Section
4 Design aspects
4.1 Design cases
4.1.1 The positional mooring system, with or without thruster-assist, is to be
designed for the following:
- Intact Case:
- Damaged Case:
- This case involves the failure of a single component,
i.e., failure of an anchor line or anchor point, or failure of a
component in the case of thruster-assist.
Note
- a single failure in the thruster
system could lead to stoppage of several, or all, of the thrusters.
This generally encompasses all non-redundant equipment in the chain
of control and power supply to the thruster system or equipment
which ensures the good operation of the thruster system.
Note
- loss or flooding of a buoyancy
element or clump weight on a mooring line could lead to loss of
effective restoring capacity of the line (as well as lead to loss of
clearance of the line with adjacent subsea structures).
- Transient Case:
- The transient case will not normally require to be
investigated.
- A transient quasi-dynamic analysis can be used to
investigate whether a transient dynamic case requires to be
considered in the design. When the maximum line tension and offset
from the quasi-dynamic transient case does not exceed the maximum
tension and offset from the corresponding quasi-dynamic damaged
case, full dynamic transient load case will not normally be required
to be investigated.
4.1.2 Sensitivity analyses on proposed PMS design are to encompass the level
of accuracy of proposed inspection techniques and procedures, tolerances and margins
on component properties (inclusive over the service life), as well as installation
tolerances. Upon consideration in the design of such variations, the design should
still satisfy the requirements of this Chapter.
4.1.3 A load case considering the failure of any one line adjacent to the
first failed line should be run to assess the consequence of such a serial failure
considering design environmental combinations not less onerous than 10 year return
period sea state + 10 year return period wind and associated current at 95 per cent
non-exceedance level. The results of the analyses of the positional mooring system
with two lines failed should indicate this abnormal configuration does not lead to
progressive collapse or incidents of substantial consequences such as loss of life,
uncontrolled outflow of hazardous or polluting products, collision, or sinking. For
this load case, a factor of safety of one is to be applied to the line minimum
breaking load at the ‘end of life’ condition (i.e. excluding corrosion
allowance).
The results (offsets, tensions, clearances, etc.) of the two lines
failure analyses are to be reported and used to set up the mooring line failure
response procedure for the unit.
Note 1. The mooring line failure response procedure should be referred to from the time
one line fails.
Note 2.The mooring line failure response
procedures shall include root cause assessment, repair planning, mitigations to
limit further damage, ensure safe control of the Offshore Unit after failure of one
line and ensure preparedness for further line failure will not have substantial
consequences.
4.1.4 The design shall consider at least three draughts or loading conditions
(fully ballasted, fully loaded and one between to attempt capture the most onerous
load condition).
4.2 Thruster-assist systems
4.2.1 Thrusters can be used to reduce the mean load on the mooring system,
provide damping of low frequency surge motion, and/or control the heading of the
unit, in order to limit the overall excursions. Thruster intervention allowances for
supplementary thruster-assist notations are given in Pt 3, Ch 10, 4.2 Thruster-assist systems 4.2.1.
Table 10.4.1 Thruster allowance
Case
|
Thruster allowance
|
TA(1)
|
TA(2)
|
TA(3)
|
Operating (Intact)
|
None
|
70% of all
thrusters
|
All
thrusters
|
Survival (Intact)
|
70% of all
thrusters
|
All
thrusters
|
All
thrusters
|
Operating (Single line
failure)
|
None
|
70% of all
thrusters
|
All
thrusters
|
Survival (Single line
failure)
|
70% of all
thrusters
|
All
thrusters
|
All
thrusters
|
NOTES
|
1. The conditions for assignment of supplementary
notations TA(1), TA(2) and TA(3) are defined in Pt 3, Ch 10, 13 Thruster-assisted positional mooring.
|
2. Net thrust values can be applied in the
calculations, to the extent indicated in the Table. The basis
for deductions due to thruster-hull, thruster-current and
thruster-thruster interference is to be documented and included
in the design submission.
|
3. Refer
to Pt 3, Ch 10, 4.1 Design cases 4.1.1 for the Rule basis of
failure, including thruster system failure, for damaged
case.
|
4.2.2 Units which utilise thruster-assistance, as an aid to position keeping or
as a means of reducing anchor line tensions which have a system approved by LR, may
be assigned a special features notation as defined in Pt 3, Ch 10, 1.2 Class notations.
4.3 Design combinations of return periods of
environmental parameters
4.3.1 Unless agreed otherwise with LR, the following design environmental
combinations are to be considered:
- For floating offshore installations at a fixed location:
- 100-year sea state + 100-year wind + 10-year
current.
- For mobile offshore units:
- 50-year sea state + 50-year wind + 10-year
current.
- For floating offshore installations at a fixed location:
- 100-year sea state + 10-year wind + 100-year
current.
- For mobile offshore units:
- 50-year sea state + 10-year wind + 50-year
current.
