WO1998030438A1

WO1998030438A1 - Arrangement of drilling and production ship

Info

Publication number: WO1998030438A1
Application number: PCT/NO1998/000002
Authority: WO
Inventors: Geir Kjersem
Original assignee: Lmg Marin As
Priority date: 1997-01-07
Filing date: 1998-01-07
Publication date: 1998-07-16
Also published as: BR9806735A; GB9915680D0; NO970448D0; GB2335170A; AU5683298A; GB2335170B; NO970448L; NO313794B1

Abstract

There is referred to an arrangement of drilling/production ship for hydrocarbons, comprising a drilling section having a drill stem (24), a number of anchoring lines (78, 80; 82, 84) which extends between anchorings on the bottom of the sea and a rotatable anchoring fastening (83) in the ship which can thereby be turned in a horizontal plane, the rotational axis of the drilling section (24) largely coinciding with the rotational axis of the anchoring fastening (83), and a number of anchoring lines (78, 80; 82, 84) which extend between anchorings on the sea bottom and an anchoring - line fastening (83) designed in the ship, which preferably is rotatable, for example in the form of a swivel, whereby the ship can be turned in a horizontal plane, the drilling axis (24) largely coinciding with the rotational axis of the ship and the anchoring fastening (83), and a number of riser pipes (62) for hydrocarbons lead via the sea bottom (66) upwardly to a connecting unit (70) and for further handling of hydrocarbons. The arrangement is characterised in that unit (70) of the ship for connecting the riser pipe(s) (62) is/are positioned at a distance from the drilling section of the ship, and the ship is adapted to rotate about the axis (24) in a horizontal plane over a sector of up to about 360°.

Description

Arrangement of drilling and production ship.

The present invention relates to an arrangement of a drilling and production ship comprising a drilling section having a drill stem, a number of anchorage lines which extend between anchorings on the sea bottom and an anchoring 's - line fastening designed in the ship, whereby the ship can be turned in a horizontal plane, the drilling axis substantially coinciding with the rotational axis of the ship and of the fastening of the anchoring, and a number of riser pipes leading via the sea bottom upwardly to a connecting unit on the ship.

By way of example the riser pipe is employed as a transfer conduit for hydrocarbons from a reservoir and upwards to the ship, and which thereafter undergo necessary additional handling such as their being led to the ship's own processing installation or to a storage installation, or so the riser pipe can be used for injecting fluids such as water, carbon dioxide and the like .

The present invention relates to a single hull vessel, which can carry out drilling operations and which at the same time undertakes the production of petroleum (oil and gas) via separate, flexible riser pipes (conduits), and possibly for storage and discharge of crude oil out at sea. Flexible riser pipes can have varying rigidity and can be made of steel, plastic, combinations of plastic and steel, and of a plastic/steel composite material. The invention is especially directed to two variants of the afore-mentioned type of ship, where the petroleum production can be conducted simultaneously with and independently of drilling operations. With the invention the aim is to provide an arrangement of a hull construction which makes possible the production of oil and/or gas from a single hull vessel which is anchored relative to the sea bottom at the same time as drilling operations can be carried out, and where the ship can rotate round an axis close or in the drilling centre of the ship.

Drilling and production for oil and gas at sea is accomplished either with floating drilling vessels or floor-fastened devices. The known floating types of vessel for drilling operations can either be half submergible drilling rigs, which are also called semi-subs, or can comprise drilling ships. In the following description the term "ship" is employed for a single hull ship.

Floating production devices at sea have hitherto been either based on semi-sub (half submergible) hulls or conventional tanker hulls. Semi-sub hulls have the advantage that they can combine drilling and production. This is due to the fact that they are anchored in a position, without being rotated by weather and wind. Thus the flexible riser pipe can be brought from the sea bottom up to a fixed side of the semi-sub, while the drilling operations can continue in the centre of the semi-sub, approximately independently of the production operations. Semi-subs are besides very well suited as drilling vessels. This is due to the fact that they have small water line areas and favourable response for operations in inhospitable waters. That the vessel has a favourable response means that the swings, that is to say amplitudes, during heaving, rolling and pitching are relatively small in large waves.

The drawback with sem-subs is that they have relatively little carrying capacity, and are dependent on continuous supplies of materials of consumption during the drilling operation. Semi-subs are sensitive to centre of gravity movements on deck, are expensive to build and cannot store oil without being constructed large and very expensive. To date semi-subs having a combined drilling installation and oil storage are not built.

