ANNEX L SPECIAL DESIGN PROVISIONS FOR SHIP SHAPED UNITS
L.7 FATIGUE LIMIT STATES (FLS)
L.7.1 General
General requirements and guidance concerning fatigue criteria are given in Chapter 8, and Annex C.
Evaluation of the fatigue limit states shall include consideration of all significant actions contributing to fatigue damage both in non-operational and operational design conditions.
The minimum fatigue life of the unit (before the design fatigue factor is considered) should be based upon a period of time not being less than the planned life of the structure.
Local effects, for example due to :
• slamming,
• sloshing,
• vortex shedding,
• dynamic pressures, and,
• mooring and riser systems,
shall be included in the fatigue damage assessment when relevant.
Calculations carried out in connection with the fatigue limit state may be undertaken without deducting the corrosion additions, provided a corrosion protection system in accordance with NORSOK, N-001 is maintained.
In the assessment of fatigue resistance, relevant consideration shall be given to the effects of stress raisers (concentrations) including those occurring as a result of :
• fabrication tolerances, (including due regard to tolerances in way of connections of large structural sections),
• cut-outs,
• details at connections of structural sections (e.g. cut-outs to facilitate construction welding).
L.7.2 Design fatigue factors
Criteria related to Design Fatigue Factors, are given in the Chapter 8. When determining the appropriate Design Fatigue Factor for a specific fatigue sensitive location, consideration shall be given to the following :
• Consideration of economic consequence of failure may indicate the use of larger design factors than those provided for as minimum factors.
• The categorisation : ‘Accessible / Above splash zone’ is, intended to refer to fatigue sensitive locations where the possibility for close-up, detailed inspection in a dry and clean condition exists. When all of these requirements are not fulfilled, the relevant design fatigue factor should be considered as being that appropriate for ‘Accessible / Below splash zone’, or, ‘No access or in the splash zone’ as relevant to the location being considered.
• Evaluation of likely crack propagation paths (including direction and growth rate related to the inspection interval), may indicate the use of a different Design Fatigue Factor than that which would be selected when the detail is considered in isolation, such that :
• Where the likely crack propagation indicates that a fatigue failure, from a location satisfying the requirements for a 'Non-substantial' consequence of failure, may result in a 'Substantial' consequence of failure, such fatigue sensitive location is itself to be deemed to have a 'Substantial' consequence of failure.
• Where the likely crack propagation is from a location satisfying the requirement for a given
‘Access for inspection and repair’ category to a structural element having another access categorisation, such location is itself to be deemed to have the same categorisation as the most demanding category when considering the most likely crack path. For example, a weld detail on the inside (dry space) of a submerged shell plate should be allocated the same Fatigue Design Factor as that relevant for a similar weld located externally on the plate –see Figure L.7-1.
Figure L.7-1 Example illustrating considerations relevant for selection of design fatigue factors (DFF’s) in locations considered to have ‘Non-substantial’ consequence of failure.
Notes:
1. Due to economic considerations (e.g. cost of repair to an external underwater structural element) the DFF’s assigned a value of 2 in Figure L.7-1 may be considered as being more appropriately assigned a value of 3.) 2. The unit may be considered as “accessible and above the splash zone” (DFF = 1.0), see Annex C, if the survey
extent e.g. given for main class (see DNV Ship Rules Pt.7 Ch.2) is followed i.e. drydocking for inspection and maintenance every 5 years.
L.7.3 Splash zone
The definition of ‘splash zone’ as given in NORSOK N-003, relates to a highest and lowest tidal reference. For ship shaped units, for the evaluation of the fatigue limit state, reference to the tidal datum should be substituted by reference to the draught that is intended to be utilised when condition monitoring is to be undertaken. The requirement that the extent of the splash zone is to extend 5 m above and 4 m below this draught may then be applied. (For application of requirements to corrosion addition, however, the normal operating draught should generally be considered as the reference datum).
Annex L Rev. 1, December 1998 If significant adjustment in draught of the unit is possible in order to provide for satisfactory
accessibility in respect to inspection, maintenance and repair, account may be taken of this
possibility in the determination of the Design Fatigue Factors. In such cases however, a margin of minimum 1 meter in respect to the minimum inspection draught should be considered when deciding upon the appropriate Design Fatigue Factor in relation to the criteria for ‘Below splash zone’ as opposed to ‘Above splash zone’. Where draft adjustment possibilities exist, a reduced extent of splash zone may be applicable. Consideration should be given to operational requirements that may limit the possibility for ballasting / deballasting operations.
