NO20210737A1

NO20210737A1 - Riser tensioning system

Info

Publication number: NO20210737A1
Application number: NO20210737A
Authority: NO
Inventors: Lars Pøhner
Original assignee: Mhwirth As
Priority date: 2021-06-07
Filing date: 2021-06-07
Publication date: 2022-12-08

Description

RISER TENSIONING SYSTEM

The present disclosure relates generally to the field of floating offshore platforms or vessels for the exploitation of undersea deposits of for example petroleum and natural gas. More specifically, it relates to risers tensioners and a system for providing tension to risers which extend from a subsea wellhead or subsurface structure to a floating platform or vessel.

BACKGROUND

Offshore platforms or vessels for the exploitation of undersea petroleum and natural gas deposits typically support risers that extend to the platform from one or more wellheads or structures on the seabed. These platforms or vessels can be subject to motion due to wind, waves and currents. Consequently, the risers employed with such platforms or vessels must be tensioned so as to permit the platform or vessel to move relative to the riser. Also, riser tension must be maintained so that the riser does not buckle under its own weight. Accordingly, a tensioning mechanism is usually employed, and operates to exert a substantially continuous tension force to the riser within a defined range.

Documents which may be useful to understand the field of technology include WO 2004/013452, US 4,886,397, US 3,902,319, GB 2109 036, WO 2012/016765, WO 2014/090682 and US 10,385,630.

Operation of such platforms or vessels, for example, floating drillships or rigs, is costly and there is a continuous need for improved technology to permit safe and reliable operation while minimizing operational downtime. The present disclosure has the objective to provide such improved technology, or at least alternatives to the state of the art.

SUMMARY

In an embodiment, there is provided a wireline riser tensioning system comprising a tensioning cylinder assembly, a wire rope, a sheave, wherein the tensioning cylinder assembly is configured for connection to a marine riser via the wire rope for providing a tensioning force on the marine riser, and the sheave is configured for arrangement between the tensioning cylinder assembly and the marine riser such that the wire rope runs via the sheave, wherein the sheave comprises a motor operatively connected to the sheave.

In an embodiment, there is provided a method of reducing load variations from a riser tensioning system on a marine riser, the method comprising replacing a passive idler sheave with a sheave comprising a motor operable to drive the sheave.

The detailed description below and the appended claims outline further aspects and embodiments according to the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments of the invention will now be described with reference to the accompanying drawings of which:

Figures 1 and 2 illustrate a wireline riser tensioner system arranged on a drillship, the wireline riser tensioner system having a first type of tensioner cylinder assemblies.

Figures 3 and 4 illustrate a wireline riser tensioner system arranged on a drillship, the wireline riser tensioner system having a second type of tensioner cylinder assemblies.

Figure 5 is an illustration of parts of a wireline tensioner system according to an embodiment.

Figure 6 is an illustration of parts of a wireline tensioner system according to an embodiment.

DETAILED DESCRIPTION

Figures 1 and 2 illustrate variants of a wireline tensioner (WLT) system 100,101 arranged on a floating installation. In this system, wireline riser tensioner cylinder assemblies 10 are located in a drill floor area 5. The wireline riser tensioner cylinder assemblies 10 are typically placed symmetrically around a well centre axis 16 on drill floor level. The WLT system is designed to keep close to constant tension in a marine riser 14 via a plurality of wire ropes 11. It maintains the tension in each wire rope 11, which is connected to a tension ring 13 on or connected to the riser 14. The wire ropes 11 from the tension ring 13 runs over pivot hinged idler sheaves 12 and then around sheaves on the tensioner cylinder assemblies 10. The wire ropes 11 may then run e.g. several turns around a snubber drum (not shown) and with the wire rope ends anchored to a clamp (not shown).

Figures 3 and 4 illustrates variants of another tensioning arrangement 102,103, having two riser tensioning cylinder assemblies 10a, 10b, sheaves 12a, 12b and wire ropes 11a, 11b extending between the cylinders 10a,b and the tensioning ring 13. In this embodiment, the wire rope 11a is at one end connected to a cylinder rod of the tensioning cylinder assembly 10a, runs over one or more sheaves 12a,c and at its other end be connected to the tension ring 13 which again supports a marine riser 14. A similar arrangement is employed in relation to cylinder assembly 10b, as shown. While Figs 3 and 4 only show two cylinder assemblies 10a,b and associated components, it will be understood that there may be further, i.e. more than two, cylinder assemblies for example arranged similarly as shown in Fig. 1. In Fig.3, each of the wire ropes 11a,b runs via a single sheave 12a,b between the respective cylinder assembly 10a,b and the tensioning ring 13, while in Fig.4 each wire rope 11a,b runs via two sheaves 12a-d. The functionality is otherwise the same between Figs 3 and 4.

