RISER STABILISATION

Abstract

A method of stabilizing a riser includes applying an oscillating force to the riser, using one or more thrust units on the rise, so as to counteract oscillating movement of the rise, preferably having a first component oscillating at a period between 3 and 20 seconds and a second component oscillating at a period between 1 and 5 minutes, where the thrust units may be positioned at antinodes of the oscillating movement and preferably each comprise three pairs of thrusters respectively configured to provide thrust in three evenly spaced directions.

Description

TECHNICAL FIELD

The present invention relates to stabilizing an offshore riser.

BACKGROUND

An offshore riser is a conduit that connects a subsea oil well, or the like, on the seabed to a surface platform. There are a number of types of risers including export and production risers, drilling risers and workover risers. Fatigue of risers is a common challenge for many types of risers. A combination of platform motion, wave forces and vortex induced forces from currents acting on the riser induce fluctuating tension and bending moments in the riser. The fluctuating bending moment induced stresses especially can cause fatigue damage to the riser resulting in a short fatigue life.

When a riser reaches a certain level of fatigue damage, it must be inspected and sometimes replaced (depending on the safety standard, inspection method, detected cracks etc). Inspection and especially replacement of the riser joints is an expensive process, with a riser itself often costing as much as $100 million. Furthermore, maintenance and replacement of the riser incurs further costs associated with lost operation/production time during the maintenance or replacement. It is therefore highly desirable to maximize the operational life of the riser by increasing its fatigue life, which is often a limiting factor.

One known problem for risers is vortex-induced vibration (VIV). This type of vibration occurs in elongate bodies interacting with an external fluid flow and can be responsible for fatigue damage to risers. VIV arises when a boundary layer of the fluid separates from the body causing vortices to form in the wake of the body, changing the pressure distribution along the rear surface. Due to irregularities in the flow, the vortices do not form symmetrically around the body and fluctuating lift forces develop on each side of the body, thus leading to relatively high-frequency oscillatory motion transverse to the flow direction.

A large number of solutions have been previously proposed to address the problem of fatigue caused by VIV by suppressing the formation of vortices about the riser. This is typically achieved using flow guides, such as helical strakes or fairings formed along the length of the riser.

In addition to reducing fatigue damage by reducing VIV, the operational life of a riser may also be increased by reducing the magnitude of the static forces to which it is subjected. WO 01/71153 A1 suggests a method of reducing bending forces arising at the lower portion of the riser that arise from static current forces acting along the length of the riser. WO 01/71153 A1 uses a number of propeller-driven thrusters positioned along the length of a riser to apply constant forces in predetermined directions that counteract static ocean current forces in order to maintain a desired riser course.

Thus, various devices have been proposed to address VIV and damage from static forces. However, other problems remain.

BRIEF SUMMARY

Viewed from a first aspect, the present invention provides a method of stabilizing a riser connecting a structure on the seabed, such as a wellhead, to the sea surface, the method comprising: applying an oscillating force to the riser using a thrust unit on the riser, so as to counteract an oscillating movement of the riser.

The inventors have realized that there is a significant problem of fatigue caused by oscillating motion of a riser. This has not been addressed in the prior art. The use of a thrust unit to provide an oscillating force on the riser allows for the damaging oscillating movement of the riser to be actively cancelled. By providing an active thrust to counteract the oscillations of the riser the oscillations of the riser can be negated to a far more significant degree than is possible using passive countermeasures.

In this context, a thrust unit is intended to include any device capable to producing an active force on the riser by ejection of mass, which should preferably be a device that can generate sufficient thrust to cause movement of the riser. For example, the thrust unit may comprise at least one propeller driven thruster or at least one water jet thruster. A propeller driven thruster is a thruster which generates thrust by moving water immediately adjacent to the thruster through a propeller. A water jet thruster is a thruster which ejects high-speed water that is stored in or collected from a remote location and transported by pipe to the thruster location.

It should be understood that the thrust unit may be mounted to the riser, for example being formed separately and mounted in accordance with the following method, or the thrust unit may be formed integrally with the riser, for example it could be formed together with one of the riser segments during manufacture.

The oscillating force from the thrust unit is applied at a location along the length of the riser. The precise location will vary depending on a number of factors, such as the particular riser properties, local weather conditions, number of thrust units used, etc., but the location is typically between 10% and 90% along the length of the riser.

