4.10.1.1 All welded joints
of the shells of independent tanks should be of the butt weld, full
penetration type. For dome-to-shell connections, the Administration
may approve tee welds of the full penetration type. Except for small
penetrations on domes, nozzle welds are also generally to be designed
with full penetration.
4.10.1.2 Welding joint details
for type C independent tanks should be as follows:
-
.1 All longitudinal and circumferential joints of
pressure vessels should be of butt welded, full penetration, double vee or single
vee type. Full penetration butt welds should be obtained by double welding or by
the use of backing rings. If used, backing rings should be removed, unless
specifically approved by the Administration for very small process pressure
vessels. Other edge preparations may be allowed by the Administration depending on
the results of the tests carried out at the approval of the welding procedure.
-
.2 The bevel preparation of the joints between the
pressure vessel body and domes and between domes and relevant fittings should be
designed according to a standard for pressure vessels acceptable to the
Administration. All welds connecting nozzles, domes or other penetrations of the
vessel and all welds connecting flanges to the vessel or nozzles should be full
penetration welds extending through the entire thickness of the vessel wall or
nozzle wall, unless specially approved by the Administration for small nozzle
diameters.
4.10.2 Workmanship should
be to the satisfaction of the Administration. Inspection and non-destructive
testing of welds for tanks other than type C independent tanks should
be in accordance with the requirements of 6.3.7.
4.10.3 For membrane tanks,
quality assurance measures, weld procedure qualification, design details,
materials, construction, inspection and production testing of components,
should be to standards developed during the prototype testing programme.
4.10.4 For semi-membrane tanks
the relevant requirements in this section for independent tanks or
for membrane tanks should be applied as appropriate.
4.10.5.1 For internal insulation
tanks, in order to ensure uniform quality of the material, quality
control procedures including environmental control, application procedure
qualification, corners, penetrations and other design details, materials
specification, installation and production testing of components should
be to standards developed during the prototype test programme.
4.10.5.2 A quality control
specification including maximum permissible size of constructional
defects, tests and inspections during the fabrication, installation
and also sampling tests at each of these stages should be to the satisfaction
of the Administration.
4.10.6 Integral tanks should
be hydrostatically or hydropneumatically tested to the satisfaction
of the Administration. The test in general should be so performed
that the stresses approximate, as far as practicable, to the design
stresses and that the pressure at the top of the tank corresponds
at least to the MARVS.
4.10.7 In ships fitted with
membrane or semi-membrane tanks, cofferdams and all spaces which may
normally contain liquid and are adjacent to the hull structure supporting
the membrane should be hydrostatically or hydropneumatically tested
in accordance with Recognized Standards. In addition, any other hold
structure supporting the membrane should be tested for tightness.
Pipe tunnels and other compartments which do not normally contain
liquid need not be hydrostatically tested.
4.10.8.1 In ships fitted
with internal insulation tanks where the inner hull is the supporting
structure, all inner hull structure should be hydrostatically or hydropneumatically
tested in accordance with Recognized Standards, taking into account
the MARVS.
4.10.8.2 In ships fitted with internal insulation tanks
where independent tanks are the supporting structure, the independent tanks should be
tested in accordance with 4.10.10.1.
4.10.8.3 For internal insulation
tanks where the inner hull structure or an independent tank structure
acts as a secondary barrier, a tightness test of those structures
should be carried out using techniques to the satisfaction of the
Administration.
4.10.8.4 These tests should
be performed before the application of the materials which will form
the internal insulation tank.
4.10.9 For type C independent
tanks, inspection and non-destructive testing should be as follows:
-
.1
Manufacture and workmanship
- The tolerances relating to manufacture and workmanship such as
out-of-roundness, local deviations from the true form, welded joints alignment and
tapering of plates having different thicknesses, should comply with standards
acceptable to the Administration. The tolerances should also be related to the
buckling analysis referred to in 4.4.6.2.
-
.2
Non-destructive testing
- As far as completion and extension of non-destructive testing of welded
joints are concerned, the extent of non-destructive testing should be total or
partial according to standards acceptable to the Administration, but the controls
to be carried out should not be less than the following:
-
.2.1 Total non-destructive testing referred to
in 4.4.6.1.3:
Radiography:
butt welds 100% and
Surface crack detection:
all welds 10%;
reinforcement rings around holes, nozzles, etc. 100%.
