Slender flexible marina structure for hydrocarbon production and ship mooring in deep seas

A flexible structure of controlled yieldability for use as a hydrocarbon drilling and production platform in deep seas, or as a mooring point for oil tankers, constituted by a foundation base assembly, a vertical cylindrical tubular element placed under tension by a buoyancy chamber disposed at its top in proximity to the water surface, and an overlying lattice which at its top supports the plant platform. The tubular element is connected at its lower end to the base and at its upper end to the buoyancy chamber by means of suitable profiled tapered terminal elements able to withstand the bending loads which are generated at said connections. The entire structure is constituted by four separate pieces, namely the foundation base assembly with the lower terminal element, the lower half of the cylindrical tubular element, the upper half thereof, and the upper buoyancy chamber which is connected at one end to the upper terminal connection element and at the other end to the top lattice. For transportation purposes, these four pieces are assembled telescopically in pairs. On final on-site installation, the telescopic tubular parts are connected together and to the respective profiled terminal elements by mechanical clamps which restore complete structural continuity along the entire axis of the structure.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a structure which can be installed in deep seas 
and is able to support at its top a plant complex designed for various 
industrial activities in open sea, in particular it being usable 
advantageously as a hydrocarbon production platform and a mooring and 
loading point for oil tankers for sea depths exceeding 1000 m. 
2. Description of the Prior Art 
Structures such as the braced derrick and articulated derrick have been 
proposed and designed for hydrocarbon production in deep seas. The braced 
derrick, being a "yieldable structure" with its first intrinsic period 
above the wave period range (.gtoreq.30 s) and its second intrinsic period 
below this (.ltoreq.7 s), has a range of use in terms of water depth which 
is rather limited, and cannot exceed a bed depth of 500 m. 
This structure also appears too complicated, sophisticated and thus costly 
to allow its use to extend to production in marginal (medium-small) oil 
and gas fields. 
The articulated derrick has the drawback of possessing a critical 
mechanical member, namely its universal base joint, in a zone which is 
inaccessible for direct inspection and maintenance. Moreover, the 
structural discontinuity constituted by said universal joint means that 
the oil feed conduits which run along said structure must also comprise 
hinges to allow structural rotation. If the structure is used as a 
production platform, this configuration does not allow the well heads to 
be disposed at the surface, but instead requires the use of underwater 
well heads, leading to a considerable reduction in system reliability and 
a significant increase in both installation and operating costs. 
For the deep-sea mooring of ships, certain of the authors of the present 
invention have patented (U.S. patent application Ser. No. 393,310 filed on 
June 29, 1982) a flexible monolithic structure concept with a buoyancy 
chamber close to its top, having certain apparent analogies with the 
structure proposed herein. 
The structure corresponding to the aforesaid patent has its first intrinsic 
period above the wave period (.gtoreq.30 s) and its second below the 
period of waves with a significant energy content (.ltoreq.7 s). This 
dynamic behaviour limits the application of the concept to a water depth 
which cannot exceed 500-600 m. 
Finally, its method of manufacture and installation, which require it to be 
constructed, transported and installed in a single monolithic piece, 
itself constitutes a limit to the depth which can be attained. A further 
concept which seems to have some analogy with the present invention is the 
SALM mooring buoy composed of a partially immersed buoy body connected to 
the sea bed by a vertical chain tensioned by the upward thrust on the 
buoy. This method cannot be extended to deep seas because, in such a case, 
in order to ensure the necessary rigidity of the mooring system against 
horizontal traction, a very high tension (many thousands of tons) would 
have to be applied to the anchoring line, and this could in no way be 
withstood by an element of chain type. 
SUMMARY OF THE INVENTION 
The structure according to the invention consists essentially of a long 
vertical cylindrical tubular element connected, by means of profiled or 
tapered terminal elements, at the bottom to a wide base and upperly to a 
buoyancy chamber which itself supports an emerging lattice carrying the 
plants at its top. 
The foundation base can be stabilised either by the effect of its own 
weight or by piles driven into the ground. 
The tubular column and its lower and upper terminal elements can be 
constructed of steel, reinforced concrete, composite components 
(steel-concrete-steel) or other materials. 
