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The electronic pdf versions of the documents found through http://www.dnv.com/ are the officially binding versions. Copyright Det Norske Veritas.
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DNV-OS-C105 Structural Design of TLPS (LRFD method) |
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| App.A: Certification of Tendon System |
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 may can however be carried out by the local DNV stations in accordance with the requirements of this Appendix.
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). |
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-C101 |
| — | DNV-OS-C401 |
| — | DNV-OS-B101 |
| — | DNV-OS-F101 |
| — | DNV-RP-C201/202 |
| — | DNV CN 30.4 |
| — | DNV-RP-C203 |
| — | DNV-RP-B401 |
| — | BS 7910 |
| — | BS 7448. |
DNV uses categorization in order to clearly identify the certification and approval requirements for different equipment and components.
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.
If there are any other equipment which is not defined in the following tables, categorization of the same shall be decided on a case by case basis with prior discussion with DNV.
Equipment categorization for offshore installations or units is as follows:
| I | = | Equipment/component important for
safety and integrity of the TLP and for which a DNV certificate is required. |
| II | = | Equipment/component important for
safety and integrity of the TLP and for which a works certificate prepared by the manufacturer is accepted. |
Equipment category I
For equipment category I, 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: ---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Independent test certificates or reports for the equipment,
or approval certificate for manufacturing system, are also acceptable.
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.
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. |
The following documentation will normally be issued by DNV for equipment and systems covered by certification activities (CMC):
| — | 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. |
| — | 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 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. |
| — | 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 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. |
The loads for the tendon component analysis are to be obtained from the tendon global analysis. All relevant requirements as mentioned in Section 1 to Section 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.
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.
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.
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.
A protection shield/hat above the bottom connector may be considered in order to prevent debris entering the bottom connector housing.
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: ---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
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.
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 pipe 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. |
Welding shall be performed with low hydrogen welding consumables/processes that give a diffusible hydrogen content of maximum 5 ml/100 g weld metal. Welding procedure qualification for girth welds shall be performed on the actual tendon pipe material and shall include:
| — | transverse weld tensile testing |
| — | all weld tensile testing |
| — | bend testing |
| — | impact testing of base material, weld, fusion line and fusion line +2 mm |
| — | macro examination and hardness testing |
| — | fracture toughness testing of weld metal and at fusion line. |
Adequate acceptance criteria for the service condition and minimum design temperature shall be specified in line with the requirements for tendon pipe.
Repair welding procedures shall be separately qualified on an existing weld and shall include:
| — | full and half thickness repair |
| — | shallow multipass repairs |
| — | single pass repairs. |
Full and half thickness repair welding procedures shall be tested as and meet the same acceptance criteria as the tendon pipe girth welds. Shallow multipass repairs welding procedures shall be tested as and meet the same acceptance criteria as the tendon pipe girth welds except that fracture toughness testing may not be required. Single pass repairs shall be root, face and side bend tested, macro examined and hardness tested and meet the same acceptance criteria as the tendon pipe girth welds.
Guidance note: ---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Pipe according to DNV-OS-F101, Section 6 with grade designation
SAWL xxx I SD will normally meet the required service requirements.
Suffix U may be added to enhance uniform tensile properties. Pipe
according to API 5L shall be specified with the additional requirements
for tendon pipes given above.according to TMCP process or equivalent shall
be used for the tendon pipes.
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. |
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" or during temporary phases of negative tension in ULS or ALS conditions at the bottom tendon interface. A mechanical latch may be provided on the BTC to prevent stroke-out and the risk of disengagement.
The tendon receptacle and other interfaces attached to the pile shall be subjected to all applicable loads related to pile design and installation.
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. Protection shielding shall be considered to prevent debris from entering between tendon and bottom receptacle.
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
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.
Fracture Mechanics tests shall be carried out in accordance with Sec.7.
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: ---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
DNV-RP-A203 gives an outline for the qualification procedures.
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. |
Flex bearings shall be tested to adequately characterize rotational stiffness, axial capacity and angular capacity.
Acceptance criteria for the all elements of the finished product shall be clearly specified and agreed before fabrication.
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.
Foundation design and fabrication shall be carried out in accordance with DNV CN 30.4 or other acceptable standards i.e. API RP 2A 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.
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.
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.
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 |
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.
Adequate protection (including corrosion) mechanism shall be provided for the TTI to protect the LAJ.
Connector in way of the LAJ shall be checked for strength and fatigue with the reduced cross section.
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, racketing and fretting as applicable.
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.
Suitable and reliable tendon tension monitoring devices shall be installed to obtain the actual tension during operation.
Guidance note: ---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Generally, one TTMS unit per corner (a group of tendons) is
sufficient.
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.
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.
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.
HULL interfaces with the tendons including the backup structure are to be designed for the breaking load of the tendons.
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 (under ULS condition) exceeds 67% of yield shall be subjected to more rigorous NDT than other areas.
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.
Tendon assembly shall in general 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.
Cathodic protection shall be carried out in accordance with DNV Recommended Practice DNV- RP-B401.
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.
Anode and other attachment details and welding to the tendon system shall be specifically approved by DNV.
Guidance note: ---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Design of the corrosion protection system, shall consider
the electric continuity between tendon, hull and pile.
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.
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.
The load management system shall meet the continuous availability requirement as defined in DNV-OS-D202, Ch.2 Sec.1 B200.
App.A F
| Table F1Categories for tendon systems, equipment and components | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Relevant text | Material or equipment | DNV approval categories I | II | TENDON PIPE | Pipe | 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 and Software | X | X | | | | LOAD MANAGEMENT PROGRAM (LMP) | Hardware and 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 | | | | | | |
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-C-401. 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.
The extent and the methods of NDT chosen for the tendon fabrication shall meet the requirements of DNV-OS-C401Sec.3 C105.
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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