WO2021038098A1

WO2021038098A1 - A pipe installation

Info

Publication number: WO2021038098A1
Application number: PCT/EP2020/074199
Authority: WO
Inventors: Søren Dahl PETERSEN; Anders STRAARUP; Daniel Øland VONBOE; Niels Juul
Original assignee: National Oilwell Varco Denmark I/S
Priority date: 2019-08-30
Filing date: 2020-08-31
Publication date: 2021-03-04

Abstract

Ine invention concerns a pipe installation for transportation of a fluid. The installation includes an unbonded flexible pipe and a sensor system. Tthe unbonded flexible pipe has a first and a second end-fitting, a plurality of layers extending from the first to the second end-fitting, where the plurality of layers includes a plurality of armor layers and at least two fluid tight polymer layers forming an annulus, wherein at least one of said armor layers is arranged in the annulus. The sensor system comprises an interrogator arrangement and at least one fiber sensor optically connected to said interrogator. The fiber sensor has a length and is extending from its optical connection to the interrogator, passing through said first end-fitting and into the annulus, wherein the fiber sensor includes at least one silica fiber length section and at least one polymer fiber length section and the at least one polymer fiber length section is arranged in the annulus.

Description

A PIPE INSTALLATION

TECHNICAL FIELD

The present invention relates to a pipe installation comprising an unbonded flexible pipe suitable for offshore fluid transportation, such as for transport of petrochemical fluids e.g. oil or gas or in a sub-sea environment.

Flexible pipes for offshore applications are generally known from the standard "Recommended Practice for Flexible Pipe", ANSI/API 17 B, fifth Edition, May 2014 (hereafter API17B), and the standard "Specification for Unbonded Flexible Pipe", ANSI/API 17J, Fourth edition, May 2014 (hereafter API 17 J).

Such pipes are generally referred to as bonded pipes or unbonded pipes. A bonded pipe generally is a pipe in which the steel reinforcement is integrated and bonded to a vulcanized elastomeric material. An unbonded pipe generally is a pipe comprising separate layers, including armor layer(s) and polymeric layer(s), which allow relative movement between layers. The present invention generally concerns an installation comprising an unbonded flexible pipe.

Such an unbonded flexible pipe generally comprise separate unbonded polymeric layers, such as extruded polymeric layers and armor layers, which allows relative movement between layers. The armor layers are typically helical wound armor layers, such as metallic helically wound armor layers.

A typical unbonded flexible pipe comprises from the inside and outwards an optional inner armor layer known as the carcass, an internal pressure sheath comprising an extruded polymer layer surrounded by one or more armor layers and an outer sheath (also referred to as external protective polymer sheath), such as an extruded polymer layer. The unbonded pipe may comprise additional layers, such as intermediate polymer layers, insulation layers, additional armor layers, and wound tape layers. The carcass is not fluid tight and thus, the internal pressure sheath, usually an extruded polymer layer, forms a bore in which the fluid to be transported is conveyed and thereby ensures internal fluid integrity and stability. In some unbonded flexible pipes, the carcass may be omitted.

The armor layers surrounding the internal pressure sheath may for example comprise one or more pressure armor layers comprising one or more armor profiles or strips, which are wound around the internal pressure sheath at a large angle (short pitch), e.g. an angle larger than 80°, relative to the center axis of the pipe. This or these pressure armor layers primarily compensate for radial forces in the pipe. The armor layers surrounding the internal pressure sheath may also usually comprise one or more tensile armor layers which are wound at a relative small angle (large pitch), such as between 10° and 50°, relative to the center axis of the pipe. This or these tensile armor layers primarily compensate for axial forces in the pipe. The armor layers are typically made of steel.

In general, flexible pipes are expected to have a lifetime of about 20 years in operation.

Unbonded flexible pipes are e.g. used for the transport of fluids, such as oil and gas between offshore installations, e.g.at shallow, deep or ultradeep sea depths. The fluid may comprise a hydrocarbon fluid, such as gas and oil, water, C02 or a mixture hereof depending upon the nature of the hydrocarbon reservoir. The fluid may also be an injection fluid such as water, C02 or methanol.

Thus, the internal pressure sheath forms the bore in which the fluid to be transported is flowing. The internal pressure sheath and a surrounding fluid impermeable sheath, an outer sheath or an intermediate sheath, form an annular volume, known as the annulus, which comprises one or more layers of armoring layers and an annular void. Where the unbonded flexible pipe comprises an intermediate layer the pipe may comprise two annuluses. Although the sheaths forming the annular volume in principle are impermeable, gases may migrate through the sheaths into the annular volume over time. From the bore of the pipe, gasses, such as CO2 and H2S, may permeate through the sheath into the annular volume and cause corrosion of the armoring layers in the annular volume, which are typically made from steel. In particular, CO2 and H2S become very corrosive if the annulus has a high humidity.

The annular volume may be exposed to water, such as from seawater ingress due to a damaged outer sheath, or water vapor permeating from the bore fluids through the internal pressure sheath and/or vapor permeating from the ambient seawater through the outer sheath and condensing in the annular volume.

The combination of chemicals, gases and water in the annular volume may cause changes in acidity, which will lead to an annular volume with an acidic or basic environment.

In addition, an annular volume exposed to water ingress may also prevent gas from being vented from the annulus, leading to a pressure build up which may result in a rupture in the external sheath and further corrosion issues.

For example, water may be captured in a "hog bend" region of the unbonded flexible pipe. A "hog bend" represents a local maximum in the height of a sinusoidal- or wave-shaped unbonded flexible pipe e.g. an installed riser pipe.

Monitoring systems for monitoring strain and temperature in a flexible pipe are known. Such a system is for example described in US 8891070, which discloses a flexible pipe system and a method of detecting a break of an elongated armoring element of an unbonded flexible pipe and comprises a plurality of optical fibers incorporated into one or more armoring elements, such as armor element(s) located in an annulus of the pipe.