In recent years ships have had an increasing success as floating production devices. This is due to the fact that they are relatively reasonable, they can be readily rebuilt from tanker to combined production and storage ship for oil, they can take large payloads, are little sensitive to the shifting of deck cargoes, and they can be easily moved to new oil fields after the first field is exhausted. In the oil industry it has long been desirable to combine drilling with production on board ship. If there could be produced a practically feasible solution for such a combination of drilling and production on a ship, it would mean big advantages for the oil industry. Such as for example that the ship can thereby itself drill the necessary wells instead of having to hire in expensive drilling rigs. In addition the operator himself can maintain the wells from the ship, as well as drill branch wells when the production vessel has its own drilling installation. The economic profit can be large when one has easy access to suitable wells. This has been experienced with fixed installations which have this type of plant, such as the Statfjord, Oseberg, and Ekofisk platforms and on other installations. When one has suitable drilling equipment one escapes waiting for free rigs, the working life of the field is extended, and expensive operation time, etc. is saved.

However it has been found to be unfavourable to gather in both anchoring system, riser pipe system and drilling installation on a small, limited region on board a ship. A production ship will lie anchored in a relatively complex buoy and swivel arrangement and will rotate around this in order to minimalise the environmental forces on the ship. The swivel lies normally in the middle or the front part of the ship so that wave and current forces will be able to turn the ship freely. In theory one could place the drilling systems in the buoy and swivel area, but in practice this is especially difficult and filled with risk. In patent specifications GB 2231356 and NO 171958 it is proposed to combine drilling, anchoring and drawing up of riser pipes in the same area of the vessel. The area includes in addition to the anchoring systems several hydrocarbon guiding conduits under pressure. A drilling area has a very high activity, for one thing in that almost continuously there is effected shifting and lifting of heavy equipment, like pipes, safety valves against blow-out, etc. The accident statistics reveal that this is one of the most dangerous areas on an offshore installation. The heaviest lifted can be over 250 tons. With a series of dangerous systems gathered in an area where at the same time men reside who often work with lifting and drilling operations under the pressure of time, it is clear that the risk level will be undesirably high. This is confirmed in that drilling combined with production on board swivel-anchored production ships has hitherto not been realised. The desire to combine drilling and production on board ship has however long been existent because one avoids with this the hiring in of expensive, floating drilling rigs for drilling and the maintaining of the associated underwater wells, at the same time as retaining the many advantages of the ship.

GB patent specification GB 2066758 specifies a production ship without an indication of a suitable drilling installation. The ship can maintain hydrocarbon wells through the well of a ship placed approximately amidships. Fig. 3 of the patent specification mentions the drawing up of a flexible riser on the outside of the ship, at the same time as one is able to operate equipment through the central ship's well of the ship. However the ship is not anchored, but employs instead dynamic positioning. A fixed anchoring of this ship with several lines will approximately make a solution impossible because the chosen embodiment of the ship is dependent upon being able to freely rotate 360° in order to be able to accommodate waves, currents and wind from all directions. In addition the solution described in the patent specification appears to be directed to production from a single underwater well which is desirably in brief test productions or in early productions over a limited time period.

In Norwegian Patent Application NO-940352 there is described a ship which will be able to combine production and loading of oil at sea. The ship has a service shaft forward for receiving riser pipes. For the cargo transfer of oil to another ship it is indicated that it is advantageous that the ship has a bowed stern for reducing forces from waves and wind. If the vessel described is to be able to be operated with suitable drilling installations, this will have to be placed over the service shaft forward. For reasons mentioned earlier, this is a solution of little favour. The British patent specification GB 2276353 also indicates a way for transferring hydrocarbons by employing the stern of a ship.

In Norwegian Patent Application No. NO-943085 a corresponding ship as in NO 940352 is specified, but where a placing of a well maintenance system on the outer side of the ship is proposed. The patent specifications GB-

2122140 A and NO-162062 have indicated a solution with two ship's wells, one for drawing up riser pipes and the other for well engagement. It is clear that these inventions put big demands on the positioning of the ship, and that this combined operation must occur during weather conditions which allow being able to maintain the desired position of the ship. Another solution of combining drilling and production in a ship's solution, is specified in NO 171957 (Single Buoy Moorings Inc.) . Here the ship functions only as production and storage, while swivel, drawing up of riser pipes and drilling systems are placed on board a tie bar platform. The solution is very expensive in that two floating installations are needed, where it has been found specifically that tie bar platforms are complicated and expensive . In the Norwegian Patent Specifications NO-166479, NO- 170878 and in British patent publication GB-2275230 different solutions are described connected to the drawing in of flexible riser pipes in a swivel on board a production ship. Common to these solutions is that they are relatively complicated and illustrates thereby that it will be problematic and questionable as to safety if in addition drilling systems should be installed in the same area .

It is an object of the present invention to produce a new construction of a ship, so that the ship can be employed for simultaneous drilling and production of hydrocarbons .

