GB2150178A - Leg member for a jack-up type rig and a method of manufacturing the same - Google Patents

Abstract

A leg member (8) for a jack-up type rig having a hollow columnar cord portion (7) and a rack portion (6) which is integrally formed with said cord portion and which projects from and extends in the longitudinal direction of said cord portion and a method of manufacturing the leg member are disclosed. The cord portion (7) and the rack portion (6) are integrally formed by a hot working process which is selected from hot rolling, hot forging, and hot extrusion so that there is no requirement for welding of the rack portion to the cord portion. <IMAGE>

Description

SPECIFICATION Leg member for a jack-up type rig and a method of manufacturing the same Background of the Invention The present invention relates to a leg member for a jack-up type rig for use in drilling in oil fields on the ocean floor. In particular, it relates to a method of manufacturing a leg member for a jack-up type rig in which the cord portion and the rack portion which make up the leg member are integrally formed.

In recent years, there has been active development of ocean floor oil fields, and many improvements have been made in the drilling apparatuses used for this development. Such drilling apparatuses (hereinafter referred to simply as "rigs") can be divided into a number of different types such as submersible rigs, semi-submersible rigs, fixed rigs, and jack-up type rigs.

Among these types, jack-up type rigs are used as mobile rigs primarily in sea depths of up to 100 meters for economic reasons.

As shown in Fig. 1 and Fig. 2, a jack-up type rig 1 is equipped with a hull 2, a drilling mechanism 3, a raising and lowering mechanism 4 (a jacking unit), and legs 5. As shown in Fig. 2, which is a cross-sectional view of a leg 5 taken along Line ll-ll of Fig. 1, each leg 5 is of a multitruss structure comprising leg members 8 each formed from rack portions 6 and a cord portion 7. When the jack-up type rig 1 is to be towed, the legs 5 are raised from the sea floor by a rack and pinion system so that the rig 1 can be freely moved about. On the other hand, when drilling is to take place, the legs 5 are lowered to the sea floor and the hull is raised above the surface of the water so that the entire rig is supported by and secured to the sea floor, and thus stabie operations can be performed even in severe circumstances.

As can be seen from Fig. 2, the legs 5 are formed of leg members 8 which in turn are composed of welded members. In general, 3-4 tubular leg members 8 (in the case of the drawings, 3 leg members) are connected together by braces 10 in a multitruss structure to form each leg 5. In order to permit the relative movement between the hull 2 and the legs 5, i.e., the raising and lowering of the legs 5, a rack portion is provided on the leg members 8 of the legs 5, and pinions are provided on the hull 2. By this rack and pinion mechanism, the raising and lowering of the legs 5 with respect to the hull 2 is caried out.

Because the shapes of the leg members described above are complicated, in the past, leg members have been produced entirely by welding. For example, as shown in Figs. 3A, 3B, and 3C, which are simplified cross-sectional views of different types of leg members, a structure in which a cord portion 7 is welded to rack portions 6 has been used. The blacked-in locations in the figures indicate longitudinally extending welds made along the length of the leg member. As can be gathered from the illustrations, the manufacture of this type of welded structure requires a large number of manufacturing processes such as press forming, and the number of locations which must be welded is large. Furthermore, the lengths of the welds are unavoidably long.In particular, in the case of Fig. 3A, a reinforcing plate S is welded between both rack portions 6.6 and with such a structure the manufacture of a leg member becomes extremely complex.

Thus, the manufacture of conventional leg members is extremely costly. However, in spite of this high cost, since the structure of legs 5 for jack-up type rigs is generally complicated so as to ensure adequate strength, it has been thought that the use of welding for the assembly of the entire structure is unavoidable.

In the past, conventional jack-up type rigs have been designed mainly for use in seas of up to moderate depths (about 100 meters). Recently, however, there has been a demand for jack-up type rigs suitable for use in more severe water and weather conditions. At the same time, there has been a demand for leg members of higher strength. Namely, the rack portion and the cord portion have recently been designed using HT80-Class high tensile steel, the welding of which requires great skill.

Furthermore, as the above-described longitudinally extending welds are extremely long, problems such as welding cracks, deterioration in the quality of welded parts, and welding defects have become difficult to eliminate.

Accordingly, numerous methods have been proposed of reducing as much as possible the number of locations to be welded in leg members.

For example, Japanese Laid Open Patent Application No. 51-135135 discloses a leg member which instead of being produced by the above-described method of welding rack portions to a welded steeltube is produced by welding a rack portion to a steel tube which is formed by centrifugal casting. That invention, which attempts to guarantee the strength of the tube by forming it as one member using centrifugal casting. has drawn attention, but as the rack portion must still be welded to the tube, it still has drawbacks.

Japanese Patent Publication No. 51-90101 discloses forming the rack portion as well by centrifugal casting. However, in that method, it is difficult to maintain a sufficient width for th rack portion, and it is difficult to obtain satisfactory quality with respect to strength and other factors.

