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DNV-OS-C201 Structural Design of Offshore Units (WSD method)
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APPENDIX D
Certification of Tendon System
App.D
A. General
App.D
A 100 Introduction
App.D A
101 Certification of the tendon system is accomplished through
the Certification of Material and Components (CMC) from various
manufacturers. Since the Tendon system itself is an extension of
the main load-bearing element of the TLP, it cannot be handled in
a traditional CMC manner. Design approval of these various components
need to be aligned with the global performance of the TLP and applicable
load cases. Approval of all the components of the Tendon system
and its interfaces shall be handled by the same DNV office whose
is responsible for the approval of the TLP main structure. Survey
can however be carried out by the local DNV stations in accordance
with the requirements of this Appendix.
App.D A
102 Tendon system generally consists of the following main elements:| — | Tendon pipe |
| — | Bottom Tendon Interface (BTI) |
| — | Flex Bearings |
| — | Foundation |
| — | Top Tendon Interface (TTI) |
| — | Tendon Intermediate Connectors |
| — | Tendon Tension Monitoring System (TTMS) |
| — | Tendon Porch |
| — | Tendon Cathodic Protection System |
| — | Load Management Program (LMP) |
App.D A
103 The following international standards and DNV standards are
considered acceptable standards for design and fabrication of various
components:| — | API RP 2T |
| — | API RP 2A |
| — | API RP 2RD |
| — | API RP 2R |
| — | DNV-OS-C201 |
| — | DNV-OS-C401 |
| — | DNV-OS-B101 |
| — | DNV-RP-C201/202 |
| — | DNV CN 30.4 |
| — | DNV-RP-C203 |
| — | DNV-RP-B401 |
| — | BS 7910 |
| — | BS 7448 |
App.D
B. Equipment categorization
App.D
B 100 General
App.D B
101 DNV uses categorization in order to clearly identify the certification
and approval requirements for different equipment and components.
App.D B
102 Categorization of equipment depends on importance for safety
and takes operating and environmental conditions into account. Once
assigned, the category of equipment refers to the scope of activities
required for DNV certification and approval, as consistent with
the importance of the equipment.
App.D B
103 If there are any other equipment which is not defined in the
following tables, categorisation of the same shall be decided on
a case by case basis with prior discussion with DNV.
App.D B
104 Equipment categorization for offshore installations or units
is as follows:| I | = | Equipment/component important for safety & integrity
of the TLP and for which a DNV certificate is required. |
| II | = | Equipment/component important for safety & integrity
of the TLP and for which a works certificate prepared by the manufacturer
is accepted. |
Equipment category I
For equipment category 1, the following approval procedure
shall be followed:
| — | design approval, documented
by a design verification report (DVR) or type approval certificate.
|
| — | fabrication survey, documented by issue of a product
certificate. |
Specific requirements:
| — | pre-production meeting prior
to the start of fabrication |
| — | survey during fabrication, as applicable |
| — | witness final functional, pressure and load tests, as
applicable |
| — | review of fabrication records. |
These requirements are typical and the final extent of DNV
survey required will be decided based on:
| — | complexity, size and previous
experience of equipment type |
| — | manufacturer's QA/QC system |
| — | manufacturing survey arrangement (MSA) with DNV |
| — | type of fabrication methods. |
Equipment category II
Equipment of category II is normally acceptable on the basis
of a works certificate prepared by the manufacturer. As a minimum,
the certificate shall contain the following data:
| — | equipment specification or data
sheet |
| — | operating limitation(s) of the equipment |
| — | statement from the manufacturer to confirm that the
equipment has been constructed and manufactured according to recognised
methods, codes, and standards |
| — | test records as applicable. |
Guidance note:
Independent test certificates or reports for the equipment,
or approval certificate for manufacturing system, are also acceptable. ---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
App.D
C. Fabrication Record
App.D
C 100 General
App.D C
101 Fabrication record shall be maintained by the manufacturer
in a traceable manner, so that relevant information regarding design
specifications, materials, fabrication processes, inspection, heat
treatment, testing, etc. can be checked.
