NL2004357C2

NL2004357C2 - Method for the construction and installation of an offshore platform, offshore platform.

Info

Publication number: NL2004357C2
Application number: NL2004357A
Authority: NL
Inventors: Dieter Korndorffer
Original assignee: Korndorffer Contracting Internat K C I B V
Priority date: 2010-03-08
Filing date: 2010-03-08
Publication date: 2011-09-09

Description

Title: Method for the construction and installation of an offshore platform, offshore platform.

The present invention is related to a semi permanent platform which is designed to be installed without the requirement of a crane vessel.

Such a platform is known from the Dutch patent 1005463. Also several of such platforms are being operated in the North Sea, some known 5 as Mobile Offshore Application Barge (MOAB) and others as Multi Purpose Platform (MPP). These platforms comprise a floating pontoon and four retractable legs, which can be lowered and lifted by a system which is driven by hydraulic or electrical power. The lower end of each leg is fitted with suction cans, which can penetrate the sea bottom and provide a fixed 10 anchoring to the bottom. If the legs are lowered in relatively shallow water, at some stage the legs will reach the seabed and the suction cans are penetrated in the sea bottom. As the legs are moved further by force, the pontoon will be elevated out of the water. The purpose of lifting the pontoon out of the water is to obtain a stable fixed structure, which is not subjected 15 to wave loads and which can therefore safely stay in place offshore, also during storm conditions. The pontoon provides a stable deck on which sheltered areas can be made e.g. for oil or gas production facilities or for systems of an electrical power network for wind turbines. These pontoons with retractable legs fitted with suction cans are further referred to as “Self 20 Installing Platform”, which is abbreviated as “SIP”. The SIP’s that are known have a number of disadvantages, which are described in the following.

A first disadvantage (1) of existing SIP’s is that the assembling method of the legs and the pontoon is complicated and expensive, e.g.

25 because assembly is to be done in dry-dock.

2

In the elevated position of the pontoon, the legs are rigidly connected to the pontoon as the SIP should be capable of staying in place for period of several years. Storm conditions will cause high loadings on the connections of the legs. One existing solution is to weld the leg onto the pontoon by 5 bracket plates. Welding is expensive and time consuming and moreover the welds will also have to be cut in order to release the leg before removal of the SIP, which is also time consuming and causes damage to the steel structure as the heat input of the steel cutting device deteriorates the quality of the steel. This is a second disadvantage (2).

10 Existing examples of a SIP have suction cans at the legs which limit the highest position of the leg relative to the pontoon when the top of the can is at the bottom of the pontoon. Therefore the draft of the floating unit is increased by the height of the cans. This is a third disadvantage (3) since it limits the accessibility of shallow locations such as a harbour or a dry-dock. 15 Another aspect is that certain existing examples of a SIP use hydraulic clamps around the legs to move the legs. A fourth disadvantage (4) is construction and installation of these systems is expensive.

Some known examples of SIP’s use strand-jacks acting in opposite directions for the movement of the legs relative to the pontoon. Controlling 20 the tension of the two strand jacks when moving simultaneously is complicated, which is a fifth disadvantage (5).

The object of the present invention is to at least partially eliminate one or more of the above drawbacks and/or to create a useable alternative.

The objective is achieved according to claim 1 by a simplification of 25 the assembling method of the legs, in which the requirement of expensive harbour facilities is minimized. Reference is made to the first disadvantage 1 described above.

For the assembly of certain known SIP’s, the legs are to be up-ended and lifted inside vertical holes, which are the leg guides at the corners of the 30 pontoon. After the legs have been placed, the suction cans are to be welded 3 to the legs. This is time consuming and can only be performed if the pontoon is placed in dry-dock. In this case the accessibility for welding is difficult. Alternatively, welding of the cans can be done in dry-dock, but this is expensive and time consuming. For other existing arrangements of SIP’s, 5 legs are upended and welded to a sub-frame, which connects all four legs. This procedure can also only be done in a dry-dock. In a preferred embodiment of the invention the suction cans are welded to the legs before up-ending. This can be done on shore with optimum access for the welders. The legs are up-ended after completion of welding and positioned vertically 10 at positions in recesses at the four corners of the pontoon. Preferably this is done while the pontoon is on deck of a floating cargo barge, such that enough vertical space is available for the suction cans. Box shaped structures providing a closing piece comprising the supplementary radial part of the guide element are brought into place enclosing the legs, which 15 are welded to the pontoon to form vertical tubular guides of the legs. The supplementary radial part cooperates with the radial part on the pontoon.

