3 October 2005
Modeling shows that the instantaneous peak chain tension of 2246 knots
(505 kips) is predicted on Leg 1 as the moored vessel nest responds to wind gusts.
This provides a peak instantaneous factor of safety of 1.5 on the breaking strength of
the selected chain size. For this example, the peak dynamic chain tension during the 1
hour at the peak of the design storm is 2.4 times the quasi-static tension in the mooring
leg with the highest tension, Leg 1.
Nest motions for surge, sway, and yaw are provided in Table 8-7. This
table shows that the maximum surge of the vessel nest is approximately 7.4 meters
(24.3 feet) from its equilibrium condition at no loading. Maximum sway and yaw of the
vessel nest is 3.2 meters (10.5 feet) and 1.59 degrees clockwise, respectively. During a
dynamic analysis simulation, nest motions oscillated up to 5.4 meters (17.7 feet) in
surge (for a wind direction coming from the stern), 1.9 meters (6.2 feet) in sway (for a
wind direction 30 degrees aft of broadside), and 2.1 degrees in yaw (for a wind direction
30 degrees off the stern).
Anchor Design. Using the quasi-static design mooring leg tension,
anchor capacity and loads on the embedded plate anchor are calculated using
procedures outlined in NFESC TR-2039-OCN, Design Guide for Pile-Driven Plate
Anchors and NFESC CR-6108-OCN, Anchor Mooring Line Computer Program Final
Report, User's Manual for Program CSAP2. Due to the lower shear strengths of the soft
silty upper layers at the site, a 6-foot by 11-foot mud plate anchor is specified (this
anchor is summarized in the lower line of Table 5-7). A design keyed depth of 55 feet is
selected for the plate anchor. This will provide an estimated static holding capacity of
1913 kN (430 kips).
CSAP is used to predict the mooring leg tension at the anchor. Input
requirements of CSAP include: (1) mooring leg configuration between the anchor and
the buoy or chock; (2) water depth or height of chock above the seafloor; (3) soil profiles
and strength parameters; (4) location and size of sinkers; (5) horizontal tension
component of the mooring leg at the buoy or chock; (6) horizontal distance or total
length of the mooring leg between anchor and buoy or chock; and (7) anchor depth.
Output provided by CSAP includes: (1) chain catenary profile from the anchor to the
buoy or chock attachment point; (2) angle of the mooring leg from the horizontal at the
anchor, the seafloor, and the buoy or chock; (3) tension of the mooring leg at the
anchor, seafloor, and at the buoy or chock; (4) predicted daylight location for the
mooring leg; and (5) length of mooring leg required or horizontal distance between
anchor and buoy or chock.