NO20181192A1

NO20181192A1 - A leg cutting two-axes wire saw, a method of forming a key cut and a GBS with cut platform legs

Info

Publication number: NO20181192A1
Application number: NO20181192A
Authority: NO
Inventors: Harald Ramfjord
Original assignee: 1Diamond As
Priority date: 2018-09-11
Filing date: 2018-09-11
Publication date: 2020-03-12
Also published as: WO2020055265A1

Description

The present invention concerns cut legs on gravity based structures of steel but may in some cases also be used for reinforced concrete.

Furthermore, the invention concerns a method for cutting such structures for decommissioning purposes.

Finally, the invention relates to a saw that is able to perform the cut.

When offshore oilfields are depleted or offshore platforms of some reason are taken out of service, the companies running the installations are obliged to remove or maintain at least parts of the installation. Such decommissioning and removal poses several challenges. One challenge is removal of the topside on GBS offshore installations. Many installations include a topside with a Main Support Frame or MSF fixed to the gravity based structures (GBS) legs. In many cases, these legs must be cut before the MSF can be lifted. Legs of this kind are often made of steel reinforced concrete.

Cutting of GBS legs is proposed in GB2458053 (B), (same applicant). This publication proposes providing a wire saw with a first frame surrounding the leg and moving a second frame with a sawing wire loop in relation to the first frame and cut the leg. The saw spans the entire diameter of the leg. The cut is horizontal and a cut surface is plane.

With the above solution, the cut transition between the upper part and the lower part remains plane, and there is a risk that the upper part / topside move in a horizontal plane under the influence of wind, large waves, due to motion in the structure etc..

In multi axis wire saws adapted for other purposes, are typically the saw stationary or moving along one axis while a workpiece moves along one or more axes.

A cutting process is time consuming, and it is important that the GBS maintains its integrity, only resting on its legs, even after the legs are cut. In some cases, the GBS must remain complete, with its legs cut off for extended periods of time, for instance while waiting for a weather window with a sufficiently calm weather to allow a topside to be lifted off the cut legs.

Accordingly there is a need for a gravity based structure with legs that for a time period after the legs are cut and before the topside is lifted off, maintains its structural integrity to satisfy safety requirements and to prevent a catastrophe.

It is a purpose with the present invention to provide a solution that will enable a GBS to resist a 100 year wave in the above explained cut off state, only resting on its cut off legs due to gravity. Furthermore, it is a purpose to reduce the risk that the topside move sideways due to fluctuating forces from wind and waves. Microscopic movements of the legs may result in side forces acting on the cut interface, allowing the topside to gradually work it way sideways on the legs to the extent that an unpredictable situation may be the result.

The present invention solves this issue by providing a platform with legs that are cut off, still in a horizontal plane. The upper and lower parts are formed simultaneously, and the cutting process will induce negligible motion in the cutting area.

Furthermore, the present invention provides a two axes wire saw.

Cutting of such legs involve a certain risk, and it is an advantage if the cutting can be controlled from a remote location to reduce the risk for operating personnel.

Accordingly, the present invention relates to a gravity based structure with a topside on at least one platform leg with a platform leg wall. The leg includes an upper leg portion cut off from a lower leg portion, the lower leg portion being secured to a foundation, wherein the upper leg portion is separate from and resting on the lower leg portion.

The gravity based structure may include three or more platform legs.

