Section
1 General
1.1 Application
1.1.1 This Chapter applies to offshore units with positional mooring systems.
This has been abbreviated to PMS.
1.1.2 The requirements apply to the following categories of unit and mooring
type:
- Ship units, column-stabilised units, offshore loading buoys and
other similar type moored floating structures.
- Multi-leg mooring systems, either spread-moorings or
single-point moorings.
- Catenary systems or taut-leg systems.
1.1.3 Other types of application will be specially considered.
1.1.4 The requirements of this Chapter are not applicable to the mooring
tethers on tension-leg units. For the design requirements of tension-leg units,
see
Pt 4, Ch 4 Structural Unit Types.
1.1.5 Requirements additional to these Rules may be imposed by the National
Authority with whom the unit is registered and/or by the Administration of the
coastal state(s) with territorial jurisdiction over the waters in which it is
intended to operate.
1.1.6 When other codes or standards are proposed, gap analysis and risk
assessments are to be provided by the Owner/designers to demonstrate the alternative
codes or standards provide an equivalent level of safety to the requirements of this
section. Acceptance of the alternative codes or standards will be subject to the
alternative standards being agreed by LR to give an equivalent level of safety to
the Rule requirements.
1.2 Class notations
1.2.2 For the assignment of the character letter T to floating offshore
installations at a fixed location with positional mooring systems, the requirements
of this Chapter are to be complied with. Mobile offshore units provided with a
positional mooring system which complies with the requirements of this Chapter will
be eligible for the assignment of a special features class notation as follows:
PM (Positional mooring system), or
PMC (Positional mooring system for mooring in close
proximity to other vessels or installations. This notation will apply in particular
to any unit operating adjacent to a fixed installation, e.g., crane unit,
accommodation unit, support unit, etc.).
1.2.3 The positional mooring system will be considered for classification on
the basis of operating constraints and procedures specified by the Owner or Operator
and recorded in the Operations Manual.
1.2.4 Units fitted with a thruster-assisted positional mooring system, which
complies with the requirements of Pt 3, Ch 10, 4 Design aspects, will be eligible for the
assignment of one of the following special features class notations:
1.2.5 The numeral in parentheses after the thruster-assist notation TA
in Pt 3, Ch 10, 1.2 Class notations 1.2.4 defines the thruster allowance
which may be permitted in the design of the positional mooring system and is
determined by the capacity/redundancy of the thrust/machinery installation,
see
Pt 3, Ch 10, 4 Design aspects, Pt 3, Ch 10, 13 Thruster-assisted positional mooring and Pt 3, Ch 10, 14 Thruster-assist class notation requirements.
1.2.6 Inshore/at-shore-moored units provided with a long-term terminal mooring
system, which complies with the requirements of Pt 3, Ch 10, 16 Long-term nearshore positional mooring system will be eligible for the assignment of a special features
class notation LTMOOR. Units with higher redundancy due to two line damages
that complies with ShipRight procedure Long-term Nearshore Positional Mooring
System will be eligible for a special feature class notation
LTMOOR(HR).
1.3 Definitions
1.3.3
Ship. Floating structure such as shuttle tanker or loading/offloading tanker
which is to be temporarily moored to an offshore unit.
1.3.4
Positional mooring. Station-keeping by means of multi-leg mooring system with
or without thruster-assist. The positional mooring system will consist of the
following components, as relevant:
- Anchor points:
- Drag embedment anchors.
- Anchor piles.
- Suction anchor piles.
- Gravity anchors.
- Plate anchors.
- Anchor lines.
- Anchor line fittings:
- Shackles.
- Connecting links/plates.
- Rope terminations.
- Clump weights.
- Anchor leg buoyancy elements.
- Fairleads/bending shoes.
- Chain or wire rope stoppers.
- Winches or windlasses.
Where applicable, the structural or mechanical connection of these items
to the unit is also considered to be part of the positional mooring system.
1.3.5
Thruster-assist. The use of thrusters, inclusive of their associated
equipment, to supplement the unit’s positional mooring system.
1.3.6
Catenary mooring. A mooring system which derives its compliancy mainly from
the catenary action of the anchor lines. Some additional resilience is provided by
the characteristic axial elasticity of the anchor lines.
1.3.7
Taut-leg mooring. A mooring system based on light-weight anchor lines
pre-tensioned to a taut configuration with no significant catenary shape at any unit
offset, and applying vertical and horizontal loads at the anchor points. With this
type of system, compliancy is derived from the inherent axial elastic stretch
properties of the anchor line.
1.3.8
Single-point mooring. An offshore positional mooring system arrangement in
which the offshore unit freely weathervanes about a geostationary structure,
generally using an internal or external turret, single buoy or single tower,
see
Pt 3, Ch 2, 1.2 Class notations
1.3.9
Spread mooring. A multi-line mooring system designed to maintain an offshore
unit on an approximately fixed heading.
1.4 Plans and data submission
1.4.1 The positional mooring system will be subject to review and approval. The
following information and plans are to be submitted in an agreed electronic format,
to cover the design review and class approval of the positional mooring system:
- Plans of the positional mooring system and associated equipment
are to be submitted including the following, as applicable:
- General arrangement of offshore floating unit (including
hull and topsides general arrangements).
- Layout and arrangement of deck mooring equipment and
support structures.
