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Title:
FLOATING SUPPORT
Document Type and Number:
WIPO Patent Application WO/2011/129706
Kind Code:
A1
Abstract:
A support (8) of the deep caisson type, with a longitudinal axis (O) has a lower portion (8c) in a body of water and an upper portion (8a) for support of a platform (2) above a water surface (F). The support comprises an extended portion (8b) in an area between the upper portion (8a) and the lower portion (8c), whereby the support's heave natural period is increased. The extended portion (8b) may comprise one or more internal storage chambers (18) and one or more ballast chambers (16).

Inventors:
HANNUS, Henrik, G (Bauneveien 7, Høvik, N-1363, NO)
LØKEN, Rolf (Anton Tschudis vei 53 A, Haslum, N-1344, NO)
LAUKELAND, Lars (Lensmannsjordet 12, Kolsås, N-1352, NO)
Application Number:
NO2011/000125
Publication Date:
October 20, 2011
Filing Date:
April 14, 2011
Export Citation:
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Assignee:
AKER ENGINEERING & TECHNOLOGY AS (P.O.Box 222, Lysaker, N-1326, NO)
HANNUS, Henrik, G (Bauneveien 7, Høvik, N-1363, NO)
LØKEN, Rolf (Anton Tschudis vei 53 A, Haslum, N-1344, NO)
LAUKELAND, Lars (Lensmannsjordet 12, Kolsås, N-1352, NO)
International Classes:
B63B35/44; B63B39/00
Foreign References:
US20030099516A1
Attorney, Agent or Firm:
ONSAGERS AS et al. (P.O.Box 6963 St. Olavs plass, Oslo, N-0130, NO)
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Claims:
A support (8) of the deep caisson type, with a longitudinal axis (O) for floating installation in a body of water (V) and provided with mooring lines (12, 12') for anchoring to a seabed, where the support comprises a lower portion (8c) in the body of water and an upper portion (8a) for support of a platform (2) above a water surface (F),

characterised in that the support further comprises an extended portion (8b) in an area between the upper portion (8a) and the lower portion (8c).

A support according to claim 1 , where the extended portion (8b) comprises one or more internal storage chambers (18).

A support according to claim 1, where the extended portion (8b) comprises one or more ballast chambers (16).

A support according to claim 2 and claim 3, where the storage chambers (18) are arranged inside, i.e. closer to the support's longitudinal axis (O) than, the ballast chambers (16).

A support according to any of the preceding claims, where the transitions between the extended portion (8b) and the upper (8a) and lower (8c) portions respectively are stepped, with protruding transitions (37) between the portions.

A support according to any of the preceding claims, where the extended portion (8b) is attached to the upper portion (8a) and the lower portion (8c) respectively via respective transitions (37) comprising shear plates (30), whereby the forces between the portions are transmitted as shear forces.

A support according to any of the claims 1-4, where the transitions between the extended portion (8b) and the upper (8a) and lower (8c) portions respectively are bevelled, with bevelled transitions (32) between the portions.

A support according to any of the preceding claims, where the upper portion (8a) has a dimension da perpendicular to the support's

longitudinal axis, the extended portion (8b) has a dimension db perpendicular to the support's longitudinal axis, and the lower portion (8c) has a dimension dc perpendicular to the support's longitudinal axis, and where db>da and db>dc.

A support according to claim 8, where da≥dc.

10. A support according to any of the preceding claims, where the said portions (8a, b, c) comprise circular cross sections.

11. A support according to any of the preceding claims, where the said portions (8a, b, c) comprise rectangular or square cross sections.

12. A support according to any of the preceding claims, where the extended portion (8b) is located below the water surface in which the support is installed in the body of water.

Description:
Floating support

The invention relates to an independent floating caisson with a relatively deep draft, known in the business as "deep caisson" and "spar", especially for use as a support for platforms employed in connection with recovery of hydrocarbons from subsea formations.

Background for the invention

There are a number of types of floating platform, such as for example drilling and production ships, tension leg platforms (TLPs), semi-submersible platforms (semi- submersibles) and so-called spar platforms.

An example of a tension leg platform (TLP) is disclosed in US 4 685 833

(Iwamoto). The publication describes a system for use on an isolated well on the seabed, comprising a basis unit 11 on the seabed, a buoyancy body 13 and a prestressed riser arrangement 12. The riser arrangement anchors the buoyancy body by means of pre-tensioning to the basis unit.

Another example of a tension leg platform (TLP) is disclosed in NO 316267

(B rseth), describing a method and a device for providing a stabilising moment for a TLP platform which is attached and anchored to the seabed.

US 3 408 821 (Redshaw) describes a floating, ballastable column.