In Figs 1-4, the cylinder assemblies 10,10a,b are operated to provide a substantially constant tension on the wire ropes 11,11a,b. This is typically achieved by designing the cylinder assemblies 10,10a,b as hydraulic cylinders supplied with hydraulic fluid with substantially constant pressure. Gas-filled accumulators can be used for this purpose, whereby a gas pressure and/or amount of gas in e.g. a bladder or piston-type accumulator is controlled in order to set the desired hydraulic pressure level, and thereby the desired tensioning force applied by the cylinder assemblies 10,10a,b.

The tensioning cylinder assembly 10,10a,b is thus operatively connected to the marine riser 14 via the wire rope 11 for providing a tensioning force on the marine riser 14. The sheave 12,12a,b is arranged between the tensioning cylinder assembly 10,10a,b and the marine riser 14 such that the wire rope 11 runs via the sheave 12,12a,b. The sheave 12,12a,b may be arranged to change the operative direction (orientation) of the wire rope 11,11a,b, for example from a substantially horizontal orientation to a downwardly orientation towards the tension ring 13 as in Figs 1 and 2, or from a substantially vertical orientation to a nearly reversed vertical orientation as in Figs 3 and 4.

The cylinder assemblies 10,10a,b are designed and operationally configured to provide a continuous tensioning force on the wire ropes 11,11a,b. The tensioning force is ideally held substantially constant. The cylinder assemblies 10,10a,b may comprise passive hydraulic cylinders with pneumatic accumulators to maintain the desired riser tension. The setpoint for the desired tension may be set by an operator, for example by adjusting the pneumatic supply pressure and/or the amount or pressure of gas in the pneumatic accumulators. Fig. 5 illustrates such a hydraulic cylinder and auxiliary components 21 for this purpose. The floating installation’s response to environmental conditions, mainly heave and horizontal motions which create changes in position relative to the marine riser 14, will then cause the hydraulic cylinders to stroke in and out. The spring effect resulting from the gas compression or expansion during platform vertical and horizontal movement relative to the seabed, partially isolates the riser from the floating installation’s motions.

At least one of the sheaves 12,12a-d in Figs 1-4 comprises a motor 20 (not illustrated in Figs 1-4 but shown in Figs 5 and 6), operable to drive the sheave 12,12a-d. The motor 20 may be, for example, an electric or hydraulic motor. The motor 20 is operable to provide a drive input on the sheave 12,12a-d. As the wire rope 11,11a,b is partially wrapped around the sheave 12,12a-d with mechanical friction between them, a drive input force can thereby be effected on the wire rope 11,11a,b by the motor 20. With knowledge of the wrap angle of the wire rope 11,11a,b over the sheave 12,12a-d and specific design of the components, the maximum design force and power of the motor 20 can be calculated for example based on Eytelwein's formula. For example, with a sheave having a wrap angle of just below 90 degrees, as in the examples of Figs 1, 2 and 4, it may be possible to alter the force in the wire rope 11,11a,b by about 15% by input power from the motor 20.

Illustrated in Fig.6, a controller 25 can be arranged to dynamically control and adjust the input from the motor 20 to the sheave 12,12a-d. The controller 25 may receive an input from a sensor 26, the sensor 26 being arranged to measure a parameter indicative of the tension in the wire rope 11,11a,b. The sensor 26 may, for example, be a sensor arranged in or on the wire rope 11,11a,b to measure the tension directly, or it may, as indicated in Fig. 6, be arranged in relation to the sheave 12,12a-d, for example measuring the vertical force acting on the sheave 12,12a-d or associated components (such as the connection to the surrounding structure) and thereby obtain a measurement indicative of the instantaneous tension in the wire rope 11,11a,b. Alternatively, the controller 25 may receive an input signal from other systems, such as a central rig control system, providing a control signal for driving the motor 20.

It is desirable to provide a tensioning force which is as close to constant as possible, in order to minimise load variations on the marine riser 14 and associated components. For example, load variations acting on the marine riser 14 from the floating installation may produce fatigue loads on a subsea wellhead to which the lower end of the marine riser 14 is connected, thereby reducing the operational lifetime of the well.

For this purpose, the motor 20 can be operated to counteract load variations present in the tensioner system, for example friction, momentum or stick-slip type effects during the cyclic motion of the system when the floating installation is exposed to weather or other environmental conditions. This can be achieved by operating the motor 20 in a cyclic manner to produce an input force on the wire rope 11,11a,b when the floating installation moves in a first direction, a braking force on the wire rope 11,11a,b when the platform moves in a second, opposite direction, or both an input and a braking force. In this manner, cyclic variations in the tension of the wire rope 11 below the sheave 12,12,a-d (i.e., between the sheave 12,12a-d and the marine riser 14) can be reduced by the motor 20 manipulating the tension in the wire rope 11 compared to what would be the situation with a passive idler sheave.

