Field Technology The invention relates to improvements in mooring systems for vessels, which may be used in deep water production applications.

Background The existing technologies utilized today for positioning a vessel in deep water are spread mooring and turret mooring. For limited time operations, dynamic positioning is used (typically suitable for operations of less than one year).

Spread mooring is used when it is permissible from a metocean (metrology and oceanography) standpoint to fix the heading of the floating production, storage, and offloading vessel (FPSO) in one direction. The vessel is moored by anchor legs from the bow and stern of the vessel in groups. Risers (connecting subsea pipelines/wellheads to the sea surface for production) are installed along the length of the FPSO hull. Figure 1 shows an FPSO 2 with mooring lines 4 and risers 6.

Spread mooring has the disadvantage that the vessel is "locked" in one direction. This is achieved with e.g. 24 mooring lines with tension in the magnitude of 5,000 tonnes arranged in 4 clusters; two in the bow and two in the stern of the FPSO. The large horizontal distance between the mooring lines, in combination with the tension, keeps the FPSO torsional / yaw stable in a fixed direction. Due to (for example in Brazil) some directional variation in incoming weather the mooring system needs to be able to handle quite large forces, as the mooring loads go up in beam sea conditions, both due to wave drift, current and wind loads. Large roll and heave motions may also be experienced in beam sea conditions, which are a challenge for riser design, and increase the cost of the topside structure.

With spread mooring, there is no possibility to adjust the heading of the vessel to accommodate incoming weather. When the heading cannot be adjusted, the mooring system will typically, in moderate environments, have to be beefed up considerably to handle loadcases where the weather does not come in line with the vessel long ship direction (i.e. along the longitudinal axis of the vessel). In addition, the topside structure needs to be increased to handle higher accelerations. So, spread mooring has disadvantages that add costs, up twice the cost for mooring lines compared to weather vaning FPSOs plus increased topside structure. A weather vaning FPSO or vessel is able to rotate to accommodate incoming weather, so as to align the direction of the ship to that of the weather.

In areas where severe weather may occur from several directions, or where it is omnidirectional, a turret with a swivel system may be used. Turret mooring allows the vessel to "weathervane" about an axis in response to the environmental conditions. That is, the vessel can adapt its orientation relative to the direction of the weather, in order to reduce the loading on the mooring. Risers and mooring lines are accommodated within the turret system, so that they are not strained as the vessel turns. That is, the risers are attached to the turret, which remains stationary with respect to the FPSO. The turret system may allow a full 360 degree turning of the vessel.

Turret and swivel systems are extremely expensive, and the cost may be in the same order of magnitude as the FPSO hull. There are limitations on the number risers and the pressure rating of the swivel paths.

Summary of the Invention The invention provides a mooring system for and a method of controlling the direction of a vessel whilst moored on a sea surface, as set out in the accompanying claims.

Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings.

Brief Description of the Drawings

Figure 1 is a top view schematic diagram of spread moored ship; and Figure 2 is a top view of a thruster assisted spread moored ship, according to an embodiment.

Detailed Description

In some parts of the world, e.g. in Campos and Santos, the possibility of some limited heading adjustment is very desirable. Due to the cost and the complexity, it is desirable to avoid a turret solution, which may cost between 400 million and 1 billion dollars.

Embodiments may solve this problem by providing a yaw / torsion unstable (or less stable) system that is controlled by thrusters at the longitudinal extremes of the FPSO (i.e. at the bow and the stern). With an unstable mooring system, the control forces needed to adjust the heading are relatively moderate. The thruster size may be in the standard range used for heading control on turret moored FPSOs. The mooring lines hold the FPSO in position in the linear directions (in the x - y plane), while the thrusters hold the vessel in position in the yaw direction (i.e. rotationally). The number of mooring lines may be reduced considerably, while still maintaining control of the heading. The specific mooring line and thruster arrangement depends on the water depth, the number of risers, the type of risers, the site specific metocean conditions etc. Indicatively ±30 degrees is desirable in the Santos basin, and may be achievable with such an unstable mooring system in the water depth in question (i.e. about 2,000 m).

