Section
6 Anchor lines
6.1 General
6.1.1 Anchor line length is to be sufficient to avoid uplift forces occurring
at the anchor point for damaged condition loads, unless the anchor point is
specially designed to accept a vertical component of loading.
6.1.2 An anchor line integrity monitoring system or device is to be provided
for floating unit mooring systems, to detect line breakage and significant tension
and offset irregularities under ambient environmental conditions as well as more
severe storms within the envelope of design environmental conditions. This is
generally not a requirement for offloading buoy systems. The following are to be
taken into account:
- The precision and accuracy of the monitoring of the tension irregularities
are to be documented for a load range up to at least 90 per cent of the
breaking strength of the mooring lines. The precision and accuracy of the
monitoring of the offset irregularities are to be documented for 100 per
cent of the offset range.
- The availability and reliability of the system or device is to be taken into
account in the mooring line failure response plan. A system is not required
to be fully autonomous or to detect tension in each anchor line. A system
can comprise monitoring offset irregularities combined with manual
inspection to identify which line has broken. Accurate inclinometers on the
mooring lines or unit position monitoring systems can be considered as a
suitable alternative to a mooring line tension monitoring system.
- The mooring line integrity monitoring system shall be able to detect failure
of any part along the line (between attachment point to Offshore Unit to at
least the seabed touch down or embedment point. Ability to detect line
failure beyond seabed touch down or embedment should be assessed and
documented. The results should be taken into consideration when setting the
scope of Offshore In Water Survey.
- Detection of tension anomalies or line breakage is to raise an alarm (at
least visual). The system should be able to be interrogated on demand and
present sufficient redundancy so that the system remains operational after
failure of any one component and to enable inspection or testing,
maintenance and repair without loss of operability.
Calibration checks are to be carried out at least once a year.
Calibration and maintenance procedures and schedule are to be documented in the
Operation Manual of the unit.
6.1.4 In general, the break strength of an anchor or mooring lines is not to
be greater than the load bearing capacity of the structure it connects to. Unless
when specifically designed as a weak link in the mooring line chain or rope
fittings, sockets, shackles, H or Y type links, connectors etc. shall be designed
based on mooring line pull at least equal to the as new nominal minimum break
strength of the mooring line main component (steel wire rope, chain or fibre rope)
applying a minimum contingency factor of 1,1.
In long term positional mooring systems, H or Y type links are often used as
connectors in preference to standard D joining shackles, as their design is not
standardised and they are specifically designed to suit the components they connect.
When standard D type joining shackles are used in positional systems, their designs
shall be checked and confirmed suitable for the specific connection arrangement, fit
and resulting constraints.
For fairleads, bending shoes, stoppers and their supporting structures
see
Pt 3, Ch 10, 10 Fairleads, bending shoes and stoppers. For
supporting structure see
Pt 4, Ch 6, 1.1 General 1.1.6.
6.1.5 In general the mooring analyses should provide all of the loading
parameters required for the detailed design of the mooring lines components and the
associated supporting structures they interact with (pad-eyes, fairleads, bend shoes
etc). The detailed structural or mechanical design of complex or non-standard (e.g.
special D-shackles with dimensions not conforming with ISO 1704, or special
connectors) component is generally substantiated by finite element calculations.
Suitable elastic plastic models need to be used to model elastic plastic behaviour
(e.g. Ramberg-Osgood law) at the contact points. Convergence should be demonstrated
for the large displacement nonlinearities, contact related nonlinearities as well as
nonlinear material properties. Alternatively elastic analysis is also acceptable.
The detailed design calculations of components should address both
strength and fatigue aspects. For fatigue calculations principal stresses at the
model mesh are to be refined at hot spots locations and at surface of the modelled
component to ensure characteristic mean principal stresses in the surface plane are
captured.
Special non-standard mooring components shall be designed so that local
yielding only occur for a few load cycles imparting a shake-down effect after which
no further yielding occurs. The analysis shall be based on cyclic material
properties and cyclic loading shall demonstrate an effective shakedown after few
cycles.
Deformation under design loads from (intact and one line damage case)
shall not adversely affect the performance of the component.
