LINE

BACKGROUND OF THE INVENTION

The present invention relates to the transport of oil/gas/chemical liquid from floating production storage and offloading to an offloading buoy, for instance in the context of deep-water offshore fields.

The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section. Furthermore, all embodiments are not necessarily intended to solve all or even any of the problems brought forward in this section.

In deep-water offshore fields, the produced oil is generally offloaded to shuttle tankers via oil offloading lines (OOL) connected between the floating production storage and offloading (FPSO) and offloading buoy.

Three types of oil offloading lines (OOL) are often used for the transfer of stabilized crude oil between the FPSO and the offloading buoy:

- bonded pipe or

- unbonded pipe. - steel pipe

This non-convergence to a unique type of concept shows that an ideal concept in terms of cost, constructability, efficiency, and safety has not been exhibited within the industry yet.

Most of the available OOL are very complex to bring on the producing sites and to install between the FPSO and the offloading buoy: heavy boats should be used to transport these pipes and to install them for production. The cost of such installation may be up to 8-10k€ per meter (the length of a normal OOL is about 2500m). Therefore, there is a need to find a way to reduce the costs of such OOL to the minimum while maintaining the safety of the OOL (i.e. avoiding any damaging of the OOL during the normal / abnormal operations).

SUMMARY OF THE INVENTION

The invention relates to an offloading line comprising an intermediate portion, said intermediate portion comprising:

- a first tube formed by more than 60% of High Density Polyethylene and having ratio between a diameter of the first tube and a thickness of a side of the first tube below 15;

- a second tube adapted to be installed inside the first tube and being removable from the first tube by pulling said second tube from the first tube.

The intermediate portion may be the middle portion of the offloading line. The offloading line may be an OOL.

As a matter of fact, the first tube may be also formed by 70%, 80%, 90% or even 100% of High Density Polyethylene.

By designing an offloading line comprising said two tubes, it may be cost effective to create an offloading line. The first tube comprising High Density Polyethylene is quite cheap to extrude and may be extruded in various locations (e.g. near the production site).

The diameter of the first tube may be the outer diameter of the first tube. By ensuring that the ratio between the diameter of the first tube and the thickness of a side of the first tube (i.e. named SDR) is below (or equal to) 15, it is possible to design an offloading line with adequate mechanical properties to support the offset(s) of FPSO / the buoy. The SDR may also be below (or equal to) 10, 7 or 4.

By having a second tube adapted to be removed from the first tube, it is possible to design a cost effective OOL in case of any leakage of the second tube. The first tube does not need any replacement while the second tube may be replaced. In a specific embodiment, the second tube may be a hose.

Therefore, it is very simple to install/remove said second tube into/from the first tube even if the first tube is bent.

In addition, an inner side of the second tube may be at least partially formed of thermoplastic polyurethane or polyvinylidene difluoride.

Thus, this second tube may be filled with oil without damaging its structure or with the one from the first tube.

A space between the first tube and the second, called annulus, may be greater than 5 mm.

Therefore, the first tube and the second tube may be independent and one may move relative to the other one during the production phase or the installation phase.

In a given embodiment, said intermediate portion may comprise at least two openings or at least two valves. Theses valves/openings may ease the installation process: one valve/opening is used to fill the annular volume with water while the other valve/opening is used to flush the air out.

The invention is also directed to a method of installing an offloading line, said offloading line comprising an intermediate portion, said intermediate portion comprising:

- a first tube formed by more than 60% of High Density Polyethylene and having ratio between a diameter of the first tube and a thickness of a side of the first tube below 15 ;

- a second tube adapted to be installed inside the first tube and being removable from the first tube by pulling said second tube from the first tube ; wherein the method comprises:

- installing plugs in the offloading line so that to create a hermetic volume in the offloading line; - towing at least the intermediate portion to a production site;

- connecting an extremity of the offloading line to a production pipe;

- removing at least one plug to fill a volume between the first tube and the second tube with seawater.

The diameter of the first tube may be an outer diameter. By creating a hermetic volume in the offloading line, it is possible to have a buoyant offloading line. No important ship is needed to transport the OOL as it can be towed by a small ship.