The arrangement of the ship according to the present invention is characterised in that the unit of the ship for connecting the riser pipe(s) is positioned at a distance from the drilling section of the ship, and the ship is adapted to rotate about the axis in a horizontal plane over a sector of up to about 360°. The ship can thereby operate relative to the environmental forces like wind, waves, current and which come in from a sector of 360°, that is to say round the whole circle, so that the whole weather sector of 360° is covered. Normally it is preferred that the ship is turned with the bow or the stern towards the weather direction occurring at any time (that is to say the resultant force of waves, wind and current) , or that the longitudinal direction of the ship is adjusted in a slightly oblique position relative to the direction of the weather.

Especially preferred constructions of the arrangement according to the invention are defined in the following dependent claims .

The ship according the invention is applied for combining the production of oil/gas with drilling on board the ship so that the established, strong demands for safety on board can be met, at the same time as the ship and the systems on board can be operated the year round, approximately independently of the weather conditions.

This is achieved according to a preferred construction of the invention in that the ship is equipped with drilling systems, where the drilling preferably occurs approximately at the mid-point of the ship, and also that the axis of the drilling ( the drill stem) will largely correspond to the axis of rotation of the ship in the horizontal plane. At the same time the ship will be anchored to the sea bottom so that it can rotate a minimum of about 180° round this axis, especially due to both the bow and stern of the ship being so constructed that this option can be positioned up against the weather direction occurring at any time, (that is to say the wind and wave direction) . Thereby the whole weather sector of 360° is covered.

It is advantageous the anchor lines are fastened to a rotatable anchoring fastening, for example a rotating swivel, where the swivel has the same rotational axis as the ship. With anchoring amidships, the ship will be dependent upon a propulsion installation in the one or both ends, where the propulsion installation can provide a pushing force sideways, so that the ship can be held in the correct position against the weather the whole time. With anchoring adjacent the bow/stern the ship will be held largely automatically in the correct position against the weather, and there will be less use of the propulsion installation. Since the ship is anchored, the need for a pushing force will be far less than that which is required with dynamic positioning. In order that the ship will be able to operate with varying wind and sea conditions which can arise over a sector of 360° round the ship, at the same time as the rotation of the ship is limited to about 270°, preferably 180°, and according to a preferred example of 120°, it is necessary that both the bow and stern portion of the ship are designed for the purpose of being able to withstand the weather conditions which are expected to occur in the region of interest, and from the directions in point. A complete symmetry of bow and stern portion is however not required, which for example is built into pendulum ferries. Both the bow and stern portion of the ship ought however to be designed to be directed up against the wave/wind direction in question.

Another important feature of the invention is that the flexible riser pipes are guided up to the ship outside the rotational axis of the ship, either from the outside of the ship or inside the ship, and are hung up in a suitable pipe suspension from which the riser pipes are led further for necessary handling, to the processing installation of the ship or for further transportation.

According to an especially preferred construction the flexible riser pipes are guided up through an opening in the ship outside the rotational axis of the ship, by way of example upwardly through a well ( moon pool ) . In this alternative it is favourable as to safety that the pipe suspension for the riser pipes is localised as far as possible away from the dwelling quarters of the ship. The pipe suspension shall be designed so that the riser pipes will be able to be led in towards the ship from a sector of about 180°, possibly up to 270°, relative to the side of the ship. A preferred construction of the pipe suspension, regardless of placement, is that it is designed with a rotatable anchoring fastening (swivel) which can absorb said rotation of up to about 180°. A number of riser pipes are led from the frame of the well, which preferably is to be found on the sea bottom near to the rotational axis of the ship, first in a direction away from the well frame and thereafter up between two of the anchor lines of the ship, and thereafter are led over the anchor lines, preferably below the surface of the sea in a direction towards the side of the ship and finally vertically upwards towards the pipe suspension. The position of the flexible riser pipes can be regulated with necessary floating and submersible bodies, possibly together in combination with single anchoring systems. This type of control of the leading in of flexible riser pipes is well known and is used inter alia on the Troll B platform in the Norwegian sector of the North Sea. An advantageous construction of the pipe suspension on the outside of the ship, is when this can also function as a wave shield of the riser pipes where these are led vertically through the surface of the sea. If the riser pipes are led up through an opening/well in the ship, it is not essential to have extra wave protection. A preferred embodiment of the invention is that the pipe suspension on the outside of the ship, can in addition be displaced along the side of the ship to a sufficient length which results in the incoming riser pipes not contacting the anchoring lines of the ship. If the riser pipes are led up through an opening/well in the ship, such a displacement is not necessary.