In this arrangement the access for welding at the pontoon is optimum. In this way the tubular guides are formed through which the legs are guided during adjustment of the legs between the transport position and the 20 installation position, wherein a radial part of the guide element is arranged on at least one corner of the pontoon and a connecting structure connects the radial part to the pontoon. The radial part on the pontoon can be arranged as a recess at a corner of the pontoon to improve strength and minimize the steel volume.

25 Another preferred embodiment of the invention comprises wedges between the legs and the pontoon and vertical tension bolts. The wedges can be moved vertically to eliminate the clearance between the legs and the pontoon. The wedge movement can be done e.g. by the application of bolts and nuts or by hydraulic cylinders. As the horizontal clearance has been 30 eliminated, the horizontal relative movement between the legs and the 4 pontoon is no longer possible. At the top of the legs a yoke structure is present which is used to hang-off the weight of the pontoon vertically by tension bolts. By the wedges and the tension bolds the horizontal and the vertical connection between the legs and the pontoon is fixed, such that the 5 unit will stand stable. By using the described wedges and tension bolts, the fixed connection between the legs and the pontoon is obtained without welding. Reference is therefore made to the second disadvantage 2 described above.

In another embodiment of the invention suction cans are welded 10 eccentrically to the legs wherein the radial part on the guides at the pontoon are arranged elevated with respect to the pontoon, such that the legs with the suction cans can be moved further up. As a result the draft of the floating unit is reduced, such that the access to shallow water locations is enabled. Reference is therefore also made to the third disadvantage 3 15 described above.

For the lifting of the pontoon and the lowering of the legs, the SIP according to the invention utilizes hydraulic lifting units known in the industry as “strand jacks”, instead of hydraulically activated clamps around the legs; reference is made to the fourth disadvantage (4). The advantage of 20 these strand jacks is that they are smaller and cheaper than the hydraulic clamps of the legs. Also strand jacks are multi-purpose tools, which can be rented on the market and which are to be removed after the installation of the SIP. This is much more economic than permanent systems for the movement of the legs. It is noted that the invention described in patent 25 1005463 uses clamps which can also be removed after installation, but these clamps are not multi-purpose, but one set is specifically for one or more SIP units, and therefore has to be designed and manufactured for this purpose.

A double set of strand jacks working in opposite directions is used to move the legs up or down, such that forces can be generated both upward, to 30 lift the pontoon up, and downward, to lower the legs. The strand jacks for 5 lifting and lowering can each work on a separate wire bundle. In a preferred embodiment of the invention the two or more strand jacks for lifting and lowering work in line on one wire bundle. The advantage of this arrangement is that the wire bundles can be pre-tensioned by a separate 5 system such that no slack wire occurs during lifting or lowering of the legs. Some existing SIP’s have strand jacks in an arrangement where oppositely working strand-jacks are not in line and are not working on the same wire bundle. In such a prior art arrangement the wires pre-tension is to be controlled actively by the strand jacks, also during movements. This control 10 system is more complicated, which is a fifth disadvantage 5.

In a special embodiment of the invention, a base frame is installed on the seabed on which the legs of the platform are lowered. The base frame can provide extra strength and it allows the installation of the platform in larger water depth.

15 In certain applications one or more tie-ins of cables or pipelines are required which are running from the sea bottom to the pontoon. In order to support and guide these cables or pipelines, and also to protect them from impact loads, the installation of a protection structure surrounding the cables or pipelines is required. The construction and installation of these 20 structures is expensive en time consuming. A preferred embodiment of the invention reduces time and costs by utilizing the fact that the legs of SIP’s are large diameter pipes; one or more legs are fitted with J-tubes, which guide one or more pipes or cables in a controlled curve inside the leg, such that no additional protection structures are required.

25 In another embodiment of the invention the protection structure comprises a large diameter pipe which is installed vertically at one side of the pontoon. The protection pipe is connected at the sea bed e.g. by one or more suction cans which are connected to the protection pipe. The protection pipe can be connected to the pontoon to provide a stable support. The 30 protection pipe can be installed before the platform is installed; this offers 6 the option that pipelines or cables are also installed before the platform is installed, such that the connections to the platform can be made faster and operation can start earlier. The protection pipe can also be installed after the platform is installed; in this case the advantage is that the protection 5 pipe can be transported to location on the platform and the installation can also be done from the platform. The cables pipelines are guided into the protection pipe similar to the J-tubes described above. An alternative connection for pipelines is a flange connection to pre-installed piping inside the protection pipe.