Once near the production site (e.g. near the FPSO), it is possible to sink the OOL by removing the plug(s) (e.g. opening a valve) and letting the seawater enter the opening(s). A second opening may be opened by removing a second plug (e.g. opening a second valve), in order to flush the air from the annular space.

The method may further comprise:

- removing at least on another plug; - connecting a second extremity of the offloading line to a buoy.

The method may also further comprise opening a transport valve to fill the first tube with seawater. Indeed, by filling the first tube, the installation of the OOL may be eased.

The method may also further comprise: - rendering the volume between the first tube and the second tube hermetic;

- installing at least a sensor in the volume between the first tube and the second tube, at least one portion of said volume being filled with seawater and at least one portion of said volume being filled with a compressible fluid. Having a compressible fluid in the annular volume (i.e. the volume between the first tube and the second tube) may ease the detection of any leakage: In case of a leakage, the compressible fluid is compressed and

- the pressure may vary in the annular volume or,

- an interface between seawater in the annular volume and the compressible fluid may vary.

Other features and advantages of the method and apparatus disclosed herein will become apparent from the following description of non-limiting embodiments, with reference to the appended drawings. BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by way of limitations, in the figures of the accompanying drawings, in which like reference numerals refer to similar elements and in which:

- Figure 1 is a representation of a configuration of the OOL in a possible embodiment of the invention;

- Figure 2 is a representation of a longitudinal section of the OOL in a possible embodiment of the invention;

- Figure 3 is a representation of a portion of the OOL in a possible embodiment of the invention; - Figures 4a-4d are different steps of the installation of the OOL in a possible embodiment of the invention;

- Figure 5 is a representation of sensors inserted in the OOL to supervise the leakage of the oil.

DESCRIPTION OF PREFERRED EMBODIMENTS

Figure 1 is a representation of a configuration of the OOL in a possible embodiment of the invention.

In deep-water offshore fields, the produced oil - which is extracted by the floating production storage and offloading 101 (FPSO) - is generally offloaded to shuttle tankers via oil offloading lines 104, 103, 105 (OOL) connected between the FPSO 101 and offloading buoy 102. The distance between the FPSO 101 and the offloading buoy 102 is generally about D~2000m - 2500m.

Nevertheless, due to the depth of the ocean, the FPSO 101 does not remain still and its position may vary of a given offset Ad: therefore the distance between the FPSO 101 and the buoy 102 may vary with time. As a consequence, it is useful to use an OOL (104, 103, 105) that can allow such offset without suffering damages with years of use.

Previously, the OOL was designed to have the same mechanical properties (resistance, flexibility, etc.) all along the line: indeed the structure of the whole line was quite identical at each point / for each segment of said line. In the present embodiment, it is proposed to have a plurality of types of OOL that are connected together in order to form a single OOL with different mechanical properties along the line.

For instance, the 104 section and the 105 section of the OOL may be formed by regular OOL which is highly flexible and resistant (e.g. any OOL from the prior art, or similar, may have the required properties).

The 103 section of the OOL may be formed by a less flexible tube (e.g. as described in relation of Figure 2). The length of the 104 section and the 105 section may vary depending on the configuration but can be of 200-400m each for instance. Indeed, it has been assessed that even if the flexibility of the OOL may be - in specific situations - important in the zones close to the FPSO 101 and the buoy 102 (i.e. zone \- _\ and zone l ₂ ), the flexibility in the intermediate zone L (103) is not a key point of the overall safety of the OOL.

Figure 2 is a representation of a longitudinal section of the OOL in a possible embodiment of the invention.

In this embodiment, the longitudinal section of the OOL (portion 103) comprises two tubes 201 and 203, the tube 203 being inserted into the tube 201 . The tube 201 provides the mechanical structures of the OOL. For instance, the wall of the tube 201 may be formed of a 7" of High Density Polyethylene (HDPE) for an external diameter of the tube 201 in [24";28"] (i.e. the inner diameter is in [17";21 "]). The density of HDPE is typically higher than 0.92 g/cm ³ but less than 1 .00 g/cm ³ As the Polyethylene is the most common plastic, it is very cheap and very simple to manipulate. High Density Poly Ethylene (HDPE) pipes have a lot of advantages in the present use: Strong and very durable, lightweight, buoyant and flexible, no corrosion, no leak, low repair costs, almost unlimited life time underwater, weldable, etc.

