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BEHAVIOR OF PILES SUPPORTING OFFSHORE STRUCTURES
material characteristics of the piles and geometry and material characteristics
of the layers of soil. For the soil, the shear-wave velocity, Poisson’s ratio, mass
density, and damping ratio are required. In addition, for pile groups the mass
of the pile cap, a damping ratio, and a latéral stiffness can be input. With this
code, the engineer can obtain a family of nonlinear curves showing pile stiffness
as a function of frequency. For a single pile or a pile group, the curves will show
the horizontal stiffness the vertical stiffness. For the pile group, the curves will
also présent the rocking stiffness in the x and y directions. With such sets of
curves at hand, the engineer can proceed to analyze the dynamic response of
the superstructure.
PROBLEMS
11.1 Compute a point on the curve of axial load versus settlement for the
example in the text using a base settlement of 0.02 in. (instead of 0.05 in.).
Compare the resuit with the computer solution in Figure 11.3.
11.2 Refer to the American Petroleum Institute’s (1993) Recommended
Practice and compute the allowable load that can be sustained by the axially
loaded pile in the example.
11.3 Given a normally Consolidated clay with an undrained shear strength
that increased from zéro at the mudline to a value of 1250 psf at a depth of
100 ft, an average submerged unit weight of 45 pcf, and an £50 of 0.02, make
necessary computations and plot the p-y curves for both static and cyclic loading
for depths of 0 ft, 6 ft, and 12 ft for a pile with a diameter of 24 in. and a wall
thickness of 1.0 in. The pile is assumed to be at an offshore location; select an
appropriate value of the submerged unit weight in your computations.
11.4 Assume a fixed-head, open, steel-pipe pile with a diameter of 30 in.
and a wall thickness of 0.75 in. and assume further that the pile behaves as a
long pile. Compute the magnitude of the latéral load at the mudline to yield
a maximum bending moment of 20 ksi if Epy = kpyx , where kpy = 12 lb/in.3.
Assume no axial load and a constant El with depth. How long should the pile
be? Use the nondimensional method.
11.5 Repeat Problem 11.4 with the assumption that the head of the pile
is free to rotate.
11.6 For Problems 11.4 and 11.5, find the rotational restraint such that
the maximum négative moment at the pile head and the maximum positive
moment are equal. Use the latéral load found in Problem 11.5.
11.7 Use data from Problem 11.3 and assume a latéral load is applied at
10 ft above the mudline. Solve for the energy from a docking boat that can be
sustained by the pile if the maximum allowable pile bending stress is 30 ksi. Use
the nondimensional method of analysis and assume the number of répétitions
is small so that the p-y curves for static loading are appropriate. (Hints: The
latéral load to cause the maximum allowable bending stress is about 40 kips.
Because several computations are required, the work may be divided among
several students.)
BEHAVIOR OF PILES SUPPORTING OFFSHORE STRUCTURES
material characteristics of the piles and geometry and material characteristics
of the layers of soil. For the soil, the shear-wave velocity, Poisson’s ratio, mass
density, and damping ratio are required. In addition, for pile groups the mass
of the pile cap, a damping ratio, and a latéral stiffness can be input. With this
code, the engineer can obtain a family of nonlinear curves showing pile stiffness
as a function of frequency. For a single pile or a pile group, the curves will show
the horizontal stiffness the vertical stiffness. For the pile group, the curves will
also présent the rocking stiffness in the x and y directions. With such sets of
curves at hand, the engineer can proceed to analyze the dynamic response of
the superstructure.
PROBLEMS
11.1 Compute a point on the curve of axial load versus settlement for the
example in the text using a base settlement of 0.02 in. (instead of 0.05 in.).
Compare the resuit with the computer solution in Figure 11.3.
11.2 Refer to the American Petroleum Institute’s (1993) Recommended
Practice and compute the allowable load that can be sustained by the axially
loaded pile in the example.
11.3 Given a normally Consolidated clay with an undrained shear strength
that increased from zéro at the mudline to a value of 1250 psf at a depth of
100 ft, an average submerged unit weight of 45 pcf, and an £50 of 0.02, make
necessary computations and plot the p-y curves for both static and cyclic loading
for depths of 0 ft, 6 ft, and 12 ft for a pile with a diameter of 24 in. and a wall
thickness of 1.0 in. The pile is assumed to be at an offshore location; select an
appropriate value of the submerged unit weight in your computations.
11.4 Assume a fixed-head, open, steel-pipe pile with a diameter of 30 in.
and a wall thickness of 0.75 in. and assume further that the pile behaves as a
long pile. Compute the magnitude of the latéral load at the mudline to yield
a maximum bending moment of 20 ksi if Epy = kpyx , where kpy = 12 lb/in.3.
Assume no axial load and a constant El with depth. How long should the pile
be? Use the nondimensional method.
11.5 Repeat Problem 11.4 with the assumption that the head of the pile
is free to rotate.
11.6 For Problems 11.4 and 11.5, find the rotational restraint such that
the maximum négative moment at the pile head and the maximum positive
moment are equal. Use the latéral load found in Problem 11.5.
11.7 Use data from Problem 11.3 and assume a latéral load is applied at
10 ft above the mudline. Solve for the energy from a docking boat that can be
sustained by the pile if the maximum allowable pile bending stress is 30 ksi. Use
the nondimensional method of analysis and assume the number of répétitions
is small so that the p-y curves for static loading are appropriate. (Hints: The
latéral load to cause the maximum allowable bending stress is about 40 kips.
Because several computations are required, the work may be divided among
several students.)