and a riser subject to water current

The present invention deals with a method of drilling or exploiting a well using an offshore platform, and a riser intended to extend along a vertical direction between the platform and a seabed.

An offshore platform is a known large structure used to drill a well from the seabed in order to extract for example oil or natural gas.

A riser is a conduit that provides an extension of the well up to the platform. Marine risers are used with a subsea blowout preventer (or BOP) located at the exit of the well, just above the seabed.

The riser has a large diameter, low pressure main tube with external auxiliary lines that include high pressure choke and kill lines for circulating fluids to the BOP, and usually power and control lines for the BOP.

In normal operation, the riser extends approximately vertically from the BOP to the platform. A lower flexible joint connecting the riser to the BOP and an upper flexible joint connecting it to the platform allow a small angle with respect to a vertical direction. The platform generally delimits an opening, known as the "moon pool", for the riser.

There are recommended practices in the industry which provide guidelines pertaining to the riser angles during operation, for example those of the American Petroleum Institute (or API). For example, if an angle between the riser and a vertical direction at the upper flexible joint is larger than 2°, rotary drilling operations should be stopped; if said angle is larger than 4°, all drilling operations should be suspended in preparation for riser disconnection; and if said angle is larger than 9°, the riser is disconnected from the BOP.

Several solutions have been developed in order to meet such constraints, and to prevent the riser from colliding the offshore platform, for example when it is suddenly disconnected from the BOP.

These solutions are mostly based on the principle of a "restrain" system, in which the riser comes into contact with an external envelope incorporated in the moon pool. The envelope is for example made of resilient material such as rubber or truck tires.

The use of a bumper joint has also been described in order to prevent or limit damages caused by the riser coming into contact with the structures of the platform.

Tensioning systems or brake systems adapted to prevent or limit vertical movements of the riser have also been implemented. Such techniques are adapted to deal with dynamic, i. e. fluctuating, efforts applied on the riser due to current, wind, waves and dynamic movements of the platform with respect to the seabed.

In the meantime, there has been a growing interest for promising offshore areas where strong water currents may exist. However, the above-mentioned processes of drilling a well cannot be implemented in such areas, given the stringent requirements regarding the position of the riser with respect to the platform or the BOP, and more particularly the requirement to avoid a collision of the riser with the platform either when the riser is connected to the well, or during a planned or emergency disconnection of the riser. Additional strong currents may generate stresses at some points of the riser which are beyond safe mechanical limits thereby creating a threat to the riser.

One aim of the invention is to provide a simple method of drilling or exploiting a well using an offshore platform and a riser in an area having strong water current.

To this aim, the subject-matter of the invention is a method of drilling or exploiting a well using an offshore platform, and a riser intended to extend along a vertical direction between the platform and a seabed and subject to a water current, wherein the water current applies mechanical efforts on the riser, the method comprising the step of applying compensation mechanical efforts on the riser, wherein the compensation mechanical efforts have a static horizontal component intended to be the opposite of a horizontal net force of the mechanical efforts applied by the water current.

The method according to the invention comprises one or more of the following technical features, taken solely or according to any possible technical combination:

- the riser is connected to the platform via an upper flexible joint, wherein the riser extends along a first longitudinal direction in the vicinity of the upper flexible joint, wherein the first longitudinal direction and the vertical direction define an upper deviation angle, and wherein the static horizontal component has an intensity adapted to maintain the upper deviation angle lower than or equal to 9°, preferably lower than or equal to 4°, and more preferably lower than or equal to 2°;

- the riser is connected to a blowout preventer via a lower flexible joint, wherein the riser extends along a second longitudinal direction in the vicinity of the lower flexible joint, wherein the second longitudinal direction and the vertical direction define a lower deviation angle, and wherein the static horizontal component has an intensity adapted to maintain the lower deviation angle lower than or equal to 9°, preferably lower than or equal to 4°, and more preferably lower than or equal to 2;

- the method further comprises the steps of: - obtaining at least one initial parameter representative of a strength of the water current, wherein the initial parameter is a measurement result or a forecast, and

