3 October 2005
For this example, a keyed anchor depth of 55 feet was selected. Input
data included: (1) configuration of the mooring leg (30 feet of 3-inch wire attached to 2+
shots of 2.75-inch chain); (2) height of seafloor to vessel chock (46 feet stern and 64
feet bow); (3) soil profile and strength for the site (shear strength increases linearly at 10
pounds per ft2 per foot of depth); (4) information on the sinkers (2 each 20-kip sinkers
placed a horizontal distance of 170 feet away from the anchor; (5) horizontal tension
component of the mooring leg from the quasi-static results (195 kips); (6) horizontal
distance between anchor and chock (280 feet) from the quasi-static results; and (7)
depth of anchor (55 feet).
CSAP results for this design leg at this anchor depth indicate that the
predicted daylight location of the mooring leg is approximately 99 feet (30 meters) from
the anchor location and the leg tension at the anchor is 166 kips. A profile of this leg is
shown in Figure 6-6. Note that the interaction between the chain and the soil accounts
for a 25 percent reduction in tension on the mooring leg at the anchor. This gives a
predicted quasi-static anchor holding factor of safety of 2.6.
Based on the CSAP results, 6-foot by 11-foot plate anchors are specified.
Based on predicted keying distances required for this anchor, as outlined in NFESC TR-
2039-OCN, Design Guide for Pile-Driven Plate Anchors, the anchors should be installed
to a tip depth of 70 feet (21 meters) below the mudline to ensure that the anchor is
keyed at a minimum depth of 55 feet (16.8 meters). Figure 8-16 provides a comparison
between tip depth, keyed depth and ultimate capacity for this size anchor.
Further information concerning this design is provided in NFESC SSR-