WO2001072580A1

WO2001072580A1 - Device for positioning of a ship

Info

Publication number: WO2001072580A1
Application number: PCT/NO2001/000139
Authority: WO
Inventors: Geir Kjersem; Torbjørn BRINGEDAL
Original assignee: Lund, Mohr & Giæver-Enger Marin As
Priority date: 2000-03-30
Filing date: 2001-03-30
Publication date: 2001-10-04
Also published as: BR0105426A; AU4488201A; NO20001666D0; NO20001666L

Abstract

A device on a ship for positioning in relation to the extraction of hydrocarbons at sea is described. The device is characterised in that the ship, approximately at its mid-section or towards one of its end points, on at least one of the ship's sides, is fitted a strength improving jacket/side member construction with a given fore-and-aft length and thickness, said jacket comprises means for storage of anchor lines which run upwards from the bottom of the sea (11), and that the jacket possibly comprises means for carrying out ballasting functions and/or the taking up of riser pipes. The device is in particular suited for rebuilding of tankers, which originally were meant for other transport tasks, to production ships for the extraction of hydrocarbons.

Description

Device for positioning of a ship.

The present invention relates to a device as described m the introductions to the subsequent claims 1 and 2. The invention also relates to an application.

More precisely, the invention relates to a device for the positioning of ships, and in particular ships that are used in activities connected with production of hydrocarbons offshore. The invention also relates to an application of the device.

The invention is especially meant for a production ship based on a conventional ship's hull, but is also particularly suitable when the production ship is formed as a barge.

In particular, an object of the invention is to be able to make use of used ships or hulls which originally are built for tanker shipping for fluids such as oil, chemicals, etc., or other cargo types. The invention relates to a new construction such that these types of ships can now easily be converted so that they can be used as production ships.

Today, different methods are employed to anchor a production ship. One method is to secure the bow and the stern of the ship to anchor lines, with riser pipes being led up either through a turntable in the bottom of the ship, or up m dedicated facilities along the sides of the ship . Another solution is to connect the bow of the ship to a permanent anchored float containing a turntable (i.e. an external turntable) and which then directs the riser pipes onboard the ship by way of this bow section. A turntable is also denoted a «turret», around which the ship can rotate. A turntable can also be inside the ship. However, turret constructions are very costly.

Another solution is to use an internal turntable which is connected to the anchor lines, with riser pipes being led into the ship through the turntable.

It is an object of the invention to provide a new construction for production ships. In particular, the object is to improve the safety of the production snip, and to improve the operating procedures for connecting and disconnecting to the production facilities and the riser pipes from the wells.

It is an object of the invention to adapt older tankers, which originally were not considered for the task of being production ships, to a new positioning device and in a simple and inexpensive way to be able to carry out a more direct connection of the riser pipes and anchoring system, directly from the sea and up into the tanker without the use of a turntable. Both of these systems represent a considerable weight and add large dynamic forces to the construction, and can therefore not be directly fitted to an older ship. In other words, it will result in damaging loads on the basic construction of the tanker (frame, bulkhead and similar strength parts) .

Furthermore, one aims to strengthen the construction of the ship so that it will be able to absorb the dynamic forces, without having to go m and affecting (rebuild) the original basic construction of the ship.

It is also an aim of the invention that it shall dampen the movement of the ship, especially during rolling in the sea.

The device according to the invention is characterised by the features which become evident from the characteristics in the subsequent claims 1 and 2. Further features of the invented devices are described in the independent claims 2-11.

According to the invention, the device is applied as mentioned above to adapt/rebuild tankers to production ships for oil/gas.

A new construction is therefore provided, which can convert older tankers into useful production ships. Furthermore, the extent of which the hull construction is affected by the new, relatively heavy assembly, can be compensated for by suitable ballasting of the set of ballast tanks which constitute a part of the device according to the invention.

