EM 1110-2-1100 (Part II)
30 Apr 02
(e) Vessel interactions with other vessels and with the channel bank must be accounted for in deter-
mining required navigation channel widths. For example, guidance given in Engineer Manual
(EM) 1110-2-1613, which specifies channel width as a function of the design vessel beam dimension for
various channel conditions, includes these effects. The total channel width consists of a maneuvering lane,
bank clearance, and ship clearance, if two-way traffic is involved. This specification is based on model
studies of large vessels in deep-draft navigation channels and discussions with ship pilots (Garthune et al.
1948).
c. Mooring.
(1) Wave forcing mechanism.
(a) Infragravity waves (wave periods typically between 25 and 300 sec) force long-period oscillations
or seiche in harbors. If the natural period of the ship corresponds to a harbor resonance mode and they are
moored in the vicinity of the node, excessive ship motion can prevent loading and unloading of the ship for
a number of days. In some cases, extensive damage to the ship and pier can result if the mooring lines fail.
(b) Infragravity energy can be divided into bound and free wave energy. Bound or forced infragravity
waves are nonlinearly coupled to wave groups, traveling at the group velocity of the wind waves, and phase
locked to sea and swell waves. Free infragravity waves radiate to and from deep water after being reflected
from the shoreline or are generated by nonlinear interactions and wave breaking of incident wind waves and
are refractively trapped in shallow water, propagating in the longshore direction. According to numerous
investigators (Herbers et al. 1992; Elgar et al. 1992; Okihiro, Guza, and Seymour 1992), bound and free wave
energies increase with increasing swell energy and decreasing water depth. Bound wave contributions are
usually more significant when energetic swell conditions exist, but free waves dominate when more moderate
conditions prevail.
(2) Mooring configurations.
(a) A vessel moored in a harbor or at some point offshore commonly has one of three types of mooring
arrangements:
C
A single-point mooring where the vessel is tied to a buoy by a single line from the bow and is thus
free to rotate around the buoy (i.e. weathervane) in response to environmental forces.
C
A multiple- point mooring where the ship is tied by several fore and aft lines to anchors or buoys.
C
A conventional pier anchorage, where the vessel is tied fore and aft to the pier and separated from
the pier by a fendering system.
(b) For a deep-draft vessel at a pier, the mooring line system will typically consist of 8 to 12 lines in a
symmetrical pattern, half from the bow and half from the stern of the ship. One to two breast lines are
positioned on both bow and stern. The breast line(s) is perpendicular to the ship and dock and presses the
ship against the dock and fenders. Two head lines make an angle of 60-70 deg to the breast line and go
forward from the bow. Two stern lines are analogous to the head lines, but originate from the stern of the
ship. These four lines are on the order of 100 ft long between ship and dock attachment points. Finally, two
or four spring lines make an angle of 85 deg to the breast line and go toward midships. These lines can vary
in length from 100 to 200 ft. The spring lines, in combination with the breast lines, provide the most efficient
ship mooring. The deck of the vessel is typically 3 to 8 m above the pier, with the bow being higher than the
stern.
II-7-64
Harbor Hydrodynamics
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