SUBMARINE MANOEUVRING: CORRELATING SIMULATION WITH MODEL TESTS AND FULL SCALE TRIALS
3. FULL-SCALE TRIALS 1 HISTORY
QinetiQ and its predecessor organisations have been involved in submarine manoeuvring trials for over 50 years. Table 1 lists submarine trials conducted during that period. These are limited to those dealing with manoeuvring aspects; there are of course many other aspects of a submarine's operations which undergo trials.
The description column in Table 1 is a little generic;
Contractor Sea Trials, First of Class and general manoeuvring trials will incorporate a range of standard manoeuvres such as propulsion performance, turning circles and autopilot depth changing. Periscope depth keeping trials are self-explanatory while emergency recovery trials will typically investigate the response to, and recovery from, an after hydroplane jam.
For many of these trials, and certainly the more recent ones, QinetiQ maintains an electronic database of the manoeuvres. The range of parameters recorded during each trial varies but will typically include:
x speed
x roll, pitch, heading x depth
x angular rates x accelerations
x control surface angles x rpm
x range track data (if available)
The method of recording these parameters is detailed in the next section.
3.2 INSTRUMENTATION
Measurement techniques have evolved over the years.
Early records were made to tape or UV paper and had to be post-processed and digitised back in the office. Since 1993 it has been standard practice to capture calibrated data directly to a laptop or PC.
The source of each measurement varies, and the intention is always to be as non-interfering with Ship Systems as possible. Motion data is typically taken from a Ship's System synchro repeater unit and converted to an analogue voltage. Some devices, such as the EM-log, may have spare serial outputs which duplicate the measurement signal. Control surface deflections are typically measured by fitting independent transducers to the rams, while shaft rpm is measured using an independent tachometer.
All these analogue signals require calibration. For the Ship Systems, there are procedures to inject dummy measurements, while the independent transducers require the actual motion of the control surfaces, which is easily enough done when alongside. Calibration of shaft rpm, however, has to be done at sea.
The sources of these measurements are distributed throughout the length of the submarine and all have to be brought to one location for synchronised recording.
Fortunately, RN submarines are fitted with a network of cables specifically for use during trials which solves the problem of passing signals through watertight bulkheads.
Each signal only requires a local cable run to the nearest junction box and from there internal wiring can route the signals to a convenient central location.
A full instrumentation rig, including cable runs and calibration, can take a team of three people around three to four days to install. Recent trials have made use of existing data highways to extract digital data from Ship Systems (e.g. over a MIL-STD-1553 network). This can save a lot of rigging effort, but the data buses do not always carry all the required information.
For some trials, it is possible to gather data using manual records alone, for example, propulsion trials. If records are only required of rpm and speed, these values can usually be noted by hand from local displays at a rate sufficient for analysis (from a manoeuvring point of view).
Warship 2011: Naval Submarines and UUVs, 29 – 30 June, 2011, Bath, UK
© 2011: The Royal Institution of Naval Architects
Year Boat Description
Digital data available 1958 HMS PORPOISE Dynamic stability and turning trials
1963 HMS DREADNOUGHT First of Class trials 1964 HMS PORPOISE Manoeuvring trials 1965 HMS DREADNOUGHT Speed trial 1967 HMS VALIANT First of Class trials
1968 HMS RESOLUTION Manoeuvring trials at AUTEC 9 1970 HMS WARSPITE Emergency recovery
1970 HMS OTTER Acceleration and deceleration 1970/1 HMS CHURCHILL Contractor sea trials
1971 HMS REPULSE Emergency recovery and stability 1972 HMS SWIFTSURE Contractor sea trials
1973 HMS SWIFTSURE First of Class trials 1974 HMS SOVEREIGN Contractor sea trials
1974 HMS SWIFTSURE Manoeuvring trials at AUTEC
1974 HMS CONQUEROR Manoeuvring trials at AUTEC 9
1976 HMS SOVEREIGN First of Class trials 9
1976 HMS SUPERB Contractor sea trials 1976 HMS OCELOT Periscope depth keeping 1978 HMS SWIFTSURE Frequency response trial 1981 HMS SOVEREIGN Periscope depth keeping 1982 HMS VALIANT Emergency recovery
1983/4 HMS TRAFALGAR First of Class trials 9
1985 HMS SPARTAN Periscope depth keeping 9
