Section
3 Design requirements
3.1 General
3.1.1 The
arrangement of water jet units is to be such that the ship can be
satisfactorily manoeuvred to a declared performance capability. The
operating conditions covered are to include the following:
-
Maximum continuous
shaft power/speed to the impeller in the ahead condition at the declared
steering angles and conditions.
-
Manoeuvring speeds
of the impeller shaft and/or reversing mechanism in the ahead and
astern direction at the declared steering angles and sea conditions.
-
The ability of
the machinery to reverse the direction of thrust of the propeller
in sufficient time, under normal manoeuvring conditions, and so bring
the ship to rest from maximum service speed. Results of the trials
are to be recorded.
-
Astern running
conditions for the ship.
3.1.2 The
mean loadings are those loadings induced by the water jet absorbing
the mean torque supplied by the prime mover.
3.1.3 Fluctuating
loads are defined as those loads which occur during one revolution
of the impeller due to cyclic variations. For example, the spatial
flow variations and torsional vibration at nominally steady state
operating conditions.
3.1.4 Transient
loads are defined as those loadings resulting from acceleration and
deceleration of the ship, manoeuvring, seaway conditions and other
similar forms of loading. This also includes any significant back-pressure
effects developed from the operation of the reversing bucket, if fitted.
3.1.5 To ensure
self-priming of the water jet unit, the shaft centreline of the unit
is to be lower than the light draught static waterline of the ship.
In cases where this is either impracticable or undesirable, the distance
of the impeller shaft centreline above the ship’s light draught
waterline is to be less than or equal to 10 per cent of the pump inlet
diameter.
3.1.6 Provision
is to be made to allow for the in-service visual inspection of the
complete blade surfaces of both the impeller and stator blades, using
either a direct visual or borescope inspection technique.
3.2 Shaftline
3.2.1 The
diameter of the shaftline components is to comply with Vol 2, Pt 3, Ch 2 Shafting Systems. For calculation purposes,
the shaft carrying the impeller is to be taken as equivalent to a
screwshaft.
3.2.2 Where
it is proposed to use carbon or carbon manganese steel shafts which
may be in contact with sea-water, these are to be protected.
3.2.5 Where
lengths of shafts are joined using couplings of the shrunk element
type, a factor of safety, based upon the mean plus the vibratory and
transient torques, against slippage of 2,0 is to be achieved for couplings
which are located inboard and 2,5 for couplings which are located
outboard.
3.2.6 Where
shaftline components are bolted together, a factor of safety of 1,5
is to be achieved for the design of the bolted connection when considered
in the context of the mean, fluctuating and transitory loadings.
3.2.8 Where
it is proposed to fit a keyless impeller, the fitting is to comply
with the requirements of Pt 5, Ch 7, 3.2 Keyless propellers of
the Rules and Regulations for the Classification of Ships, as applicable,
excluding the requirements for Ice Class. Use of the words ‘propeller’
and ‘screwshaft’ are to be taken as meaning ‘impeller’
and ‘impellershaft’, respectively.
3.3 Shaft support system and guide vanes
3.3.1 In cases
where the shaft requires support from the tunnel walls ahead of the
impeller or, alternatively, where guide vanes are required to assist
the flow around a bend in the ducting system, the supports or guide
vanes are to be suitably aligned to the flow and have suitably rounded
leading and trailing edges or be of an aerofoil section.
3.3.2 In general,
the fillet radius should be greater than or equal to the maximum thickness
of the vane or support at that location. Smaller radii may be considered,
for which the results of an approved measurement programme or calculation
procedure are to be submitted. In all cases, a factor of safety of
at least 1,5 is to be demonstrated for the maximum designed operating
conditions.
3.3.3 A facility
for the inspection of the supports or guide vanes is to be provided
which will allow either direct visual or borescope inspection of these
components and their transition to other members.
3.4 Impeller
3.4.1 A calculation
to determine the stresses within the impeller blades is to be carried
out, which takes into account the mean blade loading, fluctuating
loadings, transient loads and centrifugal force. The computations
may be accomplished by either classical methods or numerical analysis.
Designs of water jet systems which have been based on a combination
of computational fluid dynamics and finite element methods will be
considered. However, it will be necessary to demonstrate to the satisfaction
of LR that the formulation of the methods used has been correlated
with previous full scale measurement or other calculation experience.
3.4.2 For
the purposes of the calculation required by this sub-Section, the
fluctuating stresses during one revolution of the impeller are to
be taken as 20 per cent of the maximum mean stress, and the stresses
from transient loadings are to be taken as 15 per cent of the hydrodynamic
mean stress, unless otherwise specified by the designer.