Furthermore, as used herein the terms oscillating force and oscillating movement are intended to refer to a force and a displacement, respectively, having a repetitive variation in time. For example, the oscillation may be a repeated sinusoidal type variation in time.

It has been identified that the most damaging oscillations occur at periods much longer than those typically observed in VIV. Therefore, whilst the method can be applied to vortex induced vibrations, the oscillating movement of the riser to be counteracted preferably has a period of over 1 second.

The oscillation also preferably has a period of less than 30 minutes. In preferred embodiments, the oscillating motion has a period of between 3 seconds and 20 seconds, and more preferably between 6 seconds and 15 seconds, which correspond to typical periods for ocean waves. In other preferred embodiments the oscillating motion has a period of between 1 minute and 5 minutes, which corresponds to typical periods of slow drift oscillation of a surface vessel caused by wind and wave drift forces.

Furthermore, oscillations at resonant frequencies of the riser are amplified by this resonance making them particularly damaging; typically the lowest resonant frequency has the highest degree of amplification. Thus, preferably the oscillating movement of the riser to be counteracted is at a resonant frequency of the riser, and more preferably is at the lowest resonant frequency of the riser.

The oscillating movement is often not a single-mode vibration, but rather includes components oscillating at a number of different modes. The oscillating force may therefore also be applied to the riser so as to further counteract oscillating movement of the riser at one or more additional, different frequency, preferably also at a resonant frequency of the riser. For example, the oscillating force may be to counteract oscillating movement of the riser having one component with a period of between 3 seconds and 20 seconds (preferably between 6 and 15 seconds) and another component with a period of between 1 minute and 5 minutes.

The method may comprise the step of locating the thrust unit on the riser, for example by mounting the thrust unit on the riser. Preferably the method comprises locating the thrust unit on the riser at a predetermined location, this location being selected to maximize the corrective effect of the thrust unit on expected oscillating movements of the riser.

The oscillating movement will typically form a standing wave having at least two nodes (points where the wave has zero amplitude) and at least one antinode (points where the amplitude of the standing wave is a maximum). It is desirable therefore that the thrust unit is located on the riser about at an antinode of the oscillating movement of the riser because the thrust generated by the thrust unit will provide the maximum cancelling effect when applied at the antinode of the standing wave.

The method may therefore further comprise: modelling the oscillating movement of the riser; predicting the location of an antinode of the oscillating movement based on the modelling; and locating the thrust unit on the riser at the predicted antinode location. The method may include predicting the locations of multiple antinodes and locating multiple thrust units on the riser at two or more of the predicted antinode locations.

Typically, at the frequencies of interest, the ends of riser may be considered to be nodes of the riser, as they remain relatively still compared with mid-points of the riser. Therefore, the thrust unit is not located at an upper end or a lower end of the riser, and preferably not within 10% of the length of the riser from the upper or lower ends of the riser.

In a further aspect therefore, the present invention may also be seen to provide a method of locating a thrust unit on a riser, the method comprising: modelling oscillating movement of the riser; predicting the location of an antinode of the oscillating movement based on the modelling; and locating the thrust unit on the riser at the predicted antinode.

A further oscillating force may be applied to the riser using a second thrust unit configured to be located on the riser at a location axially offset from the first thrust unit. As mentioned above, the second thrust unit is preferably also positioned at an antinode of the oscillating movement of the riser. However, it may of course be positioned elsewhere. The antinode made be an antinode of a different mode of vibration of the oscillating movement of the riser from that of the first thruster.

The or each thrust unit preferably comprises a pair of thrusters on opposite sides of the riser, wherein first and second thrusters of the pair may be aligned and hence provide thrust in parallel with each other. By using a pair of thrusters on opposing sides of the riser, the pair of thrusters can generate an uninterrupted flow (i.e. not interrupted by the riser itself) without inadvertently generating a torsion force about the riser, which could cause damage to the riser.

More preferably, the or each thrust unit comprises at least two pairs of thrusters with the pairs being arranged to provide thrust in non-parallel directions, and even more preferably the thrust unit comprises at least three pairs of thrusters, each pair being arranged to provide thrust in a direction non-parallel with the direction of thrusts of the other pairs.

