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Lecture 15A.6: Foundations

OBJECTIVE\SCOPE

• to classify different types of piles
• to understand main design methods
• to cover various methods of installation

PREREQUISITES

Lecture 1B.2.2: Limit State Design Philosophy and Partial Safety Factors

Lectures 10.6: Shear Connection

Lectures 12.4: Fatigue Behaviour of Hollow Section Joints

Lecture 15A.12: Connections in Offshore Deck Structures

Lecture 17.5: Requirements and Verifications of Seismic Resistant Structures

A general knowledge of design in offshore structures and an understanding of offshore installation are also required.

SUMMARY

In this lecture piled foundations for offshore structures are presented. The lecture starts with the classification of soil. The main steps in the design of piles are then explained. The different kinds of piles and hammers are described. The three main execution phases are briefly discussed: fabrication, transport and installation.

1. INTRODUCTION

1.1 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:

• friction angle.
• undrained shear strength C_u.

Since the least significant of either of these parameters is often neglected, soils can be classified within "ideal" categories:

• granular soils.
• cohesive soils.

1.2 Granular Soils

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 C_u 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 ´ C_u.
• 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.

3. DIFFERENT KINDS OF PILES

Driven piles are the most popular and cost-efficient type of foundation for offshore structures.

As shown in Figure 2, the following alternatives may be chosen when driving proves impractical:

• insert piles.
• drilled and grouted piles.
• belled piles.

3.1 Driven Piles

Piles are usually made up in segments. After placing and driving the first long segment, extension segments called add-ons are set on piece by piece as driving proceeds until the overall design length is achieved.

In recent years one-piece piles have been widely used in the North Sea since the offshore work is considerably reduced.

Wall thickness may vary. A thicker wall is sometimes required:

• in sections from mudline down to a specified depth within which bending stresses are especially high,
• at the pile tip (driving shoe) to resist local bearing stresses while driving.

Uniform wall thickness is however preferable thus avoiding construction and installation problems.

3.2 Insert Piles

Insert piles are smaller diameter piles driven through the main pile from which the soil plug has been previously drilled out. They are therefore not subjected to skin friction over the length of the main pile and can reach substantial additional penetration.

The insert pile is welded to the main pile at the top of the jacket and the annular space between the tubes is grouted.

This type of pile is used:

• in a preplanned situation: performance is good although material and installation costs are higher than for normal driven piles.
• as an emergency procedure: when scheduled piles cannot be driven to the required penetration, resulting therefore in one of the following drawbacks.

× a thicker wall section of the main pile will be within the jacket height instead of below the mudline.

× reduced friction area and end bearing pressure,

× difficulties often noted for the setting-in of all the required volume of grouting, i.e. the concern is the leakage of grout or the impossibility to fill with the calculated volume of grout.

3.3 Drilled and Grouted Piles

This procedure is the only means of installing piles with tension resistance in hard soils or soft rocks; it resembles that for drilling a conductor well.

An oversized hole is initially drilled to the proposed pile penetration depth. The pile is then lowered down, sometimes centred in the hole by spacers and the annular space between the pile shaft and the surrounding soil is grouted.

Design uncertainty results because:

• hard soil formation softens when exposed to the water or mud used during drilling and exhibits lower skin friction resistance.
• in case of calcareous sand, external grouting just crushes the sand, slightly extending the effective pile diameter but not increasing the friction significantly.

3.4 Belled Piles

While belled piles, on land, are used to decrease the bearing stress under a pile, offshore belled piles provide a large bearing area to increase tip uplift resistance.

The main pile, normally driven, serves here as a casing through which a rig drills a slightly oversized hole ahead. A belling tool (underreamer) then enlarges the socket to a conical bell with a base diameter a few times that of the main pile. A heavy reinforcement cage is lowered inside the bell which is subsequently filled with concrete made using fine aggregate (maximum size 10mm).

4. FABRICATION AND INSTALLATION

4.1 Fabrication

The piles are usually made up of "cans" - cylinders of rolled plate with a longitudinal seam. Single cans are typically 1,5m long or more. Longitudinal seams of two adjacent segments are rotated 90° apart at least.

Bevelling is mandatory should the wall thickness difference exceed 3mm between adjacent cans. Maximum deviation from straightness is specified (0.1% in length).

Commonly used steel grade is X52 or X60.

The outside surface of grouted piles should be free of mill scale and varnished.

In certain instances, steel piles are protected underwater by sacrificial anodes or by impressed current. In the splash zone additional thickness to allow for corrosion (3mm for example) and epoxy or rubberized coating, monel or copper-nickel sheeting are provided.

