Manoeuvring
Dr J. Bate (Vosper Thornycroft) and Mr P. Redhead (MoD SS221).
Vosper Thornycroft UK Ltd, Victoria Road, Woolston, Southampton, England. SO19 9RR.
Presented at the 11th Ship Control Systems Symposium, University of Southampton, 1997.
Abstract
As part of the hunt class mid-life update a manoeuvrability study has been undertaken. This was to investigate the feasibility and relative manoeuvring performance enhancements of incorporating various alternative rudder and thruster combinations. The study was undertaken in association with other Hunt class mid-life update studies. The strategies of dynamic positioning and position control by manoeuvring were investigated to determine the relative capability of various equipment solutions, and an environmental sensitivity analysis was undertaken. Advice and guidance was sought throughout the study from ship’s operators and thus a practical assessment of the various manoeuvring solutions was made.
The Hunt Class Mid-Life Update Feasibility Study (HMLU FS) was undertaken to investigate the most suitable course of action for upgrading the class. The manoeuvring study was one component of this study, whose aim was to assess the means of improving the manoeuvrability of the MCMVs, whilst serving the requirements of the weapons and ship systems.
It is a well-established fact that the Hunt Class have poor manoeuvring performance, primarily due to the large lateral area forward, this means that a large moment is created due to the vessel's windage. In conjunction, the pump-type bow-thruster is inefficient and delivers little thrust. It is therefore primarily operated at high power levels and is frequently saturated. If the bow thruster is not used then the ship is limited in the number of headings in which it can hover.
A Pre-feasibility study had previously been undertaken by Vosper Thornycroft UK Ltd.[1]; this highlighted the key issues to be addressed, and the scope of the study.
Currently the Hunt Class employs very simple manual hovering instruments; the scope of the study included an assessment of alternative hover enhancement equipment and specifically an Automatic Manoeuvring Control System (AMCS).
The aim of this study was to determine what manoeuvring devices should be considered and to assess, using mathematical models and simulations, what improvements in performance could be achieved. Mathematical models of equipment highlighted by the pre-feasibility study were incorporated, and two separate simulation models were used; a static model and a dynamic model, these are discussed further in Section 2.7.
During the study advice and guidance was constantly given by the ships' operators, thus a practical and realistic approach to the study could be made.
Two philosophies have been established regarding the methods of position keeping [2]:
a) Dynamic Positioning (DP) using thrust from the bow thruster and propeller-rudder combination. In this lower-environment mode the ship is limited to those headings where a quasi-static balance of forces can be achieved.
b) Position Control by Manoeuvring (PCM). This assumes that the bow thruster is not used and the ship’s heading is limited to the Bow Favorable Heading (BFH), determined by the equilibrium of the ship’s propeller-rudder forces and the environment. This method takes advantage of the higher environmental forces, but as a result the Command Desired Heading (CDH) may not be achievable.
The two position-keeping methods have very different effects on the operator. The ships' requirements will depend on the type of sonar to be fitted and on the tactical philosophy employed during prosecution of a mine. For versatility and flexibility, both DP and PCM options should be available.
The type of sonar to be fitted would have a large effect on the manoeuvring performance of the ship. Primarily, the choice would be between a Hull Mounted (HM) and Vertically Deployed Sonar (VDS). The latter having associated cable drag forces to consider. The most significant problem envisaged with the HM sonar was the noise interference from the bow thruster. This noise would be due to water swirling as well as cavitation and aeration noise. This aspect was considered a major driver in the determination of the manoeuvring solution, as the HM sonar performance is massively degraded when trying to ‘look’ through the bow thruster wake. It was also recognized that this situation would also arise when the VDS was operated in hull mounted mode.
During this study the effects of navigation-aid errors (GPS glitches, etc.) where not incorporated in the manoeuvring models. This was because the relative effects of the equipment solutions in the ship were relevant, rather than the specific ship capability.
It was agreed that the limiting environment would be represented by the limiting mine hunting environment, which is sea state 4. The following wind and tide angles were used in the simulations:
a) Wind angles of 0, 45, 90, 135, 180 degrees.
b) Tide angles of 0, 45, 90 and 180 degrees.