3.4.3 The
fatigue assessment of the impeller blades is to be based on the stress
in the root sections, excluding the influence of the blade root fillets.
This assessment is to include the following components:
- the maximum stresses derived from the mean loading, including
both the hydrodynamic and centrifugal components;
- the amplitude of the fluctuating stresses during one revolution
of the impeller;
- the stresses derived from transient loading and an allowance for
any residual stresses in the material.
It is permissible to combine the variable components of stress
in a linear fatigue damage accumulation assessment procedure. A factor
of safety of at least 1,5 against fatigue failure is to be demonstrated
for the maximum continuous rating condition or any other more onerous
condition, see
Vol 2, Pt 4, Ch 2, 3.1 General 3.1.1.
3.4.4 In general,
the fillet radius is to be greater than the maximum thickness of the
impeller blade at that location. Composite radiused fillets or elliptical
fillets which provide an improved stress concentration factor are
preferred.
3.4.5 Where
an impeller has bolted-on blades, consideration is to be given to
the distribution of stress in the palms of the blade and in the boss
and bolting arrangements.
3.4.6 Where
an impeller has welded blades, the welds are to be of the full penetration
type or of equivalent strength. Where laser welding is to be used,
details are to be submitted for consideration.
3.4.7 The
blades are to be provided with hydrodynamically faired leading and
trailing edges which may be either of simple radius or of a more complex
aerofoil edge form. The tip clearance, whilst being kept to a minimum
for hydrodynamic purposes, is to be sufficient to allow for any transient
vibrational behaviour, axial shaft movement or differential thermal
expansion.
3.4.8 A calculation
of the blade natural frequency for the impeller blades is to be undertaken.
The fundamental natural frequency in water of the blade is to be shown
to lie outside any expected excitation frequencies within a speed
range of the water jet unit and up to 10 per cent above the maximum
impeller speed.
3.5 Stator
3.5.1 The
stator blades, where fitted, are to be designed to be capable of withstanding
the combined hydrodynamic mean, fluctuating, transient and mechanical
loads, including any loads transmitted via shaft bearings, developed
by the unit and reacted through the blades when the impeller is absorbing
full power. Consideration is to be given to situations when the vessel
is either free running or in a condition specified by Vol 2, Pt 4, Ch 2, 3.1 General 3.1.1 or undergoing stopping, accelerating
or decelerating manoeuvres. A factor of safety against mechanical
failure by yielding of the blades of 1,5 is to be demonstrated.
3.5.2 In general,
the fillet radius is to be greater than the maximum thickness of the
blade at that location. Composite radiused fillets or elliptical fillets
which provide improved stress concentration factors are preferred.
3.5.3 If the
stator ring comprises a segmented assembly, consideration is also
to be given to the distribution of stress in the various adjacent
members of the overall assembly.
3.5.4 A calculation
of the relative blade-passing frequency between the rotor and stator
blades is to demonstrate that this does not coincide with the natural
frequency of the stator blades over the speed range of the water jet
unit and up to 10 per cent above maximum impeller speed.
3.5.5 The
stator blades are to be provided with hydrodynamically faired leading
edges which may have either a simple radius or a more complex aerofoil
edge form.
3.6 Tunnel and securing arrangements
3.6.1 The
tunnel is to be adequately supported, framed and fully integrated
into the hull structure. The critical locations and integrity of the
supports and framing are to be as specified in the Risk Assessment,
see Vol 2, Pt 4, Ch 2, 2.4 Risk Assessment (RA) 2.4.1, and agreed by the
Shipbuilder and LR.
3.6.2 The
tunnel and supporting structure scantlings are to be not less than
the Rule requirements for the surrounding structure. The strength
of the hull structure in way of tunnel(s) is to be maintained. The
structure is to be adequately reinforced and compensated as necessary.
All openings are to be suitably reinforced and have radiused corners.
3.6.3 Consideration
is to be given to providing the inlet to the tunnel with a suitable
guard to prevent the ingress of large objects into the rotodynamic
machinery. The dimensions of this guard, if fitted, are to strike
a balance between undue efficiency loss due to flow restriction and
viscous losses, the size of object allowed to pass and the susceptibility
to clog with weed and other flow-restricting matter.
3.6.4 The
inlet profile of the tunnel is to be designed so as to provide a smooth
uptake of the water over the range of vessel operating trims and avoid
significant separation and/or cavitation of the flow which may then
pass downstream into the rotating machinery.
3.6.5 Design
consideration is to take account of pressures which could develop
as a result of a duct blockage, as well as in relation to the axial
location of rotating parts.
3.6.6 The
strength of the tunnel and supporting structure is to be examined
by direct calculation procedures.