The motor 20 can be controlled in a feedback loop to provide an input to the motor 20 based on direct readings from a sensor 26, or the motor 20 may be controlled to e.g. provide a sinusoidal force profile which is aligned in frequency (and, if applicable, phase shift) with the motion of the floating installation (which may also be generally sinusoidal-like) provided to the controller 25. Alternatively, the motor 20 may be operated in on-off mode or with a different operational (force) profile.

Some or all of the sheaves 12a-d may be pivotable about a vertical axis. In use, the sheaves 12a-d may be fixed at a suitable point in the structure of the floating installation, preferably the sheaves 12a-d are fixed above or in the vicinity of a moon pool 7 (see Fig. 2).

The motor 20 may be a separate motor connected to the sheave 12,12a-d via a drive mechanism such as a belt or chain drive (as schematically illustrated in Figs 5 and 6), or it may be a motor integrated in the sheave 12,12a-d. In the latter case, the motor may be, for example, an electric motor operating directly on the sheave 12,12a-d.

In any of the embodiments described herein, the sheave 12,12a-d can be a sheave which is separate from and independent of the cylinder assemblies 10,10a,b.

Advantageously, the teaching of the present disclosure is particularly suitable for retrofitting or upgrading older wireline tensioner systems to improve operational performance, such as load variations from the riser tensioning system on a marine riser 14. According to an example, a method comprises replacing a passive idler sheave with a sheave 12,12a,b comprising a motor 20 operable to drive the sheave 12,12a,b such as to produce a wireline tensioner system 100-103 as described in any of the examples above.

In the preceding description, various aspects of the apparatus according to the invention have been described with reference to the illustrative embodiments. For purposes of explanation, specific numbers, systems and configurations are set forth in order to provide a thorough understanding of the apparatus and its workings. However, this description is not intended to be construed in a limiting sense. Various modifications and variations of the illustrative embodiment, as well as other embodiments of the apparatus, which are apparent to persons skilled in the art to which the disclosed subject matter pertains, are deemed to lie within the scope of the present invention.

Claims

1. A wireline riser tensioning system (100-103) comprising:

a tensioning cylinder assembly (10,10a,b),

a wire rope (11,11a,b),

a sheave (12,12a-d),

wherein the tensioning cylinder assembly (10,10a,b) is configured for connection to a marine riser (14) via the wire rope (11) for providing a tensioning force on the marine riser (14),

and the sheave (12,12a,b) is configured for arrangement between the tensioning cylinder assembly (10,10a,b) and the marine riser (14) such that the wire rope (11) runs via the sheave (12,12a,b),

wherein the sheave (12,12a,b) comprises a motor (20) operatively connected to the sheave (12,12a,b).

2. The wireline riser tensioning system (100-103) of claim 1, wherein the sheave (12,12a,b) is arranged such as to change the direction of the wire rope (11,11a,b), for example to change the direction by more than 60 degrees, more than 80 degrees, or more than 90 degrees.

3. The wireline riser tensioning system (100-103) of claim 1 or 2, comprising a controller (25) operatively connected to the motor (20) and configured to adjust a force from the sheave (12,12a-d) applied on the wire rope (11,11a,b).

4. The wireline riser tensioning system (100-103) of claim 3, comprising a sensor (26) arranged to measure a parameter representative of an instantaneous tension in the wire rope (11,11a,b) and wherein the controller (25) is configured to adjust the force based on a measured signal provided from the sensor (26).

5. The wireline riser tensioning system (100-103) of claim 3 or 4, wherein the controller (25) is configured to adjust the force so as to provide a cyclic force applied by the sheave (12,12a-d) on the wire rope (11,11a,b).

6. The wireline riser tensioning system (100-103) of claim 5, wherein the controller (25) is configured to control the cyclic force in response to a cyclic motion of a vessel to which the sheave (12,12a-d) is fixed.

7. The wireline riser tensioning system (100-103) of any preceding claim, wherein the wire rope (11,11a,b) extends horizontally between the tensioning cylinder assembly (10,10a,b) and the sheave (12,12a-d).

8. A method of reducing load variations from a riser tensioning system on a marine riser (14), the method comprising replacing a passive idler sheave with a sheave (12,12a,b) comprising a motor (20) operable to drive the sheave (12,12a,b) to produce a wireline tensioner system (100-103) according to any preceding claim.