By default, the tube 201 is buoyant (e.g. PE section). Therefore to get proper dynamic behavior and optimize strain in the tube, ballasts/weights may be used.

For instance, it is possible to install in a close location from the FPSO an extruder so that to be able to extrude the HDPE tube. Therefore the transport cost is reduced to its minimum. In the prior art solution, the OOL cannot be simply extruded as they are quite complex (e.g. multiple layers of various components, etc.). Therefore it is impossible to move the fabrication supply chain close to the FPSO.

The Polyethylene is known to be degraded if it is in contact with crude oil or other chemical components. As a matter of fact the Polyethylene is disregarded when the transport of crude oil is considered.

This is not the case in the present embodiment. Indeed, the inner tube 203 is designed to be in contact with the crude oil / chemical components while the annular space 202 between the tube 201 and the tube 203 may be filled with neutral liquid (for instance, seawater).

It is not important that the inner tube 203 does not provide any mechanical structure (or very little one) to the OOL: the primary function (but not necessarily the only one) of said tube 203 is to confine and transfer the crude oil / chemical liquid into the space 204 and to avoid that the crude oil / chemical liquid is in contact with the tube 201 .

Although the tube 201 may be restricted to a given portion of the OOL (i.e. the intermediate portion 103 of the OOL), the inner tube 203 may be in every portion 103, 104 and 105. Therefore, it may be easier to connect the standard OOL pipes (portions 104/105) to the outer tube 201 as no seal is needed (the oil/chemical product being contained in the inner tube 203 which extends from the FPSO 101 to the buoy 102 ). The inner tube 203 may be a tube with a diameter in [16" ; 20"]. For instance, a tube that is not damaged in contact of crude oil/chemical products may be used as the inner tube 203.

One of the proper candidates for such tube 203 may have an inner layer with Thermoplastic Polyurethane (TPU) or polyvinylidene difluoride (PVDF) to avoid any damages due to the contact with the crude oil / chemical fluid. The tube 203 may also have an outer layer with polyethylene sheets or with TPU-BASF. Finally, the tube 203 may also have a Kevlar fabric layer as an intermediate layer so to increase its resistance to pressure.

The pressure loss in the inner liner may be calculated to check whether the pressure at the export buoy is enough to offload to export tanker manifold via floating hoses (main, tail and tanker rail hoses). A minimum of 5 bars at the buoy may be used for operating conditions (oil offloading).

The pressure loss due to wall friction is calculated thanks to the following formulation: Λ pV ²

Δρ = - .-— L

D 2 where Δρ is the linear pressure loss in Pascal, p is the fluid density in kg/m ³ , V is the fluid velocity in m/s, D is the inner hose internal diameter in mm, L is the inner hose length in meter and Λ is the pressure loss coefficient which is calculated thanks to the Colebrook's formulation (--= = -2 log ₁₀ (— .4= +— ), k is the roughness factor in mm (0.000028mm for the TPU), Re is the Reynolds number).

Therefore, it is possible to determine the maximum pressure of the liquid inside the inner tube 203 and to assess if the inner tube may support such pressure. The internal pressure (in bar) is defined by: intih) = PoughlO- ⁵ + DP - Ap where p _oa is the oil density in kg/m ³ , g is the standard acceleration due to gravity which is 9.81 m/s ² , h is the water depth in m, DP is the pressure at FPSO level in bar.

The external pressure (in bar) is defined by:

Pext(h) = p _W ate _r ghlO ^~5 where p _water is the seawater density in kg/m ³ , g is the standard acceleration due to gravity which is 9.81 m/s ² , h is the water depth in m.

