ijreiss volume2, issue 6(june 2012) issn: 2250-0588 finite ...connection of jack-up platforms the...

IJREISS Volume2, Issue 6(June 2012) ISSN: 2250-0588

International Journal of Research in Engineering, IT and Social Sciences 1 www.indusedu.org

FINITE ELEMENT MODELING OF JACKINGSYSTEM IN JACK-UP

PLATFORMS

Reza Dezvareh

Ph.D Student, School of Civil Engineering, College of Engineering, University of Tehran,

Tehran, IRAN

Prof. KhosrowBargi Professor, School of Civil Engineering, College of Engineering, University of Tehran,

Tehran, IRAN

YasinMoradi

Ph.D Student, School of Civil Engineering, College of Engineering, University of Tehran,

Tehran, IRAN

ABSTRACT

Iran with its vast reserves of oil and gas rich is one of the rich countries in the world. In

order to explore this vast underground oil and gas resources in offshore areas, using jack-up

platforms according to the cost and portability, will play a significant role. Mechanical

systems built on these platforms can deploy them in different locations and transportation

and transmission for oil drilling in the various fields will make possible. Mechanical systems

based connection to the deck, in order to lift the deck is one of the challenging parts of the

structures. For better understanding of Lift joints mechanisms in these structures, it is

necessary to review the research in this field and modeling to investigate the connections. In

this regard, various systems have suggested while reviewing them, Model of common

connection and the rack pinion made in the finite element software (Abaqus) and it has been

studied.

Keywords: Jack-up platform, Leg connection to the deck, Jacking System, Pinion, Rack.



INTRODUCTION

Jack-up platforms as one of the most affordable options in the offshore industry, Mining and

oil production companies are concerned. Jack-up platforms in offshore oil and gas industry

plays an important role in their excavations. Thus, the more accurate of these structures in a

better understanding of their performance and prevent accidents and to find more efficient

ways to structure future, designers will provide plans for future platforms. Also the

maintenance of existing platforms of this research is not without interest and under the

influence of this research will develop.

One of the challenging aspects of these structures is existing mechanical connections for

jacking. That studying available papers and research, lack of specific research in this field is

clearly understandable. Appropriate and effective for research, survey research and develop

case studies to address this deficiency is the first step. This paper describes the issue; we tried

to do things in a list that leads to understanding of the connections of these platforms to be

included. In addition to connecting systems, connections modeling in the finite element

software in turn require methods that considering the massive elements of the simulated

model, the equilibrium model ensure reasonable accuracy and ease.

The finite element model of the connection between rack and pinion, which is common in

mechanical system, was modeled by Abaqus software. And forces the pinion and rack has

been investigated. The lift systems, plans as there are creative ideas and inventions that in this

research also mentioned a few projects. Research and tests are needed to industrialization the

plans.

MATERIAL AND METHODS

Connection of Jack-up Platforms

The establishment of Jack up, it’s floating in order to change the platform position and

drilling in different areas, is possible by mechanical systems built on these platforms. Raising

and lowering the grades is done by inside connection between leg and deck (jack case). Basic

Connection to the deck is usually designed in two different types that according to the

conditions and design aspects are used in construction .

Transfer of forces in these types of connections so that all vertical forces that are exchanged

between deck and basic are via connection between pinion and rack. In Connection a chord to

the pinion, the driving force by an electric motor and gearbox is converted to jacking force.

The jacking force and velocity are extracted from the following relations .



(1)

(2)

The horizontal forces also are transferred directly from the chords and racks to the jack-case

by the systems called guides. Guides typically built in the beginning and the end jack-case

and Except for the transmission of lateral forces, they maintain their leg in jack-case. And

prevent displacement of the leg outside of the designed gap.

Single Rack Connection

In this type of connection lifting force by lifting pinion applies one rack in corner legs. The

compressive force and the weight of structures bear by rack and triangular box made of plates

welded together. This type of connection according to the force applied by the pinion, in

addition to vertical force, applies amounts of horizontal force to the rack .