Trials in low, medium and high environments were as follows:
a) Low environment: Wind 10: Tide 0 [Knots]
b) Medium environment: Wind 20: Tide 1 [Knots]
c) High environment: Wind 25: Tide 3 [Knots]
The low and medium environments corresponded to the DP control mode, whilst the medium and high environments corresponded to the PCM strategy.
The current position keeping capability of the Hunt class was discussed with operators, and a box size was determined, within which the existing ship could remain. This was then modified to incorporate the Mine Damage Radius (MDR) and sonar classification performance as shown in Figure 2.5.1:
Figure 2.5.1: Mine hunting hover box.
The generic manoeuvring equipment solutions under consideration were as follows:
a) Existing fit (Baseline). This included a pump-type bowthruster and spade-rudders.
b) New bow tunnel thruster (55 kW) and new high-lift rudders.
c) Retractable azimuthing thruster (130 kW) situated forward plus high lift rudders.
The tunnel thruster was of less power than the azimuthing thruster but it was anticipated that this unit could be fitted further forward in the ship, thus similar turning moments could be achieved. Importantly, the tunnel thruster was situated in front of the sonar compartment, whereas the azimuthing thruster was located aft of this space.
During pre-feasibility study work, a simple thruster model had been used in the simulations. During this phase of work it was decided to develop more accurate models [3].
Two methods of ship simulation were employed; a quasi-static force analysis method and a time-based simulation that incorporated a set of semi-empirical equations. These two simulation models were used to assess the potential benefits of the alternative equipment fits.
This model was developed as documented in [4]; it was used to predict the envelope of where Quasi Dynamic Positioning (QDP) could achieve hovering capability. In this initial model an optimization process was incorporated to determine the thrust allocation (rather than applying direct Thrust Allocation Logic (TAL)) and it was assumed that full thrust was available, i.e. there were no noise limitations imposed by sonar listening requirements. The thrust requirements were balanced against the environmental forces, with a small error envelope (hence quasi-static). The following is a breakdown of the components within the model:
1. Control Module. This controls the overall calculation and the pass/fail determination. The most significant elements within this are the search algorithm and selection of threshold value for the residual forces.
a) Search algorithm. This was based on a genetic method. A random population was generated, the optimum offspring of which, were selected. The values selected for population size, number of generations and number of offspring in each generation were critical. Numerous sensitivity studies were carried out to select the appropriate parameters.
b) Residual threshold. A residual threshold function was defined which was a function of the fore and aft and lateral residual forces, and residual moment. A weighting function was applied to this, based on the Slow Speed Drive (SSD) power setting.
2. Current Force Module. This assumes a constant current and optionally may include the force on the VDS cable.
3. Wind Force Module. This assumes a constant wind.
4. Propeller Module. Based on [1].
5. Rudder Module. Two options are available, the baseline rudders, based on [1], or high-lift rudders, using the model described in [3].
6. Bowthruster Modules. Three modules are incorporated:
a) Existing thruster. Modeled using data from [1].
b) Tunnel bowthruster. This unit has been fitted in SRMH and a model already existed [1].
c) Azimuthing bowthruster. Based on the model provided by BMT, but with modified KT, KQ curves for low J values.
Results of this method were given on capability plots that show the current angle relative to the ship against wind to current angle. For each position the ship is shown as being capable of hovering or not. The primary use however was in the determination of the capability ratio, i.e. the area that each equipment solution could successfully QDP. The capability ratio of each equipment solution was then compared with the baseline. Results are provided in Section 3.1.
The dynamic modeling was undertaken by Vosper Thornycroft Controls Division (VTC) [5]. This included modular components for the following:
a) Aerodynamic model. This calculated the aerodynamic forces on the ship, including any turbulence and gust components.
b) Hydrodynamic Model. This calculated the hydrodynamic forces on the ship, using semi-empirical polynomial equations in the velocity components and yaw.
c) Propulsor model. A combination of the rudder and propeller models in one module. It incorporated the propeller models (KT and KQ), rudder models, propeller/rudder interaction, paddle wheel effect, hull straightening effect, Slow Speed Drive (SSD) machinery.
d) Environmental model. This incorporated a random wind gust generator. Wind gusts fluctuated by up to 20% of average, with shifts of 15º RMS.
e) Machinery model. To reflect engine and equipment response times.