It is preferable that the or each thrust unit be able to generate a net thrust in any horizontal direction. In order to achieve this, for example, either a single thruster must be able to rotate or at least two thrusters must be able to generate thrust in two non-parallel directions. Generating thrust in non-parallel directions is preferable to a rotatable thruster as the force applied can be adjusted more rapidly in response when a change in thrust direction is required. The generation of thrust in at least three non-parallel directions (preferably three evenly-spaced directions) is preferred because this provides redundancy. With the use of three non-parallel directions the thrust unit can still operate even if thrust in one direction is unavailable, for example if a thruster is damaged or is providing insufficient thrust, because only thrust in two non-parallel directions is required to generate a net thrust in any horizontal direction.

In one embodiment, the pairs of thrusters are axially offset from one another so that their respective output streams do not significantly interfere with one another. In the present context, the term axially has been used in reference to the axial direction of the riser.

In an alternative embodiment, a first thruster of each pair of thrusters is located in a first thrust plane and a second thruster of each pair of thrusters is located in a second thrust plane, which is axially offset along the axis of the riser. The thrust planes are preferably substantially horizontal, or perpendicular to the axis of the riser.

This embodiment results in a more compact configuration. Furthermore, this configuration avoids axial bending moments from being induced by the thrust unit because the center of thrust for each pair of thrusters is the same.

The first thrusters in the first thrust plane and the second thrusters in the second thrust plane may have the same configuration rotated by 180°. By this configuration, torsion generated in one thrust plane is cancelled by an equal and opposing torsion generated in the other thrust plane.

Viewed from a second aspect, the present invention provides a riser stabilization system for stabilizing a riser connecting a structure on the seabed, such as a wellhead, to the sea surface comprising: at least one sensor for detecting movement of the riser; a thrust unit for applying a force to the riser; and a controller configured to, in use, cause the thrust unit to apply an oscillating force to the riser so as to counteract an oscillating movement of the riser detected by the at least one sensor.

The controller is preferably configured to, in use, cause the thrust unit to perform the method of the first aspect, and optionally any of its preferred features.

The oscillating movement of the riser to be counteracted preferably has a period of over 1 second. The oscillation also preferably has a period of less than 30 minutes. In preferred embodiments, the oscillating motion has a period of between 3 seconds and 1 minute, and most preferably between 6 seconds and 15 seconds. In other preferred embodiments, the oscillating motion has a period of between 1 minute and 5 minutes.

Preferably the oscillating movement of the riser to be counteracted is at a resonant frequency of the riser, and more preferably is at the lowest resonant frequent of the riser. The oscillating force may also be applied to the riser so as to further counteract oscillating movement of the riser at a second, different frequency, which may be a second resonant frequency of the riser.

The thrust unit may comprise at least one propeller driven thruster or at least one water jet thruster. The thrust unit preferably comprises a pair of thrusters mounted on opposite sides of the riser.

The thrust unit may be configured to be mounted to the riser, or the thrust unit may be formed integrally with the riser or a segment of the riser.

It is preferable that the thrust unit be able to generate a net thrust force in any horizontal direction. Thus, the thrust unit may comprise at least two pairs of thrusters arranged to provide thrust in non-parallel directions, and more preferably at least three pairs of thrusters, each being arranged to provide thrust in a non-parallel direction.

In one embodiment, the pairs of thrusters are axially offset from one another so that their respective output streams do not significantly interfere with one another. In an alternative embodiment, a first thruster of each pair of thrusters is mounted in a first thrust plane and a second thruster of each pair of thrusters is mounted in a second, axially offset, thrust plane. The thrust planes are preferably substantially horizontal, or perpendicular to the axis of the riser.

The system may further comprise a second thrust unit, optionally including any of the preferred features of the first thrust unit. Thus, the oscillating force may be a force applied using two or optionally more than two thrust units at the same time. The controller may then further be configured to, in use, control each thrust unit independently to apply an oscillating force to the riser so as to counteract the oscillating movement of the riser detected by the at least one sensor.

The sensor is preferably arranged to detect at least a horizontal velocity of the riser. The sensor may be an accelerometer.

Viewed from a third aspect, the present invention further provides a thrust unit for mounting to a riser, the thrust unit comprising: a frame configured to be mounted to the riser; and a plurality of pairs of thrusters, each being mounted to the frame, wherein each pair of thrusters generates a net thrust in a respectively non-parallel direction.