A variant of the invention is to place the drilling installation in the front portion of the ship, and anchor the ship in a rotatable anchoring fastening (a swivel), so that the drilling occurs approximately in the centre of the anchoring fastening. In this case the ship will be able to lie free towards the weather, and be in a condition to rotate approximately close to 360°. The ship needs its own propulsion installation which functions transversely, so that the ship can be rotated back when the limit of rotation is reached. In this case the pipe suspension is placed in the bow portion of the ship, on the outside of the hull, as in the construction described first, and preferably ought to be able to be pushed around the bow portion and along the side of the ship on both sides of the bow, to a length which results in the incoming riser pipes not contacting the anchoring lines of the ship when the ship lies with the bow up towards the weather and makes a necessary rotation. When the riser pipe suspension is displaced this can take place via a rail system which is either mounted within on the deck of the ship (compare Fig. 6a) or along the side of the ship. Also in this case a number of riser pipes will be led from the well frame, which preferably are located on the sea bottom near the rotational axis of the ship, first in a direction away from the well frame and thereafter up between two of the anchor lines of the ship, and be led thereafter over the anchor lines, preferably below the surface of the sea in a direction towards the side of the ship and finally vertically upwards towards the pipe suspension. The vertical and horizontal position of the flexible pipe risers can be regulated with necessary floating and submersible bodies, possibly in combination with simple anchoring systems. This variant will involve that to a special degree attention must be paid to the design of the ship with regard to movements, because pitching movements of the ship mean more the longer forward the drilling is to take place.

The detail solutions for the design of the hull will be different depending upon whether the drilling installation stands in the centre or forward of the ship. When the drilling installation stands forward attention must be paid to a great degree to pitching, while with the drilling installation amidships one must give priority to reducing the heaving movements of the ship.

An aim setting and a main feature of the invention is to provide a ship which can combine drilling and production at the same time as all demands for safety on board can be taken care of. This is achieved according to the invention in that the area for drilling on board ship is kept separate from the area for the drawing in of flexible riser pipes. This contributes at the same time to utilising the many priorities which a hull of a ship has for operations at sea, not least economic, compared with semi-sub drilling rigs.

The ship according to the invention for combining drilling and production on board ship will be explained further in the following description with reference to the accompanying Figures, in which:

Figure 1 shows a perspective view of a combined drilling and production ship according to the invention according to a first construction where two first alternatives for connecting the riser pipes of the ship, are shown.

Figures 2 and 3a show respectively a side view and a plan view of the combined drilling and production ship according to Figure 1 according to the first alternative mentioned in Figure 1. Figure 3b shows a plan view of the combined drilling and production ship according to the second alternative mentioned in Figure 1.

Figures 4 and 5 show respectively a side view and a plan view of a combined drilling and production ship according to the invention according to a second construction.

Figure 6a shows schematically a possible solution for the mechanism for guiding on board a riser pipe to a suspension outside the side of the ship, plus an arrange- ment for transferring the suspension along the side of the ship.

Figure 6b shows schematically an alternative on board guiding unit where the suspension is mounted to a rail unit along with the deck of the ship itself. Figure 6c shows a cross-section of the swivel suspension in which the riser pipe can be turned about an vertical axis. Figure 7 shows a cross-section through a hull portion with well, and where a number of riser pipes are directed up into the well.

Figure 8 shows an example of the rotary movement of a drilling ship when two anchor lines are arranged with an angular spacing of 120°.

Like parts of the drawn details are given the same reference numerals in the different Figures.

By way of introduction reference shall be made to Figure 1 which shows a perspective view of a drilling and production ship which is built designed on the basis of the hull 10 according to the invention.

Figure 1 shows a drilling and production ship with hull 10 with an amidships side portion 12, keel portion 14, a bow portion 16 and a stern portion 18. Middle aisle 12 of the hull can have largely perpendicular sides of the ship through the surface of the water. The drilling and production ship comprises a derrick 22 from where an oil/gas well, which leads down into the sea bottom, is drilled or is operated by means of a drill stem 24 or similar equipment. The stem 24 passes from the drilling tower 22 of the ship downwardly through a vertically extending well 28 through the ship. The stem 24 defines a vertical axis which the ship largely turns about. The ship is anchored relative to the sea bottom 23 via a number of anchor lines, in this instance four lines 76, 78, 80. 82. These anchor lines can be fastened to a rotatable anchoring fastening, for example a swivel 83, which is arranged within the ship, and stretch obliquely outwards and downwards in the form of a fan to a fastening on the sea bottom 23. At the starting point the ship can rotate 360° in the horizontal plane about the swivel 83. The swivel thus has the same rotational axis 24 as the ship. Besides the application of a swivel is optional since the anchoring lines can be securely anchored in/at the keel portion of the ship so that they can take up without problem a partial rotation of the ship, for example over up to 270°.

In addition to the well 28 the ship can comprise additionally a well 26 or 30 (aft or forward) . In Figure 1 the connecting of the flexible riser pipe is shown according to two alternatives.