1985 HMS TURBULENT First of Class trials 9
1986 HMS TURBULENT Periscope depth keeping 9
1989 HMS UPHOLDER Contractor sea trials 1992/3 HMS VANGUARD Contractor sea trials
1993 HMS SUPERB Depth keeping trials 9
1993 HMS UPHOLDER First of Class trials 9
1994 HMS VANGUARD Depth keeping / frequency response 9
1994 HMS VANGUARD First of Class trials 9
1995 HMS TRIUMPH Emergency recovery 9
1995 HMS TRIUMPH Trim and compensation 9
1996 HMS SCEPTRE Depth keeping 9
2000 HMS TRIUMPH Manoeuvring trials 9
2001 HMS TRIUMPH Peak motion measurement
2002 HMS TORBAY Post-refit manoeuvring trials 9 2003 HMS TRENCHANT Post-refit manoeuvring trials 9
2004 HMS SPARTAN Depth keeping 9
2005 HMS TRENCHANT Manoeuvring / emergency recovery 9 2006 HMS TALENT Post-refit manoeuvring trials 9
2006 HMS TRAFALGAR Propulsion trial 9*
2008 HMS VIGILANT Manoeuvring / emergency recovery 9
2010 HMS TALENT Propulsion trial 9*
2010 HMS TRIUMPH Post-refit trials 9*
2010 HMS VICTORIOUS Manoeuvring trials 9
* manual records only
Table 1: Submarine manoeuvring trials over the past 50 years
Warship 2011: Naval Submarines and UUVs, 29 – 30 June, 2011, Bath, UK
© 2011: The Royal Institution of Naval Architects 3.3 TYPES OF MANOEUVRES
A typical manoeuvring trial will consist of two phases – the first to establish some basic characteristics, and the second to conduct the more "exciting" manoeuvres. The types of runs are described in the following sections.
3.3 (a) Preparation/Calibration
Initial checks will consist of simple acceleration, deceleration and braking runs in order to determine propulsion performance, as it relates to manoeuvring.
One very important aspect of the analysis of manoeuvring trials is the understanding of the submarine's trim condition prior to any manoeuvre [3].
To assist with this, a real-time Trim Advisory System (TAS) is used during trials to indicate the submarine's condition. The algorithm underlying this observes the control surface angles employed to hold pitch and depth.
The lift from the control surfaces is assumed to be known from the constrained model experiments, and hence any unexpected lift required to maintain pitch or depth can be attributed to a trim or compensation error. Calibrating such a system at sea is achieved by making known trim and compensation changes and correlating these with the resulting changes in the control surface angles. Another test conducted is to maintain a fixed trim condition and accelerate the submarine. If the TAS output does not remain constant then a simple correction can be made which essentially accounts for the lift and moment due to the hull itself.
All submarines are compressible to some extent and lose buoyancy with increasing depth. Over time, the crew will learn how much compensation is required to maintain neutral buoyancy following routine depth changes. For a first-of-class submarine, initial estimates are made theoretically, and these are validated at sea by conducting slow speed trimming exercises over a range of depths.
All the above preparation runs are essential if there is an intention to use the data to validate the mathematical model and simulation codes. Trim condition and compressibility effects must be taken into account when simulating the subsequent manoeuvres.
3.3 (b) Open-loop Manoeuvring
A number of standard open-loop manoeuvres are typically conducted as part of Contractor Sea Trials or First of Class trials.
Turning circles are the most basic type of manoeuvre, and these may be augmented with pull-outs (i.e. return rudder to mid-ships) to measure horizontal stability.
Zig-zags are also a standard manoeuvre and for submarines can be conducted in both the horizontal and vertical planes.
Free-turns are turning circles with no depth control.
These are often conducted to establish a submarine’s natural response in the vertical plane at different speeds.
Pulse manoeuvres, particularly in the vertical plane, are conducted to demonstrate a control surface’s ability to generate a pitch moment.
Frequency response manoeuvres, where the pitch or heading is made to follow a sinusoidal track, are basic system identification tools used to measure the response between a control surface and the subsequent motion.
All these simple open-loop manoeuvres provide useful data against which specific parts of the mathematical model can be validated.
3.3 (c) Pseudo Emergency Manoeuvres
Several of the trial descriptions in Table 1 include the term ‘emergency recovery’. These refer to manoeuvres where the after hydroplanes are deliberately forced to
‘jam’ at a fixed angle, and then the response options to such a scenario are explored. They are described in more detail in [4].
