Clasification Society Rulefinder 2016 - Version 9.25
Clasification Society Rules and Regulations - Rules and Regulations for the Classification of Offshore Units, January 2016 - Part 3 FUNCTIONAL UNIT TYPES AND SPECIAL FEATURES - Chapter 13 Buoys, Deep Draught Caissons, Turrets and Special Structures - Section 4 Mooring arms and towers

Section 4 Mooring arms and towers

Parent topic: Chapter 13 Buoys, Deep Draught Caissons, Turrets and Special Structures

4.1 General

4.1.1 Mooring arms and towers are to be designed for the maximum mooring loads and direct wave loading to which they may be subjected in service, and design calculations are to be submitted. The loadings on lattice type structures are to be specially considered and agreed with LR.

4.1.2 The structure is to be designed for the most unfavourable of the following combined loading conditions:
  1. maximum gravity and functional loads.
  2. design environmental loads and associated gravity and functional loads.
  3. design environmental loads and associated gravity and functional loads after credible failures.

4.1.3 The structure is to be investigated for loading condition 4.1.2(c) to assess the effect of the failure of a single slender tubular (or similar) member. The permissible stress levels after credible failures are given in Pt 4, Ch 5, 2.1 General When stress levels in the structure exceed permissible levels the slender tubular member is considered to be ‘non-redundant’, see 4.1.4 . This requirement does not apply to stiffened plate structures or mechanical items.

4.1.4 When the requirements of 4.1.3 are not met the intact structure is to be further investigated for loading condition 4.1.2(b) under the action of a 10000 year return period mooring load and associated gravity and functional loads. Non-redundant slender tubular (or similar) members should in general have sufficient ductility to resist failure, i.e., strain up to 21 per cent.

When this criterion is not met the following mitigating measures are required:
  1. clear identification of high stress areas.
  2. welding in high stress areas to be full penetration, as far as practicable.
  3. NDE in accordance with an agreed plan,see also Table 9.2.1 inPt 4, Ch 8 Welding and Structural Details.
  4. minimum factor of safety of 2 on fatigue life, see Pt 4, Ch 5, 5.6 Factors of safety on fatigue life.
  5. inspection and test plans (fabrication and in-service) to be submitted for LR approval.
  6. operations manual to clearly specify critical areas and inspection requirements.

4.1.5 The permissible stresses in mooring arms and the attachment to floating units are to comply with Pt 4, Ch 5 Primary Hull Strength.

4.1.6 Attention is to be paid to the detail design in fatigue sensitive areas. Mooring arms, towers, articulated and sliding joints are to be assessed for fatigue damage due to cyclic loading in accordance with Pt 4, Ch 5, 5 Fatigue design.

4.1.7 All structures are to have adequate buckling strength and comply with Pt 4, Ch 5 Primary Hull Strength. Special attention is to be paid to the torsional buckling of mooring arms and design calculations are to be submitted.

4.1.8 Mooring towers are to be designed in accordance with an internationally recognised Code or Standard, see Appendix A.

4.1.9 Mechanical items and bearings are to comply with Pt 3, Ch 13, 6 Mechanical items.

4.1.10 Foundations to mooring towers are to comply with the requirements of Pt 3, Ch 14 Foundations.

4.1.11 If a classed unit is attached to a mooring tower which is not classed by LR, the mooring tower and its foundations are to be certified by LR or another acceptable organisation, see Pt 1, Ch 2, 2.1 General definitions.

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