New Carl Büttner vessels benefit from fuel-saving hull design

Efficiency and flexibility

Carl Büttner paid particular attention to the fuel and emissions savings that this Green Project quartet of tankers could achieve. The basic design is that of the Green Dolphin Handysize bulk carrier concept developed by the Shanghai Merchant Ship Design & Research Institute (SDARI), together with their partners. This meets the needs for fuel consumption efficiency and operational flexibility. The design was presented in 2012 and since 2015 various deliveries of newbuildings have proven that this type meets the necessary environmental requirements.

In 2016, New Hantong Shipyard and Carl Büttner approached SDARI to improve the Green Dolphin concept and use it for a Handysize 38,000 dwt chemical and oil tanker. On the basis of SDARI’s first design, a computational fluid dynamic (CFD)-based potential analysis has been conducted by class society DNV GL through its Advisory Service.

During the DNV GL consultancy process a potential for improvement was found in the flow to the propeller. In further CFD-optimisations by DNV GL an asymmetric stern was developed, promising better efficiency.

Computing power

This entire process requires enormous computing power: “We are now using highly sophisticated mathematical models on our High Performance Computing cluster, with about 7000 CPU’s in the basement of the DNV GL building, to develop several hundreds of candidates in parallel,” explains DNV GL head of department – fluid engineering Dr Karsten Hochkirch. When all computer work is done, a real-life scale model of the final design is built and taken to a traditional towing tank to verify the data. According to DNV GL’s analysis, the asymmetric stern will increase the efficiency of CB Adriatic by around 3%.

The model tests at the Hamburg Ship Model Basin (HSVA) confirmed the excellent energy efficiency of the design done by SDARI. As Dr Hochkirch says in his paper ‘Optimization of Ships with Asymmetric Aftbodies’, (Karsten Hochkirch, Bardo Krebber): “High fidelity CFD and CAD design has overcome the natural resistance of shipyards to building innovative hull designs. [There is] no longer resistance to building twisted stern in shipyards and it is possible to directly engineer optimum twisted sterns with higher fuel efficiency gains.”

The DNV GL consultancy models calm- and rough-water simulations using unsteady Reynolds-Averaged Navier-Stokes equations and the mechanical impact across the propeller and the hull.

The optimised hull design is pushed through the water by an equally optimised powertrain. The engine is a MAN Energy Solutions MAN 6S50-C9.6-HPSCR, rated at 6500 kW at 89 rpm and built under MAN licence by STX, which follows the principles of the large-bore engine series with a long stroke that reduces engine speed and allows ship designs with extreme high efficiency.

This lower optimum engine speed allows the use of a larger propeller and is more efficient in terms of engine propulsion. The 6.6-m diameter propeller turns a MAN-Kappel-CP-Propeller VBS 1450 mk 5 which was made in Denmark. The propeller diameter and propeller itself has been optimised in close co-operation between the manufacturer MAN and SDARI, DNV GL and HSVA.

The performance of the engine is closely monitored by a Noris NORIMOS 4 alarm and monitoring system. The LZYN Coriolis fuel meters from Shanghai Yinuo Instrument were chosen for their ability to precisely measure the consumption of the main engine. Combined with a torque meter on the shaft, the crew can monitor fuel consumption, the condition of the main engine and possible influences by the hull, such as marine growth.

To inhibit fouling, the hull is coated with PPG Sigma Sailadvance RX and equipped with anodes supplied by Cathelco.

Scrubber options

In keeping with the emissions reduction theme, the exhaust gas can be further cleaned by selective catalytic reduction systems (SCRs – supplied by Hitz and Yanmar) which are compliant with Tier III of Marpol Annex VI’s NOx requirements and a Saacke exhaust gas cleaning system, to comply with existing sulphur requirements. The auxiliary engines are supplied by Yanmar.

The Saacke exhaust gas cleaning system can reach the 0.1%S limit and is able to operate in closed-loop condition with zero discharge.

The large electric motors for bow thruster, cargo and ballast pumps are controlled by frequency converters. These are used to control the process and not specifically for energy saving. CB Adriatic can efficiently regulate the energy consumption for individual demands by aligning the motor speed. Kawasaki supplied the bow thruster.

Engine cooling systems are designed for tropical conditions. If used in northern regions the cooling capacity and energy consumption of the system is too high. CB Adriatic can efficiently regulate the energy consumption of the sea water cooling pumps by aligning the motor speed, according to the cooling requirements of the engine and the sea water temperatures.

The speed of the scrubber water pumps can be adopted to the required water quantity by frequency controllers.

Kockum Sonics has supplied the computerised Trim Optimisation System which is directly connected to the loading computer. This supports the officers to reach a loading condition with an ideal trim and saves up to 2% in fuel consumption.

On the bridge, Navtor’s advanced voyage planning systems and ENC solutions are used for optimal route planning. A fully integrated bridge system, comprising ECDIS, radar, AIS, depth sounder and other navigation equipment, was supplied by Furuno, which also supplied all the communication and satellite equipment.

When symmetry isn’t always best

The asymmetric stern is not a new idea, but the modern variation is attributed to Ernst Nönnecke, a trained shipbuilding engineer, sculptor and accomplished opera singer born in Hamburg in 1921, with a reputation for an intuitive, somewhat artistic approach to shipbuilding. In the late 1960s he came up with the brilliant idea of designing a ship with an asymmetric stern, to account for the different flow conditions to the right versus left side of the propeller. His concept for optimising flow and propulsion efficiency came more than a generation too early however, as the calculations were hardly possible at the time and model tests extremely tedious. Implementing the design at the yard was even more tricky – and costly.

The feature was revived in the 1960s and again in the 1980s after the oil price shocks of the 1970s. The aim was to reduce fuel consumption and increase efficiency. It seems that each generation of naval architect re-discovers the design advantages of the asymmetric stern, helped along by exponential advances in computing power. Formal parametric optimisation and CAD-based manufacturing have helped decrease the cost of designing and building the asymmetric stern. Recently, a breakthrough in the technology was made, facilitated by high-performing computers and advanced software. “What computers can do so much better than people,” explains DNV GL’s Dr Karsten Hochkirch, “is find the right balance between improved propulsion efficiency and increased resistance.” This can be accomplished only by performing complex iterative calculations for a large number of design variants – a perfect job for computers.