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

1.2.1 The Regulations for classification and the assignment of class notations are given in Pt 1, Ch 2 Classification Regulations, to which reference should be made.

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:

TA(1)
TA(2)
TA(3)

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.

1.3 Definitions

1.3.1 The definitions given in this Section are for Rule application only and will not necessarily be valid in any other context, see also Pt 1, Ch 2, 2 Definitions, character of classification and class notations

1.3.2 Offshore Unit. See Pt 1, Ch 2, 2.1 General definitions 2.1.13 and for the definitions of specific types relevant to this section such as Mobile offshore unit see Pt 1, Ch 2, 2.1 General definitions 2.1.10 and Floating offshore installation see Pt 1, Ch 2, 2.1 General definitions 2.1.9.

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.
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:
- Prime movers, gearing, shafting, propellers and thrust units,see also Pt 5 MAIN AND AUXILIARY MACHINERY.
- Machinery piping systems.
- Electrical installations.
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.