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ESDEP WG 15A

STRUCTURAL SYSTEMS: OFFSHORE

Lecture 15A.6: Foundations

OBJECTIVE\SCOPE

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:

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

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

1.2 Granular Soils

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

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:

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:

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:

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:

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:

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 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:

 

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:

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:

× 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:

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:

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:

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:

4.4.2 Pile connections

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

- 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.

- breech block (twisting method).

- lug type (hydraulic method).

4.4.3 Hammer placement

Figure 5 shows the different steps of this routine operation:

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:

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:

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:

The installation report shall mention:

- 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.

- 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:

5. CONCLUDING SUMMARY

This lecture has described:

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).

A. Air/Steam Hammers

Make

Model

Rated

Energy

(ft-lbs)

Ram

Weight

(kips)

Max.

Stroke

(m)

Std. Pilecap

Weight

(kips)

Typical

Hammer Weight

(w/leads) (kips)

Rated Operating

Pressure

(psi)

Steam

Consumption

(lbs ht)

Air

Consumption

(lbs ht)

Hose

ST/F

.....

Rated

BPM

Conmaco

6850

5650

5300

300

200

510.000

325.000

150.000

90.000

60.000

85

65

30

30

20

72

60

60

36

36

57,5

59,0

12,7

12,7

12,7

312

262

92

86

74

180

160

160

150

120

31.500

 

8.064

6.944

5.563

7.500

 

1.711

1.471

1.195

2 @ 4

3 @ 4

4

3

3

40

45

46

54

59

Menck

(MRBS)

12500

8800

8000

7000

5000

4600

3000

1800

850

1.582.220

954.750

867.960

632.885

542.470

499.070

325.480

189.850

93.340

275,58

194,01

176,37

154

110,23

101,41

66,14

38,58

18,96

69

59

59

49

59

59

59

59

50

154,32

103,62

85,98

92,4

66,14

52,91

33,07

22,05

11,5

853

600

564

583

335

313

205

125

64

171

150

142

156

150

142

142

142

142

53.910

32.400

30.860

30.800

20.940

19.840

12.130

7.060

3.530

26.500

16.700

15.900

14.830

10.400

9.900

6.000

3.700

1.950

2 @ 6

8

8

4 @ 4

6

6

5

4

3

36

36

38

35

40

42

42

44

45

MKT

OS-60

OS-40

OS-20

18.000

120.000

60.000

60

40

20

36

36

36

 

 

 

38,65

 

 

150

   

 

 

3

 

 

60

C. Hydraulic Hammers

Make

Model

Rated Energy

 

(ft-lb)

Ram Weight

 

(kips)

Standard

Pilecap Weight

(kips)

Hammer Weight

 

(kips)

Typical Operating

Pressure

(psi)

Rated

Oil Flow

(gal. min)

Rated

BPM

HMB

4000

3000A

3000

1500

900

500

1.200.000

800.000

725.000

290.000

170.000

72.000

205

152

139

55

30,8

9,5

 

 

33

17,6

 

1,1

490

414

 

172

88

27,5

   

40-70

Menck

MRBU

MHU 1700

MHU 900

MH 195

MH 165

MH 145

MH 120

MH 96

MH 80

760.000

1.230.000

650.000

141.000

119.000

105.000

87.000

69.000

58.000

132

207

110

22,0

19,0

16,5

13,9

11,0

9,3

84

77

 

6,0

6,0

6,0

6,0

1,9

1,9

415

617

386

59

51

46

40

27

24

3400

3400

3100

3550

3190

2755

2320

2830

2465

845

845

580

98

103

102

103

75

75

50-80

32-65

48-65

38

42

42

44

48

48

TABLE 1 Properties of some hammers used offshore

Hammer

 

Type

 

Blows per Minute

 

Weight including Offshore Cage, if any (metric tons)

Rated Striking Energy

Expected Net Energy (ft-lb x 1000)

(ft-lb x 1000)

KNm

On Anvil

On Pile

Vulcan 3250

Single-acting steam

60

300

750

1040

673

600

HBM 3000

Hydraulic underwater

50-60

175

1034

1430

542

542

HBM 3000 A

Hydraulic underwater

40-70

190

1100

1520

796

796

HBM 3000 P

Slender hydraulic underwater

40-70

170

1120

1550

800

800

Menck MHU 900

Slender hydraulic underwater

48-65

135

-

-

651

618

Menck MRBS 8000

Single-acting steam

38

280

868

1200

715

629

Vulcan 4250

Single-acting steam

53

337

1000

1380

901

800

HBM 4000

Hydraulic underwater

40-70

222

1700

2350

1157

1157

Vulcan 6300

Single-acting steam

37

380

1800

2490

1697

1440

Menck MRBS 12500

Single-acting steam

38

385

1582

2190

1384

1147

Menck MHU 1700

Slender hydraulic underwater

32-65

235

-

-

1230

1169

IHC S-300

Slender hydraulic underwater

40

30

220

300

-

-

IHC S-800

Slender hydraulic underwater

40

80

580

800

-

-

IHC S-1600

Slender hydraulic underwater

30

160

1160

1600

-

-

IHC S-2000

Slender hydraulic underwater

-

260

1449

2000

-

-

IHC S-2300

Slender hydraulic underwater

-

-

1566

2300

-

-

TABLE 2 Large pile driving hammers

Pile Outer Diameter

Wall Thickness

Hammer Energy

(in.)

(mm)

(in.)

(mm)

(ft-lb)

(kN-m)

24

30

36

42

48

60

72

84

96

108

120

600

750

900

1.050

1.200

1.500

1.800

2.100

2.400

2.700

3.000

5/8 - 7/8

¾

7/8 - 1

1 - 1¼

17- 1¾

17 - 1¾

1¼ - 2

1¼ - 2

1¼ - 2

1½ - 2½

1½ - 2½

15-21

19

21-25

25-32

28-44

28-44

32-50

32-50

32-50

37-62

37-62

50.000 - 120.000

50.000 - 120.000

50.000 - 180.000

60.000 - 300.000

90.000 - 500.000

90.000 - 500.000

120.000 - 700.000

180.000 - 1.000.000

180.000 - 1.000.000

300.000 - 1.000.000

300.000 - 1.000.000

70 - 168

70 - 168

70 - 252

84 - 120

126 - 700

126 - 700

168 - 980

252 - 1.400

252 - 1.400

420 - 1.400

420 - 1.400

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.

TABLE 3 Typical values of pile sizes, wall thickness and hammer energies

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