In present-day jack-up type rigs, not only is increased strength of the leg members required, but the conflicting demand of decreased weight for the leg members is also made. In order to satisfy both of these requirements, it must in general first be possible to increase the thickness of, for example. the rack portion or the cord portion The dominant external forces acting on the leg members of a jack-up type rig are generally axial forces (particularly compressive forces). and therefore a leg member must be designed so as to have resistance to buckling. If the inner diameter of the cord portion is increased and the section thickness thereof is decreased, the section stiffness and the resistance to buckling can be increased, and an increase in strength together with a decrease in weight can be expected.Depending on the case, it may be necessary to instead decrease the inner diameter and projected area of the leg members in the horizontal direction in order to decrease the external forces such as wave forces acting on the leg members.

The hollow center of the cord portion greatly contributes to heat treatment effectiveness (maintaining uniform quenching of the inner and outer surfaces) as a secondary role. Therefore, a certain minimum inner diameter for the cord portion must be maintained.

In the rolling of profiles such as so-called "shapes", large-sized steel ingots, i.e., steel ingots having a relatively small aspect ratio (the ratio of the width to the height of an ingot measured along a vertical cross section) have conventionally been used. Since large-sized ingots require a great deal of rolling reduction, the yield is necessarily poor and the manufacturing costs are high.

In recent years, while all manner of products have come to be manufactured by the rolling of continuously cast slabs as a result of the increased use of continuous casting, these products are all manufactured by reducing the initial thickness of the slabs. In the manufacture of so-called "profiles", when the desired dimensions of the product are larger than the initial dimensions of the slab, a large-sized ingot having a small aspect ratio must be used. which is undesirable.

Accordingly, if a method can be developed whereby profiles, and in particular leg members for jack-up type rigs, can be manufactured from a continuously cast slab having a large aspect ratio, increases in manufacturing yield can be expected and the advantages of the method will be very great.

Summary of the Invention It is an object of the present invention to provide a leg member for a jack-up type rig which has fewer of welded portions and which can be manufactured using a high tensile steel such at HT80-85 Class steel.

It is another object of the present invention to provide a method of manufacturing a leg member for a jack-up type rig in which the strength can be maintained and/or a weight decrease can be achieved by suitably altering the shape and the dimensions of the rack portion and the cord portion.

As a result of numerous experiments aimed at achieving the above objects, the present inventors found that by using hot working, in particular hot rolling, hot forging, hot extrusion, and the like as means for integrally forming the cord portion and the rack portion, it is possible to form a leg member without the use of any welding whatsoever. The present inventors made use of this finding and thereby completed the present invention.

As mentioned earlier, attempts at decreasing the number of welded portions in a leg member as much as possible have already been reported. However, the present inventors have further developed and improved upon those attempts, and in order to achieve the above objectives have employed hot working.

In this manner, according to the present invention, although the assembly of the legs of a jack-up type rig using leg members must be carried out by welding, the manufacture of the leg members themselves can be performed without any welding at all, a result totally unexpected from prior technology. Furthermore, the benefits gained by not performing welding are remarkable.

In one aspect, the present invention resides in a leg member for a jack-up type rig formed from a hollow columnar cord portion and a rack portion which is integrally formed with the cord portion and which projects from and extends in the longitudinal direction of the cord portion, characterized in that the cord portion and the rack portion are integrally formed by a hot working process. The hot working process is selected from hot rolling, hot forging, and hot extrusion.

In another aspect, the present invention resides in a method of manufacturing a leg member for a jack-up type rig having a hollow columnar cord portion and a rack portion which is integrally formed with the cord portion and which projects from and extends in the longitudinal direction of the cord portion, comprising the steps of:: (a) heating a steel ingot to a prescribed temperature; (b) blooming the steel ingot which was heated in Step (a) using grooved rolls followed by rolling with grooved rolls with the widthwise direction of the steel ingot pointing up and down so as to obtain an intermediate product having a solid columnar cord portion and a rack portion which is integrally formed with the cord portion and which projects from and extends in the longitudinal direction of the cord portion; (c) forming a longitudinally-extending through hole in the center of the cord portion of the intermediate product; and (d) forming rack teeth in the rack portion using suitable means.

In yet another aspect, the present invention resides in a method of manufacturing a leg member for a jack-up type rig having a hollow columnar cord portion and a rack portion which is integrally formed with the cord portion and which projects from and extends in the longitudinal direction of the cord portion, comprising the steps of: (a) heating a steel ingot to a prescribed temperature; (b) forming a longitudinally-extending cavity in the center of the steel ingot; (c) rough rolling the steel ingot with a mandrel bar inserted into the cavity: (d) hot rolling the thus obtained rough rolled material using grooved rolls so as to obtain a prescribed shape, reducing the portion of the rolled material having the greatest cross-sectional width to nearly final dimensions, changing the direction of rolling by 90 degrees, and rolling to produce a final shape; and (e) forming rack teeth in the rack portion using suitable means.

In still another aspect, the present invention resides in a method of manufacturing a leg member for a jack-up type rig having a hollow columnar cord portion and a rack portion which is integrally formed with the cord portion and which projects from and extends in the longitudinal direction of the cord portion, comprising the steps of:: (a) heating a slab having an aspect ratio of at least 2 to a prescribed temperature; (b) reducing both end portions of the slab in the thickness direction and forming longitudinally-extending protruding portions; (c) increasing the thickness of the central portion of the thus obtained roughly shaped material by reducing the roughly shaped material in the widthwise direction while restraining the projecting portions of the roughly shaped material; (d) again reducing the thus obtained rough shaped material in the thickness direction so as to obtain a prescribed final shape; and (e) forming rack teeth in the rack portion using suitable means.