App.D C
102 Fabrication record for category I equipment shall be available
for review. The following particulars shall be included, as applicable: | — | manufacturer's statement
of compliance |
| — | reference to design specifications and drawings |
| — | location of materials and indication of respective material
certificates |
| — | welding procedure specifications and qualification test
records |
| — | location of welding indicating where the particular
welding procedures have been used |
| — | heat treatment records |
| — | location of non-destructive testing (NDT) indicating
where the particular NDT method has been used and its record |
| — | load, pressure and functional test reports |
| — | as-built part numbers and revisions. |
App.D
D. Documentation Deliverables for
Certification of Equipment
App.D
D 100 General
App.D D
101 The following documentation will normally be issued by DNV
for equipment and systems covered by certification activities (CMC): - Design verification
report, (DVR)
| — | DVR will be issued by the design
approval responsible for all equipment of category I, unless covered by
a valid type approval certificate. |
| — | In addition to each individual equipment, DVRs shall
be issued for each system not covered by plan approval. |
The DVR shall contain all information needed to be followed
up by the surveyor attending fabrication survey and installation
of the equipment, and as a minimum include: | — | Design codes and standards used
for design verification |
| — | Design specification (e.g. temperature, pressure, SWL,
etc.) |
| — | Follow-up comments related to e.g. testing, fabrication
and installation of the equipment or system.
An approval letter may be issued instead of a DVR, however
such a letter shall as a minimum contain the same information as
listed above. |
- Inspection release note, (IRN)
| — | An IRN shall only be issued
if the component is delivered prior to issuance of final product
certificate (PC). A final PC shall not be issued if there are non-conformances
to the equipment or system. The IRN shall be used with detailed
description of the non-conformances, and shall always be replaced
by a certificate when all non-conformances are closed. |
- Product certificate, (PC)
| — | PC should be issued for all
category I equipment or systems |
| — | PC will be issued upon successful completion of design
verification, fabrication survey and review of final documentation.
As stated above, PC can not be issued if design verification or
non-conformances are outstanding. |
- Survey report
| — | Survey report shall be issued for all category
I equipment or systems upon satisfactory installation, survey and
testing onboard. A survey report may cover several systems or equipment
installed. The survey report shall contain clear references to all
DVRs and PCs on which the survey report is based, and shall state
testing and survey carried out. |
App.D
E. Tendon Systems and Components
App.D
E 100 General
App.D E
101 The loads for the tendon component analysis shall be obtained
from the tendon global analysis. All relevant requirements as mentioned
in Sec.1 to Sec.8 as applicable for the component shall be followed.
The requirements specified below are some additional requirements
that are specific to some of the components.
App.D E
102 As most of these connectors are complex in design and fabrication,
detailed linear elastic finite element analysis (FEA) shall be carried
out using industry recognized FE programs. In general, a 3D finite
element analysis using solid/brick elements will be required
unless a 2D analysis can realistically represent and simulate the
connectors, applicable loads and interfaces. Testing will be required
where necessary to justify and document the FEA.
App.D E
103 The design and construction shall cover all applicable load
conditions transportation, lifting, installation and operation etc.
The effects of fabrication tolerances, fit-up misalignment etc.
shall be included. All connectors must be designed and fabricated
with due consideration for installation and removal of damaged tendons.
App.D E
104 If the transportation and installation phase of the tendons
are not certified by DNV, information shall be submitted to DNV
to document the fatigue damage, locked-in stresses etc. resulting
from the lifting, transportation, free-standing tendon etc.
App.D E
105 A higher safety margin shall be considered for the tendon
components than for the tendon pipes due to the complexity of the
components and uncertainties in the response calculation.Guidance note:
In general a Design Fatigue Factor of minimum 10 shall be
used for fatigue design of tendon and tendon components provided
that the analyses are based on a reliable basis as described above.