- calculating an intensity parameter using said initial parameter, wherein the step of applying the compensation mechanical efforts on the riser includes a substep of using the intensity parameter as a set point for the static horizontal component of the compensation mechanical efforts to be applied on the riser;

- the method further comprises the step of providing a connection system fixed to the riser and adapted to apply at least part of the compensation mechanical efforts on the riser, wherein the riser has a vertical extension, and wherein the connection system is located at a distance from the upper flexible joint, wherein the distance is lower or equal to one third of the vertical extension of the riser, and is not less than 2 meters, preferably not less than 20 meters;

- the method further comprises the step of providing at least one tensioning, and at least one traction member attached to the tensioning system and to the connection system, wherein the step of applying the compensation mechanical efforts on the riser includes a substep of applying a traction on the traction member using the tensioning system;

- the tensioning system is adapted to maintain a static component of the applied traction at a certain value, the value being adjustable within a certain range;

- the tensioning system is fixed to the platform, and includes a winch, or a piston connected by a hydraulic fluid to a piston of a pneumatic chamber;

- the step of obtaining the tensioning system includes a substep of modifying an existing vertical tensioning system intended to apply a vertical tension to the riser in order to obtain the tensioning system;

- the tensioning system includes at least one ship;

- the tensioning system includes at least one buoy and at least one mooring traction member, both mechanically connected to the traction member;

- the connection system comprises a fixed part with respect to the riser, and a ring rotatably mounted on the fixed part with respect to the riser around a longitudinal axis of the riser, and wherein the traction member is attached to the ring;

- the fixed part comprises a central core and clamping elements, wherein the clamping elements are attached to the core and extend longitudinally from the core (66) along the riser on both sides of the core; - the method further comprises the step of providing at least one propulsion system fixed on the connection system, wherein the step of applying the compensation mechanical efforts on the riser includes a substep of applying a force on the connection system using the propulsion system; and

- the method further comprises the step of providing at least one water ejection system fixed on the riser, wherein the step of applying the compensation mechanical efforts on the riser includes a substep of applying a force on the connection system by ejecting a stream of water out of the ejection system.

The invention will be better understood, upon reading of the following description, given only as an example, and made in reference to the appended drawings, in which:

- figure 1 is a diagram showing a typical profile of a strong water current for which methods according to the invention are adapted;

- figure 2 is a side view of a platform and a riser subject to the water current represented in figure 1 in various positions;

- figure 3 is a schematic side view of a first installation adapted to perform a method according to a first embodiment of the invention;

- figure 4 is a schematic upper view of the installation shown in figure 3;

- figure 5 is a schematic view illustrating a tensioning system of the installation represented in figures 3 and 4;

- figure 6 is a schematic perspective, partly exploded, view illustrating a connection system within the installation shown in figures 3 and 4;

- figure 7 is a schematic side view of a second installation adapted to perform a method according to a second embodiment of the invention;

- figure 8 is a schematic side view of a third installation adapted to perform a method according to a third embodiment of the invention; and

- figure 9 is a schematic side view of a fourth installation adapted to perform a method according to a second embodiment of the invention.

Figure 1 is a diagram where a curve C provides a relationship between a horizontal velocity V of typical strong water current and a depth D of the water. The velocity V reads on a horizontal axis of the diagram, and a depth D on a vertical axis.

A water current is considered "strong" for example when the horizontal velocity V is greater than 0.5m/s in a given point of the curve C.

In the example shown in figure 1 , the horizontal velocity V is about 2m/s at the surface and still about 1 m/s at a depth of 400m.

The curve C can be obtained by known measurement techniques.

The curve C represents a water current profile at a moment of time. However, a water current may change in terms of velocity profile and direction of the flow. Hence it is advantageous to obtain forecasts of the water current.

The velocity of a water current may also have a vertical component (not shown) which is usually much smaller than the horizontal one.

Figure 2 shows an installation 1 according to prior art, in three different configurations A1 , A2 and A3 (from left to right of figure 2).

The installation 1 comprises an offshore platform 3, a BOP 5 attached to a subsea wellhead connected to the tubular providing a structural foundation with the seabed 7, and a riser 9 connected to the platform and to the BOP. The installation 1 is subject to a water current F, for example horizontal and from left to right.