Figure -1

Schematic plan and section of single rack Connection1

This Connection modeled according to its characteristics is done using elements with

equivalent stiffness. General models of jack-up platform with very high volume components

for basic modeling elements use circular cross sections. In this model the real characteristics

of the rack and the effects of current and wave forces are applied in characteristics of the

equivalent element. Existing offshore codes has recommended a method for modeling

connection in finite element software. In this method for original connector are used beam

elements with equivalent stiffness. Also the junction of the pinion to the rack is modeled by

spring element with equivalent stiffness. This stiffness value is obtained by connection unit

test or manufacturer offers this amount. Spring element at the junction is considered by angle

equal to the pinion tooth angles. Modeling method is shown in Figure 2.



Figure – 2

The modeling of single rack system

Opposed Rack Connection

As in previous case, this connection helps pinion and rack for force transmission. But the

main difference that is two racks is located on the sides. Actually the lift force in each point

was applied to chord by two pinions. One of the advantages of this type can be said the

transmission type between pinion and rack. Because the horizontal forces are neutral, Forces

from the pinion to the rack in this case due to the placement of two pinions in the front, only

applied in the axial direction of members. Figure 3 shows how the forces. Another point that

is each pinion force in the system is produced separately by the gearbox and the engine. So a

mechanical system in this type of lift has the greater number of pinion, gearbox and engine .

Figure – 3

Schematic plan and section of opposed rack Connection1

Modeling in this type of connection is made with slightly different than the previous

connection. In other words, due to placement of the front pinions, balance occurs in the forces



from the pinion to the rack. Thus the induced horizontal forces cancel each other and

resultants of the vertical forces exerted on the chord are applied on their axis. In modeling can

utilize this property and the connection was a little simpler model. In general, the modeling of

the passage as follows, using the spring element with equivalent stiffness is done. In the

remaining part, the modeling is similar to the previous model. Also in chord modeling should

be the characteristic features of the real cross section applied to model. A view of the

connection is shown in Figure 4.

Figure – 4

Rack and pinion in opposed rack connection1

Connection Characteristic and Force Transmission Mode

Profile connections inside the jack-case and the performance of members of transmission of

forces in the jack-up platforms are very importance. In general, the forces are transmitted by

two members located inside the jack-case. Rack and pinions are used for the transfer of

vertical force and the base displacement. And guides transfer shear forces between the leg and

the deck. Figure 5 is a schematic view of the transmission of forces inside the jack-case at the

moment of transfer.



Figure – 5

The transition moment in the

Other Jacking Systems

Nowadays the other jacking system except rack and pinion system, such as hydraulic system,

as lifting equipment in some jack-up platforms are designed and installed. A view of this

system is shown in Figure 6.

Figure – 6

A view of hydraulic jacking

A Sample of Presented Projects for Jacking System

The jacking system has been registered plans that two plans are the following:

The system that was named Thomas F. McDuffie is shown in Figure 7. Force leg with

hydraulic jacks that are connected to pinions is done. The advantages of this scheme can



reduce the required fifty percent referred to the rack. Another benefit of this design is low

connection weight and high speed leg movement on this system. Moving pinion between two

rack that one connected to the leg and is moving and another is fixed on jack-case, Leg

moves .

Figure – 7

Thomas F. McDuffie jacking

The second design has been registered by Felix S. Radovan. The jacking system is designed

to boost the leg of the hydraulic cylinder is used. Hydraulic jacks to be connected to chord by

appendices and it will move. In this system, there is no rack and pinion system and the

problems of corrosion. A view of the scheme is shown in Figure 8 .

Figure – 8

Felix S. Radovan jacking



RESULTS AND DISCUSSION

Modeling of Connection between Pinion and Rack

Connection between rack and pinion was modeled in finite element software Abaqus. For

modeling, use solid element with eight-point integration and rack and chord was modeled

accurately. At first, pinion inhibited by its own axis as tangy and the rack is loaded in the

longitudinal direction. After establishing the connection point between the pinion and rack,

applied to the driven pinion, rack moves and actually jacking was modeled.

Values of Young's modulus E = 2e5 MPa, shear modulus G = 8e4 MPa and Poisson

coefficient v = 0.3 are considered for modeling. Modeling in jacking system includes one

hundred steps that figure 9 in steps 0, 30, 60 and 90 are shown. Load applied from the bottom

of the rack, and its value is 4 Mega Newton. Pinion angles radians are shown in the diagrams.

And the amount of rotational motion at the end of step is one hundredth of 1.5 radians and

pinion goes through about 3-pitches during the period.