The side members of a given length extension, according to the invention can be placed over most of the longitudinal axis of the ship, they can be moved from midships and forward to the bow, or towards the stern, i.e. that end of the ship with is meant to be lying against the weather direction. It is preferred that the side members are placed approximately mid-ship. Furthermore, the side members can comprise an open truss construction or a plate construction with watertight wall structure, or a combination of truss- and plate constructions which can form closed hollow spaces for ballasting . Calculations have shown that by using the invention, the rolling movements of the ship can become so moderate and favourable that one can use cheap steel riser pipes in preference to expensive pipes such as those made from cross-spun steel or plastic. The riser pipes can be pulled up in the side members of the jacket on the one or both sides of the hull, but they can also be pulled up elsewhere m the ship.

With a fore-and-aft placing of the anchor systems, one can omit the use of turntables, as the ship will still to a certain extent turn ± 90° against the weather. It is possible to use the ship' s own propulsion facility or extra newly fitted thrusters for this, if required. The centre of rotation will lie within the section of the ship which is surrounded by the jacket side members, i.e. between the outer walls of the side members. The device according to the invention provides a cheaper and safer solution without any affects worth mentioning to the main construction. The movement characteristics of the ship, especially n rolling, are also improved.

Surprisingly, it has been found that by employing the invention an improved rotation flexibility is achieved during offshore loading of oil m that a connected tanker has more freedom, within smaller angle sectors, to pull tne production ship aside when the current conditions change.

Analyses have also shown that when employing the invention, the drift of the ship in the horizontal plane will be very small with respect to slack anchorage m combination with turntable, and also l relation to adjustment possibilities m scattered anchoring, denoted DICAS.

In general, the device according to the invention comprises a U-shaped frame construction, comprising two side members, which the ship can be led into and secured to so that the frame construction encloses the keel of the ship and sides of the ship with it being built in and integrated as a permanent part of the ship itself, and that it can be pre-fabπcated and be secured to the sides of the hull by welding or other suitable fastening methods. With the device according to the invention, the ship can easily take up anchor lines and possible riser pipes m association with the device itself.

The device according to the invention can also be used on new ships which are under construction.

The device according to the invention shall be explained further m the subsequent description with reference to the enclosed figures, in which:

Figure 1 shows a side view of the production ship. Figure 2 shows a plan section of the ship. Figure 3 shows a cross-section of the ship. The same parts of the drawn details are given the same reference numbers on the different figures. Initially, reference is made to figure 1 which shows the ship 10 m a side view. The ship comprises a bow 12, stern 14, and a superstructure 16. The cargo hull part of the ship is divided into a number of tanks 18 which can contain the oil cargo. The cargo tanks 18 are mutually separated by bulkhead 20 m the usual way.

The ship shown m the figures comprises the device according to the invention in the form of a strength improving jacket of two side stringers on the outside of the ship's sides where it is possible to pull in anchor lines. The lower part of the jacket construction, i.e. the part which is to be under the keel, can be associated with a strengthening of the bottom construction of the ship, i.e. inside the original hull. The ship is shown m figures 1 and 2 at anchor with a number of anchor lines 30 to the sea bottom at 15 (figure 3) . The lines (which can comprise rope, steel wire, chains and the like) are connected to the mid-section of the ship m association with the new device according to the invention which shall be described m more detail below. The ship can be rotated with a relatively large degree of freedom with respect to the prevailing weather direction, by active operation of the drive motors for the anchor lines . The jacket construction has an approximate crib-like U-shape, adapted to the main cross-section of the ship. This means that the construction (seen m cross-section) comprises two primarily vertically running jacket stringers 22,24, the mutual distance of which approximately corresponds to the width of the ship, and a horizontal part 26 which mutually connects the underside of the jacket stringers 22,24, and which remains lying under the keel of the ship. It is horizontal m the widest sense, as the jacket with its given thickness will, of course, follow the contours of any slanting underside surfaces of the keel. For strengthening reasons, it can be practical that the horizontal section is integrated into the bottom construction of the ship, i.e. without it being seen from the outside. It is clear that lying on the outside, it can m a negative way affect the movement of the ship at sea. However, the main task of the ship is to lie steady as a production ship, and then this factor is not so important. Each of the two jacket stringer sections 22,24, and the bottom section 26 can be formed as a plate- or truss construction, they are of a width which is dependent on the lengtn of the ship and the basic construction of the ship at the time it was built, with the width of each side member possibly being 10-15 meters in some cases. Also the fore-and-aft length of the side sections depends on the length of the ship, and on which hull construction that has been used, etc.