The unbonded flexible riser pipe is usually subjected to severe environmental and functional loads during use due to e.g. actions from water waves, vessel movements and pressure inside and outside the pipe and it is essential to detect any damage or risk of damage as early as possible. WO 03/056313 describes a sensor system for use in the detection or measurement of at least one characteristic value relating to a chemical environment in a flexible pipe. The system comprises incorporating an optical glass fiber along a flexible pipe-e.g. in an armoring wire; letting a gas derived from the chemical environment diffuse into the optical fiber, thereby altering the optical properties of the optical fiber; detecting and analyzing light from the optical fiber so as to determine changes in the optical properties of the optical fiber due to the in-diffusion of the gas; and deriving the at least one characteristic value representing the chemical environment from the determined changes. The optical fiber comprises a coating comprising reaction elements for reacting with the gas to be determined. This system, which has never been used in practice in a flexible pipe, is however fairly complicated to use as it reacts relatively slowly and it is difficult to obtain separate determination of desired fluid components.

US 8590365 discloses a pipe system in which detection of the presence and preferably the amount of water vapor and/or one or more of the components selected from oxygen, hydrogen, methane, hydrogen sulphides and carbon dioxides may be performed. The pipe system comprises a pipe, a gas sensing station and a remote output system. The pipe comprises a pipe gas cavity extending lengthwise in part or all of the length of the pipe. The gas sensing station comprises a sensing gas cavity which is in gas communication with the pipe gas cavity, the sensing gas cavity comprises a photoacoustic spectroscope, the pipe system comprises at least one optical feeding fiber for feeding light to the photoacoustic spectroscope and a transmission path for transferring a signal from the photoacoustic spectroscope to the remote output system. The transmission path from the gas sensing station to the remote output system is an optical transmission path. This system has never been practically operable and is specifically not capable of performing local determinations.

WO 2014/177152 discloses an assembly of an unbonded flexible pipe and an end-fitting, wherein the flexible pipe comprises at least one optical fiber for mounting to a processing system or another waveguide. The optical fiber is arranged in a layer of the flexible pipe and the optical fiber comprises an overlength located in the end-fitting for making it simpler to mounting the fiber to a processing system.

WO2013/135244 discloses an unbonded flexible pipe comprising an optical fiber for use in monitoring at least one parameter of the pipe during operation. The optical fiber is incorporated into a tape layer of the unbonded flexible pipe where the length of the fiber is at least 3 times the length of the unbonded flexible pipe.

DISCLOSURE OF INVENTION

An objective of the invention is to provide a pipe installation in which at least one characteristic value relating to a chemical environment in an annulus of an unbonded flexible pipe of the installation may be determined and wherein at least one of the drawbacks of the above described prior art systems has been alleviated.

In particular, it is desired that the pipe installation is capable of performing one or more local determinations along the length of the pipe annulus and wherein, desirable, the length of the pipe annulus is 300 meters or longer, such as 500 meters or longer, such as 1000 meters or longer, such as 2000 meters or longer.

These and other objects have been solved by the invention and embodiments thereof as defined in the claims and as described herein below.

It should be emphasized that the term "comprises/comprising" when used herein is to be interpreted as an open term, i.e. it should be taken to specify the presence of specifically stated feature(s), such as element(s), unit(s), integer(s), step(s) component(s) and combination(s) thereof, but does not preclude the presence or addition of one or more other stated features. Reference made to "some embodiments" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with such embodiment(s) is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases "in some embodiments" or "in an embodiment" in various places throughout the specification is not necessarily referring to the same embodiment(s). Further, the skilled person will understand that particular features, structures, or characteristics may be combined in any suitable manner within the scope of the invention as defined by the claims.

The term "substantially" should herein be taken to mean that ordinary product variances and tolerances are comprised.

Throughout the description or claims, the singular encompasses the plural unless otherwise specified or required by the context.

All features of the invention and embodiments of the invention as described herein, including ranges and preferred ranges, may be combined in various ways within the scope of the invention, unless there are specific reasons not to combine such features.

The phrases "long pitch" and "short pitch" are relative terms i.e. the short pitch is shorter than the long pitch. A winding with an angle of about 75 degrees or larger relative to the pipe axis is generally considered to be a short pitch and a winding with an angle of about 55 degrees or shorter is generally considered to be a long pitch.

The term "cross-wound layers" means that the layers comprises wound elongate elements that are wound in opposite direction relatively to the longitudinal axis of the pipe where the angle to the longitudinal axis can be equal or different from each other.

The term 'seabed' is generally used to denote the subsea floor. The phrase "winding angle" means a winding angle relative to the longitudinal center axis of the pipe (or merely called axis) when the pipe is in straight and unloaded condition.

The pipe installation of the invention uses a combined silica and polymer fiber sensor, thereby being capable of determining at least one characteristic value relating to a chemical environment in a desired location of an annulus, even far below sea surface level, while simultaneously having a high and accurate signal.

Due to the very high tensile and radial forces, which the unbonded flexible pipe may be subjected during use, the annular space may vary during use and is generally very narrow. Most of the annular space is occupied by elongate armor elements - usually helically wound wires of the armor layer(s) located in the annulus.

The pipe installation is suitable for transportation of a fluid, such as C02 and/or a petrochemical fluid e.g. hydrocarbon containing liquid and/or gas.

The installation comprises an unbonded flexible pipe and a sensor system.

Unbonded flexible pipes are well known in the art and are e.g. as described in the standards API17B and API17J.

