CN1035782C - A lower support structure for a marine structure with a central column - Google Patents

Abstract

A lower support structure for a marine structure (e.g. a sea-based tower) having a central column which is arranged to extend vertically upwardly when the structure is installed at sea and wherein there are at least three feet for resting on the sea floor and at least three legs extending upwardly and inwardly from respective feet to a top end, the top ends of the legs being below the sea-wave affected zone when the structure is installed at sea, the top ends of the legs being supported at a mid-height point, and the bottom ends of the columns being stabilized by tensioning of at least three legs extending from a position between the feet and the top ends, such that the bottom ends of the columns are supported at a position remote from the sea floor.

Description

A lower support structure for a marine structure with a central column

The present invention relates to offshore structures, in particular to a lower supporting structure of an offshore structure with a central column.

The development of offshore oil and gas fields has prompted the design of structures used to support drilling, production and carry equipment. Fluctuating energy prices lead to the need for the most complete offshore structures possible.

For large deck loads, gravity platforms or steel jackets with multiple support columns have been used as economical substructures. However, for relatively light deck loads, substructures formed of single columns penetrating the sea have proven attractive. This single column substructure has been planned for the development of oil and gas fields in the North and South Seas and offshore Australia.

In the case of single-column substructures, most of the weight and cost is in the column itself, as the column has to withstand all the forces applied to the deck. In order to reduce the bending stress in the column, it is necessary to minimize the dead length of the column.

When first installed (and not yet fixed to the seabed), in order to make the building have seabed stability, a triangular base can be properly set on the seabed, and a tripod support can be placed on this base.

Heretofore, tripod-supported tower structures have been proposed in which a column is supported by three isolated legs fixed to the subsea formation. Such a building is disclosed in British Patent Specification 2,116,237. However, this construction has the disadvantage of requiring the columns to extend the full height of the deck from the seabed and the support members needing to be fixed to the seabed formation.

Usually, the exploitation of subsea oil and gas fields is to use a fixed drilling/production platform to transport oil and gas to the shore wharf through the subsea oil inlet pipeline. pumping.

In order to exploit small subsea reservoirs at long distances, it is not economical to provide oil pipelines from the reservoir location all the way to the onshore wharf. In these cases, floating vessels (eg shuttle tankers) can be used to transfer the oil to the terminal.

A problem occurs when crude oil is pumped from subsea wells (and/or subsea reservoirs) into pontoons used to transport the oil to shore terminals when the crude oil flows through areas affected by sea waves. This problem arises because any equipment used to transfer oil from a subsea well through an area affected by waves to a pontoon will be subject to forces due to the effects of wind, waves, currents and tides.

The equipment must be designed to withstand "extraordinary events" caused by these environmental stresses. These events could range from 100-year swells to freakish Category 12-plus hurricanes. Designing a transshipment system to withstand these extraordinary events is a complex task because the pontoons that bring the oil ashore are essentially "weathervanes" or headwinds. Therefore, at a fixed point on the seabed, the oil must be transferred through equipment robust enough to withstand extraordinary events and also have the ability to use the pontoon as a "weather vane".

Installations including such equipment must additionally protect the "riser" or risers from damage by collision with the ship. (The word "riser" has a special meaning in the offshore industry and refers to a pipeline that carries oil from the seabed through the wave-affected area to the surface.)

Since then, pontoons have been known for loading offshore oil from large loading buoys or articulated columns with flexible risers or connected transfer lines. (A device for loading oil from buoys is disclosed in patent specification U.K. 2,250,253). Both the buoy and column have historically been damaged by being out of their intended position, resulting in loss of equipment and corresponding loss of product.

It has been proposed to make a fixed tower with a swivel in which the load arm is mounted on top of the swivel. The fixed tower structure must be a larger member than the load buoys or hinged columns. In order to constitute an economical installation, it has proven feasible to simplify the design of the fixed tower so that it comprises only a minimum number of necessary elements.