In a further aspect, the present invention resides in a method of manufacturing a leg member for a jack-up type rig having a hollow columnar cord portion and a rack portion which is integrally formed with the cord portion and which projects from and extends in the longitudinal direction of the cord portion, comprising the steps of:: (a) heating a steel ingot to a prescribed temperature; (b) forging the steel ingot so as to form a rough shaped material having a solid columnar cord portion and a rack portion which is integrally formed with the cord portion and which projects from and extends in the longitudinal direction of the cord portion; (c) piercing the solid cord portion by forcing a mandrel bar in the longitudinal direction into the center of the rough shaped material; (d) forming an intermediate product having a hollow columnar cord portion and a rack portion which is integrally formed with the cord portion and which projects from and extends in the longitudinal direction of the cord portion by performing passage of the intermediate product through a ring die having a suitably shaped groove at least one time; and (e) forming rack teeth in the rack portion of the intermediate product.

In yet a further aspect, the present invention resides in a method of manufacturing a leg member for a jack-up type rig having a hollow columnar cord portion and a rack portion which is integrally formed with the cord portion and which projects from and extends in the longitudinal direction of the cord portion and which has rack teeth formed therein, comprising the steps of:: (a) heating a steel ingot having a nearly rectangular cross section to a prescribed temperature; (b) piercing the steel ingot by forcing a mandrel bar in the longitudinal direction into the center of the cross section of the steel ingot using a container to house the steel ingot which does not restrain at least the central portion of the steel ingot, thereby allowing the spreading of the central portion; (c) performing forward hot extrusion at least one time so as to form an intermediate product having a hollow columnar cord portion and a rack portion which is integrally formed with the cord portion and which protrudes from and extends in the longitudinal direction of the cord portion; and (d) forming rack teeth in the rack portion.

Brief Description of the Drawings Figure 1 is a schematic explanatory diagram of a jack-up type rig.

Figure 2 is a cross-sectional view taken along Line Il-Il of Fig. 1.

Figures 3A through 3C are simplified cross-sectional views for explaining the conventional methods of manufacturing the leg members of a jack-up type rig.

Figures 4 to 16 relate to specific embodiments and examples of the invention which are described below by way of example, and in particular: Figure 4 is a simplified perspective view of a leg member for a jack-up type rig according to the present invention.

Figures 5 through 7 are cross-sectional views illustrating various leg members for a jack-up type rig according to the present invention.

Figures 8A through 8F are schematic diagrams illustrating one example of a manufacturing method utilizing hot rolling according to the present invention.

Figures 9A through 9Fare schematic diagrams illustrating the change in cross section of a material being rolled using a manufacturing method employing hot rolling according to the present invention.

Figure 10 is a schematic diagram showing the cross-sectional shapes of grooved rolls for use in the method illustrated in Fig. 9.

Figures 1 1A through 1 1 G are schematic diagrams illustrating the change in cross-sectional shape of a material being rolled using a manufacturing method employing hot rolling according to the present invention.

Figure 12 is a schematic diagram illustrating the cross-sectional shapes of grooved rolls for use in the method illustrated in Fig. 11.

Figure 13 is a flow chart illustrating a different mode of the method according to the present invention.

Figures 14A through 14D are schematic diagrams illustrating a portion of the processes in the mode of the present invention illustrated in Fig. 1 3.

Figure 15 is a flow chart illustrating yet another mode of the method according to the present invention.

Figures 1 6A and 1 6B are cross-sectional views showing the change in cross-sectional shape of a steel ingot having a nearly rectangular vertical cross section housed inside a container for extrusion at the time of piercing according to the method of the present invention.

Detailed Description of the Preferred Embodiments As shown in Figs. 4 through 7, a leg member according to the present invention has a relatively thick-walled tubular cord portion 7 with a central cavity 11 which is integrally formed with a rack portion 6 which projects from the cord portion 7, the two being formed by the hot working processes of rolling, forging, or extrusion.

A leg member according to the present invention can have rack portions 6 symmetrically protruding from both sides of the cord portion 7 as shown in Fig. 5. Alternatively, as shown in Fig. 6, it can have a single rack portion 6' asymmetrically located on one side of the cord portion 7. Furthermore, as shown in Fig. 7, the cord portion 7 may be of a hexagonal shape or of any other desired shape with the rack portions 6" positioned on either side of the cord portion 7 as in Fig. 5. If the cord portion 7 has a hexagonal shape, then braces (such as element number 10 in Fig. 2) having perpendicularly cut ends can be perpendicularly butt welded to a cord portion where two leg members are welded together, and a decrease in the number of man-hours required for assembly of a leg can be expected.

Next, one example of a method of manufacturing this type of leg member using hot rolling will be explained.

In one method according to the present invention, hot rolling is first performed using a blooming mill having grooved rolls. If normal grooved rolling is first carried out, various problems occur such as material twisting at the time of roll contact, material overturning during edging, reduction in the effectiveness of heat treatment caused by remaining forged texture due to insufficient forming and an insufficient reduction ratio, and in particular, a decrease in the Charpy value. Accordingly, in the present invention, rolling is carried out in the manner illustrated in Fig. 8.