Embodiments may be particularly useful in benign to moderate environmental conditions, but may also have potential for moored drill-ships in harsher environmental conditions. Although preferred embodiments in the description are limited to deep water production facilities, the skilled person will appreciate that embodiments may be used for other applications. In order to achieve an "unstable" mooring system (or to at least reduce the torsional stiffness), the mooring lines may be adjusted to change the heading. However, chain jacks (which are used to control the length of the mooring lines) are very slow, and may lead to complex and time consuming operations when there are many mooring lines. Such a system may be too slow to handle the incoming weather and any changes in the weather direction. Embodiments may solve this problem by way of how the mooring system is laid out, i.e. in the arrangement of the mooring lines. Instead of fixing the mooring lines to the vessel extremes in the bow and the stern, the mooring system may be fixed to the vessel around midship. This may reduce the torsional stiffness considerably.

Figure 2 shows a FPSO 20 (also referred to as 'the vessel' 20) with two thrusters 24a and 24b (e.g. assymetric or rotational thrusters) in the stern 26 of the vessel 20, and two thrusters 28a and 28b in the bow 30 of the vessel 20. The distance between the bow 30 and the stern 26 (i.e. the distance between the two ends of the vessel) is referred to as the longitudinal length 31 of the vessel 20. Mooring lines 32 are arranged in four groups 34, and are fixed (e.g. by shackle bolts or fairlead winches) along the sides 36a and 36b of the vessel 20. Risers 38 are attached to the side 36a of the ship between two groups 34 of mooring lines 32. The risers 38 may be separated from each other by about 2 m. The thrusters 24a, b and 28a, b are engaged to turn the vessel 20 about a central axis 40. The central axis 40 is at the mid-point between fixture points 42 of the four groups 34 of mooring lines 32. Although the axis 40 is at the centre of the fixture points, it may be positioned laterally off-centre relative to the vessel 20 (i.e. closer to the bow or stern) by moving the fixture points 42 laterally along the longitudinal axis of the vessel 20. The distance between two fixture points 42 (i.e. the distance between two groups 34) along one side 36a of the vessel 20 is referred to as the lateral distance 43 between fixture points. The vessel 20 turns in the direction indicated by arrows 44a and 44b, i.e. counter clockwise (CCW), towards a target position 46 (at a target angle 48 relative to the original position/heading of the vessel 20). The target position 46 and target angle 48 may be determined by the direction of the weather (e.g. the direction of wind, the direction of propagation of surface waves, the current etc.), and the constraints imposed by the mooring lines 32 and the risers 38. For example, if the angle of incidence of surface waves is 10 degrees, than the target angle 48 may also be 10 degrees, so that the longitudinal axis 50 of the vessel 20 lines up with the direction of propagation of the waves. However, if the angle of incidence is 40 degrees, the target angle 48 may be 30 degrees, or some other maximum allowed angle of turning about the central axis 40. Alternatively, the target angle 48 may be determined by offloading conditions, e.g. in order to align the vessel 20 with an offloading vessel (not shown) to increase the offloading regularity. The maximum allowed angle depends on, amongst other things, the positions of the fixture points 42 along the sides 36a, b of the vessel 20, tension in the mooring lines 32 and the arrangement of risers 38. For example, a greater number of risers 38, spread over a larger extent of the ship's side 36a, may restrict the maximum allowed angle of turning to a lower (absolute) value. In order to accommodate the changes in weather the system should have a maximum allowed angle of turning no less than ±10 degrees, or preferably no less than ±20 degrees, and more preferably no less than ±30 degrees. In general the absolute of the maximum angle may be in the range of 5 degrees to 40 degrees. This is different from a turret moored system, in which the vessel may freely rotate 360 degrees about the axis of the turret, i.e. a turret moored system does not have a maximum allowed angle as described herein.

The thrusters 24a, b and 28a, b may hence be used to change the direction of the vessel 20 by causing the vessel 20 to rotate to a target angle 48 (less than the maximum allowed angle). Once the target angle 48 has been reached, the thrusters 24a, b and 28a, b may be engaged to keep the direction of the vessel 20 fixed (or substantially fixed). That is, while the vessel 20 has the desired heading, the thrusters 24a, b and 28a,b may be used to impede further rotation. However, it is impossible to eliminate all rotation and there will be some tolerance, for example ±2 ^S , in the direction of the vessel 20.

