Section 4 Mooring and Anchoring

401. General

The mooring legs and anchors of an SPM are to be designed to protect against the failure of the underbuoy hose or riser with one anchor line broken, for the design environmental conditions de- scribed in 104. 1

Ch 3

402. Anchor points

The anchors are to be designed to have adequate holding capacity. For mooring systems with drag anchors, the holding capacity of each anchor is to be determined by using the soil characteristics of the bottom samples. The type of anchorage for the anchor leg(s) is to be selected according to conditions of the seabed and the maximum design anchor load.

The design factor of safety with respect to the holding capacity of the anchor is not to be less than 2.0 for the operating condition with the vessel moored to the SPM, and not less than 1.50 for

the design storm condition(without the vessel). Where lower factors of safety of anchor desired with additional mooring analysis for any one line broken case as indicated in

leg(s) are

403., the

factor of safety on the holding capacity of the anchor in a broken line case for the operating con-

dition should not be less than 1.60 In case of an SPM system using anchor piles, it is recom- mended that pile foundations be designed to comply with the appropriate Sections of API RP 2A. A pile driving record or pile grouting record is to be taken and submitted for each pile. The meth- od of installation of the piles and the equipment employed is to be included in the pile driving record. Where the anchoring system uses gravity boxes, resistance against sliding, uplifting, and overturning of the gravity boxes are to be analyzed. The forces due to environmental, gravity and mooring are to be taken into account appropriately. Scour effects on the gravity boxes are to be considered in the design.

On deployment of the mooring system with drag anchors, each mooring line is to be pull tested to

80 % of the maximum design load and is to be held at that load for 30 minutes in presence of the Surveyor. Calculations or data from prior experience are to be submitted to support the choice

of anchorage system and number of anchor points.

403. Anchor leg(s)

The components of each anchor leg are to be designed with the following safety factors of against breaking.

Design storm condition without vessel All lines intact 2.50

Design operating condition with vessel All lines intact 3.00

Lower factor of safety of 2.50 against the minimum breaking strength of the anchor leg compo- nent(s) will be allowed for the design operating condition if a mooring analysis of the mooring system with any one line broken meets a factor of safety of 2.00 with respect to the minimum breaking strength of anchor leg component(s).

The mooring structure of a fixed SPM system is to be designed in accordance with 313.

404. Anchor and chains

Anchors and chains are to comply with the requirements of Pt 4, Ch 8. of "Rules for Mobile Offshore Units". Equipment designed to other standards will be specially considered.

405. Mooring between vessel and SPM

When hawsers are used as the connecting links, they are to be designed using the following factors of safety on the breaking strength of the weakest part. The strength of ropes or hawsers is to be determined in accordance with and certified to the latest version of "OCIMF Prototype Rope Testing". The breaking strength of spliced rope is to be established by appropriate testing. The breaking strength of the hawser to be used is to be the lower value of the hawser in wet or dry condition.

with one fairlead, F.S. = 1.67 with multiple fairleads, F.S. = 2.50

Where the vessel is moored to the SPM using hawsers running through more than two fairleads on

Ch 3

the vessel, the hawser loads are to be calculated as if there are only two fairleads. Hawser manu-

facture is

Hawsers". the SPM,

to comply with the "OCIMF Quality Control and Inspection during the Production of

When a rigid mooring structure is used as the mooring structure between the vessel and the connecting structures are to comply with 3.7.5 of the ABS "Rules for Building and

Classing Mobile Offshore Drilling Units".

406. Structural components

If not indicated elsewhere in this Guidances, the structural and mechanical components (mooring hardware, e.g., connecting links, shackles, etc.) which transmit the mooring loads are to be designed to the greater of the following two loads:

2.50 times the maximum design anchor leg (or mooring line) load in design storm condition.

3.00 times the maximum design anchor leg (or mooring line) load in operating condition.