TECHNICAL FIELD

The present invention relates to method and an arrangement for providing pressure relief in a flow line or a riser for conveying hydrocarbons. A pressure relief arrangement according to the invention is used for mitigating the effects of a sudden depressurization of the main fluid flow in the riser or flow line.

BACKGROUND ART Conduits to transfer materials from the seafloor to production and drilling facilities at the surface, as well as from the facility to the seafloor, are commonly termed risers. Subsea risers are a type of pipeline developed for this type of vertical transportation. Risers can serve as production or import/export means and are the connection between the subsea field developments and production and drilling facilities. Similar to pipelines or flow lines, risers transport produced hydrocarbons, as well as production materials, such as injection fluids, control fluids and gas lift. Usually insulated to withstand seafloor temperatures, risers can be either rigid or flexible

There are a number of types of risers, including attached risers, pull tube risers, catenary risers, top-tensioned risers, riser towers and flexible riser configurations, as well as drilling risers.

A flexible riser is a hybrid that can accommodate a number of different situations. Flexible risers can absorb large curvatures, making them ideal for use with floating facilities. This flexible pipe was originally used to connect production equipment aboard a floating facility to production and export risers, but now it is found as a primary riser solution as well. There are a number of configurations for flexible risers, including the steep S and lazy S that utilize anchored buoyancy modules, as well as the steep wave and lazy wave that incorporates buoyancy modules.

In general, a flexible riser comprises a central stainless steel or metal- alloy carcass covered by a pressure barrier or pressure sheath that provides hydraulic integrity. Numerous layers of flexible armour surround the sheath, or pressure vault, to provide tensile- and hoop- stress strength. The armour layers are usually separated by cushioning layers of composite or thermoplastic material to prevent them from rubbing against one another. The number and size of armour layers is a function of the pressure and tensile strength specifications imposed by the particular application for which the riser is designed. A final thermoplastic outer sheath provides protection from external sources such as seawater ingress, attack from marine growth, or abrasion.

During operation of risers a sudden loss of pressure, or depressurization, can occur. For risers in general this should not be a problem, but for flexible pipes with multilayer pressure barrier a problem with collapsing carcasses has occurred. In the case of flexible pipes with traditional multilayer pressure barrier design, a pressure equal to the bore pressure also occurs in the annular gap between the innermost layer, termed the sacrificial sheath, and the layer to the sacrificial sheath, termed the pressure sheath. Both the sacrificial sheath and the pressure sheath are fluid tight, but opposed to the pressure sheath the sacrificial sheath is not sealed at the end fitting, allowing bore fluid to enter the annular gap between. At depressurization of the bore, a pressure difference between the annular gap and bore will occur due to the time delay in evacuating the long and narrow annular gap. In case of high depressurization rates the pressure difference between the annular gap and bore can threaten the integrity of the pipe. Hence, there is a need for an arrangement that avoids the above problems. The object of the invention is to provide pressure relief through the sacrificial sheath itself.

DISCLOSURE OF INVENTION The above problems are solved by an arrangement and a method as described in the attached claims.

In the subsequent text, the term "flexible pipe" is defined as comprising flexible risers, flowlines and jumpers. The term "riser" or "subsea riser" is defined as comprising a flexible riser.

The invention relates to a flexible pipe used for conveying fluids, such as hydrocarbons. The flexible pipe comprises, from inner to outer layer, an internal load bearing structural layer arranged to withstand external pressure, often termed a carcass, a fluid tight pressure barrier, at least one external load bearing structural layer, such as pressure and tensile armours, and an outer sheath or protective sheath.

The carcass forms the innermost layer of the flexible pipe and is commonly made from stainless steel flat strip that is shaped and wound into an interlocking profiled tube. The carcass prevents collapse of the pressure barrier due to decompression and/or external hydrostatic pressure. Common stainless steel grades for use in a carcass are AISI grades 304, 316 and Duplex. The inner fluid flows freely through the carcass, which has to be corrosion resistant and resistant to the conveyed fluid.