The purpose of the upper buoyancy chamber is to place the vertical tubular 
element under high tension and thus ensure that the structure is able to 
sufficiently oppose horizontal forces applied to its top. 
Compared with the SALM buoy systems, the present structure when using a 
steel tube as its vertical tensioning element enables very high tensions 
of the order of 10,000 tons or more to be attained, so providing the 
necessary overall system rigidity even in sea depths exceeding 1000 m. 
The emerging upper lattice, connected rigidly to the buoyancy chamber, 
supports at its top the plants required for the use to which the structure 
is put. 
The conduit or conduits which convey the crude oil from the sea bed to the 
surface run along the axis of the structure, either on the inside or 
outside of this latter, and are supported thereby. The central part of the 
tubular column is of constant cross-section, and when in operation is 
subjected practically only to axial tensile stress. The lower and upper 
terminal connection elements are however also subjected to considerable 
bending stresses, both static and dynamic, and their rigidity increases 
towards the joint so as to be able to support said bending stresses. 
The internal structure is constructed in four separate pieces, of which the 
first is constituted by the foundation base and lower terminal element, 
the second by the lower half of the cylindrical column, the third by the 
upper half of the cylindrical column, and the fourth by the buoyancy 
chamber connected at one end to the upper terminal element and at the 
other end to the emerging lattice. For transportation purposes the second 
and third pieces are inserted telescopically into the first and fourth 
piece respectively. For site installation purposes, the telescopic parts 
are withdrawn and connected together and to the other two parts, namely 
the lower and upper part, by mechanical clamps which are located in zones 
not subjected to bending moments and which re-establish the complete 
structural continuity of the structure from the sea bed to the surface. In 
contrast to articulated derricks, the structural continuity of the present 
invention enables the oil feed conduits to be run along the structure in a 
structurally continuous manner as in the case of conventional fixed 
structures, and thus, if used as a production platform, the well heads can 
be disposed on the surface platform. 
This makes the proposed structure suitable for exploiting marginal 
(medium-small) oil and gas fields in very deep seas, both because it 
represents a very low-cost design, and because it enables the same plants 
and operational methods already used in fixed shallow sea structures to be 
utilised. 
It should also be noted that the proposed transportation and installation 
method, with the structure divided into parts held together 
telescopically, enables a much greater depth to be reached than in the 
case of similar monolithic structures. In the current art, marine 
structures have a dynamic behaviour characterised by very short intrinsic 
periods (.ltoreq.4 s), less than those of waves with significant energy 
content, in order to prevent resonance phenomena. Other structures, such 
as the braced derrick, have their first intrinsic period longer than the 
wave periods and their second intrinsic period shorter than the period of 
waves with appreciable energy content. 
In contrast, from this aspect the structure according to the present 
invention has no limitation. By virtue of its very high flexibility, its 
dynamic behaviour approaches that of a taut cable, or that of a drilling 
riser tensioned at its top, and it can therefore also withstand intrinsic 
periods which lie within the wave period range (typically from 7 to 20 s) 
without consequent resonance phenomena creating unacceptable states of 
stress. A typical configuration of this structure for 1000 m of sea depth 
has the following intrinsic periods: T.sub.1 =90 s, T.sub.2 =20 s, T.sub.3 
=12 s, T.sub.4 =8 s, from which it can be seen that the periods T.sub.2 
and T.sub.3 can generate resonance phenomena in that they lie clearly 
within the wave period range. Careful dynamic analysis has shown that such 
resonance phenomena are in reality very small, both because of the high 
degree of structure motion damping in water, and because the wave forces 
vary along the vertical (F.sub.2, F.sub.3 of FIG. 2) in a manner which 
opposes the shape of the mode of vibration (M.sub.2, M.sub.3 of FIG. 2) 
corresponding to the resonance period.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1, the tubular central element of constant cross-section is divided 
into two parts, an upper part 1 and a lower part 2. The two parts are 
connected together by a mechanical connection, 3. The upper part is 
connected by a mechanical connection 4' to the mechanical connection 4 on 
to the upper terminal connection element 5 and has a smaller diameter than 
the element 5 so that it may be positioned or telescoped therein. The 
lower part is connected by a mechanical connection 6 to the lower terminal 
connection element 7 and has a smaller diameter than the element so that 
it may be positioned or telescoped therein. This arrangement is useful in 
transporting and constructing the structure as more fully set forth below. 
