EP1339921B1

EP1339921B1 - Offshore platform

Info

Publication number: EP1339921B1
Application number: EP01991787A
Authority: EP
Inventors: Harke Jan Meek; Pieter Gerardus Frank Sliggers
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 2000-12-05
Filing date: 2001-12-03
Publication date: 2007-04-04
Anticipated expiration: 2021-12-03
Also published as: MY131158A; WO2002046537A1; EP1339921A1; AU2002231658B2; AU3165802A; US20020081157A1; DK1339921T3

Abstract

Offshore platform comprising a bottom structure (1) comprising a polygonal base (5) resting on the sea floor (2) having suction piles (10) joined to its corners and a first support frame (8) connected to the polygonal base (5) including interconnected open-ended pipes (15), conductors (33) extending from above the water surface (36) into the seabed through the open-ended pipes (15) of the first support frame (8), and at least one further support frame (22, 24) including interconnected open-ended pipes (25) extending around the conductors (33), wherein each further support frame (22, 24) is secured to the conductors (33) at a predetermined level.

Description

The present invention relates to a method of installing an offshore platform. In particular the present invention relates to installing an offshore production platform, which is an offshore platform that is provided with wellhead and/or production equipment only and not with drilling equipment. The platform is suitably installed in water depths in the range of from 15 to 100 m. The present invention further relates to an offshore platform.
USA patent specification No. 4 739 840 discloses a method of installing an offshore platform. The known method comprises lowering to the sea floor a support frame including interconnected open-ended vertical pipes; lowering conductors into the seabed to a predetermined depth through the open-ended pipes of the support frame such that the upper ends of the conductors extend above the water surface; and lifting the support frame to a predetermined level and securing the support frame to the conductors.
Because the support frame is in one piece, this method allows installation of an offshore platform in relatively shallow water only, wherein relatively shallow waters have water depths in the range of from 15 to 50 m.
British patent specification No. 2 222 189 discloses a method of installing an offshore platform, which method comprises lowering to the sea floor a first support frame including interconnected open-ended vertical pipes; lowering conductors into the seabed to a predetermined depth through the open-ended pipes of the support frame such that the upper ends of the conductors extend above the water surface; and attaching to the upper ends of the conductors a further support frame that is provided with a platform deck.
The later method is more flexible and can be used in deeper waters, because the platform includes two separate support frames.
In both methods, the conductors serve as foundation piles and as the legs of the offshore platform.
Yet another method of installing an offshore platform is known from US-A-5 356 239 whereby the platform comprises a plurality of support frames and includes a bottom structure with a polygonal base. The support frames are assembled one above the other and subsequently piles are driven through open ended vertical pipes to which the piles are connected.
It is an object of the present invention to improve the foundation of the offshore platform.
To this end the method of installing an offshore platform according to the present invention comprises the steps of:

(a) providing the sea floor with a set of releasably interconnected support frames comprising a bottom structure including a polygonal base having suction piles joined to its corners and a first support frame connected to the polygonal base including interconnected open-ended pipes that are arranged perpendicular to the base, and at least one further support frame, wherein each further support frame includes interconnected open-ended pipes and has the same cross-section as the first support frame, and wherein the suction piles are fixed in the seabed;
(b) lowering conductors into the seabed to a predetermined depth through open-ended passages formed by the open-ended pipes of the at least one further support frame and the first support frame such that the upper ends of the conductors extend to above the water surface; and
(c) lifting the at least one further support frame to a predetermined level and securing the at least one further support frame to the conductors.

The invention will now be discussed by way of example in more detail with reference to the accompanying drawings, wherein

Figure 1 shows schematically an initial stage of the method of installing an offshore platform according to the present invention;
Figure 2 shows schematically a further stage of the method according to the present invention; and
Figure 3 shows schematically the platform according to the present invention.

