INTRODUCTION 1 Classification of Soils

The stratigraphy of the sea bed results from a complex geological process during which various materials were deposited, remoulded and pressed together.

Soil texture consists of small mineral or organic particles basically characterized by their grain size and mutual interaction (friction, cohesion).

The properties of a specific soil depend mainly on the following factors:

• density.

• water content.

• over consolidation ratio.

For design purposes the influence of these factors on soil behaviour is expressed in terms of two fundamental parameters:

Granular soils are non-plastic soils with negligible cohesion between particles. They include:

• sands : characterized by large to medium particle sizes (1mm to 0,05mm) offering a high permeability,

• silts : characterized by particle sizes between 0,05 and 0,02mm; they are generally over-consolidated; they may exhibit some cohesion.

1.3 Cohesive Soils

Clays are plastic soils with particle sizes less than 0,002mm which tend to stick together; their permeability is low.

1.4 Multi-Layered Strata

The nature and characteristics of the soil surrounding a pile generally vary with the depth. For analysis purposes, the soil is divided into several layers, each having constant properties throughout. The number of layers depends on the precision required of the analysis.

2. DESIGN

Steel offshore platforms are usually founded on piles, driven deep into the soil (Figure 1). The piles have to transfer the loads acting on the jacket into the sea bed. In this section theoretical aspects of the design of piles are presented. Checking of the pile itself is described in detail in the Worked Example.

2.1 Design Loads

These loads are those transferred from the jacket to the foundation. They are calculated at the mudline.

2.1.1 Gravity loads

Gravity loads (platform dead load and live loads) are distributed as axial compression forces on the piles depending upon their respective eccentricity.

2.1.2 Environmental loads

Environmental loads due to waves, current, wind, earthquake, etc. are basically horizontal.

Their resultant at mudline consists of:

• shear distributed as horizontal forces on the piles.

• overturning moment on the jacket, equilibrated by axial tension/ compression in symmetrically disposed piles (upstream/downstream).

2.1.3 Load combinations

The basic gravity and environmental loads multiplied by relevant load factors are combined in order to produce the most severe effect(s) at mudline, resulting in:

• vertical compression or pullout force, and

• lateral shear force plus bending.

2.2 Static Axial Pile Resistance

The overall resistance of the pile against axial force is the sum of shaft friction and end bearing.

2.2.1 Lateral friction along the shaft (shaft friction)

Skin friction is mobilized along the shaft of the tubular pile (and possibly also along the inner wall when the soil plug is not removed).

The unit shaft friction:

• for sands: is proportional to the overburden pressure,

• for clays: is calculated by the "alpha" or "lambda" method and is a constant equal to the shear strength Cu at great depth.

Lateral friction is integrated along the whole penetration of the pile.

2.2.2 End bearing

End bearing is the resultant of bearing pressure over the gross end area of the pile, i.e. with or without the area of plug if relevant.

The bearing pressure:

• for clays: is equal to 9 Cu.

• for sands: is proportional to the overburden pressure as explained in Section 6.4.2 of API-RP2A [1].

2.2.3 Pile penetration

The pile penetration shall be sufficient to generate enough friction and bearing resistance against the maximum design compression multiplied by the appropriate factor of safety. No bearing resistance can be mobilized against pull-out: the friction available must be equated to the pull out force multiplied by the appropriate factor of safety.

2.3 Lateral Pile Resistance

The shear at the mudline caused by environmental loads is resisted by lateral bearing of the pile on the soil. This action may generate large deformations and high bending moments in the part of the pile directly below the mudline, particularly in soft soils.

2.3.1 P-y curves

P-y curves represent the lateral soil resistance versus deflection. The shape of these curves varies with the depth and the type of soil at the considered elevation. The general shape of the curves for increasing displacement features:

• elastic (linear) behaviour for small deflections,

• elastic/plastic behaviour for medium deflections,

• constant resistance for large deflections or loss of resistance when the soil skeleton deteriorates (clay under cyclic load in particular).

2.3.2 Lateral pile analysis

For analysis purposes, the soil is modelled as lumped non-linear springs distributed along the pile. The fourth order differential equation which expresses the pile deformation is integrated by successive iterations, the secant stiffness of the soil springs being updated at each step.

For large deformations, the second order contribution of the axial compression to the bending moment (P-Delta effect) shall be taken into account.

2.4 Pile Driving

Piles installed by driving are forced into the soil by a ram hitting the top. The impact is transmitted along the pile in the form of a wave, which reflects on the pile tip. The energy is progressively lost by plastic friction on the sides and bearing at the tip of the pile.

2.4.1 Empirical formulae

A considerable number of empirical formulae exist to predict pile driveability. Each formula is generally limited to a particular type of soil and hammer.

2.4.2 Wave equation

This method of analysing the driving process consists of representing the ensemble of pile/soil/hammer as a one-dimensional assembly of masses, springs and dashpots:

• the pile is modelled as a discrete assembly of masses and elastic springs.

• the soil is idealized as a massless medium characterized by elastic-perfectly-plastic springs and linear dashpots.

• the hammer is modelled as a mass falling with an initial velocity.

• the cushion is represented by a weightless spring (see Figure 3).

• the pile cap is represented by a mass of infinite rigidity.

The energy of the ram hitting the top of the pile generates a stress wave in the pile, which dissipates progressively by friction between the pile and the soil and by reflection at the extremities of the pile.

The plastic displacement of the tip relative to the soil is the set achieved by the blow. Curves can be drawn to represent the number of blows per unit length required to drive the pile at different penetrations.

The wave equation, though representing the most rigorous assessment to date of the driving process, still suffers a lack of accuracy, mostly caused by the inaccuracies in the soil model.