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:
- maximum gravity and functional loads.
- design environmental loads and associated gravity and
functional loads.
- 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 to be in accordance with
Table 5.2.1 Factors of safety for the
combined load cases – load case (d) 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:
- clear identification of high stress areas.
- welding in high stress areas to be full penetration, as far as
practicable.
- NDE in accordance with an agreed plan,see also Table
9.2.1 in Pt 4, Ch 8 Welding and Structural Details.
- minimum factor of safety of 2 on fatigue life, see
Pt 4, Ch 5, 5.6 Factors of safety on fatigue life.
- inspection and test plans (fabrication and in-service) to be
submitted for LR approval.
- operations manual to clearly specify critical areas and
inspection requirements.
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.