The present invention relates to a leg cutting two-axes wire saw. The wire saw includes a saw main frame and a U-shaped saw frame movably secured to the saw main frame. U-shaped is in the context of the application intended to indicate a structure with two sides and a bottom portion holding the two sides apart and forming a U-shaped cutting area that can be led over a workpiece/ a leg. The shape of the bottom portion may be square or rounded. A sawing wire loop is driven by at least one drive wheel driven by a drive motor. The sawing wire loop includes a first wire loop guide wheel at a first end of the U-shaped saw frame and a second wire loop guide wheel at a second end of the U-shaped saw frame on the U-shaped saw frame whereby the saw wire extends from the first wire loop guide wheel to the second wire loop guide wheel and closes the a U-shaped cutting area. At least at least two first axis slider bars is movably secured along a first axis to at least one slider bar guide. At least one first axis feed actuator is arranged to slide the at least one slider bar guide along the at least two first axis slider bars, and arranged to move the U-shaped saw frame in relation to the saw main frame along the first axis. At least one second axis feed actuator is arranged to move the U-shaped saw frame in relation to the saw main frame along a second axis different from the first axis. A leg gripper claw is secured to the main frame.

The at least two first axis slider bars may be secured to the main frame.

The leg cutting two-axes wire saw may include two slider bar guides, each movably secured along the first axis to a first axis slider bar.

The U-shaped saw frame may be secured to at least one first second axis slider bar, movably secured along the second axis to a first slider bar guide, and least one second - second axis slider bar, movably secured along the second axis to a second slider bar guide.

The U-shaped saw frame may be secured to at least four first second axis slider bars, movably secured along the second axis to the first slider bar guide, and least four second - second axis slider bars, movably secured along the second axis to the second slider bar guide.

The leg cutting two-axes wire saw may include two second axis feed actuators, each secured to one of the slider bar guides.

The leg gripper claw secured to the main frame may include at least one leg gripper claw actuator adapted to operate the leg gripper claw between a gripping position around a leg and a released position.

The drive motor may be secured to the U-shaped saw frame.

Each of the actuators may be electric, self-locking actuators.

Furthermore, the invention relates to method of forming a key cut during a cutting of legs of a gravity based structure on a plurality of platform legs. The method includes the steps of: locating a two-axes wire saw at a cutting location along one of the plurality of platform legs, gripping the one out of the plurality of platform legs with the leg gripper on the two-axes saw, cutting partly through the one of the plurality of platform legs along a first axis along a component at a first angle in relation to a longitudinal axis of the one of the plurality of platform legs, continue cutting partly through the one of the plurality of platform legs along a second axis along a component at a second angle or parallel in relation to the longitudinal axis of the one of the plurality of platform legs, continue cutting partly through the one of the plurality of platform legs along a first axis along a component at a first angle in relation to a longitudinal axis of the one of the plurality of platform legs, continue cutting partly through the one of the plurality of platform legs along a second axis along a component at a second angle or parallel in relation to the longitudinal axis of the one of the plurality of platform legs, continue cutting to completely cut through the one of the plurality of platform legs along the first axis along the component at the first angle in relation to the longitudinal axis of the one of the plurality of platform legs, whereby the leg is cut into a platform leg upper part and a platform leg lower part with keying cut surfaces with a first keying direction, manually or automatically cutting the sawing wire and release the leg gripper, install a new cutting wire on the two-axes wire saw, locate the wire saw on another of the plurality of platform legs, and continue cutting according to the above sequence until all the platform legs are cut.

Subsequent keying directions of the keying cut surfaces of each of the plurality of legs may be equidistant from each other to produce a keying effect in all horizontal directions.

If for instance the GBS includes four legs, then the keying directions are distributed with 90° intervals (360°/4 legs).

Furthermore, the invention concerns a gravity based structure on a plurality of platform legs. Each leg is cut into a platform leg upper part with a keying cut surface resting on a complementary keying cut surface on a platform leg lower part.

The keying cut surface may be shaped as a hat with three horizontal portions and two vertical portions.

“a keying cut surface” is intended to describe a surface on a first structure that upon contact with a corresponding complementary surface on a second structure will prevent the structures to slide in relation to each other at least in two directions.