Figure – 9

Four time step of model of the rack and pinion



One of the items that in the connection system design enjoys of importance is how to transfer

forces. As mentioned at the single rack, pinion is on one side of the rack and hence the force

applied on one side of the rack. The vertical force exerted on the rack during jacking as

shown in Figure 13. The four mega-Newtonian forces applied to the rack in a vertical,

maximum horizontal force exerted on the connection is 2.88 Mega Newton that is about 72

percent of the vertical force. The horizontal remarkable force into the chord cause shear and

bending stresses in the range of pinions and ultimately reduces the amount of leg bearing.

This chart shows changing horizontal force values in connection to the pinion rotation angle

that clearly indicates the maximum horizontal force is at the beginning of the pitch. The

amount of loss force occurs in about half the length of pitch and this change occurs for two

reasons:

The first reason: the angle that the line passing through the junction and the pinion axis makes

with the horizon. And the second reason: the connection depends on the angle of curvature of

the rack pitch and pinion.

For stress evaluation, nodes 5168, 3736 and 2304 are selected and shown in Figure 10. The

nodes located on the surface of pinions and pinion during its movement through the nodes,

makes contact with the rack.

Figure -10

Location of integration of three points on pinion

For more accurate evaluation of stress in the inner parts of the pinion ribs, nodes 5552, 4120

and 2688 were selected in the pitches that the exact position illustrates in Figure 11.



Figure – 11

Location of integration of three points within the pinion pitch

In this part, the Vonmises maximum stress is used for studying stresses of a point. For nodes

on the pinion level, increasing stress with approaching the point to the contact location is

visible in Figure 12. Also the point in 3736 given the close location in the rack with the

pinion, the more stress is experienced at some angles.

Figure -12

Maximum stresses in three-point integration on pinion

The moment applied to the pinion that can bear vertical force is shown in Figure 14. As

shown in this figure, the angle that force to the pinion makes with the pinion axis, at the

beginning of pitch connection, is great. And continued the amount of rotation angle will

decrease and as a result, a moment is reduced during of the pitch conflict.



Figure – 13

Horizontal and vertical force curve relative to angle

(Force unit is N and the angle unit is radians)

Figure – 14

Axis pinion moment curve relative to angle

(Moment unit is N.m and the angle unit is radians)

The obtained values for the type of connection represent the geometry of the connection and

its efficiency. Another important factor in the joints is weight, required space for connection

and related facilities that it has a direct relationship with the connection system.



CONCLUSIONS

Connections in the jack-up platforms have high importance and sensitivity and construction

and design of such connections require sufficient of scientific knowledge. Better

understanding of current connections is obtained with numerical modeling and experimental

study.

1- Exchange forces on the pinion connections of the modeling show high levels of horizontal

force into the connection so that the horizontal force into the connecting is 72 percent of

the vertical force. So it is necessary to consider these forces in the design and modeling.

2- Study the amount of torque that exerted on the pinion axis during its rotation depending

on the angle of pinion position is different. So that moment during the rotation will

increase to 22 percent of initial amount.

3- The Vonmises maximum stresses on the pinion during its rotation in the parts that are in

direct contact with the rack because of stress concentration are higher than other parts of

the pinion.

Attention to the connections in the jack-up rigs that is very affordable for drilling in the

Persian Gulf and the Caspian Sea is important step in the localization of manufacturing and

designing this type of platform. Now the design and manufacture parts such as connections it

is quite the monopoly of a few countries.

It is hoped that with all plans offered in this area, necessity of further investigations and

studies on new inventions and industrial designs lead to design and construction structures

with more efficient and cost-effective in the future.

REFERENCES

1. Chakrabarti, Subrata. K., “Handbook of offshore engineering,” Elsevier, (2005)

2. X.M. Tan, J. Li, C. Lu, Structural behavior prediction for jack-up units during jacking

operations, Computers and Structures 81, 2409–2416(2003)

3. SNAME, “RECOMMENDED PRACTICE FOR SITE SPECIFIC ASSESSMENT

OF MOBILE JACK-UPUNITS,” (1994), Revision 2 January (2002)

4. www.GustoMSC.com. [Online]

5. http://www.freepatentsonline.com/

ijreiss volume2, issue 6(june 2012) issn: 2250-0588 finite ...connection of jack-up platforms the...

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