AU623085B2 - Large diameter closed-end pipe piles for offshore platforms - Google Patents

Description

I CI---cp~ COMMONWEALTH OF AUSTRALIA PATENTS AC, 1952 COMPLETE SPECIFICATION FOR OFFICE USE Form Short Title: Int. Cl: 0 S900 ot S S. S S Application Number: Lodged: Complete Specification-Lodged: Accepted: Lapsed: Published: Priority: Related Art: 6 9

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S TO BE COMPLETED BY APPLICANT I Ir *t 0 Name of Applicant: Address of Applicant: Actual Inventor: Address for Service: PETROLEO BRASILEIRO S.A. PETROBRAS Avenida Republica do Chile, 65 Rio de Janeiro, BRAZIL Alvaro Maia Da Costa; Lino Francisco Rodrigues Moreira; Cipriano Jose De Medeiros Junior and Luiz Hissashi Hassui GRIFFITH HACK CO.

71 YORK STREET SYDNEY NSW 2000

AUSTRALIA

Complete Specification for the invention entitled: LARGE DIAMETER CLOSED-END PIPE PILES FOR OFFSHORE PLATFORMS The following statement is a full description of this invention, including the best method of performing it known to me/us:- 8243A:rk IC-L-~L-CL3CIII _1 XIU 1A "LARGE DIAMETER CLOSED-END PIPE PILES FOR OFFSHORE PLATFORMS" Field of the Invention This invention concerns a pipe pile designed mainly for use with oil exploring and producing platforms, said platforms lying within very deep water.

Background of the Invention Most of the foundations for offshore oil exploring and producing platforms are made up of large diameter pipe piles driven into the ground. They may be as much as 2 meters (80 inches) in diameter and be driven in as far as 150 m.

The load bearing capacity of the piles is made up of two parts: one is the lateral friction between the pile and the soil and the other is the bearing of the tips.

This invention is intended to overcome the problem of i 4' the lack of load bearing strength of the piles that were known theretofore.

t Description of Prior State of the Art I, Closed end piles have long been known and have been used for different purposes, which includes small size I platforms on and offshore.

For a long time much has been said about pile-driving difficulties. Many suggestions have been put forward to overcome such difficulties. An example was Brazilian Patent'No. 63.600 for "pile driving tips", now in the public domain. The inventor proposed the use of tips provided with fins which would act like a milling tool and make it easier for the pile to be driven into rock.

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2 However for the purpose that this present invention is intended the suggestion referred to under Brazilian patent no.

63.600 is no good because the tip is not one piece with the pile shank. Furthermore, the stress developed particularly where the pile tip and shank meet would be so high that the pile shank would become damaged.

The first large diameter piles to be made were of concrete, some of them reinforced with steel at their closed end. However, such piles cannot be used where water is very deep nor in soils hard to pierce. Load stresses would be such as to seriously damage them when being driven.

Owing to the need to look for oil in ever deeper waters, the tendency throughout the world has become to use steel piles that are very long and openended. It was believed that open-ended pile shanks would not withstand being driven into the soil pile driving loads upon pile tips are somewhere in the region of 7,000 tons.

44,, ltt 4 1 One kind of large diameter pile is described under Brazilian application PI 8704565 of September 3, 1987, where the axial load bearing capacity of the pile is increased with the aid of a S cross-wall introduced into a foundation pile driven into the soil and provided inside with retaining rings meant to hold the S cross-wall when pile is being driven.

The closed end pile which is the subject of the present invention is a technological milestone as regards feasibility of driving such large size piles, which are able to withstand stress without becoming damaged.

Thus, the present state of the art is piles open all the way or piles open but provided with an inside cross-wall (figure 1).

In the case of such open-ended pipe piles the strength of the tip is a question of the thickness of the pile (figure or of the soil/pile cross-section when the strength between the sides of the soil and the pile at the inside face of the pile is strong 3 enough for the column of soil to remain steady when the pile is being driven (figure This second method depends on the properties of the soil, the dissipation of neutral pressure (pressure transferred to water) during driving, uniformity of soil and kind of soil/pile contact.

A disadvantage of this method is doubt as to the degree of compactness of the soil and during the useful life of the platform the inside plug of soil may become affected by flow or slump according to what work loads the pile has to bear. Therefore designs rely more on lateral strength, great lengths of piles being driven into the soil.

Pipe piles with an inside cross-wall are one way of attempting to ensure that the inside column of soil remains steady.

When the pile is being driven the soil rises within the pile until it gets to the cross-wall where it is stopped. As driving goes ahead the soil within is 2 compacted and becomes a plug, which makes the tip of the pile stronger (figure 3).

Disadvantages are the same as those referred to for a ,soil plug without a cross-wall, the most doubtful point being a lack of knowledge as to uniformity of the soil within the pile.

