Reliability and Integrity Engineering

In push over analysis, soil is modeled using soil disc. Something likes this:

The disc diameter represents the relative strength of the soil layers, weighting the P-Y capacity to a factor of 1.0 and T-Z capacity to a factor of 100.00. In layman term, the bigger the soil disc the higher your soil capacity is. The maximum soil disc diameter is 10 times the maximum pile diameter and the minimum soil disc diameter is 2 times the maximum pile diameter. The relative lateral movement as well as punch through or pull out mechanism and rate of utilization is visualized in the model.

Layers of soil need to be modeled in the structural foundation. Like any other analysis, the higher the samples the better the results become. In our case, putting a small segment of soil layers will definitely give better results and accuracy but at expense of running time. In my practice, I always put very fine layers from 0 to 20 meter beneath mudline and then the layers become more separated towards the soil penetration depth.  Now there is one good reason using this technique. Most of soil failure happened at the point of contra flexure whereby the moment started to change from positive to negative, and this point usually located between 0 to 20 meter layers. Another reason is because we always have weak topsoil condition. As we go deeper, generally the shear strength and end bearing will increased tremendously compare to the first 20 m layers. Negative sign the depth shows the layers are actually beneath the mudline:

 Examples of how P-Y, T-Z and Q-Z curve look like are:

P-Y curve

Q-Z curve

T-Z curve

Usually for fixed offshore structure with depth from 10 m to 200 m two common types of piles are used. There are main and skirt pile.  Main pile is the pile that runs from pile-leg connection point just below the topside down to targeted penetration depth.  Yes, you are right! The one that inside the jacket leg! Main pile will resist and transfer both axial and lateral loading from environmental and topside load. Skirt pile is installed at the bottom bay of the structure. Not so much for transmitting axial loading, it main objective is to counter excessive lateral loading from environmental condition. Skirt pile has shorter targeted penetration compared to main pile but in most of the time it has larger diameter and thickness. The difference between skirt and main pile can is illustrated below: