Performance of dynamically penetrating anchor (torpedo pile) by Physical Modelling

S.A. Brum Jr., F. Saboya Jr, S. Tibana, R.M. Reis, R.R. Sobrinho, J. Vieira & V. M.

Del’Aguila State University of Norte Fluminense Darcy Ribeiro - UENF, Department of Civil Engineering, Campos dos Goytacazes, Brazil

ABSTRACT: This paper presents the results of a research that have been carried out in the Centrifuge Laboratory of UENF, which deals with dynamic penetrating anchors (or as known “torpedo piles”) with reduced physical models. The experimental program consisted of the preparation of the reduced physical models, consolidated at 1g and centrifuge pullout tests at 50g. The initial effort was toward in the investigation of soil material to compose the physical models. Mixtures with different content of kaolin and meta-kaolin were investigated in order to reach a clayey mud with low plasticity. The mixture chosen was that of 5% of kaolin with 95% of meta-kaolin whose plastic index was 14.2%. As the second main objective, three different geometries of torpedo piles were testes in order to assess their performance as anchoring. The results of all tests are presented herein.

1 INTRODUCTION

Floating structure is the most common solution used

by offshore oil industries in Brazil, since when the

deep-water oil fields in Campos Basis (BRAZIL)

were discovered. Many solutions to anchoring these structures have

been investigated to guarantee the steady position of those structures. Suction cassion, VLA (Vertically Loaded Anchor), DPA (Deep Penetrating Anchor), Torpedo Pile and SEPLA (Suction Embedded Plate Anchor) are some examples of it.

The Offshore Technology Research Center (OTRC) in Austin (EUA) has been tested different types of anchors; suction cassion, VLA and Torpedo Piles (see, Aubeny & Murff, 2003; Audibert et al., 2006).

The Centre of Offshore Foundations System of UWA (COFS) has been involved in successfully investigation of performance of the DPA, drag anchors and also suction cassion (see, Randolph et al., 2005).

The high costs of all solutions has led the Brazilian Oil Company (PETROBRAS) to develop its own dynamically penetrating anchors called Torpedo Piles (see Medeiros Jr. et. al., 1996). It is launched without propulsion of about 200 meters from the seabed. Basically, this anchor must reach the seabed with the highest possible energy to penetrate into the soil as deeper as possible, and when tensioned, it has to work as an anchor, mobilizing also friction, passive and active thrust. The external geometry of this anchor must attend these different situations.

The torpedo piles has three main parts; tip, shaft and flukes that must be weight-balanced to keep the gravity center as closed as possible to the tip of the penetrometer and far from the center of pressure. This ideal configuration allows the torpedo to dive vertically into the seabed and in case of a lateral pressure to deviate it from its original path it can have it rectified by itself.

2 TORPEDO PILES ANCHORS

2.1 Hydrodynamic assessment of the Torpedo Pile

Previously, a hydrodynamic study in a small scale

laboratory tests (1:200 of the 15 m high anchor) was

carried out at UENF Laboratory to define which

would be the best geometry of the tip and the flukes

(see, Izola, 2007). Fifteen torpedoes were built with different

configurations varying the type and number of flukes and geometry of the tips. Three of them were built with the same geometry that those used by PETROBRAS.

In these preliminary tests the torpedoes were launched in a water-filled strong-box without propulsion, in two positions: vertical and horizontal. These situations have a different proposal: the vertical lunch is to verify the velocity and the trajectory of the rockets at the bottom of the strong-box, and the horizontal lunch is to verify if the rockets have the capacity to recover the vertical initial path.

As the aim of this study is to compare the performance of torpedo with different geometries, the torpedoes that showed the best results in this study, that one proposed by O’Loughlin et al. (2004), as well as that used by PETROBRAS were copied in a scale of 1:125.

In Figure 1 the torpedoes built in a scale of 1:125 are presented. The number one is a torpedo with a fluke geometry similar to that used by PETROBRAS. The number two is a torpedo that has a fluke geometry proposed by O’Loughlin et al. (2004), and, finally, the number three is that one that has shown the best performance in the early hydrodynamic studies.

Figure 1 Miniature of dynamic anchors used in these tests

All three models are 120mm length and possess

four flukes equally spaced by 90o and were designed

to have equal shaft area (6680 mm2). The pile

number one has 7.5mm diameter shaft, whilst piles two and three has a 9.45 mm pile diameter.

The gross mass of the pile one is 28.4g that is equivalent to a actual mass of 55.5 tons of the prototype. Similarly, piles two and three that have gross mass of 32.4 g is equivalent to a prototype mass of 63.3 tons.