In a prior art riser, the main function of the carcass is to prevent pipe collapse due to hydrostatic pressure or build up of gases in the riser annulus located between the internal-pressure-retaining pressure barrier and the external outer protective sheath. The build-up of gases in the riser annulus can be a potential failure mode for the riser and is caused by hydrocarbon gases diffusing through the pressure barrier into the riser annulus. In the case of a well shut-down and a subsequent depressurization of the inner conduit in the carcass, the pressure of the accumulated gas in the riser annulus could cause the riser to collapse. The wound steel carcass is designed to withstand this pressure. In a flexible pipe according to the invention, the function of the carcass is the same. However, the build-up of pressure occurs between layers in the pressure barrier surrounding the carcass. This is caused by hydrocarbons entering an annular gap between an inner and an outer fluid tight layer of the pressure barrier through an end fitting. The annular gap is open to the bore in the end fitting at both ends of the pipe. In the case of a well shut-down and a subsequent depressurization of the inner conduit in the carcass, the pressure of the accumulated gas in the annular gap is arranged to be released through the sacrificial sheath and into the carcass to prevent a flexible pipe collapse. The wound steel carcass is designed to withstand the pressure in the annular gap until the pressure can be released.

The pressure barrier comprises at least two layers separated by an annular gap, which annular gap is open to the internal pressure of the flexible pipe at the end fittings. A first layer is provided with weakened portions along at least a major part of the longitudinal extension of the flexible pipe. The weakened portions are arranged to open if the pressure difference across the first layer exceeds a predetermined limit during an internal pressure drop. A second layer is arranged to contain any liquid hydrocarbons under pressure within the annular gap. The first and second layers can be encased in an anti-creep sheath to prevent formation of notches on the outer surface of the pressure sheath. In the subsequent text, the first and second layers are also referred to as a sacrificial sheath and a pressure sheath, respectively, to clarify the function of said layers. The pressure sheath, i.e. the second layer, needs to be fluid tight..

The pressure barrier is a polymer sheath that makes the pipe leak proof. The materials are selected based on fluid exposure concentrations and fluid temperatures. Common materials used for the pressure barrier include polyamide-1 1 , high density polyethylene (HDPE), cross-linked polyethylene (XLPE) and polyvinylidene difluoride (PVDF). The material selected may have to withstand fluid temperatures up to 130 °C. For flexible risers, the maximum allowable strain (%) must also be considered. The average thickness of a pressure barrier is about 5-8 mm, but can be up to 13 mm.

The pressure barrier is encased by external load bearing structural layer. Examples of such structural layers are pressure amours and tensile armours.

The pressure armour is arranged to withstand the hoop stress in the riser wall caused by the inner fluid pressure or external forces. The pressure armour is wound around the pressure barrier and comprises interlocking wires. The wires have a profile that allows bending flexibility and controls the gap between the armour wires to prevent extrusion of the polymer pressure barrier through the armour layer. In order to resist radial loads in the riser wall, the pressure armour is wound at a relatively small angle. The winding angle relative to the longitudinal axis of the riser can be about 89°. The material for the pressure armour is typically high strength carbon steel, where the quality is chosen depending on the fluid in the riser (sweet or sour gas).

The tensile armour comprises layers cross-wound in pairs and is used to resist tensile load on the flexible riser. Tensile armour layer typically comprise flat, rectangular wires wound at an angle between 30-55° to the longitudinal axis. For instance, a winding angle of 55° relative to the longitudinal axis results in a torsionally balanced riser and is used when the tensile armour takes up the hoop stress, whereby no pressure armour is required. Tensile armour layers support the weight of all the riser layers and transfers the load through an end fitting to a vessel structure or similar. High tension in a deepwater riser may require two pairs of cross-wound layers. The material for the tensile armour wires is typically high strength carbon steel, as for the pressure armour. Local conditions determine the material selection in the same way as for the pressure armour. The outer sheath or protective sheath is an outer polymer sheath that can be made from the same material as the pressure barrier. The outer layer is a barrier against seawater and provides a level of protection for the armour against chafing with other objects or during installation of the riser. Additional, minor layers may include anti-friction tapes around the armour layers, to reduce wear caused by friction between armour layers, and anti- wear tape to ensure that wires in the armour does not twist out of their preset configuration.

As stated above, the pressure barrier is provided with an inner layer having weakened portions arranged to open if the pressure difference across the first layer exceeds a predetermined limit. For instance, if a sudden depressurization of the fluid being conveyed in the carcass occurs then there will be insufficient time for pressure equalization between the inner conduit formed by the carcass and the annular gap between the two layers in the pressure barrier. In order to limit damage to the riser in this situation, the weakened portions will open and allow the relatively higher pressure in the annular gap to escape through the carcass into the depressurized inner conduit.