10 In another embodiment of the invention, the protection pipe is part of the base frame mentioned above. The protection pipe is also used as a leg of the pontoon. The protection pipe is fitted with strand jacks, as described above, to lift the pontoon out of the water.

In a special embodiment of the invention, the internal compartments 15 of the pontoon are utilized for the installation of equipment. In case of equipment which is sensitive to the temperature, moisture and saltiness of the environment, such as electrical and electronic equipment, the closed compartments are fitted with air conditioning systems. Access of large components of the equipment into and out of the compartments is created by 20 hatches in main deck of the pontoon.

In another special embodiment the pontoon is fitted with an opening through the deck to the bottom of the pontoon, generally known as a moon pool. This opening allows e.g. oil- or gas drilling operations or subsea installation work from main deck through the moon pool.

25 The invention further relates to an offshore platform.

Further preferred embodiments are laid down in writing in the remaining sub conclusions.

Further explanation of the invention will be given by means of some application examples represented in figures below, which will give a 7 practical embodiment of the invention, but may not be considered in a limiting sense, wherein:

Figure 1 shows a side view of a SIP comprising the main components: pontoon, legs and suction cans in a floating condition according 5 to the prior art;

Figure 2 shows a side view of a SIP in an installed condition according to the prior art;

Figure 3 shows a side view of one stage of the conventional assembling procedure in dry dock of a leg of a SIP when lifting the leg in 10 vertical position through the guide in the pontoon according to the prior art;

Figure 4 shows a side view and top view of assembling a leg following the same procedure as in figure 3 in a floating condition according to the prior art;

Figure 5 shows a side view of one stage of a conventional assembling 15 procedure of the base frame connection of the legs according to the prior art;

Figure 6a and Figure 6b show a top view and a side view of one stage of the assembling when a leg is placed at the pontoon according to the invention;

Figure 7a and Figure 7b show a top view and a side view the next 20 step after figure 6a and figure 6b when the closing piece is aligned and connected to the pontoon according to the invention;

Figure 8 shows a side view of two strand-jacks acting in line on one strand bundle according to the invention;

Figure 9 shows a side view of two strand-jacks acting on separate 25 strand bundles according to the prior art;

Figure 10 shows a cross section of a clamp with wedges for horizontal fixed connection between the leg and the pontoon and tension bolts to hang-off the weight of the pontoon vertically, according to the invention; 8

Figure 11 shows a side view of suction cans which are placed eccentrically and leg-guides at an elevated position according to the invention;

Figure 12 shows a cross section of J-tubes in one leg for tie-in of 5 cables or pipelines according to an aspect of the invention;

Figure 13 shows a SIP with a base frame on the sea bed according to an aspect of the invention;

Figure 14 shows a SIP with a protection pipe at one side of the pontoon according to an aspect of the invention; 10 Figure 15 shows a SIP with a base frame and a protection pipe which is a fixed part of the base frame and which replaces one leg according to an aspect of the invention;

Figure 16 shows a cross section of a SIP at the location of a moon pool according to the invention.

15 It is noted that the figures are only schematic representations of embodiments of the invention that are given by way of non-limiting example. In the figures, the same or corresponding parts are designated with the same reference numerals.

The pontoon 1 and legs 2, suction cans 3 and longitudinal guides 4 in 20 floating condition are presented in figure 1. The legs 2 are in transport position.

Figure 2 shows that the legs 2 are lowered to the sea bottom, whereby the suction cans 3 penetrate the sea bottom; the legs 2 are moved further down by which the pontoon 4 is lifted out of the water. Finally the legs 2 and 25 the pontoon 1 are connected whereby the structure becomes a stable platform 100. The legs 2 are in the installation position.

Figure 3 shows a side view of one stage of the conventional prior art assembling procedure of the legs 2 to the pontoon 1 in a dry dock. The pontoon 1 is placed on a support structure 6 such that the bottom of the 30 pontoon 1 is high enough above the floor to put the suction can 3 under the 9 pontoon. The leg 2 is lifted by a crane 5 in vertical position through the guide 4 in the pontoon and is to be aligned with the suction can 3 in order to make a connection at location 7, for example by welding.

Figure 4a and Figure 4b show the same procedure as in figure 3, also 5 according to the prior art, but in a floating condition where the pontoon 1 is placed on a barge 6, such that the connection between the legs 2 and the suction cans 3 can be made above the water. The positioning of the floating pontoon 1 and floating suction can 3 is more complicated than in dry dock and the access for connecting the suction can 3 to the leg 2 is more 10 restricted.