3.7 Nozzle/steering systems
3.7.2 Where more than one steerable water jet is fitted, Vol 2, Pt 6, Ch 1, 4.1 General 4.1.2 is considered to be
met when:
- Each steerable water jet fulfils the requirements for main steering
gear (Vol 2, Pt 4, Ch 2, 3.7 Nozzle/steering systems 3.7.3), and
- Each steerable water jet is provided with the ability to position
and lock the steerable water jet in a neutral position after a failure of its own
power unit(s) and actuator(s). These arrangements are to be of sufficient strength
to hold the steerable water jet in position at the ship's manoeuvring speed to be
taken as not less than 7 knots. Instructions displayed at the locking mechanism’s
operating position are to include a directive to inform the bridge of any
limitation in ship's speed required as a result of the securing mechanism being
activated.
3.7.3 The main steering gear is to be:
- Of adequate strength and capable of
changing direction of the ship’s steerable water jet from one side to the other in
accordance with the declared steering angle limits at an average turning speed of
not less than 2,3 deg/s with the ship running ahead at maximum ahead service speed
which shall be demonstrated in accordance with Vol 2, Pt 1, Ch 3, 13.3 Testing, Vol 2, Pt 1, Ch 3, 17.3 Performance testing requirements for naval vessels
and Vol 2, Pt 4, Ch 2, 7.3 Sea trial requirement; and
- Operated by power; and
- So designed that they will not be
damaged at maximum astern speed; this design requirement need not be proved by
trials at maximum astern speed and declared steering angle limits.
3.7.4 The auxiliary steering gear is to be:
- Capable of being brought speedily into
action in an emergency; and
- Of adequate strength and capable of
changing the direction of the ship’s water jet nozzles from one side to the other
in accordance with the declared steering angle limits at an average turning speed
of not less than 0,5 deg/s, with the ship running ahead at one half of the maximum
ahead service speed or 7 knots, whichever is the greater.
- Operated by power for ships having
propulsion power of more than 2500 kW per water jet unit and for all ships, where
it is necessary to meet the requirements of Vol 2, Pt 4, Ch 2, 3.7 Nozzle/steering systems 3.7.4.(b).
3.7.5 In addition to the requirements in Vol 2, Pt 4, Ch 2, 2.1 Water jet arrangement 2.1.1, for
ships fitted with a single steerable water jet, where the main steering gear comprises
two or more identical power units and two or more identical steering actuators,
auxiliary steering gear need not be fitted provided that the steering gear:
- Is capable of satisfying the requirements in Vol 2, Pt 4, Ch 2, 3.7 Nozzle/steering systems 3.7.3.(a) while
any one of the power units is out of operation; and
- Is arranged so that after a single failure in its piping system or in one of the
power units, steering capability can be maintained or speedily regained.
3.7.6 For ships fitted with more than one steerable water jet, where each main steering system
comprises two or more identical steering actuating systems, auxiliary steering gear need
not be fitted provided that each steering gear:
- Is capable of satisfying the requirements in Vol 2, Pt 4, Ch 2, 3.7 Nozzle/steering systems 3.7.3.(a) while
any one of the power units is out of operation; and
- Is arranged so that after a single failure in its piping or in one of the steering
actuating systems, steering capability can be maintained or speedily regained (e.g.
by the possibility of positioning the failed steering system in a neutral position in
an emergency, if needed). Consideration will be given to alternative arrangements
providing equivalence can be demonstrated.
The above capacity requirements apply regardless of whether the steering systems are
arranged with shared or dedicated power units.
3.7.7 Nozzles can be either of a fixed or steerable form. The design of the
nozzle is to take into account fully the change in pressure distribution along its inner
surface, together with the other mechanical loads (e.g. stator assembly loads) and
transient loads caused by the flow directing attachments which may be reacted through
the body of the nozzle. In this analysis, the changes to the pressure distribution
caused by transient manoeuvres are to be considered.
3.7.9 Consideration is to be given to all transient loads which the steering unit
is likely to experience from manoeuvring, accelerating, decelerating and the sea
conditions.
3.7.10 The nozzle/bucket is to be given mechanical protection by the Shipbuilder
from other impact damage such as collision.
3.8 Bolts
3.8.1 Detailed
consideration and analysis are to be given to essential bolting arrangements
in critical locations, as specified in the Risk Assessment, see
Vol 2, Pt 4, Ch 2, 2.4 Risk Assessment (RA) 2.4.1, and where indicated by the
manufacturer or Shipbuilder and agreed by LR. These are to include
bolts used in the securing of blades or guide vanes, assembly of the
unit in the ship and any conduit components.
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