According to one example, each weakened portion comprises a recess extending a predetermined distance into the outer surface of the first layer. Each recess is at least partially filled with a plug attached by means of an adhesive or heat meltable plug. The first layer and each plug comprise a suitable thermoplastic material, wherein the melting point of the plug is lower than that of the first layer. The weakened portions in this example can comprise drilled or milled recesses. When subjected to a predetermined pressure difference between the annular gap and the inner conduit, the remaining material will open and create a fluid passage past the plug.

According to a second example, each weakened portion comprises a cut that extends completely through the first layer which cut is provided with a seal to form a plug. The cut can comprises an elongated straight line, with a predetermined width and extension, a number of holes with a predetermined diameter, located along a predetermined linear extension, or an at least part circular, cylindrical or conical slot. When subjected to a predetermined pressure difference between the annular gap and the inner conduit, the sealing material will open and create a fluid passage through the cut.

Alternatively, the cut forming each weakened portion can comprise an at least part circular cut extending a predetermined distance into the outer surface of the first layer to form a plug. The cut comprises an at least part circular, cylindrical or conical slot. When subjected to a predetermined pressure difference between the annular gap and the inner conduit, the remaining material will open and create a fluid passage past the plug.

The cut in the second example can be formed by a water jet, a laser or a heated blade tool. The internal load bearing structural layer, or carcass, comprises a helically wound strip.

The weakened portions are arranged to at least partially overlap gaps between adjacent portions of the metal strip making up the wound carcass.

Each weakened portion has an outer surface flush with the outer surface of the first layer, or sacrificial sheath. Depending on the shape of the recess or cut, or whether a plug has been used, it is desirable to remove or cover any sharp edges on the sacrificial sheath, in order to prevent notching or damage to the second layer, or pressure sheath. This can be achieved by machining, by melting, or by a suitable filler or patch applied over the sharp edge. The material used in a filler or patch is preferably a low strength material that can dissolve in hydrocarbons. Patches can be provided in the form a sticky tape or as shaped adhesive patches, being sufficiently adhesive to prevent accidental removal during production of the pressure barrier. Alternatively, the patches can comprise a deformable, hardening material that can be applied to the outer surface of the sacrificial sheath by rollers. According to a further alternative, if a plug is used then the outer surface of the plug can be shaped to conform to the outer surface of the sacrificial sheath. The filler/plug material can be made from a water soluble salt, as used in metallic moulding, or sodium silicate (water glass). Another water soluble polymer is polyvinyl alcohol (PVA) which can be a filler alternative. Low density polyethylene, EPDM rubber are known to swell and lose mechanical properties when exposed to hydrocarbons, making them suitable as plug materials. Some silicon rubbers will also degrade rapidly in hydrocarbons at elevated temperatures and can be used as filler materials.

As indicated above, a flexible pipe or riser according to the invention can comprise an external load bearing structural layer comprising at least one first external load bearing structural layer arranged to absorb longitudinal forces, and at least one optional second load bearing structural layer arranged to withstand internal pressure. The pressure barrier can further comprise an anti-creep sheath encasing the first and second layers. The anti- creep sheath is in contact with the innermost of the external load bearing structural layers.

The invention further relates to a method for manufacturing a flexible pipe component with a pressure relief system, which method comprises the steps of

- winding a profiled metallic strip into a tubular internal load bearing structural layer arranged to withstand internal pressure,

- extruding at least a first and a second, thermoplastic layer over the load bearing structural layer, which layers are separated by an annular gap, wherein weakened portions are cut or machined into the first layer prior to the extrusion of the second layer.

According to a first example, the method involves machining weakened portions by milling or drilling recesses extending a predetermined distance into the outer surface of the first layer. After the machining step, each recess is at least partially filled with a plug attached by means of an adhesive or using an adhesive, at least a part of the plug or a cooperating surface in the recess is coated with a suitable adhesive. The plug is then inserted into the recess and the adhesive is allowed to harden. When using a heat meltable plug, the plug is inserted into the recess and subjected to a heat source. When the plug or parts thereof begins to soften, a tool applies a force to at least partially deform the plug and force it into contact with a predetermined axial or radial surface in the recess. Subsequently, any sharp edges or radially extending portions are removed by machining, so that the plug conforms to the cylindrical outer surface of the first layer.

According to a second example, the method involves cutting perforations or straight slots or part circular, cylindrical or conical slots extending through the outer surface of the first layer to form weakened portions and sealing or partially filling the cut perforations or slots. After the cutting step, each cut perforations or slots is sealed or partially filled with a thermoplastic material having less strength and lower melting point than the first layer. Alternatively, weakened portions are cut by cutting perforations or straight slots or part circular, cylindrical or conical slots extending a predetermined distance into, but not through, the outer surface of the first layer. If necessary, any sharp edges or radially extending portions are removed so that the weakened portion conforms to the cylindrical outer surface of the first layer. The cut making up the said perforations or slots can be formed by a water jet, a laser or a heated blade tool.

