Offshore Structure Platform Design - Offshore Structures: Design, Construction and Maintenance - page 110

2. Apply the loads on the structure jacket with different load cases, but the
software cannot be run because there are no supports applied to the struc-
ture system
3. Order the nodes that reflect the degree of freedom in K, F so that the nodes
p that will connect the piles are together at the end of the stiffness matrix
and follow the nodes j that are slowly connected to jacket members. The
stiffness matrix and force vector can be:

2 3 8

<

9
=

8
<

9
=

j 1 j 2 p 1 p 2

4 5

δ j F j

= (3.1)

: ; : ;

k jj k jp

δ p

k pj k pp F p

4. In order to analyze foundation behavior, the stiffness of the jacket and load-
ing on the jacket as felt by the piles are required. Detailed behavior of the
jacket is not required at this stage, as an equation of form K s δ p = F s is
wanted, where K s and F s represent the stiffness and applied forces on the
structure as seen at the pile connecting nodes p.
5. Form the model of the foundation by developing the stiffness matrix for the
foundation at zero deflection.
6. Assume no load is applied directly to the piles and the only load is applied
through the structure. The foundation forcing vector contains the element
from the forces on the structure but located in the appropriate positions
for the same degree of freedom in the foundation stiffness matrix.
7. Add the foundation and the jacket substructure and solve, then recalculate
the pile foundation stiffness and displacement, as the first nodal displace-
ment along the pile and at connection to the jacket is only a first estimate
because it has been based on step 5, in which the stiffness of p-y and t-z
curves is at zero deflection. Note that once the pile deflections have been
estimated, a better estimate of p-y and t-z stiffness can be made. The model
of the foundation is shown in Figure 3.14 . The stiffness may be represented
by either a secant or tangent stiffness. Note that the secant stiffness method
is generally slower but more stable than the tangent stiffness approach.
8. Repeat the sequence from step 5 until the stiffnesses have converged; then
the nodal deflections can be used to determine the forces, shears and
moment in the piles.
9. The deflections at the link to the jacket are also known now. They can be
applied as prescribed deflection to the pile nodes on the original jacket
model for step 2.
10. Given the jacket deflections, the jacket member forces are calculated from
separate member stiffness properties.
Figure 3.15 illustrates the global structure analysis procedure in a flowchart.