WO2017174083A1

WO2017174083A1 - Riser retention system and drillship with the same

Info

Publication number: WO2017174083A1
Application number: PCT/DK2017/000006
Authority: WO
Inventors: Konstantin PUSKARSKIJ
Original assignee: Maersk Drilling A/S
Priority date: 2016-04-04
Filing date: 2017-04-04
Publication date: 2017-10-12

Abstract

The present invention relates to a drillship (100) for performing offshore drilling, the drillship (100) having a hull (160), a main deck (110) and a moon pool (120) through which a marine riser (140) may extend into the sea (150) and towards the seabed and through which offshore drilling may be performed, the drillship (100) comprising a riser retention system (300) for retaining a retention point (170) of the marine riser (140) within a predetermined lateral area, the riser retention system (300) having a riser retention element (180) defining the retention point (170) substantially flush or under the hull (160).

Description

Riser retention system and drillship with the same

Field of the invention

The invention relates generally to a riser retention system for a drillship for performing offshore drilling, the drillship having a hull, a main deck and a moon pool having a substantially vertical opening through which a marine riser may extend into the sea and towards the seabed, the drillship comprising a riser retention system.

Background

Certain offshore drilling operations may be carried out by various types of drillships typically comprising an opening or so-called moon pool through which a marine riser may extend into the sea and towards the seabed.

The marine riser is typically connected at the upper end to a diverter under the drillfloor via ball joint (also referred to as a flex joint). As discussed below, a tensioning system and telescopic joint is typically installed to put the marine riser in vertical tension and compensate for heave motions. In some instances the rig further comprises a retention system arranged to apply horizontal forces at a point below the ball joint in order ensure that the riser's deflection angle at the ball joint is tolerable.

Patent specification US 4,616,708 e.g. discloses an example of a riser tensioning system combined with a retensioning system where a number of riser tensioning cables are provided within a moon-pool and connected to a riser and to a collar that is slidably arranged around the riser.

Existing riser retention systems often need reinforced structure or integrity and/or specific measures in order to safely accommodate stress caused by handling or retaining a marine riser.

Furthermore, drillships having a marine riser may comprise a marine riser tensioner system, heave system, etc. that aims at decreasing influence of the heave motion of the drillship on the marine riser.

The present invention does not relate to such systems but rather to riser retention systems and embodiments thereof that address relative lateral motion between a drillship and a marine riser while it is to be understood that non-lateral movement typically will be allowed and typically still take place during operation , which may be addressed by other measures, as already known.

It would be beneficial to provide a riser retention system providing reliable control of the lateral movement of a marine riser.

Furthermore, there is a need for a riser retention system that accommodates stress caused by handling or retaining a marine riser in particularly expedient manner(s).

Summary It is an object to alleviate at least one or more of the above mentioned drawbacks at least to an extent by one or more aspects of the present invention.

Aspects of the invention are set out in the appended claims. These and other aspects of the invention are discussed below.

Accordingly, in some embodiments the present invention relates to a drillship for performing offshore drilling, the drillship having a hull a main deck and a moon pool through which the drillship is arranged to operate a marine riser and drill into a seabed through said marine riser wherein the drillship comprises (i) a riser retention system for retaining a retention point of the marine riser within a predetermined lateral area, the riser retention system comprising a riser retention element defining the retention point substantially flush or under the hull and under the moon pool of the drillship, and (ii) a plurality of retention elements connected at one end to the drillship, such as above the moon pool or at the main deck, and at another end to the riser retention element. In this way, control of the lateral movement of a marine riser is provided compensating for a relative lateral motion between a drillship and the marine riser in an expedient way. That the retention point of the marine riser is located below the hull and under the moon pool, e.g. substantially centrally, of the drillship enables reliable control of the lateral movement of the marine riser so that it may be ensured at least to a certain extent that the marine riser e.g. does not hit any inner side of the moon pool and that deviation angles of a flex joint or similar of the marine riser does not exceed required or specified maximum angles. Excessive angles may prevent or complicate running of tubulars within the marine riser or may impose risks to its integrity. In some embodiments above (or below) the water or sea level is taken to mean above (or below) the average waterline for which the ship is designed to operate or operates. Typically a drillship a ballasted so the vessel sits deeper in the water during drilling (and other operations) relative to the waterline in transit. In some embodiments above or out of the water is taken to mean above the splash zone where a wave may hit the equipment such as the largest wave in the operational window of the ship.

