GB2286167A - Bearing for mooring turret - Google Patents

Abstract

A bearing 22 for a mooring turret of the kind which is anchored to the sea bed and is used for transferring mineral oil or gas to a vessel, is arranged so that the rotational resistance of the bearing is greater than the sum of the yaw moment of motion and the spring moment of the anchor lines with respect to dynamic high-frequency rotational movements less than 2 DEG , but less than the spring moment of the anchor lines with respect to low-frequency rotational movements greater than 10 DEG . The arrangement is intended to reduce deterioration of the bearing due to high frequency low amplitude wave induced oscillation, but allow relatively free rotation in response to larger magnitude low frequency effects. The bearing may comprise wheels 17 supported in slide bearings 22 or wheels supported in roller bearings provided with a brake (Fig. 8). <IMAGE>

Description

TURRET BEARING The present invention concerns a bearing for a turret in a vessel for drilling for, storing and/or extracting oil and/or gas from a geological formation under the sea bed, which vessel is adapted to be anchored to the sea bed via the turret and anchor lines.

A turret of the type concerned can have a diameter of up to 20 m or more and weigh up to 3000 tonnes or more, depending on the design and the materials used. Significant axial forces act on the turret's bearing and foundation, partly on account of the weight of the turret and partly on account of its anchorage to the sea bed. The radial forces are relatively small during normal operation but can be considerable in rough seas. Sea currents, waves and wind rotate the vessel around the turret's axial longitudinal axis with swings in both directions which can be up to approximately 20 in the course of 5-15 minutes. However, the vessel's rotation is composed of both low-frequency, long rotational movements and high-frequency, superimposed rotational movements.These high-frequency, superimposed rotations are undesirable because they cause a lot of frequently large axial changes in load above the bearing and its foundation. Over time, this can lead to material fatigue and wear, with the consequent risk of cracks and damaging irregularities in the bearing, bearing wheels and foundation of the turret.

Two types of bearings between the turret and the vessel are already known, namely slide bearings, and wheel bearings where the wheels are supported in roller bearings.

In the first of these the rotational resistance is high on account of the high friction between the slide element and the sliding surface in the slide bearing, which results in a high torque being required for the turret to turn in relation to the vessel. On account of these the turret will follow the rotation of the vessel up to 20 or more. This is undesirable because high accelerations and uncontrolled movements occur when the turret "slips" and begins to rotate in relation to the vessel, which leads to reduced safety on the vessel/ turret. Furthermore, the risers, which are suspended from the turret, can be subject to torsion, which is undesirable. Excessive angles between the turret and the anchor lines can also occur.Torsion in the risers can, at worst, cause them to break, and excessive angles between the turret and the anchor lines can lead to the destruction of the anchor lines' guide wheel on the turret. A slide bearing is also more exposed to corrosion which can lead to increased friction with wear and destruction of the bearing function as a consequence.

With the second type of bearing the rotational resistance is very small as a result of the use of wheels with roller bearings in the hub. The wheels run on a bearing which is fastened to the foundation. Therefore the vessel can freely rotate around the longitudinal axis of the turret, because the frictional force between the wheels on the turret and the rail is very small.

However, this triggers high-frequency wave-induced rotations between the turret and the vessel, which leads to the above-mentioned frequent changes in load on the bearing and the foundation. The turret rotates so easily that it is even possible for resonance oscillations to occur with an oscillation system in which the turret constitutes the mass and the anchor lines constitute the spring(s) in the system. Resonance oscillations may be triggered by the waves, which often have a frequency which is identical with the natural frequency of the oscillation system. Such resonance oscillations could, at worst, result in the anchor lines breaking and the destruction of the turret and its bearing.

To void the above-mentioned problems in connection with the known bearings, it has also been suggested that a drive device be arranged in connection with the turret on a vessel, for example by arranging an electric motor or a hydraulically-driven motor on the vessel which meshes with a toothed ring on the turret to control the movement of the turret in relation to the vessel's sea-induced or wind-induced rotation.

However, this represents a costly and not very safe way of controlling the vessel's movements in relation to the turret, as such a rotation arrangement is expensive to use, build and maintain and will be exposed to high stress which may, at worst, lead to breaks and fatigue.