Short description of the enclosed drawings:

Fig. 1 is a perspective view of a two-axes saw of an embodiment of the invention; Fig. 2a is a side elevation of the saw of fig.1 with a wire saw frame in a first starting position;

Fig. 2b is a side elevation of the saw of fig.1and 2a with the wire saw frame in a second advanced and elevated position;

Fig. 2c shows a detail of dig. 2a;

Fig. 2d shows a detail of fig 2b;

Fig. 3a is a side elevation of the saw as shown in fig.2a in an inverted position and attached to a platform leg;

Fig. 3b is a perspective view of the saw and the platform leg shown in fig.3a; and Fig.3c is a perspective view of the saw and the platform leg with the wire saw frame in an advanced and final position with the sawing wire ready to be cut before removal of the two-axes saw.

Figs. 1-3c show an embodiment of a saw of the invention, and figures 3a-3c additionally show an offshore platform of the invention with a cut platform leg.

The platform leg is a part of an offshore platform (not shown in its entirety).

Fig. 1 is a perspective view of a two-axes platform leg wire saw 1 of the invention. The wire saw incudes a movable U-shaped saw frame 3 with a U-shaped cutting area 6 including an open end. A first guide wheel 8 and a second guide wheel 9 for a sawing wire loop are located at each side of the open end. A drive wheel 5 driven by an electric motor at a corner portion opposite the open end drives the sawing wire loop. Free wheeling idler wheels inside the saw frame 3 provides an extra wire path inside the saw frame to ensure a sawing wire of sufficient length to contain the required amount of diamonds to complete a cut. A tightening pulley (not shown) controls and monitors the wire tension.

The movable U-shaped saw frame 3 is secured to two front slider bars and two rear slider bars at each side of the movable U-shaped saw frame 3. One electric linear second axis actuator 12 is located at each side of the movable U-shaped saw frame 3 between the two front slider bars and the two rear slider bars.

The two front second axis slider bars and the two rear second axis slider bars at each side of the movable U-shaped saw frame 3 are inserted into a slider bar guide 4 at each side of a saw main frame 11 including a first axis slider bar 7 at each side of the saw main frame 11. Each slider bar guide 4 includes one first axis slider bar bore and a second axis slider bar bore for each second axis slider bar. Each second axis linear actuator 12 is secured a respective slider bare guide and to the U-shaped saw frame 3.

The two second axis linear actuators 12 are synchronized to move the U-shaped saw frame 3 up and down along the second axis slider bars.

A first axis feeder linear actuator with a threaded bar 20 drives the wire saw frame 3 along the first axis slider bars 7 allowed to slide by the two slider bar guides 4.

The first axis is perpendicular to the second axis.

Figs. 2a-2d show the saw of the invention from the side in two different positions.

Figs. 2a and the enlarged fig.2c show the wire saw frame 3 in a retracted and low position to the far right in relation to the saw main frame 11. This is the position of the saw before the sawing commences.

Figs. 2b and the enlarged fig.2d show the wire saw frame 3 in an elevated and midway position in relation to the saw main frame 11. This a position of the saw during operation/cutting. The saw could also start cutting in an elevated position

The saw frame could then be positioned at an elevated position at the outer end of the main frame 11.

Figs. 2a, 2b also show the self-locking electric linear actuator 10 actuating the leg gripper claw 2 for holding the saw to an offshore platform leg. The first axis slider bars 7 extend through the longitudinal bores in the slides bar guides 4. The drive motor 17 is fixed to and moves along with the wire saw frame 3.

Figs. 2c and 2d show the slider bar guides 4, the first axis slider bars 7, the second axis slider bars 14-16 and the self locking electric linear actuators 12 in greater detail. These side elevations only show one slider bar guide 4 with actuator 13, first axis slider bar 7 second axis slider bars 14-16 etc as the other slider bar guide with related components are located directly behind the shown slider bar guide 4. The slider bar bores for the first axis slider bar and the four second axis slider bars in each slider guides 4 include low friction materials and / or rolling elements to ensure low friction between the slider bars and each slider guide. The second axis linear actuator 13 includes a longitudinal axis parallel to a longitudinal axis of the second axis slider bars. The second axis linear actuator 13 enables the wire saw frame 3 and thus the sawing wire to be lifted or lowered a distance D in relation to the saw main frame.