At a connection point with the platform 3, the riser 9 respectively defines angles oc1 , oc2 and oc3 with a vertical direction V in the configurations A1 , A2 and A3. At a connection point with the BOP 5, the riser 9 respectively defines angles β1 , β2, β3 with the vertical direction V in the configurations A1 , A2 and A3.

As it can be noticed, the angles oc1 and β3 may be particularly significant. Even if the platform 3 is located above the BOP 5, such as in configuration A2, due to the water current F applying mechanical efforts along the riser 9, the angles oc2 and β2 may be also significant, for example larger than 2°.

A first installation 10 is illustrated in figures 3 and 4. The installation 10 allows performing a method according to a first embodiment of the invention. The installation 10 is adapted to drill a well 12 in a seabed 14.

The installation 10 comprises an offshore platform 16, a BOP 18 connected to the well 12, and a riser 19 extending along a vertical direction V between the BOP and the platform and subject to a water current F.

The installation 10 also comprises an assembly 20 adapted to apply a mechanical effort F1 (figure 3 and 4) on the riser 19 in order to compensate the loading induced by current.

The platform 16 for examples floats on a sea above the well 12. The platform 16 is adapted to be positioned in a selected location with respect to the well 12 in a known way.

The platform 16 further comprises means 22 for vertical tensioning of the riser 19.

The platform 16 comprises several half immersed columns 24 and defines an opening 26 along the vertical direction V, often called "moon pool", for the riser 19.

As shown in figure 4, the platform 16 tends to align along the water current F at the water surface. A platform forward part (left of figure 4) is upstream the water current F, while an aft part (right of figure 4) is downstream. The riser 19 is subject to a water current F. The riser 19 is known in itself and includes a main tube 30, an upper flexible joint 32 and a lower flexible joint 34. The upper flexible joint 32 and lower flexible joint 34 allow an angular rotation between the riser 19 and the platform 16 (alpha) and between the riser 19 and the blow out preventer 18 (beta).

The riser 19 has a vertical extension E0 between upper flexible joint 32 and a lower flexible joint 34 along the vertical direction V.

The extension E0 may be subject to small variations, for example due to the tide or a heave of the platform 16.

The riser 19 extends along a first longitudinal direction L1 in the vicinity of the upper flexible joint 32, and along a second longitudinal direction L2 in the vicinity of the lower flexible joint 34.

For example the water current F flows horizontally and has the profile shown in figure 1 .

The first longitudinal direction L1 and the vertical direction V define an upper deviation angle a.

The second longitudinal direction L2 and the vertical direction V define a lower deviation angle β.

There are six columns 24 in the example shown. Advantageously the four outer columns form a support for the assembly 20. In a variant (not shown), the assembly 20 is used in a platform having a different number of columns.

The assembly 20 comprises independent subassemblies (figures 4 and 5) corresponding to the supporting columns 24, and a connection system 36 connecting each of the subassemblies with the riser 19.

Advantageously the subassemblies are similar with each other. Each of the subassemblies comprises a traction member 38, and a tensioning system 40 adapted to apply a traction T on the traction member. Each subassembly also comprises at least one guiding member 42 for the traction member 38.

Each traction member 38 is for example a cable, a chain, or any mechanism known by the skilled person. Each traction member 38 is respectively adapted to transmit the traction T to the connection system 36 in the form of a mechanical traction T1 , T2, T3, T4.

As a variant (not shown), some, or all of the subassemblies comprise a cylinder directly applying part of the compensation mechanical efforts on the riser 19.

Advantageously, the tensioning system 40 is obtained by modifying an existing vertical tensioning system of the riser 19. As a variant (not shown), the tensioning system 40 is a dedicated system installed on purpose.

The tensioning system 40 is configured so that the traction T provided by each of the traction members can be adjusted as a function of actual current conditions in order to maintain the riser 19 within the maximum allowable excursion. The tension can be either adjusted manually or regulated by a mechanism or a control system. The tension is adjustable within a certain range, for example between 50 and 900 kN.