An object of the present invention is to obtain an arrangement with a bearing between a turret and a vessel which does not have the above disadvantages, i.e. which has a moment of friction which results in the turret and the vessel not rotating relative to each other before the vessel and the turret have rotated together at least 2", but less than 10 . A purpose of this was to eliminate the high-frequency rotational movements between the turret and the vessel, and, as a result, the frequent changes in load on the bearing and the foundation.

A further object of the present invention is to provide a bearing which is simple, reasonable, in terms of cost, to build and maintain and which is better than the known solutions.

In accordance with the present invention, this is achieved by means of a turret as stated in the introduction, whereby the friction in the turret's bearing is of such a magnitude that the rotational resistance of the bearing, MtUrrett in response to dynamic high-frequency rotational movements which are less than 2" (amplitude), is greater than the sum of the yaw moment of motion, Ml and the spring moment, Anchor of the anchor lines, but less than the spring moment, Anchor of the anchor lines in response to low-frequency rotational movements which are greater than 100 (amplitude).

The dependent Claims 2-4 state the advantageous features of the present invention. The present invention will now be described in more detail with reference to the examples and to the accompanying drawings, in which: Fig. 1 shows a vessel with a turret for drilling for, storing and/or producing oil and/or gas at sea; Fig. 2 shows in somewhat larger scale a front view of a part of the vessel shown in Fig. 1; Fig. 3 shows curves which illustrate the rotational movements or yaw movements of a vessel in relation to a turret; Fig. 4 is a schematic top view of the vessel of Fig. 1 in various rotational situations; Figs. 5a are graphs showing the correlation between the and 5b direction of rotation (angle) of the vessel and the turret in degrees (Fig. 5a), as well as the torque between the vessel and the turret as a function of time (Fig. 5b); ; Fig. 6 shows one embodiment of a bearing in accordance with the invention for a turret in a vessel; Fig. 7 shows another embodiment of a bearing in accordance with the invention, the bearing comprising a brake; and Fig. 8 shows in principle the rotational resistance of a turret compared with the moment from the anchor lines at various angles of rotation for a turret, where various bearings have been used.

Figs. la and lb are sketches, seen from the side and from above respectively, of a vessel 1 for drilling for, producing and/or storing oil and/or gas at sea. The vessel is provided with a turret 8 which is arranged in the hull of the vessel 1 so that it can rotate and is designed so that it can be anchored to the sea bed by means of anchor lines 7 via the turret.

In the present application the expression "vessel" means any slip or floating construction which is anchored to the sea bed via a turret.

Fig. 2 shows in somewhat greater scale a front view of a part of the vessel 1 with the turret 8 shown in Fig. 1. The turret is supported on a bearing 5 on a foundation 3 in a well 2 in the hull of the vessel 1.

A number of anchor lines 7 extend up from the sea bed (not shown) to the lower end of the turret where they pass over a guide wheel 4 and on up to the cable stoppers or anchor winches 6. The vessel can thus rotate around the turret and be positioned freely or under control in relation to the waves, current and wind.

In an operative situation, in addition to the anchor lines 7, risers and/or drill columns 9 will extend from the turret, but these pipes and other pipes or equipment on the turret and vessel are not to be discussed in further detail here as they are not part of the invention.

The rotational motion or yaw motion of a vessel around a turret is, as stated previously, composed of slow, low-frequency movements and high-frequency, superimposed movements. Figs. 3a, b and c illustrate this in more detail.

Fig. 3a shows a graph of a theoretical calculation of yaw movements for a vessel which is anchored via a turret. The curve represents the sum of the curves shown in Figs. 3b and 3c.

Fig. 3b shows the curve for the high-frequency yaw movements. They have a yaw period which corresponds to the period for the waves, which can be from 5 to 20 seconds, depending on the state of the sea or the height of the waves. The yaw amplitudes depend, in turn, on the size of the vessel and the mean direction in relation to the waves, as well as on the design or shape of the vessel. For a vessel of a size which is the most common for a production vessel today, approximately 100,000 tonnes displacement, the greatest yaw movement (in a storm situation in the North Sea) is approximately 1.5 to 2 degrees amplitude with an oscillation period of 15 seconds.

