve the fuel efficiency of both new and existing bulk carriers. This constitutes a viable and proven solution to optimize ship designs for fuel efficiency, and should be part of all future-oriented efforts to obtain a “green design” that reduces emissions to air.
contact information:
octavi.sado@dnv.com
liv.hoem@dnv.com