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DNV-OS-C105 Structural Design of TLPS (LRFD method)
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SECTION 1
Introduction Sec.1
A. General
Sec.1
A 100 Introduction
Sec.1 A
101 This standard provides requirements and guidance to the structural
design of TLPs. The requirements and guidance documented in this
standard are generally applicable to all configurations of tension
leg platforms.
Sec.1 A
102 This standard is based on the load and resistance factor design
method (LRFD). LRFD is defined in DNV-OS-C101.
Sec.1 A
103 A TLP can alternatively be designed according to working stress
design principles, which is defined in DNV-OS-C201.
Sec.1 A
104 A TLP can also alternatively be designed to API RP 2T as it
has been accepted that it meets the safety levels required by this
Standard.
Sec.1 A
105 A Tension Leg Platform (TLP) is
defined as a buoyant unit connected to a fixed foundation (or piles)
by pre-tensioned tendons. The tendons are normally parallel, near
vertical elements, acting in tension, which usually restrain the
motions of the TLP in heave, roll and pitch. The platform is usually
compliant in surge, sway and yaw. Figure 1 shows an example of a
tension leg platform.
Fig. 1 Example of a tension leg platform
Sec.1 A
106 The standard has been written for general world-wide application.
Governmental regulations may include requirements in excess of the
provisions of this standard depending on size, type, location and
intended service of the offshore unit/installation.
Sec.1
A 200 Objectives
Sec.1 A
201 The objectives of the standard are to:| — | provide an internationally acceptable
standard of safety by defining minimum requirements for structural design
of TLPs |
| — | serve as a contractual reference document for suppliers
and purchasers |
| — | serve as guidance for designers, suppliers, purchasers
and regulators |
| — | specify procedures and requirements for TLP units subject
to DNV verification classification and certification services. |
Sec.1
A 300 Scope and application
Sec.1 A
301 A TLP is usually applied for drilling, production and export
of hydrocarbons. Storage may also be a TLP function.
Sec.1 A
302 A TLP may be designed to function in different modes, typically
operation and survival. Also horizontal movement (e.g. by use of
catenary or taut mooring) of TLP above wells may be relevant. Limiting
design criteria when going from one mode of operation to another
shall be established.
Sec.1 A
303 The TLP unit should also be designed for transit relocation,
if relevant.
Sec.1 A
304 For novel designs, or unproved applications of designs where
limited, or no direct experience exists, relevant analyses and model
testing shall be performed which clearly demonstrate that an acceptable
level of safety can be obtained, i.e. safety level is not inferior
to that obtained when applying this standard to traditional designs.
Sec.1 A
305 Requirements concerning riser systems are given in DNV-OS-F201.
Sec.1 A
306 In case of application of a catenary or taut mooring system
in combination with tendons, reference is made to DNV-OS-E301.
Sec.1 A
307 Requirements related to stability (intact and damaged) are
given in Sec.6 for ULS condition
and Sec.8 for ALS condition.Sec.1
A 400 Classification
Sec.1 A
401 Classification principles, procedures and applicable class
notations related to classification services of offshore units are
specified in the DNV Offshore Service Specifications given in Table
A1.Sec.1 A
| Table A1 DNV Offshore
Service Specifications |
| Reference | Title | DNV-OSS-101 | Rules for Classification of Offshore Drilling
and Support Units | | DNV-OSS-102 | Rules for Classification of Floating Production,
Storage and Loading Units | | DNV-OSS-103 | Rules for Classification of LNG/LPG
Floating Production and Storage Units or Installations | | | Rules for Planning and Execution of Marine
Operations | |
Sec.1 A
402 It shall be agreed with DNV, at the start of the project,
what documents from the project Master Document Register (MDR) shall
be the subject of approval. Documentation for classification shall be in accordance with
the NPS DocReq (DNV Nauticus Production System for documentation
requirements) and DNV-RP-A201.
Sec.1 A
403 It is possible to limit the classification to a selected scope
(e.g. HULL, TOPSIDE, DRILLING MODULE, ACCOMMODATION etc,). Details
of the scope for classification shall be discussed and agreed with
DNV at the start of the project.
Sec.1 A
404 Some of the design phases (e.g. hull and deck mating, transportation,
installation and decommissioning) presented in the standard are
not covered by normal classification scope, Technical requirements
given in DNV-OS-C101 Sec.8, related to Serviceability Limit States,
are not mandatory as part of classification.