One known proposal consists of a tower structure with a central column, three legs and a base member anchored to the seabed with piles. The legs are all secured at their upper ends to a set through which the central post passes and is fixedly connected. The central column extends from the sea floor to a point above the height of the strongest waves expected for an extraordinary event at a particular location (such a tower structure is disclosed in British Patent Specification 2,136,860).

This known proposal has the disadvantage that a riser must either be fixed to the outside of the central column, or it must make a small radius turn at the root of the central column, so that it is within the area of influence of the waves in the area passing through the column. Move up.

In the first case, if the riser is fixed to the outside of the column it will be impacted by the ship. In the second case, the situation is further complicated by the tight radius turn where the tubing is routed around the root of the column. Therefore, a center column extending down to the sea floor is not suitable for receiving a riser (or J-pipe).

In view of the aforementioned drawbacks of the prior art, the object of the present invention is to provide a substructure of a marine structure having a central column capable of holding risers, resistant to waves and other natural forces.

In order to achieve the above object, the present invention provides a lower support structure of an offshore building with a central column. When the building is installed on the sea, the central column is arranged to extend vertically upwards, and at least three of them are used A footing that rests on the bottom of the sea and at least three outriggers that extend upwards and inwards from the corresponding footing to a top. at the midpoint of the central column, characterized by having circumferential horizontal members extending between adjacent legs and radial horizontal members extending from the respective legs towards a central point in the same horizontal plane as the circumferential horizontal members, The bottom end of the central column is fixed against said central point against horizontal movement and is spaced from the seabed by a distance slightly greater than the bend radius of the riser or J-pipe and in a radial direction There are X-shaped braces between the junction of the horizontal member and one leg and the footing of an adjacent leg.

Preferably, there are X-shaped braces between each point where the radial horizontal member connects the legs and the footings of the adjacent legs.

Preferably, each of the X-shaped braces between pairs of adjacent legs has two longer lower arms connected to adjacent footings and two legs connected between the footings and the top ends of the legs. The shorter upper arm at a position between, wherein the upper end of the X-shaped brace is connected to said circumferential horizontal member extending between adjacent legs and said radial horizontal member extending from the corresponding leg to a center point tied together.

Preferably, when the building is installed on the sea, the central column extends to a position higher than the area affected by waves.

Preferably, the circumferential and radial horizontal members are attached to the legs at about mid-point along the length of the legs.

Preferably, it is suitable for fixing the footing to the seabed by driving piles.

Now describe a specific embodiment of the present invention by the example in conjunction with accompanying drawing, wherein:

Fig. 1 is a side view of an offshore bearing tower;

Figure 2 is a more detailed side view of the base and central column forming the base of the tower;

Figures 3 to 6 are plan views along the horizontal line III-VI in Figure 2;

Figure 7 is a side view of the central column and base showing the structure of the riser and J-tubes associated with the base and column;

Figure 1 shows an offshore load-bearing tower comprising a triangular base portion with three legs supporting a central column to which a load-bearing assembly is mounted.

The offshore structure has three footings 10 for resting on the seabed and three steel tubular legs 11 extending upwardly and inwardly from the footings to a top end 12 . When installing the structure at sea, the piles 14 are driven, for example, through the pile sleeves 15, so that the tops of the legs 12 are lower than the area affected by the waves.

Referring now to Figures 2 to 6, which show details of the support arrangement for the central column, Figure 2 is a detailed side view of the building. The lines shown in Figure 2 with Roman numerals III to VI represent elevations above the sea floor, and Figures 3 to 6 provide plan views.

X-shaped braces (generally indicated at 16 ) are disposed in respective planes defined by adjacent legs 11 . There are thus three inclined X-shaped support bases pointing directly towards the top end 12 . Each base plate has two lower arms 17 connected to adjacent footings 10 and two upper arms 18 connected at points 19 on the legs approximately midway between footing 10 and top end 12 . Point 19 draws together circumferential horizontal members 21 extending between adjacent legs (best seen in Figure 5) and radial horizontal members 22 extending from each leg to a central point (or node) 23 . The elements 21 and 22 form a horizontal planar bracing layer.