"2 Outline of the Invention According to the invention there is provided a method of designing a large diameter, closed-end pipe pile for offshore oil platforms, said pile having a tip which is 0 closed by means of a thick-walled, conical shell, said method' comprising the steps of: simulating the dynamic behaviour of an interaction between the pile and the soil using a threedimensional model which is symmetrical about the pile axis; '009 S:14823J I _I a -DI~ -4a examining a history of transient stresses i occurring along said pile, and isolating critical stress ii regions for determining the size of said pile; and i determining the pile size and the size and shape of the conical tip by means of a non-linear static analysis of different outline configurations, using the history of Stransient stresses isolated under step (b) According to a further aspect of the present invention there is provided a large diameter, closed-end pipe pile for offshore oil platforms located in deep water regions, and able to bear an impact of at least 7000 tons during driving thereof, the pile comprising a pile body in the form of a large diameter, substantially cylindrical pile; a pile tip for closing off an end of said pile body which is driven into the soil, the pile tip including a thickwalled, conical shell which is symmetrical about its axis, Sand joining to said pile body so as to form a continuous union having a smooth outer surface; wherein the pile is designed according to the above method.

The pile tip is closed by means of a thick-wall, conical shell which is symmetrical about its axis. Closing V the tip of the pile makes such tip stronger and it is so designed as to enable it to pierce the soil without being damaged.

S2, cs i, Brief Description of the Drawings "."Figure 1 shows open-end piles used in the old method; Figures 2 and 3 are cross-section view of an open end SrI* pile showing points where resistance builds up; SFigure 4 is a sketch of the closed-end pile according to this invention; S:148231 Figures 7 and 8 are cut-away section views of Figures and 6 respectively.

Of course the figures shown are just meant to illustrate and therefore are not to be regarded as in any way limiting this invention.

In this way Figures 5 and 6 are shown as two of such closed end piles designed for the Northeast Hub of the Production Field of the Campos Basin, lying offshore of the State of Rio de Janiero coast in Brazil. Figures 7 and 8 are cut-away views of such piles.

Detailed Description of the Invention As referred to above the invention is of a large diameter pile with its tip closed and designed mainly for use in very deep waters.

The leading feature of this pile is that its tip is closed off by a conical, thick-wall shell which is symmetrical about its axis. Such kind of shell enables the pile to be driven into the soil without becoming damaged.

.'2C1. The thickness and conical shape of the pile tip arise 1 out of a logical series of studies and evaluations, stages of which are briefly as follows: Pile-driving check; Review of stress and strain during driving; S* Numerical Modelling of the Tip; Size of the Tip Structure; and S 0 Check on part of soil plasticised by the piledriving so as to gauge the effect of one pile upon another during driving.

:3d In more detail the process involves: simulate soil/pile interacting dynamic behaviour, in terms of a three dimensional model symmetrical about its axis, and by the finite element method; S:14823J i 3 SI-i .i 6 examine the record or history of transient stresses along the pile, separating critical stages for the purpose of the size of the structure; work out the size of pile and its conical tip in terms of non-linear analysis, for different kinds of outline, with aid of the record of transient stress separated out under the dynamic analysis referred to above; and check plasticised bulb caused by driving a closed end pile, and its effect upon the behaviour of other piles driven.

Closed-end, large diameter pipe piles need threedimensional simulation to suitably represent soil/pile interaction behaviour. Because of the dimensions of the pile, field of displacement and spread of stress at soil/pile contact, it would be difficult to design it along the usual uniaxial model lines. Any record of stress and strain in terms of time would be considerably affected by any topological change in the actual structural model.

Applicant has a program (software) specially worked out for such purpose.

S* Using this basic analysis approach, the finite element method was employed with discreteness of the continuum i (soil and pile), by isoparametric elements with a variable number of nodal points (4 to 8 knots), with the aid of the formula for symmetrical axis solids. Since, during i 2 driving, there is impact stress, representing a shortlasting transient, the explicit central difference method is used to integrate time in the dynamic equilibrium equations. Also physical behaviour of pile and soil matter may be'non-linear. For piles von Mises plastic flow rule is followed and for the soil the Mohr-Coulomb flow rule.

7 Soil/pile interacting behaviour is arrived at by use of interface type special finite elements, which allow for plastic tangential sliding and plastic behaviour (non-linear) in a normal direction. Such elements also simulate the existence of a gap between the soil and the pile, that is, the unsticking of soil from the pile.

Dissipation of dynamic energy through the continuum is arrived at through the use of special finite elements in the outline, known as "infinites". Such elements extend geometrically towards infinity, with a drop in displacements suitable for the phenomenon under study and a mass matrix suitable for the spread of impact waves.

For dynamic excitation the force versus time curve is used, measurement taking place by means of power transducers fitted to the top of piles that have already been put in and are being monitored. Several simulations were carried out, changes being made in the diameter of the piles and in the soil properties, particularly where connected with soil/pile contact. The outcome of such simulations was a record in terms of time, of stresses, strains, displacements, speeds and accelerations, along the pile, and soil/pile contact.