According to the invention, the weakened portions are machined or cut so that they at least partially overlap a gap between adjacent portions of the wound metallic strip of the carcass. The wound metallic strip makes up a carcass, as described above.

In order to position the respective tool relative to the carcass gap, a number of methods can be used. If the thickness of the first layer is less than a predetermined value, it is possible to sense the geometry of the underlying carcass on the outer surface of the extruded layer. If the material used in the extruded layer is not opaque, then a laser or light detector can be used to detect the carcass gap. Alternatively, an ultrasound detector can be used for detecting the location of the carcass gap along the outer surface. Finally, the invention relates to a flexible pipe component with a pressure relief system manufactured according to the above method.

BRIEF DESCRIPTION OF DRAWINGS

In the following text, the invention will be described in detail with reference to the attached drawings. These schematic drawings are used for illustration only and do not in any way limit the scope of the invention. In the drawings:

Figure 1 shows a riser extending from the sea floor to a production vessel;

Figure 2 shows a schematic illustration of the layers making up a flexible riser;

Figure 3 shows a cross-section through the flexible riser in Figure 2;

Figure 4A-C show alternative methods for making weakened portions in a pressure barrier according to a first example;

Figure 5A-C show alternative methods for making weakened portions in a pressure barrier according to a second example;

Figure 6 A-C show alternative methods for making weakened portions in a pressure barrier according to a third example; and

Figure 7 A-C show alternative methods for making weakened portions in a pressure barrier according to a fourth example. EMBODIMENTS OF THE INVENTION

Figure 1 shows a riser extending from the sea floor to a production vessel. Oil is produced by subsea wells via a manifold, which passes through rigid flow lines and then flexible risers into a floating production, storage and offloading system. The vessel shown in this figure is a ship, but the arrangement is applicable on any type of floating, semi-submersible of permanent production platform.

As shown in Figure 1 , the riser assembly comprises a manifold 1 1 located on the seabed, connected to the lower end of the flexible riser 12. The flexible riser 12 comprises a carcass, a pressure barrier, pressure and tensile armours, and an outer protective sheath (not shown). Figure 1 indicates both a lazy wave and a steep wave arrangement of the flexible riser.

Figure 2 shows a schematic illustration of the layers making up a flexible riser 20. In this example, the riser comprises, from the inner to the outer layer, an internal load bearing structural layer 21 arranged to withstand internal pressure, often termed a carcass, a fluid tight pressure barrier 22, external load bearing structural layer 23, 24 and an outer layer 25 or protective sheath. The carcass forms the innermost layer of the flexible riser and is commonly made from stainless steel flat strip that is shaped and wound into an interlocking profiled tube.

The pressure barrier comprises first and second layers separated by an annular gap, which layers comprise an inner sacrificial sheath and an outer pressure sheath. The first and second layers are encased in an anti-creep sheath for preventing formation of notches on the outer surface of the pressure sheath. These layers will be described in detail in Figure 3.

The pressure barrier is encased by a pressure amour and a tensile armour. The pressure armour 23 is arranged to withstand the radial loads in the riser wall caused by the inner fluid pressure. The pressure armour is wound around the pressure barrier and comprises interlocking wires. Figure 2 shows two pressure barrier layers 23a, 23b. The tensile armour 24 comprises layers of flat, rectangular wires cross-wound in pairs 24a, 24b and is used to resist tensile load on the flexible riser. The outer layer 25 is an outer polymer sheath that can be made from the same material as the pressure barrier. The outer layer 25 is a barrier against seawater and provides a level of protection for the armour layers.

Figure 3 shows a cross-section through the flexible riser in Figure 2. This figure shows the construction of the pressure barrier 22 located between the carcass 21 and the outer pressure armour layer 23. The longitudinal central axis of the flexible riser is denoted X.