The unbonded flexible pipe comprises a first end-fitting and a second end^¬ fitting, a plurality of layers extending from the first to the second end-fitting. The plurality of layers comprises a plurality of armor layers and at least two fluid tight polymer layers forming an annulus, wherein at least one of the armor layers is located in the annulus. Advantageously at least the two or more fluid tight polymer layers are anchored in the first and second end- fittings. Generally, it is desired that all load bearing layers, such as armor layers, are anchored in the first and second end-fittings.

Optional Insulation layers and/or anti-wear layers or similar may be anchored in the end-fittings, however is some embodiments this is not required. The sensor system comprises an interrogator arrangement and at least one fiber sensor optically connected to the interrogator.

The fiber sensor has a length and is extending from its optical connection to the interrogator, passing through the first end-fitting and into the annulus. The fiber sensor comprises at least one silica fiber length section and at least one polymer fiber length section, the at least one polymer fiber length section is arranged in the annulus.

The phrase "silica fiber length section" designates a fiber length section comprising an optical fiber having a silica core comprising or consisting essentially of silica. Advantageously the silica fiber length section also comprises a silica cladding. The silica may comprise impurities and/or doping materials, such as it is generally known in the art for silica fibers. In addition, the silica fiber length section may comprise a coating, such as a polymer coating for mechanically protection of the fiber section.

The silica fiber length section may be a length section of a photonic crystal fiber, comprising a core region and a surrounding cladding region, said cladding region comprises an arrangement of voids extending along at least a part of the length of the silica fiber length section.

The phrase "polymer fiber length section" designates a fiber length section comprising an optical fiber having a core comprising or consisting essentially of polymer. The polymer fiber length section advantageously comprises a polymer cladding. The polymer may comprise impurities and/or doping materials e.g. for modifying the refractive index. In addition, the polymer fiber length section may comprise a coating, such as a polymer coating for mechanically protection of the fiber section.

The fiber sensor may for example be passed through the first end-fitting and into the annulus as described in EP2992261.

The annulus normally extends in the entire length of the unbonded flexible pipe from end-fitting to end-fitting. For many installations very long pipes are required, e.g. to reach a desired depth. It has been found that the pipe installation is very advantageous where the pipe is long and hence, the annulus is long. In an embodiment, the annulus has a length extending from the first to the second end-fitting, which is at least 300 m, such as at least 500 m, such as at least 1000 m, such as at least 2000 m or even longer.

The fiber sensor may advantageously extend relative long into the annulus, such that the fiber length located in the annulus is at least 25 m, such as at least 100 m, such as at least 500 m or even at least 1000 m or even longer. The attenuation of light in the silica fiber length section is relatively low, whereas the light attenuation in the polymer fiber length section is generally much higher. By providing that most of the length of the fiber sensor located in the annulus is silica fiber, the light signals propagating in the fiber may retain its strength even where it is propagating through a relatively long fiber sensor.

Advantageously, the part of the fiber sensor outside the annulus is silica fiber, e.g. such that the fiber sensor portion extending from the interrogator and through the first end-fitting is free of polymer fiber length section(s).

In an embodiment, the fiber sensor is extending into at least 50 % of the annulus length, such as at least 75 % of the annulus length, such as 90 % of the annulus length, such as substantially the entire annulus length. Thereby it is possibly to determine at least one characteristic value relating to a chemical environment in the annulus at practically any desired location in the annulus.

As mentioned above, most of the annular space is occupied by the helically wound elongate armor elements. During use these elongate armor elements are moved relative to each other due to varying radial and tensile forces provided by the action from e.g. water waves, vessel movements and the fluid transported in the pipe, thereby making the annulus a rather disturbed location. It has surprisingly been found that a reliable optical connection may be provided between the silica fiber length section and the polymer fiber length section within the annulus. Thereby it has now become possibly to perform chemical determinations at greater distance from the end-fitting, such as at hog bend locations, where the risk of undesired accumulation of corrosive fluid constituents may take place.

In an embodiment, at least one of the at least one polymer fiber length section is located at a distance from the first end-fitting which distance is at least 300 m, such as at least 5000 m, such as at least 1000 m, such as at least 2000 m. Chemical determinations, such a local annulus value(s) of humidity and/or pH may thus be determined at location far from the first end-fitting, such as at location of the annulus which are far below sea level and where the unbonded flexible pipe is subjected to very high hydrostatic pressure and/or very high bore pressure.

Advantageously, the at least one polymer fiber length section comprises a measuring site comprising a fiber section with a Fiber Bragg Grating (FBG).

The term "measuring site" is generally used to designate a local length of the fiber where the fiber is sensitive, preferably where the fiber comprises a grating.

The FBG (and thereby the measuring site of the fiber) may have a length of from a few mm to meters. Generally, it is desired that the FBG length is from 0.2 to 50 cm, such as from 2-20 cm. To ensure very local determinations it may be desired that the FBG length is 5 cm or less, such as 3 cm or less.

In an embodiment, the fiber sensor may comprise at least one silica fiber length section located in the annulus and the at least one silica sensor in the annulus may comprise a measuring site comprising a FBG. This silica fiber measuring site may be configured for determining strain and/or temperature. Thereby the fiber sensor may comprise polymer fiber measuring site(s) configured for determining a characteristic value relating to a chemical environment in an annulus and silica fiber measuring site(s) configured for determining strain and/or temperature. A silica fiber measuring site means a measuring site (preferably comprising a FBG) located on a silica fiber length section and corresponding meaning for a polymer fiber measuring site.

In an embodiment, the fiber sensor comprises two or more polymer fiber length sections located in the annulus, each preferably comprising at least one measuring site with a FBG. Thereby, measurement may be performed at two different locations in the annulus.

The fiber sensor may comprise two or more silica fiber length sections located in the annulus. One or more of these silica fiber length section may comprise a measuring site.