According to a first alternative the riser pipe is led upwards on the outside of the ship 10 to a suitable pipe suspension 70, via a possible swivel 72, following which the riser pipes 62 are conducted further in to the processing or storage installation of the ship on board.

This is shown by the reference numeral 62.

Alternatively then the riser pipe, shown by the reference numeral 62 ' , can be led upwardly within the ship through the extra well, 30 or 26.

The riser pipes (62 or 62') lead upwards, from equipment in the form of a well frame 60 on the sea bottom

23.

The pipe suspension 70 or 70' for the two alter- natives is besides designed so that the riser pipes are led in towards the ship from a horizontal sector of about

180° relative to the side of the ship.

A number of riser pipes are led from the well frame

60, which according to one construction can be found on the sea bottom near to rotational axis 24 of the ship, first in direction PI (see Figure 1) away from the well frame 60, and thereafter upwardly P2 between two 80 and

82 anchor lines of the ship, and thereafter are led over

P3 the anchor lines, preferably below the surface (20 in Figure 2) of the sea in a direction towards the side 64 of the ship and finally vertically (P4) upwardly towards the pipe suspension 70. The position of the flexible riser pipes 62 can be regulated with necessary floating and submersible bodies 66 which form a winding shape, possibly in combination with single anchoring systems 68. As is evident from Figure 2 the suspension 66 is placed over the anchoring line 80, but in a position below the surface of the sea.

In the illustrated construction the ship is anchored with the four anchor lines 76, 78, 80, 82 there being formed, according to an idealised placement, an angle sector of 90° between each of these. The riser pipe 62 is preferably led away from the well head 60 in a direction PI which divides the sector VI between two anchor lines 80 and 82 into two largely equal part-sectors. The riser pipes are led thereafter upwards and further backwards forward towards the respective suspension 70 and 70' in a respective direction P3 and P3 ' (see Figure 1) which is approximately opposite to the direction PI. There is achieved thereby an optimal riser pipe avoidance relative to the two adjacent anchor lines 80,82, and the foundation is prepared for the ship to be able to be turned over a sector of above 180° without the conduit 62 coming into conflict with or contacting the anchor lines 80,82.

If the rotation of the ship is limited to a sector equal to the opening between two chosen anchor lines, for example 120°, the introduction of the flexible riser pipes 62,62' is simplified in that they can then be led vertically up to the ship, for example between the anchor lines 76,78. The position of the ship which is shown in Figure 3 represents therefore the one outer position of the ship, and the ship can now turn in the direction of the arrow F until the bow 16 faces the opposite way towards the left which is the other outer position. When the riser pipes are guided up into a centrally placed well in the hull, as is illustrated in Fig. 3b, the suspension 70' draws an arc b2 correspondingly over a semi-circle having a radius r2. Here lies the ship, in its outer positions, largely across the the upwards directed pipe 62, while according to the solution in Figure 3 the ship is arranged along the upwards directed pipe when it is positioned in its outer positions . When a suspension is used which can be moved along a guide rail 90 on the side of the ship, the ship can be turned still more outwards these outer points, that is to say close upon 270°. There is also a possible solution that the flexible riser pipes are suspended in a crane mounted on the deck of the ship, and which comprises a turnable boom, which can project outside the side of the ship, and to which the riser pipes are stored.

Corresponding judgements apply to the cases where there is used for example only three anchor lines forming sectors of 120°, or for example more than 4 anchor lines. When the ship is constructed so that both ends, that is to say both stern and bow, can be adjusted up against the weather, the ship can now be correctly adjusted to handle weather conditions (wind/currents) over the whole circular sector of 360°, by only being turned 180°.

If a starting point is now taken in a stationary rotational axis, the suspension swivel 72 for the riser pipes can, draw an arc bl which is larger than a semi- circle, and with the radius rl (see Figure 3a), when the ship is turned over 180°, that is to say close to 270°, about the axis 24. When the suspension 70 for the riser pipes is stationary to (the one) side 64 of the ship, the most favourable suspension position is that point on the side of the ship which provides the shortest distance to the drilling axis 24. When the suspension 70 for the riser pipes is placed within the ship the suspension 70 will draw an arc greater than a semi-circle having a radius equal to the distance from the rotational centre 24 to the suspension 70. When the axis is placed amidships, rl is approximately equal to half the breadth of the ship.

