NL2005370A

NL2005370A - Floating marine structure with suction piles and vessel.

Info

Publication number: NL2005370A
Application number: NL2005370A
Authority: NL
Inventors: Mark Riemers
Original assignee: Suction Pile Technology B V
Priority date: 2009-12-09
Filing date: 2010-09-17
Publication date: 2011-06-15
Also published as: WO2011071385A1; EP2510156A1; NL2005370C2

Abstract

A marine structure floating upright or in its operating position in a body of water, said structure consisting of a platform above the water and having at least three spaced upright supporting legs extending downwards from the platform towards the water, at the lower side of which a suction pile is connected, which marine structure is loaded onto a separate floating body or floater or vessel, such that the suction piles are external from the vessel and penetrate with their lower parts the water into which the vessel floats or are completely submerged while the platform of the marine structure is at a distance above said water and also above the vessel deck. There are at least three or four upward extending panels that define, seen in top view of the marine structure as supported by the vessel, the sides of a triangle or rectangular or multi angle, one of these panels being penetrated by the vessel. These panels are delimited by the supporting legs and these panels contain horizontal and/or inclined braces which provide mutual connection of two adjacent supporting legs, except for the panel that is penetrated by the vessel. This allows vertical movement of the supporting legs to completely lower the suction piles onto the sea bottom while supported by the vessel.

Description

Title: Floating marine structure with suction piles and vessel.

The invention concerns a marine structure, a method of installing a marine structure and a suction pile foundation. Particularly, the invention concerns application to a so called "marginal field" i.e. an offshore field for which the predicted economical exploitation is limited to less then 10 years.

Suction piles and their way of installing are o.a. known from GB-B-2300661 and EP-B-0011894, which disclosures are enclosed in here by reference. Briefly, a suction pile is a thin walled steel cylinder, closed at at least one longitudinal end, that is located on the subsea bottom with the opposite end and penetrates the subsea bottom with the aid of a suction created within the cylinder. The creation of the suction can be with the aid of a suction source, such as a pump, being on, or close to or at a distance from the suction pile. The applied level of the suction can be e.g. at least substantially constant, smoothly increase or decrease or else pulsate, for which there are convenient means. After use, the suction pile can easily be removed by creating an overpressure within the cylinder, e.g. by pumping in (sea) water.

A self installing platform applying suction piles which provide buoyancy is known from e.g. W099/51821 (SIP1) or EP-A-1 101 872 (SIP2) of the present inventor.

Instead of installing a suction pile into the under water bottom by generating a fluid pressure difference between the inside and outside of the suction pile, it is also feasible that the suction pile at least partly penetrates the under water bottom by a weight resting on it, e.g. the platform and/or a ballast body .

Therefor according to the invention it is proposed to make the marine structure self foundating or self installing by providing it with one or more suction piles. So the hoisting device and the plant for installing the foundation, e.g. hammering device, can be eliminated.

Since the structure is provided with one or more suction piles, removal after use is made easier in that by pressing out the suction pile, the anchoring of the structure to the underwater bottom can be removed.

In this respect the marine structure typically will be rather large, e.g. a production platform with equipment, and made of steel. Due to its own weight, such marine structure is designed to be applied with a foundation of pile bodies to be pressed in the bottom. The marine structure according to the present invention will typically weigh not more than about 5,000,000 kilo, although structures with a much higher weight of e.g. 10,000,000 kilo or more are also feasible.

For the purpose of transporting to the final destination the marine structure is loaded onto a ship or barge or other separate floating and/or sailing body with buoyancy of its own. "Barge" preferably means at least a vessel known as such with one or more floating spaces at least substantially hermetically delimited from the environment. Preferably the barge has no equipment of its own for propelling and/or directional control. The ship on the other hand has equipment of its own for propelling and/or directional control.

If herein the term "substructure" is used, it means the assembly comprising the supporting legs and suction piles and also, if present, the horizontal and/or inclined braces. The marine structure is the assembly comprising the substructure and the upper structure, such as platform or wind turbine.