In operation, the two-axes saw 1 can saw in steps, operating the first axis feeder actuator and the second axis actuators 12 in a sequence to form a stepped cut. Alternatively, the first axis feeder actuator and the second axis actuators 12 can be operated simultaneously to formed a cut.

Fig. 3a-3c show the saw describes above gripping on to a leg / jacket leg on an offshore installation. The offshore installation includes an upper installation part 27 and a lower installation part 26. A “key” cut provides a keying cut surface 21 performed by a two-axes saw 1 has been lowered by wires or ropes and has been manoeuvred in place to position the two-axes saw 1 at the right location. The saw is placed upside down to locate the leg gripper claw 2 at a location not interfering with bracings to provide the cut surface 21 at the intended location. The cutting is performed above sea level, but the saw can also be made to operate subsea and the cut can be performed below sea level. The key cut surface 21 includes (assuming that the leg to be cut includes a vertical centreline as a reference) a first horizontal portion 22, a first vertical portion, a lowered second horizontal portion 23, a second vertical portion 24 and a third horizontal portion 25. The height of the first and second vertical portions 22, 24 (at both sides of the hollow leg) is up to the distance D the wire saw frame 3 can be lifted in relation to the main frame 11. The first vertical portion 22, the second horizontal portion 23 and the second vertical portion 24 form a hat-shaped leg wall forming a key cut locking the leg in two directions. The key cut of each leg can be located at for instance a 90° degree angle to the neighbouring cut leg to produce locking in four directions. The key cut surface 21 is actually two complementary surfaces resting on each other.

A method of cutting a plurality of legs on an offshore jacket includes:

locating a two-axes wire saw 1 at a cutting location along one of a plurality of jacket legs 30;

gripping the one out of the plurality of jacket legs 30 with a leg gripper 2 on the two-axes saw 1;

cutting partly through the one of the plurality of jacket legs 30 along a first axis along a component at a first angle in relation to a longitudinal axis of the one of the plurality of jacket legs 30,

continue cutting partly through the one of the plurality of jacket legs 30 along a second axis along a component at a second angle or parallel in relation to the longitudinal axis of the one of the plurality of jacket legs;

continue cutting partly through the one of the plurality of jacket legs along a first axis along a component at a first angle in relation to a longitudinal axis of the one of the plurality of jacket legs;

continue cutting partly through the one of the plurality of jacket legs along a second axis along a component at a second angle or parallel in relation to the longitudinal axis of the one of the plurality of jacket legs;

continue cutting completely cut through the one of the plurality of jacket legs along the first axis along the component at the first angle in relation to the longitudinal axis of the one of the plurality of jacket legs,

manually or automatically cutting the sawing wire and release the leg gripper; install a new cutting wire on the wire saw;

locate the wire saw on another of the plurality of jacket legs;

continue cutting according to the above sequence until all the jacket legs are cut.

Claims

1. A leg on gravity based structure-cutting two-axes wire saw comprising:

a saw main frame (11);

a leg gripper claw (2) secured to the main frame (11), wherein the leg gripper claw (2) is secured to the saw main frame (11) and includes at least one leg gripper claw actuator (10) adapted to operate the leg gripper claw (2) between a gripping position around the leg and a released position;.

a U-shaped saw frame (3) with a U-shaped cutting area (6) movably secured to the saw main frame (11);

a sawing wire loop driven by at least one drive wheel (5) driven by a drive motor (17), the sawing wire loop including a first wire loop guide wheel (8) at a first end of the U-shaped saw frame (3) and a second wire loop guide wheel (9) at a second end of the U-shaped saw frame (3) on the U-shaped saw frame whereby the saw wire extends from the first wire loop guide wheel (8) to the second wire loop guide wheel (9) and closes the a U-shaped cutting area (6);

at least one first axis feed actuator (18) arranged to move the U-shaped saw frame (3) in relation to the saw main frame (11) along a first axis,

c h a r a c t e r i z e d i n:

at least one slider bar guide (4) movably secured along a first axis to at least at least two first axis slider bars (7), wherein the at least one first axis feed actuator (18) is arranged to slide the at least one slider bar guide (4) along the at least two first axis slider bars (7); and;

at least one second axis feed actuator (12) arranged to move the U-shaped saw frame (3) in relation to the saw main frame (11) along a second axis perpendicular to the first axis.