In order to optimise the conduit guidance when the suspension 70 is placed on the outside of the ship, "clear" of the anchor lines, the suspension is adapted to be able to be displaced horizontally along the ship's side of the vessel. This is carried out in that the suspension 70 is movably mounted in a rail system 90, schematically shown in Figure 2 and 3, where the rail system is either placed within on the deck or along the side of the ship. In the position of the suspension which is shown in Fig. 3a, the suspension 70 is pushed completely to the right along the rail 90, designated position A in Figure 3. Now one supposes that the direction of wind/current changes so that the ship is displaced about the axis 24 so that the bow swings in the direction of arrow F. When the ship is turned 90°, the suspension 70 is displaced towards the left so it is about in position B on the rail. When the ship is turned an additional 90°, that is to say 180° total, it will be favourable that the suspension is in position C in Figure 3. The ship is thus now turned 180°, without the conduits 62 coming into conflict with the anchor lines 80,82 because the conduits move the whole time over the lines with good clearance. The angle sector which the ship can be turned about, will in reality be well over 180°, (that is to say close upon 270°) for the vessel can swing without problems additionally about 45° to each side, beyond 180°.

Normally there will be used 6 - 8 anchor lines in order to anchor a drilling and production ship. With the organisation of the riser pipes upwards from the well head as according to the invention, the turning of the ship with the bow/stern up against the weather, can take place over 180° without the conduits contacting the lines. In order to turn the vessel to the correct position the vessel's own propulsion machinery is used.

According to Figure 3b the riser pipes 62 ' are led up through an opening/well into the ship. Then the horizontal distance between the rotation centre 24 and the pipe suspension 70' can be increased relative to the solution where the suspension is placed at the side of the ship, that is to say that r2 can be greater than rl . In that the anchor lines 78 - 84 form an abrupt angle downwards from the keel towards the sea bottom the danger is reduced of conflict between riser pipes and anchor lines. In addition the rotation sector of the ship will be able to be increased right up towards 270°, depending upon the angle of the anchor lines downwards towards the bottom, the rotational radius r2 and the number of riser pipes 62 ' . According to an alternative construction of the invention the drilling and production equipment is arranged at the bow/stern of the vessel as is evident from the side view of Fig. 4 and the plan section of Fig. 5. The drilling installation 22,24 is arranged in the forward portion of the ship, that is to say in the forepart (alternative in the stern) , and the ship can be anchored, for example via a swivel, to the sea bottom by means of anchoring lines 76 - 82. The production conduit suspension 70 is mounted on the outside of the bow portion of the ship, at a distance from the axis 24. In this case the ship will be able to lie free of the weather, and be in condition to rotate approximately 360°. The ship has its own propulsion installation which also functions in the transverse direction, so that the ship can be rotated backwards when the limit for rotation is reached. In this instance the pipe suspension is placed in the bow portion 16 of the ship, on the outside of the hull and can be pushed for example on a rail system mounted to the side of the ship or within the bow deck (the forecastle head) of the ship (for example as is shown in Figure 6a), in an arc round the bow and aft a distance on both sides of the bow portion, see especially Figure 5, in a length which results in the incoming riser pipes 62 not contacting the anchoring lines 80,82 of the ship when the ship rotates. Also in this case a number of riser pipes 62 can be guided from the well frame, which is preferably to be found on the sea bottom 23 close to the rotational axis 24 of the ship, first in a direction away from the well frame and thereafter up between two of the anchor lines 80,82 of the ship, and thereafter are guided over the anchor lines, preferably below the surface of the sea in a direction towards the side of the ship and finally vertically upwards towards the pipe suspension 70. The position of the flexible riser pipes can be regulated with necessary floating 66 and submersible bodies 67, possibly with single anchoring systems in combination. This variant will involve having to pay attention to a particular degree to the design of the ship with thoughts of movements, because pitching movements of the ship mean more the further forward the drilling is to take place. In this case (with two bow constructions) the ship can handle weather conditions from all sides, by rotating up to 360°.

In Figure 6a there is shown an enlarged perspective section of a conduit suspension 70 which can be displaced on and along the deck of the ship. On the deck 65 there are arranged two rails 94,95 (very simply sketched in the Figure) extending largely parallel with the railing of the ship. The suspension unit 70 comprises a frame-shaped housing portion in which there is arranged a swivel construction 72 for reception of the end to a flexible riser pipe 62 in a rotatable manner, plus means for anchoring to the deck/ship's side of the ship, for example to a rail system. The storage unit 70 projects a suitable distance outside the railing of the ship. A second pipe 96 leads further from the swivel and to the production installation or storage tank on board. The unit 70 comprises for example a wheel-going unit 98, which can be moved on/along the rail by means of a drive unit not shown further. The drive unit is employed to control the movement and the fixing of the unit 98 on/along the rail 94 relative to the rotational requirement of the ship, and the control can occur from the central control room of the ship.

The rail unit 94 can also be mounted to the side 64 of the ship as is shown in Figure 6b, in the form of a single rail which extends along the side 64 of the ship at a suitable height above the water line 20. Besides the connecting unit is mounted as is shown in Figure 6a. In Figure 6c there is shown a cross-section through the outermost section of the suspension unit 70 according to the Figures 6a/6b, through that region where the riser pipe is led up. Both the riser pipe 62, and the pipe 96 leading further are rotatably mounted to the swivel 72 of the connecting unit 70. The swivel will prevent twisting of the flexible riser pipes, and that the directional orientation is largely maintained, both on the underside and the upper side of the swivel, when the ship turns. The rail systems which are shown in Figure 6a and 6b, can also be applied around the bow portion of the ship in the constructions which are evident from Figures 4 - 5.