In some embodiments the riser retention system is arranged to keep the deviation angle of marine riser at the ball/flex joint relative to the center position (i.e. perpendicular to the drill floor) at an angle of 10 degrees or less, such as 8 degrees or less, such as 6 degrees or less, such as 4 degrees or less, such as 3 degrees or less, such as 2 degrees or less such a 1 degree or less.

In some embodiments the retention point is substantially flush with the underside of the hull on the starboard and port sides of the moonpool such that the retention point is substantially level with a line drawn across the moon pool from the bottom of the hull on the starboard and port sides. In some embodiment substantially flush is taken to mean within 3 meters above this line, such as within 2 meters, such as within 1 meter, such as within ½ a meter. In some embodiments substantially flush is taken to mean at least ½ a meter below, such as at least 1 meter, such as at least 2 meters such as at least 3 meters. In some embodiments, these measurements are for the ship at rest i.e. without any relative motion between the ship and the riser retention element.

Furthermore, in some embodiments stress related to lateral retention of the marine riser is not applied to any point inside the moon pool whereby less structural reinforcement or capability is needed.

The retention elements connects the riser retention element to the ship and provide the (mainly parallel to the main deck) forces to retain the riser. Typically, each retention element is arranged so that it can pull in the riser retention element from single direction relative to the ship. In some embodiments this is provided by a wire or cable connect to the riser retention element and engaged in a sheave mounted on the ship thereby defining the direction from which the wire can pull on the riser retention element. From the sheave the wire typically run to and is connected to or engages with a restriction element optionally via one or more further sheaves, fairlead or similar, but the restriction element may in principle also be connected directly to the riser retention element. The restriction element is the component use for applying pull to the wire, such as tensioner similar to tensioners similar to those used for providing vertical riser tension, a piston or a spring where a spring is an example of a passive restriction element. This may be in line with the wire so that the wire is connected to one of the restriction element, which in turn is fixed to the ship. The restriction element may also provide the pull by pushing or pulling on the side of the wire thereby (at least locally) extending the path of the cable which transfers into a pull on the riser retention element (see e.g. US4616708 and the way the piston- cylinder assembly is arranged to apply for force/pull to the cable).

In some embodiments, at least some of the plurality of retention elements comprises tensioners or other passive or active dampening and/or restriction elements.

In some embodiments, the riser retention system comprises four retention elements that are secured to the riser retention element with a respective angle between them of about 90 degrees. In some embodiments, the riser retention system comprises at least one guide element for each retention element, wherein the guide elements restrict lateral movement along a first predetermined direction of the retention element and/or is installed or located subsea at least during use.

In some embodiments, the guide elements each comprises a securing element adapted to allow a guide element to be retracted, folded, and/or moved between a subsea use location and a non-use location located above sea level.

In some embodiments, the securing element is secured to the hull of the drillship at a location below sea level when the drillship is submerged in water.

In some embodiments, the retention elements comprise wires running at least partly along the outside of the hull and under the hull. In some further embodiments, the plurality of retention elements each comprises a wire or the like that extends along two substantially opposite outer sides of the hull. The term wire is intended to be synonymous with cable, chain and line suitable for withstanding a pull of 10 tons or more, such as 50 tons or more, such as 75 tons or more, such as 100 tons or more.

This allows retention forces to be accommodated by the hull thus requiring less structural reinforcement or capability. In particular no stress for retention needs to be applied to the inside of the moon pool.

In some embodiments, at least some, e.g. two, of the plurality of retention elements comprise tensioners or other passive or active dampening and/or restriction elements while at least some, e.g. two, other of the plurality of retention elements do not comprise tensioners or other passive or active dampening and/or restriction elements.

