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DNV-OS-C102 Structural Design of Offshore Ships

Sec.12: Additional Provisions for Floating Production, Storage and Offloading Units

SECTION 12
Additional Provisions for Floating Production,
Storage and Offloading Units

Sec.12
A. Introduction

Sec.12
A 100   Scope and application

Sec.12
A 200   Definition

Sec.12 A

Table A1 Decision criteria for benign waters or harsh environmental areas
Benign waters	Harsh environmental areas
Hs £ 8.5m	Hs ³ 8.5m
Hs = significant wave height at site specific with a 100 year return period

Sec.12 A
202   If the unit is defined for "benign waters operation", the "main class requirements" to the midship section modulus are by definition more stringent than the design principles based on the direct calculations applied to "benign waters". If hull structures comply with the minimum midship section modulus and moment of inertia given in the DNV Rules for Classification of Ships Pt.3 Ch.1 Sec.5, no direct calculations of wave bending moments and shear forces are required in such cases.

Sec.12 A
203   When the hull is designed according to the direct calculations, the wave loads shall be derived from a direct calculation site specific scatter diagram(s) with a 100 year return period.

Guidance note:
The significant wave height in the extreme storm can be estimated using 2-parameter Weibull parameters (a_s b_s) for different scatter diagram as present in DNV-RP-C205 Appendix C Table C-1 together with the formula:

, where "N" is the number of maxima for the sea state in a time period "t". N = t/t and "t" is the is the duration of each short-term variation (normally taken as 3 hours). E.g. for a time period of 100 year the value N is then N=100*365*24/3= 292000.

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Sec.12
B. Design Principles

Sec.12
B 100   General

Sec.12 B

Table B1 Design principles for floating production and storage units
Design condition		Design basis and environmental loads
Transit	Actual transit route	Actual route with a 10 years return period and equal probability of headings (0-360) or on the Rules from a recognised Marine Warranty
	Worldwide transit	DNV Rules for Classification of Ships Pt.3 Ch.1 Alternatively see Sec.3 B500
Survival condition	Harsh environmental areas	Direct calculations based on the specified sea state or site specific scatter diagram(s) with a 100 year return period, ref. Sec.3 B500
	Benign waters	Alt. 1: DNV Rules for Classification of Ships Pt.3 Ch.1
		Alt. 2: Direct calculations based on the specified sea state or site specific scatter diagram(s) with a 100 year return period, ref. Sec.3 B500

Guidance note:
Operation condition needs normally not to be considered for FPSO's as the weight distribution of the topside structure in the operation and in the survival condition is assumed to be similar.

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Sec.12
C. Design Loads

Sec.12
C 100   Mooring loads

Sec.12
C 200   Green sea

Sec.12 C

Table C1 Green sea design loads for weather deck, topside supports and deckhouses
Area	Benign waters Hs < 8.5 m	Harsh Environment Hs > 8.5 m
Weather deck	Pt.3 Ch.1 Sec.8 B100	Area forward of 0.15 L from F.P., or forward of deckhouse front, whichever is the foremost position, the design pressure, P1 = a (Pdp- (4+ 0.2ks) h0, given in Pt.3 Ch.1 Sec.8 B100 shall be increased with a factor as following:
		8.5m £ Hs £ 12.5 m At unit's side: b= 1+ 0.5*Hs/12.5 At the centre line: b= 1+ 0.75*Hs/12.5	Hs ³ 12.5 m b =1.5 b =1.75
		Linear interpolation shall be used for intermediate locations between the unit's side and the centre line.
Unprotected front bulkheads	Pt.3 Ch.1 Sec.10 C100	The pressure, p1 = 5.7 a (k Cw - h0) c, defined in the ship rules P.3 Ch.1 Sec.10 C100 shall be increased with a factor as following:
		8.5m £ Hs £ 12.5 m 1+ 0.5*Hs/12.5	Hs ³ 12.5 m 1.5
Unprotected bulkheads elsewhere and topside supports	Pt.3 Ch.1 Sec.10 C100	Pt.3 Ch.1 Sec.10 C100
Linear interpolation shall be used for intermediate locations between the unit's side and the centre line. Speed V = 8 knots is to be used as minimum for moored or dynamically positioned units to ensure sufficient minimum pressure.

Sec.12 C
202   The required local scantlings shall be according to the DNV Rules for Classification of Ships Pt.3 Ch.1 Sec.10 using the design pressure as given in 201.

Sec.12 C
203   Glass thickness of windows in unprotected front bulkheads according to DNV Rules for Classification Pt.3 Ch.3 Sec.6 L, as well as the design of the fastening arrangement to the bulkheads shall be considered using the design pressures given in Table C1.

Sec.12 C
204   Topside members located in the midship or aft area of the unit shall be based on p₄ in Table C1 of the DNV Rules for Classification of Ships Pt.3 Ch.1 Sec.10.

Guidance note:


It is advised that provisions are made during model testing for suitable measurements to determine design pressures for local structural design. This implies that model tests should be performed at design draught, for sea states with a spectrum peak period approximately 70 to 100% of the pitch resonance period of the unit. The unit model should be equipped with load cells on the weather deck at positions of critical structural members or critical topside equipment.