The marine structure is loaded onto the vessel such that the suction piles are external from the vessel and may or may not penetrate with their lower parts the body of water into which the vessel floats or are completely submerged while preferably the platform of the marine structure is at a distance above said body of water and also preferably above the vessel deck.

More preferably, one or more of the following features apply: the marine structure is loaded onto the vessel in the upright orientation or the orientation equal to the operating position of the marine structure, which is typically the vertical orientation; from each suction pile a supporting leg extends upward; the platform has a rectangular shape in top view; at each corner of the platform and possibly also at more locations along the side of the platform a supporting leg and/or suction pile is present; the platform is connected to the supporting legs by connection means that allow the platform to move along the supporting legs; the supporting legs have a length of at least 10 or 20 metres (e.g. 20 metre for a water depth of 5 metre and 100 metre for a water depth of 60 metre); each suction pile has a diameter of at least 5 metres and a height of at least 5 metres and a wall thickness of at least 1 centimetre; the longitudinal axis of the suction pile and the relevant supporting leg are substantially in line; the longitudinal axis of the suction pile and the relevant supporting leg are eccentric, preferably such that the longitudinal axis of the suction pile extends outside the relevant supporting leg and/or the supporting leg extends within the suction pile or its extension and/or the supporting leg is connected to the suction pile at or near the circumferential edge of the suction pile such that e.g. part of the external radial walls of both merge, at least where supporting leg and suction pile join; between all supporting legs horizontal and/or inclined braces extend which braces preferably provide mutual connection of two adjacent supporting legs, preferably except for one side of the substructure where such braces are absent, at least at the level above the vessel in case the structure is loaded onto the vessel; the sides of the substructure with the braces are outside the vessel, at least at the level above the vessel; the side of the substructure without the braces extends across the vessel or is penetrated by the vessel; the substructure provides at least three or four upward extending panels that define, seen in top view of the marine structure as supported by the vessel, the sides of a triangle or rectangular or multi angle; each of the two upward extending sides of each of the panels is defined by the relevant supporting leg; only one of the panels is penetrated by the vessel while the other panels are outside the vessel; the panel that is penetrated by the vessel is free from projecting parts projecting from any if its two upward extending sides; a panel that is not penetrated by the vessel contains one or more horizontal or inclined or crossed braces mutually connecting the supporting legs of said panel; projecting parts projecting from a panel into the space delimited by the panels are absent; a projecting part, such as the platform, that can move upward relative to the substructure or otherwise can move out of the way while the substructure is lowered onto the sea bottom without the vessel having moved horizontally relative to the marine structure, may be present; the marine structure is loaded at a longitudinal end of the vessel and/or comprises one two, three, four or more suction piles.

Additionally, one or more of the following applies: the marine structure is split in at least two or precisely two separate substructures, e.g. an upper and a lower substructure, while supported by the common vessel during transport to the installation site, which substructures are individually or next to each other supported by the vessel and are mutually stacked or superpositioned during installation of the marine structure onto the subsea bottom to obtain the complete marine structure; a substructure comprises length parts of all supporting legs; a substructure comprises all suction piles or the platform; one of the substructures is loaded at a longitudinal end of the vessel; the substructures are assembled by stacking or locating their supporting leg parts on top of each other; the length of the supporting leg parts of a substructure, onto which another substructure has to be located or stacked to obtain the complete marine structure, is such that during installation said supporting leg parts project above (e.g. several metres such as between 2 and 3 metre) or below (e.g. below the bottom of the vessel or several metres such as between 4 and 5 metres below) the water level when the suction piles are installed into the subsea bottom at their final depth and said supporting legs are supported by the suction piles in their final configuration; a substructure contains all braces connecting two adjacent supporting legs; a substructure contains all panels; the substructures are located at the vessel in an orientation equal to their orientation in the completed marine structure; to obtain the completed marine structure the corresponding supporting leg parts of the substructures are brought in mutual elongation and then united; the substructures are united by e.g. quick-connectors or welding or some other mechanical fixing method.