All sloping walls (17 and 18) and horizontal members (21 and 22) are made of steel pipes of smaller diameter and are welded to the legs 11 and to each other as appropriate.

A relatively large diameter hollow steel column 24 extends from node 23, through top 12, up through sea level 25 to a deck support or swivel seat 26 (to which is attached a swivel assembly 27 - the exact construction of which is irrelevant to the present invention) . The mid-section of the column 24 passes through the sea so that only a single column prevents the passage of waves through the tower. In this case, the wave loads acting on the tower are minimal. The upper ends of the legs 11 support a column 24 at the top end 12 which is halfway up the height between the node 23 and the sea level. Radial members 22 merging at nodes 23 position the bottom ends of columns 24 a greater distance above the seafloor.

The swivel assembly 27 is located on top of the swivel base 26 and supports a load arm 28 with oil pipes. In use, a reciprocating tanker (not shown) receives oil from the oil pipeline for delivery to an onshore offloading terminal.

The structure shown has a particular advantage in that the inclined X-shaped braces 16 and the horizontal braces 21/22 between the legs constitute a structurally effective support for the lower end of the column 24 . This structure does not require full length (ie distance from sea floor to tower) columns as the bending stresses in the columns can act in the plane of the tip 12 and parts 21/22. In addition, the structure does not require planar stays on the seabed formation so that the footings 10 can independently rest on the seafloor.

FIG. 7 is a view substantially corresponding to FIG. 2, showing a riser 29 and J-tube 31 entering at a level close to the sea floor and turning through an appropriate angle to enter the central column at the level of the meridional member 22. 24 lower end.

By properly locating the lower end of the column 24 above the seafloor, the riser and J-pipe can adopt an appropriate radius of curvature to penetrate directly up the interior of the center column to the swivel seat 26, swivel assembly 27 and load arm 28.

Claims (8)

1. A lower supporting structure of an offshore building with a central column, when the building is installed on the sea, the central column is arranged to extend vertically upwards, and wherein at least three foundations are used to rest on the seabed feet and at least three outriggers extending upwards and inwards from the corresponding footings to a top. When the building is installed on the sea, the tops of the outriggers are lower than the area affected by waves, and the tops of the outriggers are supported on the middle point of the central column. characterized by having circumferential horizontal members extending between adjacent legs and radial horizontal members extending from the respective legs to a central point in the same horizontal plane as the circumferential horizontal members, the bottom end of the central column is fixed horizontal movement is prevented at the center point, and the bottom end of the center column is separated from the seabed by a distance slightly greater than the bend radius of the riser or J-pipe, and at the distance between a radially horizontal member and a leg There is an X-shaped brace between the junction and the footing of an adjacent leg. 2. The lower supporting structure as claimed in claim 1, characterized in that there are X-shaped braces between each point where the radial horizontal members connect the legs and the footings of the adjacent legs. 3. A lower support structure as claimed in claim 1 or 2, wherein each of the X-shaped braces between pairs of adjacent legs has two longer ones connected to adjacent footings. The lower arm and two shorter upper arms are connected to the legs between the base feet and the top ends, wherein the upper ends of the X-shaped braces are connected to said circumferential horizontal members extending between adjacent legs and from Said radially horizontal members with respective legs extending to a center point are bundled together. 4. The lower support structure of claim 1, wherein when the structure is installed offshore, the central column extends to a position above the area affected by waves. 5. The lower support structure of claim 4, wherein the circumferential and radial horizontal members are connected to the legs at about mid-point along the length of the legs. 6. The lower support structure of claim 1, wherein the footings are adapted to be secured to the sea floor by piling. 7. The lower support structure of claim 3, wherein the footings are adapted to be secured to the sea floor by piling. 8. A lower support structure as claimed in claim 4 or 5, wherein the footings are adapted to be secured to the seabed by piling.

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