Preferably, for each pair of thrusters, a first thruster of the pair of thrusters is configured so as to be on an opposite side of the riser to a second thruster of the pair of thrusters, when the frame is mounted to the riser. Preferably the plurality of pairs of thrusters comprises three pairs of thrusters.

In one preferred embodiment, the pairs of thrusters are mounted to the frame so that, when mounted to the riser, each of the pairs of thrusters are axially offset from the other pairs of thrusters.

In another preferred embodiment, a first thruster of each pair of thrusters is mounted in a first thrust plane and a second thruster of each pair of thrusters is mounted in a second thrust plane that is axially offset along the axis of the riser, wherein the thrusters in each of the first and second thrust planes are configured to generate a net thrust in any direction within the thrust plane.

The invention extends to a riser for connecting a structure on the seabed, such as a wellhead, to the sea surface, comprising a system or a thrust unit as described above, wherein the thrust unit is mounted on the riser. The thrust unit is preferably mounted on the riser at about the location of an antinode of a mode of oscillating movement of the riser.

The thrust unit mounted at a location along the length of the riser, i.e. the thrust unit is not located at an upper end or a lower end of the riser, and preferably between 10% and 90% along the length of the riser.

Viewed from a fourth aspect, the present invention provides a computer program product comprising computer readable instructions that, when executed, will cause a processor to perform a method comprising: receiving indications of a movement of a riser connecting a structure on the seabed, such as a wellhead, to the sea surface from at least one sensor; and causing a thrust unit to apply an oscillating force to the riser so as to counteract oscillating movement of the riser detected by the at least one sensor.

The computer program product preferably comprises computer readable instructions that, when executed, will cause a processor to perform the method performed by the controller in the first and second aspects, and optionally any of the preferred aspects.

Preferably, the computer readable instructions are for controlling a thrust unit or a stabilization system according to the second or third aspects.

The oscillating movement of the riser to be counteracted preferably has a period of over 1 second. The oscillation also preferably has a period of less than 30 minutes. In preferred embodiments, the oscillating motion has a period of between 3 seconds and 20 seconds, and preferably between 6 and 15 seconds. In other preferred embodiments, the oscillating movement has a period of between 1 minute and 5 minutes.

Preferably the oscillating movement of the riser to be counteracted is at a resonant frequency of the riser, and more preferably is at the lowest resonant frequent of the riser. The oscillating force may also be applied to the riser so as to further counteract oscillating movement of the riser at a second, different frequency, which may be a second resonant frequency of the riser.

Viewed from a fifth aspect, the present invention provides a computer program product comprising computer readable instructions that, when executed, will cause a processor to perform a method comprising: receiving parameters of a riser; modelling oscillating movement of the riser when connecting a structure on the seabed to the sea surface; and determining a location for a thrust unit on the riser to counteract the oscillating movement based on the modelling. The determined location is preferably an antinode of the oscillating movement and may be determined by modelling as discussed above in relation to the methods of the invention and preferred embodiments.

The location for the thrust unit should not be at an upper end or a lower end of the riser, and preferably not within 10% of the length of the riser from the upper or lower ends of the riser.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain preferred embodiments of the invention will now be described in greater detail, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 shows a prior art offshore installation including a riser;

FIG. 2 shows an offshore installation including a riser stabilized by a thrust unit;

FIG. 3 shows a side view of the thrust unit shown in FIG. 2;

FIG. 4 shows a plan view of the thrust unit shown in FIG. 2;

FIG. 5 shows a perspective view of an alternative thrust unit;

FIG. 6 shows a plan view of the thrust unit shown in FIG. 4; and

FIG. 7 shows an offshore installation including a riser stabilized by a thrust unit.

DETAILED DESCRIPTION

FIG. 1 shows a simplified diagram of a conventional offshore installation 10. The installation 10 comprises a floating platform 12 (formally a floating production, storage, and offloading system), a production riser 14, and a wellhead 16. The production riser 14 connects the floating platform 12 to the wellhead 16 via a lower riser package 18 (also known as a Christmas tree) and a blow-out protector (BOP) 20. The riser 14 is typically tensioned by a heave compensating system, which is a system that tries to compensate for the relative movement of the floating platform 12 with respect to the seabed, in order to improve the stability of the riser 14.