The jacket/stringer structure preferably has a fore- and-aft length of about 1/2-1/6 of the total length of the ship, and particularly preferred is that the mentioned ratio is about 1/4-1/5. For a ship of a length 300 metres, the section can have a length of up to, for example, 70 metres, and mentioned width of 10-15 meters. The ratio between width of the stringer and the original width of the ship can be up to 0.3. For a tanker with the abovementioned length and an original width of 50 metres, the ratio between the width of the stringer and the ship' s initial width will be in the range 0.2-0.3 (i.e. 10/50-15/50). The jacket construction, for example, with the abovementioned dimension (s) , can be positioned m many places along the fore-and-aft axis of the ship depending on the dominating weather direction at the anchorage location. This means, for example, much towards the bow if the storms primarily come from one dominating weather sector. If bad weather comes equally from approximately all directions, it can be convenient to place the jacket construction roughly mid-ships. This means that for most cases the jacket will be positioned a place between mid-ships and bow. For re- location, the jacket can be removed by being burnt off

(i.e. a welding operation) and a new jacket constructed at a different place on the ship adapted for the new conditions. In milder waters, such as outside South Africa, one can visualise a passive system while one can adjust the anchor lines in rougher waters.

The transition area between foremost- and rear side stringer and the hull itself, forms a slanting surface 31 (vertical) , to decrease the resistance at speed and the strain from currents and from waves that hit against the side of the hull.

The two stringer sections are preferably divided into a series of smaller mutually separated sections, and some of these are, especially m the sections which are in the

(the side box's) corner of the «crιb» 32,34,36,38 primarily arranged, for example, to contain the drive system/chain winches and coupling bodies for securing and releasing of the anchor lines, and the necessary automation and control systems for the operation of these. Figure 3 shows that these sections are located in the area m which the side members (legs) 22,24 run over into the horizontal bottom section 26. In figure 1 there is also shown two approximately vertically rotating pulleys 40,42 for the respective anchor lines 30. From the area in which the anchor lines run into the section, the lines can be led further vertically upwards to the ship' s deck where the line/chain winch can be fitted.

A series of the sections m the side stringers/legs and in the horizontal section are arranged for ballasting functions, i.e. one can in a controlled fashion fill the tanks with water and empty them of water. This applies both for the tanks that are below and over the water line 13. For this there are channels/pipes which enable a liquid connection to be made between the tank volumes and the sea outside. The channels/pipes can comprise valves which can be regulated to let water into the spaces, and pumping systems to replace the water m the tanks with air. All these elements are controlled by means of data systems so that the ballasting systems can be used to neutralise the additional forces that the ship is exposed to, due to increased strains from different anchorage tension and load contributions from various fitted systems. According to the invention, the two side stringers and the bottom horizontal section can comprise a truss construction of pipes fitted together, beams, rails, etc. Another solution is that the sections comprise a combination of these open truss elements and closed ballast tanks. If the whole construction is a plate construction, the stringers and the bottom part consist of a closed unit with watertight side walls.

According to a preferred embodiment, the new device according to the invention comprises an optional system for guiding up riser pipes 50 from the wellheads 52 on the sea bed 11. The riser pipes are directed up into the bottom part of one or both the two side stringers 22,24 and are directed further up to the upper deck of the ship where the individual riser pipes are further handled in known manner. According to the invention, there are several ways to adapt an older ship (for example an ordinary tanker) for its new task as a production ship.