4.2 Transportation

4.2.1 Barge transportation

Pile segments are choked and fastened to the barge to prevent them from falling overboard under severe seastates. Pile plate should be thick enough to prevent any deformation caused by stacking.

4.2.2 Self floating mode

This method is attractive where long segments of pile are to be lifted and set in guides far below the sea surface (skirt piles for example).

The ends of the piles are sealed by steel closure plates or rubber diaphragms which should be able to resist wave slamming during the tow.

4.2.3 Transport within the jacket

The piles are pre-set inside the main legs or in the guides/sleeves, generating additional weight and possibly buoyancy (if closed). They are held in place by shims which prevent them from escaping from their guides during launch and uprighting of the jacket.

Several piles are driven immediately after the jacket has touched down, providing initial stability against the action of waves and current.

4.3 Hammers

Piles are positioned:

• either inside the jacket legs, extending the full height of the jacket,
• or encased in sleeves protruding at the bottom of the jacket, running vertical or parallel to the legs (typical batter 1/12 to 1/6).

Piles can then be driven using any type of hammer (or a combination of types). Hammers are illustrated in Figure 3.

4.3.1 Steam hammers

Steam hammers are widely used for offshore installation of jackets. They are generally single acting with rates of up to 40 blows/minute. Energies of current hammers range from 60 000 to 1 250 000 ft lb/blow. (82KNm to 1725KNm per blow).

During driving, the hammer with attached driving head rides the pile rather than being supported by leads. The hammer line from the crane boom is slackened so as to prevent transmission of impact and vibration into the boom.

4.3.2 Diesel hammers

Diesel hammers are much used at offshore terminals. They are lighter to handle and less energy consuming than steam hammers, but their effective energy is limited.

4.3.3 Hydraulic hammers

Hydraulic hammers are dedicated to underwater driving (skirt piles terminating far below the sea surface).

Menck hydraulic hammers are widely used. They utilize a solid steel ram and a flexible steel pile cap to limit impact forces. They are double acting. Hydraulic fluid under high pressure is used to force a piston or set of pistons, and in turn, the ram up and down.

Properties of some hammers used offshore are shown in Table 1. A selection of large offshore pile driving hammers driving on heavy piles is also shown in Table 2.

4.3.4 Selection of hammer size

Selection of hammer size is based on:

• experience of similar situations (see Quality Control: Section 4.6),
• numerical modelling of driving for each particular site (see Pile Driving: Section 2.4)

Typical values of pile sizes, wall thicknesses, and hammer energies for steam hammers are shown in Table 3.

4.4 Installation

4.4.1 Pile handling and positioning

Figure 4 shows the different ways of providing lifting points for positioning pile sections. Padeyes are generally used (welded in the fabrication yard; their design should take into account the changes in load direction during lifting). Padeyes are then carefully cut before lowering the next pile section.

Sketch E shows the different steps for the positioning of pile sections:

• pile or add-on lifted from the barge deck.
• rotation of the crane to position add-on.
• installing and lowering of the pile add-on.

4.4.2 Pile connections

Different solutions for connecting pile segments back-to-back are used:

• either by welding, Shielded Metal Arc Welding (SMAW) or flux-cored, segments held temporarily by internal or external stabbing guides as shown in Figure 4. Welding time depends upon:

- pile wall thickness: 3 hours for 1in. thick (25,4mm); 16 hours for 3in. thick, (76,2mm) (typical).

- number and qualification of the welders.

- environmental conditions.

• or by mechanical connectors (as shown in Figure 4):

- breech block (twisting method).

- lug type (hydraulic method).

4.4.3 Hammer placement

Figure 5 shows the different steps of this routine operation:

• lifting from the barge deck.
• positioning over pile by booming out or in (the bell of the hammer acts as a stabling guide... very helpful in rough weather).
• alignment of the pile cap.
• lowering leads after hammer positionment.

Each add-on should be designed to prevent bending or buckling failure during installation and in-place conditions.

4.4.4 Driving

Some penetration under the self weight of the pile is normal. For soft soil conditions, particular measures are taken to avoid an uncontrolled run.

Piles are then driven or drilled until pile refusal.

Pile refusal is defined as the minimum rate of penetration beyond which further advancement of the pile is no longer achievable because of the time required and the possible damage to the pile or to the hammer. A widely accepted rate for defining refusal is 300 blows/foot (980 blows/metre).