- Structural arrangement of mooring equipment, support
structure and attachment point to the main structure or hull of the
offshore unit.
- Mooring layout.
- Field layout.
- Anchor lines and fittings assembly.
- Anchor points.
- Fairleads/bending shoes, including associated
mechanism, articulation or stopper.
- Cable (i.e. mooring line, steel wire or fibre rope or
chain) stoppers or connectors.
- Winches, windlasses or tensioners.
- Deck equipment used in support of the mooring line
failure response plan.
- General arrangement showing moorings for tandem and side-by-side
offloading. This is to include the maximum and minimum dimensions
and main particulars for the range of shuttle tankers that are
permitted to attend. For each mooring line, the breaking load and
the maximum and minimum angles (horizontal and vertical) between the
line and the offshore unit are to be stated.
- Location of control stations.
- For thruster-assisted positional mooring systems, plans of the
following together with particulars of ratings, in accordance with the
relevant Parts of these Rules are to be submitted:
- In addition, details of proposals for the redundancy provided in
machinery, electrical installations and control systems are to be submitted.
These proposals are to take account of the possible loss of performance
capability should a component fail. Where a common power source is utilised
for thrusters, details of the total maximum load required for
thruster-assist are to be submitted.
- Plans of control, alarm and safety systems including the
following are to be submitted:
- Functional block diagrams of the control
system(s).
- Functional block diagrams of the position reference
systems and environmental sensors.
- Details of electrical supply to the control system(s),
the position reference system(s) and the environmental sensors.
- Details of the monitoring functions of the controllers,
sensors and reference system together with a description of the
monitoring functions.
- List of equipment with identification of the
manufacturer, type and model.
- Details of the overall alarm system linking the
centralised control station, subsidiary control stations, relevant
machinery spaces and operating areas.
- Details of control stations, e.g., control panels and
consoles, including the location of the control stations.
- Factory and customer acceptance test schedules which are
to include the methods of testing and the test facilities
provided.
1.4.2 The following supporting plans, data, calculations or documents are to
be submitted in an agreed electronic format:
- General:
- Mooring design premise or basis of design.
- Moored unit details (dimensions and main
particulars).
- Corrosion protection strategy and/or corrosion
rates.
- Specifications:
- Materials.
- Mooring line components, mooring equipment and
fittings.
- Model testing.
- Data reports:
- Environmental criteria (covering extreme as well as
ambient conditions and all applicable operating environmental
limits) and in addition for floating offshore installations at a
fixed location:
- Detailed specialist environmental reports.
- Sea bed conditions.
- Soil and soil conditions.
- Design reports and calculations:
- Hydrodynamic/motion analysis.
- Mooring analysis.
- Model test report with results
- Design load report.
- Anchor line components: strength and fatigue, including
as applicable, detailed design at points of constraints (e.g. in and
out of plane bending analysis, in the case of top chain
connection).
- Anchor point: strength and fatigue.
- Anchor point holding capacity.
- Fatigue.
- Equipment/ancillaries including the associated
equipment, stoppers and fairleads: strength and fatigue.
- Corrosion protection and/or corrosion allowance.
- Other information:
- In-service inspection programme.
and in addition for floating offshore installations at a
fixed location:
- Installation procedures.
- Installation records for piles and anchors, see
also
Pt 3, Ch 14, 5 Drag embedment anchors – General.
- Plan and schedule for PMS Initial Installation
Survey,
- Mooring line components datasheets, inclusive of LR
certificate of manufacturing and testing.
- LR certificate of manufacturing and testing of Deck
mooring equipment including those used in support of the mooring
line failure response plan.
- PMS Initial Installation Survey records.
and in addition for mobile offshore units:
- Anchor point holding capacity.
1.4.3 An Operations Manual, as required by Pt 3, Ch 1, 3 Operations manual, is to be submitted and the manual is to
contain all necessary information and instructions regarding positional mooring and,
where relevant, thruster-assisted positional mooring. It would normally also contain
descriptions of the following:
- Mooring systems.
- Laying the mooring system.
- Anchor pre-loading.
- Pre-tensioning anchor lines.
- Tension adjustment.
- Mooring line tensions/ offset/integrity monitoring.
- Winch/windlass performance.
- Winch/windlass operation.
- Procedure in event of failure or emergency.
- Procedure for operating thrusters.
- Fault-finding procedures for thruster-assist system.
- Maintenance procedures. see also
Pt 3, Ch 10, 1.4 Plans and data submission 1.4.4.
- Mooring line failure or loss of station keeping capability
failure response procedure.
1.4.4 A PMS Inspection, Maintenance and Repair Manual (PMS IMMR Manual) is to
be submitted covering frequency or scheduling, procedures and techniques of such
activities for each component, related equipment and support structures. Due
consideration is be given to the Oil and Gas UK Mooring Integrity Guidance.
Calibration and testing of monitoring equipment (position monitoring, line integrity
monitoring etc.) and associated alarms are also to be addressed. The PMS IMMR Manual
is to report pertinent inspection, fault or defect detection, efficiency or
degradation measurement methods (and associated error margins or accuracy), ways of
recording the results. Inspection records should aim at enabling tracking and
trending of degradation processes. This Manual is to address all inspections
required for Periodical Surveys.
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