38
STRUCTURE-ENVIRONMENTAL force interactions
Substituting vt from équation (2.34 ) into (2.35), the resuit is
mv + Civ + k^v — mvg
(2.36)
In this structural model, the excitation force pi(t) is identifier! as the horizontal
ground accélération of magnitude -mvg. In using this model to compute the
structural responses v = v(t) to typical earthquake motion, which will be done
in Chapter 5, the négative sign on the right side of équation (2.36) is of no
conséquence and is generally ignored.
Base shaking of an offshore structure is just one of several types of seismicinduced ground motions (Hudson, 1970). For instance, piles may shift or the
structure’s foundation may be undermined and fail due to fault displacements
or large-scale mud slides. Such large-scale earth motions occur over a period
of time which is relatively long compared to the natural period of an offshore
structure. Thus these earth motions are essentially static as far as the structure
is concerned and are not a part of a structural dynamic analysis.
There is a vast amount of literature describing the geological nature of earthquakes and the effects of soil and rock strata on structural response, including
linear and nonlinear effects. For such information the reader may consult the
following classical works: the book edited by Wiegel (1970); the concise présentation of the seismic phenomenon, including the deterministic and statistical
analyses of structural responses, by Gould and Abu-Sitta (1980); and the compilations of references pertinent to earthquake engineering of offshore structures
by Bea et al. (1979) and Marshall (1981).
Ice Impact Forces
Ice is a hazard to fixed offshore structures which are located in polar seas
such as the Gulf of Alaska. Gaythwaite (1981) summarized ice hazards and
ways to minimize associated structural damage. Current field studies about ice
are published yearly in the Proceedings of the Offshore Technology Conférence,
Houston, Texas.
To analyze for the impact hazard, it is necessary to know ice speed, size,
and material properties. Drifting ice travels at speeds from 1 to 7 percent of
the wind speed. A typical ice island in Cook Inlet, Alaska, for instance, may
be 1 km in diameter, 1 m thick, and travel with a speed of 3 knots. In general,
the ratio of the height of a drifting ice block above water to that below is about
1.2; but may vary from 1:1 up to 1:7. With the usual concentrations of Na2SÛ4,
sea ice has a compressive or rupture strength from 200 to 400 psi, but this does
varj with sait concentration and the rate of impact loading (Peyton, 1968).
The American Petroleum Institute (1979, 1997) has recommended the following
formula far calculating the horizontal force, Fh, on structures subjected to the
impact of ice:
Fh = CicrciA0
(2.37)
Here Q , is a coefficient in the range of 0.3 to 0.7 which accounts for loading rate;
'T . is the compressive strength or rupture stress for the ice; and _40 is the area
STRUCTURE-ENVIRONMENTAL force interactions
Substituting vt from équation (2.34 ) into (2.35), the resuit is
mv + Civ + k^v — mvg
(2.36)
In this structural model, the excitation force pi(t) is identifier! as the horizontal
ground accélération of magnitude -mvg. In using this model to compute the
structural responses v = v(t) to typical earthquake motion, which will be done
in Chapter 5, the négative sign on the right side of équation (2.36) is of no
conséquence and is generally ignored.
Base shaking of an offshore structure is just one of several types of seismicinduced ground motions (Hudson, 1970). For instance, piles may shift or the
structure’s foundation may be undermined and fail due to fault displacements
or large-scale mud slides. Such large-scale earth motions occur over a period
of time which is relatively long compared to the natural period of an offshore
structure. Thus these earth motions are essentially static as far as the structure
is concerned and are not a part of a structural dynamic analysis.
There is a vast amount of literature describing the geological nature of earthquakes and the effects of soil and rock strata on structural response, including
linear and nonlinear effects. For such information the reader may consult the
following classical works: the book edited by Wiegel (1970); the concise présentation of the seismic phenomenon, including the deterministic and statistical
analyses of structural responses, by Gould and Abu-Sitta (1980); and the compilations of references pertinent to earthquake engineering of offshore structures
by Bea et al. (1979) and Marshall (1981).
Ice Impact Forces
Ice is a hazard to fixed offshore structures which are located in polar seas
such as the Gulf of Alaska. Gaythwaite (1981) summarized ice hazards and
ways to minimize associated structural damage. Current field studies about ice
are published yearly in the Proceedings of the Offshore Technology Conférence,
Houston, Texas.
To analyze for the impact hazard, it is necessary to know ice speed, size,
and material properties. Drifting ice travels at speeds from 1 to 7 percent of
the wind speed. A typical ice island in Cook Inlet, Alaska, for instance, may
be 1 km in diameter, 1 m thick, and travel with a speed of 3 knots. In general,
the ratio of the height of a drifting ice block above water to that below is about
1.2; but may vary from 1:1 up to 1:7. With the usual concentrations of Na2SÛ4,
sea ice has a compressive or rupture strength from 200 to 400 psi, but this does
varj with sait concentration and the rate of impact loading (Peyton, 1968).
The American Petroleum Institute (1979, 1997) has recommended the following
formula far calculating the horizontal force, Fh, on structures subjected to the
impact of ice:
Fh = CicrciA0
(2.37)
Here Q , is a coefficient in the range of 0.3 to 0.7 which accounts for loading rate;
'T . is the compressive strength or rupture stress for the ice; and _40 is the area