WO2001086074A1

WO2001086074A1 - Device and method for the removal of offshore platforms at sea

Info

Publication number: WO2001086074A1
Application number: PCT/DE2001/001752
Authority: WO
Inventors: Marten Fluks; Theo Frans Henri Werners; Marinus Wilhelmus Cornelis Maria Van Den Oetelaar
Original assignee: Mannesmann Rexroth Ag
Priority date: 2000-05-09
Filing date: 2001-05-09
Publication date: 2001-11-15
Also published as: DE10022322A1; AU7386901A; DE10191831D2

Abstract

The invention relates to a method and a device for the removal of the upper section of an offshore platform substructure. According to the invention, the upper section is lifted from the substructure by means of lifting gear and the upper section and the substructure are connected together by means of controllable drawing gear, at least during a load transfer phase and the beginning of the lifting process. The weight and/or the centre of gravity of the upper section are determined from the forces acting on the controllable drawing gear and the lifting gear and the lifting gear and drawing gear are optionally correspondingly controlled. After said load transfer phase the drawing forces introduced by the drawing gear are reduced such that the upper section is lifted from the substructure in a controlled manner by the forces generated by the lifting gear.

Description

description

Device and method for removing offshore platforms at sea

For the exploration and extraction of hydrocarbons stored under the sea floor, such as oil and natural gas, stationary systems are used, which have a jacket and a platform. The jacket is a construction that rests on the sea floor and on which the platform is attached. The platform contains the equipment and systems with which the exploration, extraction and extraction are carried out.

Many of the existing platforms have reached the end of their calculated lifespan and will need to be transported from their location to a coastal shipyard for disassembly or renovation. It is preferred if the platforms can be transported as a whole instead of being broken down into smaller units on site.

In recent years there has been a tendency for the platforms to be transported as a unit. Due to the limited lifting capacity and availability of ship cranes, special ships are required for this purpose, as described in the applicant's subsequently published DE 100 26 727.

Particular difficulties in separating from

Platform and jacket occur in the transition phase between the situation in which the platform is already being carried to a large extent by the ship and only partially by the jacket, and the situation in the . the platform is completely lifted off the jacket. If the forces on the ship are unfavorably distributed, stability and seaworthiness can be severely impaired.

A method for such a separation of platform and jacket according to the prior art is described below with reference to FIGS. 1 and 2.

In Fig. 1 there is a platform 1 on a jacket 2 in the sea. To remove the platform 1 from the jacket 2, two side sections 3a and 3b of a ship are now arranged below end sections of the platform. At least two lifting cylinders 4a and 4b are located on the side sections 3a and 3b of the ship. These lifting cylinders 4a, 4b are connected to hydropneumatic accumulators and act as preloaded springs in the loaded state.

If the side sections 3a and 3b of the

Ship are in a predetermined position with respect to the platform 1, the lifting cylinders 4a, 4b are extended into a position in which the cylinder rods of the lifting cylinders touch the underside of the platform 1. The lifting cylinders 4a, 4b act due to their spring action in such a way that the end portions of the cylinder rods, despite the wave movement of the sea, the platform

1 touch. As a result of the gradual increase in the gas pressure in the hydropneumatic accumulator connected to the lifting cylinders 4a and 4b and the resulting simultaneous increase in the pressure in each lifting cylinder 4a, 4b, an increasing part of the weight of the platform 1 is released from the legs of the jacket

2 to the lifting cylinders 4a, 4b and thus transmitted to the ship. This also requires simultaneous

Adjustment of the ballast situation of the ship. 1 and 2 must be completed at a time when the total load on the lifting cylinders 4a, 4b is less than the weight of the platform 1 and a compressive force still acts on the jacket 2.

In the fast lifting phase that then follows, in which the lifting cylinders 4a, 4b are quickly extended, the relief process is continued and the platform 1 is moved away from the jacket 2 upwards. The draft of the ship will increase temporarily. Each individual cylinder load then increases to a value that results from the mass of platform 1 and the center of gravity. The amount of the cylinder stroke, the extension speed and the change in draft from the ship can only be controlled to a certain extent, so that an impact between the platform 1 and the jacket 2 can occur with undesirable consequences.