As mentioned above the drilling operation occurs by the drill stem 24 being led from the deck of the ship and through a through-going well 28 in the ship and downwards to the sea bottom. The hull form can comprise one or more additional forward and rear wells 26,30. The wells 26,28,30 extend connectedly from the upper deck vertically down through the whole ship and debouch into the sea beneath the keel. The water will thus stand a distance upwards in the well and in a stationary condition is adjusted to a level which is called the water line level, and as is most clearly evident in Figure 7 by the reference numeral 20. Figure 7 shows, sketched simply, a preferred solution according to the invention, in the form of a cross-section of the hull form 10 about amidships where there is designed a through-going well 30. The surface of the water is indicated at 20. The Figure shows schematically how a number of flexible riser pipes 62' are guided from below and upwardly through the well 30 for mounting to a suspension unit 70' (that is to say a swivel unit) which is positioned in the upper part of the well 30, for example adjacent the deck of the ship. Fastening bar 71' of the suspension 70' to the hull of the ship is also shown schematically in Figure 7. The suspension unit 70', which is shown schematically, comprises a swivel frame 70' which via mountings 72' and a socket unit 71' can be turned relative to the hull 10 of the ship. The flexible riser pipes 62' are in addition turnably mounted relative to the frame 70'. By applying such a double turning system, the ship can be turned without altering the relative position of the riser pipes.

A variant of the invention is to employ an anchoring where there is established an open sector of up to about 120° between two of the anchor lines, as is shown in Figure 8. The well frame on the bottom of the sea is shown at 60, the riser pipe at 62, and the axis of rotation at 83. This variant is specifically well suited when the flexible riser pipes are led largely vertically straight upwards to a suspension within the ship. This can be achieved for example by employing a three point anchoring of 3 x 120°, that is to say 120° between each of the three anchor lines 100,102,104. This anchoring pattern can be sufficient to maintain the ship having two bows 18,20 in position in some ocean regions of the ocean. If rotation of the ship 10 is limited to one of the open sectors SI, in this case 120°, the ship with its two bows will be able to cover a weather sector SI + S2 of 2 x 120° = 240°, see Figure 8, but cannot be turned beyond this because then the riser pipe 62 will be able to collide with the anchoring lines 100,102. The remaining weather sector S3 + S4 of totally 360° - 240° = 120° must therefore " be distributed " on each of the two bows of the ship with 60°, or +/- 30°. With this rotational limitation of 2 x 60° the ship must thus be in a condition to maintain the outer positions 100 or 102 with a weather direction of twice +/- 30° = 120°. Estimates have shown that the ship can maintain the position, but not always carry out drilling, when the weather comes from the two weather sectors S3 and S4 each of twice 60°. For weather directions within S3 and S4, the said twice 60° of the circular sector the ship is held in its angle-outer positions 100,102 by means of its propulsion installation. This applies specifically in mild and medium weather-hard waters. If dominating wind directions are provided, the limitation can also be advanced for weather-hard regions. The object with this variant is that the flexible riser pipes can then be led vertically upwards to the ship between the two anchor lines which form a sector of about 120°. This greatly simplifies the introduction of the flexible riser pipes by avoiding having to hold them over the anchor lines in the upper part, close to the introduction to the ship. Thereby it becomes simpler to apply buoyancy bodies and weights to the flexible riser pipes in order to ensure movements within a defined angle sector, and in order to avoid collisions between anchor lines and riser pipes.

According to a variant of the example of Figure 6, the lines 100 and 102 can have the said angle spacing SI of 120°, while the ship for the remaining sector S2, S3 and S4 is anchored with a larger number of anchor lines, for example 4 - 8 lines spread in the form of a fan over he sectors S2, S3, S . This can be well suited when the weather V towards the ship in sector SI is of the mild type, while the weather in the opposite direction is normally more severe. Then the operation of the ship can be organised so that the oil production occurs as shown in Figure 8 from the well frame 60, while the anchor lines in sector S2, S3 and S4 keep the ship steady against the weather and maintain its horizontal position even when the weather becomes more severe the reverse of the weather direction V, and the propulsion installation of the ship makes the necessary adjustment of direction at any time within the sector SI.