The pressure barrier 22 comprises first and second layers 31 , 32 separated by an annular gap 33, which annular gap is open to the internal pressure of the riser at the end fitting 35. The second layer 32, i.e. the pressure sheath, needs to be fluid tight.. During normal operation, the pressure of fluid flowing into the annular gap will adapt to and substantially corresponds to the pressure of the fluid in the carcass, also termed the bore pressure. A first layer 31 , or sacrificial sheath, is provided with weakened portions (see Figures 4-7) along at least a major part of the longitudinal extension of the flexible riser. The weakened portions are arranged to open if the pressure difference across the first layer exceeds a predetermined limit during a pressure drop in the fluid the carcass. A second layer 32, or pressure sheath, is arranged to contain any liquid hydrocarbons within the annular gap 33. The inner and outer layers are encased in an anti-creep sheath 34 to formation of notches on the outer surface of the pressure sheath 32.

If collapse occurs, the effect on the flexible riser is mitigated by the collapse of the carcass 21 and the sacrificial sheath 31 . The pressure sheath 32 and the armour layers can contain remaining fluid in the riser and prevent an uncontrolled release of hydrocarbons. Figure 4A shows a method for making weakened portions according to a first example. According to the first example a recess 42a is milled 41 a or drilled a predetermined distance into the outer surface of a first layer, which forms a sacrificial sheath 31 . The recess is located in line with a gap 43a between adjacent portions of a wound metal strip making up the carcass 21 . The recess is filled with a plug 44a inserted into the recess and is attached by means of an adhesive 45a deposited into the recess prior to the insertion. Excess adhesive or any part of the plug extending out of the recess is then machined away to provide a smooth outer surface 46a. If a sudden depressurization occurs, excess pressure in the annular gap outside the sacrificial sheath will open 47a the sheath adjacent the plug and at the remaining section between the recess and the inner surface of the sacrificial sheath. Pressurized fluid can then escape from the annular gap and flow radially inwards through the carcass. Figure 4B shows an alternative first example where a recess 42b is milled 41 b or drilled a predetermined distance into the outer surface of the sacrificial sheath 31 . The recess is located in line with a gap 43b between adjacent portions of a wound metal strip making up the carcass 21 . The recess is filled with a hot-melt plug 44b, having a lower melting temperature than the sacrificial sheath, which is inserted into the recess. Heat is applied to the hot- melt plug to fuse it into the recess 45b. Excess material from the melting of the plug extending out of the recess is then machined away to provide a smooth outer surface.

If a sudden depressurization occurs, excess pressure in the annular gap outside the sacrificial sheath will open a central portion 46b of the plug and the remaining section 47b between the recess and the inner surface of the sacrificial sheath, as indicated in Figure 4B. Pressurized fluid can then escape from the annular gap and flow radially inwards through the carcass. Figure 4C shows a further alternative first example where a stepped recess 42c is milled 41 c or drilled a predetermined distance into the outer surface of the sacrificial sheath 31 . The recess is located in line with a gap 43c between adjacent portions of a wound metal strip making up the carcass 21 . The recess is filled with a hot-melt plug 44c, having a lower melting temperature than the sacrificial sheath, which is inserted into the recess to rest on the outer stepped surface in the recess. Heat is applied to the hot-melt plug to fuse it into the recess 45c. Excess material from the melting of the plug extending out of the recess is then machined away to provide a smooth outer surface.

The method in Figure 4C could also be adapted so that the plug is attached by means of an adhesive applied to the underside of the plug or onto the outer stepped surface of the recess.

If a sudden depressurization occurs, excess pressure in the annular gap outside the sacrificial sheath will open a central portion 46c of the plug and the remaining section 47c between the recess and the inner surface of the sacrificial sheath, as indicated in Figure 4C. Pressurized fluid can then escape from the annular gap and flow radially inwards through the carcass.

Figure 5A shows a method for making weakened portions according to a second example. According to the second example a cut 51 a is made that extends completely through a first layer, which forms a sacrificial sheath 31 . The cut can comprise an elongated straight line, with a predetermined width and extension, or a number of holes with a predetermined diameter, located along a predetermined linear extension, as shown in Figure 5C. Each hole or slot 51 a is located in line with a gap 52a between adjacent portions of a wound metal strip making up the carcass 21 . The hole or slot 51 a is provided with filler to form a plug 53a extending a predetermined distance into the hole or slot. The filler has a lower melting temperature than the sacrificial sheath. Excess filler extending out of the recess is then machined away to provide a smooth outer surface 54a. Figure 5B shows an alternative second example, where the filler shown in Figure 5A is replaced by an adhesive patch 53b covering the hole or slot 51 b. The cut can comprise an elongated straight line, with a predetermined width and extension, or a number of holes with a predetermined diameter, located along a predetermined linear extension, as shown in Figure 5C. Each hole or slot 51 b is located in line with a gap 52b between adjacent portions of a wound metal strip making up the carcass 21 . As the patch is flush with the outer surface of the sacrificial sheath 31 , no after-treatment is required after application of the patch 53b. The patch can be shaped to conform to the shape of the hole or cut, or be applied as a strip of adhesive tape.