- In locations subject to squalls:
- For floating offshore installations at a fixed
location:
- 100-year squall + 1-year sea-state + 1-year
current.
- 100-year squall + no other environment.
- For mobile offshore units:
- 50-year squall + 1-year sea-state + 1-year
current.
- 50-year squall + no other environment.
When specialist environment reports adequately provide determined joint
probabilities of occurrence of the various local environmental actions (wind waves,
swell, wind, current), the design may be based on investigation of the mooring
system response to each dominant environmental action with return period of 100
years and associated other actions (e.g. 100 year wind wave plus associated swell,
wind and current).
Combinations of environmental parameters of lower return period may
govern the response of the positional mooring system and as such combinations of
environmental parameters with lower return period may need to be considered to
ensure the worst response of the positional mooring system is captured in the design
analyses.
In locations where both wind driven waves and swell prevail, the sea
state report is to consider the 100 years (or 50 years for Mobile Offshore Units)
wind driven waves with associated swell and 100 year (or 50 year as applicable)
swell with associated wind driven sea.
In locations subject to cyclonic events the above combinations are to be
extended to investigate both cyclonic and non-cyclonic conditions.
4.3.2 When the specialist environmental data report indicates stronger
correlation between wind, wave and current, (e.g. concurrent occurrence of all
environmental parameters at same return period) the above design environmental
condition may need to be amended.
4.3.3 For 100-year (or 50-year for mobile offshore units) waves, a range of
different wave height and period combinations shall be considered.
- To ensure that the most critical combinations of low frequency
and wave frequency responses are determined, a broad range of sea states
represented by significant wave heights and peak periods will required to be
investigated and be preferably based on the use of a contour of significant
wave height and peak period joint frequency distribution at 100-year return
period (or 50-year contour for mobile offshore units).
- When contours of significant wave heights and peak periods are
not reported in the environmental data report, a conservative range of wave
heights and period range will require to be applied in the design
(see
Pt 3, Ch 10, 3.4 Environmental parameters 3.4.3.(e)).
- As the wave spectrum is a combination of wind-driven waves and
swell, consideration will need to be given for certain locations to the
joint occurrence and angular separation between these two components.
Appropriate hindcast data can be used to this effect.
See also
Pt 3, Ch 10, 3.3 Metocean data 3.3.2 and Pt 3, Ch 10, 3.4 Environmental parameters 3.4.3.
4.3.4 For a unit and/or ship designed to disconnect from the mooring system,
appropriate lower limiting environmental conditions can be applied for the connected
cases.
4.3.7 Note that account is to be taken in the design of build-up of marine
growth on the anchor lines, riser system and/or the hull, and the resulting increase
in load and damping.
4.4 Design directional combinations of
environmental parameters
4.4.1 Sufficient combinations of directions of wind and current relative to
wave direction are to be investigated to ensure the critical cases are found. Swell
is to be superimposed from the worst case direction, see
Pt 3, Ch 10, 3.4 Environmental parameters 3.4.3.(d). The following combinations are envisaged as a minimum for
design (unless joint directional probabilities of the various environmental actions
are available and can be adequately documented or more onerous directional
combinations are reported in a specialist report):
- Wave, wind and current collinear.
- Wind and current at 30° to waves.
- Wind at 30° to waves, and current at 90° to waves.
Note.
For case (c) above, only combination (a) given in Pt 3, Ch 10, 4.3 Design combinations of return periods of environmental parameters 4.3.1 has to be considered.
For locations where swell direction differs from that of the wind driven
waves this directional separation is to be considered.
4.4.3 For locations subject to cyclonic events, the directionality of the
dominant environmental parameters may change rapidly, resulting in transient
responses of the offshore unit and its positional mooring system which are to be
investigated as required.
4.5 Environmental directions relative to unit
and mooring system
4.5.1 For spread moored units, at least head, quartering, beam and down-line
directions are to be considered in mooring analysis. Depending on the symmetry of
topside structure, super-structure and positioning mooring system, as well as on
response analysis and wind, wave and current force/moment calculations, other
directions may require to be considered, see also
Pt 3, Ch 10, 4.4 Design directional combinations of environmental parameters.
4.5.2 For weathervaning units, the following cases must be considered as a
minimum requirement:
- Wave direction along mooring line.
- Wave direction between mooring lines.
Additionally for locations subject to squalls:
- Squalls blowing in direction along mooring line.
- Squalls blowing in direction between mooring lines.
4.5.3 Where the mooring lines are grouped, additional wave directions will
require to be considered at intermediate headings between the directions given
above.
4.5.4 For a positional mooring system without thruster assist:
- the single line failure case shall investigate:
- loss of highest loaded line leading to highest
excursions; and
- loss of second highest loaded line leading to highest
line tensions.
- the two lines failure case shall investigate:
- loss of either line adjacent to the first failed
line.
