Section
5 Fatigue design
5.1 General
5.1.1 For the purposes of these Rules, the definitions of environmental terms,
e.g. squall and cyclone, given in ‘ISO 19901-1, Specific Requirements for Offshore
Structures - Part 1 Metocean design and operating considerations' apply.
5.1.2 Fatigue damage due to cyclic loading is to be considered in the design of
all unit types. The extent of the fatigue analysis will be dependent on the mode and
area of operation.
5.1.3 Where any unit is intended to operate at one location for an extended
period of time, a rigorous fatigue analysis is to be performed using the long-term
prediction of environment for that area of operation with the unit at the intended
orientation. Due allowance is to be made of any previous operational history of the
unit.
5.1.4 The two basic methods of fatigue analysis available are Deterministic
Fatigue Analysis and Spectral Fatigue Analysis. Both are acceptable to LR.
5.1.5 Factors which influence fatigue endurance and should be accounted for in
the design calculations include:
- Loading spectrum.
- Detail structural design.
- Fabrication and tolerances.
- Corrosion.
- Dynamic amplification.
5.1.6 The following important sources of cyclic loading should be considered
in the design:
- Waves (including those which cause slamming and
variable-buoyancy effects).
- Wind (especially when vortex shedding is induced, e.g. on
slender members).
- Currents (where these influence the forces generated by waves
and/or induced vortex shedding).
- Mechanical vibration (e.g. caused by operation of
machinery).
- Temperature (thermal cycles) for areas of significant
temperature variation.
- Gravity and functional loads (e.g. caused by loading and
unloading, and lifting operations).
For sites susceptible to cyclones, the fatigue design of units which
remain on-station during cyclones is to include cyclic loading from both the
cyclonic and non-cyclonic environments.
5.1.7 Where a fine mesh finite element analysis is carried out to determine
local geometric stress concentration factors, selection of associated S-N curves
will be specially considered. Account is to be taken of fatigue stress direction
relative to the weld. In general, the element mesh size adjacent to the weld detail
under consideration is to be of the order of the local plate thickness. Mesh
arrangement and analysis methodology are to be agreed with LR.
5.1.9 Fatigue damage due to lifting operations is to be based on an estimate of the profile
of lifts when in operation.
5.2 Fatigue life assessment
5.2.1 Fatigue life assessment of all relevant structural elements is required
to demonstrate that structural connections have a fatigue endurance consistent with
the planned life of the unit and compliance with the minimum requirements. The
following structural elements are to be included:
- Column-stabilised and tension-leg units:
- Bracing structure.
- Bracing connections to lower hulls, columns and
decks.
- Column connections to lower hulls.
- Column connections to deck.
- Mooring structure and associated hull structure
integration.
- General structural discontinuities.
- Surface type units:
- Hull longitudinal stiffener connections to transverse
frames and bulkheads.
- Toe area of main structural brackets.
- Hopper knuckle connections.
- Main openings in the hull envelope.
- Mooring structure and associated hull structure
integration.
- General structural discontinuities in the primary hull
structure.
- Self-elevating units:
- Lattice legs and connections to footings.
- Leg support structure.
- Raw water towers.
- Other unit types:
- Special consideration will be given to the hull
structure of other unit types on the basis of this Section.
- General: Hull, deck and supporting structure in way of topside
facilities, e.g.:
- Module support.
- Process plant support stools.
- Crane pedestals and boom rests.
- Flare structures.
- Offloading station.
- Drilling derrick and substructures.
- Structure supporting pipe-laying systems.
- General: Other structures subjected to significant cyclic
loading.
5.2.2 Fatigue life is normally governed by the fatigue behaviour of welded
joints, including both main and attachment welds. Structure is to be detailed and
constructed to ensure that stress concentrations are kept to a minimum and that,
where possible, components may deform without introducing secondary effects due to
local restraints.
5.2.3 The minimum design fatigue life of a unit is to be specified by the
Owner, but is not to be less than 25 years, unless agreed otherwise by LR. See
also
Pt 10 Ship Units for ship units.
5.3 Fatigue damage calculations
5.3.1 The fatigue damage calculations are to be based on the long-term
distribution of the applied stress ranges. A sufficient number of draughts and
directions are to be included.
5.3.2 An appropriate wave spectrum is to be used and representative percentages
of the total cumulative spectrum included for each direction under consideration.
When using a limited number of directions, account is to be taken of symmetry within
the structure.