First, as shown in Figs. 8A to 8F, a steel ingot 1 5 is heated to a prescribed temperature of at least 1 200 degrees C. Using grooved rolls with the widthwise direction of the ingot 1 5 pointing up and down, the ingot is formed into a rolled material 1 7 having rounded corners 16, as shown in Fig. 8B.

Next, this material 1 7 is reheated, then rotated 90 degrees so that the widthwise direction points up and down, and then is rolled between grooved rolls K-l, as shown in Figs. 8C and 8D. Here, the central portion of the material bulges to the left and right, and rack portions 18 are formed together with a cord portion 19..

Next, the rolled material is again rotated 90 degrees and edging is performed on the end portions of the rack portions 1 8 using grooved rolls K-2 and the lengths of the rack portions 18 are fixed, as shown in Fig. 8E.

Then, the rolled material is once again rotated 90 degrees, and the cord portion 1 9 is shaped by grooved rolls K-3 to form an intermediate product 20, as shown in Fig. 8F.

With this type of rolling, the cord portion 1 9 is directly reduced so that complete filling of the grooves is possible, material twisting and overturn are made infrequent, and the number of passes can be made small. Furthermore, the reduction ratio for steel ingots can be made large, and a uniform product of high toughness can be obtained. Thus, in the finishing stage it is desirable to roll the cord portion between grooved rolls.

Once the above-described profile rolling is caried out, the rolled material undergoes the following processing in the order given.

(1). If necessary, the rolled material is reheated and then bend removal is performed.

(2). The end surfaces of the intermediate product 20 are made plane by machining, and then a central cavity is formed in the center of the cord portion 1 9 by percing.

(3). The rolled material is then heated to approximately 900 degrees C for 2-3 hours, quenched in water, and then tempered at around 600 to 630 degrees C.

(4). If necessary, bend removal is performed using a press machine in a hot or cold state.

(5). When cold straightening has been carried out, annealing is performed, if necessary.

(6). Shot blasting is performed.

(7). NC gas cutting is performed to produce a tooth profile in the rack portions 1 8. When two set of teeth are to be cut, it is desirable that they be cut at the same time so as to increase accuracy.

(8). In preparation of the welding assembly of the legs, both ends of the resulting leg member are bevelled.

(9). The leg member is inspected for mechanical properties, and the hardness and accuracy of the teeth.

This type of leg member is formed in unit lengths. At one end of each member, a backing strip is attached by welding so that the end of another leg member can be inserted into the central cavity of the first leg member. The end portions of leg members are then welded together to obtain a desired length.

In the manner described above, the present invention provides the following effects.

(1). Since each leg member is integrally formed, there are no longitudinal welds which require a great quantity of welding, and so the problems arising from welding are avoided.

(2). Unlike conventional leg members, the quality does not depend on the skill of a welder, and so a product of uniform quality can be obtained.

(3). Any desired cross-sectional shape can be obtained. For example, it is possible to increase the thickness of the rack and cord portions so that the strength of the leg member can be much greater than conventional strengths.

(4). Reductions in weight are possible. In general, the leg members in a jack-up type rig of truss construction are subjected predominantly to axial forces (particularly compressive forces), and therefore buckling is a problem. However, when compared to a conventional leg member of the same shape, a leg member according to the present invention has more of its cross-sectional area on its outer sides, which increases its resistance to buckling. Therefore, a leg member according to the present invention can be lighter than a conventional leg member of equal strength.

(5). In general, the resistance to punching shearing is large, so it is not necessary to specially reinforce the inside of the cord portion.

(6). During manufacture, no bending work is carried out so that no decrease in toughness due to bending occurs.

(7). The number of locations requiring inspection is small. During manufacture of the leg member, no welding is performed, so inspection of welds is completely unnecessary. Only inspections of material quality and dimensions need be performed.

(8). The manufacturing time is short. Because the processes used are simple, because use is made of mechanical force, because the amount of manual labor is small, and because product quality is consistent, the manufacturing time is shorter than with conventional methods.

(9). Block construction is made easy.

Next, another mode of the present invention will be described.

Figs. 9A through 9F are schematic diagrams showing the change in cross-sectional shape of a steel ingot 20, in which a central cavity 21 has been previously formed during a casting step, as the ingot 20 is rolled according to the method of the present invention so as to form the abovedescribed leg member.

The central cavity 21 can be formed in the ingot 20 after casting by forcing the belowdescribed mandrel bar into the ingot 20 using a press machine, for example.

As shown in Fig. 9B, according to the present invention, while a mandrel bar 22 is inserted into the cavity 21 of the ingot 20. the ingot 20 is first reduced in the thickness direction by rolls K-1. The thus obtained rolled material 23 is then turned 90 degrees so that the widthwise direction points up and down, and then rolling is carried out between grooved rolls K-2 with the direction of reduction turned 90 degrees so that the direction of reduction is as shown by the arrows in Fig. 9C. Next, repeated passes through grooved rolls K-2 are performed so as to obtain rack portions 24 having the shape shown in Fig. 9D. The rolled material is again turned 90 degrees, and is rolled between grooved rolls K-3 so as to obtain a cord portion 25 as shown in Fig. 9E. At this time, light rolling of the rack portions 24 takes place. Next. as shown in Fig.