When considering utilisation of Remotely Operated Vehicle ( ROV) inspection consideration should be given to the limitations imposed on such inspection by the action of water particle motion (e.g. waves). The practicality of such a consideration may be that effective underwater inspection by ROV, in normal sea conditions, may not be achievable unless the inspection depth is at least 10 metres below the sea surface.
L.7.4 Structural details and stress concentration factors
Fatigue sensitive details in the FPSO should be documented to have sufficient fatigue strength.
Particular attention should be given to connection details of the following:
• Integration of the mooring system with hull structure
• Main hull bottom, side and decks
• Main hull longitudinal stiffener connections to transverse frames and bulkheads
• Main hull attachments; seats, supports etc.
• Openings in main hull
• Transverse frames
• Flare tower
• Riser interfaces
• Major process equipment seats
Selections of local details and calculations of stress concentration factors may be undertaken in accordance with DNV Classification Note 30.7, /11/. For details not covered in this document, stress concentration factors should be otherwise documented. Detailed finite element analysis may be utilized for determination of SCF’s, according to procedure given in DNV Classification Note 30.7, /11/.
L.7.5 Design actions and calculation of stress ranges L.7.5.1 Fatigue actions
An overview of fatigue actions is given in L.4.7. Site specific environmental data shall be used for calculation of long term stress range distribution. For units intended for multi field developments the site specific environmental actions for each field should be utilised considering the expected duration for each field. The most onerous environmental actions may be applied for the complete lifetime of the unit, as a conservative approach.
A representative range of action conditions shall be considered. It is generally acceptable to consider two action conditions, typically: ballast condition and the fully loaded condition, with appropriate amount of time at each condition, normally 50 % for each condition unless otherwise documented.
An appropriate range of wave directions and wave energy spreading shall be considered. For weather waning units, and in absence of more detailed documentation, the head sea direction shall be considered with the spreading taken as the most unfavourable between cos2 – cos10, see
NORSOK, N-003. Maximum spacing should be 30 degrees. Smaller spacing should be considered around the head-on heading, e.g. 15 degrees.
Typically, most unfavourable spreading will be cos2 for responses dominated by beam sea e.g.
external side pressure, and cos10 for responses dominated by head sea e.g. global wave bending moment.
The following dynamic actions shall be included in a FLS analysis as relevant:
• Global wave bending moments
• External dynamic pressure due to wave and ship motion
• Internal dynamic pressure due to ship motion
• Sloshing pressures due to fluid motion in tanks for ships with long or wide tanks
• Loads from equipment and topside due to ship motion and acceleration L.7.5.2 Topside structures
The following actions shall be considered for the topside structure:
• Hull deformations due to wave bending moment acting on the hull
• Wave induced accelerations (inertia actions)
• Vortex induced vibrations from wind
• Vibrations caused by operation of topside equipment
Additionally, the following low cycle actions should be considered where relevant for the topside structure:
• Hull deformations due to temperature differences
• Hull deformations due to change in filling condition e.g. ballasting / deballasting
The relevance of combining action components is dependent on the structural arrangement of the topside structure. Relevant stress components, both high cycle and low cycle, shall be combined, including phase information, when available. If limited phase information is available, the design may be based on ‘worst-case’ action conditions, by combination of maximum stress for each component.
L.7.5.3 Turret structure
The turret structure will normally be exposed to high dynamic action level. The choice of fatigue design factor for the turret should reflect the level of criticality and the access for inspection at the different locations, L.7.2.
The following actions shall be considered for the fatigue design of turret structures:
• Dynamic fluctuations of mooring line tension.
• Dynamic actions (tension and bending moment) from risers
• Varying hydrodynamic pressure due to wave action
• Varying hydrodynamic pressure due to unit accelerations (including added mass effects)
• Reactions in the bearing structure due to the other effects
• Inertia actions due to unit accelerations
• Fluctuating reactions in pipe supports due to thermal and pressure induced pipe deflections Typical critical areas for fatigue evaluation listed in L.6.3.3.
Annex L Rev. 1, December 1998 L.7.5.4 Calculation of global dynamic stress ranges
Global stress ranges shall be determined from the global hull bending moments. If applicable, both vertical and horizontal bending moments shall be included. Shear lag effects and stress
concentrations shall be considered.