A spar platform has a support which substantially comprises a relatively long, columnar structure floating in an approximately vertical position in the water, with one or more buoyancy chambers in an upper part and a stabilising weight in the spar support's lower part. An upper part of the spar support extends above the water line where it supports a platform with, e.g. a drill deck, processing plant or the like. The spar support's relatively slim, elongate shape and relatively deep draft permit this type of support to tackle heave motions better (i.e. longer heave natural periods) than other types of floating platform.

A spar support of this type is described in patent publication US 4 702 321

(Horton). Further examples of spar type supports in different variants are described in patent publications WO 2005/113329 (Horton), US 5 722 797 (Horton), US 4 630 968 (Berthet et al.), US 6 309 141 (Cox et al.), WO 98/29299 (Allen et al.), and US 6 161 620 (Cox et al.).

A platform with a spar support is a well-established structure, employed

substantially in maritime areas with relatively low wave periods, such as off

Malaysia and in the Mexican Gulf. In these waters waves are encountered with a typical period (Tp) of 13-15s for a 100-year state. A design restriction for the spar support is excitation of heave motion at resonance, and the combination of heave and roll/pitch motions for waves with a long period. In maritime areas such as, e.g. the North Atlantic, the wave conditions are considerably more challenging than in the Mexican Gulf, and floating supports designed for the Norwegian Sea have to be designed for wave periods (Tp) of between 15 and 19s in a 100-year state.

When the spar support's heave natural period is excited by waves, it will produce unacceptable motions. Another resonance lies in the coupling of heave and roll/pitch motions, often referred to as Mathieu instability or parametric excitation. This effect occurs at different sum frequencies of heave and roll/pitch.

Thus there is a need for a platform support of the spar type which is better suited for installation in maritime areas with long waves (i.e. high wave periods) than is the case with the known spar supports.

Summary of the invention

The invention is specified in the independent claim(s), and the dependent claims indicate other characteristics of the invention.

Thus a support of the deep caisson type is provided with a longitudinal axis for floating installation in a body of water, with a lower portion in the body of water and an upper portion for support of a platform above a water surface, characterised in that the support further comprises an extended portion in an area between the upper portion and the lower portion, whereby the support's heave natural period is increased.

The extended portion preferably comprises one or more internal storage chambers and one or more ballast chambers. In an embodiment the storage chambers are arranged inside, i.e. closer to the support's longitudinal axis than, the ballast chambers.

In an embodiment the transitions between the extended portion and the upper and lower portions respectively are stepped, with protruding transitions between the portions. In an embodiment the extended portion is attached to the upper portion and the lower portion respectively via respective transitions comprising shear plates, whereby the forces between the portions are transmitted as shear forces.

In an embodiment the transitions between the extended portion and the upper and lower portions respectively are bevelled, with bevelled transitions between the portions.

The upper portion has a dimension d a perpendicular to the support's longitudinal axis, the extended portion has a dimension d perpendicular to the support's longitudinal axis, and the lower portion has a dimension d c perpendicular to the support's longitudinal axis, and db>d a and db>d c . In an embodiment da≥d c .

In an embodiment the said portions comprise circular cross sections. In an embodiment the said portions comprise rectangular or square cross sections.

The extended portion is located below the water surface in which the support is installed in the body of water.

The invention involves an alteration of the geometry on the upper part of the underwater hull, resulting in an increase in the diameter of a section under water relative to the rest of the hull. This provides increased mass without increasing the heave rigidity, but the hull's buoyancy increases and heave mass increases, and thereby also the heave period. The device according to the invention increases the platform's heave natural period, thereby permitting the use of spar platforms in the North Atlantic. The hull's buoyancy and diameter in the water line are reduced, resulting in a reduction in the water line rigidity, which also helps to increase the heave period.

Brief description of the figures

These and other characteristic features of the invention will be explained in the following description of an embodiment of the invention, presented as a non- limiting example, with reference to the accompanying schematic drawings, in which:

Figure 1 is a side view of a first embodiment of the support according to the invention;

Figure 2a is a sectional view along line A-A in figure 1 ;

Figure 2b is a sectional view along line B-B in figure 1 ;

Figure 3 is a sectional view along the support's longitudinal axis, as indicated by lines C-C in figures 2a and 2b.

Figure 4 is a side view of a second embodiment of the support according to the invention;

Figure 5 is a perspective and partly intersected view of a further embodiment of the support according to the invention;

Figure 6 is a perspective and partly intersected view of a further embodiment of the support according to the invention;

Figure 7 illustrates heave response RAO (Response Amplitude Operator) as a function of wave frequency for a spar support according to the prior art and for the spar support according to the invention, compared with a representative wave spectrum;

Figure 8 illustrates heave response RAO for a spar support according to the prior art and for the spar support according to the invention, compared with a representative wave spectrum, for wave frequencies round heave resonance; and

Figure 9 illustrates extreme values for heave response for sea states along a typical 100-year contour in the Norwegian Sea.