However, if the fatigue life assessment is associated with a larger
uncertainty, a higher Design Fatigue Factor for complex tendon components
may be recommended. In such cases, a higher Design Fatigue Factor
should be determined based on an assessment of all uncertainties
in the fatigue analysis with due consideration to the consequence
of a fatigue failure. Before increasing the Design Fatigue Factor
one should aim to reduce the uncertainties in the design basis as
much as possible.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
App.D
E 200 Tendon pipe
App.D E
201 Pipe manufacturer shall be an approved manufacturer or shall
be certified by DNV. The pipes must be adequately specified for
the service conditions. The following as a minimum shall be specified
as applicable:| — | the pipe shall be formed from
Thermo-Mechanically Controlled Process (TMCP) plates |
| — | Submerged Arc Welding (SAW) process shall be used for
the manufacturing of the pipes |
| — | the steel shall be fully killed and melted to fine grain
practice. |
| — | tensile and compression testing shall be performed also
in the longitudinal direction. |
| — | the variation in yield stress should be limited |
| — | material fracture resistance properties shall be specified. |
| — | the impact toughness of base material, weld and Heat-Affected
Zone (HAZ) must be acceptable considering the service temperature. |
| — | the hardness of welds must not exceed 330 Brinell's
Hardness Number (BHN) and tendons and weld areas must have a high
grade coating to prevent hydrogen embrittlement (especially important
for high tensile steels) |
| — | NDT should be performed to ensure freedom of imperfections
especially transverse to the direction of stress in the weld and |
| — | base material as little variation as possible in wall
thickness, diameter and out of roundness to reduce stress concentrations
around welds. |
The girth weld of the tendon pipes shall be qualified on the
actual pipe material.
Guidance note:
API 5L pipes with additional service requirements shall be
specified. Alternatively, Line pipes as mentioned in DNV OS-F101,
Sec.6 with agreed applicable properties can be used. ---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
App.D
E 300 Bottom tendon interface (BTI)
App.D E
301 The bottom tendon interface assembly must provide a secure
connection throughout the design life of the TLP. The connector
shall be designed adequately against yielding, fatigue and corrosion.
BTI normally consist of the following main elements:| — | a receptacle which will be welded
to the pile |
| — | a bottom tendon connector (BTC) which locks in to the
receptacle |
| — | a flex bearing element |
| — | a tendon extension piece that is welded to the tendon
pipe. |
App.D E
302 The maximum angular stiffness of the connection shall be specified
consistent with the tendon design. There shall be no disengagement
of the load bearing surfaces assuming a minimum tendon tension of "zero" at
the bottom tendon interface.
App.D E
303 The tendon receptacle and other interfaces attached to the
pile shall be subjected to all applicable loads related to pile
design and installation.
App.D E
304 BTI and flex bearing design shall allow for rotation between
the tendon and pile considering all applicable operation and installation
conditions. Maximum installation angle shall be specified for the
BTC to enter and lock in to the receptacle.
App.D E
305 Pile installation loads and applicable impact loads for all
components that are relevant during the installation and transportation
phase shall be considered in the design
App.D E
306 Guidance on fatigue methodology is defined in DNV RP-C203.
If no documented S-N curve exists for the material selected, S-N
curves shall be selected from DNV RP-C203.
App.D E
307 Fracture mechanics tests shall be carried out in accordance
with Sec.13 H. App.D
E 400 Flex bearings
App.D E
401 The selected material and manufacturer should have adequate
prior experience with successful in-service history to demonstrate
adequacy for its intended purpose. If a new material or manufacturer
without sufficient prior experience for similar application is selected,
the material and manufacturing process shall go through an adequate
level of qualification.Guidance note:
DNV-RP-A203 gives an outline for the qualification procedures.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
App.D E
402 Manufacturer shall demonstrate by in-depth analysis and testing
that the product meets the specified properties for the flex element
including but not limited to:| — | specified tendon loads |
| — | maximum rotational stiffness |
| — | minimum axial stiffness |
| — | design life |
| — | internal pressure (if applicable) |
| — | other properties as specified. |
App.D E
403 Flex bearings shall be tested to adequately characterize rotational
stiffness, axial capacity and angular capacity.