Another example of the prior art is described below with reference to FIG. 3.

For various reasons, it has been found useful to transport not only the platform but also an upper section 21 of the jacket together with the platform, the unit consisting of the platform and the upper section 21 of the jacket being referred to below as the platform upper part. In other words, to transport the platform upper part, the upper section 21 of the jacket is detached from the lower section of the jacket. The lower section of the jacket is referred to below as the substructure 22. The platform upper part and the substructure 22 can for example by tie rods 23a, 23b, as shown in FIG. 3.

More specifically, the legs of the jacket are divided in such a way that a cutting line 24 runs between the base 22 and the upper section 21 of the jacket. On opposite sides of the upper section 21 and the lower section 22, support elements 21a, 21b, 22a, 22b are provided near the cutting line 24, which are preferably designed in a triangular shape. The tension rods are fastened to the sections of the support elements which are distant from the upper section 21 or substructure 22 in such a way that a support element 21a, 21b of the upper section 21 from the jacket is connected to a support element 22a, 22b from the substructure 22. The connection between the support elements and the tension rods takes place on one side of the tension rods via a quick release device, e.g. Detonating bolts 24a, 24b in FIG. 3.

3, a force is generated in the hydraulic lifting cylinders (4a, 4b) which is greater than the weight of the platform upper part. A tensile force thus acts on the tension rods. The forces transmitted from the lifting cylinders to the platform are set in such a way that an estimated total mass of the upper platform part and the estimated position of the center of gravity are taken into account by this. Since the actual values deviate from the estimated values, there are negative effects on the heeling and the trim of the ship after lifting off the platform. This is particularly noticeable when catamaran semi-submersibles are used. In the device from FIG. 3, the lifting process begins with the detonating bolts being triggered. The ship rises from the water. At the same time, the lifting cylinders continue to extend due to the pressure conditions in the hydropneumatic accumulator until a state of equilibrium is reached between the forces on the lifting cylinders and the weight of the platform upper part. Due to the complete separation of the platform upper part from the substructure and due to the distance between the platform upper part and the substructure, the platform upper part and the ship can move as a unit. The disadvantage in this case is that the movements of the upper part of the platform and the ship cannot be controlled during the lifting process.

The invention has for its object to provide a method and an apparatus for lifting an upper part from a substructure of an offshore platform, by means of which a controlled lifting of the upper part of the platform is possible even under unfavorable environmental conditions, such as strong wind or sea conditions.

This object is achieved in terms of the method by the combination of features of claim 1 and in terms of the device by

Combination of features of the independent claim 4 solved.

According to the invention, the upper part and the substructure are connected to one another by controllable pulling devices during the lifting process. The stroke submissions are controlled in such a way that the lifting force applied exceeds the weight of the upper part and the pulling devices are loaded under tension. That is, , the top is practical between the Clamping and pulling devices clamped. The center of gravity and the weight of the upper part can then be determined from the resulting loads on the lifting and pulling devices, so that a defined position of the upper part can be set by suitable control of the lifting and / or pulling devices. This is also possible when the sea is comparatively strong or windy. After this load transmission phase, the effective length of the pulling devices is increased and thus the transmitted pulling force is reduced so that the upper part lifts off the substructure in a controlled manner.

The advantage of the solution according to the invention is that not only is the upper part held in a controlled position by the pulling and lifting devices during the entire load transmission phase, and that a stable equilibrium position is ensured even under unfavorable environmental conditions, but especially in that by a controlled movement the towing equipment enables controlled lifting. The pulling devices are only decoupled when the upper part is stably supported on the lifting devices.

According to the invention, it is preferred if the lifting devices are formed by hydraulically actuated lifting cylinders which are prestressed in the lifting direction by means of a pressure accumulator, preferably a hydropneumatic accumulator.

The traction devices are also preferably formed by hydraulically actuated traction cylinders, with in a preferred embodiment each leg of the platform being assigned two traction cylinders arranged diametrically to one another. It is also possible to arrange more than two pull cylinders per leg.

