WO2013045640A1

WO2013045640A1 - A structure for offshore operation and a method for installation of an offshore floating structure

Info

Publication number: WO2013045640A1
Application number: PCT/EP2012/069231
Authority: WO
Inventors: Kjell Håvard BELSVIK; Anders DALSLAAEN
Original assignee: Aker Engineering & Technology As
Priority date: 2011-09-29
Filing date: 2012-09-28
Publication date: 2013-04-04
Also published as: NO20111327A1; NO334831B1

Abstract

An offshore platform comprises a deck (1) supported on a plurality of vertically movable legs (4) which can be displaced relative to the deck by a jack-up mechanism (2, 3). The jack-up mechanism (2, 3) comprises an upper portion which connects an upper end of the legs to the upper side of the deck and a lower portion which connects a lower end of the legs to the underside of the deck. Both portions of the jack-up mechanism are pre-tensioned such that when moving the deck relative to the deck, both portions are permanently tension loaded.

Description

A structure for offshore operation and a method for installation of an offshore floating structure

FIELD OF THE INVENTION

The present invention in general relates to an improved structure for offshore operation, which is adapted to be lowered and elevated relative to the sea bed in a substantially stable manner.

In particular, the present invention relates to a technology for lowering and elevating a structure such as an offshore platform, ensuring that its jack up legs and jacks are not damaged by heave action and sudden jerk, triggered by high sea wind and wave action.

More particularly, the present invention relates to a structure for offshore operation according to the preamble of claim 1 and to a method according to the preamble of claim 10.

TECHNICAL BACKGROUND OF THE INVENTION

Installation of structures such as offshore platforms in the offshore regions, especially for exploration and production of oil and gas, has been known to be cumbersome, due to the constant weather challenges at the sea. To be precise, maintaining substantial stability of the structure and associated equipments during installing and de-installing of such structures in the high sea, is something, which is yet to be sorted out .

Various technologies are already being applied for installation of such offshore structures. Lifting the platform sections by crane vessels, followed by in situ assembling is known. However, sustained operation applying this technology is not always possible, due to the huge involvement of cost.

Attaching buoyancy blocks to the structures for floating them to the site and installing the structures by ballasting, with or without the additional use of jack- up legs, is another known technique. The abovementioned techniques have been found to be unsatisfactory due to the involvement of complicated machineries for installation, consequently entailing high cost.

Additionally, removing the structures to a different installation site after completion of operation in one location has been consequently expensive as well. GB2330854A discloses a technique of transporting and installing an offshore structure by application of barge. It also discloses attaching the structure to the barge by means of strand jacks, the strand jacks also serving to lower the structure on the sea bed. However, in this technology, involvement of barge constitutes a major element for transporting and installing the structure, which is expensive. That apart, it fails to disclose how the lifting and lowering operation of the structure is done in a stable manner, so that the structure, the barge and the strand jacks are not damaged due to heave by the wave action.

US 2010/0135728A1 discloses a technology of application of jack up legs, strand jacks, mat located below the hull shaped structure to be installed and ballast tanks. The hull structure is elevated by lowering of the mat, the legs and the caisson to the sea bed, by activating the strand jacks. The elevation is further enhanced by de-ballasting the hull and ballasting the mat. During lowering of the hull to the sea level for towing it to a second site, the strand jacks are deactivated and the mat and legs are jacked up. This operation is further enhanced by de-ballasting the mat and ballasting the hull.

GB 1 551 859 describes a jack-up platform that is designed to be floated to a site of installation, where the jack-up legs are lowered by linear jacks, each having a tie that extends from the upper portion of the leg, through a duct in the deck ant to the lower portion of the leg. The technology as disclosed in the preceding paragraph, does to some extent attempt to solve the problem of high costs, however, it fails to teach how the age old menace of maintaining substantial stability during installing and deinstalling of such structures in the high sea is ensured. Moreover, it fails to disclose in particular how the legs, the strand jacks, the mat and thus the hull, withstand the wave actions during lowering and lifting operations.

US 3986368 teach how to ensure stability once the structure is installed on a jack up leg. However, it does not teach how substantial stability during installing and de-installing of the structure in the high sea is ensured. Moreover, the arrangement involved in the load equalizing system taught by this patent, appears to be quite complicated. Rack and pinion jacks are very expensive and they cannot be removed after installation for use on another structure. US20060062637 teaches minimizing leg deformation and damage to the hull structure and jacking machinery by detecting the extent of leg bending, differential chord bending and hull inclination, but strictly speaking, it does not teach how substantial stability during installing and de-installing of the structure in the high sea is ensured.