9F, finishing rolling is performed between grooved rolls K-4 having finishing grooves. After that.

the cord portion 25 is finished so as to give it a prescribed circular shape, and the mandrel bar is removed.

Thus, in the pass schedule of the illustrated example. the rolled material is turned 90 degrees when it is transferred from rolls K-l to grooved rolls K2, and it is again turned 90 degrees when it is transferred from grooved rolls K-2 to grooved rolls K-3. At the end of each step, additionai rolling between flat rolls like rolls K-l in Fig. 9B may also be applied.

Thus, in the above-described mode, the present invention is a hot rolling method in which a steel ingot having a central cavity formed therein is subjected to rough rolling while having a mandrel bar inserted into the cavity, the thus obtained rough rolled material is then subjected to hot rolling with grooved rolls so as to obtain a prescribed shape with the portion having the greatest cross-sectional width being reduced to nearly final dimensions, the direction of rolling is preferably changed by 90 degrees. and rolling to produce a final shape is carried out.

Fig. 10 is a schematic diagram showing the change in cross-sectional shape of grooved rolls used in the mode illustrated in Fig. 9.

As a method of rolling with a mandrel bar inserted into the central cavity of a steel ingot, there is the mandrel mill method of manufacturing seamless steel tubes. However, that is a manufacturing method for tube material in which after insertion of a mandrel bar, simultaneous reduction of the uuter diameter and of the wall thickness of the tube material is carried out by continuous rolling. Furthermore, in that method, rolling is performed in a manner so as to produce no thickness deviations.

Figs. 1 A through 1 1 G are schematic diagrams showing the change in cross-sectional shape of a slab produced by continuous casting as the slab is rolled according to the method of the present invention.

As shown in Fig. 1 lA. a slab S first has its end portions 30 (which correspond to rack portions) reduced in the thickness direction by grooved rolls K-i. Next, as shown in Figs. 11C through 11 E, the rolled material S is turned 90 degrees and rolled between grooved rolls K-2 with the protruding portions 30 of both ends being restrained in special grooves. By multiple passes through the rolls, edging is gradually carried out.

As edging is performed, the distance between the rolls is successively decreased and the rolled material S spreads out in the widthwise direction. The size and the shape of the protruding portions 30 of the ends of the slab S are made such that neither overturning nor twisting of the rolled material S takes place during rolling in the grooved rolls K-2. Once the thickness of the central portion of the rolled material S reaches a predetermined value, the material S is once again turned 90 degrees, i.e., is returned to its original orientation and is then rolled between grooved rolls K-3 so that reduction of the central portion takes place in its thickness direction (Fig. 11 F). The necessary reduction ratio is obtained, and the cord portion is shaped. At this time, the rack portions 30 undergo light reduction.

Next, the material S undergoes finishing rolling in grooved rolls K-4 having finishing grooves, and the central portion of the material S is given a prescribed circular shape, as shown in Fig.

1 1 G.

Thus, in the pass schedule of the illustrated example, the rolled material S is turned 90 degrees when it is transferred from grooved rolls K-1 to grooved rolls K-2, and it is once again turned 90 degrees when it is transferred from grooved rolls K-2 to grooved rolls K-3.

In the present invention, the aspect ratio of the continuously cast slab should be at least 2 and is preferably 5.0. If the initial thickness of the cast slab is t, and if the maximum thickness of the finally obtained profile is D, then D should be greater than t. Preferably, D is greater than (1.5-3.0)t. In other words, rolling is performed in which the final thickness is at least one times its initial value and preferably at least 1.5 to 3.0 times its initial value.

According to the above-described mode of the present invention, rolling of a profile from a continuously cast slab having a large aspect ratio of at least 2 can be carried out easily and efficiently. The number of grooved rolls required for rolling can be made small. Moreover, during rolling, the dimensions in the thickness direction can be increased, so that this method is particularly advantageous for the manfacture of integral leg members for jack-up type rigs.

Fig. 1 2 is a schematic diagram illustrating the change in cross-sectional shape of the grooved rolls used in the above-described mode of the present invention.

Fig. 1 3 is a flow chart of Process 1 through Process 5 of yet another mode of the manufacturing method according to the present invention which employs hot forging. Figs. 1 4A through 1 4D are schematic diagrams illustrating a forging process, a piercing process, and a passage-through-a-die process in accordance with the present method.

Process 1: After heating a steel ingot to a prescribed temperature, the top portion and the bottom portion of the ingot are cut off, possibly after rough forging, and a roughly shaped material 44 (for example, the material shown in Fig. 14A having a solid cord portion 40 and a rack portion 41 integrally formed with the cord portion 40) is formed by hot forging, using a horizontal or a vertical press machine, for example, Fig. 1 4B is a cross-sectional view taken along Line b-b of Fig. 1 4A.

Process 2: The above-described roughly shaped material 44 is secured in a receiving tool 43 having a steel securing jig 42 which has a shape corresponding to the shape of the material 44 produced in Process 1. After it is secured, a mandrel bar 45 (a piercing mandrel) having an appropriate diameter is forced into the material 44 using a press machine so as to pierce the solid cord portion 40. Fig. 1 4C is a schematic diagram of an example of an arrangement of the equipment used when the piercing takes place.