The azimuthing thruster and the high-lift rudder models were incorporated, but it was established that the tunnel thruster option was not viable due to ship-fit considerations.
The dynamic simulation model was used to assess the DP and PCM capabilities of the ship. The results were plotted to show the movement of the ship on a grid, with the hover box marked. In PCM mode, the ship was always started outside the hover box at a given position and heading, with 2 knots ground speed. This was to simulate a situation in which the ship started from a low-speed track-keeping mode and entered a hover box on one side of the track. The test allowed 1000 seconds for the ship to enter the hover box, and the ship had to maintain position inside the box for a 600 second period once inside. Success or failure was recorded for each test, as well as graphs of position, heading and machinery settings.
In the DP mode the vessel was started on the hover point, on the CDH, with zero ground speed. Pass or failure was determined by whether the vessel could remain in the hover box for 600 seconds. The range of CDH over which the vessel was capable of hovering was also assessed.
The TAL for this model was based on a ‘best operator’ algorithm, which could hold the hover position competently (so that the test was of the equipment, not the algorithm), it was automated, so that the tests were consistent and repeatable.
The following is a summary of the results and observations [4]:
|
|
|
Environment 2 |
Environment 3 |
||
|
Bowthruster |
Rudder |
Capability Ratio % |
% Baseline |
Capability Ratio % |
% Baseline |
|
Existing |
Existing |
38 |
100 |
9 |
100 |
|
|
High-lift |
42 |
111 |
12 |
133 |
|
Tunnel |
Existing |
45 |
118 |
10 |
111 |
|
|
High-lift |
50 |
132 |
13 |
144 |
|
Azimuthing |
Existing |
60 |
158 |
27 |
300 |
|
(Free) |
High-lift |
62 |
163 |
29 |
322 |
|
Azimuthing |
Existing |
64 |
168 |
26 |
288 |
|
(Lateral only) |
High-lift |
67 |
176 |
29 |
322 |
A sample output plot is given in Figure 3.1.1.
In all cases the limitation was due to lack of side-force. The thrusters can usually provide enough thrust to maintain the ships heading, and fore and aft forces were not a problem.
It was seen that to counteract the environmental forces most effectively (in DP) a thruster should be fitted at the end of the ship facing the environment. This unit would then resist the lateral force and provide the necessary moment on the ship. In other words, the ship should be made to ‘hang’ into the environment.
The relative capability improvement offered by these thrusters is as follows (original rudders used):
|
Bowthruster |
Relative Capability % |
|
|
|
Environment 2 |
Environment 3 |
|
Existing |
100 |
100 |
|
Tunnel thruster |
118 |
111 |
|
Fixed azimuthing |
158 |
300 |
|
Free azimuthing |
168 |
288 |
The effect of changing the rudders depends on the bowthruster fitted. It can be seen from the table below that the benefit of high-lift rudders is reduced as the amount of bow thrust is increased.
|
Bowthruster |
Relative Capability % |
|
|
|
Environment 2 |
Environment 3 |
|
Existing |
111 |
133 |
|
Tunnel thruster |
132 |
144 |
|
Fixed azimuthing |
163 |
322 |
|
Free azimuthing |
176 |
322 |
The difference in rudder performance is not significant when the ship's stern is held into the environment. It was therefore realized that the choice of rudders was a secondary feature in improving hovering performance.
The PCM component of this baseline simulation (current ship configuration) resulted in 0/2 passes in the low environment, 7/10 passes in the medium environment, and 9/10 passes in the high environment [5].
The results of the baseline DP simulation showed that there was no problem in achieving DP in the low environment. In the medium environment the vessel was also able to DP, although with some limitation on CDH. In the high environment the vessel failed to DP in 3/5 simulations [5].