According to a solution which is also shown in Figure 1, the ship is equipped with three wells having largely rectangular plan sections, and which extend longitudinally along the longitudinal axis of the ship. A distribution or arrangement of the wells, that is to say the well area, along the mid-section of the ship has proved to be favourable. Alternatively the hull form can comprise a single longitudinal well, and which can have the same water line area as the three wells 26, 28 and 30 together. The wells can be designed with even perpendicular well walls 36. As an alternative to wells having rectangular or square cross-sections they can also be designed with other cross-sectional forms such as oval, circular or other more irregular forms. The precise cross-sectional form will be able to be varied all according to the actual hull construction, in order for example to pay attention to necessary frame constructions.

The invention will be well suited to drilling and production ships of all sizes, especially for ships which have a length of over 100 metres, a breadth of over 15 metres.

Claims

Patent Claims

1. Arrangement of drilling/production ship comprising: a drilling section intended for the mounting of a drill stem (24) , a number of anchoring lines (78,80; 82,84) which pass between anchorings on the bottom of the sea and an anchoring - line fastening (83) designed in the ship, whereby the ship can be turned in a horizontal plane, the drilling axis (24) being largely coincident with the rotational axis of the ship and the anchoring fastening, and a number of riser pipes (62) lead via the sea bottom (66) upwardly to an connecting unit (70) on the ship (10), characterised in that the unit (70) of the ship for connecting the riser pipe(s) (62) is positioned at a distance from the drilling section of the ship, and the ship is adapted to rotate about the axis (24) in a horizontal plane over a sector of up to about 360┬░.

2. Arrangement in accordance with claim 1, characterised in that the drilling section (24) and the connecting unit

(70) are arranged in mutually separated areas of the ship, such as at the longitudinal central axis of the ship in connection with respective wells (28,30) extending mutually separated through the hull of the ship, or that the drilling section (24) and the connecting section (70) are positioned with a mutual spacing in connection with a single well extending through the hull of the ship.

3. Arrangement in accordance with claim 1 and 2, characterised in that the connecting unit (70) is positioned connected to the side of the ship.

4. Arrangement in accordance with claim 1 and 3, characterised in that the connecting unit (70) is adapted to be able to be displaced a given length along the longitudinal direction of the ship, such as movably mounted along a rail unit (90) extending on the side (64) of the ship or on/along the deck (65) of the ship, or that the connecting unit is led via a swivel suspended in a rotatable crane.

5. Arrangement in accordance with one of the preceding claims, characterised in that both the bow and stern portions (16,18) are constructed to be positioned " towards the direction of the weather", and the ship is adapted to turn about 180┬░ in order thereby to cover the whole weather sector of 360┬░.

6. Arrangement in accordance with one of the preceding claims, characterised in that the riser pipe(s) (62) extend (s) from a well head (60) in a direction upwardly to a position (66) approximately centrally between two anchor lines (80,82), from which position the conduit (s) (62) extend largely horizontally, upwardly and forwardly to the connecting unit (70), so that the riser pipe(s) is (are) not in conflict with the anchoring lines (80,82) on rotation of the ship over a sector of about 360┬░.

7. Arrangement in accordance with one of the preceding claims, characterised in that the ship is adapted to rotate in a horizontal plane over an angle sector of up to about 270┬░, preferably over an angle sector of 180 - 210┬░, whereby the connecting unit (70) can draw a sector of a circle with radius rl when the connecting unit (70) is arranged adjacent to the side of the ship, or draw a sector of a circle with radius r2 when the connecting unit (70') is positioned connected to a well.

8. Arrangement in accordance with claim 1, characterised in that the drilling section of the ship with drilling axis (24) is arranged adjacent to the bow (16) or stern area (18), and that the connecting unit (70) of the ship for the riser pipe(s) (62) is positioned in the bow/stern of the ship, and the ship is adapted to rotate about the axis (24) in said horizontal plane over an angle of 360┬░, in order thereby to cover the whole weather sector of 360┬░.

9. Arrangement in accordance with claim 8, characterised in that the connecting unit (70) is, by means of displacement device (90), adapted to be able to be displaced in an arcuate form round the bow/stern portion and a given length further along the hull on both sides of the bow/stern portion.

10. Arrangement in accordance with claim 8 - 9, characterised in that the displacement device (90) comprises a rail unit to which the connecting unit (70) is slidably mounted, preferably by means of a drive unit.

11. Arrangement in accordance with claim 1-2, and 5 characterised in that the bow and stern portion (16,18) of the ship (10) is constructed to be positioned "towards the weather direction" (V) , that at least two adjacent anchor lines form a mutual angle sector of up to about 120┬░, and that one or more riser pipe(s) of the ship is (are) led largely vertically upwards to suspension (70) of the ship (10) between the anchor lines, with which the ship is adapted to be turned over a weather sector of about 2 x 120┬░ = 240┬░, while for a weather direction within the remaining twice 60┬░ of the circular sector the ship is held in its angle-outer positions (100,102) by means of its propulsion installation.