Accordingly there has been a desire for a structure for offshore operation which has a simple construction, is economic and simultaneously is technically effective to maintain substantial stability during installing and de-installing of the structure in the high sea, and has the possibility to be easily shifted from one location to another without the assistance of other vessels, such as heavy crane vessels or barges.

The present invention meets above long felt need and other associated needs. OBJECTS OF THE INVENTION

The primary object of the present invention is to provide a structure for offshore operation which has a simple construction, is economic and technically effective to maintain substantial stability during installing and de-installing of the structure in the high sea.

It is yet another object of the present invention to provide a structure for offshore operation which is capable to perform year round operation.

It is a further object of the present invention to provide a structure for offshore operation which is adapted to ensure that its jack up legs and the strand jacks mounted thereon are not damaged by sudden impact of wave action, during lifting and lowering operation.

It is a further object of the present invention to provide a structure for offshore operation which is adapted to be towed or self propelled to the installation site, without the need for any heavy crane vessel and the like.

It is a further object of the present invention to provide a structure for offshore operation which is adapted to be brought down to the sea level after completion of operation for towing to another installation site. It is yet a further object of the present invention to provide an offshore platform to act as an exploration and/or production facility, which has a simple construction, is economic and technically effective to maintain substantial stability during its installing and de-installing in the high sea. It is another object of the present invention to provide a method for installing and de-installing a structure in an offshore site, maintaining substantial stability during installing and de-installing of the structure in the high sea.

All through the specification including the claims, the words "foundation member", "hull",, "tower ", "frame", "jack up legs", "ballast tanks", "strand jacks", "jack house", "hydraulic cylinder", "platform", "structure", "offshore operation", "hydrocarbon" are to be interpreted in the broadest sense of the respective terms and includes all similar items in the field known by other terms, as may be clear to persons skilled in the art. Restriction/limitation, if any, referred to in the specification, is solely by way of example and understanding the present invention. SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided a structure for offshore operation comprising a foundation member having a base and a top, said foundation member being movably supported by one or more vertically elongated legs, the legs being connected to a jacking system which is adapted to move the legs vertically in either direction with respect to the foundation member, characterized in that the upper portion of each leg is removably connected to an upper jacking system and the lower portion of each said leg is removably connected to a lower jacking system and that the jacking system is adapted to be pre-tensioned during the movement of the legs with respect to the foundation member. This ensures lifting and lowering of the foundation member with substantial stability.

Preferably, the foundation member is a platform adapted to function as an offshore hydrocarbon exploration and/or production facility.

According to a second aspect of the present invention there is provided a method for installation of an offshore floating structure. According to the method one or more jack up legs are secured to said structure having removably mounted thereon pre-tensioned upper strand jacks extending from the foundation member to the upper portion of the legs and pre-tensioned lower jack strands extending from the foundation member to the lower portion of the legs. Thereafter, the method involves towing or self propelling said structure having said jack-up legs and said strand jacks secured thereon with sea fastening means, to the desired installation site after adjusting the ballast tank contents so as to achieve the desired draft. On reaching the desired site, the sea fastening means are removed as applicable and de-ballasting action of the structure is initiated. Simultaneously with, or immediately thereafter, the strand jacks at both the upper and lower portions of the jack up legs are activated for causing the legs to be gradually lowered on the sea bed, whereby the structure is gradually raised to adequate height above the sea level to provide adequate air gap, to make it ready for operation.

Preferably, the strand jacks are deactivated and disconnected after lifting operation is over for application on a second structure. More preferably, the ballast tanks in the structure are filled up after completion of operation of said structure and the jack up legs are detracted in upward direction relative to sea bed by means of said strand jacks now operating in opposite direction. This facilitates lowering said structure on sea water and moving it to a second installation site, applying the steps as described hereinbefore.

In a preferred embodiment is mounted a skirt for wave impact damping around the circumference of the lower part of the structure. Said skirt, which may includes several bottomless compartments, is adapted to carry water by suction as the structure is lifted out of the water. Hence slamming forces are prevented. The skirt may be controlled to contain as much water or air as desired for the operation to be performed, either lifting the structure out of water or lowering it into the water.

BRIEF DESCRIPTION OF THE DRAWINGS

Having described the main features of the invention above, a more detailed and non-limiting description of an exemplary embodiment will be given in the following with reference to the drawings, in which: Figure 1 is a side elevation view of a preferred embodiment of the structure according to the present invention in transport and pre-installation state.