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Sec.12
D. Hull Girder Longitudinal Strength

Sec.12
D 100   General

Sec.12
D 200   Design loading conditions

Sec.12 D

Table D1 Design loading conditions for hull girder longitudinal strength
Load case	Draught	Global load	Local load
		Bending moment	External pressure	Internal pressure	Topside load	Turret load
LC1	Full load draught	Max. sagging (limit value)	Static - dynamic	Static + dynamic	Vertical load	Vertical load
LC2	Ballast draught	Max. hogging (limit value)	Static + dynamic	Static + dynamic	Vertical load	Vertical load
All dynamic loads are to be determined based on a 100 year return period, ref. Sec.3 B500 For internal dynamic pressure, the vertical acceleration at the centre of hold/cargo tank induced by the heave and pitch motion is to be applied. The height of air pipe should normally not be taken less than 0.76m. The specific gravity of each cargo tank and water ballast tank is to be 1.025 t/m3 Turret load is to be added, if applicable.

Sec.12
E. Transverse Strength

Sec.12
E 100   General

Sec.12
E 200   Design loading conditions

Guidance note:
DNV Classification Note No.31.3 is normally used for a cargo hold FE analysis for 3 standard types of tanker. Harbour conditions need normally not to be considered.

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Sec.12
F. Local Detail Stress Analysis

Sec.12
F 100   General

Sec.12 F

Table F1 Areas to be checked
Hull	— toe of girder bracket at typical transverse web frame — toe and heel of horizontal stringer in way of transverse bulkhead — local stiffener in way of transverse bulkhead subject to relative deformation — opening on main deck, bottom and inner bottom, e.g. moonpool corner.
Hull-topside interface structure	— topside stools and support structures — turret and supporting structures — riser interfaces — crane pedestal foundation and supporting structures — gantry foundation and supporting structures — flare tower foundation and supporting structures — fairlead support.

Sec.12
F 200   Turret and moonpool structure

Fig. 1   Global stress concentration in hull


Sec.12 F
202   Turret interface structure should be calculated by FE calculations considering the relevant loads. The combination of loads from mooring, internal tank filling and hull girder loads should be taken as unfavourable for the design of the turret/moonpool area and the adjoining hull structure.

Sec.12 F
203   The extent of the FE calculations should be appropriate to evaluate the effect of loads on the hull girder, transverse girders and local plate and stiffeners. For guidance of extent of FE model, see Figures 2 and 3.

Guidance note:


For guidance of the extent of modelling, see Figure 2 and 3.

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Fig. 2   External turret

Fig. 3   Internal turret

Sec.12
G. Fatigue Strength

Sec.12
G 100   Design loading conditions

Guidance note:
Normally 50% in full load and 50% in ballast may be used for the operation, unless otherwise documented. Partial filling may be considered depending on operational characteristics.

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Sec.12
G 200   Mean stress effect

Sec.12 G

Table G1 Mean stress effect for permanently installed units
Base material	Welded structure
DNV-RP-C203 or DNV Classification Notes No.30.7	Not allowed

Guidance note:
Mean stress effect for welded structures is normally allowed for sailing ships that are in dry dock each 5 year and can be easily repaired. For permanently installed ships where traditionally a rather high safety margin with respect to fatigue is required and repair of these structures on the field can be rather costly, use of mean stress effect for welded structures is thus normally not allowed.

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Sec.12
G 300   Design fatigue factors

Sec.12 G

Table G2 Design fatigue factors (DFFs)
1	Internal structure, accessible and not welded directly to the submerged part
1	External structure, accessible for regular inspection and repair in dry and clean conditions.
2	Internal structure, accessible and welded directly to the submerged part.
2	External structure not accessible for inspection and repair in dry and clean conditions.
3	Non-accessible areas, areas not planned to be accessible for inspection and repair during operation

Fig. 4   Example of design fatigue factor


Sec.12 G
302   The units can normally be ballasted to different draughts, and the term "splash" zone has thus no significance. Sufficient margin in respect to the lowest inspection waterline should however be considered depending on the expected wave heights during the inspection periods.

Guidance note:
Normally 1-2 m is considered sufficient margin on the lowest inspection waterline in world wide operation.

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The DFF applied will therefore be dependent on the accessibility for inspection and repair and the position of the lowest inspection waterline.

Sec.12
G 400   Areas to be checked

Sec.12 G

Table G3 Areas to be checked
Hull	— main deck, including deck penetrations, bottom structure and side shell — longitudinal stiffener end connections to transverse webframe and bulkhead — shell plate connection to longitudinal stiffener and transverse frames with special consideration in the splash zone. — hopper knuckles and other relevant discontinuities — openings and penetrations in longitudinal members — toe and heel of horizontal stringer in way of transverse bulkhead — bilge keels.
Hull-topside interface structure	— attachments, foundations, supports etc. to main deck and hull — topside stools and supporting structures — caissons — turret and supporting structures — riser interfaces — crane pedestal foundation and supporting structures — flare tower foundation and supporting structures.

Sec.12
G 500   Class Notation FMS

The analysis should be performed in accordance with the general principles stated in DNV-RP-C206. Alternatively, specific methodology given in DNV Classification Note No.30.7 may be used.

Sec.12
H. Special Consideration

Sec.12
H 100   Bilge keels

The bilge keel should normally be welded directly onto the shell plate without doubling plates. Adequate transverse supporting brackets, or an equivalent arrangement, are to be provided.

Sec.12 H
102   For bilge keels of a closed type design material yield, buckling and fatigue strength shall be documented. The transfer functions for stress responses from the wave dynamics and motion induced drag forces shall be determined separately. The transfer functions shall be combined in the cumulative damage calculations.

Sec.12
H 200   Support of mooring equipment, towing brackets etc

Sec.12
H 300   Loading Instrument

Sec.12
I. Inspection Principles

Sec.12
I 100   General

Sec.12 I

Table I1 Inspection categories
Inspection category	Material class/hull elements
I	IV
II	III
III	I & II

Sec.12
J. Corrosion Control

Sec.12
J 100   General