Figure 5 shows a side view at one stage of another conventional prior art assembling procedure in dry-dock wherein the legs 2 are connected to a base frame 8. The pontoon 1 is placed on a support structure 6 such that the bottom of the pontoon is high enough above the floor to put the suction can 3 15 under the pontoon 1. The leg 2 is moved sideways in vertical position and as such is placed at the base frame 8, e.g. by a crane as in figures 4a and 4b, and welded to the base frame. Before the connection is made, the base frame 8 and the leg 2 are to be aligned. The base frame 8 is welded to the leg 2 at locations 9. In this prior art technique the leg 2 is not enclosed and guided 20 by tubular guides through the frame 8, but is simply positioned aside of the frame 8 and welded to the frame 8.

Figure 6a and figure 6b show the assembling of a leg 2 with the pontoon 1 according to the invention. The suction can 3 has been connected to the leg 2 prior to the assembling with the pontoon. In this way, the dry 25 dock or a second barge are not required as the connections which are to be made are above the water. The leg is positioned at a recess 12 of the pontoon and kept in place at the mounting location e.g. by slings or temporary hooks. The recess 12 comprises a radial part of the guide element 4.

Figure 7a and figure 7b show the next step of the assembling 30 procedure of Figure 6a and figure 6b. The closing piece 13 is aligned and 10 connected to the pontoon 1, e.g. by bolts or by welding. The closing piece 13 comprises the supplementary radial part to form the guide element 4 to cooperate with the radial part 12 on the pontoon to form the guide element 4. The weight of the closing piece 13 is relative small, which makes it easier 5 to handle if compared to the large parts which are to be lifted in conventional assembling methods, as shown in previous figures 3, 4, 5.

Figure 8 shows two strand-jacks 14 and 15 in an arrangement according to the invention, wherein the strand jacks 14, 15 are acting in line in opposite directions on one wire bundle 16. The strand jack 14, which is 10 mounted such that is capable of pulling a bundle of strands 16 upward (according to the arrow), and a strand-jack 15, which is mounted such that is capable of pulling the same bundle of strands 16 downward. The bundle of strands 16 is pre-tensioned by a pulling device 17 between the top and bottom of the leg 2 in order to prevent the strands 16 to become slack if a 15 vertical load is acting on the support 18, which is rigidly connected to the pontoon 1. Slack strands are to be avoided for proper functioning of the strands and also to avoid impact loads when the slack strands are tensioned after reversal of the load. In this arrangement, an upward force at the pontoon 1 is by the action of strand jack 15, while de strand jack 14 is not 20 active. A downward force of the pontoon is by the action of strand jack 14, while de strand jack 15 is not active. Therefore only one strand jack is to be controlled at the time, depending on the direction of the load acting on the pontoon.

Figure 9 shows two strand jacks 14 and 15 in a prior art 25 arrangement wherein the strand jacks are acting in opposite directions, as in figure 7, but the strand jacks are acting on separate bundles of strands 16. For the proper functioning of the system, the bundle of strands is to be pre-tensioned by pulling at both strand jacks at all times. Therefore both strand jack forces F14 and F15 are to be controlled simultaneously and also 11 the movement of both units is to be synchronized by active control of both strand jacks.

Figure 10 shows a cross section of a clamp with wedges 19 for applying a horizontal fixed connection between the leg 2. In this example 5 the wedges are engaged by turning the nuts 21 of the tension bolts 20; alternatively the wedges 19 are engaged e.g. by hydraulic cylinders. The pontoon 1 and one or more tension bolts 22 are used to hang-off the weight of the pontoon vertically from a yoke 24 on top of the leg 2, according to the invention. In this example the length of the bolts 20 can be adjusted by the 10 nuts 23; alternatively this can be done e.g. by hydraulic cylinders.

Figure 11 shows a side view of a SIP 100 with the legs 2 in the transport position whereby the suction cans 3, 27 are placed eccentrically and leg-guides 4, formed by the connecting structure 25 and a closing piece 13, 26, are at a higher elevation according to the invention. An advantage in 15 this arrangement is that the suction cans 3 can be lifted higher, such that the draft of the floating structure is smaller.

Figure 12 shows a cross section of J-tubes 29 in one leg for tie-in of cables or pipelines 28. As the cables or pipelines are inside the leg 2, they are protected against external loads.

20 Figure 13 shows a SIP with a base frame 8, 30 on the sea bed. The base frame is pre-installed, e.g. by crane vessel, and connected to the sea bottom e.g. by suction cans 3, 31 (as in the figure) or by piles driven vertically in the soil. The base frame 8, 30 is used as a stable support for the SIP 100 and enables the installation in deeper water.

