EARTHQUAKES, ICE IMPACT, AND WAVE SLAMMING
39
of the structure exposed to the impacting ice. An example of a structure that
tolérâtes impacting ice is a concrète gravity platform with one wide-based, coneshaped leg (Bercha and Stenning, 1979). This leg geometry leads to effective
stress rupturing of the impacting ice with minimal damage to the structure.
Besides the impact of floating ice, there are other ice hazards to a structure
in the polar seas. Most of these hazards are well understood and can be minimized by careful structural design. For instance, one of these hazards is the
uplift force on the deck due to the buoyancy of accumulated ice attached to
the structure around the water line. (Note that the spécifie gravity of sea ice
ranges from 0.89 to 0.92). Although this uplift force may be offset by added
gravity loads on the superstructure due to ice accumulation there, this accretion
also increases wind loads because of the increased exposed area. Further, ice
accumulation on the legs increases wave and current loads for the same reason.
Ice also causes abrasion in varions forms. Cyclic freezing and thawing leads
to cracking and spalling of offshore concrète structures, phenomena common in
our highways. This is usually due to the expansion of freezing water in cracks,
pores, or capillary cavities. Some of this entrained water is the excess required
for hydration of the cernent and may be minimized by careful choice of the mix
ingrédients.
Using a fracture mechanics approach, the quasi-static pénétration and fracture of floating ice plates was investigated by Bazant and Kim (1998). Related
studies were reported by DeFranco and Dempsey (1994). These ideas are beginning to be applied to the design of offshore structures in arctic régions, to
mitigate their vulnerability to the multiple hazards of ice.
Wave Slamming Forces
Although general descriptions of offshore waves and their associated loadings
of structures are considered in Chapters 3 and 4, wave slamming is an important
enough hazard that it is now considered separately. Unlike the steady wave train
models addressed in Chapters 3 and 6, slamming refers to the impact of a single,
occasional wave with a particularly high amplitude of energy. Sarpkaya and
Isaacson (1981) reviewed the classical research on the slamming of water against
circular cylinders, of which the work of Miller (1977, 1980) seems particularly
applicable. Based on water-tank experiments, Miller found that the peak wave
slamming force on a rigidly held, horizontal, circular cylinder is correlated by
the following équation:
Fs = ^CsPDeu2
(2.38)
Here, the coefficient Ca is in the range of 3.5 to 3.6; D and t are the cylinder
diameter and length, and p and u are the water mass density and the peak
horizontal water particle velocity, respectively. If the cylinder is not rigid but
a flexible, elastic body (a tubular brace of a jacket-template platform. for instance), then Sarpkaya and Isaacson (1981) recommend the following procedure
for computing the cylinder load: let Ca = 3.2 and then multiply the resulting
force calculated from équation (2.38) by the force-impact magnification factor
39
of the structure exposed to the impacting ice. An example of a structure that
tolérâtes impacting ice is a concrète gravity platform with one wide-based, coneshaped leg (Bercha and Stenning, 1979). This leg geometry leads to effective
stress rupturing of the impacting ice with minimal damage to the structure.
Besides the impact of floating ice, there are other ice hazards to a structure
in the polar seas. Most of these hazards are well understood and can be minimized by careful structural design. For instance, one of these hazards is the
uplift force on the deck due to the buoyancy of accumulated ice attached to
the structure around the water line. (Note that the spécifie gravity of sea ice
ranges from 0.89 to 0.92). Although this uplift force may be offset by added
gravity loads on the superstructure due to ice accumulation there, this accretion
also increases wind loads because of the increased exposed area. Further, ice
accumulation on the legs increases wave and current loads for the same reason.
Ice also causes abrasion in varions forms. Cyclic freezing and thawing leads
to cracking and spalling of offshore concrète structures, phenomena common in
our highways. This is usually due to the expansion of freezing water in cracks,
pores, or capillary cavities. Some of this entrained water is the excess required
for hydration of the cernent and may be minimized by careful choice of the mix
ingrédients.
Using a fracture mechanics approach, the quasi-static pénétration and fracture of floating ice plates was investigated by Bazant and Kim (1998). Related
studies were reported by DeFranco and Dempsey (1994). These ideas are beginning to be applied to the design of offshore structures in arctic régions, to
mitigate their vulnerability to the multiple hazards of ice.
Wave Slamming Forces
Although general descriptions of offshore waves and their associated loadings
of structures are considered in Chapters 3 and 4, wave slamming is an important
enough hazard that it is now considered separately. Unlike the steady wave train
models addressed in Chapters 3 and 6, slamming refers to the impact of a single,
occasional wave with a particularly high amplitude of energy. Sarpkaya and
Isaacson (1981) reviewed the classical research on the slamming of water against
circular cylinders, of which the work of Miller (1977, 1980) seems particularly
applicable. Based on water-tank experiments, Miller found that the peak wave
slamming force on a rigidly held, horizontal, circular cylinder is correlated by
the following équation:
Fs = ^CsPDeu2
(2.38)
Here, the coefficient Ca is in the range of 3.5 to 3.6; D and t are the cylinder
diameter and length, and p and u are the water mass density and the peak
horizontal water particle velocity, respectively. If the cylinder is not rigid but
a flexible, elastic body (a tubular brace of a jacket-template platform. for instance), then Sarpkaya and Isaacson (1981) recommend the following procedure
for computing the cylinder load: let Ca = 3.2 and then multiply the resulting
force calculated from équation (2.38) by the force-impact magnification factor