Conservative plastic strain and stress curve and characteristics plastic
strain limit shall be reported for the selected material with reference to
recognised code or standard and substantiated by material test records.
6.1.6 Kenter links are not permitted on long term permanent mooring systems.
Connectors purposely designed (for project specific strength and fatigue loading)
(e.g. H-Links) and manufactured under LR Survey shall be preferred.
6.1.7 Locking mechanisms of pin parts of mooring line component connections on
long term positional mooring systems should be redundant and not be located within
the main load path.
6.2 Factors of safety – Strength
6.2.1 Minimum factors of safety applicable to the steel wire rope, chain and
polyester anchor lines of moored floating units are given in Pt 3, Ch 10, 6.2 Factors of safety – Strength 6.2.1. For fibre ropes, see
Table 10.9..
Table 10.6.1 Minimum factors of safety
for anchor lines for floating offshore installations at a fixed
location
| Design case
|
Factor of safety, see Note 2
|
| Intact
|
Damaged
|
|
Extreme storm, or maximum
environment, with floating unit attached
|
1,67
see Note 1
|
1,25
see Note 1
|
| NOTES
|
| 1. The factors of safety given in this Table are
associated with the following conditions:
|
| (a) Arrangements being available to shut down
production and/or transfer of oil or gas through risers in event
of anchoring system failure.
|
| (b) The floating unit being located in an
open sea area. Special consideration will be given to factors of
safety when the unit is in close proximity to another
installation, or is located near the shore.
|
|
2. Factor of safety = 
Anchor line minimum breaking strength basis to
be documented.
A reduction factor may require to be applied to
the standard assigned minimum breaking strength of anchor
line components in some cases (e.g., where component test
database is small: for non-standard components), or where
anchor line components are not new.
|
| 3. Maximum tension to be based on assessment by
dynamic analyses. See also
Pt 3, Ch 10, 5.5 Combination of low and high frequency components – Design values 5.5.3 on maximum
tension.
|
6.2.4
PMC notation (including PMC TA(1), PMC TA(2) and PMC
TA(3)). Minimum factors of safety applicable to steel wire rope and chain
anchor lines for mooring system for mobile offshore units analysed quasi-statically
and dynamically are given in Table 10.6.4 Factors of safety for PMC
notation - Quasi-static analysis and Table 10.6.5 Factors of safety for PMC
notation - Dynamic analysis respectively.
Table 10.6.2 Minimum factors of safety
for anchor lines of offshore loading buoys
| Design case
|
Factor of safety
|
| Intact
|
Damaged
|
|
Extreme storm, or maximum storm
condition with ship attached
|
1,85
|
1,35
|
| NOTES
|
|
1. For special cases where allowable offset
criteria for risers cannot be met in a Damaged Case (single
line break) (e.g. in offshore loading buoy systems for
shallow water), the Damaged Case can be omitted in design
and an increased intact factor of safety applied. A minimum
factor of safety of 2,3 is to be applied in this case.
Failure of any one mooring line should still be shown not
lead to progressive collapse or incidents of substantial
consequences such as loss of life, uncontrolled outflow of
hazardous or polluting products, collision, sinking.
2. Maximum tension to be based on assessment by
dynamic analyses. See also
Pt 3, Ch 10, 5.5 Combination of low and high frequency components – Design values 5.5.3
on maximum tension.
|
Table 10.6.3 Factors of safety for PM
notation
| Design case
|
Description
|
Factors of safety for
PM notation, see Note 1
|
|
Quasi-static
analysis
|
Dynamic
analysis
|
| 1
|
Operating (Intact)
|
2,7
|
2,3
|
| 2
|
Survival (Intact)
|
1,8
|
1,5
|
| 3
|
Operating (Single line
failure)
|
1,8
|
1,5
|
| 4
|
Survival (Single line
failure)
|
1,25
|
1,1
|
| NOTES
|
| 1. The factors of safety given in this Table apply to
units positioned at least 300 m from another unit.
|
| 2. The unit is to be positioned to avoid contact with
another unit in any of the design cases.