Figures 2 to 6 illustrate different stages involved in undertaking installation operations of the structure, illustrated in figure 1 .

DETAILED DESCRIPTION OF THE INVENTION The following describes a preferred embodiment of the invention which is exemplary for the sake of understanding the present invention and non-limiting to the scope of protection. It is also hereby clarified that the structure according to the present invention, acts as a foundation for offshore operation, such as exploration and production of hydrocarbons but is not limited to this function. It is equally effective to achieve its objectives, in all such similar operations as will be known to persons skilled in the art. Further, hereinafter the structure is referred to as a foundation member/platform, just for the sake of brevity and clear understanding and not by way of any sort of limitation. The accompanying figure 1 illustrates a side elevation view of the platform 1 according to the present invention. At the installation site, it plays the role of a foundation structure for oil and gas exploration and/or production. It comprises three or more jack-up legs 4 (in this example four jack-up legs), a platform crane 12 extending upwards from its flat top portion 1 '. The legs 4 extend upwardly from the flat top portion V of the platform 1 and downwardly beyond the bottom face of the platform 1 .

In the above context it is hereby clarified that top face/upper portion and bottom face/lower portion of the platform 1 and legs 4 are defined hereinbefore and hereinafter, with reference to the sea bed 5. The top face is the one which is facing away from the sea bed 5, while the bottom face is the one which is near the sea bed 5. This should be clear from the figures 1 to 6. Preferably, the platform may be designed as a hull unit with self propulsion or be adapted for towing by one or more assist vessels.

The features of the platform will be further understood from the accompanying figure 2. It clearly shows the jack up legs 4 having mounted thereon strand jacks 2, 3. Of course, the number and shape of the strand jacks 2, 3 and legs 4 are not limited to the embodiment shown. The strand jacks are represented by the reference number 3 and the strands are represented by reference number 2.

Figure 2 clearly shows that each of the legs 4 are provided with strand jacks 3, both attached to their upper portion and lower portion. The strand jacks 3, cause movement of the legs 4 vertically with respect to the platform 1 . How, this is achieved is explained later. To maintain stability during transportation or lifting/lowering operation both sets of upper and the lower strand jacks are pre- tensioned. In other words, the cables 2 of the strand jacks are always kept taut.

Advantageously, there are ballast tanks 6 at the basal portion of the platform 1 and extending downwardly wherefrom is a skirt arrangement 7, having perforations therein. The extreme lower portion of each leg is provided with pads 9 for resting on the sea bed 5, underneath the sea water 8, and may also have skirts that can penetrate the seabed.

The platform may be removably attached with a drilling template (not shown).

The materials which are applied for construction of the platform and the features associated therewith are conventional and known to persons skilled in the art and such materials are not consequential to the present invention. As such those are not elaborated herein.

How the features interact, and the advantages achieved thereby are now explained with reference to the installation procedure, described in relation to the figures 2 to 6. In all these figures, like reference numerals indicate like features.

At the first stage, three or more jack up legs 4 are secured to the platform 1 . Pre-tensioned strand jacks 3 are mounted both in the area of the top and bottom face of the platform 1 , with the strands 2 of the jacks being suitably attached to the top or bottom, respectively of each jack up leg.

All the strands 2 of the jacks are always kept in tension, to provide stability to the structure during transportation or lifting/lowering. These are secured at the initial stage with the platform utilizing suitable sea fastening means, as known per se. At the first stage, the legs 4 are supported by the platform 1 , contrary to the stages shown in figures 3 to 6, where the legs 4 gradually takes over the support of the platform 1 .

In the next stage, since the platform 1 is designed as a hull unit, it is towed by one or more vessels or by a self propelling arrangement to the installation site. For this purpose the ballast tanks 6 are adjusted to bring the platform on the sea level so as to achieve the desired draft. The ballast tanks 6 may be filled with water or air to adjust the buoyancy, and this is not consequential to the present invention.

On reaching the desired installation site, the sea fastening means are removed, the platform is de-ballasted and the strand jacks 3 are activated for tensioning the strands 2 and moving the legs 4 vertically downward relative to the hull/platform 1 and towards the seabed. This stage is shown in figure 2.