25 Figure 14 shows a SIP 100 with a protection pipe 32 at one side of the pontoon. The protection pipe 32 is connected at the sea bed e.g. by one or more suction cans 33, as shown in the figure 14. The figure 14 also shows J-tubes 29 at the bottom as described above.

Figure 15 shows a SIP 100 with a base frame 8, 30 and a protection 30 pipe 34 which is also used as a leg supporting the pontoon 1. As the base 12 frame 8, 30 with the protection pipe 34 are pre-installed, cables or pipelines can be installed and tied-in some time before the SIP is installed; this speeds up the start of operation.

Figure 16 shows a cross section of a SIP 100 at the location of a moon 5 pool 35 according to the invention.

Claims

A method for the construction of an offshore platform, comprising a floating pontoon with legs that is built and assembled in the port or on land as a complete unit and that is towed to a location at sea and installed by temporary systems for raising and lowering the legs, the legs being mounted on the pontoon by placing the legs in substantially vertical position at the mounting locations of the pontoon, after which the legs are enclosed by forming a longitudinal guide of the legs by aligning and attaching at least one retaining piece to the pontoon, such that the legs can be moved in the longitudinal direction of the legs through the longitudinal guides.

A method for the construction and installation of the invention according to claim 1, wherein hydraulic cable jacks are used to move the legs relative to the pontoon in a composition where at least two hydraulic cable jacks operate in opposite directions on the same bundle of cables, which bundle of cables is kept under pre-stress by a separate system.

The method for the construction and installation of the invention according to claim 1, wherein wedges are used for the semi-permanent horizontal attachment of the legs to the pontoon after the pontoon has been raised to the required elevation above sea level.

The method for the construction and installation of the invention according to claim 1, wherein bolts are used for the semi-permanent vertical attachment of the legs to the pontoon after the pontoon has been raised to the required elevation above sea level.

5. Method for the construction and installation of the invention according to claim 1, wherein suction anchors are attached eccentrically with respect to the axis of the legs and the guides on the pontoon are placed higher, such that the top of the suction anchors is above the bottom of the pontoon can be lifted.

6. Method for the construction and installation of the invention according to claim 1, wherein at least one cable or pipeline is pulled from a horizontal position on the seabed to a vertical position in the legs by a tube that forms a uniformly curved guide for the cable or pipeline.

7. Method for the construction and installation of the invention according to claim 1, wherein compartments in the pontoon are used for the installation of devices for the production of oil or gas or devices for the transformation of electrical power of at least 500 kW.

8. Method for the construction and installation of the invention according to claims 1 and 7, wherein the compartments in the pontoon are accessible to devices through at least one hatch in the upper deck of the pontoon, which is at least one meter long or wide .

9. Method for the construction and installation of the invention according to claim 1, wherein the pontoon is equipped with an opening through the deck up to the bottom of the pontoon.

The method for the construction and installation of the invention according to claim 1, wherein the legs are lowered onto a base frame that is pre-installed on the seabed.

11. Method for the construction and installation of the invention according to claim 1, wherein a tube is placed vertically against one side of the pontoon between the pontoon and a mounting on the seabed, which is intended for pipeline or cables that are retracted from enclose, guide and protect the seabed to the pontoon.

The method for the construction and installation of the invention according to claims 1, 10 and 11, wherein the tube is attached to the base frame for protection.

Method for the construction and installation of the invention according to claims 1, 10 and 12, wherein the protection tube forms part of the basic frame and replaces one leg.

14. Offshore platform comprising a floating pontoon and at least one leg, the leg being adjustable between a transport position, in which the leg is raised relative to the pontoon, and an installation position, in which the leg is lowered relative to the pontoon , further comprising a tubular guide element through which the leg is guided during adjustment of the position of the leg between the transport position and the installation position, wherein a radial portion of the guide element is arranged at at least one corner of the pontoon.

The offshore platform according to claim 14, further comprising a closing piece comprising the additional radial portion of the guide element cooperating with the radial portion of the pontoon to form a tubular guide for at least one leg.

The offshore platform according to claim 14 or 15, wherein the radial portion on the pontoon is arranged as a recess in the corner of the pontoon.

17. Offshore platform as claimed in any of the claims 14-16, wherein the radial part on the pontoon is placed raised relative to the pontoon.

18. Offshore platform as claimed in any of the claims 14-17, wherein a mounting construction fixes the radial part to the pontoon.

19. Offshore platform according to claims 14-18, wherein the leg has a suction anchor.