L.7.5.5 Calculation of local dynamic stress ranges
Local stress ranges are determined from dynamic pressures acting on panels, accelerations acting on equipment and topside and other environmental actions resulting in local stresses to part of the structure.
Dynamic pressures shall be calculated from a 3D sink-source wave load analysis. The transfer function for the dynamic pressure could either be used directly to calculate local stress transfer functions and combined with the global stress transfer function, or a long-term pressure distribution could be calculated. As a minimum, the following dynamic pressures components shall be
considered:
• Double hull stresses due to bending of double hull sections between bulkheads
• Panel stresses due to bending of stiffened plate panels
• Plate bending stresses due to local plate bending
For a description of calculation of local stress components, reference is made to DNV Classification Note 30.7, /11/.
L.7.5.6 Combination of stress components
Global and local stresses should be combined to give the total stress range for the detail in question.
In general, the global and the local stress components differ in amplitude, phase and location. The method of combining these stresses for the fatigue damage calculation will depend on the location of the structural detail. A method for combination of actions is given in DNV Classification Note 30.7, /11/.
L.7.6 Calculation of fatigue damage L.7.6.1 General
The basis for determining the acceptability of fatigue resistance, with respect to wave actions, shall be appropriate stochastic fatigue analyses. The analyses shall be undertaken utilising relevant site specific environmental data and take appropriate consideration of both global and local (e.g.
pressure fluctuation) dynamic responses. (These responses do not necessarily have to be evaluated in the same model but the cumulative damage from all relevant effects should be considered when evaluating the total fatigue damage.)
Simplified fatigue analyses may form the basis of a ‘screening’ process to identify locations for which a detailed, stochastic fatigue analysis should be undertaken. Such simplified fatigue analysis shall be calibrated, see L.7.6.2.
Local, detailed FE-analysis (e.g. unconventional details with insufficient knowledge about typical stress distribution) should be undertaken in order to identify local stress distributions, appropriate SCF’s, and/or extrapolated stresses to be utilised in the fatigue evaluation, see Annex C for further details. Dynamic stress variations through the plate thickness shall be documented and considered in such evaluations.
Explicit account shall be taken of any local structural details that invalidate the general criteria utilised in the assessment of the fatigue strength. Such local details may, for example be access openings, cut-outs, penetrations etc. in structural elements.
Principal stresses (see Annex C) should be utilised in the evaluation of fatigue responses.
L.7.6.2 Simplified fatigue analysis
Provided that the provisions stated in L.7.6.1, are satisfied, simplified fatigue analysis may be undertaken in order to establish the general acceptability of fatigue resistance. In all cases when a simplified fatigue analysis is utilized a control of the results of the simplified fatigue evaluation, compared to the stochastic results, shall be documented to ensure that the simplified analysis provides for a conservative assessment for all parts of the structure being considered.
The fatigue damage may be calculated based on a cumulative damage utilizing Miner-Palmgren summation, applying a Weibull distribution to describe the long-term stress range. The stress range shape parameter and the average zero-crossing frequency may be taken from the long-term stress distribution, utilizing the stress transfer function and environmental data for the operating area. A simplified method of fatigue analysis is described in DNV Classification Note 30.7, /11/.
L.7.6.3 Stochastic fatigue analysis
Stochastic fatigue analyses shall be based upon recognised procedures and principles utilising relevant site specific data.
Providing that it can be satisfactorily documented, scatter diagram data may be considered as being directionally specific. In such cases, the analyses shall include consideration of the directional probability of the environmental data. Relevant wave spectra shall be utilised. Wave energy spreading may be taken into account if relevant.
Structural response shall be determined based upon analyses of an adequate number of wave directions. Generally a maximum spacing of 30 degrees should be considered. Transfer functions should be established based upon consideration of a sufficient number of periods, such that the number, and values of the periods analysed :
• adequately cover the site specific wave data,
• satisfactorily describe transfer functions at, and around, the wave ‘cancellation’ and
‘amplifying’ periods. (Consideration should be given to take account that such ‘cancellation’
and ‘amplifying’ periods may be different for different elements within the structure), and,
• satisfactorily describe transfer functions at, and around, the relevant excitation periods of the structure.
The method is described in DNV Classification Note 30.7, /11/.
Annex L Rev. 1, December 1998