As mentioned earlier, to impart stability to the entire process, the strands of the individual jacks are always kept in tension. To bring the legs 4 down, the strands of the jacks 3 mounted near the top face of the platform 1 are pulled. As the legs 4 come down, this generates a further tension on the strands of the jacks mounted at the bottom face of the platform 2. These strands are then slightly loosened to allow the legs 4 to come down but the lower jacks 3 always keep the lower strands 2 at a certain tension. The steps are repeated again and again, to lower the legs. As the legs 4 are gradually brought down relative to the platform 1 towards the sea bed 5, the platform 1 is gradually lifted.

In figure 3, the pads 9 of the legs 4 have touched the sea bed 5 by gradual lowering of the legs by continuous activation of the strand jacks 2, 3 as well as by increased ballast water/suction.

In figure 4, the legs 4 have further penetrated the sea bed 5 lifting the platform 1 further. Figure 5 indicates the next stage when the legs 4 have further penetrated the sea bed 5, lifting the platform 1 further and above the water level.

In figure 6, the platform 1 has been lifted to a height above the sea level, where there is adequate air gap. This is followed by final preloading by ballasting or diagonal jacking and finally, jacking to the ready-to-operate position.

It is amply clear from all the figures 2 to 6 that strand jacks 2, 3 are activated at both the upper and lower portions of the jack up legs 4, during lifting/lowering or transportation, with the strands always in tension. So, during the lifting operation both the upper strand jacks and lower strand jacks are kept in tension. This effectively balances excessive loads on the legs 4 and the platform 1 , when the platform 1 is exposed to heave motion by sea waves and wind. This aspect effectively works as well, while the platform 1 is lowered.

To be precise, if the platform is subjected to waves, as it is likely to happen in the high seas, the strand jacks may be damaged due excessive forces if they are subject to a sudden impact. Damage to parts of the structure may also occur. With tension in the strands all the time, the strands do not get slack due to the heave and a sudden jerk on the platform 1 and legs 4 is avoided.

It should be understood that in all the stages of installing the platform 1 as illustrated in figures 2 to 6, the lifting of the platform is caused by the legs 4 being activated by the strand jacks 2, 3 as well as by the de-ballasting action of the platform. These two actions either take place simultaneously or the strand jacks are activated immediately after initiation of de-ballasting action. Another advantageous feature, as best shown in figure 2 is a skirt member 7 extending downwardly from the base of the ballast tanks 6. The skirt member 7 is perforated. It prevents the waves from entering under platform 1 and slam against the underside. It acts as a cushion. Thereby, a sufficient air gap is created, before the wave enters under the platform 1 . Hence, the perforated skirt member 7 extending downwardly below the ballast tanks 6 rapidly increases the air gap, allowing trapped air or water as the case may be, to be released slowly during lifting or lowering of the platform.

Once, the platform is installed so as to have safe air gap above sea level and a sufficient pressure against the seabed from the legs, as shown in figure 6, the strand jacks 2, 3 are deactivated and disconnected for use with another platform.

While bringing the platform 1 down to reach the sea level, the strand jacks 2, 3 now work in the manner opposite to what has been described with reference to figures 2 to 6.

Once the operation of the platform 1 is over after a passage of time, the ballast tanks 6 in the platform 1 are filled up, the strand jacks 2, 3 are made to operate in opposite direction so that the jack up legs 4 are retracted vertically upward relative to the platform 1 . This action facilitates lowering the platform 1 on the sea to an adequate level to have a desired draft, so that it may be towed or self propelled to a second installation site. Alternatively, it may support a production facility at the site of installation.

The working of the strand jacks 2, 3 is now explained in greater detail. As stated before the strand jacks 2, 3 are activated for locking and moving the legs 4 vertically down relative to the hull/platform 1 towards the sea bed.

Strand jacks are generally known in the art and will not be explained in detail here. However, a detailed description of stand jacks can be found in a brochure from Dorman Long Technology at:

http://www.dormanlonqtechnoloqy.com/Download files/DLT%20strand%20iack %20svstems 1 .0 600%20dpi.pdf.

In the exemplary non-limiting embodiment illustrated herein, the body of the jacks 3 are secured to the platform 1 and the ends of cables passing through the jacks are secured to the top or bottom ends of the legs 4.

Now to understand the working of the strand jack, let the top jacks be considered first. It should be noted that all jacks in the top array of jacks are not operated simultaneously, otherwise the basic idea of always keeping the strands in tension, will be defeated. The jack 3 is activated and it firmly holds the strand 2. Once the unit contracts, it pulls down the strand and along with it, the leg 4 since the end of the strand 2 is attached to it. Now, let the bottom jacks be considered. As locking leg 4 is pulled down, the strands are held by the bottom jacks under further tension. So a few of the clamps of the array of bottom jacks are released, the strands of those then move some amount to release the tension. These are clamped again and a further set is released in a similar fashion. As a result, a certain amount of tension is always maintained in the strands, thus adding stability to the platform and legs against heave motion. During lowering of the platform, the strand jacks operate in the opposite manner to what has been explained above. The lowering is also enhanced by ballasting of the platform as previously explained.