Reference is now made to Figure 1. The first step of the method of installing an offshore platform according to the present invention comprises lowering a bottom structure 1 to the sea floor 2. The bottom structure 1 comprises a polygonal base 5 and a first support frame 8 that is connected to the polygonal base 5. The polygonal base 5 has suction piles 10 joined to its corners 11. In Figure 1 only two of the three suction piles 10 are shown. The first support frame 8 is connected to the polygonal base 5 by means of structural elements 12 and structural elements 13, and the first support frame 8 includes open-ended pipes 15 that are arranged perpendicular to the polygonal base 5. The open-ended pipes 15 are interconnected by means of horizontal structural elements 17 at their lower ends, horizontal structural elements 19 at their upper end and inclined structural elements 20.
Lowering the bottom structure 1 is done using a crane on a barge or the hoisting means of a drilling rig that is arranged on a jack-up platform. Neither the barge nor the jack-up platform are shown.
In the position as shown in Figure 1 the lower ends of the three suction piles 10 rest on the sea floor 2.
Next pumps (not shown) on the suctions piles 10 are activated to remove water from the interior of the suction piles 10 so as to cause the suction piles 10 to enter into the seabed, driven by the pressure differential. When this step is completed, the polygonal base 5 rests on the sea floor 2, and the suction piles 10 are fixed in the seabed below the sea floor 2.
To discuss the next steps of the method according to the present invention, reference is now made to Figure 2. For the sake of clarity we do not include the reference numerals for all the features that were discussed with reference to Figure 1.
The polygonal base 5 rests on the sea floor 2, and two further support frames 22 and 24 are lowered to the bottom structure 1 so as to form a set of support frames 8, 22 and 24 that are releasably interconnected. Each further support frame 22 and 24 includes open-ended pipes 25, which open-ended pipes 25 are interconnected by means of horizontal structural elements 27 at their lower ends, horizontal structural elements 29 at their upper end and inclined structural elements 30. For the sake of clarity not all structural elements have been referred to by means of a reference numeral.
In order to allow the support frames 8, 22 and 24 to get releasably interconnected, each further support frame 22 and 24 has the same cross-section as the first support frame 8. Elements that facilitate the interconnection are not shown.
When the further support frames 22 and 24 have been put on the bottom structure 1, conductors 33 are lowered into the seabed to a predetermined depth below the sea floor 2. The conductors 33 are driven through the open-ended passages formed by the open- ended pipes 25 and 15 of the two further support frames 24 and 22 and the first support frame 8. The length of the conductors 33 is so selected that the upper ends 34 of the conductors 33 extend to above the water surface 36.
Driving the conductors 33 is done using suitable pile-driving equipment from the barge or the jack-up platform (not shown).
When the driving operation is completed, the two further support frames 24 and 22 are lifted to a predetermined level above the sea floor 2. In this case, the further support frame 22 is raised to about half way the distance between the sea floor 2 and the water surface 36, and the further support frame 22 is raised to the top of the conductors 33. Having raised the further support frames, they are secured to the conductors 33 by any suitable means, such as welding, swaging and grouting.
It will be understood that the offshore platform according to the invention is used in the production of hydrocarbons from an underground formation via a well. In order to get the well a borehole is drilled and completed in the known manner. This borehole is drilled when the platform is in the stage as shown in Figure 2, thus before lifting the further support frames 22 and 24. To drill the borehole, a jack-up platform provided with a drilling rig is used, and drilling and completing the borehole is done through the conductors 33. The lateral spacing of the conductors 33 is limited by the lateral skidding of the drilling rig. If applied in relatively deep water, suitably, the further support frames are raised and the lowermost further support frame is secured at its predetermined position before drilling commences. This way of supporting the conductors improves the stability of the conductors during drilling.
The fact that the drilling rig on a jack-up platform can only be displaced over a limited distance limits the lateral spacing of the conductors. With the known offshore platform, the conductors form the foundation of the known offshore platform, and consequently, if the foundation needs to be improved, the lateral spacing of the conductors has to be enlarged. However, there is a limitation to the lateral spacing. In contrast to the known offshore platforms, the foundation of the offshore platform of the invention is improved because it comprises the conductors and the suction piles. Consequently, there is no need to enlarge the lateral spacing of the conductors.
In the embodiment as discussed with reference to the Figures, the first step of the method comprises lowering to the sea floor the bottom structure; allowing the suction piles to enter into the seabed; and lowering the at least one further support frame to the bottom structure so as to form the set of interconnected support frames. Alternatively, the first step of the method comprises lowering to the sea floor the set of releasably interconnected support frames comprising the bottom structure and the at least one further support frame; and allowing the suction piles to enter into the seabed.
In order to place the wellhead and/or production equipment, the platform is provided with a deck. Schematically the deck is shown in dashed lines in Figure 3, and the deck is referred to with reference numeral 40. The deck can be placed on the uppermost further support frame 24 after the support frame 24 has been lifted to its predetermined level and secured to the conductors 33. Alternatively the deck is already present on the further support frame 24, when this support frame is lowered to the bottom structure 1. When lifting the further support frame 24, the deck is raised as well.
In the embodiment of the invention as described with reference to the Figures, the polygonal base of the bottom structure is a triangle. Alternatively, the polygonal base is a square or a rectangle. The same applies to the shape of the cross-section of the first support frame and the further support frames.
In the embodiment of the invention as described with reference to the Figures, each structural element 13 is connected to the middle of the corresponding open-ended pipe 15. Alternatively the structural element 13 is connected to the top of the open-ended pipe 15 where it intersects with the structural elements 19.
In the embodiment of the invention as described with reference to the Figures, there are two further support frames, 22 and 24. An advantage of having a small number of further support frames is that the wave loading on the platform is reduced. Thus the number of further support frames is so selected that an optimum is found between wave loading and structural integrity. Suitably the number of further support frames is at least two, and more suitably 2, 3 or 4.
In the specification and the claims, the term suction piles is used. This term is intended to cover as well devices that are known in the art as suction anchors or suction cans.

Claims

Method of installing an offshore platform on a sea floor (2), which method comprises the steps of:
(a) providing the sea floor (2) with a set of releasably interconnected support frames comprising a bottom structure (1) including a polygonal base (5) having suction piles (10) joined to its corners and a first support frame (8) connected to the polygonal base (5) including interconnected open-ended pipes (15) that are arranged perpendicular to the base (5), and at least one further support frame (22, 24), wherein each further support frame (22, 24) includes interconnected open-ended pipes (25) and has the same cross-section as the first support frame (8), and wherein the suction piles (10) are fixed in the seabed;

(b) lowering conductors (33) into the seabed to a predetermined depth through open-ended passages formed by the open-ended pipes (25, 15) of the at least one further support frame (22, 24) and the first support frame (8) such that the upper ends (34) of the conductors (33) extend to above the water surface (36); and

(c) lifting the at least one further support frame (22, 24) to a predetermined level and securing the at least one further support frame (22, 24) to the conductors (33).
The method according to claim 1, wherein step (a) comprises lowering to the sea floor (2) the bottom structure (1); allowing the suction piles (10) to enter into the seabed; and lowering the at least one further support frame (22, 24) to the bottom structure (1) so as to form the set of interconnected support frames.
The method according to claim 1, wherein step (a) comprises lowering to the sea floor (2) the set of releasably interconnected support frames comprising the bottom structure (1) and the at least one further support frame (22, 24); and allowing the suction piles (10) to enter into the seabed.
The method according to any one of the claims 1-3, wherein the set of releasably interconnected support frames (22, 24) comprises at least two further support frames (22, 24).
The method according to claim 4, wherein an uppermost further support frame (24) is provided with a platform deck (40).