Fig. 3c shows the curve for the low-frequency, slow yaw movements. These arise as a result of a vessel being slightly directionally unstable in the anchorage system, or as a result of the anchorage system, which consists of anchor lines, being subject to one or more resonances with weather-induced forces which act on the vessel with irregular strength and direction. These oscillations depend on a number of factors and can be difficult to predict. They depend, among other things, on the geometric shape and the size of the vessel, its draught, mean direction in relation to the vessel's weather direction, the depth of the water at the location, the rigidity of the anchor lines and the relative directions between the wind, waves and sea water current. Another important factor is the location of the turret in relation to the bow of the vessel.A typical period for a vessel of approximately 100,000 tonnes, moored at 300 m depth with a catenary chain system in the North Sea is 60 to 300 seconds with a yaw angle of up to 15-20 degrees in a storm situation.

The low-frequency yaw movements can be reduced considerably by using propellers or thrusters, which are often located in a transverse direction to the ship.

This enables the greatest yaw angles for the low-frequency movements to be reduced to about 5 degrees in a storm situation.

As stated previously, it is the high-frequency yaw or rotational movements which contribute to the accumulated wear and fatigue damage in the bearing and it is, therefore, important to eliminate these movements.

In accordance with the present invention this can be done as described below.

Fig. 4a shows a top view of a vessel which is anchored so that it can rotate, via a turret, by means of anchor lines 7. The vessel is facing the direction of the weather (wind, waves, sea water current) illustrated by the arrow in front of the bow.

Fig. 4b shows that the vessel has turned through an angle a1 on account of a change in the direction of the weather. Now, after the rotation, a tangential force from the anchor lines will act on the turret which will cause a torque Manchor in the turret.

The turret will move through an angle astat together with the vessel. In other words, a1 is the same as a start in this situation.

Fig. 4c shows a vessel rotated further to an angle a2. The torque from the anchor lines Anchor has now become so large that it will overcome the rotational resistance caused by the friction in the bearing and the turret will attempt to rotate back to the initial position. As the friction is generally lower after the movement in the bearing has started, the turret now assumes a somewhat smaller angle a* in relation to the vessel. If further rotation takes place, the turret will continue to have this angle in relation to the sea bed and it should not exceed 5-10 , depending on the type and size of the ship and turret.

Fig. 5 shows graphs of the correlation (a) between the direction of rotation (angle of the vessel and the turret in degrees and (b) the moment between the vessel and the turret as a function of time.

The graphs apply for slow (static) movement of the vessel.

Up to the time t1 the turret rotates together with the vessel and a = astat, c. f. the previous sections.

The rotational resistance in the bearing Turret at this time is the same as the rotational resistance from the anchor lines Anchor At the time t2 the turret has "slipped" and rotated back to angle ad, and the moment from the anchor line Nanchor is correspondingly reduced.

It is important that the angle between astat and does not become too large as in bearing solutions in which slide bearings are used as stated above; in such case the moment from the anchor line N anchor will be very large, which will lead to the turret rotating back rapidly with the risk of damage to equipment and persons working on board the vessel.

Of importance to the present invention is, therefore, that the rotational resistance, or more precisely the torque caused by the friction in the bearing Nturret must be less than the torque from the anchor lines Anchor with astat > 10".

Slow, low-frequency rotations or yaw movements as stated above are called "static" movements. With high-frequency, superimposed rotational movements, called "dynamic" movements, the moment of inertia Ml of the turret will have a considerable influence and must be included (the moment of inertia has little or no influence with "static" movements).

Another important condition for the solution in accordance with the present invention is, therefore, that the bearing friction for the bearing in the turret is of such magnitude that the rotational resistance of the turret Turret is greater than the sum of the moment of inertia of the turret Ml and the torque from the anchor line Anchor i Nturret > M1 + Manchor for high-frequency rotations of the vessel < 20.