4.5 Pile-to-Jacket Connections

4.5.1 Welded shims

The shims are inserted at the top of the pile within the annulus between the pile and jacket leg (see Figure 6) and welded afterwards.

4.5.2 Mechanical locking system

This metal-to-metal connection is achieved by a hydraulic swaging tool lowered inside the pile and expanding it into machined grooves provided in the sleeves at two or three elevations as shown on Figure 7.

This type of connection is most popular for subsea templates. It offers immediate strength and the possibility to re-enter the connection should swaging prove incomplete.

4.5.3 Grouting

This hybrid connection is the most commonly used for connecting piles to the main structure (in the mudline area). Forces are transmitted by shear through the grout.

Figure 8 shows the two types of packers commonly used. The expansive, non-shrinking grout must fill completely the annulus between the pile and leg (or sleeve).

Bonding should be excellent; it is improved by shear connectors (shear keys, strips or weld beads disposed on the surface of the sleeve and pile in contact with the grout).

The width of the annulus between pile and sleeve should be maintained constant by use of centralizers and be limited to:

• 1,5in. minimum, (38,1mm)
• about 4in. (101,6mm) maximum (to avoid destruction of the tensile strength of the grout by internal microcracking).

Packers are used to confine the grout and prevent it from escaping at the base of the sleeve. Packers are often damaged during piling and are therefore:

• installed in a double set.
• attached to the base of the sleeve to protect them during pile entry and driving.

Thorough filling should be checked by suitable devices, e.g. electrical resistance gauges, radioactive tracers, well-logging devices or overflow pipes checked by divers.

4.6 Quality Control

Quality control shall:

• confirm the adequacy of the foundation with respect to the design.
• provide a record of pile installation for reference to subsequent driving of nearby piles and future modifications to the platform.

The installation report shall mention:

• pile identification (diameter and thickness).
• measured lengths of add-ons and cut-offs.
• self penetration of pile (under its own weight and under static weight of the hammer).
• blowcount throughout driving with identification of hammer used and energy, as shown in Figure 9.
• record of incidents and abnormalities:

- unexpected behaviour of the pile and/or hammer.

- interruptions of driving (with set-up time and blowcount subsequently required to break the pile loose).

- pile damage if any.

• elevations of soil plug and internal water surface after driving.
• information about the pile/structure connection:

- equipment and procedure employed.

- overall volume of grout and quality.

- record of interruptions and delays.

4.7 Contingency Plan

Contingency documents should provide back-up solutions in case "unforeseen" events occur such as:

• impossibility to get the required pile penetration.
• mechanical breakdown of the hammer.
• grout pipe blockage.

5. CONCLUDING SUMMARY

This lecture has described:

• the difficult aspects of foundations in a variety of soils.
• the multiplicity of solutions and the different kind of piles and hammers.
• the complexity of the process from design to installation.

6. REFERENCES

[1] API-RP2A, "Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms", American Petroleum Institute, Washington, D.C., 18th ed., 1989.

7. ADDITIONAL READING

1. McClelland, B. and Reifel, M. D., Planning and design of fixed offshore platforms, Von Mostrand Reinhold Company (1982).
2. Bowles, J. E., Foundation analysis and design, MacGraw Hill Book Company (4th edition 1988).
3. Bowles, J. E., Analytical and computer methods in Foundation Engineering, MacGraw Hill Book Company (1983).
4. Poulos, H. G. and Davis, E. H., Pile foundation analysis and design, John Wiley and Sons (1980).
5. Graff, W. J., Introduction to offshore structures, Gulf Publishing Company (1981).
6. Le Tirant, P., Reconnaissance des sols en mer pour l'implantation des ouvrages Pétroliens, Technip (1976)
7. Pieux dans les formatines carbonates - Technip ARGEMA (1988).
8. Capacité patante des pieux - Technip ARGEMA (1988).
9. Dawson, T. H., Offshore Structural Engineering, Prentice Hall Inc (1983).
10. Gerwick, Ben C., Construction of Offshore Structures, John Wiley and Sons (1986).

Note 1: With the heavier hammers in the range given, the wall thicknesses must be near the upper range of those listed in order to prevent overstress (yielding) in the pile under hard driving.

Note 2: With diesel hammers, the effective hammer energy is from one-half to two-thirds the values generally listed by the manufacturers and the above table must be adjusted accordingly. Diesel hammers would normally only be used on 36-in. or less diameter piles.

Note 3: Hydraulic hammers have a more sustained blow, and hence the above table can be modified to fit the stress wave pattern.