MISSION BASED HYDRODYNAMIC DESIGN OF A HYDROGRAPHIC SURVEY VESSEL

Transcription

1 MISSION BASED HYDRODYNAMIC DESIGN OF A HYDROGRAPHIC SURVEY VESSEL S.L. Toxopeus 1, P.F. van Terwisga 2 and C.H. Thill 1 1 Maritime Research Institute Netherlands (MARIN) Haagsteeg 2, Wageningen, NL 2 Royal Netherlands Navy (RNLN) V.d. Burchlaan 31, The Hague, NL Artist impression of the new hydrographic survey vessel ABSTRACT This paper presents the requirements and design for a new hydrographic survey vessel for the Royal Netherlands Navy (RNLN). Based on the mission of the ship, a dedicated integrated hydrodynamic study was conducted at MARIN to verify compliance of the design with all requirements. Using this approach, an optimum balance between hydrodynamic, operational and economical requirements was found. KEYWORDS Hydrodynamics, Mission Based Design, Hull Form Optimisation, Manoeuvring, Seakeeping

2 INTRODUCTION The operational and technical necessity to replace two existing North Sea hydrographic survey vessels (HSV) of the Royal Netherlands Navy initiated a materiel project for two new vessels. The primary mission of these vessels will be carrying out hydrographic surveys to comply with civil and NATO commitments. These tasks will in particular be carried out in the Netherlands part of the Continental Shelf and in the area of the Netherlands Antilles and Aruba. Secondary tasks consist of general military support tasks, assistance in calamities, and support in typical coast guard operations. The development of a more rational design method (see Wolff (2000)) as well as the operational experience within the hydrographic service of the RNLN have resulted in a clear and complete set of operational requirements regarding the hydrodynamic performance. The approach taken and described has lead to a balanced design between those hydrodynamic performance aspects and cost. THE REQUIREMENTS Propulsion A) Cruising speed at least 13 kn in calm water with 6 months fouling allowance. B) The ship should be able to sail for prolonged periods at low speeds, e.g. 1.5 to 2 knots and at intermediate speeds during surveying, i.e. between 4 and 9 knots. Manoeuvrability A) Good manoeuvrability and course stable in particular for typical surveying speeds (4 9 kn). B) For 95% of the time within 5 m lateral distance deviation during trackkeeping for conditions up to Bf5, 2 kn current and sea state 4 (H sig = 1.88 m, T 1 = 6 s). C) During autopilot sailing, less than 5 heading deviation, in the same environmental conditions. D) The ship should be able to turn 180 and change tracks that are 100 m apart within 5 minutes. E) Unassisted berthing or unberthing should be possible in wind speeds corresponding to up to Bf7. F) Buoy handling in winds up to Bf7 combined with up to 2 kn current, with less than 10 m positioning deviation relative to the sea floor. Seakeeping The ship has to be able to perform measurements during 128 days/year on the Netherlands part of the Continental Shelf (NCS). Given per year a total of 210 sailing days and 30 days transit to and from the measurement location, 10 days loss due to technical reasons and 16 days for secondary tasks, there is a number of 154 survey days available. This leads to the conclusion that the maximum allowable downtime due to ship motions on the NCS equals 17%. The seakeeping requirements for the new hydrographic vessel are much more stringent than for the current survey vessels. The ships to be replaced are on average capable of 103 measurement days/year corresponding to a downtime of 33%. Sea conditions Looking at available wave statistics on the operational area, a probability of exceedance of 17% for the Netherlands coastal waters leads to a significant wave height of 2m. In the more northerly part of the NCS this probability leads to a significant wave height of 3 m, so the requirement of 128 hydrographic survey days can be met if the ship motion behaviour meets the criteria for seastates up to maximum sea state 4