Sec.1
B. Normative References
Sec.1
B 100 General
Sec.1 B
101 DNV documents in Table B1 and recognized codes and standards
in Table B2 are referred to in this standard.
Sec.1 B
102 Other recognised standards may be applied provided it can
be demonstrated that they meet or exceed the level of safety of
actual DNV Offshore Standards. Sec.1 B
| Table B1 DNV Reference
documents |
| Reference | Title | DNV-OS-A101 | Safety Principles and Arrangement | | DNV-OS-B101 | Metallic Materials | | DNV-OS-C101 | Design of Offshore Steel Structures, General
(LRFD
method) | | DNV-OS-C103 | Structural Design of Column Stabilised Units
(LRFD method) | | DNV-OS-C106 | Structural Design of Deep Draught Floating
Units | | DNV-OS-C201 | Structural Design of Offshore Units
(WSD
method) | | DNV-OS-C301 | Stability and Watertight Integrity | | DNV-OS-C401 | Fabrication and Testing of Offshore Structures | | DNV-OS-C501 | Composite Components | | DNV-OS-C502 | Offshore Concrete Structures | | DNV-OS-D202 | Instrumentation and Telecommunication Systems | | DNV-OS-E401 | Helicopter Decks | | DNV-OS-E301 | Position Mooring | | DNV-OS-F201 | Dynamic Risers | |
Sec.1 B
| Table B2 Recognised
codes and standards |
| Reference | Title | API RP 2A | Recommended Practice for Planning, Designing
and Constructing Fixed Offshore Platforms - Working Stress Design | | API RP 2T | Planning, Designing and Constructing Tension
Leg Platforms | | API RP 2R | Recommended Practice for Design, Rating and
Testing of Marine Drilling Riser Couplings | | API RP 2RD | Design of Marine Risers for Floating Production
System and TLPs | | N-004 | NORSOK - Design of Steel Structures | | API SPEC 2H | Specification for Carbon Manganese Steel Plate
for Offshore Platform Tubular Joints | | API RP 2L | Recommended Practice for Planning, Designing
and Constructing Heliports for Fixed Offshore Platforms | | BS 7910 | Guide on Methods for Assessing the Acceptability
of Flaws in Fusion Welded Structures | | BS 7448 | Fracture Mechanics Toughness Tests | |
Sec.1
C. Definitions
Sec.1
C 100 Verbal forms
Sec.1 C
101 Shall: Indicates a
mandatory requirement to be followed for fulfilment or compliance
with the present standard. Deviations are not permitted unless formally
and rigorously justified, and accepted by all relevant contracting
parties.
Sec.1 C
102 Should: Indicates a
recommendation that a certain course of action is preferred or particularly
suitable. Alternative courses of action are allowable under the
standard where agreed between contracting parties but shall be justified
and documented.
Sec.1 C
103 May: Indicates a permission,
or an option, which is permitted as part of conformance with the
standard.Sec.1
C 200 Terms
Sec.1 C
201 Heave restrained platform (HRP): A
platform which is free to roll and pitch, but restrained in the
heave eigenmode.
Sec.1 C
202 High frequency (HF) responses: Defined
as TLP rigid body motions at, or near heave, roll and pitch eigenperiods
due to non-linear wave effects.
Sec.1 C
203 Low frequency (LF) responses: Defined
as TLP rigid body non-linear motions at, or near surge, sway and
yaw eigenperiods.
Sec.1 C
204 Mini TLP: Small tension
leg platform with one, or multiple columns.
Sec.1 C
205 Ringing: Defined as
the non-linear high frequency resonant response induced by transient
loads from high, steep waves.
Sec.1 C
206 Roll,
pitch, and yaw: Rotational modes around
surge, sway and heave axis, respectively.
Sec.1 C
207 Springing: Defined
as the high frequency non-linear resonant response induced by cyclic
(steady state) loads in low to moderate seastates.
Sec.1 C
208 Surge, sway, heave: Translatory
displacements of TLP in horizontal planes (surge, sway) and vertical plane
(heave).
Sec.1 C
209 TLP deck structure: The
structural arrangement provided for supporting the topside equipment
or modules. Normally, the deck serves the purpose of being the major
structural component to ensure that the pontoons, columns and deck
act as one structural unit to resist environmental and gravity loads.
Sec.1 C
210 TLP foundation: Defined
as those installations at, or in, the seafloor which serve as anchoring
of the tendons and provides transfer of tendon loads to the foundation
soil.
Sec.1 C
211 TLP hull: Consists of buoyant columns, pontoons and intermediate
structural bracings, as applicable.