The assessment of the highest and second highest loaded line are to be
based on stable statistics. For asymmetrical mooring configuration due consideration
is to be given to the determination of the most onerous line breakage case leading
to worst offset and line tension. Additional single and two lines failure cases may
need to accounted to check minimum clearances (e.g. in case the worst offset cases
do not correspond to the worst clearance). See also
Pt 3, Ch 10, 4.3 Design combinations of return periods of environmental parameters 4.3.1.
4.5.5 For locations subject to squalls, the environmental directions relative
to the unit are to be based on a specialist report or a full 360° screening to
ensure the worst condition is captured. As the direction of the wind during a squall
can significantly and rapidly change, the effect of shift in the wind direction as
the squall reaches the offshore unit is to be investigated giving due consideration
transient shift in heading of offshore units that weathervane.
4.6 Other design aspects
4.6.1 Anchor lines are to have adequate clearance from subsea equipment such
as templates, flowlines, adjacent fixed structures or other floating units and their
subsea equipment. In general a minimum clearance of 10m is to be maintained at all
times between the Offshore Unit inclusive of its other mooring lines and all other
neighbouring floating, fixed or subsea structures. Acceptability of smaller
clearance would need to be substantiated by an appropriate risk assessment. Where
mooring line failure could lead to fouling of other structures (e.g. PLEM,
pipelines, risers, flow-lines etc) a risk assessment is to be carried out. It is the
responsibility of the Owner to notify the local authority or regulator, LR and the
Owner of the other structures of the low clearance and any associated risks for the
nearby asset. The design shall also check clearance (considering most onerous offset
and damaged stability conditions, connection or disconnection operation etc.)
between the Offshore Unit and its positional mooring system between components of
the positional mooring system or account for the interaction through detailed
design.
4.6.2 The design of the mooring system is to take account of the offset limits
required by the drilling string or riser system, and the avoidance of contact
between risers and anchor lines or other structures.
4.6.3 Where normal production, or other normal operational activity, is
intended to be continued during periods where an anchor line is disconnected for
planned inspection, maintenance or repair etc., specific environmental limitations
are to be established to ensure that safety factors are maintained even with one
line out of action. Such arrangements and the specific environmental limitations are
to be reflected in the Inspection, Maintenance and Repair Manual and Operation
Manual. The PMS Inspection, Maintenance and Repair plan is to ensure that scheduling
of such IMMR activities be subject to an HAZOP type risk assessment (and account for
potential hazard from squall, cyclonic event, etc.).
A similar procedure applies when machinery and equipment cannot remain
fully functional during maintenance and inspection.
4.6.4 Wherever practicable, permanent moorings are to be designed to allow
removal for repair in reasonable weather of damaged components without seriously
reducing the overall safety of the unit as a whole.
4.6.5 In cases where the mooring system is intended to be actively controlled
by adjustment of line lengths and tensions, satisfactory evidence must be submitted
to show that the adjustment procedure is practical, taking account of winch control
and prevailing environmental conditions. The risk associated with such arrangements
and operational practice would need to be assessed using HAZID, HAZOP, etc.
4.6.6 Where units are moored in areas where high velocity currents occur,
dynamic excitation due to vortex shedding associated vortex and wake induced
vibrations are to be considered. This will affect both the global response of the
Offshore Unit as well as the mean line tensions and the line dynamics of its
positional mooring system.
The effects may be more significant as water depth increases.
4.6.7 The mooring line interaction with support structures such as stoppers,
fairlead, hawse pipes, guide tubes is to be subject to detailed assessment of
actions and reactions between mooring components and the support structure to enable
detailed design of the interacting structures.
Aspects such as compression, bending, torque, friction, bearing pressure,
grip pressure, chaffing, locking, wear, electrical continuity and effect on
corrosion control of the components should be considered and documented as
appropriate in the detailed design of the components.
4.6.8 The maximum allowed thickness of marine growth taken into account is to
be stated in the Operations Manual. The actual marine growth should be monitored in
service and the plan for regular cleaning (consistent with design assumptions) is
also to be included in the Operation Manual. Marine growth is not to exceed the
maximum allowed thickness in service.
4.6.9 When a positional mooring system is found damaged, it shall be promptly
reported to the LR and a Condition of Class will generally be recorded. For normal
production, or other normal operational activities to continue under Class, the
Owner shall reassess the normal operations and demonstrate that the Offshore Unit
still meets after damage the level of safety required by Class Rules for intact and
damaged (i.e. with one further line failed) conditions allowing for agreed
documented mitigation measures (as per mooring failure response procedures) to be
put in place. The Offshore Unit operating on Positional Mooring System with one line
damage shall not present a risk of major hazard.
4.6.10 Consideration should be given to providing redundancy in the positional
mooring system, to avoid potential disruption of normal production or other normal
operation when a single failure or damage is found. This applies to the PMS in
general (i.e. mooring lines, mooring equipment, integrity monitoring system and
instrumentation etc.).
|