Analysis of the baseline PCM simulations showed that the failures occurred in the transition phase, i.e. as the vessel entered the hover box it would overshoot whilst attempting to reach the BFH. The PCM capability clearly improved as the environment increased; this is not only due to the better balance of forces achievable, but also the propeller wash was increased, making the rudders more effective. Machinery cycling was seen to decrease in magnitude, but increased in frequency as the ship responded more swiftly to adjustments.
Results of the DP simulations showed that the machinery cycling frequency increased as the hovering accuracy improved. The vessel tended to oscillate from side to side about the CDH. It was seen that the bow thruster was frequently at it's RPM limit. The limit on DP capability (CDH capability compared to BFH) was limited by the bow thruster size.
The PCM simulations were conducted with the high lift rudders incorporated [6]. Results showed a marked similarity to the baseline results, with one additional pass in the high environment. The high lift rudders enabled the craft to reach the hover point more rapidly. The high lift rudders had little effect in terms of position error from the hover point and deviance from the BFH.
The DP simulations showed that with the azimuthing thruster incorporated, the vessel was capable of maintaining the CDH in all conditions tested. Figure 3.2.2.1 shows the BFH offsets at which the vessel was able to DP successfully.
Having established the relative performance of the current and azimuthing bow thrusters, it was decided to incorporate the cavitation noise limitations.
Following discussions with the ship’s operators it was possible to determine the thrust levels at which the current thrusters pump cavitation (and possibly aeration) noise left the present sonar ineffective. The azimuthing thruster was assumed to be noise limited at 250 RPM in the bollard-pull condition, when tip-vortex cavitation was predicted to commence.
Both QDP and dynamic DP simulations were undertaken to assess the effect of this noise limitation.
It can be seen in Figure 4.1.1 how the ship's capability ratio varies with thrust, for the various thruster and rudder options [7],[8]. Two vertical lines are drawn to represent the noise limits.
Figure 4.1.1: QDP Comparison of Manoeuvring Solutions.
The effect of the high-lift rudders in both environments is distinctly shown by the vertical displacement of the two curves for the azimuthing thruster. It can be seen that the effect of changing the current bow thruster to an azimuthing thruster is to increase the capability area ratio by 33% in environment 2, and by 19% in environment 3.
In environment 2 (where DP is most required) there is a very small increase in capability area ratio beyond the 250 rpm noise limit, about 2.5 %. Initially, this could imply that the unit is oversize for the application, however there is a margin against cavitation and other possible forms of noise production that make this unit preferable.
These simulations were undertaken in the same manner as the previous dynamic DP simulations, except that the azimuthing thruster was noise (RPM) limited [9].
Results showed that the vessel was capable of hovering at an offset from the BFH in all but one of the environments tested. Other results showed similar trends to those previously recorded. Figure 4.2.1 shows that in the medium environment the noise limited thruster did not offer any significant performance improvements (in terms of deviation from BFH) compared to the baseline. In the high environment the noise limited thruster gives considerable performance improvements over the baseline.
Having undertaken the aforementioned simulation studies, a reasonably comprehensive understanding of the relative DP behavior of the ship was gained. A further study was required to ascertain the performance of the vessel in PCM control mode as a function of wind speed, tide speed and wind to tide angle. This would identify the environmental limits of the PCM control mode [10].
For these simulations the machinery fit included the addition of high lift rudders. The model was used as described in Section 2.4 and Section 2.7.2, with the following environmental conditions:
1. Wind Speed 0 to 40 Knots, in 5 Knot increments.
2. Tide speed 0 to 3 Knots, in 0.5 Knot increments.
3. Wind to tide angle 0 to 180 degrees in 20 degree increments.
The hoverbox pass/fail criteria were used and the simulation was started on the hoverpoint, on the BFH with zero ground speed.
The simulation runs were analysed and a series of graphs were produced. The graphs illustrate PCM hover performance as a function of wind and tide speeds, for wind-tide angles of 0 to 180 degrees. Figure 5.1 illustrates these results. On each graph the bright areas indicate environments under which the vessel had most difficulty hovering, or failed the simulation.