A) The first solution can be to arrange a jacket as described above, but only on the outside of each of the two sides of an older tanker.

B) The second solution is also to place a jacket under the keel part of the ship, and then it is preferred to incorporate the horizontal strengthening part, possibly including ballasting tanks and -systems, inside the hull down towards the keel part of the ship. This depends to a large extent on the workshop or shipyard which will fit the jacket construction to the ship.

When this new solution is incorporated into a new ship during the design stage, the U-shaped strengthening part will be mounted in the same way as that described above for older ships .

The new jacket constructions can be secured to the ship's hull with the aid of well known methods, such as welding, other mechanical fastening procedures or the like.

Claims

P A T E N T C L A I M S

1. Device of a ship for positioning m relation to the extraction of hydrocarbons at sea, characterised in that the ship, approximately at its mid-section or towards one of its end points, on at least one of the sides of the ship, is fitted a strength-improving jacket/side member construction with a given fore-and-aft length and thickness, sa d jacket comprising means for storage of anchor lines which run upwards from the bottom of the sea (11), and that the jacket optionally comprises means for performing ballasting functions and/or the taking up of riser pipes.

2. Device of a ship for positioning m relation to the extraction of hydrocarbons at sea, characterised m that the ship, approximately at its mid-section (around the point of gravity area) , on at least one of the sides of the ship, is fitted with a strength-improving jacket/side member construction, and that the keel section of the ship comprises an approximately similarly formed jacket, with the ship's side jackets and keel jackets having a given fore-and-aft length and thickness, said jackets, especially m the transition area between the ship's side and keel, comprise means for storage of anchor lines which run upwards from the bottom of the sea (11), and that the jacket possibly comprises means for carrying out ballasting functions and the taking up of riser pipes.

3. Device according to claims 1 and 2, characterised m that the jacket forms a U-shaped construction unit with two side members running alongside the outer side of the ship, with the side members and the bottom jacket section are secured to the ship by welding or by other fastening methods .

4. Device according to one of the preceding claims, characterised in that the keel contacting-forming jacket is built into and integrated into the inner basic construction (ribs, bulkhead, etc) inside in the bottom of the ship.

5. Device according to one of the preceding claims, characterised in that the jacket is secured to the sides of the ship by welding or by other fastening methods.

6. Device according to claim 1, characterised m that the jackets are divided into a number of smaller mutually separated sections, whilst some of these are arranged, for example, to contain the drive system/chain winch and coupling bodies for securing and releasing of the anchor lines, whilst the tank sections can be used for ballasting functions .

7. Device according to claim 2, characterised in that the U-shaped jacket construction is initially formed as a section which can be hoisted and lowered to be positioned surrounding, close up to and round, the hull of the ship (for example an older tanker adapted for tasks other than that of a production ship for oilfields offshore) .

8. Device according to one of the preceding claims, characterised m that the side member construction has a fore-and-aft length which is about 1/2-1/6 of the length of the ship, especially preferred is a length which is about 1/4-1/5 of the length of the ship.

9. Device according to one of the preceding claims, characterised m that the section can have a length of up to 70 metres for a 300 meters long ship, and is placed approximately at the mid-section of the ship.

10. Device according to one of the preceding claims characterised in that the ballasting systems are arranged to be used for neutralising additional strains that the ship is exposed to because of increased loads from changing anchorage tensions and the extra load contribution from the different newly mounted systems.

11. Device according to one of the preceding claims, characterised in that the ratio between the width of the side members and the original width of the ship is up to 0.3 and preferably in the range 0.2 to 0.3 (10/50-15/50).

12. Application of device according to claims 1-10, to adapt/rebuild tankers to production ships for oil/gas.