When a former tensioning system of the riser, powered by a pressurized air capacity is used, the tensioning system 40 is configured to have a certain elasticity. This means that, if the traction member 38 pulls on the tensioning system 40, the traction T increases, and conversely, if the traction member 38 yields, the traction T decreases.

As shown in figure 5, the tensioning system 40 comprises two cylinders 44 and 45. The cylinder 44 has a piston 46 defining a hydraulic chamber 50.

The cylinder has a piston 47 defining a hydraulic chamber 54 and pneumatic chamber 55.

The chambers 50, 54 are filled with a hydraulic liquid 52 and are in fluidic communication with each other.

The pneumatic chamber 55 is filled with pressurized air 56 coming from a source 58 of compressed air.

The piston 46 is connected with the traction member 38 via a sheave 60 in order to produce the traction T from the pressure in the two chambers 50 & 54.

When the traction T on the traction member 38 increases, the piston member 46 goes down and pushes the hydraulic liquid 52 in the cylinder 45, thus reducing the compressed air volume and increasing the air pressure.

The guiding member 42 is fixed to the platform 16 and is for example located underwater as shown in fig 3. Advantageously, the guiding member 42 is located so that it is higher than the connection system 36 when the riser 19 is connected to the BOP 18.

The connection system 36 is located at a distance E1 from the upper flexible joint 32 along the vertical direction V.

As shown in figure 6, the connection system 36 (also shown in figure 3) comprises a fixed part 62 connected to the riser 19, for example to a main tube 63, and a ring 64 rotatably mounted on the fixed part with respect to the riser 19 around a longitudinal axis Δ of the riser.

The connection system 36 is adapted to receive the mechanical tractions T1 , T2, T3, T4 from the traction members 38 and to transform them into the compensation mechanical effort F1 applied to the riser 19. The fixed part 62 comprises a central core 66, two flanges 68, 70 located on either sides of the central core and surrounding the main tube 63 around the longitudinal axis Δ, and hollow pipes 72 longitudinally extending between the flanges and going through the central core. The hollow pipes 72 allow continuity of the auxiliary lines of the riser 19, i.e. the choke line, kill line, booster line, hydraulic conduit and any other line which could be part of the particular design of the riser 19 which is provided with the platform 16. The flanges 68, 70 are similar or provide an adequate interface with the components of the riser 19.

The fixed part 62 can also comprise, longitudinally on each sides of the central core 66, and for example successively from the central core towards each of the flanges 68, 70, additional elements such as clamping elements 76, 78, and spacers 80, 82.

For example there are four hollow pipes 72 fixed to the flange 68 and the same four hollow pipes 72 are rigidly fixed to the flange 70. The hollow pipes 72 can for example be secured in longitudinal housings of the central core 66 in order to mitigate vibrations induced by current and waves. The hollow pipes 72, are advantageously angularly spaced around the main tube 63 in a regular manner compatible with the configuration of the riser 19

The clamping elements 76, 78 are adapted to grip the riser 19 and advantageously enhance the mechanical strength of the main tube 63. The clamping elements prevent the central core 66 from sliding along the tube 19bis.

The flanges 68, 70 advantageously surround the riser 19 completely. The flanges 68, 70 are for example separated from each other by a distance E2 along the longitudinal axis Δ, for example ranging between a few meters and 25 meters (the length of a typical riser joint).

The central core 66 for example has bearing plates 84 distributed around the longitudinal axis Δ, and tapered extremities 86, 88.

The ring 64 is for example made of two half-rings 90, 92 adapted to be attached to each together and advantageously symmetric.

The two half-rings 90, 92 are for example movable between an open position (figure 6) enabling to install them around the fixed part 62, and a close configuration in which the ring 64 is formed.

The ring 64 is configured to freely rotate around the central core 66 along the longitudinal axis Δ on the bearing plates 84.