Advantageously, the inner tube may be an inner hose. Therefore it may be very easy to manipulate during the transportation (it can be flattened and coiled so that the dimensions of the flatten/coiled hose may be very small compared to a rigid tube/pipe): 2500m of the inner hose may be transported in a small boat as its dimension may be for instance about 3mx3mx2m. In addition, an inner hose may be easy to insert into the outer tube 201 , even if said outer tube is already in place (e.g. if the inner hose should be replaced during the production): due to the bending of the portion 103 (see Figure 4c or 4d, for instance), it may be difficult to insert into the outer tube 203 a rigid tube/pipe. Due to the fact that the inner tube 201 is an inner hose in a specific embodiment, it is far simpler to insert it into the outer tube which is bent: A robot may simply draw the inner hose into the portions 104, 103 and 105 regardless the bending of the portions. In specific conditions, it may be important to avoid any inner hose crushing (i.e. if the internal pressure in the inner hose full of oil is lower than the hydrostatic pressure (external pressure)) because the outer tube 203 will thus support a positive pressure toward its center: the outer tube 203 is often not designed to support such constraints and may be damaged. An acceptable water depth h _acc may thus be defined by the following formula:

AP - DP

d vPoil ^~ Pwater)

As the OOL shall be deeper than 40m below the sea surface in order to avoid any problem with the maritime navigation, it is possible to assess if the chosen inner hose/pipe may satisfy the above condition.

Simulations have been carried out to ensure that the mechanical properties of the above embodiments conform to the expectations during:

- Operating conditions: inner tube 203 full of oil / annulus 202 full of sea water / operating temperature 40 °C / Long-term condition fa creep modulus (or cold flow modulus) of the PE ;

-Non operating conditions : inner tube 203 full of seawater / annulus 202 full of sea water / temperature 20 °C / Short-term condition for creep modulus (or cold flow modulus) of the PE ;

- Abnormal conditions ; Replacement conditions: inner tube 203 removed / outer tube 201 full of sea water / temperature 20 °C/ Short-term condition for creep modulus (or cold flow modulus) of the PE.

The following table shows the maximum Von Mises stress that is supported by the outer tube 201 in the section 103, and for an offset of 4.4% of water depth: Operating conditions Non operating conditions

Maximum Von Mises stress

on the section 103 - outer 3.06 MPa 5.57 MPa

HDPE tube 201

The following table shows the maximum Von Mises stress that is supported by the outer tube 201 in the section 103, and for an offset of 5% of water depth:

Operating conditions Non operating conditions

Maximum Von Mises stress

on the section 103 - outer 3.12 MPa 5.75 MPa

HDPE tube 201

The following table shows the maximum Von Mises stress that is supported by the outer tube 201 in the section 103, and for an offset of 8% of water depth:

Operating conditions Non operating conditions

Maximum Von Mises stress

on the section 103 - outer 3.34 MPa 6.5 MPa

HDPE tube 201

Considering that the maximum stress considered as acceptable is 4.5MPa for the operating conditions and 7.5MPa for the non-operating conditions, the HDPE tube may accept a very large range of offset before being damaged.

Figure 3 is a representation of a portion of the OOL in a possible embodiment of the invention.

The portion 103 of the OOL may have elements 301 / 302 adapted to be connected to the OOL portions 104 / 105. This element may be specific to the type of OOL used in these portions.

In addition, as the tube 103 may be lighter than the seawater (e.g. HDPE is lighter than seawater) and as the crude oil may be lighter than the seawater, it may be advantageous to use ballasts to increase the linear weight of the portion 130 and to ensure that the OOL would be under the surface (at least 40m under the surface). The weight of the ballast may be determined so that the ballasted tube 103 remains buoyant if the tube is filled with air (e.g. 80% filled with air): If the tube 103 remains buoyant in such configuration it may be easier to install it (see Figure 4a, for instance). It has been determined that with HDPE tube as mentioned above, a ballast of 36kg/m (+/-10%) may be advantageous.

Optionally, it is possible to design openings / valves 303 and 304 to be used during a possible installation process (see Figure 4b). In a possible embodiment, these openings / valves 303 and 304 may be opened / closed on demand.

Figures 4a-4d are different steps of the installation of the OOL in a possible embodiment of the invention.