As an alternative, an unlockable check valve is arranged between each pressure accumulator and the lifting cylinder, so that retraction of the lifting cylinder is reliably prevented during the lifting process.

Further developments according to the invention are the subject of the dependent claims.

The invention is described below with reference to the accompanying drawings, in which:

1 shows a schematic representation of a platform mounted on a jacket in the sea according to the prior art,

2 shows the basic structure of a ship with lifting cylinders according to the prior art, FIG. 3 shows a device according to the prior art,

Figure 4 shows a device according to the present invention, and Figures 5a and 5b show the forces acting on the platform.

The structure of an offshore platform according to the present invention corresponds in essence to the structure shown in FIGS. 1 and 2. In contrast to the prior art, as shown in FIG. 3, the tension rods from FIG. 3 are replaced by tension cylinders 33a, 33b. These pull cylinders 33a, 33b are, as shown in FIG. 4, fastened to support elements 21a, 21b and 22a, 22b. A pulling force is exerted via these pulling cylinders, which acts between the upper part 21 and the substructure 22. The attachment to the support elements 22a, 22b takes place in the same way as in FIG. 3 via quick release mechanisms, for example explosive bolts, via which the pull cylinders 33a, 33b can be decoupled from the substructure 22.

Cylinder spaces 34a, 34b and annular spaces 35a, 35b of the traction cylinders 33a, 33b can be connected to a pressure medium source (pump) or a tank, so that pressure medium can be supplied or removed in order to extend or retract the cylinders and thus the acting tensile forces depending to control the lifting force of the lifting cylinders 4a, 4b, the weight and center of gravity of the upper part 21 and the external forces (rough sea, wind, ship movements).

The lifting cylinders 4a, 4b shown in FIG. 2 for lifting the upper platform part, which are also used in the exemplary embodiment according to the invention in accordance with FIG. 4, must be designed such that they can support the upper platform part plus the tensile force applied by the pulling cylinders 33a, 33b, whereby disturbances are taken into account over a tolerance range. A force must be absorbed by the pull cylinders 33a, 33b, which overall is substantially less than the weight of the upper part 22, so that this fact must be taken into account when designing the pull cylinders with a view to saving costs.

The load transmission phase and the lifting process for an offshore platform with a removal device according to the exemplary embodiment according to the invention from FIG. 4 will now be described.

To lift the platform, the side sections 3a and 3b of the ship are brought into the position shown in FIG. 2 with respect to the platform 1 and the Lift cylinder 4a, 4b extended so that lifting forces are transmitted to the upper part. The lifting cylinders 4a, 4b are biased against the platform 1 due to the spring action of the or the associated hydropneumatic reservoir, even in rough seas. Due to the gradual increase in gas pressure in the hydropneumatic accumulators connected to the lifting cylinders 4a and 4b and the consequent increase in pressure in each lifting cylinder 4a, 4b, an increasing part of the weight of the platform 1 becomes from the legs of the jacket 2 to the lifting cylinders 4a, 4b and thus transmitted to the ship. This also requires a simultaneous adaptation of the ship's ballast situation. This load transmission phase is carried out until the lifting cylinders 4 deliver a total force which exceeds the weight of that of the upper part 21 and thus the cylinders 33 pass from the originally unloaded state into a tensile stress phase. The pressures in all cylinders can now be measured using pressure transducers attached to the cylinders. In this way, results are obtained even when the sea is rough, which allows the weight of the upper part 21 and its center of gravity to be determined.

Alternatively or additionally, instead of the pressures in the cylinders, load cells or the like can also be used. measure the forces on the cylinders to determine the weight and location of the center of gravity.

It should be noted that the determination of the weight and position of the center of gravity takes place at a time when the platform upper part is still connected to the substructure via the closing cylinders 33a, 33b. In order to increase the reliability of the result obtained, the pressures are measured in all Lift cylinders and pull cylinders at the same time and not only at one point in time, but practically for a longer time, so that a larger amount of data is available in order to obtain correct mean values of the measured lifting and pressure forces.