In some embodiments, the drillship comprises at least two of the retention elements that

- do not comprise tensioners or other passive or active dampening and/or restriction elements, and

- do comprise a wire running at least partly along the outside of the hull and under the hull, wherein at least two of the wires are joined at a load sharing yoke being connected at one end above the moon pool, e.g. at the main deck.

In some embodiments, at least some of the plurality of retention elements are adapted to apply a lateral force to the marine riser according to a nonlinear force profile according to which increasing non-linear force is applied in response to a deviation of the marine riser from a center position.

In some embodiments, at least some of the plurality of retention elements are adapted to apply a lateral force to the marine riser so that it is ensured that the marine riser does not hit any inside part of the moon pool or the risk thereof is minimized or reduced and/or that movement of a flex joint of the marine riser does not exceed any maximum movement angles of the flex joint as specified or recommended by the American Petroleum Institute.

In some embodiments, the riser retention system is adapted to be used for drillships designed to operate at water depths of more than 500 meters, such as of more than 1000 meters, such as of more than 2000 meters, such as of more than 3000 meters, such as of more than 4000 meters.

In some embodiments, the riser retention system is adapted to be used for drillships designed to operate at water depths commonly designated as 'deep water' within the offshore community. Definitions

All headings and sub-headings are used herein for convenience only and should not be constructed as limiting the invention in any way.

The use of any and all examples, or exemplary language (e.g., "such as") provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

Definitions may be made elsewhere in the specification.

This invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law.

Brief description of the drawings

FIG. 1 schematically illustrates a cross-sectional view of a drillship in the form of a drillship and one embodiment of a riser retention system ;

FIG. 2 schematically illustrates a cross-sectional view of the drillship and riser retention system of FIG. 1 with the riser retention system being shown at different positions; FIG. 3 schematically illustrates a perspective view of the riser retention system of FIG. 1 and 2; and

FIG. 4 schematically illustrates a perspective view of the riser retention system according to an alternative embodiment.

Detailed description

Various aspects and embodiments of the riser retention system, and drillships with the same, as disclosed herein, will now be described with reference to the figures.

When/if in the following terms, such as "top" and "bottom", "right" and "left", "horizontal" and "vertical", "clockwise" and "counter clockwise" or similar relative expressions are used, these refer to the appended figures and not necessarily to an actual situation of use. The shown figures are schematic representations for which reason the configuration of the different structures as well as their relative dimensions are intended to serve illustrative purposes only.

Some of the different components are only disclosed in relation to a single embodiment of the invention, but is meant to be included in the other embodiments without further explanation.

FIG. 1 schematically illustrates a cross-sectional view of a drillship in the form of a drillship and one embodiment of a riser retention system.

Shown is a drillship here as an example in the form of a drillship 100 where the drillship 100 comprises a hull 160 and a main deck 1 10 comprising a moon pool 120 having or being a substantially vertical through-going opening 130 from an upper side to a lower side of the hull 160.

Further illustrated is a marine riser 140 secured to the ship where the marine riser 140 extends through the moon pool 120 into the sea 150 and towards the seabed. Offshore drilling may be performed through the moon pool 120 using the marine riser 140 in any suitable manner e.g. as traditionally known. When performing offshore drilling, the marine riser 140 may e.g. be connected to an offshore well located on the seabed, be connected to a (pre- deployed) BOP, be disconnected from being connected to a lower marine riser package (LMRP) of a BOP stack, and/or other. The drillship 100 further comprises a riser retention system 300 for retaining a retention point 170 of the marine riser 140 within a predetermined lateral area as defined in relation to the retention point 170 (as will be explained further in the following) where the retention point 170 is located under the hull 160 and, e.g. more or less centrally, under the moon pool 120 of the drillship 100. The riser retention system 300 may be regarded as a lateral riser retention system while it is to be understood that some non-lateral movement of the retention point 170 (and thereby the predetermined lateral area) may be allowed and will typically take place during operation.