3 PHYSICAL MODEL OF CLAYEY SOIL

3.1 Container and the steel frame

A cylindrical (465mm diameter and 480mm height)

container was used to build the physical model of

the seabed. It was built with two drainage lines, one

at the base and another one close to the top. All

these drainage lines allow the control of the

consolidation. A rigid plate is used to apply a

uniform load on the top of physical model. The reaction structure is a 50ton steel frame that

has a 50ton hydraulic cylinder fixed at the upper

frame. This hydraulic cylinder was connected to an electrical hydraulic-pump and a low pressure air control panel. An air-oil interface was used to regulate the low pressure when necessary. A valve between the high pressure controller system and the low pressure control panel was installed in order to avoid damages to this device.

Figure 2 shows all the apparatus assembly in order to perform the consolidation of the physical model; (a) the steel frame, (b) electrical hydraulic oil pump, (c) low pressure control panel, (d) cylindrical container.

Figure 2. Consolidation apparatus

3.2 Clayey mud

Physical models of clayey mud were prepared with

admixture of meta-kaolin and kaolin. After an

experimental evaluation of which admixture would

be the most suitable a proportion with 95% of meta-

kaolin plus 5% of kaolin, was chosen. The grain size distribution curve of Meta-Kaolin,

Kaolin and the admixture are presented in Figure 3.

0

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30

40

50

60

70

80

90

100

0,0001 0,001 0,01 0,1 1 10

Diameter (mm)

% f

inn

er b

y w

eig

ht

Meta-kaolin

Kaolin

Mixture

Figure 3 Grain size distribution curve

In the first step meta-kaolin and kaolin were weighed and a dry mixture was prepared. In the second step, the mixture was placed in the colloidal mixer with de-aired water in order to produce a homogeneous mud. The water content of the mixture was about 1.5 times the liquid limit of the mud (84.45%). After one hour the mud was placed in other mixer with adapted line of vacuum source in order to have a de-aired clayey mud. The de-aired process takes about one hour. So, the mud was carefully placed in the container with a shallow water column, 20 mm, in order to prevent air from being entrapped in the slurry during pouring, as recommended by Robinson et al. (2003). The initial layer thickness was 400 mm.

Table 1 presents a summarized of geotechnical characterization parameters of the clayey mud and Meta-Kaolin and Kaolin used to make the mixture.

Table 1 Summarizes of geotechnical characterization

parameters

Material

Grains size

distribution G

Atterberg

Limits

Clay silt sand LL PL

Meta-kaolin 6.9 83.4 9.7 2.60 NP NP

Kaolin 9.8 89.1 1.1 2.67 72.2 41.6

Clayed Mud 8.0 84.2 7.8 2.60 56.3 42.1

3.3 Oedometer consolidation test

The consolidation test was carried according to

standard ABNT MB-3336/1990, in a soil with initial

water content of about 1.5 times the LL, resulting in

the following relationship between void ratio (e) and

effective vertical stress (σ’v, in kPa):

e = 2.16 – 0.12 ln(σ’v) (1)

Falling head Permeability tests were conducted for each stress increment. The relationship between void ratio and permeability (k, in cm/s) is given by Eq, 2, sa follows:

e = 4.90 + 0.31 ln(k) (2)

The consolidation coefficients obtained by Taylor’s method are presented in Table 2.

Table 2 Values of coefficient of consolidation of clayed mud

Pressure Range

kPa

Coefficient of Consolidation

cm²/s

6.25 – 12.5 2.40 x 10-2

12.5 - 25 3.22 x 10-2

25 -50 3.59 x 10-2

50 -100 4.52 x 10-2

The compression and swelling index (Cc) and (Cr) are 0.30 and 0.04, respectively

3.4 Consolidation of physical model

Since the material has the consistency of mud, the

first attempt to consolidate it was to consider a

Hydraulic Consolidation Test (HCT) procedure,

which was supposed to be the most suitable

technique to consolidate mixture with this initial

water content. However the presence of pore-

pressure transducer inside the sample represented a

great shortcoming for this technique that was later

discarded So, the physical model was prepared in the

container and a static load consolidation was carried out, considering two surface drainages: one at the top and another at the bottom of it. The thickness of the soil in the container was being measured during all these consolidation steps.

Initially, a low pressure of 23kPa was applied with the pressure control panel just to correct the plate position. After that, a vertical pressure is increased to 103 kPa using an electrical oil pump. This pressure was kept constant until the displacement stabilization. The final vertical displacement was about 83.5mm, which represents a layer thickness of 316.5mm.