The patterns show that high wind speeds, combined with low tide speeds are very difficult to cope with. It was seen that the bright (difficult) areas do not extend below about 20 Knots of wind, even with slow tides. This suggests that the low environments do not present a problem to PCM so long as the wind is light. Furthermore, the bright (difficult) areas are generally larger when the wind-tide angle reduces, i.e. wind opposing tide.
1. The Manoeuvring study provided a variety of useful data pertaining to the manoeuvrability of the Hunt class MCMVs. This data could be incorporated in the COEIA model for appraisal and incorporation in the final ship-upgrade decision process.
2. Section 3.1, Section 3.2.1 and Section 4.1 showed that the current bow thruster is totally inadequate to control the ship's position in QDP and DP. Furthermore, Section 4.1 shows that as the bow thruster rpm are reduced, the ship's capability to QDP is rapidly degraded.
3. If increased hovering capability was required then the azimuthing thruster should be fitted. Section 4.1 shows that this thruster can achieve large increases in Capability Area Ratio (CAR), and can do this at reduced rpm, thus avoiding noise from tip-vortex cavitation.
4. Sections 3.1 and 3.2.2 indicated that high-lift rudders and the associated steering gear that could operate to ±65 degrees should be fitted. Although they do not provide a significant increase in CAR, they would certainly reduce the requirement for differential thrusting of the main propellers, thus improving the noise characteristics and also reducing machinery cycling.
5. The environmental sensitivity study discussed in Section 5 showed that generally, hover capability in PCM decreases with wind speed, and increases with tide speed.
6. The profile shape of the ship is fundamentally important for hovering. A good balance of aerodynamic forces to try and keep the transverse aerodynamic centre of pressure amidships is important. The Hunt class is particularly limited by the large windage forward. The PCM simulations described in Section 3.2 and Section 5 highlighted this.
7. Ideally, the bow thruster should be as far forward as possible. This would allow the largest possible moment to be generated for heading and control; the ship could then ‘hang’ into the environment. The limitation of this configuration is that the thruster wake could cause noise for the sonar. Flow noise was identified as a problem, even if cavitation and aeration do not occur.
References.
1. Courts, M.D. and Winter, N.J. ‘Hunt Class Mid-Life Update: Methods Used and Results Obtained for MCMV Hovering Assessment’, VT Library No. D/92-777, September 1992.
2. Bate, J.P., ‘Hunt Mid-Life Update Feasibility Study: Interim Manoeuvrability Report’, VT Document No. 0068r1aa.doc, July 1995.
3. Dand, I.W., ‘Hunt Class Mid-Life Update: Improvements in Manoeuvrability’, BMT-Seatech Commercial Report Project No. C3005, June 1995.
4. Courts, M.D., ‘Hunt Mid-Life Update Feasibility Study: Calculation of Hovering Envelopes’, VT Library No. D/95-1139, July 1995.
5. Jones, S., ‘Baseline Simulation Report for the Hunt Mid-Life Update Feasibility Study’, VT Controls Report No. 67604/PTN/001, August 1995.
6. Jones, S., ‘Simulation Report for the Hunt Mid-Life Update Feasibility Study: High Lift Rudder and Azimuthing Bow Thruster’, VT Controls Document No. 67604/PTN/002, October 1995.
7. Bate, J.P., ‘Hunt Mid-Life Update Feasibility Study: Manoeuvring Study Final Report’, Document Number 0176r1ab.doc, January 1996.
8. Courts, M.D., ‘Hunt Mid-Life Update Feasibility Study: Calculation of Hovering Envelopes with Reduced Bowthruster Revolutions’, VT Library No. D/95-1140, November 1995.
9. Jones, S., ‘Simulation Report for the Hunt Mid-Life Update Feasibility Study: High Lift Rudder and Noise Limited Azimuthing Bow Thruster’, VT Controls Document No. 67604/PTN/003, November 1995.
10.Jones, S. and Robinson, H., ‘Environmental Sensitivity Study Report for the Hunt Mid-Life Update Feasibility Study’, VT Controls Report No. 6760/PTN/004, November 1995.
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