Section
6 Mechanical items
6.1 General
6.1.2 This Section covers mechanical items of turrets and swivels including
bearings, hinges, universal joints and seals, etc. Turret structures are to comply
with Pt 3, Ch 13, 3 Turret structures.
6.1.3 Sufficient plans, data and specifications are to be submitted to enable
the mechanical arrangements to be assessed and approved.
6.1.4 Plans and data covering the following items are to be submitted for
approval, as relevant:
- Structural arrangements.
- Materials specification.
- Lubrication system.
6.1.5 The following supporting plans and documents are to be submitted:
- General arrangement.
- Design specification.
- Design calculations.
- Surveillance program.
6.2 Design
6.2.1 The design of joint and hinges should minimise any stress concentrations,
particularly where significant dynamic loadings may occur.
6.2.2 Suitable strength and fatigue analyses of joint or hinge assemblies are
to be carried out, where appropriate.
6.2.3 It is to be considered that vibration levels in the associated pipe work
and structure of the swivel are to be kept to a minimum level to avoid
bearing-associated failures.
6.3 Bearings
6.3.1 Components in swivel support systems are to be designed for the
operating forces, moments and pressures intended, taking into account, where
necessary, survival, tow out, damaged, fatigue and other operating conditions.
Design calculations are to be submitted.
6.3.2 Rolling element, pad and journal bearings used in swivel units are to be
designed for the static and dynamic loadings which are expected in service. Bearing
pressure and fatigue life calculations are to be submitted.
6.3.3 Bearings, joints, etc., are to be suitable to withstand the application
of all loads expected during service life. The effect of construction tolerances of
the bearing and bearing supports is to be considered. The maximum tolerances
recommended by the bearing supplier should be used. The maximum design loadings are
to be determined in accordance with Pt 4, Ch 3, 4 Structural design loads.
6.3.4 The design of bearings, joints, etc., is to be in accordance with an
acceptable design method or an internationally recognised Code or Standard. For
acceptable Codes for roller and ball bearings, see
Pt 3, Ch 19 Equipment Categories.
6.3.5 Bearing design is to include the effects of low and high frequency
response loadings, where appropriate.
6.3.6 The effects of motions, for a range of typical operating modes, are to
be considered in the design.
6.3.7 Where necessary, suitable lubricating arrangements are to be fitted to
all adjacent bearing surfaces to maintain an adequate and continuous supply of
lubricant to the surfaces during all unattended periods. Gravity-fed or
non-power-operated systems are to be preferred for non-manned installations.
6.3.8 Consideration is to be given to monitoring turret roller bearings in
service by condition monitoring the bearing lubrication fluid. Details to be
submitted to LR.
6.3.9 Primary bearing surfaces are to be adequately protected from
deterioration caused by the ingress of seawater and other contaminants by a system
of seals or other suitable alternative methods. Sealing arrangements for bearing
systems are to contain lubrication and are to be designed for their intended service
life or field life of the installation as applicable.
6.3.10 Data should be submitted to substantiate the fitness of the bearing for
the field life of the installation or 20 years, whichever is greater. Consideration
will be given to the reduction of this life where an agreed change-out programme is
implemented.
6.3.11 Classification will be based on a review of the designers
calculations.
6.3.12 In all cases where the bearing dynamic load is more than 50 per cent of
the basic load dynamic rating, supporting justification is to be submitted.
6.3.13 The suitability of bearings selected for heavily loaded applications
should be checked to ensure that their basic static load rating is adequate, taking
into account their static safety factor.
6.3.14 Consideration is to be given to the use of lubricants with EP additives
where the bearing loads are high.
6.3.15 Consideration is to be given to rolling element bearings; those which
cannot be replaced whilst vessel/buoys are at location are to be designed for L5
bearing life.
6.3.16 Consideration is to be given to ensuring that excessive lubrication is
avoided in tilting pad bearings and that the Pressure Velocity is within the
recommended limits. For acceptable limits, see
Pt 3, Ch 19 Equipment Categories.
6.3.17 Where grease lubrication is being used on a loading buoy bearing,
frequent grease sampling and system monitoring are to be considered.
6.3.18 Turret bearings which carry the operating hawser load, rotating structure load and
mooring load are to be designed with a safety factor of not less than 2 without
destructive yielding of the bearing surfaces. Bearing mounting bolts are to be
designed in accordance with recognised industry standards acceptable to LR. For high
tension bolts stress corrosion cracking is to be considered (bolting with minimum
yield stress above 355N/mm2 and tensile stress above 500 N/mm2
will be considered as high strength bolting and hardness value to be limited to
300HV for avoidance of stress corrosion cracking).
6.3.19 Swivel bearings that do not carry the hawser load are to be designed in accordance
with Anti-Friction Bearing Manufacturers Association (AFBMA) Codes or other industry
standards deemed appropriated by LR.
6.3.20 The swivels are to be coated on the outside with a suitable corrosion resistant
coating. This coating will not be required for parts made of corrosion resistant
material. The possibility of corrosion due to the presence of CO2,
O2, or H2S in the cargo or product fluid is to be
considered in the swivel design.
6.4 Bearing support structures
6.4.3 A fatigue analysis of structural items is to be carried out in accordance
with Pt 4, Ch 5, 5 Fatigue designFactors of safety on fatigue life is to be determined after
consideration of the redundancy of the structure, the accessibility of the item
being considered, the consequence of failure, etc. Minimum required factors of
safety are given in Pt 4, Ch 5, 5 Fatigue design.
