ANNEX N SPECIAL DESIGN PROVISIONS FOR TENSION LEG PLATFORMS
N.2 STRUCTURAL CLASSIFICATION AND MATERIAL SELE CTION
N.2.1 General
Selection of steel quality, and requirements for inspection of welds, shall be based on a systematic classification of welded joints according to the structural significance and the complexity of the joints/connections as documented in Chapter 5.
In addition to in-service operational phases, consideration shall be given to structural members and details utilised for temporary conditions, e.g. fabrication, lifting arrangements, towing and
installation arrangements, etc.
N.2.2 Structural classification
The structural classification guidance given below assumes that the tendon system is demonstrated to have residual strength and that the TLP structural system satisfies the requirements of the ALS condition with failure of the tendon (or a connection in the tendon) as the defined damage. (See also NORSOK N-004, Table 5-1). If this is not the case then design classes, DC3 and DC4 stated below should be substituted by design classes DC1 and DC2 and the inspection categories respectively upgraded in accordance with the requirements of NORSOK N-004, Tables 5-3 and 5-4 as applicable.
Examples of typical design classes applicable to the hull and deck structures of a TLP are given in Annex M.
Examples of typical design classes applicable to the pile foundation structures are given in Annex K.
Examples of considerations with respect to structural classification of tendons, tendon interfaces are given below. These examples provide minimum requirements and are not intended to restrict the designer in applying more stringent requirements should such requirements be desirable.
Typical locations - Design class : DC3
Locations in way of complex structural connections should be classified as DC3, such connections may occur at :
• tendon interfaces with the foundation and the TLP hull, and,
• complex tendon / tendon connections.
DC3 areas may be limited to local, highly stressed areas if the stress gradient at such connections is large.
Typical locations - Design class : DC4
Except as provided for in the description for DC3 structural categorisation, the following listed connections may appropriately be classified as being DC4 :
• simple tendon / tendon connections,
• interface arrangements outside locations of complex connections including general stiffened plate fields (e.g. at hull interface)
Consideration of the economic consequence of failure may however indicate the utilisation of a higher design class than the minimum class as given above.
Annex N Rev. 1, December 1998
Typical locations - Design class : DC5
DC5 locations are not normally relevant for tendons or tendon interfaces.
N.2.3 Material selection
Material specifications shall be established for all structural materials utilised in a TLP. Such materials shall be suitable for their intended purpose and have adequate properties in all relevant design conditions. Material selection shall be undertaken in accordance with the principles given in NORSOK M-DP-001.
When considering criteria appropriate to material grade selection, adequate consideration shall be given to all relevant phases in the life cycle of the unit. In this connection there may be conditions and criteria, other than those from the in-service, operational phase, that provide the design requirements in respect to the selection of material. (Such criteria may, for example, be design temperature and/or stress levels during marine operations.)
Selection of steel quality for structural components shall normally be based on the most stringent Design Class of the joints involving the component.
Through-thickness stresses and low temperature toughness requirements shall be assessed.
The evaluation of structural resistance shall include relevant account of variations in material properties for the selected material grade (e.g. Variation in yield stress as a function of thickness of the base material)
N.2.4 Inspection categories
Welding, and the extent of non-destructive examination during fabrication, shall be in accordance with the requirements stipulated for the appropriate 'inspection category' as defined in NORSOK, M-101. Determination of inspection category should be in accordance with the criteria given in Table 5-5.
The lower the extent of NDT selected, the more important is the representativeness of the NDT selected and performed. The designer should therefore exercise good engineering judgement in indicating mandatory locations for NDT, where variation of utility along welds is significant.
N.2.5 Guidance to minimum requirements
Figures N.2-1 and N.2-2 illustrate minimum requirements to selection of Design Class, and Inspection Category for typical TLP unit, structural configurations.
Segment of Column DC2 A 1)
DC2 B 2)
Figue N.2-1 Examples of typical hull porch design classes and inspection categories
1. Inspection Category A is selected in accordance with Table 5.4 assuming high fatigue utilisation and principal stresses in the transverse direction.
2. Inspection Category B is selected in accordance with Table 5.4 assuming high fatigue and principal stresses longitudinal to the weld.
Annex N Rev. 1, December 1998
Welded
Threaded
Longitud. weld DC2 B 1)
Girth weld DC2 A 2)
Figure N.2-2 Examples of design classes and inspection categories for typical tendon connections
1. Inspection Category B is selected in accordance with Table 5.4 assuming high fatigue utilisation and principal stresses in the longitudinal direction.
2. Inspection Category A is selected in accordance with Table 5.4 assuming high fatigue and principal stresses transverse to the weld. Stricter acceptance criteria as per footnote 2 in Table 5.4 will apply.