The torsional stiffness depends on (amongst other things) the lateral distance 43 between the fixture points 42 on each side 36a, b of the vessel 20. For large distances, where the fixture points 42 are at or close to the stern 26 and bow 30 of the vessel 20, the system tends to towards a traditional spread mooring system. However, if the mooring lines 32 are fixed too close together, then there is insufficient torsional stiffness and the vessel 20 may be difficult to control even with thrusters 24a, b and 28a, b. If the distance 43 between the fixture points 42 is correctly set, then the thrusters 24a,b and 28a, b may not have to be used on days with normal weather conditions. The vessel 20 will naturally weather vane and align according to the weather (e.g. the waves, current and wind) given an appropriate amount of torsional stiffness.

A FPSO 20 may have a longitudinal length 31 (from bow to stern) between 100 m and 450 m. In an embodiment the lateral distance 43 between fixture points 42 is between ¼ (25%) of the longitudinal length and ¾ (75%) of the longitudinal length 31 . More preferably the lateral distance 43 between fixture points 42 is 1 /3 of the longitudinal length 31 or about 1/3 of the longitudinal length 31 of the vessel 20. For example, for a vessel 20 with a longitudinal length 31 of 300 m, the lateral distance 43 between fixture points 42 may be 100 m or about 100 m (e.g. 100 m ± 20 m). In another embodiment, where greater torsional stiffness may be required, the lateral distance 43 between fixture points 42 may be (about) half of the longitudinal length 31 of the vessel 20. For example, if the longitudinal length 31 of the vessel 20 is 300 m, the lateral distance 43 may be 150 m ± 20 m. As the mooring lines 32 within a group 34 are fixed/attached at slightly different points laterally along the side 36a, b of the vessel 20, the fixture point 42 is taken to be located at the midpoint (average position) of the group 34 of mooring lines 32. Although the lateral distance 43 between fixture points 42 on one side 36a of the vessel 20 is generally equal (or substantially equal) in length to the corresponding lateral distance 43 between fixture points 42 on the opposite side 36b of the vessel 20, the lateral distances 43 may vary in some embodiments. The lateral distances 43 may be considered to have substantially equal length if their respective lengths do not differ by more than 5 m.

The central axis 40 is located at the centre of the four fixture points 42. The axis 40 may be shifted laterally along the longitudinal axis 50 towards the bow 30 or stern 26 of the vessel 20. This may improve natural weather vaning (i.e. weather vaning without thrusters), due to the increased torque (i.e. rotational force) experienced by the vessel 20. Moving the central axis 40 may also improve offloading, as the mooring lines 32 may be located in a more convenient position. In one embodiment the central axis 40 is located between the centre of the vessel 20 and a distance of no more than 3/7 of the longitudinal length 31 of the ship 20 laterally away from the centre. For example, for a vessel 20 with a longitudinal length 31 of about 300 m, the lateral distance from the centre of the vessel 20 to the central axis 40 may be 50 m. Each of the thrusters 24a, b and 28a, b may have a power of no less than 2 MW and preferably about 2 MW. The thrusters may be rotational thrusters that can turn 360 ^s in order to direct the force exerted by the thrusters on the vessel 20. The thrusters may be controlled manually (e.g. by the captain of the ship) or may be adjusted automatically by a thruster control system though feedback from various sensors (e.g. weather sensors, GPS, proximity sensors etc.) Although Figure 2 only shows risers 38 on one side 36a of the vessel 20, in another embodiment there are risers located between the fixture points 42 on the other side 36b of the vessel 20 as well. In this way the production rate can be increased.

In addition to the arrangement of mooring lines 32 and thrusters 24a, b and 28a, b, a fairlead that can handle larger variations in the x-y plane angels may be used. It may also be advantageous to add a mooring balcony (not shown) to allow for larger changes of the heading angle by increasing the clearance to the hull of the vessel. That is, the mooring lines 32 may be fixed to fixture points 42 located at a distance from the hull over the sea surface. This may increase the ability of the mooring system to handle larger variations in heading angle.

Hence, the above embodiments provide arrangements of the mooring lines and/or arrangements of the thrusters and a thruster control system in a spread moored application that allow some weather vaning. The embodiments may allow a change in heading without any adjustment of the mooring line tension (hence avoiding the use of winches or chain jacks). This may cause significant cost reductions, compared to alternative solutions.

In deep water, the heading of the vessel may be adjusted rapidly to decrease global mooring forces, to decrease global motions, and to improve offloading regularity. This allows the more expensive turret mooring system to be avoided in some areas. Each feature disclosed or illustrated in the present specification may be incorporated in the invention, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.