The perpendicular forces are shown schematically in Figures 5a and 5b. FIG. 5 a shows a greatly simplified plan view of the platform, the lifting forces applied by the lifting cylinders 4a, 4b being designated by Fc, while the tensile forces applied by the pulling cylinders 33a, 33b are identified by Ft. The larger circles shown hatched each represent a section through a leg 21, 22 of the platform. The larger unshaded circles represent the points of attack of the lifting cylinders 4a, 4b, while the smaller unshaded circles indicate the points of attack of the pull cylinders 33a, 33b. Accordingly, four lifting cylinders are provided in the model shown in FIG. 5a, by means of which the upper part 21 is supported. The substructure is supported on the seabed by four legs. Each of the legs are assigned two pull cylinders 33, so that the upper part

21 during the load transmission or lifting process via a total of eight pull cylinders 33 relative to the substructure

22 is held.

The result of the pressure / force determinations in the course of the load transmission phase is shown in dash-dot lines in FIG. 5a in a greatly simplified form. Accordingly, the resulting lifting cylinder force Fc does not lie in the middle between the points of application of the lifting cylinders, but in the illustrated embodiment is displaced to the bottom right, while the resulting tensile force Ft is displaced somewhat to the top left in the load transmission phase. The resulting tensile and compressive forces are connected by the dash-dotted line. 5a, the lifting force Fc moves towards the viewer, while the pulling force Ft is directed downwards away from the viewer.

At the end of the load transfer phase, the upper part is practically clamped between the lifting cylinders on the one hand and the traction cylinders on the other. FIG. 5b shows the resulting forces with their respective calculated positions. That is, , The pulling force Ft is removed with a (still unknown) lever arm b from the weight of the upper part, while the resulting lifting force Fc has a (known) lever arm with respect to the pulling force Ft.

According to figure 5b:

F _G = F _ct -F _tt and b = - - - a

so that the weight and the center of gravity can be determined from the measured values mentioned.

The calculated weight and position of the center of gravity is now used to calculate and set target values for the cylinder forces of the lifting cylinders 4a, 4b, by means of which a uniform lifting of the platform upper part can be ensured and an undesired heeling and change of the trim can be prevented.

Then the pressures in the memories of the lifting cylinders 4a, 4b are adjusted accordingly to the to generate calculated cylinder forces within a certain tolerance range. At the same time, the pressure conditions in the pull cylinders 33a, 33b also change, so that the ballast situation of the ship must also be checked and adjusted if necessary. The load distribution on the platform can also be adapted to the relevant requirements. Then, however, the weight and position of the center of gravity of the upper platform part must be determined again. The entire process is controlled, while the upper platform part is still mechanically connected to the substructure. After this setting of the pressures in the lifting cylinders, the load transfer phase is complete. The upper part 21 wants to lift through the set lifting forces, but this is prevented by the pull cylinder 33. These are then controlled in such a way that they extend in a controlled manner and the upper part 21 is lifted off the jacket leg 22.

By short-circuiting the pressure chambers 34a, 35a and 34b, 35b, a free movement of the upper platform part with respect to the substructure within the lifting range of the lifting cylinders can be ensured from a certain height.

Now the explosive bolts 24a, 24b or other quick release devices are triggered, whereby the platform upper part can be moved independently of the substructure and moves with the ship. After proper seaworthy attachment of the platform upper part to the ship, the overseas transport can take place.

According to a second exemplary embodiment, the traction cylinders from FIG. 4 are replaced by pressure cylinders which absorb the tensile forces between the substructure and the upper platform part in the form of compressive forces. The other structure and function of the device of the second Embodiment essentially corresponds to that of the first embodiment, so that a detailed description can be omitted.

In a third exemplary embodiment, a releasable check valve is provided in the device according to the first exemplary embodiment between the hydropneumatic reservoir and the lifting cylinders 4a, 4b, by means of which the relative position of the upper part of the platform and the ship can be determined in one direction following a relative movement of these. The hydraulic fluid can flow from the accumulator to the lifting cylinder, but reverse flow in the opposite direction is blocked. In other words, the position of the platform top with respect to the ship is frozen. This is preferably carried out during an upward movement of the ship in the lowest position of the ship.