Fatigue problems at the upper part of the riser 14 occur at all water depths due to motion of the floating platform 12. It is predominantly roll and pitch motions (i.e. rotation about the horizontal axes) that cause bending moments in the riser 14, but surge and sway motions can also contribute. Tension variations in a heave compensating system can also lead to bending moment variations when the riser 12 is angled, for example as in the case in FIG. 1. These combined effects result in fluctuating bending moments and bending stresses, causing fatigue damage to the upper part of the riser 14.

Fatigue problems at the lower part of the riser 14, as well as in the lower riser package 18 or the wellhead 16, occur most often at shallow water depths, although fatigue problems in these regions can also occur at deep water depths when there are large tension variations in the system. A fluctuating bending moment at the bottom of the riser 14, which gives rise to fatigue stresses, is primarily induced by the following two effects.

Firstly, the lower part of the riser 14 may be at an angle relative to the wellhead 16 due to current forces, vessel offset between the wellhead 16 and the floating platform 12, a non-vertical wellhead 16, higher order oscillation of a floater due to higher order wind and wave drift forces, as well as other factors. Where the lower part of the riser 14 is angled relative to the wellhead 16, tension variations, for example due to the differential movement between the wellhead 16 and the floating platform 12 cause fluctuating bending moments.

Secondly, horizontal oscillatory motions of the riser 14 caused by wave forces and wave motion of the floating platform 12 causes a varying angle of the riser at the bottom. Eigenmodes, also known as resonant modes, of the riser 14 can result in large dynamic amplification of the horizontal riser motion.

Eigenmodes of the riser 14 at periods between 3 and 20 seconds, and more specifically between 6 and 15 seconds are particularly dangerous. This is because this range corresponds to the typical frequency of ocean waves acting on and exciting the floating platform 12. In deep water depths the drag of the water on the riser 14 provides significant damping, moving the horizontal eigenmodes outside of this range, but in shallow water they can be a big problem. Thus, the effects of horizontal oscillatory motion due to ocean waves are amplified by shallow water, whilst in deeper water horizontal motion of the lower part of the riser 14 is limited.

Furthermore, lower frequency oscillations of the riser 14, typically having a period of between 1 and 5 minutes, result from wind and wave drift forces. These movements occur in both deep and shallow waters.

FIG. 2 shows an offshore installation 100 that has been stabilized in accordance with the present invention. The installation 100 includes a floating platform 102 at the sea surface, a wellhead 104 on the seabed, and a riser 106 connecting the wellhead 104 to the platform 102. Mounted to the riser is a thrust unit 108.

The thrust unit 108 will be described in greater detail later, but is capable of generating a thrust in any horizontal direction. The riser 106 in this first embodiment is shown exhibiting oscillating movement having two nodes (at the platform 102 and the wellhead 104) and one antinode (approximately at the middle of the riser 106).

The thrust unit 108 is mounted at a location along the length of the riser 106. The precise location is selected depending on a number of factors, as will be discussed in greater detail below, such as the particular riser 106 properties, local weather conditions, number of thrust units 108 used, etc. However, the location at which the thrust unit 108 is mounted is typically between 10% and 90% along the length of the riser 106 from the wellhead 104 to the platform 102, i.e. from the seabed to the sea surface.

A method of mounting the thrust unit 108 to the riser will now be discussed.

A computer, such as the controller 118, stores a computer program product for modelling the behavior of the riser 106 and determining where on the riser 106 to mount the thrust unit 108.

First, a user inputs parameters relating to the riser 106. These may include, for example, the length of the riser 106, the stiffness of the riser 106, the type of platform 102, the end-fixing conditions of the riser 106, the weather and tidal patterns for the location where the riser 106 is to be installed (i.e. the location of the offshore platform 102), etc. Many risers 106 are purchased “off-the-shelf”. Therefore, optionally, certain of the riser parameters may be pre-stored, either on the computer or at a remote location, and the user may simply select a location and/or model of the riser 106 from a list of known locations and/or models.

Next, the computer models behavior of the riser 106. The computer may, for example, generate estimates of the forces to which the riser 106 will be subjected based on the input parameters and, based on those forces, estimate the modes of oscillation of oscillating movement that the riser 106 might be expected to display.

The invention primarily relates to the cancellation of oscillatory motion along the length of the riser 106. As such, the ends of riser 106 may be treated as nodes of the waveform, as they remain relatively still compared with mid-points of the riser at the frequencies of interest.

