EP0071297A1

EP0071297A1 - A tower for forming a drilling and/or production island

Info

Publication number: EP0071297A1
Application number: EP82200913A
Authority: EP
Inventors: Hendrik Veth
Original assignee: Ingenieursbureau H Veth BV
Current assignee: Ingenieursbureau H Veth BV
Priority date: 1981-07-23
Filing date: 1982-07-16
Publication date: 1983-02-09
Also published as: NO822460L; NL8200232A

Abstract

@ A tower for forming addrilling and/or production island, which tower is completely assembled from annular steel parts, the lowermost part being constructed as or being connected to an also annular supporting foot, which is adapted to carry the weight of the tower, and is provided with seats for accomodating anchoring means, which foot can, in particular, be lowered directly onto the sea bottom. At least the foot is constructed with double walls, and is subdivided by means of partitions into water-tight ballast chambers.

Description

In a prior patent application NL-A 7805368 a method is described for drilling a source of mineral oil and/or natural gas situated under water, and for winning therefrom oil and/or gas, as well as an assembly for executing this method.
Therein use is made of a tower which is assembled from annular steel elements, and is, at least in the lower portion, frusto-conically shaped, the lower part being connected to at least three diverging legs and to a bottom closing the inner space, which legs are to be connected to a substructure anchored to the sea bottom and surrounding the well site, which bottom should be resistant to the water pressure at the water depth in question, and is provided with water-tight passages for ducts to be connected to the well, said legs being provided, at their lower ends, with ballast means such as compartments to be internally filled with water. Such a tower, when provided with a superstructure or platform assembly, comprising an engine room, crew housing etc., which can be constructed as a landing place for helicopters, is adapted to be towed floating in the vertical position towards the drilling site, and to be lowered there on the substructure.
Such a tower can be assembled from unit elements manufactured elsewhere, and, after completing the legs and at least the lowermost element on a slipway or another suitable building site, the whole is lowered into the water, and, if required, is weighted by means of the ballast spaces in the legs, until the upper side will slighty protrude above the water, and then a next element is supplied by means of a crane or pontoon, and is welded to the portion protruding above the water. Finally the superstructure can be floated on pontoons above the completed tower and connected thereto, after which the whole assembly can be towed away by means of these pontoons. If necessary a completed portion can be towed towards deeper water between times in order to be finished. On arrival in the point of utilisation the whole assembly can be lowered by weighting the pontoons, and then the latter can be taken away.
This manner of operation has many advantages over the manners usual up till now. Although the total cost of manufacturing and erecting of such a tower will be substantially lower than when using the current manners of operation, it may, nevertheless, be desirable to obtain a further saving of cost by simplifying the construction and/or shortening the duration of manufacturing and erection.
The invention provides, in this respect, an improvement of the tower according to the above-mentioned prior patent application, the tower being completely assembled from annular elements, the lowermost annular element being constructed as or provided with a supporting foot which is annular as well, and which is designed to carry the weight of the tower, said supporting foot being provided with seats for taking up anchoring means.
Such a tower can be supported as such on a flat sea bottom, and generally no substructure is required. Manufacturing and erecting, and also maintenance, of such a tower are considerably more economic than in the case of the current towers made from steel columns or reinforced concrete.
At a very large water depth, however, objections against such a tower which is directly supported on the bottom may exist, e.g. if the forces exerted thereon by water currents will give rise to undesired vibrations. In that case first a substructure, e.g. of concrete, can be lowered on the sea bottom, which remains anchored thereon by its own weight, said substructure being provided with anchoring means on which the foot of the tower lowered on said substructure can be fixed. The height of the substructure is, then, chosen so that the height of the tower to be placed thereon will be sufficiently reduced for avoiding said undesired phenomene. The cost of such an assembly are, then, still considerably more favourable than when using towers of the current type.
In particular the inner space of the tower is, at least for a substantial part, not closed so that this space is pervaded by water. For the bottom of the tower of the above-mentioned prior patent application should, in particular in deep water, withstand considerable pressures, so that manufacturing it can be expensive. Also making duct passages through said bottom which are resistant against such pressures can be expensive. Although in a tower not pervaded by water the ducts are protected against any corrosion by the sea water, said ducts in a water pervaded tower are, at any rate, shielded from water currents so that suspension and support thereof is simple, and protection against corrosion is relatively simple in the stagnant water, and can favourably compare with the cost of a pressure resistant bottom. This holds the more if previously drilling from a substructure provided on the sea bottom is not required or not sensible, and then drilling by means of a tower which is internally open will be possible as such. Another advantage of a water pervaded tower is that lowering diving apparatus through the interior of the tower for executing works at or near the well head is simple since such apparatus are, then, not subjected to water currents.
The inner wall of the supporting foot and/or of the annular elements can be provided with stiffeners which, in particular, can be constructed as parts of a double wall which, in this manner, is divided into compartments so that, apart from a higher stiffness, also the possibility is provided to use these compartments as floating or as ballast compartments.
In the centre of the supporting foot and also of annular elements situated at a higher level a supporting ring can be provided which is adapted to surround the well head means and ducts respectively present there, and is connected to the circumferential part of the foot or the element in question respectively, and said rings can also be used for supporting and shielding such well head means or ducts respectively.
The interior of the tower can be provided with one or more transverse partitions so as to allow to maintain during transport one or-more air compartments, in particular in the upper part of the tower, said compartments contributing to the buoyancy, and said transverse partitions are not subjected to very high water pressures, and, therefore, do not need to be made very strong, said partitions being eventually removed for clearing the interior for passing drilling strings, ducts etc. If the tower is to be transported floating in a substantially horizontal position, the inner space can also be closed near the other extremity by means of additional removable partitions.
The invention will be elucidated below by reference to a drawing, showing in:

Fig. 1 a lateral view with parts broken away of a tower according to the invention;
Figs.2 and 3 partial sections on lines II - II and III - III resp. of Fig. 1 at a larger scale; and
Fig. 4 a partial section corresponding to Fig. 2 of an other embodiment of the lower portion of a tower according to the invention.
In Figs. 1..3 a first embodiment of a tower 1 according to the invention is diagrammatically shown with a superstructure 2 provided thereon.

This tower consists of a plurality of superposed substantially frusto-conical elements 3 with an annular cross-section, adjoining edges thereof having the same diameter, and being interconnected by means of a welding connection. The cone angle of the different elements decreases upwardly, this in such a manner that an optimal resistance against bending moments as a consequence of forces acting on the upper end is obtained. Besides by loading the overhanging parts of the superstructure 2, such moments are also generated by the wind forces, wave beat and water currents acting on the tower 1.
The lowermost element 3 joins an annular supporting foot 5 having a bottom 6 bearing on the sea bottom 7. By the weight of the tower 1, the foot 5 will, at a plane sea bottom, penetrate somewhat into the bottom 7. The weight of the tower 1 will then be uniformly distributed over the supporting foot 5. The diameter of the foot 5 and, therefore, also of the lower edge of the adjoining element 3, is sufficiently large for temporarily ensuring the lateral stability of the tower 1.
The supporting foot 5 is, moreover, provided with sleeves or seats 8 distributed along the circumference, and being adapted for taking up ram piles 9, and the upper ends 10 of the sleeves 8 can be widened for facilitating the introduction of the piles 9. The piles 9 are fixed in the foot in a suitable manner, e.g. by means of cement. These piles 9 not only serve for anchoring the supporting foot 5 against lateral displacements, but also for taking up tilting moments, the piles then being loaded by traction.
Inside the foot 5 a ring 11 is present which is connected, by means of transverse connections 12, to the outer side of the foot. This ring comprises an area 13 in which ducts and/or well head closure means are present, which are diagrammatically indicated at 14 in Fig. 2. This ring 11 can serve for supporting and fixing purposes, and laterally shields the well head portion.
Also the higher wall elements can be provided with similar rings 11' and transverse connections 12', by means of which ducts can be supported. The higher elements 3 reach, eventually, a diameter which substantially corresponds to that of the rings 11 and 11' so that, there, such rings will be superfluous. In particular these rings 11 and 11' can be made so that the bundle of ducts led upwardly will have a gradually reduced diameter.
The supporting foot 5 can, furthermore, provide an inner wall 15 defining with the outer wall an interspace which is divided, by means of transverse partitions 16, into compartments 17, the whole then being additionally stiffened, and, moreover, these compartments can be completely closed by means of terminal walls 18 so that they can serve as ballast spaces. Also the elements 3 can be provided with an inner wall 15' and partitions 16', as well as with terminal walls 18' so as to obtain a wall stiffening, and the spaces 17 and 17', in particular in the lower wall elements, can be connected to pumps so as to be filled at will with water and to be emptied again so as to control the buoyancy.
In Fig. 3 a cross-section of the upper part of the tower is shown, in which the wall can be singular, and only vertically directed stiffening means 19 are provided at the inner side which, if required, can be interconnected by means of annular transverse stiffeners 20.
The inner space 21 of the tower remains open so that this space is filled with water. This tower can, for instance, with the superstructure provided thereon, be towed in the upright position towards the utilisation point. The buoyancy is provided i.a. by the air-filled spaces 17 and 17' of the supporting foot and the remaining elements 3 respectively, and, if required, additional floats can be connected to the tower. When arriving in the utilisation point, water can be admitted into the compartments 17, and, as the case may be, also in the compartments 17' of the lower elements, so as to allow the tower to be sunk in the upright position until the foot 5 reaches the bottom, and then the foot can be anchored by the ram piles 9.
In order to provide possibly required additional buoyancy, it may sometimes be favourable to close the upper portion of the space 21 in a water-tight manner by means of a transverse partition, so that, then, no water can penetrate into the overlying portion of the inner space. This partition is to be removed before the ducts are provided. Such a partition is preferably arranged as high as possible so as to be submitted to a possibly low water pressure.
Such a tower can be assembled in the same manner as the tower of the above-mentioned prior patent application, viz. by assembling at first the lower portion on a slipway or an other suitable construction site, launching it into the water, and thereafter providing the consecutive elements 3, the buoyancy always being controlled so that a next element can be placed on a preceding one by means of pontoons or a crane, and can be welded thereto above the water level. If necessary a completed portion can be towed towards deeper water for being finished there. Finally the superstructure 2 is supplied in parts, and is, depending on circumstances, mounted on the finished tower, which, if required, can also be done after having lowered the tower on the sea bottom in the point of utilisation.
It is also possible to transport the tower without the superstructure horizontally floating, and then additional partitions can be provided for obtaining a larger air space and, thus, a better buoyancy. The superstructure will then be transported separately towards the point of utilisation, and will be fixed there on the tower after the latter is sunk in the upright position.
At a very large water depth it can happen that, caused by forces exerted by the sea water, periodical movements will occur which are troublesome for the crew and/or will lead to fatigue phenomena in the material. In that case a substructure consisting, for example, of concrete can be provided on the sea bottom first, which is lowered on the sea bottom by flooding ballast spaces so that this substructure will remain anchored to the sea bottom by its own weight. The height of this substructure is chosen so that the tower to be lowered thereon has a height at which said undesired phenomena will no longer take place. The substructure is then provided with fastening means which can be fixed in the sleeves 8 of the foot 5.
An additional advantage of such a substructure is that pile ramming at a large depth is no longer necessary, and anchoring of the foot 5 can take place at a smaller depth,and, of course, the strenght of the tower is to be adapted to a smaller depth. The total cost of such an assembly will, nevertheless, be more favourable than when using a tower of the current type.
For the rest such a substructure can also be used if the sea bottom is not suitable for anchoring the foot of the tower directly thereon, and, of course, the^fheight of the substructure should be chosen as small as possible as will be compatible with the required weight of the substructure.
Fig. 4 shows a slightly modified embodiment of the lower portion of such a tower, in which the foot 5 is widened outwards, and the sleeves 8 are situated inside its circumference. In the case shown these sleeves form a part of the transverse partitions 16.
The various ducts can be supported within the tower in a simple manner, and are completely shielded by the tower against the influence of water currents.
Since the interior of the tower, after removing the possibly present transverse partitions, is completely open, drilling can be done from the superstructure 2 without difficulties, and the rings 11 and 11' can be provided with additional means for supporting the drilling string.