The said mechanical connections are used to connect the various parts of 
the structure together during the installation stage, and are such that 
when the connection is made they provide structural continuity between the 
elements. 
Structure stability on the sea bed is provided by the foundation base 
assembly, which is composed of a tubular element lattice structure 8 and 
foundation bases 9. 
If the gravity method is used, the foundation bases must contain the 
necessary ballast to ensure stability on the sea bed. As an alternative to 
this method, stability can be provided by foundation piles driven into the 
ground. 
The upper terminal connection element is rigidly connected to the positive 
buoyancy chamber 10 which is positioned in proximity to the sea surface. 
The emerging structure 11 connected to the buoyancy chamber is composed of 
a tubular element lattice structure or a single tubular element. On the 
upper end of the emerging structure is installed the platform 12 
containing the plants necessary for the use of the structure. The conduits 
13 for conveying the crude oil from the sea bed to the surface run along 
the axis of the structure over its entire length and may pass through the 
member 14 as shown in FIGS. 1A and 1B. In addition, the conduits 13 may be 
situated internally of the tubular elements as shown in FIG. 1C. 
A description is given hereinafter of the procedure for constructing, 
transporting and installing the structure according to the invention. As 
the structure is of telescopic design and is divided into two structures 
to be connected together on the installation site, the installation can be 
carried out over two different time periods. With reference to FIG. 3, the 
formation stages are as follows: In stage I, the lower portion of the 
central tubular element of constant cross-section 2 is inserted into the 
lower structure formed from the foundation base 8, 9 and lower terminal 
connection element 7 as shown in FIG. 5. The first sub-structure assembled 
in this manner is transported horizontally (stage II). In stage III, 
certain compartments are progressively flooded in order to rotate the 
structure into a stably floating vertical position. 
Further ballasting with water (stage IV) enables it to be installed on the 
sea bed with the aid of surface means. 
At this point, stability of the structure on the sea bed must be ensured, 
and this in the case of a gravity method is done either by feeding solid 
ballast into the foundation bases or, if the bases already contain 
ballast, by flooding the base buoyancy chambers which were kept empty 
during transportation. 
In the case of the method using piled bases, piles must be driven in in 
order to ensure structural stability under any sea condition. In stage VI, 
the upper portion of the central element 1 of constant cross-section is 
inserted into the upper sub-structure formed from the upper terminal 
connection element 5, positive buoyancy chamber 10 and emerging lattice 
structure 11 as shown in FIG. 4. 
The second sub-structure assembled in this manner is transported 
horizontally (stage VII). 
In stage VIII, certain compartments are progressively flooded in order to 
rotate the structure into a stably floating vertical position. 
In stage IX, the lower portion of the central element 2 contained inside 
the lower sub-structure is made to rise by pulling it from the surface, 
until the already prearranged mechanical connection, 6 between said 
element and the lower terminal connection element is implemented. 
Simultaneously with this, by flooding suitable compartments and with the 
aid of winches inside the buoyancy chamber, the upper portion of the 
central element 1 contained in the upper substructure is lowered until the 
already prearranged mechanical connection 4 between said element and the 
upper terminal connection element is implemented. 
At this point, by partially flooding the buoyancy chamber, the upper 
sub-structure is made to emerge until the mechanical connection 3 between 
the two sub-structures is implemented. 
On termination of this operation, ballast water is removed from the 
buoyancy chamber to give the structure its final operating tension. A 
continuous structure from the sea bed to the surface is thus formed in 
which the three mechanical connections which have enabled installation to 
be carried out are able to re-establish the structural continuity between 
the connected elements. 
In stage X, the vertical conduits for the crude oil flow from the sea bed 
to the surface plants are launched. The superstructures containing the 
necessary plants 12 are also installed.