2. The leg cutting two-axes wire saw of claim 1, wherein the at least two first axis slider bars (7) are secured to the main frame.

3. The leg cutting two-axes wire saw of claim 1-2, including two slider bar guides (4), each movably secured along the first axis to a first axis slider bar (7).

4. The leg cutting two-axes wire saw of claim 3, wherein the U-shaped saw frame (3) is secured to at least one first second axis slider bar, movably secured along the second axis to a first slider bar guide (4), and least one second second axis slider bar, movably secured along the second axis to a second slider bar guide (4).

5. The leg cutting two-axes wire saw of claim 4, wherein the U-shaped saw frame (3) is secured to at least four first second axis slider bars, movably secured along the second axis to the first slider bar guide (4), and least four second second axis slider bars, movably secured along the second axis to the second slider bar guide (4).

6. The leg cutting two-axes wire saw of claim 4-5, including two second axis feed actuators (12), each secured to one of the slider bar guides.

.

7. The leg cutting two-axes wire saw of any of the preceding claims, wherein the drive motor (17) is secured to the U-shaped saw frame (3).

8. The leg cutting two-axes wire saw of any of the preceding claims, wherein each of the actuators are electric, self-locking actuators.

9. A method of forming a key cut during a cutting of legs of a gravity based structure on a plurality of platform legs with a leg cutting two-axes wire saw of claim 1, including the steps of:

locating a two-axes wire saw (1) of claim 1 at a cutting location along one of the plurality of platform legs (30);

gripping the one out of the plurality of platform legs (30) with the leg gripper 2 on the two-axes saw (1);

cutting partly through the one of the plurality of platform legs (30) along a first axis along a component perpendicular to a longitudinal axis of the one of the plurality of platform legs (30);

continue cutting partly through the one of the plurality of platform legs (30) along a second axis along a component parallel in relation to the longitudinal axis of the one of the plurality of platform legs;

continue cutting partly through the one of the plurality of platform legs (30) along the first axis along the component perpendicular to the longitudinal axis of the one of the plurality of platform legs (30);

continue cutting partly through the one of the plurality of platform legs along the second axis along the component parallel in relation to the longitudinal axis of the one of the plurality of platform legs (30);

continue cutting completely cut through the one of the plurality of platform legs along the first axis along the component at the first angle in relation to the longitudinal axis of the one of the plurality of platform legs (30), whereby the leg is cut into a platform leg upper part (27) and a platform leg lower part (26) with keying cut surfaces with a first keying direction;

manually or automatically cutting the sawing wire and release the leg gripper (2); install a new cutting wire on the two-axes wire saw (1);

locate the wire saw on another of the plurality of platform legs (30); and continue cutting according to the above sequence until all the platform legs are cut.

10. The method of forming a key cut during a cutting of legs of a gravity based structure on a plurality of platform legs of claim 10, wherein subsequent keying directions of the keying cut surfaces of each of the plurality of legs are equidistant from each other to produce a keying effect in all horizontal directions.

11. A gravity based structure on a plurality of platform legs, wherein each leg (30) is cut into a platform leg upper part (27) with a keying cut surface (21) resting on a complementary keying cut surface on a platform leg lower part (26),

c h a r a c t e r i z e d i n that:

the keying cut surface (21) is shaped as a hat with three horizontal surface portions and two vertical surface portions.