Claims

1. A tower for forming a drilling and/or production island, assembled from substantially annular steel elements with, at least near the lower side, a frusto-conical cross-section, which tower is adapted to be transported floating towards the point of utilisation, and to be lowered and anchored there, and within this tower vertical ducts are or can be provided which can be connected to a well head, characterised in that the tower (1) is completely assembled from annular elements (3), and the lowermost element is constructed as or is connected to a supporting foot (5) which is annular as well, and which is adapted to carry the weight of the tower (1), which supporting foot -(5) is provided with seats (8) for taking up anchoring means (9).

2. The tower of claim 1, characterised in that the supporting foot (5) is adapted to be lowered directly on the sea bottom (7), and to be anchored thereon by means of anchoring means (9) to be driven into the sea bottom (7).

3. The tower of claim 1, characterised in that the supporting foot (5) is adapted to be lowered on a substructure and to be anchored on anchoring means present thereon, said substructure having previously been lowered on the sea bottom (7) and is fixed thereon by its own weight.

4. The tower of any one of claims 1..3, characterised in that its inner space (21) is not closed for at least a substantial part.

5. The tower of any one of claims 1..4, characterised in that the inner wall (15, 15') of the supporting foot (5) and/or of the annular elements (3) is provided with stiffening means (19).

6. The tower of claim 5, characterised in that at least the supporting foot (5) is of a double-walled construction, and is subdivided into water-tight compartments (17) by means of partitions (16).

7. The tower of any one of claims 1..6, characterised in that in the centre of the supporting foot (5) and/or of the annular element (3) a ring (11, 11') is provided which is connected to the wall of the element in question, and is adapted for supporting and/or shielding well head means and/or ducts or drill strings or the like (₁4).

8. The tower of any one of claims 1..7, characterised by one or more water-tight transverse partitions in its inner space (21).