Section 4 Design aspects

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 serial failure. 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, sinking. Mooring line failure response procedure should be referred to from the time one line fails.

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.

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).

4.2.2 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.

4.2.3 The requirements of Pt 3, Ch 10, 13 Thruster-assisted positional mooring and Pt 3, Ch 10, 14 Thruster-assist class notation requirements are to be complied with and for the majority of offshore units with positional mooring systems which utilise thruster-assistance the class notation TA(3) will be applicable. Thruster-assist notations TA(1) and TA(2) will only be considered for applications of low criticality.

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.

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.

4.3.5 The mooring system with the unit and/or ship disconnected is normally required to be designed for the criteria specified in Pt 3, Ch 10, 4.3 Design combinations of return periods of environmental parameters 4.3.1 to Pt 3, Ch 10, 4.3 Design combinations of return periods of environmental parameters 4.3.3.

4.3.6 Specific combinations of environmental conditions need to be set as design limits for temporary operations where these operations may overload the positional mooring system in environmental conditions less severe than those considered in Pt 3, Ch 10, 4.3 Design combinations of return periods of environmental parameters 4.3.1 to Pt 3, Ch 10, 4.3 Design combinations of return periods of environmental parameters 4.3.3. The design shall check and confirm that specific operations (e.g. side by side or tandem loading or offloading or connection or disconnection of OU from PMS (disconnectable cases)) carried out within specific environmental limits, do not result in overloads in the positional mooring system. The limiting environmental criteria for specific operations should be reported in the operation manual.

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.

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.

4.4.2 For locations subject to squalls events, squalls are to be considered. For all possible directions relative to waves and current unless directionality of squall event is sufficiently substantiated in a specialist report (See Pt 3, Ch 10, 3.4 Environmental parameters 3.4.2). The range of concomitant wave and current directions is to be agreed with LR (See Pt 3, Ch 10, 4.3 Design combinations of return periods of environmental parameters 4.3.1). At the approach of squalls the directionality of the wind may change rapidly. The resulting transient responses of the offshore unit and its positional mooring system are to be investigated as required.

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.

4.5.1 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.

4.5.3 Where the mooring lines are grouped, additional wave directions will require to be considered at intermediate headings between the directions given above.

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.

4.6.1 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.).