Sec.1 C
212 TLP tendon system: Comprises
all components between, and including the top connection(s) to the
hull and the bottom connection(s) to the foundation(s). Guidelines,
control lines, umbilicals etc. for tendon service and or other permanent
installation aids are considered to be included as part of the tendon
system.
Sec.1 C
213 Vortex induced motions (VIM): Vortex
induced motion (VIM): Transverse (cross) and in-line, current induced
floater motions.
Sec.1 C
214 Vortex induced vibrations (VIV): The
in-line and transverse oscillation of a tendon, riser, or floater
in a current induced by the periodic shedding of vortices.
Sec.1 C
215 Wave frequency (WF) responses: TLP
linear rigid body motions at the dominating wave periods.Sec.1
D. Abbreviations and Symbols
Sec.1
D 100 Abbreviations
Sec.1 D
| Table D1 Abbreviations |
| Abbreviation | In full | ALS | Accident limit states | | AUT | Automatic ultrasonic testing | | BTI | Bottom tendon interface | | BTC | Bottom tendon connector | | DFF | Design fatigue factors | | DNV | Det Norske Veritas | | FLS | Fatigue limit states | | HF | High frequency | | HRP | Heave restrained platform | | IC | Inspection category | | LAJ | Length adjustment joint | | LAT | Lowest astronomical tide | | LMP | Load management program | | OS | Offshore standard | | OSS | Offshore service specification | | LF | Low frequency | | LRFD | Load and resistance factor design | | NDT | Non-destructive testing | | QTF | Quadratic transfer function | | RAO | Response amplitude operator | | TLP | Tension leg platform | | TLWP | Tension leg wellhead platform | | TTI | Top tendon interface | | TTMS | Tendon tension monitoring system | | ULS | Ultimate limit states | | VIM | Vortex induced motion | | VIV | Vortex induced vibrations | | WF | Wave frequency | |
Sec.1
D 200 Symbols
Sec.1 D
201 The following Latin symbols are used:| xD | load effect |
| D | number of years |
| FX(c) | long-term peak distribution |
| Hs | significant wave height |
| ND | total number of load effect maxima during D
years |
| Tp | wave period. |
Sec.1 D
202 The following Greek symbols are used:| gf,D | load factor for deformation loads |
| gf,E | load factor for environmental loads |
| gf,G,Q | load factor for permanent and functional loads |
| gm | material factor. |
Sec.1
E. Description of the Tendon System
Sec.1
E 100 General
Sec.1 E
101 Individual tendons are considered within this standard as
being composed of three major parts:| — | interface at the platform |
| — | interface at the foundation (seafloor) |
| — | link between platform and foundation. |
| — | in most cases, tendons will also have intermediate connections
or couplings along their length, see Figure 2. |
Sec.1 E
102 Tendon components at the platform interface shall adequately
perform the following main functions:| — | apply, monitor and adjust (if
possible) a prescribed level of tension to the tendon |
| — | connect the tensioned tendon to the platform |
| — | transfer side loads and absorb bending moments or rotations
of the tendon relative to TLP. |
Sec.1 E
103 Tendon components providing the link between the platform
and the foundation consist of tendon elements (tubulars, solid rods
etc.), termination at the platform interface and at the foundation
interface, and intermediate connections of couplings along the length
as required. The intermediate connections may take the form of mechanical
couplings (threads, clamps, bolted flanges etc.), welded joints
or other types of connections. Figure 2 shows a typical TLP tendon
system.
Fig. 2 Typical TLP tendon system
Sec.1 E
104 Tendon components at the foundation interface shall adequately
perform the following main functions:
| — | provide the structural connection
between the tendon and the foundation |
| — | transfer side loads and absorb bending moments, or rotations
of the tendon |
| — | tolerate certain level of tendon slacking without disengaging
or buckling the tendon |
| — | allow for future change-out of tendons (if required). |
Sec.1 E
105 The tendon design may incorporate specialised components,
such as:| — | corrosion-protection system
components |
| — | buoyancy devices |
| — | sensors and other types of instrumentation for monitoring
the performance and condition of the tendons |
| — | auxiliary lines, umbilicals etc. for tendon service
requirements and/or for functions not related to the tendons |
| — | provisions for tendons to be used as guidance structure
for running other tendons or various types of equipment |
| — | elastomeric elements |
| — | intermediate connectors with watertight bulkheads for
tendon compartmentation (if needed). |
Sec.1 E
106 Certification requirements for tendon system are specified
in Appendix A.
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