Fabrication and Installation - Offshore Structures: Design, Construction and Maintenance - page 363

The usual requirement for the distance clearance between maximum sta-
tic draft and minimum water depth should not be less than 2 m or 10% of the
maximum static draft, whichever is lesser, plus an allowance for motion. The
maximum static draft should be the actual measured draft at the deepest
point, with allowance for errors in measurement, initial trim, and water den-
sity change. The motion allowance should include the maximum increase in
draft due to towline pull, wind heel, roll, pitch and heave. These values can
best be determined by model tests. In practice, most fixed offshore platform
structures will be governed by the 2-m minimum clearance.
The dynamic accelerations of the towed structure should generally be lim-
ited to 0.2 g, to minimize forces acting on the tie-downs and to minimize
adverse effects on personnel.
The towing horsepower selected should be sufficient to hold the towed
structure against significant wave heights, 40-knot sustained wind speed, and
current speed for the region of work. Obviously, these arbitrary parameters
have to be adjusted to the region involved.
The metacentric height (GM) is the distance between the center of gravity of
a ship and its metacenter ( Figure 5.39 ). (When a ship is heeled, the center of
buoyancy of the ship moves laterally. The point at which a vertical line through
the heeled center of buoyancy crosses the line through the original, vertical cen-
ter of buoyancy is the metacenter, as shown in Figure 5.38.) The GM is used to
calculate and to ensure the stability of a ship and it must be determined before
the ship proceeds to sea. The GM must equal or exceed the minimum required
GM for that ship for the duration of the forthcoming voyage.
The GM should have a positive value, usually 1

-

2 m for a large offshore
structure. The maximum inclination of the towed structure under conditions

M

M

G

G

B B

B

FIGURE 5.39 Metacentric height.