The riser retention system 300 comprises a riser retention element 180 that also defines the retention point 170 of the marine riser 140. The riser retention element 180 may in some embodiments be secured or locked (potentially with a release option) to the marine riser 140. In alternative embodiments, e.g. in combination with secured or locked embodiments, the riser retention element 180 is slidably or otherwise movably attached to the marine riser 140 so the marine riser 140 e.g. can move vertically at least to a certain extent within the riser retention element 180. The riser retention element 180 may e.g. be a collar or any other suitable retention element for retaining the marine riser 140. As the riser retention element 180 defines the retention point 170, the predetermined lateral area will generally follow the movement of the riser retention element 180. The riser retention system 300 further comprises a plurality of retention elements 190, where at least one or more of the retention elements 190 are connected at one end at respective connection point(s) or element(s) above the moon pool, e.g. at the main deck 1 10, and where some, e.g. all, are connected at another end to the riser retention element 180. A function of the plurality of retention elements 190 is to keep the riser retention element 180 and thereby the retention point 170 of the marine riser 140 within the predetermined lateral area. This provides control of the lateral movement of the marine riser 140 and thereby compensate for the relative lateral motion between the drillship 100 and the marine riser 140. That the retention point 170 of the marine riser 140 is located below the hull 160 and under the moon pool 120 of the drillship 100 enables reliable control of the lateral movement of the marine riser 140 so that it may be ensured that it e.g. does not hit any inside part of the moon pool 120 and that deviation angles of a flex joint or similar of the marine riser 140 does not exceed required or specified maximum angles during operations that could prevent or complicate e.g. running of tubulars within the marine riser, etc. The riser retention element 180 is advantageously movable to some extent, e.g. as explained further in connection with FIG. 2, while still allowing sufficient control of the marine riser 140.

Even though it is simpler and adequate with a single riser retention element 180, the riser retention system 300 may in principle comprise a plurality of riser retention elements 180 with the retention elements 190 being connected to them in a suitable way.

The number of retention elements 190 may e.g. depend on specific implementation. In some embodiments, the number of retention elements 190 is e.g. three while in other embodiments the number of retention elements 190 is e.g. four (e.g. as shown in FIG. 3 and 4). The number may alternatively be any other suitable number e.g. two or more than four.

Each or at least some of the retention elements 190 is/are secured to a riser retention element 180 in a suitable way, e.g. using a lockable hook and eye bolt arrangement or similar. A retention element 190 may in principle be connected to multiple locations (e.g. by being split) of the riser retention element 180 and/or multiple retention elements 190 may in principle be connected to a single location of the riser retention element 180. In one example a retention element 180 may comprise a double run of wire or the like and the wire may extend around a sheave or the like secured to the riser retention element. Not all retention elements 190 need to be connected to the riser retention element 180 in the same way. In some embodiments, the retention elements 190 comprise tensioners, e.g. wire-line tensioners. In alternative embodiments, the retention elements 190 comprise passive or active (movement) dampening and/or restriction elements 220 such as springs, winches, actively controlled hydraulic cylinders like pistons or the like, or any other suitable dampening and/or restriction elements. The springs, winches, actively controlled hydraulic cylinders, pistons, etc. may (also) be located above the moon pool, e.g. at the main deck 1 10 and thus are likely to be readily accessible for maintenance and repair. In some embodiments, the capacity of one dampening and/or restriction element 220 is 10 ton or more, such as 25 ton or more, such as 50 ton or more, such as 75 ton or more, such as 100 ton or more, such as 150 ton or more, such as 200 ton or more. .

The part of the retention elements 190 above the moon pool, e.g. at the main deck 1 10, may e.g. comprise fairleads, sheaves, or similar.

In some embodiments and as shown, the plurality of the retention elements 190 comprises wires, ropes, chains, or the like that extends along two substantially opposite outer sides 165 of the hull 160 and towards the underside of the hull. The retention elements 190 may e.g. comprise anything that translates a force from the riser retention element 180 to a retention element 190 on substantially a one to one basis. Alternatively, the retention elements 190 may in some cases e.g. comprise an elastic element such as an elastic pennant, etc.

Practically, multiple tensioners or passive or active dampening and/or restriction elements and multiple wires, ropes, chains, or the like may be used and connected at a single location of the riser retention element 180.

In some embodiments and as shown, the plurality of retention elements 190 (comprising wires or the like) further extends, e.g. at least partly, around and under the hull 160.