Finally, the computer will determine the optimal location(s) on the riser 106 in which to mount a desired number of thrust units 108 so as to minimize the oscillation of the riser 106. This may, for example, be at the antinodes of the determined oscillating movement. The number of thrust units 108 available may also affect the optimal locations. As the ends of the riser remain relative stationary, the thrust unit will not be located at an upper end or a lower end of the riser 106, and normally not within 10% of the length of the riser 106 from the upper or lower ends of the riser 106.

Turning now to the details of thrust unit 108, an embodiment of a thrust unit 108 is shown in greater detail in FIGS. 3 and 4. The thrust unit 108 comprises a sensor (not shown) and three pairs of axially offset (with respect to the axis of the riser 106) propeller-driven thrusters 110, 112, 114. The thrusters are mounted to a frame 116 that is in turn releasably mounted to the riser 106. The frame 116 is configured so as not to move axially along the riser 106, nor to rotate about the riser 106, when mounted thereto.

Each pair of thrusters 110, 112, 114 is controllable independently of the other pairs of thrusters 110, 112, 114, and may be controlled to generate a forward or backward thrust along a respective thrust direction. Each thrust direction passes through an axis of the riser 106 so that no torque is generated by thrust along that thrust direction.

Each pair of thrusters 110, 112, 114 comprises a first thruster 110a, 112a, 114a and a second thruster 110b, 112b, 114b that are arranged respectively on opposite sides of the riser 106. Each first thruster 110a, 112a, 114a is positioned with its direction of thrust parallel to the direction of the corresponding second thruster 110b, 112b, 114b. The direction of thrust of each thruster 110a, 112a, 114a, 110b, 112b, 114b is substantially horizontal and each thruster 110a, 112a, 114a, 110b, 112b, 114b can be operated in both a forward thrust and a rearward thrust mode.

The pair of thrusters 110, 112, 114 are arranged such that the thrust direction of each pair of thrusters 110, 112, 114 is non-parallel with the thrust directions of the other two pairs of thrusters 110, 112, 114. In the present embodiment, the thrust directions of the pairs of thruster 110, 112, 114 are oriented at equal intervals of 120°.

By selectively controlling the magnitude and directions of thrust generated by each of the pairs of thrusters 110, 112, 114, a resulting thrust force can be generated in any horizontal direction on the riser 106. The thrust unit 108 has designed redundancy because only two non-parallel thruster pairs are required to generate a resulting thrust in any horizontal direction. Thus, if any thruster pair 110, 112, 114 fails, for example by one of the thrusters 110a, 112a, 114a, 110b, 112b, 114b becoming ineffective or less effective, the thrust unit 108 can still operate at maximum capacity.

The thrusutilized 08 is to be utilized to counter oscillating movement of the riser 106 occurring at the resonant modes of the riser 106. Typically, the greatest resonance occurs at the resonant mode having the lowest natural frequency (the fundamental mode). In order to maximize the efficiency of the thrust applied by the thrust unit 106, and to minimize the inadvertent stimulation of other resonant modes, the thrust unit 108 is positioned at the antinode of the vibration. Whilst drag forces and end conditions of the riser 108 may cause the antinode position to move, the antinode of the fundamental mode of vibration is typically about midway along the length of the riser 106.

The sensor detects at least the horizontal velocity of the riser 106 at the location of the sensor. The sensor may include an accelerometer for determining horizontal velocity. At the location of the sensor, the sensor may additionally detect the horizontal position of the riser 106, the acceleration of the riser 106, the relative speed of the water surrounding the riser 106, the tension in the riser 106 and the bending moment in the riser 106.

The thrust unit 108 and sensor are connected to a controller 118. The controller 118 is configured to receive inputs from the sensor and to control the thrusters 110a, 112a, 114a, 110b, 112b, 114b of the thrust unit 108 to apply a force to the riser 106. The controller 118 may be positioned either locally to the thrust unit 108, for example mounted on the frame 116, or positioned elsewhere, for example on the floating platform 100.

In use, the controller 118 controls the thrust unit 108 to provide an oscillating force to counteract oscillating movement of the riser 106 detected by the sensor. In one example, this comprises applying a force to the riser 106 proportional and opposite to the horizontal velocity of the riser 106.

Various implementations of how to counteract the oscillating movement based on the inputs from the sensor and the model of the riser 106 will be apparent to those skilled in the art. Further examples will therefore not be set forth in length.