To ensure flexibility of the unbonded flexible pipe with low risk of damaging the fiber sensor, the fiber sensor is advantageously helically wound.

Preferably at least one of the armor layers of the unbonded flexible pipe is located in the annulus and comprises a helically wound armor element, wound with an angle a relative to a pipe axis and wherein the fiber sensor is wound with an angle a ± 10 degrees, preferably substantially the winding angle a. Thereby the fiber sensor may for example be located in a groove or cavity in the armor element or in a folding of the armor element. The risk of damaging the fiber sensor is thereby reduced.

The fiber sensor in the annulus may for example be mounted in an armor element according to the method of mounting a sensor arrangement as described in any one of EP 1407243, EP2745037 or US 2010/089478.

Advantageous the winding direction of the fiber sensor is the same as an amor layer containing the fiber sensor or below the fiber sensor in axial direction. The winding direction of an armor layer means the winding direction of the helically wound elongate armor elements of the armor layer.

In an embodiment, the fiber sensor is located in a groove of a helically wound elongate armor element. In an embodiment, the fiber sensor is located in a protection tube, in at least a part of its length. To ensure that the polymer fiber measuring site is exposed to the fluids in the annulus, the protection tube advantageously comprises one or more perforations located at measuring site(s) of the polymer fiber length section.

The protection tube may in an embodiment comprise a protection house arrangement in which at least one of the measuring site(s), e.g. a polymer fiber measuring site, is located. The protection house arrangement may ensure a desired contact to the fluid in the annulus, while simultaneously ensuring a high mechanical protection of the fiber sensor.

The pipe installation may comprise one or more additional fiber sensors. Such one or more additional fiber sensors may comprise one or more all silica fiber sensors and/or one or more silica-polymer fiber sensor comprising a silica fiber length section and a polymer fiber length section.

In an embodiment, the pipe installation comprises at least one additional fiber sensor located in the protection tube and the additional fiber sensor comprises a FBG measuring site located in the protection house arrangement, such that the protection house arrangement comprises two measuring sites, wherein the protection house arrangement is configured for ensuring that the polymer fiber sensor measuring site of the fiber sensor is in physical contact with the fluid in the annulus. The measuring site of the additional fiber sensor may advantageously be protected from coming into contact with the fluid in the annulus, e.g. by being fully encapsulated. Thus, the protection house arrangement may be configured for protecting the FBG measuring site of the additional fiber sensor from coming in physical contact with fluid(s) in the annulus. When the measuring site of the additional fiber sensor is a polymer fiber measuring site and this polymer fiber measuring site of the additional fiber is not in contact with annulus fluid, it is advantageous to use the measurement of the polymer fiber measuring site of the additional fiber to compensate for noise (measurement) related to temperature changes of measurement relating to a chemical environment of the annulus determined by the polymer fiber sensor measuring site of the fiber sensor. At the same time, the measurement of the polymer fiber measuring site of the additional fiber may be used for temperature determinations.

For some determination relating to the chemical environment in the annulus, such a pH determinations and/or certain gas concentration determinations, it may be advantageous that the polymer fiber length section comprises a coating comprising a chemically sensitive element. The coating with the chemically sensitive element may located at the measuring site. The chemically sensitive element may advantageously be chemically sensitive to concentration changes of H2S, CO2 and/or reaction products thereof and/or chemically sensitive to pH changes.

The chemically sensitive element may e.g. change color and/or absorbance and/or chemical structure when the concentration of the ion atom or molecule to which it is sensitive changes in the annulus. The change of color and/or of absorbance and/or of chemical structure advantageously results in a change of the wavelengths reflected by the FGB and/or a change of the peak power reflected by the FGB.

In a preferred embodiment, the polymer fiber length section is sensitive to changes of humidity in the annulus. It has been found that at least the measuring site(s) comprising the FBG of the polymer fiber length section is sensitive to changes of humidity and changes of temperature in the annulus. This may be determined as a change in the FBG reflected wavelength(s) compared to the light fed into the fiber from the light source arrangement of the interrogator.

Change in the FBG reflected wavelength(s) due to change of temperature, may be determined by a polymer fiber measuring site protected from coming into contact with fluid in the annulus, e.g. as described above. Change in the FBG reflected wavelength(s) due to change of humidity, may be determined by a polymer fiber measuring site in contact with fluid in the annulus, e.g. as described above, and optionally compensated for change in the FBG reflected wavelength(s) due to change of temperature.

Where the polymer fiber measuring site is embedded and/or anchored in an armor element of the armor, strain may be determined analogues.

In an embodiment, it is desired to determine temperature and/or strain via a silica fiber measuring site.

In an embodiment, it is desired to determine humidity and/or temperature via a polymer fiber measuring site.

It has been found that polymer fiber measuring site has a very operable sensitivity for change in humidity and that the humidity may be determined relatively fast.

The sensor system of a pipe installation may be very effective for performing local determinations of the annulus environment condition. This is very beneficial, since various parameters, such as temperature, humidity and/or pH in the annulus may vary largely along the length of the pipe.

The polymer fiber length section has a core or a core region of polymer and a surrounding cladding and/or cladding region, which may advantageously also be of polymer. The polymer may comprise impurities and/or doping materials. In addition, the polymer fiber length section may comprise a coating, such as a polymer coating for mechanically protection of the fiber section.

Advantageously, the polymer fiber length section comprises a polymer photonic crystal fiber (PCF), comprising a core region and a surrounding cladding region, the cladding region comprises an arrangement of voids extending along at least a part of the length of the polymer fiber length section.

The voids may advantageously be arranged in pattern selected from a hexagonal pattern, a circular pattern or a square pattern. The polymer fiber length section may be obtained by preparing a preform, having a desired pattern of voids and drawing the fiber in a drawing tower. To control the diameter and/or shape of the voids, the voids may advantageously be closed at a location at or near the end opposite to the drawing end. For additional control of the diameter and/or shape of the voids the pressure inside the voids may be controlled during the drawing e.g. as describer in US 7,320,232.