3 Criteria A) Significant vertical motion amplitude of the echosounder/sonar < 1.0 m for heave compensation B) Allowable roll angle of echosounder/sonar and for launching and retrieving RIB < 5 C) Criteria for security / operability of the crew Significant pitch amplitude <3.5 At the location of the RIB / sloop, measurement analysis work spaces, deck spaces for launch and retrieval of buoys and the bridge: Significant amplitude vertical accelerations < 2 m/s 2 Significant amplitude of lateral accelerations <1.5 m/s 2 (on the bridge < 2 m/s 2 ) D) General criteria: Green water over the bow < 30 times/hour, on the aft deck < 1 time/hour Slamming < 20 times/hour DESK STUDY Extensive desk studies have been conducted in the early phase of the project. The first analysis comprised a feasibility study of the application of an existing conventional, single-screw, single-rudder ship for the purpose. Compliance with the staff requirements was verified for this ship. The desk study showed that the existing ship could be adapted in such a way that all requirements were met. The second, more elaborate, desk study was conducted to determine the operability of the ship and which propulsion and steering arrangements could be applied. Pods and thrusters prove to be promising for application to various ships and therefore the idea arose that those concepts might be advantageous. The propulsion and steering arrangements comprised the following concepts: Single-screw single-flap rudder Single pod Twin screw/twin rudders Twin pods Single propeller with wing thrusters arrangements. Seakeeping The main dimensions of the hull form resulted from operability requirements, the required deck width and stability requirements. Operability is defined here as the proportion of time the ship is able to successfully accomplish its missions for given combinations of sea area, speeds, and headings, see Lloyd (1989) and NATO STANAG The complement of operability is referred to as downtime. For the above mentioned ship motion criteria and wave statistics the downtime in bow quartering and head waves is governed by the vertical motion limits of echosounder and sonar. For beam and stern quartering waves, the vertical motions continue to affect the downtime together with lateral acceleration limits. The limiting roll angle affects the downtime in stern quartering waves as well. In a hull form variation study, length, displacement, C p and the draught were varied to investigate their effects on operability. Also, a pram shaped aft hullform was evaluated and applied because of its beneficial effect on vertical motions (Blok and Beukelman (1984) and Kapsenberg and Brouwer (1998)). Finally, the study resulted in the following dimensions and coefficients for further model testing. TABLE 1: MAIN PARTICULARS OF THE SHIP. L pp 75 m 1850 t C b 0.47 B 12.8 m LCB m C m 0.80 T 4 m C wl 0.80

4 The calculations also showed the importance of effective anti roll tanks (ART) for beam and stern quartering waves, which would therefore receive special attention in the seakeeping experimental program. Hull form optimisation and propeller verification The mission profile of this vessel was rather complicated. Good propulsive efficiency is required while at the same time the wake field of the ship has to allow for the design of a low noise signature propeller. Above all, excellent steering ability was demanded. The concept with wing thrusters was believed promising, as the wing propulsors might be sufficient to achieve the survey speed, whereas the central propeller needs to deliver the thrust for normal transition speed. For full speed all three propulsors would be used, filling the gap between 9 and 13 knots by the thruster power enabling direct diesel drives for all shafts. Unfortunately, it was found that the power for the wing thrusters was too little to fill the gap. The single pod arrangement was dismissed because of the high associated costs and minimal advantage of applying a single pod over a single propellerrudder configuration. Finally, the twin screw exposed shaft arrangement failed in efficiency compared to a single screw arrangement, so that the single screw concept was chosen. The shaft power demand of all the four concepts can be found in Figure 1 below. As the cavitation inception speed of the design affects the noise signature to a large extent, special attention was paid to the wake field of the vessel. In this regard, a single screw design usually suffers from a wake peak in the top and bottom position. Aiming at a minimisation of this peak, an open shaft layout was designed. At MARIN, good experiences exist with such a design from former projects and therefore it was applied in this project. Shaft power [kw] Single screw Twin screw Single pod Wing thrusters Speed [kn] Figure 1: Shaft power of the four investigated concepts. The lines of the ship were verified by applying MARIN s potential flow code RAPID. The wave patterns have been evaluated for design speed and survey speed. It appeared that further optimisation of the lines was not profitable. Dynamic Tracking, position keeping and manoeuvring During the desk study, the following calculations were carried out to verify compliance with the requirements, for all steering arrangements: Standard zig-zag, turning circle and reversed spiral manoeuvres, to verify the directional stability and controllability of the ship. Track change ability simulations, to determine the time required sailing from one survey track to the next.

5 Dynamic tracking manoeuvres, to verify the ability of the ship to follow a pre-defined track, in wind, waves and current. Dynamic positioning simulations, to verify the ability of the ship to hold station in wind and current. Based on the standard manoeuvres and the track change ability manoeuvres with the SurSim simulation program, it was concluded that all steering arrangements provided a controllable ship, with a slight preference to podded propulsion. The application of a sufficiently large centre line skeg however, proved to be necessary to ensure the directional stability of the ship. Using dynamic tracking and positioning simulations, conducted with the MARIN program DPSIM, the deviations from the track and steering actions were compared for all steering arrangements. By applying the appropriate controller coefficients, the ship met the requirements, irrespective of the steering arrangement. The difference between the final results of the configurations was small and therefore no conclusions were drawn regarding the best arrangement. Besides the above mentioned simulations, also calculations were conducted to determine the required bow thruster power in order to be able to berth or unberth without tug assistance up to BF7. MODEL TESTS Based on the desk study results, in combination with operational and economic requirements, the single-screw, single-rudder option was selected for the model test phase of the project. The complete model tests program was based on the results of the desk study, concentrating on risk areas and therefore minimising the number of tests. Calm water A paint smear test was conducted in order to position the appendages such that least resistance and disturbance of the flow was achieved. The speed-power relation and propeller hull interaction was determined by means of resistance and propulsion tests. Finally, a wake survey was conducted to determine the essential wake distribution. Figure 2: Result of the wake survey The results presented in Figure 2 were found to be very favourable for a single screw vessel, showing that the solution of exposed shafts even for a single screw vessel is worthwhile, when cavitation

