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DNV-OS-C102 Structural Design of Offshore Ships

Sec.11: Additional Provisions for Well Service and Drilling Units

SECTION 11
Additional Provisions for Well Service and Drilling Units

Sec.11
A. Introduction

Sec.11
A 100   Scope and application

Sec.11
B. Design Principles

Sec.11
B 100   General

Sec.11 B

Table B1 Design principles for well service and drilling units
Design condition	Design basis and environmental load level
Transit condition (World Wide)	DNV Rules for Classification of Ships Pt.3 Ch.1 Sec.5. Alternatively see Sec.3 B500
Operating condition 1)	Direct calculations using the maximum allowable sea state (max Hs) criteria for aborting the operation, ref Sec.3 B500
Survival condition	DNV Rules for Classification of Ships Pt.3 Ch.1 Sec.5 2) Alternatively direct calculations based on the specified sea state or site specific scatter diagram(s) with a 100 year return period, ref. Sec.3 B500
The operation condition is mainly only relevant for the drillfloor interface to the hull structure Using Ship Rules values Pt.3 Ch.1 Sec.5 for the longitudinal strength in operation is allowed provided that MwRule-20 year > MwSite-100year Where: MwRule-20 year : Rule wave bending moment based on a 20 year return period in the North Atlantic MwSite-100year : Direct wave bending moment based on a 100 year return period at the specified site

Sec.11 B
103   Extreme conditions to be included when relevant, ref Sec.3 B500.

Guidance note:
When the unit is intended to operate in areas where the 100 years significant wave height (Hs) in operation is less than 8.5 m (Hs £ 8.5m), the Ship Rules Pt.3 Ch.1 Sec.5 may be used for the longitudinal strength check. 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.11
C. Design Loads

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C 100   Green sea

Sec.11
D. Hull Girder Longitudinal Strength

Sec.11
D 100   General

Sec.11
D 200   Design loading conditions

Sec.11 D

Table D1 Design loading conditions for hull girder longitudinal strength
Load case	Draught	Global load		Local load
		Bending moment	Shear force	External pressure	Internal pressure	Drill floor, Topside load and Turret
LC1	Maximum draught	Max. sagging	Max. sagging	Static - dynamic	Static + dynamic	Vertical load
LC2	Minimum draught	Max. hogging	Max. hogging	Static + dynamic	Static + dynamic	Vertical load
All dynamic loads are to be determined based on the design principles given in Table B1. 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 is normally not installed for well service and drilling vessels but the loads shall be included if relevant.

Sec.11
E. Transverse Strength

Sec.11
E 100   General

Sec.11
E 200   Design loading conditions

Sec.11
F. Local Detail Stress Analysis

Sec.11
F 100   General

Sec.11 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	— drill floor substructure and supporting structures — topside stool and support structures — crane pedestal foundation and supporting structures — foundation and supporting structures for rail of gantry crane — foundation and supporting structures for heavy equipment like BOP, Xmas, etc.

Sec.11
G. Fatigue Strength

Sec.11
G 100   Design loading conditions

Guidance note:
The fraction of the total design life using 80% in operation and 20% in Transit may be used if no other information is available.

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Sec.11
G 200   Mean stress effect

Sec.11 G

Table G1 Mean stress effect for well service and drilling vessels
Base material	Welded structure
DNV-RP-C203 or DNV Classification Notes No.30.7	DNV Classification Notes No.30.7

Sec.11
G 300   Design fatigue factor

Sec.11
G 400   Areas to be checked

Sec.11 G

Table G2 Areas to be checked
Hull	— openings on main deck, bottom and inner bottom structure including deck penetrations — longitudinal stiffener end connections to transverse web frame and bulkhead — shell plate connection to longitudinal stiffener and transverse frames with special consideration in the splash zone. — hopper knuckles and other relevant discontinuities — attachments, foundations, supports etc. to main deck and bottom structure openings and penetrations in longitudinal members.
Hull-topside interface structure	— attachments, foundations, supports etc. to main deck and hull — hull connections including substructure for drill floor — topside stool and supporting structures — crane pedestal foundation and supporting structures.

Sec.11
H. Inspection Principles

Sec.11
H 100   Hull and topside structure

The extent of non-destructive testing during fabrication of the hull shall be in accordance with DNV Rules for Classification of Ships Pt.2 Ch.3 Sec.7.

Sec.11 H
102   Topside structure

The extent of non-destructive testing during fabrication of the topside shall be in accordance with DNV OS-C401.

Sec.11
I. Corrosion Control

Sec.11
I 100   Hull and topside structure