The preform may for example be prepared by casting, drilling of holes and/or by stacking of polymer tubes.

When the polymer fiber length section is a PCF, the core region and the cladding region are advantageously of identical material.

Example of suitable polymer materials for the polymer fiber length section includes poly(metyl methacrylate) (PMMA); polystyrene (PS); polycarbonate (PC); amorphous Perfluoropolymer, such as Cytop®; cyclic olefin copolymer (COC), such as Topas® COC resin.

The fiber sensor may be a single mode fiber or a multimode fiber. Advantageously the fiber sensor is a single mode fiber sensor supporting only one guided mode per polarization direction. Generally, a single mode fiber sensor provides a better signal-to-noise determination than multimode fiber sensors.

In an embodiment, the fiber sensor is a few mode fiber sensor e.g. comprising up to six linearly polarized modes. In this embodiment, the measurements are preferably performed using only the fundamental mode.

The silica fiber length section(s) and the polymer fiber length section are optically coupled to each other via a ferrule, the ferrule is preferably a ceramic ferrule. The optically coupling may advantageously be an adiabatic coupling. This may for example be provided by tapering of the core of one or more of the silica fiber length section or the polymer fiber length section adjacent to the coupling site.

In an embodiment, a fiber end of the silica fiber length section and a fiber end of the polymer fiber length section are mounted in the ferrule.

In an embodiment, the ferrule is a ceramic ferrule, such as a ferrule having an outer diameter of less than 2 mm, such as about 1.25 mm or less.

The fiber end of the silica fiber length section and the fiber end of the polymer fiber length section may for example be mounted in the ferrule by an adhesive. Preferably the ferrule has a lumen extending from a first to a second end of the ferrule and the fiber end of the silica fiber length section extend into the lumen via the first end and the fiber end of the polymer fiber length section extend into the lumen via the second end.

The fiber end of the silica fiber length section has a silica end facet and the fiber end of the polymer fiber length section has a polymer end facet.

To reduce the risk of undesired reflections at the silica facet and/or the polymer facet, one or both of these may comprise an anti-reflection coating.

Advantageously the silica end facet and the polymer end facet are in physical contact with each other.

Since the diameter of the lumen of the ferrule is advantageously only slightly larger in diameter, such as about 1 mm or less, such as about 100 pm or less, than respectively the fiber end of the silica fiber length section and the fiber end of the polymer fiber length section extending into the lumen, it may in some embodiments be difficult to bring the silica end facet and the polymer end facet into physical contact and there may be a small gab, such as up to about 3 mm, preferably less than 1 mm, such as less than 100 pm, between the silica end facet and the polymer end facet. This gab may e.g. be filled up with a material that is transparent for the wavelength(s) used in the measurement(s), such as the adhesive used for mounting the fiber end of the silica fiber length section and the fiber end of the polymer fiber length in the ferrule.

The adhesive may e.g. be an epoxy adhesive, such as an epoxy adhesive obtainable from Kientec Systems Inc. or Thorlabs Inc.

The interrogator comprises a light source arrangement for feeding light to the fiber sensor and for receiving reflected light. Advantageously, the light source arrangement is configured for emitting laser light comprising one or more selected wavelength(s) and/or a range of wavelengths.

In an embodiment, the light source arrangement comprises a broadband light source such as a light source spanning at least about 100 nm, such as about 300 nm or more. Advantageously, the light source arrangement comprises a light source emitting light comprising wavelength(s) in the visible range (380- 780 nm) and/or in the NIR range (780-1800 nm).

In an embodiment, the light source arrangement may comprise a supercontinuum laser, a superluminescent diode, a semiconductor optical amplifiers, an amplified spontaneous emission source and/or one or more single-wavelength lasers.

In an embodiment, the light source arrangement comprises a light source configured for emitting narrow wavelength laser light comprising a wavelength range spanning up to about 50 nm, such as up to about 25 nm.

In an embodiment, the light source arrangement comprise a "swept source" light source that can emit broadband light by scanning through an amount of narrowband light emitting steps.

The light source arrangement may be optically coupled to the fiber sensor for delivering light to the fiber sensor optionally via an optical distribution network. The optical distribution network may comprise various optical components, such as one or more filters, one or more beam-splitters, one or more gratings, one or more amplifiers and/or one or more optical circulators. The light source arrangement and the fiber sensor are advantageously adapted such that the fiber sensor is capable of propagating at least one wavelength from the light source arrangement. Advantageously, light source arrangement and the fiber sensor is matched to each other such that the fiber sensor is capable of propagating a substantial portion of the light power delivered to the fiber sensor from the light source arrangement, such as 20 % or more, preferably 50 % or more of the light power delivered to the fiber sensor from the light source arrangement.

The interrogator advantageously also comprises a spectrum analysis arrangement. Such spectrum analysis arrangements are well known in the art and are generally configured for receiving and analyzing the spectrum of the light reflected from the measuring site(s) of the fiber sensor. Advantageously, the spectrum analysis arrangement is further configured for receiving and analyzing the spectrum of the light fed to the fiber sensor from the light source arrangement and for comparing the reflected light with the light fed to the fiber sensor from the light source arrangement.

Advantageously, the spectrum analysis arrangement is optically coupled to the fiber sensor for receiving reflected light from the fiber sensor. The optically coupling may for example be via an optical distribution network. The optical distribution network may comprise various optical components, such as one or more filters, one or more beam-splitters, one or more gratings, one or more amplifiers and/or one or more optical circulators. One or more of the optical components of the optical distribution network for delivering light to the fiber sensor and the optical distribution network for receiving reflected light may be shared, such as the circulator.