Process 3: The roughly shaped material 44 is removed from the above-described receiving tool 43, and with the mandrel bar 45 still inserted inside the material 44, a passage-through-aring-die process is carried out with the mandrel bar and a press machine using a ring die 46 having grooves of appropriate shape. This process can be caried out either by drawing or by sinking. At least one pass should be made through the ring die 46, and usually 5 to 6 passes are performed to achieve the final shape. Fig. 1 4D is a simplified cross-sectional view illustrating this process. The illustration shows so-called "drawing" being performed, and there is no effective change in the inner diameter of the rough shaped material 44. However, when socalled "sinking" is carried out, the inner diameter of the material decreases.

Process 4: The mandrel bar is removed from the material, and the thus obtained leg member is subjected to heat treatment consisting of quenching and tempering.

Process 5: Teeth are formed in the rack portions protruding from the cord portion with which they are integrally formed using melting (gas cutting, arc cutting, etc.), for example.

Effects (1). In the above-described mode of the present invention, a solid body is produced by forging. Since this mode is not subject to the limitations imposed by roll installation in the case of rolling, a leg member having a large cross section can be easily manufactured. In addition, changes in the thicknesses of the cord portion and the rack portions can be made if necessary, and increases in thickness are easy to achieve.

(2). This mode of manufacture can be advantageously used even for a leg member whose cord portion has relatively thin walls and a large-diameter central cavity.

(3). In the manufacturing method according to the present invention, a boring operation using a boring machine or the like is unnecessary. Accordingly, work efficiency is improved and yield is increased.

Fig. 1 5 is a flow chart illustrating Process 1 through Process 6 of still another mode of the present invention. Fig. 1 6 comprises cross-sectional views showing the change in cross-sectional shape of a steel ingot having a nearly rectangular cross section at the time of piercing according to the method of the present invention. Fig. 1 6A shows the shape prior to piercing. and Fig.

16B shows the shape subsequent to piercing.

Process 1: This is a heating process which is carried out in order to make piercing easier. In general, heating is carried out at 1000-1100 degrees C. As shown in Fig. 16A, if the ingot 50 has a rectangular cross section, there are no restrictions on its dimensions, but in order to make it possible for the outer diameter of the cord portion to be made fairly large, an ingot with a rectangular cross section whose thickness is considerably smaller than the intended outer diameter of the cord portion can be used.

Process 2: After the heating of the ingot 50, the ingot 50 is housed in a container 51, a mandrel bar is inserted into the ingot 50, and piercing accompanied by spreading of the ingot 50 are performed. The container 51 is shaped such that the ends sections 52 of the container 51 act as restraining molds for the end portions of the ingot 50 corresponding to the rack portions, while the middle portion 53 of the container 51 acts as a non-restraining mold for the portion of the ingot 50 corresponding to the cord portion. In this manner, the spreading action of the ingot 50 into the empty space 54 within the container 51 at the time of piercing with the mandrel bar is all the more enhanced.

Process 3: After the formation of a pierced and widened roughly shaped material, heating is again performed at 1000-1100 degrees C in preparation for hot extrusion.

Process 4: Using normal means, hot extrusion is performed on the roughly shaped material which was heated to a prescribed temperature in Process 3. Glass powder is used as a lubricant, and forward extrusion is carried out using a die having a shape conforming to the final desired shape.

If necessary, the heating of Process 3 and the hot extrusion of Process 4 are repeated a number of times until a finished shape is achieved.

As shown in Fig. 16, with a leg member manufactured according to the method of the present invention, if the diameter of the cord portion at the time of spreading by piercing is He, and if the diameter of the cord portion after the finished shape is achieved is H,. then He can be made larger than Hc, which means that the required thickness of the steel ingot can be reduced.

Furthermore, if the thickness of the steel ingot is H, and the thickness of the rack portions of the finished leg member is H" the ratio of H, to H, can be reduced. This means that the extrusion ratio can be reduced and that forming by extrusion can be carried out with a minimal number of passes. In Fig. 1 6B, H,' indicates the thickness of the rack portions, which is substantially equal to H,.

Process 5: The mandrel bar is removed from the thus obtained leg member and heat treatment consisting of usual quenching and tempering is carried out.

Process 6: In order to finish the leg member, rack teeth are formed by melting (gas cutting, arc cutting, etc.), for example, in the rack portions which are integrally formed with and project from the cord portion.

Thus, in the method according to the present invention, the spreading of the central portion which occurs at the time of piercing is constructively used by putting a rectangular steel ingot in a container having semicircuiar projections in its portions corresponding to the cord portion of the leg member and by piercing the ingot by insertion of a mandrel bar into the center of the steel ingot so as to produce spreading which completely fills the inside of the container. In this manner, spreading by at least the diameter of the mandrel bar can be achieved.For example, in the case of a leg member having a cord mernber with an outer diameter of 450 mm and an inner diameter of 1 50 mm and rack portions each having a length of 1 90 mm and a thickness of 1 80 mm, in the previously described processing method using reduction of area, a steel ingot having a thickness of at least 500 mm is necessary. In the present method, however, an ingot with a thickness ot 350 mm is sufficient. Furthermore, the extrusion ratio for the rack portions can be reduced from 2.8 to 1.9, and the extrusion ratio for the entire cross section can also be greatly improved.