Survey of such results serves to show that piles of about 1.40 to 2.00 meters (56 to 80 inches) in diameter can be driven and reveals plasticized points in the soil during pile-driving.

Points plasticized during pile-driving is a limiting factor in the use of such piles since the bulb plasticized by one pile must lie outside the pile-driving radius of other piles, if design covers use of a group of piles. In the case of the northeast hub such bulb did not become a limiting factor as 't regards use of the method.

Examination of the record of stresses along the pile, particularly of the axial component, enables structural dimensions to be arrived at. In the case in point of conical tips, sizing thereof calls for a discrete model with a higher degree of refinement than that used to simulate dynamic behaviour. With the aid of such finite element model along with considerably better refinement, an elasto-plastic static analysis was undertaken, loads having been the maximum dynamic stresses drawn from the simulations.

The geometric model and the grid of finite elements of the pile and its tip were done with the aid for a specific program FEMS (Finite Element Modeling System) on an Intergraph CAD System. Features were made to apply, such as: use of six and eight nodal point isoparametric element's symmetrical about the same axis; keeping to a length/width ratio for each element within suitable ranges; densification of grid at places of concentrated stresses; automatic generation of knots and elements starting at arbitrary borders; and exact agreement with the physical model of the pile.

The pile tip itself is made basically of steel like that of the pile itself, as regards composition and physical strength. It may be made in two ways: .a single all cast part.

.made of plates rolled into the shape of a cone, plus a cast tip.

Such part is basically cone-shaped, its height and angle of attack varies according to soil features and the surface finish of the con'e is smooth for which it may be ground or blasted. The outside shape must be as smooth as possible at any changes, the end of the tip to be rounded and have no edges where it joins the pile body.

I The cone may be heat treated so as to raise its soil penetrating performance. The cone is of uniform thickness but it may be provided with inside stiffeners in the shape of fins and rings. The thickness of the cones varies in terms of the following factors: strength of soil, diameter of pile, height of cone, capacity of pile driving hammer.

ABSTRACT

Invention Patent: "LARGE DIAMETER CLOSED-END PIPE PILES FOR OFFSHORE PLATFORMS" A pile for use on oil exploring and producing platforms, such platforms lying in very deep water areas. The end of the pile is closed by means of a conical shaped, thick-walled shell which is symmetrical about its axis. Such a design enables the pile to be driven into the soil without being damaged.

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Claims (8)

1. A method of designing a large diameter, closed- end pipe pile for offshore oil platforms, said pile 'a.-ing a tip which is closed by means of a thick-walled, coni,-, shell, said method comprising the steps of: simulating the dynamic behaviour of an interaction between the pile and the soil using a three- dimensional model which is symmetrical about the pile axis; examining a history of transient stresses occurring along said pile, and isolating critical stress regions for determining the size of said pile; and determining the pile size and the size and shape of the conical tip by means of a non-linear static analysis of different outline configurations, using the history of transient stresses isolated under step

2. A method according to claim 1 including the further step of carrying out a simulation to determine the dimensions of the plasticised bulb formed in the soil by driving the closed-end pile and in turn compacting the soil, and determining the effect of said bulb upon any adjacent piles subsequently driven.

3. A method according to claim 1, wherein the step of determining the size and shape of the conical tip includes varying the angle of attack, as well as the height, of the conical tip depending on the soil characteristics. i S:14823J -U 1C- i I 11

4. A method according to claim 1, wherein the step of determining the size and shape of the conical tip includes determining a thickness of said conical tip as a function of soil strength, pile diameter, height of the conical tip and pile driving hammer capacity.

A large diameter,, closed-end pipe pile for offshore oil platforms located in deep water regions, and able to bear an impact of at least 7000 tons during driving thereof, the pile comprising a pile body in the form of a large diameter, substantially cylindrical pile; a pile tip for closing off an end of said pile body which is driven into the soil, the pile tip including a thick-walled, conical shell which is symmetrical about its axis, and joining to said pile body so as to form a continuous union having a smooth outer surface; wherein the pile is designed according to any preceding claim.

6. A large diameter, closed-end pipe pile according to claim 5, wherein the conical shell is formed by a plurality of steel plates rolled into a conical shape and a cast steel tip.

7. A large diameter, closed-end pipe pile according to claim 5, wherein the pile body has a diameter in the range of 1.40 m to 2.00 m.

8. A large diameter, closed-end pipe pile designed according to any one of claims 1 to 4 and substantially as described with reference to figures 4 to 8. DATED this 18th day of February 1992 PETROLEO BRASILIERO S.A. PETROBRAS By their Patent Attorneys i GRIFFITH HACK CO. Y.