The ring 64 comprises a metallic structure 94, 96, and pivots 98, 100, 102, 104 respectively mounted between the metallic structure 94, 96 around rotation axis Δ1 , Δ2, Δ3, Δ4 that are substantially parallel with the longitudinal axis Δ. The pivots 98, 100, 102, 104 are also respectively rotatably mounted on the traction members 38 around rotation axis D1 , D2, D3, D4 that are substantially orthogonal with the longitudinal axis Δ.

The distance E1 is for example lower or equal to one third of the vertical extension E0 of the riser 19 and is not less than 2 meters, preferably not less than 20 meters.

The compensation mechanical effort F1 have a static horizontal component R (figures 3 and 4) adapted to be the opposite of a horizontal net force R' of the mechanical efforts applied by the water current F on the riser 19. The operation of the installation 10 and a method according to the invention will be now described.

In terms of drilling and exploiting the well 12, the platform 16 operates as any other offshore platform when the riser 19 is connected to the BOP 18. The main difference comes from the water current F applying large mechanical efforts on the riser 19.

The mechanical efforts applied on the riser 19 may have a horizontal net force R' typically between 0 kN and 900 kN. The horizontal net force will be a function of the current velocity profile across the water column.

The compensation mechanical effort F1 is applied on the riser 19 in order to compensate the action of the water current F. They include a static horizontal component R, and possibly other components such as a vertical component, and dynamic (fluctuating) horizontal or vertical components.

The static horizontal component R is the opposite of the horizontal net force R' of the water current F.

Advantageously, the static horizontal component R has an intensity adapted to maintain the upper deviation angle a of the riser 19 lower than or equal to 9°, preferably lower than or equal to 4°, and more preferably lower than or equal to 2°.

The static horizontal component R advantageously has an intensity adapted to maintain the lower deviation angle β lower than or equal to 9°, preferably lower than or equal to 4°, and more preferably lower than or equal to 2°.

The intensity of the static horizontal component R to be applied on the riser 19 is advantageously determined by obtaining at least one initial parameter representative of a strength of the water current F.

The initial parameter is advantageously a measurement result, for example a profile as shown in figure 1 , or a forecast, or a combination of a measurement result and a forecast.

Then the intensity is calculated using said initial parameter. The calculated intensity is for example used as a set point for the static horizontal component R to be applied on the riser 19.

In practice, the compensation mechanical effort F1 is advantageously applied using the assembly 20.

The connection system 36 is fixed to the riser 19. The tensioning system 40 is attached to the connection system 36.

The tensioning system 40 is advantageously obtained by modifying an existing vertical tensioning system of the platform 16 intended to apply a vertical tension to the riser 19.

The traction T is applied on each of the traction members 38 in order to obtain the mechanical tractions T1 , T2, T3, T4 applied to the connection system 36.

The connection system 36 transmits the mechanical tractions T1 , T2, T3, T4 to the riser 19 in the form of the compensation mechanical effort F1 .

The intensity and direction of the static horizontal component R is obtained by selecting the intensities and directions of the mechanical tractions T1 , T2, T3, T4.

When using a modified vertical tensioning system of the platform 16 powered by pneumatic pressure, the pressure in the chambers 50 and 54 generates a force on the piston 46 which is converted into the traction T.

The pressure is set by introducing a certain amount of compressed air 56 from the source 58 into the pneumatic chamber 55. The pressure on top of the hydraulic fluid 52 in the pneumatic chamber 55 is converted into the traction T. The pressure value allows controlling the value of the traction T.

In order to increase the static value of the traction T, higher air pressure is set into the pneumatic chamber 55. Conversely, in order to decrease the traction T, some compressed air is withdrawn from the pneumatic chamber 55.

Each tensioning system 40 maintains a static component of the applied traction T at a certain value. The value is adjustable within a certain range, for example by modifying the pressure in the pneumatic chamber 55.

Advantageously, the tensioning systems 40 also have some elasticity. If one of the traction members 38 is pulled by the riser 19 via the connection system 36, the pressure increases in the pneumatic chamber 55 and the value of the traction T increases, which tends to increase the traction of said traction member on the riser 19. Conversely, if one of the traction members 38 yields, the pressure decreases in the pneumatic chamber 55 and the value of the traction T decreases, which tends to reduce the traction T of said traction member on the connection system 36. The magnitude of the elasticity of the tensioning systems 40 is created by the limited internal volume of the pneumatic chamber 55. For example, the internal volume is less than 250 liters, preferably less than 16 liters. The smaller the pneumatic chamber 55, the higher the stiffness of the tensioning system 40. The position of the piston 47 in the cylinder 45 conditions the volume of compressed air which has a direct bearing on the stiffness of the system.