These runs form an important part of crew training, and an equally important part in the validation of the mathematical model. The operational limits placed upon a submarine’s manoeuvring envelope (in terms of speed and depth) are generated as a result of computer simulation predictions of ‘worst-case’ scenarios. Clearly, these limits are never tested at sea but it is necessary to establish that the computer models are not over- optimistic in assessing a submarine’s chances of recovery.
Full-scale mathematical model validation trials have to be performed well within the existing safe limits of operation. The only option for truly exploring the boundaries is to return to a free-running model. First, though, it is necessary to establish the correlation between the full-scale submarine, the physical model and the simulation. One example is described in section 4.
3.4 DEVELOPMENT OF ‘BEST PRACTICE’
Over the years, a great deal of experience has been gained in planning, conducting and analysing trials.
Much of the process is covered in [3] but essentially, once Trials Orders are written, they are subject to scrutiny by a multi-disciplinary board of Suitably Qualified and Experienced Personnel (SQEP). The impact of each planned manoeuvre is assessed and precautionary measures are suggested. If considered necessary, some manoeuvres are practiced in a Submarine Control Trainer (or simulator) to establish the best procedure. The objective is to generate a set of Trials Orders which are clear to follow, provide all the necessary guidance, and deliver the objectives of the
Warship 2011: Naval Submarines and UUVs, 29 – 30 June, 2011, Bath, UK
© 2011: The Royal Institution of Naval Architects trial. The remainder of this section contains a few lessons learned.
One of the greatest issues faced during analysis is having a good understanding of the initial conditions prior to each manoeuvre. This problem is nothing new – the report [5] on the 1958 trial listed at the top of Table 1 states:
"considerably more attention must be given in future to the state of the ship at the beginning of the manoeuvre"
This remains a challenge which is faced today. There is always pressure in the Control Room to "crack on"
through a trials programme and this can often mean that one manoeuvre is begun before the submarine has sufficiently recovered from the previous one. The solution is to state explicitly in the Trials Orders that a specific period of time must elapse between the submarine being declared ready for a manoeuvre and the actual execution.
Real-time time-history displays in the Control Room are a strongly recommended means of communicating the required initial conditions to the crew. During propulsion trials, for example, a five-minute scrolling display of speed will indicate when steady conditions have truly been reached, i.e. the display will show a flat line.
Alternatively, the TAS output can be used to show when a steady trim condition has been achieved. If such observations are made at the time of conduct, it will save a lot of difficulty during subsequent analysis.
The Trials Orders should always plan to conduct repeat runs. This is in recognition that initial conditions will never be exactly the same twice, despite best intentions.
A mathematical model will always give the same output for a given input but the real-life response is likely to vary due to external disturbances. Repeats should be conducted at different times, possibly with different on- watch operators.
For the pseudo-emergency recovery manoeuvres, a particular tool has been developed to aid the repetition process – the stick limiter device. Not all the jams are required to use the maximum deflection, indeed it is part of the safety case that the manoeuvres only increase in small steps from a known response. In order to provide a limited deflection jam, a temporary stop device is fitted which allows the helmsman to rest the control stick against an adjustable lug (Figure 1). This also prevents
‘wandering’ of the control surface during the jam. When full authority control is required, the lug can be flipped out of the way. An advantage of this system is that repeat runs can ensure that the control stick is placed at the exact same deflection each time.
Alongside all the data gathered by computer, equal importance must be given to manual records. There is a great deal which goes on in a submarine’s Control Room
and Manoeuvring Room which cannot be captured electronically. This can range from trimming operations to changes of on-watch personnel, and must include all decisions taken as to why things happened the way they did.
Figure 1: Control surface limiter device An example of the need to conduct repeat runs is given in Figure 2. This was an emergency recovery manoeuvre, with the after hydroplanes ‘jammed’ at 6° to rise. The response was to apply astern rpm in order to reduce the pitch and depth excursions. This manoeuvre was conducted three times over a two-day period.
From the perspective of the actions taken in the Control Room, the procedure followed was identical. However, the resulting pitch and depth trajectories were not. One of the runs (solid line) shows some undesirable control surface activity prior to the manoeuvre, and also the astern rpm being applied in a different way. The other two lines (dashed and dotted) show agreement in the inputs, but not in the outputs. The cause of this may be simply due to different trim conditions (the runs were conducted 48 hours apart) or perhaps a chaotic hydrodynamic effect due to the astern propulsion.
4. FREE RUNNING MODEL EXPERIMENTS