Running the retention elements 190 along the outer sides of the hull and/or around or under the hull 160 facilitates retrofitting of the system 300 to an existing drillship 100. For example, it may be possible to fit the system without piercing the hull 160 and elements of the system 300 may be mounted to the surface of the hull 160 or to the deck 1 10. Is should be noted that not all retention elements 190 need to be connected to or comprise tensioners or other passive or active dampening and/or restriction elements 220 such as springs, winches, actively controlled hydraulic cylinders, pistons, etc., with one example being shown and explained in connection with FIG. 4.

This allows the hull 160 to receive a significant part of the stress and forces generated by the movement of the riser 140, which will reduce the stress and forces received at the connection point(s) or element(s) of the retention elements 190 located above the moon pool, e.g. at the main deck 1 10. Furthermore, stress related to lateral retention of the marine riser 140 is not applied to any point inside the moon pool 120, which is an advantage as less structural reinforcement or strength then is needed. This is further facilitated by having the retention point 170 of the marine riser 140 being located below the hull 160 and under the moon pool 120 of the drillship 100. In some embodiments, the riser retention system 300 comprises at least one guide element 200 for each or for a number of the retention elements 190, where the guide element(s) 200 is installed or located subsea at least during use. In this way, the retention elements 190 may be guided around the lower part of the hull up to their respective connection point(s) or element(s). In some further embodiments, the guide element(s) 200 also restrict lateral movement along a first predetermined direction of the retention element 190. In the shown and similar embodiments, the first predetermined direction is in and out of the plane of FIG. 1 . The guide element(s) 200 may e.g. be or comprise fairleads, sheaves, or similar. In some examples the guide elements 200 may comprise channels or tubes and the bearing surfaces of the elements may be formed or coated with a low friction material.

The retention elements 190 may experience bending or flexing at the guide elements 200, for example where a retention element 190 changes direction and passes from a side of the hull to below the hull. To minimize the stress experienced by the retention elements 190 the guide elements 200 may describe relatively large radii. The retention elements 190 may be reinforced or otherwise configured to minimize any risk of fatigue failure at the guide elements 200. Alternatively, or in addition, the system 300 may be configured to reposition the retention elements 190 such that a different portion of an element 190 passes around a guide element 200. In other examples the guide elements 200 may be configured to accommodate changes in alignment of the retention elements 190 without requiring bending of the retention elements 190, for example by pivotally mounting a rigid guide element to the hull and attaching end portions of a retention element 190 to corners of the guide element. Such an arrangement would also permit the use of rigid rods and the like as retention elements 190.

A guide element 200 may e.g. and as shown be secured to the hull 160 of the drillship 100 by a suitable securing element 210.

In some embodiments, the securing element 210 is secured to the hull 160 of the drillship 100 at a location below sea-level when the drillship 100 is submersed in water.

In some further embodiments, the securing element 210 of the guide element 200 is adapted to be retracted, folded, and/or moved between a sub-sea use location and a non-use location being located above sea level. This enables that the guide element(s) 200 may be put Out of the way' and thereby not induce drag and be less susceptible to damage, etc. e.g. when the drillship 100 is sailing. In one example the securing element 210 may comprise a hull-mounted frame which may be extended to move a guide element 200 from a retracted position to an operational position below sea level. In a retracted configuration the securing element 210 and/or the associated guide element 200 may lie within a recess or the like in the hull 160 to provide a degree of protection for the elements of the system 300 when the system is not in use.

In some embodiments and as shown, the retention elements 190 comprise wires running at least partly along the outside of the hull and towards the underside of the hull. Is should be noted that not all wires need to be connected to a retention element 190, one example being shown and explained in connection with FIG. 4.

As mentioned above, the riser retention system 300 is adapted to retain the retention point 170 of the marine riser 140 within a predetermined lateral area in relation to the retention point 170.

