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DNV-OS-C102 Structural Design of Offshore Ships
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APPENDIX A
Conversion of Tanker
to Floating Offshore Installation
App.A
A. Introduction
App.A
A 100 General
App.A A
101 This section provides specific requirements and guidance applicable
for the conversion from tanker for oil to floating offshore production
and/or offshore storage unit, ref. DNV-OSS-C102.
App.A A
102 The basis of the vessel to be converted is a vessel with hull
structures that comply with the structural requirements of 1A1 Tanker
for Oil or equivalent from a vessel transferring into the DNV classification.
App.A A
103 Prior to conversion it is to be evaluated for the followings:| — | Steel wastages should be identified. |
| — | Fatigue cracking and possible repair should be evaluated. |
App.A A
104 Any major change as lengthening of the unit, increased draft,
increased static loads, etc, shall be evaluated.
App.A A
105 For the new structures added to the converted vessel, material
selection and inspection principles shall comply with the requirement
given in Sec.2. The structural renewal
including material should be replaced by steel of the same or higher
grade according to the approved design scantling or greater.
App.A A
106 Loading manual containing all operational modes shall be submitted
for approval. The permissible limit curves for the hull girder still
water bending moments and shear forces used for a trading vessel
need to be checked for all operating conditions. New permissible
limit curve for operation as an FPSO may need to be established,
as found relevant.App.A
B. Strength
App.A
B 100 General
App.A B
101 Existing hull structure may be accepted "as is" provided
that it comply with class requirements in force for 1A1 Tanker for
Oil at the date of construction of the vessel. However, additional
requirements with respect to longitudinal strength and fatigue capacity
shall be complied with based on site specific environmental conditions.
App.A B
102 New structures added to the converted vessel or existing structures
affected by the new structures shall comply with the requirements
of this standard. This will typically include but will not be limited
to:| — | installation of turret or mooring arrangement |
| — | modification of super-structure |
| — | installation of topside and modification to topside
interface |
| — | installation of helideck |
| — | installation of lifeboat davits, cranes, etc. |
App.A
B 200 Local scantling requirement
App.A B
201 The plate and stiffener may be accepted with lower scantlings
than original given that they comply with the local scantlings requirements
in DNV Rules for Classification for Ships Pt.3 Ch.1.App.A
B 300 Transverse strength
App.A B
301 Transverse strength of the vessel due to tank filling arrangement
may be accepted "as is" if the following conditions
are satisfied with:| — | Loading conditions in production and storage mode
are not more severe than what the vessel has been basis for the
trade as a tanker. |
| — | Minimum scantlings are within the minimum thickness
list provided as a tanker. |
App.A B
302 In case the vessel does not comply with 301, transverse strength
in accordance with the principles in Sec.5
D shall be performed.App.A
B 400 Longitudinal strength
App.A B
401 If the unit is defined for "benign waters operation",
ref Sec.12 Table A1, hull structures
shall comply with the requirements given in the DNV Rules for Classification
of Ships Pt.3 Ch.1 Sec.5. No direct wave load analysis is required.
App.A B
402 For operation in "harsh environment", ref. Sec.12 Table A1, or if direct calculations
are used as an alternative to the longitudinal strength requirements
in the DNV Rules for Classification
of Ships Pt.3 Ch.1 Sec.5, hull
girder longitudinal strength shall comply with the requirements
given in Sec.5 C.
App.A B
403 Reduced scantlings due to wastage of longitudinal members
may be accepted given that the vessel complies with the longitudinal
strength requirement based on direct calculations of site specific
environment in Sec.5 C.App.A
B 500 Topside and topside interface to hull
structure
App.A B
501 Topside structure shall be calculated according to Sec.6
App.A B
502 Topside interface to hull structure shall be calculated according
to Sec.7.App.A
C. Fatigue
App.A
C 100 General
App.A C
101 The fatigue capacity for conversions shall be considered,
and is a function of the following parameters:| — | results from survey and assessment of critical
details |
| — | service history of the vessel |
| — | duration of the intended stay on a specific location
and environmental conditions at the location. |
App.A
C 200 Previous trade
App.A C
201 Fatigue damage during the previous unrestricted trade should
be evaluated for critical areas of the hull. Service history with
actual trading route can be used in the assessment of fatigue damage.
App.A C
202 Previous repair and damage history should be evaluated with
the focus on critical areas and how to remove these potential failures
in the operation phase as floating offshore installation.App.A
C 300 Operation
App.A C
301 The fatigue capacity shall be evaluated in accordance with Sec.8. Minimum 15 years fatigue life
in the intended operational site shall be basis for the fatigue
calculations. "As is" scantlings shall be used
in the assessment.
App.A C
302 S-N curves in air in the DNV Classification Notes No.30.7
(CN30.7) may be used for the specified design life of unit in case
the corrosion protection system is maintained according to the principles
in DNV-OS-C101/C201.App.A
C 400 Areas to be checked
App.A C
401 Fatigue sensitive details
in the existing hull structure and newly installed structures shall
be documented to validate sufficient fatigue strength. Particular
attention should be given to the following details:| — | longitudinal stiffener end connections
to transverse frames and bulkheads |
| — | shell plate below the draught in full load condition |
| — | new foundations and supports to main deck, side and
bottom structure (e.g. topside, flare tower, riser balcony, turret,
etc). |
App.A C
402 For units intend to stay on location without going to dry-dock
every 5th year, the
design fatigue factors as specified in Sec.12
G300 shall be followed.App.A
C 500 Mean stress effect
App.A C
501 Mean stress effect according to Table C1 shall be used.App.A C
| Table C1 Mean stress
effect for conversion of tankers to floating offshore installation |
| Operating profile | Base material | Welded structure | Trading as tanker | DNV-RP-C203 or CN30.7 | CN30.7 | | Operation at site | DNV-RP-C203 or CN30.7 | Not allowed | |
App.A
D. Topside Interface to hull structure
App.A
D 100 General
App.A D
101 When new topside structure is placed on the existing deck
structure the following items need to be specially considered: Uplift
of the topside structure may introduce high tensile stresses perpendicular
to the plate of the deck plate and replacing existing plate with
z-quality steel may be necessary.Due to fatigue, the existing fillet welds between transverse
frame/bulkhead and deck plate may need to be replaced with
full penetration welds when the topside support structure is placed
above the deck plate.
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