Effects (1). In the present invention, a hollow body is formed making use of the spreading action occurring at the time of piercing so that there are none of the restrictions generally imposed by rolling equipment when rolling is carried out. In addition, the thickness of the rack portion and the cord portion can be changed as necessary, and increases in thickness are simple to achieve.

(2). In the manufacturing method according to the present invention, a boring operation using a boring machine or the like is unnecessary. Accordingly, work efficiency is improved and yield is increased.

(3). Since a rectangular steel ingot having a relatively small thickness can be used as a starting material, a continuously cast slab can be used as a starting material just as it is.

Example 1: Using a steel ingot (900 X 1 100 mm) in the center of which a 1 80 mm diameter cavity was formed at the time of casting, rough rolling of a leg member for a jack-up type rig was carried out. As a rolling method for a solid ingot, rolling can be carried out by unidirectional reduction beginning with grooved rolls K-3 of Fig. 9 so as to obtain a desired shape. However, in the case of the present invention, a 1 70 mm diameter mandrel bar is inserted into the cavity of the above-described steel ingot, and as shown in Fig. 9, the cord portion is first reduced using grooved rolls K-2. After a sufficient reduction ratio is obtained, the material is turned 90 degrees, and the direction of reduction is changed by 90 degrees. at this time, spaces develop between the mandrel bar and the material in the up and down direction.

Next, as a result of reduction in the thickness direction of the rack portions and the cord portion in grooved rolls K-3, the above-mentioned spaces decrease in size, and during finishing rolling in grooved rolls K-4, they nearly disappear. It is therefore possible to form a central cavity having a shape which is nearly a perfect circle. In addition, it is difficult for the location of the cavity to deviate from the center of the cord portion. If it is necessary to have a perfectly circular cavity, the cavity formed in the above manner can be enlarged by piercing and then finished. After finishing rolling, the mandrel bar is removed, and if necessary, in the case of the above-described leg member, rack teeth are wormed in the rack portions by melting (gas cutting, arc cutting, etc.).

When unidirectional reduction is carried out beginning with grooved rolls K-2, spaces develop between the mandrel bar and the rolled material in the widthwise direction, and these spaces expand in the widthwise direction. At the completion of rolling, the central cavity has the shape of an ellipse with its major axis extending in the widthwise direction. Furthermore, the center of this elliptical shape can easily deviate from the center of the cord portion, and thus the necessary wall thickness of the cord portion can not be obtained.

In the 2-high reversible roiling mill illustrated in Fig. 10, the top portion of the figure is the upper roll and the bottom portion is the lower roll. Roll K-1 is a flat roll for roughing, and roll K 4 is a grooved finishing roll.

Table 1 shows a pass schedule for rolling carried out in accordance with the present example for a leg member having rack portions with a thickness of 1 20 mm, a cord portion with an outer diameter of 380 mm and an inner diameter of 200 mm, and a total width of 760 mm.

Table 1

Thickness After Width After Pass No. Roll No. Release (mm) Release (mm) Starting Ingot (900) (1100) 1 K-1 1000 930 2 " 900 960 3 T K-l 840 920 4 " 740 940 5 T K-2 880 (Rack Portion) 750 6 .. 820 ( " ) 760 7 .. 760 ( " ) 765 8 | T K-1 | 740 | (760) 9 T K-2 700 (Rack Portion) 750 10 .. 640 ( " ) 760 11 .. 580 ( " ) 765 12 | T K-1 | 740 | (580) 13 T K-2 520 (Rack Portion) 750 14 " 460 ( " ) 765 15 T K-1 740 (460) 16 T K-2 400 (Rack Portion) 750 17 .. 360 ( " ) 765 18 | T K-3 | 680 (Cord Portion) | 750 19 " 600 ( " ) 760 20 .. 520 ( " ) 770 21 .. 440 ( " ) 775 22 " 390 ( " ) 780 23 K-4 384 ( " ) 785 Note: T : 90 Turn Example 2 Using a continuously cast slab with a thickness of 300 mm and a width of 1 550 mm, a solid roughly shaped material having a rack thickness of 120 mm, a cord diameter of 380 mm, and a total width of 760 mm was formed by rolling.The shapes of the grooved rolls used were as shown in Fig. 11. The upper roll is shown in the top of each figure and the lower roll is shown in the bottom. Although rolls K-1 and K-2 had the same shapes, the orientation of the rolled material differed by 90 degrees for K-1 and K-2. Roll K-4 is a finishing roll. The pass schedule for this example is shown in Table 2.

Table 2

Thickness After Width After Pass No. Roll No. Release (mm) Release (mm) Starting Slab (300) (1150) 1 K-1 200 (Rack Portion) 1570 2 " 130 ( " ) 1580 3 T K-2 1410 340 4 | " | 1370 | 370 5 " 1330 400 6 " 1290 420 7 U 1250 438 8 .. 1210 454 9 " 1170 470 10 " 1130 486 11 " 1090 500 12 " 1050 512 13 " 1010 522 14 1. 970 530 15 11 930 540 16 " 890 550 17 " 850 558 18 | " | 810 | 564 19 " 770 570 20 t R-3 500 (Cord Portion) 774 21 n 430 ( " ) 778 22 n 390 ( " ) 782 23 K-4 384 ( " ) 782 Note: T : 900 Turn

Claims (11)

1. A leg member for a jack-up type rig having a hollow columnar cord portion and a rack portion which is integrally formed with said cord portion and which projects from and extends in the longitudinal direction of said cord portion, characterized in that said cord portion and said rack portion are integrally formed by a hot working process.

