Section
2 General requirements
2.1 Water jet arrangement
2.1.1 In general, for a ship to be assigned an unrestricted service notation, a
minimum of two water jet systems are to be provided where these form the sole means of
propulsion. For ships where a single water jet system is the sole means of propulsion or
steering, a detailed engineering and safety justification is to be evaluated by LR,
see
Pt 5, Ch 16, 2.3 Calculations and information 2.3.23. This evaluation process will include a risk
assessment analysis using a recognised technique to verify that sufficient levels of
redundancy and monitoring are incorporated in the water jet unit’s essential support
systems and operating equipment.
2.1.3 It
is the Shipbuilder’s responsibility to ensure that all of the
installed equipment is suitable for operation in the location and
under the environmental conditions defined in Pt 5, Ch 1 General Requirements for the Design and Construction of Machinery. Where anticipated environmental conditions are outside
these limits or where additional conditions are to be considered,
such as vibration and impulsive accelerations, requirements and details
of compliance are to be submitted to LR.
2.2 Plans to be submitted
2.2.2 General arrangement plans showing details of the following:
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Shafting assembly indicating bearing positions.
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Steering assembly.
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Reversing assembly.
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Shaft sealing arrangement assembly.
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Longitudinal section of the complete water jet unit.
2.2.3 Detailed and dimensioned plans indicating scantlings, materials of
construction and where applicable surface finish of the following:
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Arrangement of the system, including the intended method of
attachment to the hull and building-in, tunnel geometry, shell openings, method of
stiffening, reinforcement, etc.
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All torque transmitting components, including the shafting system,
impeller and stator if fitted.
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Steering components, together with a description and line diagram of
the control circuit. This is to include steerable exit water jet nozzles where
fitted.
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Components of the retractable buckets where these are used for
providing astern thrust.
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The bearing or bearings absorbing the thrust and supporting the
impeller, together with the method of lubrication.
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Details of any shafting support or guide vanes used in the water jet
system.
2.2.4 Schematic plans of the lubrication and hydraulics required for
steering/reversing systems, together with pipe material, relief valves and the working
pressures required.
2.2.5 The declared steering angle limits are to be stated by the manufacturer for each
steerable water jet system.
2.3 Calculations and information
2.3.1 Strength
calculations based on fatigue considerations incorporating the maximum
continuous torque rating and the most ‘onerous’ operating
condition, see
Pt 5, Ch 16, 2.3 Calculations and information 2.3.12,
including any short-term high power operation, and including the effects
of mean and fluctuating loads, transitory loadings, residual stress
allowances, and stress raisers, for the following components:
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Impeller, stator
and any bolting arrangements supporting propulsion or steering loads.
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Shaft supports
and coupling arrangements.
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Inlet guide vanes,
if fitted.
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Steering components,
including the lugs of steerable nozzles where fitted.
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Retractable buckets
and associated mechanisms which are used to provide astern thrust.
A calculation of the hydrodynamic transient loads is to be made for
each design and is to include the full ahead to full astern condition.
The calculation procedure used is to be supported, where possible,
with full scale or model test data, or satisfactory service experience,
to validate the design method.
2.3.2 Calculations
supporting the connection method of the impeller to the shaft, including
details of the fit, push-up, securing, bolting arrangements, etc.
In addition, where lengths of shafts are joined using couplings of
the shrunk element type, full particulars of the method of achieving
the grip force.
2.3.6 Calculations
of the tunnel strength and supporting structure.
2.3.7 A calculation
to determine the stresses within the impeller blade.
2.3.8 A calculation
of the blade natural frequency for the impeller blades.
2.3.9 A calculation
of the relative blade passing frequency between the rotor and stator
blades.
2.3.10 The
value of the fluctuating stresses during one revolution of the impeller
and from transient loadings.
2.3.11 Details
of the power/speed range of operation, indicating the maximum continuous
torque rating, together with the associated thrusts; this information
may be presented in the form of a characteristic curve for the water
jet.