FIGS. 5 and 6 illustrate a thrust unit 208 having an alternative thruster configuration. The thrust unit 208 shown in FIGS. 5 and 6 is similar to the thrust unit 108 shown in FIGS. 3 and 4 and like reference numerals have been used to indicate like features. Description of those features common to both configurations and discussed previously has been omitted.

The thrust unit 208 again comprises a sensor (not shown) and three pairs of thrusters 110, 112, 114. As with thrust unit 108, the first thruster 110a, 112a, 114a of each of the pairs of thrusters 110, 112, 114 is positioned on an opposite side of the riser 106 to the respective second thruster 110b, 112b, 114b, and the first and second thrusters 110a, 112a, 114a, 110b, 112b, 114b of each thruster pair 110, 112, 114 are positioned with their direction of thrust parallel to one another

Different from thrust unit 108, each first thruster 110a, 112a, 114a is provided in a first thrust plane, which is perpendicular to the riser 106, and each second thruster 110b, 112b, 114b is positioned in a second thrust pane, which is also perpendicular to the riser 106 and is axially offset with respect to the first thrust plane. However, the thrust direction of each pair of thrusters 110, 112, 114 still passes through an axis of the riser 106 so that no torque is generated by thrust along that thrust direction.

The first thrusters and second thrusters 110a, 112a, 114a, 110b, 112b, 114b are mounted, respectively, to first and second mounting surfaces of a frame 208. The frame 208 is in turn releasably mounted to the riser 106 so as to prevent axial movement and relative rotation.

The first thrusters 110a, 112a, 114a are arranged within the first thrust plane such that their directions of thrust act are evenly spaced at 120° intervals. The second thrusters 110b, 112b, 114b are opposite to the first thrusters 110a, 112a, 114a, and are thus similarly arranged within the second thrust plane at evenly spaced 120° intervals. Thus, each plane may independently generate thrust in any horizontal direction. As the thrusters are radially offset from the riser 106, the generated thrust in each plane may generate a twisting moment on the riser 106; however, this is counteracted by the thrust generated in the other thrust plane.

FIG. 7 shows another offshore installation 100 that has been stabilized in accordance with the present invention. The installation 100 is similar to the installation 100 described in the first embodiment and like reference numerals have been used to indicate like features. Description of those features common to both embodiments and discussed previously has been omitted.

The installation 100 again includes the floating platform 102, the wellhead 104 on the seabed, and the riser 106 connecting the wellhead 104 to the platform 102. Mounted to the riser are a first thrust unit 120 and a second thrust unit 122. Both the first and second thrust units 120, 122 are similar to thrust unit 108 described above; however, thrust unit 208 may also be used in this configuration.

The first and second thrust units 120, 122 are both connected to and controlled by the controller 118. Each of the first and second thrust units 120, 122 includes a sensor (not shown) as discussed above, although only one sensor is required to determine the movement of the riser 106 as the movement of the rest of the riser can be determined based on the model. The sensors are also connected to the controller 118.

The optimal locations for mounting the first and second thrust units 120, 122 is determined by the method discussed above. The riser 106 in FIG. 7 is shown exhibiting oscillating movement having three nodes (at the platform 102, the wellhead 104 and approximately at the middle of the riser 106) and two antinodes (approximately one quarter and three quarters along the length of the riser 106). As the dominant mode of vibration has two antinodes, the first and second thrust units 120, 122 are respectively positioned at the antinodes.

The controller 118 independently controls the first and second thrust units based on the measurements from both sensors to counteract oscillating movement of the riser 108.

Whilst the invention has been described with reference to certain preferred embodiments, it will be understood by those skilled in the art that various other embodiments are also within the scope of the invention.

For example, it will be understood that the described production riser 106 of the floating platform 102 is merely an exemplary riser, and that the invention is applicable to any type of riser, such as export risers, production risers, drilling risers, and workover risers, utilized by any type of offshore platform, such as fixed tower platforms, spar platforms, tension-leg platforms, semi-submersible platforms, etc.

Additionally, whilst the described embodiment includes one sensor for each thrust unit 108, 120, 122, 208, it will be apparent to those skilled in the art that a greater number of sensors may be positioned along the riser 106, each being connected to the controller 118. Furthermore, a single sensor could be used to control two or more thrust units 108, 120, 122, 208.