As an alternative, ultrasonic testing may be accepted as a
partial substitute for the radiographic testing, if specially allowed by the
Administration. In addition, the Administration may require total ultrasonic
testing on welding of reinforcement rings around holes, nozzles, etc.
-
.2.2 Partial non-destructive testing referred
to in 4.4.6.1.3:
Radiography:
butt welds: all welded crossing joints and at least 10% of the
full length at selected positions uniformly distributed and
Surface crack detection:
reinforcement rings around holes, nozzles, etc. 100%:
Ultrasonic testing:
as may be required by the Administration in each instance.
4.10.10 Each independent
tank should be subjected to a hydrostatic or hydropneumatic test as
follows:
-
.1 For type A independent tanks, this test should be
so performed that the stresses approximate, as far as practicable, to the design
stresses and that the pressure at the top of the tank corresponds at least to the
MARVS. When a hydropneumatic test is performed, the conditions should simulate, as
far as practicable, the actual loading of the tank and of its supports.
-
.2 For type B independent tanks, the test should be
performed as required in 4.10.10.1 for type A independent tanks. In addition, the
maximum primary membrane stress or maximum bending stress in primary members under
test conditions should not exceed 90% of the yield strength of the material (as
fabricated) at the test temperature. To ensure that this condition is satisfied,
when calculations indicate that this stress exceeds 75% of the yield strength, the
prototype test should be monitored by the use of strain gauges or other suitable
equipment.
-
.3 Type C independent tanks should be tested as
follows:
-
.3.1 Each pressure vessel, when completely
manufactured, should be subjected to a hydrostatic test at a pressure
measured at the top of the tanks, of not less than 1.5 P
o, but in no case during the pressure test should the calculated
primary membrane stress at any point exceed 90% of the yield stress of the
material. The definition of P
o is given in 4.2.6. To ensure that this condition is satisfied where
calculations indicate that this stress will exceed 0.75 times the yield
strength, the prototype test should be monitored by the use of strain gauges
or other suitable equipment in pressure vessels other than simple
cylindrical and spherical pressure vessels.
-
.3.2 The temperature of the water used for the
test should be at least 30°C above the nil ductility transition temperature
of the material as fabricated.
-
.3.3 The pressure should be held for 2 h per 25
mm of thickness but in no case less than 2 h.
-
.3.4 Where necessary for cargo pressure
vessels, and with the specific approval of the Administration, a
hydropneumatic test may be carried out under the conditions prescribed in
4.10.10.3.1, .2 and .3.
-
.3.5 Special consideration may be given by the
Administration to the testing of tanks in which higher allowable stresses
are used, depending on service temperature. However, the requirements of
4.10.10.3.1 should be fully complied with.
-
.3.6 After completion and assembly, each
pressure vessel and its related fittings should be subjected to an adequate
tightness test.
-
.3.7 Pneumatic testing of pressure vessels
other than cargo tanks should only be considered on an individual case basis
by the Administration. Such testing should be permitted only for those
vessels which are so designed or supported that they cannot be safely filled
with water, or for those vessels which cannot be dried and are to be used in
a service where traces of the testing medium cannot be tolerated.
4.10.11 All tanks should
be subjected to a tightness test which may be performed in combination
with the pressure test referred to in 4.10.10 or separately.
4.10.12 Requirements with
respect to inspection of secondary barriers should be decided by the
Administration in each case.
4.10.13 In ships fitted with
type B independent tanks, at least one tank and its support should
be instrumented to confirm stress levels unless the design and arrangement
for the size of ship involved are supported by full-scale experience.
Similar instrumentation may be required by the Administration for
type C independent tanks dependent on their configuration and on the
arrangement of their supports and attachments.
4.10.14 The overall performance
of the cargo containment system should be verified for compliance
with the design parameters during the initial cool-down, loading and
discharging of the cargo. Records of the performance of the components
and equipment essential to verify the design parameters should be
maintained and be available to the Administration.
4.10.15 Heating arrangements,
if fitted in accordance with 4.8.4,
should be tested for required heat output and heat distribution.
4.10.16 The hull should be
inspected for cold spots following the first loaded voyage.
4.10.17 The insulation materials
of internal insulation tanks should be subjected to additional inspection
in order to verify their surface conditions after the third loaded
voyage of the ship, but not later than the first 6 months of the ship's
service after building or a major repair work is undertaken on the
internal insulation tanks.
4.10.18 For type C independent
tanks, the required marking of the pressure vessel should be achieved
by a method which does not cause unacceptable local stress raisers.