|
| 3. See also
Pt 3, Ch 10, 5.5 Combination of low and high frequency components – Design values 5.5.3 on maximum
tension.
|
Table 10.6.4 Factors of safety for PMC
notation - Quasi-static analysis
| Design case
|
Description
|
Factors of safety for PMC notation
Quasi-static analysis, see Notes
|
|
Unit moored 50 m or less
from other structures
|
Unit moored within 50 to 300 m
from other structures
|
| Critical line
|
Non-critical line
|
Critical line
|
Non-critical line
|
| 1
|
Operating (Intact)
|
3,0
|
2,7
|
3,0
|
2,7
|
| 2
|
Survival (Intact)
|
—
|
—
|
2,0
|
1,8
|
| 3
|
Operating (Single line
failure)
|
2,0
|
1,8
|
2,0
|
1,8
|
| 4
|
Survival (Single line
failure)
|
—
|
—
|
1,5
|
1,33
|
| NOTES
|
| 1. See also
Pt 3, Ch 10, 5.4 Analysis
|
| 2. The unit is to be positioned to avoid contact with
another unit in any of the design cases.
|
| 3. See also
Pt 3, Ch 10, 5.5 Combination of low and high frequency components – Design values 5.5.3 on maximum
tension.
|
|
|
Table 10.6.5 Factors of safety for PMC
notation - Dynamic analysis
| Design case
|
Description
|
Factors of safety for PMC
notation Dynamic analysis, see Notes
|
|
Unit moored 50 m or less
from other structures
|
Unit moored within 50 to 300 m
from other structures
|
| Critical line
|
Non-critical line
|
Critical line
|
Non-critical line
|
| 1
|
Operating (Intact)
|
2,5
|
2,3
|
2,5
|
2,3
|
| 2
|
Survival (Intact)
|
—
|
—
|
1,65
|
1,5
|
| 3
|
Operating (Single line
failure)
|
1,65
|
1,5
|
1,65
|
1,5
|
| 4
|
Survival (Single line
failure)
|
—
|
—
|
1,35
|
1,2
|
| NOTES
|
| 1. See also
Pt 3, Ch 10, 5.4 Analysis.
|
| 2. The
unit is to be positioned to avoid contact with another unit in
any of the design cases.
|
6.3 Fatigue life
6.3.1 The fatigue life of the main components in the positional mooring system
are to be verified. Calculations are to be submitted.
6.3.2 Where applicable tension bending effects are to be considered in the
fatigue calculations of the mooring line at the fairleads and stoppers (or at any
point within the line where it is subject to a constraint resulting in local
bending). The detailed methodology shall be reported and agreed with LR in the early
stages of the design. Contingencies should be included to address any uncertainties.
Torsion in the mooring line shall be avoided by design. In cases where
this is not possible the performance of the component under such loading regime
should be substantiated by a qualification programme agreed with LR.
Note: For top chain connections to stoppers, guidance can be drawn from
publications from recent joint industry research projects on Fatigue of Top Chain of
Mooring Lines due to In-Plane and Out-of-Plane Bending). Details of the methodology
shall be reported and agreed with LR at early stage of design. The associated scope
of manufacturing and testing shall be agreed with LR. The bushing performance shall
be well documented and substantiated by adequate prototype testing and confirmed by
factory acceptance tests. The design shall include contingencies to address any
uncertainties (e.g. long term performance of bushing, bush and interlink friction
coefficients etc.).
(see also
Pt 3, Ch 10, 10.1 General requirements 10.1.1).
Applicable factors of safety shall be agreed with LR, after review of the
detailed design methodology (else the default is 10).
6.3.3 Fatigue life calculations for anchor lines can be carried out in
accordance with a recognised Code, e.g., API RP 2SK: Recommended Practice for
Design and Analysis of Station keeping Systems for Floating Structures.
Note Where various wind driven wave and swell (potentially multiple)
regimes prevail concurrently, the fatigue assessment shall be shown to account
for these environmental characteristics and conservatively capture the various
peak frequencies and relative directionalities.
6.3.4 Consideration will be given to the use of alternative methods, detailed
proposals are to be submitted and agreed with LR.
|