After freezing the movement, the lifting cylinder forces are increased until the pull cylinders 33 are loaded in the pulling direction and remain constantly under tension even when the shaft is moving. This is done by supplying pressure medium into the individual lifting cylinders 4a, 4b. In principle, the determination of the dead weight and the center of gravity takes place as described above. After that, the lifting cylinder forces have to be adjusted by supplying pressure medium to ensure an optimal lifting process in the individual cylinders.

The lifting phase then takes place analogously to the exemplary embodiment described at the beginning. The lift-off phase is the same as in the first and second exemplary embodiments third exemplary embodiment ended with the release of the explosive bolts.

The invention relates to a method and a device for removing an upper part from a substructure of an offshore platform. According to the invention, the upper part is lifted off the substructure by means of lifting devices, the upper part and the substructure being connected to one another via controllable pulling devices, at least during a load transmission phase and the initiation of the lifting process. From the forces acting on the controllable pulling device and the lifting device, the weight and / or center of gravity of the upper part are determined and, if necessary, the lifting and pulling devices are readjusted accordingly. After this load transmission phase, the tensile force introduced via the traction device is reduced, so that the upper part lifts off the substructure in a controlled manner by the force acting via the lifting device.

Claims

Expectations

1. A method for removing an upper part (21) from a substructure (22) of an offshore platform with the

steps:

Raising the upper part (21) with respect to the substructure (22) by means of a lifting device (4a, 4b) supported on a floating body (3a, 3b), the upper part (21) and the substructure (22) using a controllable pulling device (33a, 33b) ) are connected to one another and the weight and center of gravity of the upper part (21) are determined from the forces transmitted via the pulling and lifting devices (4a, 4b; 33a, 33b).

2. The method according to claim 1, wherein the lifting devices (4a, 4b) are controlled until the applied lifting force exceeds the weight of the upper part (21) and the pulling devices (33a, 33b) are loaded under tension, and wherein the pulling force of the pulling devices (33a, 33b) is then reduced such that the upper part (21) lifts off the substructure (22) and the traction devices (33a, 33b) are decoupled after the upper part (21) has been lifted off.

3. The method according to any one of the preceding claims, wherein the lifting and pulling devices (4a, 4b; 33a, 33b) are formed by hydraulic cylinders which, depending on the weight of the upper part, the center of gravity of the upper part and on the ambient conditions such as rough seas, Wind force etc. and the forces acting on the upper part as a result can be controlled.

4. Device for performing the method according to one of the claims 1 to 3, with lifting devices (4a, 4b) supported on a floating body (3a, 3b), for example a ship, via which the upper part (21) can be lifted off the substructure (22) , characterized by traction devices which are arranged between the upper and lower part (21, 22) and whose tractive force can be controlled.

5. The device according to claim 4, wherein the lifting device has a plurality of lifting cylinders (4a, 4b) which are prestressed in the lifting direction by means of pressure accumulators, preferably hydropneumatic accumulators.

Apparatus according to claim 5, wherein an unlockable check valve device is provided between the lifting cylinder (4a, 4b) and the hydraulic accumulator.

7. Device according to one of claims 4 to 6, wherein the lifting / pulling cylinders (4a, 4b; 33a, 33b) are connected to a pressure medium source, for example a hydraulic pump, via a valve arrangement for controlling the entry and exit movement.

8. Device according to one of the claims 4 to 7, wherein the traction devices are hydraulically operated traction cylinders (33a, 33b) or pressure cylinders with deflection.

9. Device according to one of the claims 4 to 8, wherein each lifting and / or pull cylinder (4a, 4b; 33a, 33b) is assigned a pressure sensor or a load cell for detecting the pressure or the transmitted force.

10. Device according to one of the claims 5 to 8, wherein each leg of the platform is assigned two pull cylinders (33a, 33b) arranged on both sides of the respective leg.

11. Device according to one of the claims 5 to 8, wherein each leg of the platform is assigned more than two rotationally symmetrically arranged pull cylinders.