As indicated above, the HDPE tube 201 may be extruded onshore or offshore and may be connected to the sections 104 and 105 (jumpers). Once connected, the inner tube 203 may be pulled through the entire sections 104, 103 and 105.

It is possible to temporarily seal the extremities of the OOL (104-103-105) with plugs so that the OOL is hermetic.

The ballasts mentioned above may be installed onshore on the section 103.

Once this is done, it is possible to tow the empty OOL on the production site, close to the FPSO, with for instance a small tug 401 (the OOL may remain buoyant as detailed in reference of Figure 3). No heavy boat is needed as the towing distance is limited. An extremity of the portion 104 may be connected to the FPSO 101 (see Figure 4a).

Then, the openings / valves (e.g. 303 / 304) may be opened so that the annulus 202 is filled with seawater: one opening may be used for the seawater entering (arrow 402) while a second opening may be used for the air exit (arrow 403). To ease the sinking of the OOL (see Figure 4b), it may be useful to open a transport valve on the FPSO side in order to fill the inner tube 203 with seawater.

When the OOL is completely sunk (Figure 4c), it is possible to connect the OOL to the buoy 102.

When the OOL is connected to the buoy 102, the inner tube 203 may be filled with crude oil. The OOL thus takes the configuration of Figure 4d which is a classic configuration for operating conditions for the OOL. At this stage, it is possible to close the openings / valves 303 and 304 in order to create a closed annulus volume 202. This closing may increase the accuracy of leakage detection as presented in Figure 5. The figure 4c is a classic configuration of abnormal conditions for the OOL.

If the inner tube 203 should be replaced, it is possible to perform such replacement without de-installing the entire OOL. Indeed, it is possible ^*

- to flush the inner tube 203 with seawater for instance to come back to the configuration of figure 4c,

- to disconnect the inner tube 203 at each side (i.e. on the FPSO side and on the buoy side), - to connect a new inner tube to the inner tube to be replaced,

- to pull the inner tube to be replaced through the outer tube 201 (the new inner tube being dragged in the outer tube 201 ) ;

- to connect the new inner tube at each side (i.e. on the FPSO side and on the buoy side), - to flush the new inner tube 203 with seawater,

- to come back to the operating condition by injecting oil in the new inner tube.

Due to the curve of the outer tube 203 in the configuration of Figure 4c, it may be easier if the old/new inner tubes are hoses as mentioned previously.

Figure 5 is a representation of sensors inserted in the OOL to supervise the leakage of the oil.

The section of figure 5 is a section of a portion 104 or 105 of the OOL (i.e. close to the FPSO or close to the buy). Therefore, element 501 may be a jumper.

In this embodiment, the annulus 202 of the OOL is not completely filled with seawater but the upper part of the OOL (in the operating conditions) is filled with air (or any compressible fluid) while the lower part of the OOL (in the operating conditions) is filled with seawater.

Then, it is possible to insert into the annulus various sensors.

For instance, the sensor 502 (which is not in contact with the seawater) may be a pressure sensor: if the inner tube 203 has a leak, the oil that enters the annulus 202 will compress the air in the annulus. Therefore the pressure sensed by the sensors 502 will rise.

In another possible embodiment, the sensor 503 may be a level sensor with measure a position of the interface water-air: if this interface rises, a leakage of oil in the annulus 202 is a possible explanation.

The sensor 502 or 503 may also be a temperature sensor: as the seawater has a lower temperature than the temperature of the oil, if the temperature rises, a leakage of oil in the annulus 202 is a possible explanation.

Expressions such as "comprise", "include", "incorporate", "contain", "is" and "have" are to be construed in a non-exclusive manner when interpreting the description and its associated claims, namely construed to allow for other items or components which are not explicitly defined also to be present. Reference to the singular is also to be construed in be a reference to the plural and vice versa.

A person skilled in the art will readily appreciate that various parameters disclosed in the description may be modified and that various embodiments disclosed may be combined without departing from the scope of the invention.

For instance, it is stressed that any liquid/fluid/gas may be transported by the OOL. Therefore, the "oil" or "crude oil" may be replaced by any liquid/fluid/gas in the above description.