Advantageously, the interrogator is configured for analyzing the received light and determine at least one characteristic property relating to a local environment located adjacent to the at least one polymer fiber length section.

Advantageously, the interrogator comprises processor means and is programmed for performing the analysis. The spectrum analysis arrangement for receiving and analyzing light reflected from the measuring site(s) may for example comprise one or more photodetectors, such as a single fiber-based broadband photodetector, a charge-coupled device spectrometer or one or more photodetectors for detecting different wavelength ranges. The bandwidth of the spectrum analyzing arrangement is preferably sufficiently large to detect the required wavelengths, such as, as large as the combined wavelength(s) fed to the fiber sensor and the wavelength propagated in the fiber sensor.

Advantageously, the at least one characteristic property comprises at least one of pH value, humidity, pressure and gas content, such as the content of one or more gas selected from H2S, CO2.

In an embodiment, the sensor system is configured for sensing least one characteristic property comprising humidity, pressure, gas content, strain, pH value and/or temperature.

As mentioned above the sensor system may comprise at least one additional fiber sensor selected from an all silica fiber and a silica-polymer fiber sensor comprising a silica fiber length section and a polymer fiber length section.

The unbonded flexible pipe is advantageously a riser pipe, preferably extending partly or fully between a seabed installation, such as a production well and a sea surface installation, such as a platform or a vessel.

In an embodiment, the riser pipe being arranged to have a local maximum in its height relative to sea surface and the at least one polymer fiber length section comprising one or more measuring sites is located in the annulus at the local maximum in height. Such local maximum in height may cause a local accumulation of moisture/ water in the annulus. This may be determined by performing determination(s) of humidity in such local maximum in height.

The at least one polymer fiber length section may be located at any position in the annulus. Advantageously, the at least one polymer fiber length section is located in the annulus at its lowermost end, such as at a distance from the second end-fitting up to about 100 m, such as up to about 50 m, such as up to about 25 m.

The length of the polymer fiber length section may be selected to be sufficient long to comprise at least one measuring site comprising a FBG. The polymer fiber length section may for example have a length op to about 10 m, such as from 2 cm to 5 m, such as from 3 cm to 3 m, such as from 4 cm to 1 m, such as from 5 to 50 cm.

All features of the inventions including ranges and preferred ranges can be combined in various ways within the scope of the invention, unless there are specific reasons not to combine such features.

BRIEF DESCRIPTION OF PREFERRED EMBODIMENTS AND PARTS THEREOF

The above and/or additional objects, features and advantages of the present invention will be further elucidated by the following illustrative and non^¬ limiting description of embodiments of the present invention, with reference to the appended drawings.

The figures are schematic and are not drawn to scale and may be simplified for clarity. Throughout, the same reference numerals are used for identical or corresponding parts.

Fig. 1 is a perspective view of a section of an unbonded flexible pipe.

Fig. 2 is a side view of an exploded unbonded flexible pipe with a fiber sensor arrangement located in the annulus.

FIG.3 illustrates an embodiment of a pipe installation comprising an unbonded flexible pipe extending from a seabed installation to a sea surface installation and a sensor system.

Fig. 4 illustrates an embodiment of a of a pipe installation comprising a riser and a flowline and where a polymer fiber length section is located in an annulus of the flowline. Fig. 5 is a schematic view of a fiber connection between an unbonded flexible pipe and an interrogator.

Fig. 6 illustrates a portion of a polymer fiber length section with the polymer fiber and a mechanical protection.

Fig. 7 schematically illustrates a section of a sensor housing comprising polymer fiber length sections.

Fig. 8a is a schematic view of an arrangement of polymer fiber length sections with respective measuring sites.

Fig. 8b is a schematic view of another arrangement of polymer fiber length sections with respective measuring sites.

Fig. 9 shows a ferrule coupling a silica fiber length section and a polymer fiber length section.

Fig. 10 illustrates a cross-sectional view of an end portion of an unbonded flexible pipe with an end-fitting and a fiber sensor extending into the annulus of the unbonded flexible pipe.

Fig. 11 illustrates a portion of an unbonded flexible pipe, with a fiber sensor extending into the annulus of the unbonded flexible pipe and where a part of the outer sheath and an outermost tensile armor has been removed.

The unbonded flexible pipe of fig. 1 comprises from inside and out a carcass 1, a pressure sheath 2, a pair of cross-wound tensile armoring layers 3a, 3b and an outer sheath 4. The pressure sheath 2 and the outer sheath 4 are liquid impervious and forms an annulus in which the tensile armor layers 3a, 3b are located. The unbonded flexible pipe of the pipe installation may be as the unbonded flexible pipe of fig. 2 or it may be a variation thereof e.g. comprising an intermediate polymer layer a pressure armor layer, an insulation layer and/or any other layer normally applied in unbonded flexible pipes. It should be mentioned that the carcass may be omitted, bur is usually desired, where the pipe is for use at deep water. Fig. 2 is a side view of an exploded unbonded flexible pipe with fiber length sections 5 located in the annulus. The unbonded flexible pipe of fig. 2 comprises from inside and out a carcass 1, a pressure sheath 2, a pressure armor layer 6, a pair of cross-wound tensile armoring layers 3a, 3b and an outer sheath 4. The three armor layers 3a, 3b, 6 are located in the annulus between the pressure sheath 1 and the outer sheath 4. The fiber length sections 5 form part of a fiber sensor optically connected to a not shown interrogator. The fiber length sections 5 are located in the outermost tensile armor layer 3b. Each of the fiber length sections 5 may advantageously be arranged in a protecting tube, which is helically wound in between windings of armor wires of the tensile armor layer 3b.