Figure 4 shows the displacement measured during the consolidation of the clayey mud. These results show that the displacement reached stabilization after approximately 12 hours.

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80

90

0,01 0,1 1 10 100

Time (h)

Set

tlem

ent

(mm

)

Figure 4 Displacement measured during the consolidation step

4 1G MODELLING

4.1 Pile Installation

All tests were carried in two steps. In each step all

three piles models were inserted in the soil up to

240mm deep, which is equivalent to two times the

pile length.

The piles were inserted at a rate of 0.5mm/sec with a

servo controlled electrical actuator, at a distance of

five diameters amongst them, as proposed by

Richardson et al. (2006).

5 CENTRIFUGE MODELLING

5.1 UENF CENTRIFUGE TEST

All centrifuge tests were conducted in the UENF centrifuge whose main characteristics are:

3.5m Radius 100g x ton payload capacity Payload dimensions of 900 x 900 x

1000mm Dual basket Slip-Rings, Rotary Joint, and Fiber Optic

Rotary Joint (54 shielded signal lines, 17 power lines, 2 closed circuit tv channel, four 300 psig air ports, two 300 psig water ports and two 3000 psig oil ports)

Acceleration Range 0-200 g Further details about the UENF centrifuge can be

found in Saboya et al (2010)

5.2 Pull out centrifuge test at 50 g.

All the tests for the assessment of the anchoring capacity of the piles were carried at 50g at the depth of pile installation.

It can be stressed that, despite the fact the model piles had a 1:125 scale, structural container limitation, the tests could not surpass 50g. However, as these tests are of comparative effect only, the rule to be followed was to carried all of them at strictly the same conditions.

The anchoring capacity of each pile was determined through pull out test to a rate of 6.3mm/s and 5mm/s for piles1, 2 and 3 respectively.

These modeling pull out velocities assure undrained condition for the prototype normalized velocity V (House et al., 2001), as follows:

V = vd/cv ≥ 10 (3)

Where: v = pull out velocity, d = pile diameter and cv

= Coefficient of consolidation

In order to pull out the piles during centrifuge

spinning, it was used a nylon coated stainless steel wire for fishing purposes O’Loughlin et al. (2004).

6. – TESTING RESULT

6.1 – Anchoring (Holding) Capacity

The maximum anchoring capacity reached by each

pile is presented in Figure 5.

Figure 5 Holding capacity from anchor tests

It can be noticed that pile 1 shows holding capacity less than the others two. This is specially interesting because all of them have the same lateral area. It is also verified that pile 3 shows a higher holding capacity than pile 2, despite the fact that both have the same geometry, where only the flukes have different geometries.

Taking into account the gross mass of the piles, it is possible to assess their efficiency. It can be observed that pile 1 have a mean efficiency of 2.4 while piles 2 and 3 reached values of 2.5 and 2.7, respectively, which represent gains of 2% and 12% in relation to pile 1.

7 - CONCLUSION

These initial results presented herein, along with

others already published, indicate that the torpedo

pile geometry can be improved in order to ally

geotechnical and hydrodynamic efficiency. Hydrodynamic and holding capacity tests showed

that pile 3 presented better performance than the others two tested. The influence of the fluke geometry in the holding

capacity can be observed in the comparison between

piles 2 and 3 that possess the same gross mass and

same lateral area. Therefore it can be said that pile 1 design is

unfavorable in comparison to piles with different fluke geometry, and its design must be improved in order to reach better hydrodynamic and holding capacity conditions

ACKNOWLEDGEMENTS

The Authors are grateful for the financial support

provided by FAPERJ (Research Agency of Rio de

Janeiro State) and PETROBRAS (Brazilian Oil

Company).

REFERENCES

American Society for Testing and Materials (2005) Standard test method for laboratory miniature vane shear test for saturated fine-grained clayey soil: ASTM D4648-05. West Conshohocken.

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Aubeny, C., Murff, J.D. (2003) Simplified limit solutions for undrained capacity of suction anchors. Proceedings of International Symposium on Deepwater Mooring Systems, Houston: Texas, USA, p. 76-90.

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Saboya Jr., F., Tibana, S., Reis, R.M., Sobrinho, R.R., Brum Jr., S.A., Del’Aguila, V.M., Vieira, J. (2010) The UENF geotechnical centrifuge facility. 7th International Conference on Physical Modelling in Geotechnics, Zurich, Switzerland. (to be published)