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DNV-OS-C105 Structural Design of TLPS (LRFD method)
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SECTION 3
Design PrinciplesSec.3
A. Introduction
Sec.3
A 100 General
Sec.3 A
101 The following basic design criteria shall be complied with
for the TLP design:- The TLP shall be able to sustain
all loads liable to occur during all relevant temporary and operating
design conditions for all applicable limit states.
- Wave loading on the deck structure should not occur
in the ultimate limit states (ULS). Wave loading on the deck structure
may be accepted in the accidental limit states (ALS) condition provided
that such loads are adequately included in the design.
- Momentary (part of a high frequency cycle) loss of tendon
tension may be accepted provided it can be documented that there
will be no detrimental effects on tendon system and supporting (foundation
and hull) structures, and it would not cause the tendon to become
disengaged.
Sec.3 A
102 Operating tolerances shall be specified and shall be achievable
in practice. Normally, the most unfavourable operating tolerances
shall be included in the design. Active operation shall not be dependent
on high reliability of operating personnel in an emergency situation.Guidance note:
Active operation of the following may be considered in an
emergency situation, as applicable:| - | ballast distribution| - | weight distribution| - | tendon tension| - | riser tension. | | | |
A clearly defined and well calibrated Load Management Program
or equivalent shall be available onboard to facilitate safe management
of these parameters in normal operation and emergency situation.
Details of Load Management Program is given in Appendix
A, E1100.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.3
B. Design Conditions
Sec.3
B 100 General
Sec.3 B
101 The structure shall be designed to resist relevant loads associated
with conditions that may occur during all stages of the life cycle
of the unit. Such stages may include:
| — | fabrication |
| — | site moves |
| — | mating |
| — | sea transportation |
| — | installation |
| — | operation |
| — | decommissioning. |
Sec.3 B
102 Structural design covering marine operation and fabrication
sequences shall be undertaken in accordance with DNV-OS-C101.
Sec.3 B
103 Marine operations may be undertaken in accordance with the
requirements stated in Rules for Planning and Execution of Marine
Operations. All marine operations shall, as far as practicable,
be based upon well proven principles, techniques, systems and equipment
and shall be undertaken by qualified, competent personnel possessing
relevant experience.
Sec.3 B
104 Structural responses resulting from one temporary phase condition
(e.g. a fabrication or transportation operation) that may affect
design in another phase shall be clearly documented and considered
in all relevant design workings.Sec.3
B 200 Fabrication
Sec.3 B
201 The planning of fabrication sequences and the methods of fabrication
shall be performed. Loads occurring in fabrication phases shall
be assessed and, when necessary, the structure and the structural
support arrangement shall be evaluated for structural adequacy.
Sec.3 B
202 Major lifting operations shall be evaluated to ensure that
deformations are within acceptable levels, and that relevant strength
criteria are satisfied.Sec.3
B 300 Hull and deck mating
Sec.3 B
301 All relevant load effects incurred during mating operations
shall be considered in the design process, e.g. hydrostatic load,
lock-in stresses, tolerances, deflections, snatch/shock
loads (if applicable) etc. Sec.3
B 400 Sea transportation
Sec.3 B
401 A detailed transportation assessment shall be undertaken which
includes determination of the limiting environmental criteria, evaluation
of intact and damage stability characteristics, motion response
of the global system and the resulting, induced load effects. The
occurrence of slamming loads on the structure and the effects of
fatigue during transport phases shall be evaluated when relevant.The accumulated fatigue damage during transportation phases
shall be included in the fatigue assessment of in-place condition.
Sec.3 B
402 In case of transportation (surface or sub surface) of tendons;
this operation shall be carefully planned and analysed. Special
attention shall be given to attachment or securing of buoyancy modules.
Model testing shall be considered.
Sec.3 B
403 Satisfactory compartmentation and stability during all floating
operations shall be ensured. See details in Sec.6 for
ULS condition and Sec.8 for ALS
condition.
Sec.3 B
404 All aspects of the transportation, including planning and
procedures, preparations, seafastenings and marine operations should
comply with the requirements of the warranty authority.
Sec.3
B 500 Installation
Sec.3 B
501 Installation procedures of foundations (e.g. piles, suction
anchor or gravity based structures) shall consider relevant static
and dynamic loads, including consideration of the maximum environmental
conditions expected for the operations.
Sec.3 B
502 For novel installation activities (e.g. foundations and tendons),
relevant model testing should be considered.
Sec.3 B
503 Free standing tendon (pending TLP installation) phases shall
be considered with respect to loads and responses.
Sec.3 B
504 The loads induced by the marine spread mooring involved in
the operations, and the forces exerted on the structures utilised
in positioning the unit, such as fairleads and pad eyes, shall be
considered for local strength checks.
Sec.3 B
505 For segmented tendons, tendon buckling should also be checked
for the lifting of the segment during installation.Sec.3
B 600 Decommissioning
Sec.3 B
601 Decommissioning and removal of the unit shall be planned for
in the design stage.Sec.3
C. Design Principles, Tendons
Sec.3
C 100 General
Sec.3 C
101 Essential components of the tendon system shall be designed
by the principle that, as far as practicable, they are to be capable
of being inspected, maintained, repaired and/or replaced.
Sec.3 C
102 Tendon mechanical components shall, as far as practicable,
be designed to be "fail safe". Consideration is
to be given in the design to possible early detection of failure
for essential components, which cannot be designed according to
this principle.
Sec.3 C
103 Certain vital tendon components may, due to their specialized
functions, and if unproven, require engineering and prototype qualification
testing to determine:| — | confirmation of anticipated
design performance |
| — | fatigue characteristics |
| — | fracture characteristics |
| — | corrosion characteristics |
| — | mechanical characteristics. |
Sec.3 C
104 A TLP shall be designed with sufficient safety margin to prevent
the potential of tendon rupture. The tendon system and the securing
or supporting arrangements shall be designed in such a manner that
a possible failure or removal of one tendon is not to cause progressive
tendon failure or excessive damage to the securing or supporting
arrangement at the platform or at the foundation.
Sec.3 C
105 A fracture control strategy should be adopted to ensure consistency
of design, fabrication and in service monitoring assumptions. The
objective of such a strategy is to ensure that the largest undetected
flaw from fabrication of the tendons will not grow to a size that
could induce failure within the design life of the tendon, or within
the planned in-service inspection interval, within a reasonable
level of reliability. Elements of this strategy include:| — | adequate design fatigue life |
| — | adequate fracture toughness |
| — | reliability of inspection during fabrication |
| — | in-service inspection intervals and methods. |
See Sec.7 for guidance on
fracture control and required fatigue life for tendons.
Sec.3 C
106 Inspection to detect damage due to accidental loads or overloads
may be replaced by monitoring the loads and comparing them to the
design loads, provided that the events can be measured by the monitoring system.
If this method is used the component must be replaced after any
overload occurrence or other events exceeding the design scenario.
Sec.3 C
107 All materials liable to corrode shall be protected against
corrosion. Special attention should be given to:
| — | local complex geometries |
| — | areas that are difficult to inspect or repair |
| — | consequences of corrosion damage |
| — | possibilities for electrolytic corrosion |
| — | dissimilar metal. |
Sec.3 C
108 All sliding surfaces shall be designed with sufficient additional
thickness against wear. Special attention should be given to the
following:| — | cross-load bearings |
| — | seals |
| — | ball joints. |
Sec.3 C
109 Satisfactory considerations shall be given to settlement or
subsidence, which may be a significant factor in determining tendon-tension
adjustment requirements.
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