6.4.4 Consideration is to be given to improve bearing support structure
stiffness to prevent substantial increase in the bearing loading.
6.4.5 Consideration is to be given to the integrity of the weld attachments
for the support structures.
6.4.6 Cracking of bearing housings at stress concentrators due to bearing wear
is common in roller bearings and should be considered as a potential damage
mechanism.
6.4.7 The strength and fatigue analysis of bearing supports is to consider the
effect of construction tolerances of the bearing and bearing supports. The maximum
tolerances recommended by the bearing supplier should be used.
6.5 Seals
6.5.1 Leakage of lubrication fluid and subsequent ingress of sea-water is to be
prevented by installing a suitable system of seals.
6.5.2 The seals employed are to be of a suitable material for the intended
service.
6.5.3 Sealing elements installed are to be capable of safely absorbing the
required deflection or, alternatively, adequate provisions for slippage are to be
incorporated in the design.
6.5.4 A lubrication leakage detection system is to be installed in order to
monitor seal performance in service. The system is to provide early warning of seal
deterioration to allow appropriate remedial action to be taken.
6.5.5 Swivels and sections in the swivel stack in flammable and toxic services
are to use seal arrangements which shall provide redundancy such that leaks can be
detected before process fluid release occurs.
6.5.6 The seal fluid pressure is to be higher than the maximum well shut-in
pressure and system surge pressure.
6.5.7 A continuous seal fluid leakage detection system is to be monitored to
verify system availability and ensure hydrocarbons are not released. The system is
to be fitted with alarms to detect early seal deterioration and allow appropriate
remedial action to be taken.
6.5.8 In the event of a secondary seal failure, a production ESD is to be
initiated and the leak detection system must be capable of precisely identifying the
failed seal.
6.5.9 The supply of barrier seal oil for the swivel stack is to be from a
dedicated HPU package with its own control panel and feedback to the main control
room.
6.5.10 The seal seats and travelling surfaces should be corrosion-resistant and
of sufficient hardness to prevent excessive abrasion and wear.
6.5.11 Care is to be taken to minimise the risk of explosive decompression of
seal in the event of a catastrophic failure. Maximum decompression rates for the
seal material are to be provided by the manufacturer.
6.5.12 Prevention of contamination to dynamic seals is crucial. Seals are to be
fitted with a silt-barrier system to prevent sand or particles getting into the
seals, where applicable.
6.6 Bolted joints
6.6.1 An acceptable method for the determination of flanged bolt loads is to be
found in Verein Deutscher Ingenieure (VDI) 2230 – Systematic Calculation of High
Duty Bolted Joints. Other suitable internationally recognised Codes or
Standards may be used.
6.6.2 For joints subject to fatigue loading, the bolts are to be of ISO 898/1
Material Grade 8.8, 10.9 or 12.9, or equivalent. They are to be pretensioned by a
controlled means to 70 to 90 per cent of their yield stress. For bolt sizes greater
than M30, pre-tensioning must be carried out, in a rational order, by a hydraulic
tensioning device.
6.6.3 The torque on all bolting on bearing housing, support structures and
attachments is to be regularly inspected and checked. The maintenance plan is to be
submitted to LR for review.
6.7 Swivel stack
6.7.1 Cargo or product swivels are to be of steel construction with flanged or welded
connections. Details of the swivel connecting stationary piping with rotating piping
are to be submitted for approval. Such details are to include fixed and rotating
parts details, plate thicknesses, nozzle locations and arrangement seal and bearing
design, and welding. The swivel design is to consider the most adverse combination
of applicable loads. At least the following loads are to be considered:
- Breakaway torque required for each swivel at maximum design pressure.
- Weight of swivel and its structural components.
- Dynamic loads due to vessel motion.
- Piping loads.
- Pressure loads.
- Thermal loads.
Pressure retaining components of the swivel are to be designed in
compliance with a Recognised Industry standard deemed appropriate by LR such as the
ASME Pressure Vessel Code. Structural components of the swivel and
driving mechanism are to comply with Pt 4 Steel Unit Structures of these Rules as
applicable or recognised National and International structural codes or standards,
including those within Pt 12, Ch 1 Recognised Codes and Standards.
6.7.2 In general, the swivel stack is to be analysed by a three-dimensional
finite element method unless agreed otherwise with LR. Design calculations,
including details of the model, are to be submitted.
6.7.3 Permissible stress levels are to be in accordance with a recognised Code
or standard.
6.7.5 Special consideration is to be given to torsional loading effects for the
design of universal joints and other connections.
6.7.6 The fluid swivel is to be designed to withstand the maximum range of
operating conditions, including maximum well shut-in pressure and pressure surge
condition.
6.7.8 Electrical swivels, if installed in hazardous area, the electrical swivel is to be
certified by an independent testing laboratory as suitable for installation within
such an area, the amperage rating of the electrical swivels (slip rings) is to be
adequate to carry the full load current of the equipment supplied.
6.8 Survey
6.8.1 Joint structures are to be included in the Periodical Classification
Surveys, in accordance with the requirements contained in Pt 1 Regulations.
6.8.2 A comprehensive surveillance program, including detailed seal replacement
and overhaul procedures, is to be developed by the Owner. A sufficient number of
spare parts and required tools is to be provided for the installation.
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