5.3.3 Cumulative damage may be calculated by Miner’s summation:
where
|
s |
= |
number of stress range blocks |
 |
= |
actual number of cycles for stress range block number ‘i’ |
 |
= |
corresponding number of cycles obtained from the relevant S-N
curve for the detail under consideration |
5.3.4 Cumulative damage for individual components is to take into account the
degree of redundancy, accessibility of the structure and also the consequence of
failure.
5.3.6 Where wave scatter diagrams are used for the calculation of fatigue
damage for mobile offshore units and transit voyages of floating offshore
installations at a fixed location, the scatter diagram is to contain at least one
year of data
5.4 Joint classifications, S-N curves and
fatigue life improvement methods
5.4.2 Consideration will be given to the use of alternative methods; detailed
proposals are to be submitted and agreed with LR.
Table 5.5.1 Fatigue life factors of
safety for structural components
| Inspectable/repairable
|
Fatigue life
factor
|
| Consequence of
failure
|
| Non-substantial
|
Substantial
See Notes 1 and 4
|
| Yes, dry
See Notes 2 and 5
|
1
|
2
|
|
Yes, wet
See Note 3
|
2
|
4
|
| No
|
3
|
10
|
| NOTES
|
| 1. Substantial consequences of failure include,
inter alia, loss of life, uncontrolled outflow of
hazardous or polluting products, collision, sinking. In
assessing consequences, account should be taken of the potential
for progressive failure. This factor will be applicable for
bottom structure of oil storage tanks of single bottomed units
and side structures of oil storage tanks of single sided units.
This factor will be applicable for supports for risers,
umbilicals and caissons; stools for safety critical topside
modules; crane pedestals and crane boom rests; supports for
topside structures including drilling plants, process plants,
flare towers, pipe-lay towers and derricks; supports for
helidecks; supports for lifeboat platforms; mooring attachments;
supports for thrusters; supports for main engines; supports for
liquefied natural gas tanks and supports for offloading
stations.
|
| 2. Includes internal and external structural elements
and connections which can be subjected to dry inspection and
repair.
|
| 3. Includes external structural elements and
connections situated below the minimum operating draught of the
unit or structure which can only be inspected during in-water
surveys but dry repairs could be carried out subject to special
arrangements being provided.
|
| 4. The use of fatigue life factors for
non-substantial consequences of failure will be specially
considered provided it can be demonstrated that there is
adequate structural redundancy after fatigue failure. To
demonstrate redundancy the structure is to comply with loading
condition (d) in Pt 4, Ch 3, 4.3 Load combinations 4.3.1. The environmental loads for
this loading condition are to be taken as the same as determined
for loading condition (b) in Pt 4, Ch 3, 4.3 Load combinations 4.3.1.
|
| 5. Connections that are covered by passive fire
protection are to be considered as non-inspectable unless it can
be confirmed that the passive fire protection is to be removed
for each inspection.
|
Table 5.5.2 Fatigue life factors of
safety for anchor line and tether components
| Replaceable
|
Inspectable for fatigue
(damage/cracks etc.)
|
Fatigue life factor
|
| Yes
|
Dry
|
3
|
| Yes
|
Wet
|
5
|
| No
|
No
|
10
|
| NOTES
|
| 1.
|
Anchor line components include chains, steel wire ropes, synthetic
fibre ropes and associated fittings such as shackles, connecting
links, rope sockets and terminations. Tether components include
tubular or rod tendon elements, connectors etc.
|
| 2.
|
Inspection in the context of this table, assumes the ability to
detect onset of fatigue cracks, for example through periodical NDE
and dimensional measurements on the component to capture any rate of
wear or corrosion significantly higher than assumed in design. Such
inspection should be carried out in accordance with an approved
Inspection, Monitoring, Maintenance and Repair plan.
Inspectable dry: Includes mooring line components and
connections which can be subjected to dry inspection and repair,
thus mainly on the deck or not subject to water splash or spray.
Inspectable wet: Includes mooring lines
components and connections situated in the splash zone and may
be occasionally or at all times wet during service but can be
retrieved for periodical inspection in dry conditions.
Replaceable: A mooring line component may be
considered replaceable when such timely replacement is shown
practical subject to special arrangements within a specific time
frame, all documented in an approved Inspection, Monitoring and
Maintenance and Repair plan.
|
| 3.
|
It is recommended that the main mooring line components above the
seabed be designed for potential replacement offshore.
