Foundation Simulation

Pile Modelling
 

In an offshore structure, the piles hold them on to the sea bed. This needs to be simulated
in the structural analysis involving their inplace strength and stability. There are type of
pile system that can be used in the o®shore structures.

1.  Main Pile
2.  Skirt Pile

As it can be seen from the ¯gure that the skirt pile is always grouted with the skirt sleeve
of the jacket. But in the case of main pile, the annulus between the pile and the jacket leg
may be grouted or not grouted depending on the design water depth. Like other structural
elements of the jacket structure, pile is also a structural member and shall be modelled
according to the diameter, wall thickness and material properties. It is the load transfer
mechanism between the jacket leg and pile that requires special care in simulation of actual
load transfer.

For the case of grouted skirt piles and main piles, the model becomes much easier by simply
specifying the cross section as a "Composite Section" containing jacket leg, pile and the
annulus ¯lled with cement grout. The equivalent axial area, shear area and bending sti®ness
can be calculated using the equivalent section concept and used in the analysis.
But for the case of main pile, this cannot be done. The pile and jacket are two parallel
members physically connected at the top of jacket by means welded connections and else
where no welding but spacers are placed inside the jacket leg to provide contact points for
load transfer. These spacers are specially located at the horizontal framing such that the
lateral loads from the wave and current can be easily transferred to the piles.

Soil Simulation

Piles below seabed shall be modelled in the structural analysis to reffect the vertical and
lateral behaviour of pile soil system. This is very essential to simulate the jacket and deck
de°ections and pile stresses. This can be done in three ways.

1.  Equivalent Pile Stub -
2.  P-Y, T-Z and Q-Z Curves -
3.  Linearised Pile Stiffness Matrix -

Pile Group EFfect
The skirt piles for very large jackets normally arranged in cluster at each corner to resist the
forces from gravity and environmental loads.
These pile clusters can be arranged in various ways but due to construction limitations
usually they will arranged in closed manner as shown in the Figures 5.10 and ??. The
distance between the jacket leg and the farthest pile shall be kept to a minimum possible for
fabrication to avoid unnecessary bending on jacket legs as well on the pile sleeves.
It is a good practice to space the centre to centre of adjacent piles at a distance of 3D
where D is the diameter of the pile. This will prove a clear distance between the pile face of
2D.Even with this separation, the effect of load on one pile will affect the behaviour of the

adjacent pile. The issues to be looked into are two categories as listed below.

•  EFect Axial Capacity
•  EFect on P-Y, T-Z and Q-Z behaviour