The strand jack arrangement 2, 3 according to the present invention, thus facilitate vertical lowering (and like wise lifting) of the legs 4 relative to the platform 1 and eventually embedding the legs 4 on the sea floor 5, so that the platform 1 is lifted to adequate height above water level, from where it can be ready to operate.

The present invention also embraces a method of producing hydrocarbons by drilling a well using the platform of the present invention. It will be understood from the exhaustive disclosure above and also from the appended claims, that the platform has a simple construction, is economic and technically effective to maintain substantial stability during installing and deinstalling of the structure, in the high sea. It is capable to perform year round operation. Furthermore, the present invention ensures that the jack up legs and the strand jacks mounted thereon are not damaged by sudden impact of wave action during lifting and lowering operation.

Additionally, it is adapted to be towed or self propelled to the installation site, without the need for any heavy crane vessel and the like. Further, it can be brought down to the sea level after completion of operation for towing to a second installation site.

The present invention has been described with reference to a preferred embodiment and some drawings for the sake of understanding only and it should be clear to persons skilled in the art that the present invention includes all legitimate modifications within the ambit of what has been described hereinbefore and claimed in the appended claims.

Claims

P a t e n t c l a i m s

1.

A structure for offshore operation comprising a foundation member (1) having a base and a top, said foundation member (1 ) being movably supported by one or more vertically elongated legs (4), the legs being connected to a jacking system which is adapted to move the legs (4) vertically in either direction with respect to the foundation member (1), characterized in that the upper portion of each leg (4) is removably connected to an upper jacking system (2, 3) and the lower portion of each said leg (4) is removably connected to a lower jacking system (2,3) and that the jacking system (2, 3) is adapted to be pre-tensioned during the movement of the legs with respect to the foundation member (1 ).

2.

The structure according to claim 1, c h a ra ct e r i s ed i n that said base of said foundation member (1) is provided with one or more ballast tanks (6) for ballasting and de-ballasting said foundation member (1) during lowering and lifting operation respectively.

3.

The structure according to claim 1 or 2, c h a ra cte r i s ed i n that said foundation member (1) is provided with a perforated skirt member(2) extending downwardly below said ballast tanks (6) for rapidly increasing air gap.

4.

The structure according to any of the claims 1 to 3, characterized in that said foundation member is a platform adapted to function as an offshore hydrocarbon exploration and/or production facility.

5.

The structure according to claim 4, characterized in that said platform(1) is designed as a hull unit adapted to be floated to the deep sea installation site.

6.

A method for installation of an offshore floating structure,

ch a racte ri sed by the following steps: a) securing one or more jack up legs (4) to said structure having removably mounted thereon pre-tensioned upper strand jacks (2, 3) extending from the foundation member (1) to the upper portion of the legs (4) and pre-tensioned lower jack strands (2, 3) extending from the foundation member (1) to the lower portion of the legs (4), b) towing or self propelling said structure having said jack up legs and said strand jacks (2, 3) secured thereon, to the desired installation site after adjusting the ballast tank contents so as to achieve the desired draft, c) initiating de-ballasting of the structure, d) activating the strand jacks (2, 3) at both the upper and lower portions of the jack up legs (4) either simultaneously with, before or after initiation of said de- ballasting for causing said legs (4) to be gradually lowered to the sea bed, while keeping both the upper and the lower stand jacks (2, 3) in tension, whereby said structure (1) is raised to adequate height above the sea level to provide adequate air gap, to make it ready for operation.

7.

The method according to claim 6 is ch a racte ri sed by deactivating and disconnecting said strand jacks (2, 3) for its application on a second structure after completion of installation.

8.

The method according to claim 7, ch a racte ri sed by filling up ballast tanks (6) in said structure (2) after completion of operation of said structure, detracting jack up legs (4) in upward direction relative to sea bed by means of said strand jacks (2,3) now operating in opposite direction, for lowering said structure on sea water and applying steps (a) and (b) to move said structure (1) to a second installation site.

9.

The method according to any of the claims 6 to 8, ch a ra cterised by using said structure (1) after completion of installation, to support a production facility and/or to facilitate hydrocarbon exploration.