The connection system 36 allows the traction members 38 to adjust directions with respect to the riser 19, in order to accommodate the excursion of riser and the relative movement of the platform 16. The ring 64 being able to rotate with respect to the riser 19, this enables the orientation of the platform 16 to change with respect to the riser 19 for weather warning purposes.

The internal structure of the connection system 36, for example the distance E2, allows applying the compensation mechanical effort F1 not on a single point of the riser 19, but rather on a segment of the riser surrounded by the connection system 36.

Thanks to the above described features, the installation 10 allows fighting the effects of the water current F on the riser 19. The installation 10 allows performing a simple method of drilling or exploiting the well 12 using the platform 16 and the riser 19 despite the strong water current F.

A second installation 100 adapted to perform a method according to a second embodiment of the invention will now be described with reference to figure 7.

The installation 100 is analogous to the installation 10 represented in figures 2 to 6. Similar elements have the same numeral references. Only the differences will be described in details hereunder.

In the installation 100, the assembly 20 of the installation 10 is replaced by an assembly 120 having a tug boat 102, and the traction member 38 connected to the connection system 36.

Advantageously, the tug boat 102 comprises a tensioning system 140 adapted to generate the traction T.

The installation 100 operates in a similar way as the installation 10. The more the tug boat 102 pulls on the traction member 38, the larger the static component R.

One advantage of the installation 100 is to allow a larger distance E1 .

A third installation 200 adapted to perform a method according to a third embodiment of the invention will now be described with reference to figure 8. The installation 200 is analogous to the installation 10 represented in figures 2 to 6. Similar elements have the same numeral references. Only the differences will be described in details hereunder.

In the installation 200, the assembly 20 of the installation 10 is replaced by an assembly 220 having a tensioning system 240 connected to the traction member 38.

The tensioning system 240 comprises a first buoy 242 located at the sea surface and connected to the traction member 38, a second buoy 244 located in the water, and a mooring line 246 providing an anchoring point with the seabed 14 and connected to the traction member through a sheave arrangement.

The first buoy 242 is adapted to pull on the traction member 38, in order to create the traction T.

The second buoy 244 is adapted to make the traction T more horizontal than what would be obtained without the second buoy.

The installation 200 operates in a similar way as the installation 10.

One advantage of the installation 200 is to allow a larger distance E1 than in installation 10 and possibly to make the traction member 38 more horizontal than in installation 100.

A fourth installation 300 adapted to perform a method according to a fourth embodiment of the invention will now be described with reference to figure 9.

The installation 300 is analogous to the installation 10 represented in figures 3 to 6. Similar elements have the same numeral references. Only the differences will be described in details hereunder.

In the installation 300, the assembly 20 of the installation 10 is replaced by an assembly 320 without a traction member such as the traction members 38.

The assembly 320 comprises two or more propulsion systems such as 322, 324 attached to the connection system 36. Both of the propulsion systems 322, 324 for example include a propeller.

The propulsion systems 322, 324 respectively applies mechanical forces T1 ' and T2' to the connection system 36, resulting in the compensation of mechanical effort F1 on the riser 19.

The installation 300 operates in a similar way as the installation 10.

One advantage of the installation 300 is to allow more flexibility when selecting the location of the connection system 36 along the riser 19.

As a variant (not shown), other propulsions systems similar to the propulsion systems 322, 324 are installed on the riser 19 at several distinct depths. One advantage of this variant of the installation is to distribute compensation forces along a longer length of the riser and hence minimize the bow and also the mechanical stresses.

According to another embodiment (not represented), the propulsion systems include no propeller, but a tube adapted to eject water away from the connection system 36. The tube is connected to a pumping system of the platform 16 adapted to inject water in the tube.