Section 6 Anchor Holding Power

601. Generals

Different types of foundation systems used for floating installations are drag anchors, pile anchors, vertically loaded anchors (VLAs) and suction piles. Gravity boxes, grouted piles, templates, etc., may also be used and are considered to be within the scope of classification.

602. Drag anchor

1. For a mooring system with drag anchors, the mooring line length should be sufficiently long such that there is no angle between the mooring line and the seabed at any design condition.

2. For soft clay (in Gulf of Mexico) condition, a small angle for the damaged case with one broken line are to be as deemed appropriate by the Society.

3. Drag anchor holding power depends on the anchor type, as well as the condition of the anchor de- ployed in regard to penetration of the flukes, opening of the flukes, depth of burial, stability of the anchor during dragging, soil behavior of the flukes, etc.

4. The designer should submit to the Society the performance data for the specific anchor type and the site-specific soil conditions for the estimation of the ultimate holding capacity (UHC) of an an- chor design. Because of uncertainties and the wide variation of anchor characteristics, exact holding power is to be determined after the anchor is deployed and test loaded.

5. The maximum load at anchor, ÁŴŸẄYŸZ is to be calculated, in consistent units, as follows 301. 4

603.

Conventional pile

1. Conventional pile anchors are capable of withstanding uplift and lateral forces at the same time.

2.

3.

604.

1.

2.

Analysis of the pile as a beam column on an elastic foundation is to be submitted to the Society for review.

The analyses for different kinds of soil using representative soil resistance and deflection (p-y) curves are described in the API RP 2A and API RP 2T, as applicable. The fatigue analysis of the pile should be submitted for review.

Vertically loaded drag anchors (VLA)

VLAs can be used in a taut leg mooring system with approximately a 35° to 45° angle between the seabed and the mooring lines.

These anchors are designed to withstand both the vertical and horizontal loads imposed by the mooring line.

3. The structural and geotechnical holding capacity design of the VLA are to be submitted for review.

This is to include the ultimate holding capacity and the anchor's burial depth beneath the seabed.

Additionally, the fatigue analysis of the anchor and the connectors joining the VLA to the mooring line should be submitted for review.

4. The safety factors of VLA anchors' holding capacity are specified in Table 6-7.

TABLE. 6.7 Factor of Safety for Anchor Holding Capacities1)

605. Suction piles

1. Suction pile anchors are caisson foundations that are penetrated to the target depth by pumping out the water inside of the pile to create underpressure within the pile.

2. They may typically consist of a stiffened cylindrical shell with a cover plate at the top and an openbottom and generally have larger diameters and are shorter in length than conventional piles.

3. These piles can be designed to have a permanent top or a retrievable top depending on the re- quired vertical holding capacity.

4. The pad eye for the mooring line connection can be at the top or at an intermediate level depend- ing on the application of the suction pile. Suction pile anchors are capable of withstanding uplift and lateral forces.

5. Due to the geometry of the suction piles, the failure modes of the soils maybe different than what are applicable for long slender conventional piles.

6. The safety factors for the suction piles' holding capacity are specified in Table 6.7. Geotechnical holding capacity and structural analyses for the suction piles are to be submitted to verify the ad- equacy of the suction piles to withstand the in-service and installation loads.

7. Additionally, fatigue analysis of the suction piles are to be submitted to verify the adequacy of the fatigue life of the critical locations. Installation analyses are to be submitted to verify that the suc- tion piles can be penetrated to the design penetration and that the suction piles can be retrieved, if necessary.

8. It is suggested that a ratio of at least 1.5 between the force that would cause uplift of the soil- plug inside of the pile and the effective pile installation force be considered in the penetration analysis.