In accordance with one embodiment in accordance with the invention the bearing of the turret can be designed as a wheel bearing as shown in Fig. 6. The bearing comprises a radial wheel bearing with a radially aligned guide wheel 13 which runs along a rail 26, as well as a vertical wheel bearing with, on the same shaft 23, parallel wheels 17 which run on rails 16 on a base 3 on the vessel. The wheels 17 in the vertical bearing can be arranged to good effect in a bogie 21 which is fastened to each of the support arms 11 which extend out from the turret. Such a wheel bearing solution is actually known but the wheels in the known solution are supported in roller bearings which provide too little rolling resistance, c.f. the previous sections.

The special element of the preferred solution in accordance with the present invention is that the wheels are supported in slide bearings 22 instead of roller bearings. This enables sufficient resistance to be achieved in the bearing of the turret so that the superimposed rotational movements are avoided.

With regard to the actual calculation of the rotational resistance in a bearing as mentioned above, this will depend on the many factors such as the diameter of the slide bearing in relation to the diameter of the wheel and the weight of the turret with the anchor lines.

It will be possible for an engineer to calculate this rotational resistance when he knows the overall dimensions, i.e. the size of the ship and the shape of the ship, the diameter of the turret and the fastening point of the anchor lines etc.

In another embodiment of a bearing in accordance with the invention, wheels supported in roller bearings can be used but in this case a brake must be arranged in connection with the bearing on the turret to prevent the turret from rotating when the high-frequency rotational movements of the vessel are less than 2 and the "static" movements, and to make it possible for the vessel to rotate when the low-frequency movements are greater than 10 , as defined in Claim 1.

Figs. 7a and b show an example of such a brake, seen from the side and from the front respectively.

The figures show a band brake arranged in connection with a wheel 17 on a vertical bearing of a turret, for example a vertical bearing as shown in Fig. 6. A brake wheel 25 is arranged on a shaft 23 which is also used by the bearing wheel 17. A brake band 24 extends around the brake wheel 25, whereby the braking force can be adjusted by tightening the band. The number of wheels (17) in a wheel bearing which should be fitted with such brakes will need to be calculated by an engineer and is not to be the subject of further comment here. However, the invention is not restricted to this type of band brakes, and other types of friction brakes, hydraulic brakes or electric brakes can be used.

Fig. 8 shows in principle the rotational resistance N turret of a turret compared with the torque from the anchor lines Anchor for various bearing systems with various angles of rotation of the turret. "Bearing 1" represents a wheel bearing in which the wheels in the bearing are supported in roller bearings with very little resistance to rotation. "Bearing 3" is a slide bearing with high resistance to friction and thus high torque which approaches the maximum value for the anchor line moment Anchor "Bearing 2" represents the resistance to rotation of a bearing in accordance with the present invention with a good margin in relation to the maximum anchor lines moment but with a good rolling resistance which is large enough to ensure that the turret does not "slip" in relation to the ship with high-frequency, waveinduced yaw movements in the hull.

Claims (5)

CLAIMS:

1. A bearing for a turret in a vessel for drilling for, storing and/or extracting oil and/or gas from a geological formation under the sea bed, the vessel being adapted to be anchored to the sea bed via a turret and anchor lines, wherein the friction in the turret bearing is of such a magnitude that the rotational resistance of the bearing Mturret, in response to dynamic, high-frequency rotational movements which are less than 2" (amplitude), is greater than the sum of the yaw moment of motion N1 and the spring moment Manchor of the anchor lines, but less than the spring moment Nanchor of the anchor lines in response to low-frequency rotational movements which are greater than 10 (amplitude).

2. A bearing as claimed in Claim 1, wherein the bearing comprises a wheel bearing where each of the wheels in the bearing is supported, in turn in a slide bearing.

3. A bearing as claimed in Claim 1, wherein the bearing comprises a wheel bearing where each of the wheels in the bearing is supported, in turn, in a roller bearing, a brake being arranged in connection with the bearing of the turret.

4. A bearing as claimed in Claim 3, wherein the brake comprises a mechanical band brake arranged in connection with the bearing wheels.

5. A bearing substantially as hereinbefore described and with reference to Figure 6 or 7 of the accompanying drawings.