By making sure that the panel that is penetrated by the vessel and the space delimited by the panels is free from projecting parts projecting from a side and free from projecting parts projecting into the space delimited by the panels, respectively, except for a projecting part, such as the platform, that can move upward relative to the substructure, or otherwise can move out of the way while the substructure is lowered onto the sea bottom without the vessel having moved horizontally relative to the marine structure, the installation procedure is simplified.

It is feasible that the vessel has insufficient buoyancy of its own to keep the marine structure floating, considering the prevailing safety requirements. In such a case the one or more suction piles provide the additionally required buoyancy.

Thus a cost and time and energy and environmental saving is possible, and also the work is safer. Besides, additional dynamic stability is gained of the in the body of water floating and advancing assembly provided by the vessel and the marine structure loaded onto it, compared to the embodiments disclosed in the state of the art. Also the draft of the floating assembly is limited.

To provide the additional buoyancy, the suction pile is preferably provided with compressed air or integral floating means, preferably present inside the suction pile. The floating means can at least substantially comprise a space in open communication with the surrounding water at its under side, such as the pressure space of the suction pile.

The invention is further illustrated by way of a non-limiting, presently preferred embodiment providing the best way of carrying out the invention and shown in the drawings.

Fig. 1 shows a top view of an assembly of the invention;

Fig. 2 shows in a side view of the assembly of fig. 1 the three main steps during installation;

Fig. 3 shows a back view of the three steps of fig. 2;

Fig. 4a-c show a perspective of the three steps of fig.

2;

Fig. 5a shows the views of fig. 1-3 for a first alternative;

Fig. 5b and c show the steps of Fig. 4a and 4b for the first alternative;

Fig. 6 shows the views of fig. 1-3 for a second alternative;

Fig. 7-13 show a further alternative;

Fig. 14 show yet another alternative.

Fig. 1-4 show a marine structure consisting of a platform, rectangular in top view, each corner of which having a supporting leg at the lower side of which a suction pile is connected. The supporting legs are mutually connected by braces, however braces are absent at one side, so only present at three of the four sides as shown. Each supporting leg is connected to the suction pile such that the longitudinal axis of the one is spaced from the other. Where they join the supporting leg is internal from the relevant suction pile or its extension and the longitudinal axis of the suction pile is external from the supporting leg. Also, the supporting leg is connected to the suction pile such that part of its circumferential edge merges with the circumferential edge of the suction pile.

The braces extend horizontal at low level and diagonal crosswise and extend partly aside the vessel at all three sides.

In this example the suction piles partly penetrate the water.

During transport over water, the substructure is retracted such that the supporting legs maximally extend above the water level whilst meeting stability reguirements. The marine structure is at least partly supported by the vessel and the platform is preferably close to the water surface. Therefor, the platform 2 preferably rests directly onto the vessel, possibly through supports. The vessel provides all required buoyancy, or the vessel and the suction piles together provide the required buoyancy.

The marine structure rests on the longitudinal end of the vessel.

During installation, the supporting legs and the platform are mutually shifted in longitudinal direction of the supporting legs, such that the lower side of the supporting legs move away from the platform. The marine structure is located onto the ship such that while lowering the suction piles, the three braced sides pass the relevant side and back of the ship with close spacing.

One proceeds in the following manner during installation:

While the platform still rests onto the vessel floating in the water, one lowers the substructure onto the under water bottom such that the suction piles reach the water bottom. After the suction piles have penetrated the under water bottom, preferably to their final depth, the platform is lifted from the vessel and to its operating position by moving upward along the supporting legs . After the platform is cleared from the vessel, the vessel sails from below the platform. Lifting of the platform 2 is carried out by lifting means operating between the supporting legs and the platform. During lifting, the platform shifts along the stationary supporting legs.