The pipe installation illustrated in fig. 3 comprises an unbonded flexible pipe 11 extending from a seabed installation 12 to a sea surface installation 13 and a sensor system comprising an interrogator 14 and a fiber sensor 15. unbonded flexible pipe 11, which in this example is a riser, comprises a first end-fitting 16a connecting the riser 11 to the surface installation 13 at or near the sea surface S. The riser 11 also comprises a second end-fitting 16b connecting the pipe to the seabed installation 12.

The fiber sensor 15 is optically connected to the interrogator 14 and extends from its optical connection to the interrogator 14, passing through said first end-fitting 16a and into an annulus of the riser 11, wherein the fiber sensor 15 comprises at least one silica fiber length section and at least one not shown polymer fiber length section, where at least one polymer fiber length is arranged in the annulus.

The interrogator comprises a light source arrangement 14a configured for emitting laser light and is optically coupled to said fiber sensor for feeding light to the fiber sensor 15. The interrogator^ also comprises a spectrum analysis arrangemenl4bt, optically coupled to the fiber sensor for receiving reflected light from the fiber sensor 15, where the optically couplings are via an optical distribution network 14c. The pipe installation illustrated in fig. 4 comprises a riser flexible pipe 22 and a flowline 21, which is an unbonded flexible pipe comprising a not shown annulus. The riser 22 has a first and a second end-fittings 25a, 26b and the flowline has a first and a second end-fitting 26c, where only the first end^¬ fitting 26c is shown. The riser 21 is connected to the flowline 22 via their respective end-fittings 26b, 26c such that fluid can be transported via the bore of the flowline 21 and through the bore of the riser 22.

An optical sensor 25 of a sensor system is extending from an optical connection to a not shown interrogator, passing through the first end-fitting 26a and into the riser, such as into an annulus of the riser, passing through the riser to its second end-fitting 26b to the first-end-fitting of the flowline and into the annulus of the flowline 21.

As indicated with the dotted boxes 27 the sensor system comprises two polymer fiber length section measuring sites. The length of the fiber sensor passing from its connection to the interrogator, through the riser pipe and into the flowline is preferably a silica fiber length section and the one or more polymer fiber length sections are located in the annulus of the flowline. Thereby it is also possible to perform measurements within the annulus of connected pipes located at large distance from the interrogator.

Fig. 5 illustrates a fiber connection between an unbonded flexible pipe 23 - e.g. a riser and an interrogator 24, wherein the fiber sensor 25 extends into an annulus of the unbonded flexible pipe 23 via an end-fitting 26 of the unbonded flexible pipe 23.

Fig. 6 illustrates a portion of a length section of the fiber sensor e.g. as located in a mechanical protection 30 in an annulus of an unbonded flexible pipe of an embodiment of a pipe installation of the invention. The fiber sensor comprises a silica fiber length section 31 and a polymer fiber length section 32 optically connected via a ferrule 33. The mechanical protection 30 is advantageously a tube, comprising openings 34 at least at the sections housing the polymer fiber length section. The polymer fiber length section comprises one or more measuring sites.

The polymer fiber length sections 41, 42 may be arranged in an annulus of an unbonded flexible pipe e.g. as described elsewhere herein. Each polymer fiber length section forms part of a fiber sensor optically connected to the same interrogator and arranged to receive the same light signal from the light source of the interrogator. Each of the polymer fiber length sections 41, 42 comprises a measuring site 41a, 42a formed by respective FBG in the fibers. A first of the measuring sites 41a is located in an impermeable housing 41b, such that the measuring site 41a is not in physical contact with fluid in the annulus. The impermeable housing 41b is advantageously not thermally insulating, such that the temperature inside the impermeable housing 41 is substantially identical to the temperature immediately outside the impermeable housing 41.

Both of the measuring sites 41a, 42a are located in a mechanical protecting housing 40, which is permeable for gas and/or liquid. Thereby the first measuring site 41a is not in contact with the fluid in the annulus and the second measuring site 42a is in physical contact with the fluid in the annulus. By using measurements from the two measuring sites 41a, 42a it will be possible to distinguish between signals originating from a change of a chemical parameter, e.g. pH value, and signals originating from temperature and changes thereof.

For example by comparing the signal measured by the first measuring site 41a with the signal measured by the second measuring site, a chemical parameter, such as pH value, can be determined without any substantial noise due to temperature changes.

Fig. 8a is a schematic view of an arrangement of polymer fiber length sections with respective measuring sites. Three fiber sensors 51, 52, 53 of a sensor system pass through an end-fitting 56 of an unbonded flexible pipe 55 and into an annulus of the unbonded flexible pipe 55. Each fiber sensor 51, 52, 53 comprises a silica fiber length section 51a, 52a, 53a and a polymer fiber length section 51b, 52b, 53b, each polymer fiber length section 51b, 52b, 53b comprises one or more measuring sites preferably comprising a FGB. The arrangement has been shown with three fiber sensors, but it may comprise as many fiber sensors as desired.

Fig. 8 is a schematic view of another arrangement of polymer fiber length sections with respective measuring sites. One fiber sensor 61 of a sensor system passes through an end-fitting 66 of an unbonded flexible pipe 65 and into an annulus of the unbonded flexible pipe 65. The fiber sensor 61 comprises three polymer fiber length section 61b, 62b, 63b, with a first silica fiber length section 61a and two in between silica fiber length sections 62a, 63a. Each polymer fiber length section 61b, 62b, 63b comprises one or more measuring sites preferably comprising a FGB. The silica fiber length sections 61a, 62a, 63a may be as long as desired since the attenuation in the silica fiber length sections is relatively low. The arrangement has been shown with three polymer fiber length section with measuring site(s). The skilled person will understand that the fiber sensor could include additional polymer fiber length section with measuring site(s).