2. A leg member as claimed in Claim 1, wherein said hot working process is selected from hot rolling, hot forging, and hot extrusion.

3. A method of manufacturing a leg member for a jack-up type rig having a hollow columnar cord portion and a rack portion which is integrally formed with said cord portion and which projects from and extends in the longitudinal direction of said cord portion, comprising the steps of: (a) heating a steel ingot to a prescribed temperature; (b) blooming said steel ingot which was heated in Step (a) using grooved rolls followed by rolling with grooved rolls with the widthwise direction of said steel ingot pointing up and down so as to obtain an intermediate product having a solid columnar cord portion and a rack portion which is integrally formed with said cord portion and which projects from and extends in the longitudinal direction of said cord portion; (c) forming a longitudinally-extending through hole in the center of said cord portion of said intermediate product; and (d) forming rack teeth in said rack portion using suitable means.

4. A method of manufacturing a leg member for a jack-up type rig having a hollow columnar cord portion and a rack portion which is integrally formed with said cord portion and which projects from and extends in the longitudinal direction of said cord portion, comprising the steps of: (a) heating a steel ingot to a prescribed temperature; (b) forming a longitudinally-extending cavity in the center of said steel ingot; (c) rough rolling said steel ingot with a mandrel bar inserted into said cavity; (d) hot rolling the thus obtained rough rolled material using grooved rolls so as to obtain a prescribed shape, reducing the portion of the rolled material having the greatest cross-sectional width to nearly final dimensions, changing the direction of rolling by 90 degrees, and rolling to produce a final shape; and (e) forming rack teeth in said rack portion of said intermediate product.

5. A method of manufacturing a leg member for a jack-up type rig having a hollow columnar cord portion and a rack portion which is integrally formed with said cord portion and which projects from and extends in the longitudinal direction of said cord portion, comprising the steps of: (a) heating a slab having an aspect ratio of at least 2 to a prescribed temperature; (b) reducing both end portions of said slab in the thickness direction and forming longitudinally-extending projecting portions; (c) increasing the thickness of the central portion of the thus obtained roughly shaped material by reducing said roughly shaped material in the widthwise direction while restraining said projecting portions of said roughly shaped material; (d) again reducing the thus obtained rough shaped material in the thickness direction so as to obtain a prescribed final shape; and (e) forming rack teeth in said rack portion of said intermediate product.

6. A method of manufacturing a leg member for a jack-up type rig having a hollow columnar cord portion and a rack portion which is integrally formed with said cord portion and which projects from and extends in the longitudinal direction of said cord portion, comprising the steps of: (a) heating a steel ingot to a prescribed temperature; (b) forging said steel ingot so as to form a rough shaped material having a solid columnar cord portion and a rack portion which is integraiiy formed with said cord portion and which projects from and extends in the longitudinai direction of said cord portion; (c) piercing said solid cord portion by forcing a mandrel bar in the longitudinal direction into the center of said rough shaped material;; (d) forming an intermediate product having a hollow columnar cord portion and a rack portion which is integrally formed with said cord portion and which projects from and extends in the longitudinal direction of said cord portion by performing at least one time passage of the intermediate product through a ring die having a suitably shaped groove; and (e) forming rack teeth in said rack portion of said intermediate product.

7. A method as claimed in Claim 6, wherein said passage-through-a-ring-die is drawing.

8. A method as claimed in Claim 6, wherein said passage-through-a-ring-die is sinking.

9. A method of manufacturing a leg member for a jack-up type rig having a hollow columnar cord portion and a rack portion which is integrally formed with said cord portion and which projects from and extends in the longitudinal direction of said cord portion and which has rack teeth formed therein, comprising the steps of:: (a) heating a steel ingot having a nearly rctangular cross section to a prescribed temperature; (b) piercing said steel ingot by forcing a mandrel bar in the longitudinal direction into the center of the cross section of said steel ingot while using a container to house said steel ingot which does not restrain at least the central portion of said steel ingot, thereby allowing the spreading of said central portion; (c) performing forward hot extrusion at least one time so as to form an intermediate product having a hollow columnar cord portion and a rack portion which is integrally formed with said cord portion and which protrudes from and extends in the longitudinal direction of said cord portion; and (d) forming rack teeth in said rack portion.

10. A leg member for a jack-up type rig manufactured by a method as claimed in any of Claims 3 to 9.

11. A leg member for a jack-up type rig, substantially as hereinbefore described with reference to and as illustrated in Figs. 4 to 7 of the accompanying drawings.

1 2. A jack-up type rig having a plurality of leg members as claimed in any of Claims 1, 2, 10 and 11.

1 3. A method of mahufacturing a leg member for a jack-up type rig, substantially as hereinbefore described with reference to Figs. 8 to 1 6 of the accompanying drawings.