Alternatively one proceeds as follows: one lifts the structure from the vessel, preferably by increasing the buoyancy of the suction piles, whereafter one disengages the vessel from the structure and removes it by sailing it away from below the platform. Subsequently one lowers the suction piles together with the lower side of the supporting legs onto the under water bottom, while one shifts the platform along the supporting legs to remain at least substantially at the same level above the water surface. After the suction piles are penetrated into the under water bottom, preferably to their final depth, the platform is lifted to its final level, shifting along the supporting legs.

Fig. 7-13 show an embodiment based on the marine structure of fig. 1-3. In this embodiment the marine structure is split in two substructures, a lower and an upper, supported next to each other by a common vessel during transport to the installation site. The lower substructure is located at a longitudinal end of the vessel and contains the lower parts of the supporting legs, the suction piles and the four panels of which merely one is penetrated by the vessel. The upper substructure contains the upper parts of the supporting legs and the platform.

Fig. 7 shows steps 1-5 of the installation sequence in side and end view (step 1 also top view). Fig. 8-12 show the same steps 1-5 in perspective view. Fig. 13 is identical to fig. 12, however the vessel is removed.

After lowering the lower substructure onto the subsea bottom and penetration of the sea bottom by the suction piles to their final depth, the supporting legs of the lower substructure project above the water level (step 2) , keeping the vessel between them during step 3 and 4. In an alternative, these supporting legs are completely submerged and end below the bottom level of the vessel such that they are no obstruction to the vessel.

Both substructures are located at the vessel in the orientation they take in the completed marine structure such that from step 2 to step 3 the vessel merely has to advance in a straight line to bring the upper substructure on top of the lower substructure.

In step 3 the supporting legs are brought in mutual elongation. In step 4 the two substructures are united by mutually fixing the in mutual elongation extending supporting legs. From step 4 to step 5 the platform is lifted to its final operating level.

Fig. 14 shows a completed marine structure with additional stiffening frames at low level. The partition between the upper and lower substructure is located at a level above the upper horizontal bracings mounted between the upright supporting legs.

To be able to mutually shift the supporting legs and the platform, convenient means can be present, such as strands and strand jacks or cables and driven winches which are connected to the platform and the supporting legs.

The invention is not limited to the above described and in the drawings illustrated embodiments. E.g. the marine structure can have less than four, e.g. two or three, or more than four, e.g. five or six, suction piles. The number of supporting legs is preferably equal to the number of suction piles, but this is not absolutely necessary. E.g. three suction piles are at the corners of a structure that is triangular in top view. It is not required that the suction piles and supporting legs are at the corners of the structure. The platform can be constructed and/or shaped differently.

Claims

An offshore structure floating upright in the water, which construction comprises: a platform above the water and with at least three mutually spaced upright support legs that run downwards from the platform to the water and with a suction pile on its underside, structure is placed on a separate boat, so that the suction piles are outside the boat and with their lower end protrude into the water in which the boat sails while the platform is at a distance above the water and also above the ship deck while the structure four is raised has extending panels bounded by the support legs which, viewed in plan view of the structure while supported by the boat, define the sides of a rectangle, the boat protruding through only one of these panels (plug-in panel) while the other panels extend outside the boat and the insert panel through which the boat extends vertically moving the support legs relative of the platform resting on the boat allows the suction piles to be lowered completely onto the seabed while the structure is supported by the boat, because this panel is free of horizontal and oblique braces (in English: braces) between this insert panel limiting supporting legs, while horizontal and oblique braces run for the other panels between the supporting legs limiting those other panels.

2. Construction as claimed in claim 1, wherein the support legs bounding the insert panel through which the boat extends are equipped with additional stiffening elements, comprising stiffeners extending parallel to the relevant support leg and mounted on the relevant support leg and on a non-belonging to the respective insert panel support leg through horizontal and sloping braces / braces (fig. 14).

A structure according to claim 1 or 2, which is divided into exactly two sub-structures, an upper and a lower one, which are carried individually and next to each other by the boat and placed on top of each other during installation on the seabed to obtain the complete offshore construction wherein the lower sub-structure contains lengths of all support legs and all suction piles and all panels and is placed on a longitudinal end of the boat.