6 inception is an item that needs to be considered. Propeller design Propeller performance calculations were conducted using MARIN s computer programs TIPVCI and DESPRO, together with knowledge from individually and systematically tested propellers, such as the B-series. Parameters that were varied were the diameter, blade area, blade number and pitch (distribution). When the inception speed of tip vortex cavitation was accepted as the determining criterion, a 7 bladed propeller with a pitch of about 1.3 was found to be favourable from cavitation point of view. It was found that a diameter of 2.75 m should be applied. Although the propeller tips touch the boundary layer of the hull, the lower rotation rate in this case is beneficial. Manoeuvring and dynamic tracking The manoeuvring tests were conducted to verify the conclusions drawn during the desk study and to obtain more accurate information regarding the manoeuvring characteristics. As was predicted in the preliminary analysis, the ship complied easily with all manoeuvring requirements. Although not applicable to this ship, the manoeuvring characteristics also complied with the draft IMO Resolution A.751(18) requirements (1993). During the model test phase, also dynamic tracking (DT) tests in wind and waves with correction for current were conducted, to verify compliance with the dynamic tracking requirements. After proper calibration of the DT control system, it was found that the requirement was met: TABLE 2: FINAL RESULTS OF DYNAMIC TRACKING TESTS IN WIND, WAVES AND CURRENT σ track deviation σ rudder angle Requirement exceeded 2.3 m % of the time With the model tests uncertainties during the desk study were removed and the vessel demonstrated excellent manoeuvring and trackkeeping characteristics. It was found that with a single-screw/singleflap rudder arrangement, the dynamic tracking requirements were met, resulting in a cost-efficient solution. Seakeeping In the desk study phase of the design, two anti roll tanks were foreseen, each 2.4 m long. This was also the configuration tested. In a later stage it became apparent that it was impossible to maintain the lower ART in the general arrangement, a decision which was partly compensated for by lengthening the upper anti roll tank by 25%. The model test results on roll damping showed significantly lower roll damping characteristics than the initially predicted roll damping. Figure 3 shows the corrected final downtime analysis for the enlarged anti roll tank. The correction was made using the model test results. By a variation of the GM value in the operability calculations, the lower KG limit was derived from the criterion that at 45 and 60 degrees the 17% downtime should not be exceeded. The seakeeping test results in severe head seas also led to the decision to increase the bulwark and the addition of a breakwater on deck. A comparison with the current survey vessels showed the desired increase in operability for the final design, for the greater part caused by the main dimensions, but also through the optimised hull form as it resulted from the variation study 1. With the exception of beam waves, the operability requirement of 83% is met for all wave headings. 1 The calculated downtime for the current survey vessels was verified with the experience of the hydrographic service, and confirmed for head and bow quartering wave headings

7 G'M = 0.95 m (SHIPMO) Current ships tests with ART criterium following waves Heading [deg] head waves Figure 3: Downtime analysis for enlarged upper ART CONCLUSIONS Traditionally applicable only to propulsive performance, the hydrodynamic design based on clear definitions of operability requirements and mission criteria have made seakeeping and manoeuvring oriented design decisions easier through a quantitative description of performance throughout the design process. By utilising available knowledge, dedicated computational tools and verification by model tests, a balanced design that met all the numerous requirements was obtained. Based on the desk studies, a single-screw single-rudder configuration with open shaft arrangement was chosen for the propulsion and steering of the ship. During the model tests, it was found that this steering arrangement could be used successfully to comply with the dynamic tracking requirements, resulting in a cost-effective solution. For further improvement of the operability an anti roll tank was installed. REFERENCES Blok J.J. and Beukelman W. (1984). The high speed displacement ship systematic series hull forms seakeeping characteristics, SNAME Transactions. IMO Resolution A.751(18) (1993), Interim Standards for Ship Manoeuvrability. Kapsenberg G.K. and Brouwer R. (1998), Hydrodynamic development for a frigate for the 21 st century, PRADS 1998 Proceedings Practical Design of Ships and Mobile Units, Elsevier Science, NL. Lloyd A.R.J.M. (1989), Seakeeping: Ship Behaviour in Rough Weather, Ellis Horwood Ltd., UK. STANAG 4154 (1998, Edition 3), Common procedures for seakeeping in the ship design process, NATO Standardisation Agreement, unclassified. Wolff P.A. (2000), Conceptual design of warships, ISBN