NEWSWAVE - HSVA

More documents

Recommendations

Info

For the optimisation process two different strategies can be observed today. On the one hand most of the shipyards follow the strategy of increasing the block coefficient without increasing the resistance. On the other hand ship owners and a few shipyards investigate variants with lower block coefficient and therefore lower resistance, especially for vessels in a seaway. Having in mind the actual fuel prices which have almost doubled during the last years, this strategy may be more successful to cover future demands of ship owners and operators. The “QUICK CHECK” of the speed/power characteristics based on HSVA’s database can serve as input for a cash flow analysis comparing different design variants featuring different main parameters. The results of this analysis can be the basis on which decision makers can consider which variant shall be chosen for optimisation of the overall economy of a certain design. SERVICE SPEED AND SEA MARGIN This “QUICK CHECK” includes advise regarding a maximum economical speed for the customers vessel which should not be exceeded in service in order to avoid an excessive fuel consumption. This will help designers at shipyards and in design offices as well as decision makers at shipping companies when selecting main dimensions in an early stage of project development. Figure 2 illustrates that the economic speed of a project vessel (red line) can be expected to be in the range of FN between 0.225 and 0.255. The upper value can be achieved with well optimised lines. For service speed a sea margin of 15% is often applied to the power, independent of the type and size of the vessel and any environmental and operational requirements. Having in mind the increasing fuel prices this simple procedure can no longer be recommended. Instead, the speed/power prognosis of a project vessel should 2 NEWSWAVE 2006/2 Fig. 2 Quick Check of the economic speed take into account the actual environmental conditions expected during normal operation. The “QUICK CHECK” of additional power demand which can be expected for actual environmental conditions (wind, sea state, restricted water) will give an indication if a charter contract based on 15% sea margin as service allowance will fall short, either in respect to the guaranteed speed or in respect to the guaranteed fuel consumption. PROPELLER DESIGN AND PRESSURE PULSES The “QUICK CHECK” of the propeller parameters, the tip speed, the power density and the pressure pulses to be expected for the actual design give an indication if the selected propeller in combination with the selected main parameters of the hull may face problems with respect to efficiency and cavitation induced pressure pulses. In this case the designer should consider not only using potential flow codes for optimising the hull, but also using more advanced numerical CFD tools for calculating the wake field and the expected pressure pulses for the actual design well in advance of model tests. Furthermore this check is helpful to decide upon the necessity of cavitation tests with the design propeller. INTERACTION OF SHIP AND PROPELLER Few shipyards spend time and money to improve the interaction between the ship, the propeller and the rudder. When the results of resistance and self propulsion tests are satisfactory (i.e. the target speed has been reached), the customer is often not willing to improve a wake field of average quality by further modifications to the aft body of the vessel. It is sometimes overseen that a wake field of good quality not only helps to reduce the pressure pulses of the propeller, thus minimising the danger of propeller induced vibrations in the structure, it is also the basis for reducing frictional losses of the propeller by allowing selection of the lowest area ratio Ae/Ao possible. The “QUICK CHECK” of the quality of the wake field based on HSVA’s database will give an indication whether further improvements of the wake field can be expected.
MANOEUVRING CHARACTERISTICS Generally, the higher the block coefficient, the lower the length-breath ratio and the higher the speed in relation to the block coefficient, the higher is the risk to fail some of the IMO criteria for manoeuvrability. The “QUICK CHECK” comparison of the project vessels main parameters with the HSVA database will give an indication what manoeuvring characteristics may be expected, and if measures have to be taken to meet the IMO manoeuvring recommendations and to improve the yaw stability of the project vessel. The following Figure 3 shows the estimated and exact figures for the 1st overshoot angle during 10°/10° zig-zag manoeuvre in our database and the expected figures (red line) for a project vessel. Model tests with different hull form variants of a full block vessel have shown that a variant with a lower resistance is more exposed to the risk being unstable in yaw for a larger range of rudder angles and that for this variant the IMO criteria for the overshoot angles at 10°/10° and 20°/20° zigzag manoeuvres will be exceeded. For hull forms which have demonstrated a supe- rior speed / power characteristic during calm water tests we therefore recommend to consider either additional numerical investigations and/or manoeuvring tests to check compliance with IMO criteria. SEA-KEEPING CHARACTERISTICS Fig. 3 Quick Check of the manoeuvring characteristics Ships main dimensions and the hull form are largely determined by other design factors than ships motions in waves. After the main dimensions and hull form are fixed, there is not much to be done to reduce ship motions or the related derived responses. The “QUICK CHECK” of the most important motion components heave, pitch and roll can give an indication in an early stage of the design, when the main dimensions are selected. The uncoupled, undamped natural periods can be estimated on the basis of a few main dimensions like length, breadth, water plane area coefficient and metacentric height and are compared with the wave encounter periods to be expected for a ship in the operational sea area. As a standard, wave encounter periods of the Baltic Sea, the North Sea, the Bay of Biscay, the Mediterranean Sea and the North Atlantic are checked, but this can be extended to suit the customer needs. In cases, where the natural and the wave encounter periods are nearly the same, violent resonant ship motions may occur. For these cases it is good practice to perform sea-keeping calculations in an early stage of the design or, in a later stage, to perform sea-keeping model tests. In addition to the ship motions and loads, various associated dynamic effects or derived responses can be used in comparison of different designs. OPERATION IN ICE Due to the increasing amount of crude oil exported from Russia a quickly growing fleet of tankers is employed in seasonally ice-infested waters like the Baltic Sea, the Southern Barents Sea (Petchora), and the waters around Sakhalin (Far East). For a number of tanker projects the transit performance in brash ice channels has been proven by model tests. The power derived from the model test was in no case higher than the power calculated by the FSIR formulas. In some cases the tanker models were transiting through the brash ice channel with only 70% of the power calculated by the FSIR formulas. A “QUICK CHECK” of the required engine output based on previous model tests performed at HSVA will give advice if there is a possibility for upgrading the ice class of a project, without increasing the installed engine power. CONCLUSION The “QUICK CHECK” of a customer’s project vessel gives designers at shipyards and decision makers in shipping companies advice if there is potential for optimisation in an early design stage when main dimensions are selected and the feasibility of a project is investigated. And what is also important - the “QUICK CHECK” will take no more time than is necessary to read this article. 2006/2 NEWSWAVE 3
Page 1: NEWSWAVE T HE H AMBURG S HIP M ODEL
Page 5 and 6: Further investigations on turbulenc
Page 7 and 8: During the entire test run, the shi
Page 9 and 10: The oil produced will be transferre
Page 11 and 12: Fig. 5 Non bursting vortex, smooth

NEWSWAVE - HSVA

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?