In some embodiments, the predetermined lateral area is kept under the center of the moon pool 120. This may e.g. be provided by having retention elements 190 that apply a linearly increasing counter force being proportional to the deviation of the marine riser 140 from the center position (at the level of the riser retention element 180). This would aim at keeping the marine riser 140 exactly at the center position of the moon pool 120 even when there is a relative lateral motion between the drillship 100 and the marine riser 140.

However, this would provide a very stiff system that potentially could impose significant forces on the marine riser. In alternative and preferred embodiments, the predetermined lateral area is provided that limits the movement of the marine riser 140 so that certain predetermined criteria is fulfilled while not aiming at keeping the marine riser 140 exactly at the center position. The predetermined criteria are e.g. that it is practically speaking impossible that the marine riser 140 will or should hit any inner side of the moon pool 120 and that the marine riser 140 will not be moved in such a way that would cause a flex-joint of the marine riser 140 to exceed any maximum movement angles of the flex-joint as specified or recommended by the American Petroleum Institute (API) or other body. Examples of maximum deflection angles provided by the riser retention system are provided above. In general, the function of retaining the riser with a specific radius or deflection angle may in some embodiments be provided by having retention elements 190 that apply counter force according to a non-linear force profile where a non- linearly increasing, e.g. exponentially increasing, force is applied in response to the deviation of the marine riser 140 from the center position (at the level of the riser retention element 180). In some embodiments, the curve follow a profile similar to tensioners used for (vertical) riser tensioners. In some embodiment the force profile is arranged to so that the riser retention system provides little or no force when the retention point is within a first radius (or the riser angle from the center position, a steep increase (such as exponential) increase in force outside the first radius and a second radius and substantially blocking any further extension at the second radius. In one embodiment the second radius corresponds to a deviation angle at the ball joint of 2 degrees or more, such as 3 degree or more, such as 4 degrees or more, such as 5 degrees or more, such 6 degrees or more while the first radius correspond to a an angle of 4 degrees or less, such as 3 degrees or less, such as 2 degrees or less, such as 1 degree or less. Preferably, the non-linear force profile will not apply any or only a small amount of force for relative small or smaller deviations and from exceeding a certain deviation threshold apply a quickly increasing force. This will allow some controlled movement of the riser retention element 180/the retention point 170 from the center position without any significant counter force, which will reduce the load that the marine riser 140 is subjected to for lesser deviations.

An actual non-linear force profile may depend on a specific actual design and on certain actual parameters such as maximum allowed or preferred angles of the flex-joint of the marine riser 140, the extent (shape and size) of allowed or preferred maximum deviation from the center position, and aspects of loads that the marine riser 140 is expected to be subjected to, e.g. worst-case. Further parameters may be taken into account.

FIG. 2 schematically illustrates a cross-sectional view of the drillship and riser retention system of FIG. 1 with the riser retention system being shown at different positions. Shown is the drillship 100 and the riser retention system 300 of FIG. 1 but where the riser retention element 180 is shown at various different positions a) (being the position of FIG. 1 ), b), c), and d).

Position c) corresponds to position a) only being located further into the water, while positions a) and d) illustrate the riser retention element 180 at tilted positions. As can be seen, the riser retention system 300 allows control of the lateral movement the marine riser 140 at different positions and ensures safe operating conditions (e.g. ensuring that the marine riser 140 does not hit the moon pool 120) even for fairly tilted positions such as positions a) and d). FIG. 3 schematically illustrates a perspective view of the riser retention system of FIG. 1 and 2.

Shown is an embodiment of the riser retention system as explained in connection with FIG. 1 and 2.

As can be seen, this particular embodiment comprises four retention elements 190.

In some embodiments, and as shown, the four retention elements 190 are secured to the riser retention element 180 with a respective angle between them of about 90 degrees. This provides a great amount of control of retention of the marine riser 140 in an expedient manner and also enables distribution of forces to the marine riser 140 as evenly as possible. No matter where the marine riser 140 is moved laterally at least, two retention elements will be able to apply the majority of forces to restrain the movement of the marine riser 140.

FIG. 4 schematically illustrates a perspective view of the riser retention system according to an alternative embodiment. Shown is an exemplary riser retention system 300 corresponding to the ones shown and explained in connection with FIG. 1 - 3 except as noted in the following.