App.D E
404 Acceptance criteria for the all elements of the finished product
shall be clearly specified and agreed before fabrication.App.D
E 500 Foundations
App.D E
501 The loading for the foundation (pile or gravity based) design
should be obtained from the tendon and geotechnical analysis. The
loads resulting from all applicable load conditions including the
damaged and removed tendon cases shall be considered.
App.D E
502 Foundation design and fabrication shall be carried out in
accordance with DNV CN 30.4 or other acceptable standards i.e. API
RP 2A - WSD and API RP 2T. For gravity based concrete foundations,
interface with tendon bottom connector and receptacle is given in
DNV-OS-C502. Foundation system shall be designed for the same in-place
conditions as the tendon system it supports, including tendon damage
cases.
App.D E
503 In particular, analysis shall reflect positioning tolerances
for installation, installation loads like pile driving and installation
and in-place damage. For gravity based structures, settlements (or
uplift) needs to be taken into account.
App.D E
504 Tendon foundation receptacle and pile above the mudline need
to be protected from external corrosion by a combination of coatings
and passive cathodic protection systems.App.D
E 600 Top tendon interface (TTI)
App.D E
601 The top tendon interface assembly must provide a secure connection
throughout the life of the TLP. The connector shall be designed
adequately against yielding, fatigue and corrosion. TTI normally
consist of the following main elements:| — | tendon porch that is attached
to the HULL |
| — | the length adjustment joint (LAJ) that will be welded
to the top tendon piece |
| — | tendon connector with the flex bearing |
| — | TTMS interface |
App.D E
602 TTI and flex bearing design shall allow for rotation between
the tendon and hull connection considering all applicable operation
and installation conditions. Maximum installation angle shall be
specified for the tendon to enter and lock in to the TTI.
App.D E
603 Connector in way of the LAJ shall be checked for strength
and fatigue with the reduced cross section.
App.D E
604 Adequate protection mechanism shall be provided for the TTI
including the "corrosion cap" on the top, protecting
the LAJ.App.D
E 700 Intermediate tendon connectors
(ITC)
App.D E
701 The intermediate tendon connectors (ITC) must provide a secure
connection throughout the life of the TLP. The connector shall be
designed adequately against yielding, fatigue and corrosion, racheting
and fretting as applicable.
App.D E
702 The connectors must be sealed and form a watertight connection.
The design shall ensure that all potential damage during handling
and installation for the sealing mechanism is identified and designed
against.App.D
E 800 Tendon tension monitoring system
(TTMS)
App.D E
801 Suitable and reliable tendon tension monitoring devices shall
be installed to obtain the actual tension during operation.
App.D E
802 This system generally consists of load cells, data acquisition
system and alarm system. Load cells shall be calibrated to the required
accuracy for the range of tension anticipated accounting for all
possible system errors.
App.D E
803 Alarm shall be pre-set for the values that exceed the design
conditions so that adequate load balancing and operational measures
can be taken to ensure that the tendon tension remains within the
maximum allowable values. The alarms shall be audible and visually
represented in the room where the LMP is monitored.
App.D E
804 The load cells and all critical elements of the data acquisition
system shall be redundant. There shall be more than one load cell
per tendon. It shall monitor both tendon tension and bending moments
(requires 3 load cells).
App.D E
805 Marine quality cables shall be used and a watertight sealing
shall be arranged for the top and bottom load ring interface.App.D
E 900 Tendon porch
App.D E
901 HULL interfaces with the tendons including the backup structure
shall be designed for the breaking load of the tendons.
App.D E
902 Cast steel shall be a weldable low carbon and fine grained
steel. Test coupons representing the greatest end thickness of welding
to the HULL shall be developed from each casting to facilitate actual
production weld testing qualifications. NDT for the special areas
(welding attachment to the HULL) or wherever the stress level exceeds
67% of yield shall be subjected to more rigorous NDT than
other areas.