2.3.12 The
water jet thrust for the assessment of the strength condition being
considered is to be as follows:
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For ships which
are intended to operate predominantly in a free-running condition
and at steady service conditions, the water jet thrust is to correspond
to the absorption of the maximum continuous shaft power and corresponding
revolutions per minute, giving the maximum torque for which the shaft
system is approved.
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For ships which
are designed for several operating conditions, the maximum thrust
associated with these conditions and the absorption of the corresponding
power, in addition to the maximum continuous powering condition, are
to be used in the calculation.
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The justification
for the thrust selected is to be submitted for consideration in the
approval process and this is to include the ship type and the ship
speed at the conditions considered.
2.3.14 Specifications
of materials and NDE procedures for components essential for propulsion
and steering operation and, in the case of the impeller and stator,
the yield strength and the fatigue characteristics of the material
intended for their manufacture.
2.3.15 A
detailed weld specification where an impeller has welded blades.
2.3.16 Full
details of the means of corrosion protection in the case of carbon
or carbon manganese steel shafts. Alternatively, where it is proposed
to use composite shafts, details of the connections at flanges, materials,
resin, lay-up procedures, quality control procedures and documentary
evidence of fatigue endurance strength is to be provided.
2.3.17 Dry
impeller mass and polar moment of inertia.
2.3.18 The
prime mover type and designation.
2.3.20 The
tolerance specification, agreed between the manufacturer and the Shipbuilder
or Owner, to which the components of the unit are to be manufactured
is to be defined, together with a justification.
2.3.21 Details
of the water jet’s loading reactions together with the positions
of application within the hull and is to include the maximum applied
thrust, tunnel pressures, moments and forces imposed on the ship.
2.3.22 The
water jet unit’s rated flow and head.
2.3.23 Where
an engineering and safety justification report is required, the following
supporting information is to be submitted:
2.3.24 Recommended
installation, inspection, maintenance and component replacement procedures.
This is to include any in-water engineering procedures where recommended
by the water jet manufacturer.
2.3.25 All
transient loads which the steering unit is likely to experience from
manoeuvring, accelerating, decelerating and the sea conditions.
2.4 Failure Mode and Effects Analysis (FMEA)
2.4.1 An
FMEA is to be carried out where a single water jet system is the ship’s
sole means of propulsion, see
Pt 5, Ch 16, 2.2 Plans to be submitted 2.2.3. The FMEA is to identify
components where a single failure could cause the loss of all propulsion
and/or steering capability, and the proposed arrangements for preventing
and mitigating the effects of such a failure.
2.4.2 The
FMEA is to be carried out using the format presented in Table 22.2.1 Failure Mode and Effects
Analysis in Chapter 22 or an
equivalent format that addresses the same reliability issues. Analyses
in accordance with IEC 60812 Analysis for System Reliability
– Procedure for Failure Mode and Effects Analysis, or
the IMO Code of Safety for High Speed Craft, 2000, Annex 4 – Procedures for Failure Mode and Effects Analysis, would be
acceptable.
2.4.3 The
FMEA is to be organised in terms of equipment and function. The effects
of item failures at a stated level and at higher levels are to be
analysed to determine these effects on the system as a whole. Actions
for mitigation of the effects of failure are to be determined, see
Pt 5, Ch 16, 2.4 Failure Mode and Effects Analysis (FMEA) 2.4.1.
2.4.4 The
FMEA is to:
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identify the
equipment or sub-system and mode of operation;
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identify potential
failure modes and their causes;
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evaluate the
effects on the system of each failure mode;
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identify measures
for reducing the risks associated with each failure mode;
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identify measures
for preventing failure; and
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identify trials
and testing necessary to prove conclusions.
2.4.5 At
sub-system level it is acceptable, for the purpose of these Rules,
to consider failure of equipment items and their functions. It is
not required that the failure of components within that equipment
item be analysed, see
Pt 5, Ch 22, 2.1 General 2.1.5.
2.4.6 Where
a FMEA is used for consideration of systems that depend on software-based
functions for control or co-ordination, the analysis is to investigate
failure of the functions rather than a specific analysis of the software
code itself.
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