The shown and corresponding riser retention systems 300 comprise four retention elements 190 where only two of them comprise tensioners or other dampening and/or restriction elements 220 as explained in relation to the earlier figures.

The other two retention elements 190' are joined e.g. by a load sharing yoke 230 or similar. In this way, some of the retention element(s) 190 is/are high capacity while other retention element(s) 190' is/are low or lower capacity. It is to be understood that another number of retention elements 190, 190' may be different from four and/or also that the mix between the two types may be different from two and two.

In Fig. 4, the retention elements 190^' are joined via the yoke 230 and tension is provided by a tensioner 220 similar to that of the elements 190. However, in some embodiment this tensioner is replaced by a simpler device such as a winch.

The shown and corresponding embodiments are advantageously drillships where the vessel dynamically can be turned or otherwise positioned into the direction of the current so that a main lateral push on the marine riser 140 primarily will be from one side or direction. The riser retention system 300 should then be positioned on the ship so that the (high-capacity) retention elements 190 comprising tensioners or other dampening and/or restriction elements 220 are located upstream of the current compared to the other (low/lower capacity) retention elements 190' that are located relatively downstream of the current.

This provides a simpler riser retention system 300 while still ensuring suitable lateral control of the marine riser 140 for vessels that can be positioned into the current. It should be understood that certain embodiments or aspects of the different figures may be combined to effect while certain embodiments or aspects also may be used independently of each other.

Some preferred embodiments have been shown in the foregoing, but it should be stressed that the invention is not limited to these, but may be embodied in other ways within the subject matter defined in the following claims.

In the claims enumerating several features, some or all of these features may be embodied by one and the same element, component or item. The mere fact that certain measures are recited in mutually different dependent claims or described in different embodiments does not indicate that a combination of these measures cannot be used to advantage. It should be emphasized that the term "comprises/comprising" when used in this specification is taken to specify the presence of stated features, elements, steps or components but does not preclude the presence or addition of one or more other features, elements, steps, components or groups thereof.

Claims

Claims:

1 . A drillship (100) for performing offshore drilling, the drillship (100) having a hull (160), a main deck (1 10) and a moon pool (120) through which the drillship is arranged to operate a marine riser (140) and drill into a seabed through said marine riser wherein the drillship (100) comprises a riser retention system (300) for retaining a retention point (170) of the marine riser (140) within a predetermined lateral area, the riser retention system (300) comprising

- a riser retention element (180) defining the retention point (170) substantially flush or under the hull (160) and under the moon pool (120) of the drillship (100), and

- a plurality of retention elements (190, 190') connected at one end to the drillship and at another end to the riser retention element (180), the retention elements (190, 190') running at least partly along the outside of the hull and under the hull.

2. The drillship according to claim 1 , wherein the retention elements (190, 190') are connected at one end to the drillship above the moon pool (120) or at the main deck (1 10).

3. The drillship (100) according to claim 1 or 2, wherein one or more of the plurality of retention elements (190) comprise tensioners or other passive or active dampening and/or restriction elements (220).

4. The drillship (100) according to any one of claims 1 to 3, wherein one or more, such as all, of said retention elements (190) comprises a wire or the like.

5. The drillship (100) according to any one of claims 1 to 4, wherein two or more side retention elements (100) extend along two substantially opposite outer sides (165) of the hull (160).

6. The drillship (100) according to any one of claims 1 to 5, wherein the riser retention system (300) comprises four retention elements (190, 190') that are secured to the riser retention element (180).

7. The drillship of claim 6, wherein the retention elements (190, 190') are secured to the riser retention element (180) with a respective angle between them of about 90 degrees.

8. The drillship (100) according to any one of claims 1 to 7, wherein the riser retention system (300) comprises at least one guide element (200) for each retention element (190, 190') installed at least partly below water during operation at least during use.

9. The drillship (100) according to claim 8, wherein the at least one guide element (200) is installed at least partly below the hull (160).

10. The drillship (100) according to claim 8 or 9, wherein the guide element (200) is a fairlead, sheave, or similar.

1 1 . The drillship (100) according to claim 8, 9 or 10, wherein the guide elements (200) each comprises a securing element (210) adapted to allow a guide element (200) to be retracted, folded, and/or moved over the bottom of the hull.