Figure 5C shows a sacrificial sheath provided with a cut both in the form of an elongated straight line 51 c, with a predetermined width and extension, and a number of holes 52c with a predetermined diameter, located along a predetermined linear extension. If a sudden depressurization occurs, excess pressure in the annular gap outside the sacrificial sheath will open the sealing filler 53a or adhesive patch 53b, as indicated in Figures 5A and 5B (see references 55a; 54b). Pressurized fluid can then escape from the annular gap and flow radially inwards through the carcass. Figure 6A shows a method for making weakened portions according to a third example. According to the third example, two parallel, angled cuts or a part circular conical cut 60a; 61 a is made completely through a first layer, which forms a sacrificial sheath 31 . Each hole or slot 61 a is located in line with a gap 62b between adjacent portions of a wound metal strip making up the carcass 21 The angled cuts form slots parallel in a tangential plane through the sacrificial sheath and diverge in the direction of the carcass at a predetermined angle a relative to a plane through the central axis of the carcass, as indicated in Figures 6B and 6C. Alternatively the part circular conical cut forms a truncated cone with a predetermined cone angle β and remains attached to the sacrificial sheath along a generatrix to prevent the cone shaped plug from falling towards the carcass, as indicated in Figures 6B and 6C. The respective the angled cuts and the part circular conical cut is provided with a patch 63a to cover the cut, or with a filler to form a plug 64a extending a predetermined distance into the cut 60a; 61 a. The filler has a lower melting temperature than the sacrificial sheath. Excess filler extending out of the recess is then machined away to provide a smooth outer surface. If a patch is used, no after-treatment is required.

Figure 6B shows a plan view of the angled cuts 60a and the part circular conical cut 61 a respectively. As indicated in this figure the angled cuts and the part circular conical cut are located in line with a gap 62a between adjacent portions of a wound metal strip making up the carcass. Figure 6C shows a schematic cross-section through either of the angled cuts or the part circular conical cut in Figure 6B. The figure indicates the angle a of the parallel angled cuts 60a, and the cone angle β of the part circular conical cut 61 a.

When subjected to a predetermined pressure difference between the annular gap and the inner conduit, the sealing material, that is the filler or the patch, will open and create a fluid passage through either cut, as indicated in Figure 6A. In the case of the part circular conical cut, if the pressure difference is sufficient, the plug can separate from the sacrificial sheath 64 and be displaced towards the carcass. In the case of the parallel cuts, Fluid will open the filler or patch and escape through the said cuts 65.

Figure 7A shows a method for making weakened portions according to a fourth example. According to the fourth example a cut 71 a is made that extends a predetermined distance into a first layer, which forms a sacrificial sheath 31 . The cut is a circular conical cut forming a truncated cone with a predetermined cone angle β. The circular conical cut can be provided with an optional patch 73a to cover the cut 71 a.

Figure 7B shows a plan view of the circular conical cut 71 a. As indicated in this figure the part circular conical cut is located in line with a gap 72a between adjacent portions of a wound metal strip making up the carcass.

Figure 7C shows a schematic cross-section through the circular conical cut in Figure 7B, indicating the cone angle β of the cut.

When subjected to a predetermined pressure difference between the annular gap and the inner conduit the remaining section between the recess and the inner surface of the sacrificial sheath, as well as the optional patch, will open and create a fluid passage 74a through the cut, as indicated in Figure 7A. A plug 75a formed by the conical cut will separate from the sacrificial sheath and be displaced towards the carcass.

The above cuts, to produce holes, slots or conical cuts can be formed by a water jet, a laser or a heated blade tool.

Suitable diameters for the above recesses and holes, widths and lengths of slots, or diameters and cone angles for the conical slots are determined by the diameter and length of the flexible riser, as well as by the pressure at which the weakened portions are required to release. The invention is not limited to the above embodiments, but may be varied freely within the scope of the claims. For instance, the type of weakened portion need not be the same along the entire length of the flexible riser or pipe. Consequently it is possible to combine any one of the types of weakened portions described above along the extension of the riser or pipe. The type of weakened portion and its size can be selected dependent on its desired position along the riser and the expected pressure difference required for relieving pressure from the annular gap.