Detailed description of embodiments of the invention

With reference to figure 4, the support 8 according to the invention comprises an upper portion 8a, a lower portion 8c, provided with a weight element 10, and an intermediate extended portion 8b. The support comprises buoyancy elements and is floating in a body of water V, and above the water surface F supports a platform 2, which may comprise known units such as a drilling installation, processing plant 4, auxiliary systems 3 and living quarters 1.

The support may be anchored to the seabed (not shown) by means of mooring lines 12 etc. of a known type and in a known manner. Alternative or additional mooring lines 12' are indicated by dotted lines.

The upper portion 8a has a first width d a , the extended portion 8b has a second width db, and the lower portion 8c has a third width d c . In this context, the term "width" refers to a dimension perpendicular to the support's longitudinal axis, as illustrated in the figures. In the embodiments illustrated in figures 1-4, the support has a circular cross section, so that these said widths are diameters. Moreover, as illustrated in figures 2a, 2b and 5, the support according to the invention may have a circular cross section or a square cross section, and the invention, furthermore, is not limited to these cross sectional shapes.

Figure 4 shows how the upper portion 8a has the same diameter as the lower portion 8c, i.e. and how the extended portion 8b has a diameter which is larger than both the upper and the lower portion, i.e. db>d a and d c . Thus it is possible to insert riser 14 or a similar elongate element from installations on the seabed (not shown) into the extended portion's 8b lower edge via a penetration and up to the platform 2 along the outside of the upper portion 8a, as illustrated schematically in figure 4. In this variant the upper portion 8a and the lower portion 8c may be interconnected and in practice be a through-going structure through the extended portion 8b, thereby giving a direct strength connection between the support's upper and lower ends.

Figure 1 illustrates a variant of the support according to the invention' where the upper portion 8a has a larger diameter d a than the lower portion's 8c diameter d c . This permits the riser 14 to also be inserted internally in the upper portion 8a, in addition to internally in the extended portion 8b as mentioned above. This is illustrated in the sectional views in figures 2a, 2b and 3 et al. These figures also show how the extended portion 8b contains internal tanks 18, which may be employed, e.g. for oil storage. For reasons of safety these tanks 18 are arranged centrally in the cylinder, with ballast tanks 16 radially outside. The storage tank(s) may preferably be placed as low as practically possible in the extended portion 8b in order to make a positive contribution to the platform's stability.

The transitions 37 between the three portions 8a, b, c are illustrated in figures 1-5 as stepped transitions 37. This is advantageous from the product engineering and cost point of view. In order to be able to withstand such a sudden design change and to support the loads between the portions, the transitions include bulkheads 30 extending between the upper portion 8a and the extended portion 8b, and between the lower portion 8c and the extended portion 8b. For a circular cross section, see figure 2b, the bulkheads 30 are radial.

The bulkheads 30 act as shear plates, since they transmit the forces in the transitions as shear forces.

Figure 6 illustrates an embodiment of the support according to the invention where the transitions between the upper portion 8a and the extended portion 8b, and between the extended portion 8b and the lower portion 8c are provided as bevelled transitions 32. Such bevelled portions 32 create softer transitions between the portions and can replace the internal shear plates 30 mentioned above. However, this embodiment too may also comprise the above-mentioned internal storage tanks 18 and buoyancy chamber 20, even though these are not shown in figure 6.

As an example, an embodiment of the support according to the invention with a load-bearing capacity of 20,000 tons and an oil/condensation storage tank 18 with a volume of 50,000 barrels, may have the following parameters (see figures 1, 3 and 4 which are schematic and not on the correct scale):

d a = d c = 30 metres d = 40 metres

hi = 170 metres h 2 = 80 metres h = 50 metres

Improvement in heave response is illustrated by comparing motions with a classic spar in typical design waves for the Norwegian Sea. The heave motion's RAOs (Response Amplitude Operator) are illustrated in figure 7, showing where the wave energy is for a typical design wave in the Norwegian Sea. The dotted curve marked He indicates a classic spar support, the continuous curve marked ¾ indicates the support according to the invention, while the dotted curve marked D indicates the design wave spectrum. The curves show that both the concepts will have little response excited by the dominating wave frequencies. The problem for the classic spar support arises in the area where heave resonance starts and it is still noticeable with wave energy. This area is illustrated in figure 8.

Comparison of resulting heave motion for relevant wave periods, represented by sea states along the 100-year contour line in the Norwegian Sea is illustrated in figure 9. The classic spar support shows great sensitivity to wave period due to excitation of heave resonance at higher wave periods. The spar support according to the invention is much less sensitive to wave period, and it is excitation round the top of the wave spectrum which will dominate, and not heave resonance, with the result that this support is much less exposed to the Mathieu instability or parametric excitation between heave and pitch. With its low heave damping level, it is highly probable that a spar support of the known type will be unstable in such waves.