Also, whilst the described risers 106 have been shown exhibiting oscillations affecting the entire length of the riser 106, localized oscillations may also occur affecting only a relatively short length of the riser 106. This may particularly be the case where other counter-vibration measures are implemented.

Furthermore, whilst the described thrust units 108, 120, 122, 208 have been described as units formed separately of the riser 106, they may of course be formed integrally with the riser, for example formed together with a riser segment to be installed in the riser 106 during manufacture.

The invention is therefore not to be seen as being limited to the described embodiment, but should be understood to include all embodiments falling within the scope of the claims.

Claims

1. A method of stabilizing a riser connecting a structure on the seabed to the sea surface, the method comprising:

applying an oscillating force to the riser, using a thrust unit on the riser, so as to counteract an oscillating movement of the riser.

2. A method according to claim 1, wherein the oscillating movement of the riser has a period of between 1 second and 30 minutes.

3. A method according to claim 1, wherein the oscillating movement of the riser is at a resonant frequency of the riser.

4. A method according to claim 1, further comprising:

modelling the oscillating movement of the riser;

determining a location for the thrust unit on the riser based on the modelling; and

locating the thrust unit on the riser at the determined location.

5. A method according to claim 4, wherein the determined location is based on a predicted location of an antinode of the modelled oscillating movement.

6. A method according to claim 1, wherein the thrust unit comprises at least two pairs of thrusters arranged to provide thrust in non-parallel directions.

7. A method according to claim 6, wherein the thrust unit comprises at least three pairs of thrusters, each being arranged to provide thrust in a non-parallel direction.

8. A riser stabilization system for stabilizing a riser connecting a structure on the seabed to the sea surface, comprising:

at least one sensor for detecting movement of the riser;

a thrust unit for applying a force to the riser; and

a controller configured to, in use, cause the thrust unit to apply an oscillating force to the riser so as to counteract an oscillating movement of the riser detected by the at least one sensor.

9. A system according to claim 8, wherein the oscillating movement of the riser has a period of between 1 second and 30 minutes.

10. A system according to claim 8, wherein the oscillating movement of the riser is at a resonant frequency of the riser.

11. A system according claim 8, wherein the thrust unit comprises at least two pairs of thrusters arranged to provide thrust in non-parallel directions.

12. A system according to claim 11, wherein the thrust unit comprises at least three pairs of thrusters, each being arranged to provide thrust in a non-parallel direction.

13. A system according to claim 8, wherein the sensor is arranged to detect a horizontal velocity of the riser.

14. A thrust unit for mounting to a riser, the thrust unit comprising:

a frame configured to be mounted to the riser; and

a plurality of pairs of thrusters, each thruster being mounted to the frame,

wherein each pair of thrusters generates a net thrust in a respectively non-parallel direction.

15. A thrust unit according to claim 14, wherein, for each pair of thrusters, a first thruster of the pair of thrusters is configured so as to be on an opposite side of the riser to a second thruster of the pair of thrusters, when the frame is mounted to the riser.

16. A thrust unit according to claim 14, wherein the plurality of pairs of thrusters comprises three pairs of thrusters.

17. A thrust unit according to claim 14, wherein the pairs of thrusters are mounted to the frame so that, when mounted to the riser, each of the pairs of thrusters are axially offset from the other pairs of thrusters.

18. A thrust unit according to claim 14, wherein a first thruster of each pair of thrusters is mounted in a first thrust plane and a second thruster of each pair of thrusters is mounted in a second thrust plane that is axially offset along the axis of the riser, wherein the thrusters in each of the first and second thrust planes are configured to generate a net thrust in any direction within the thrust plane.

19. A riser for connecting a structure on the seabed to the sea surface, comprising a system as claimed in claim 8 or a thrust unit comprising a frame configured to be mounted to the riser; and a plurality of pairs of thrusters, each thruster being mounted to the frame, wherein each pair of thrusters generates a net thrust in a respectively non-parallel direction, wherein the thrust unit is mounted on the riser.

20. A computer program product comprising computer readable instructions that, when executed, will cause a processor to perform a method comprising:

receiving indications of a movement of a riser connecting a structure on the seabed to the sea surface from at least one sensor; and

causing a thrust unit to apply an oscillating force to the riser so as to counteract oscillating movement of the riser detected by the at least one sensor.

21.-23. (canceled)