App.D E
903 Acceptance criteria for weld repairs and acceptability shall
be clearly defined in the specifications and agreed upon. Casting
shall in general be in accordance with DNV offshore standard DNV-OS-B-101,
Ch.2 Sec.4. App.D
E 1000 Tendon corrosion protection system
App.D E
1001 Tendon assembly shall be protected using a combination of
coating systems, sacrificial-anodes, material selection, and corrosion
allowance considered for the life time of the platform and the inspection philosophy
during operation. Special areas like the TTI may need corrosion
inhibitors, corrosion cap etc for protecting the moving parts. An
affective corrosion protection system shall be in place from the
time the structure is initially installed.
App.D E
1002 Cathodic protection shall be carried out in accordance with
DNV Recommended Practice DNV- RP-B401.
App.D E
1003 Site specific data shall be used for the corrosion protection
design. Special considerations shall be given for the higher ambient
temperature effect for areas like West Africa.
App.D E
1004 Anode and other attachment details and welding to the tendon
system shall be specifically approved by DNV.App.D
E 1100 Load management program (LMP)
App.D E
1101 Load management program shall facilitate the safe operation
of the TLP and the tendon systems under the defined load conditions
by monitoring the weight changes and centre of gravity (CG) shifts
compared with the pre-defined envelope. It shall be possible to
automatically calculate weight redistribution of live loads and ballast
water. Other relevant variable data such as draft, wave, wind etc.
shall be used by the program as appropriate.
App.D E
1102 The system shall operate from a UPS power supply and shall
have a redundant fail safe system. Data shall be backed-up continuously
and all important data saved on a regular basis.App.D
F. Categorisation of Tendon Components
App.D
F 100 General
App.D F
101 The load management system shall meet the continuous availability
requirement as defined in DNV-OS-D202, Ch.2 Sec.1 B200.App.D F
| Table XX Categories
for tendon systems, equipment and components |
| Relevant
text | Material
or equipment | DNV approval
categories | I | II | TENDON PIPE | Pipe | X | | | | X | | | TOP TENDON INTERFACE | TTI Connector | X | | | LAJ Assembly | X | | | Top Flex Bearing | X | | | | | | | BOTTOM TENDON INTERFACE | BTI Connector | X | | | BTI Receptacle | X | | | BTI Flex Bearing | X | | | | | | | INTERMEDIATE CONNECTORS | Connectors | X | | | | | | | FOUNDATION (PILE) | Pile | X | | | | | | | TENDON TENSION MONITORING
SYSTEM | Load Cell | X | | | Hardware &Software | | X | | | X | | | LOAD MANAGEMENT PROGRAM
(LMP) | Hardware & Software | X | | | | | | | TENDON CATHODIC PROTECTION
SYSTEM | Anodes | | X | | Attachments | | X | | | | | | FLEX BEARINGS | Reinforcement | X | | | Outer/Inner
bulk metal | X | | | Elastomer | X | | | | | | | TENDON PORCH | Casting | X | | | | | | | |
App.D
G. Tendon Fabrication
App.D
G 100 General
App.D G
101 Tendon systems are critical load carrying elements and are
essential for the integrity of the TLP. Fabrication of the tendon
system in general shall meet the requirements as applicable for "special
areas". NDT requirement on all welding shall, as a minimum,
be in accordance with the butt weld requirement for inspection category
1 as defined in DNV Offshore standard DNV-OS-C401. In all cases,
where the global design requires more stringent standards than what
is outlined in the DNV standard, fabrication requirements shall
be adjusted such that the tendon joints meet those higher requirements.
App.D G
102 The extent and the methods of NDT chosen for the tendon fabrication
shall meet the requirements of DNV-OS-C401, Ch.2 Sec.3 C105.
App.D G
103 Casting, forging techniques used for the tendon fabrication
shall, as a minimum, meet the good practices as outlined in DNV-OS-B101
Ch.2 Sec.4 and Ch.2 Sec.3.
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