For the component to be considered replaceable,
the Inspection, Monitoring and Maintenance and Repair plan or
the Mooring Line Failure Response procedure shall report a
detailed replacement procedure and consistent sparing policy to
enable prompt replacement of two same components i.e.
demonstrate that replacement is practicable.
|
| 4.
|
In the vicinity of points of constraint (e.g. top chain at stopper)
tension, In and Out of Plane Bending and other cyclic loading shall
be accounted for as applicable to the specific design. The
methodology, selected fatigue damage curves, associated factors of
safety shall be demonstrated to provide conservative safety margins
at least consistent with that of the same component under cyclic
tension load only, based on recognised T-T factor of safety of 10.
The supporting documents shall be submitted to LR for review and
acceptance.
Higher factors of safety will apply
when specified by the Owner (in the design Basis).
|
| 5.
|
The safety factor of attachment point and support equipment (e.g.
chain-stoppers, bending shoes, pivots, fairleads etc.) shall be
consistent with that of the mooring line it supports.
|
5.4.3 Full penetration welds are normally to be used for all nodal joints
(i.e., tubular brace to chord connections). For full penetration welded joints,
fatigue cracking would usually be located at the weld toe. However, if partial
penetration welds have to be used where weld throat failure is a possibility,
fatigue should be assessed using the ‘W’ curve and a shear stress estimated at the
weld root.
5.4.4 For nodal joints, the stress range to be used in the fatigue analysis is
the hot spot stress range at the weld toe. For any particular type of loading (e.g.,
axial loading) this stress range is the product of the nominal stress range in the
brace and the appropriate stress concentration factor (SCF).
5.4.5 The hot spot stress is defined as the greatest value around the
brace/chord intersection of the extrapolation to the weld toe of the geometric
stress distribution near the weld toe. This hot spot stress incorporates the effects
of overall joint geometry (i.e., the relative sizes of brace and chord) but omits
the stress-concentrating influence of the weld itself which results in a local
stress distribution. Hence, the hot spot stress is considerably lower than the peak
stress but provides a consistent definition of stress range for the design S-N curve
(curve ‘T’ shown in Pt 4, Ch 12 Fatigue – S-N Curves, Joint Classification and Stress Concentration Factors). Stress ranges both for the brace and
chord sides are to be considered in any fatigue assessment.
5.4.6 For all other types of joint (e.g., welded stiffeners or attachments,
including those at nodal joints) the joint classifications and corresponding S-N
curves are to take into account the local stress concentrations created by the
joints themselves and by the weld profile. The relevant stress range is then the
nominal stress range which is to include any local bending adjacent to the weld
under consideration. However, if the joint is also situated in a region of stress
concentration resulting from the gross shape of the structure, this is to be taken
into account.
5.4.7 In load-carrying partial penetration or fillet-welded joints, where
cracking could occur in the weld throat, the relevant stress range is the maximum
range of shear stress in the weld metal. For details which are particularly
fatigue-sensitive, where failure could occur through the weld, full penetration
welding is normally to be used.
5.4.9 Normal fabrication tolerances according to good workmanship standards as
given by the Rules are considered to be implicitly accounted for in the S-N
curves.
5.4.10 The following fatigue life improvement methods are acceptable to LR:
- Weld geometry control and defect removal:
- Machining, e.g. grinding;
- Re-melting, i.e. TIG dressing;
- Weld profile control, i.e. enhanced
workmanship.
- Residual stress improvement:
- Peening;
- Thermal stress relief.
Where it is proposed to use any of these methods, a procedure detailing the method is
to be submitted to LR for approval prior to commencement of work, and is to be
referenced or included directly on the applicable structural and fabrication
drawings. The procedure and associated increase in fatigue life are to be in
accordance with Chapter 2 of the ShipRight Fatigue Design Assessment Level 1
Procedure, 2009. Where toe grinding is used to improve the fatigue life of a
fillet welded connection, the weld throat thickness after grinding is not to be less
than that required by Pt 4, Ch 8, 2 Welding. All work is to be carried out to the
satisfaction of an attending LR Surveyor. Corrosion pitting of the ground/peened
metal surface can remove the benefit of grinding/peening. Therefore, the
ground/peened surface must be adequately protected against corrosion, e.g. by means
of a suitable paint system.
5.5 Cast or forged steel
5.5.1 Fatigue life calculations for cast or forged steel structural components
are to include details of the fatigue endurance curve for the material, taking
account of the particular environment, mean stress and the existence of casting
defects, and the derivation of any stress concentration factors.
5.6 Factors of safety on fatigue life
|