12. The drillship (100) according to claim 1 1 , wherein the securing element (210) is operable to move the guide element (200) out of the water.

13. The drillship (100) according to any one of claims 1 to 12, wherein at least some, e.g. two, of the plurality of retention elements (190) comprise tensioners or other passive or active dampening and/or restriction elements (220) while at least some, e.g. two, other of the plurality of retention elements (190') do not comprise tensioners or other passive or active dampening and/or restriction elements (220).

14. The drillship (100) according to any one of claims 1 to 13, wherein the drillship (100) comprises at least two retention elements (190') that

- do not comprise tensioners or other passive or active dampening and/or restriction elements (220), and

- do comprise a wire running at least partly along the outside of the hull and under the hull (160), wherein at least two of the wires are joined at a load sharing yoke (230) being connected at one end above the moon pool (120), e.g. at the main deck (1 10).

15. The drillship (100) according to any one of claims 1 to 14, wherein the riser retention system (300) is adapted to apply a lateral force to the retention point (170) according to a non-linear force profile according to which increasing non-linear force is applied in response to a deviation of the marine riser (140) from a center position.

16. The drillship (100) according to any one of claims 1 to 15, wherein the riser retention system (300) is adapted to apply a lateral force to the retention point (170) to ensure that at least one of:

the marine riser (140) does not hit any inner side of the moon pool

(120) or the risk thereof is minimized or reduced; and

the movement of a flex-joint of the marine riser (140) does not exceed a maximum angle for the flex-joint such as a maximum angle specified or recommended by the American Petroleum Institute.

17. The drillship (100) according to any one of claims 1 to 16, wherein one or more, such as all, of said plurality of retention elements (190, 190') is connected at one end above water, and at another end to the riser retention element (180).

18. The drillship (100) according to claim 17, wherein the one or more of said plurality of retention elements (190, 190') is connected at one end above a waterline of the drillship, and at another end to the riser retention element (180).

19. The drillship (100) according to claim 18, wherein the one or more of said plurality of retention elements (190, 190') is connected at one end above the moon pool (120) or at the main deck (1 10), and at another end to the riser retention element (180).

20. A method of controlling lateral movement of a marine riser operated from a drillship, the method comprising:

securing a marine riser in a moon pool of a drillship;

coupling a riser retention element to the marine riser at a retention point under the moon pool and flush with or under a hull of the drill ship;

running a plurality of retention elements from the riser retention element under the hull and at least partly along the outside of the hull to a drillship connection, and applying retention forces to the riser retention element to retain the retention point within a predetermined lateral area.

21 . The method of claim 20, comprising applying the retention forces to the riser retention element to compensate for relative lateral motion between the drillship and the marine riser.

22. The method of claim 20 or 21 , further comprising providing a flex joint between the drillship and the marine riser above the retention point and applying retention forces to the riser retention element to restrict a deviation angle of the flex joint.

23. The method of any one of claims 20 to 22, comprising applying a pull to at least one retention element using a restriction element.

24. The method of any one of claims 20 to 23, comprising running retention elements along two substantially opposite sides of the hull.

25. The method of any one of claims 20 to 24, comprising drilling through the marine riser.

26. The method of any one of claims 20 to 25, comprising applying retention forces to the riser retention element according to a non-linear force profile where a nonlinear force is applied in response to deviation of the marine riser from a center position.

27. The method of claim 26, comprising applying little or no retention force when the retention point is within a first radius, applying an increased retention force when the retention point is between the first radius and a second radius, and applying a retention force to block deviation beyond the second radius.

28. The method of any one of claims 20 to 27, comprising positioning the drillship in the direction of a current and locating higher-capacity retention elements upstream of the riser retention element.

29. The method of any one of claims 20 to 28, comprising running a retention element over a guide element.

30. The method of claim 29, comprising retracting the guide element from an operating position to a retracted position.

31 . The method of claim 30, comprising retracting the guide element from a position at least partly below water to a position out of the water.