The arrangements shown in fig. 8a and fig. 8 b may be combined.

Fig. 9 shows a ferrule coupling a silica fiber length section 71 and a polymer fiber length section 72. The ferrule is advantageously a ceramic ferrule and comprises a bore in which an end of the silica fiber length section 71 and an end of the polymer fiber length section 72 is mounted. The silica fiber length section 71 and a polymer fiber length section 72 is sealed to the ferrule using a temperature resistant glue, such an epoxy. The end facets of the silica fiber length section 71 and a polymer fiber length section 72 should advantageously be located with a little distance as possible, but in practice, a small distance 74 between the two facets may be acceptable. Fig. 10 illustrates a cross-sectional view of an end portion of an unbonded flexible pipe 81 with an end-fitting 80 and a fiber sensor 83 extending into the annulus 84 of the unbonded flexible pipe.

The unbonded flexible pipe 81 comprises an outer sealing sheath 81a surrounding two cross-wound tensile armor layers 81b, 81c. Inside the cross^¬ wound tensile armor layers 81b, 81c, the pipe comprises a number of other layers 81d, including at least a liquid impervious inner sealing sheath and preferably additional layers, which are usually present in unbonded flexible pipes e.g. as described above or as described in "Recommended Practice for Flexible Pipe", ANSI/API 17 B, and the standard "Specification for Unbonded Flexible Pipe", ANSI/API 17J. The layers 81d inside the cross-wound tensile armor layers 81b, 81c will usually be terminated individually, as shown schematically in the drawing with the terminating unit 85.

The end-fitting 80 comprises an annular end-fitting body structure with a termination section 80a and an end-fitting body 80c with a narrow section and a mounting flange 80d with holes 80e for mounting to another part, e.g. another end-fitting or to a platform or a vessel. A housing cavity 80b is formed between the termination section 80a and the end-fitting body 80c.

The end-fitting 80 further comprises an annular outer casing 19.

The outer sealing sheath 81d is terminated at a termination area 86 in a well- known manner. The tensile armor elements of the tensile armor layers 81b,

81c are terminated and secured by securing material in the housing cavity 80b of the end-fitting 80.

A length of the optical sensor fiber is arranged in the annulus between the outer sheath 81a and the pressure sheath. The end-fitting 80 comprises a fiber exit cavity 87, with an opening 87a through which the fiber sensor 83 passes. The fiber is applied to have an overlength in the fiber exit cavity 87 to protect against movements between the interrogator and the end-fitting 80

Fig. 11 illustrates a portion of an unbonded flexible pipe, with a fiber sensor extending into the annulus of the unbonded flexible pipe and where a part of the outer sheath 91 and an outermost tensile armor layer 92 has been removed. The unbonded flexible pipe comprises an annulus located between a not shown pressure sheath and the outer sheath 91. A number of armor layers are located in the annulus, including the tensile layer 92 and a further below lying tensile armor layer 93 a length section of a fiber sensor 95 is helically wound between windings of armor wires of the below lying tensile armor layer 93. The part or a part of the sensor fiber length located in the annulus is advantageous located in a protecting housing e.g. a tube, which may be fluid permeable. The end-fitting 96 comprises a mounting flange 96a and an exit collar through which the fiber sensor passes.

Claims

PATENT CLAIMS

1. A pipe installation for transportation of a fluid, the installation comprises an unbonded flexible pipe and a sensor system, the unbonded flexible pipe comprises a first and a second end-fitting, a plurality of layers extending from the first to the second end-fitting, said plurality of layers comprises a plurality of armor layers and at least two fluid tight polymer layers forming an annulus, wherein at least one of said armor layers is arranged in the annulus, the sensor system comprises an interrogator arrangement and at least one fiber sensor optically connected to said interrogator, said fiber sensor has a length and is extending from its optical connection to the interrogator, passing through said first end-fitting and into said annulus, wherein said fiber sensor comprises at least one silica fiber length section and at least one polymer fiber length section, said at least one polymer fiber length section being arranged in said annulus.

2. The pipe installation of claim 1, wherein the annulus has a length extending from the first to the second end-fitting, the annulus length is preferably at least 300 m, such as at least 500 m, such as at least 1000 m, such as at least 2000 m.

3. The pipe installation of claim 2, wherein the fiber sensor is extending into at least 50 % of the annulus length, such as at least 75 % of the annulus length, such as 90 % of the annulus length, such as substantially the entire annulus length.

4. The pipe installation of any one of the preceding claims, wherein at least one of the at least one polymer fiber length section is located at a distance from the first end-fitting which distance is at least 300 m, such as at least 500 m, such as at least 1000 m, such as at least 2000 m.

5. The pipe installation of any one of the preceding claims, wherein the at least one polymer fiber length section comprises a measuring site comprising a fiber section with a Fiber Bragg Grating (FBG).

6. The pipe installation of any one of the preceding claims, wherein the fiber sensor comprises at least one silica fiber length section located in the annulus, said at least one silica sensor located in the annulus preferably comprises a measuring site comprising a fiber section with a FBG.

7. The pipe installation of any one of the preceding claims, wherein the fiber sensor comprises two or more polymer fiber length sections located in said annulus, each preferably comprising at least one measuring site comprising a fiber section with a FBG.

8. The pipe installation of any one of the preceding claims, wherein the fiber sensor comprises two or more silica fiber length sections located in said annulus, each preferably comprising at least one measuring site comprising a fiber section with a FBG.

9. The pipe installation of any one of the preceding claims, wherein the fiber sensor is helically wound, preferably at least one of said armor layers of said unbonded flexible pipe is located in the annulus and comprises a helically wound armor element, wound with an angle a relative to a pipe axis and wherein said fiber sensor is wound with an angle a ± 10 degrees, preferably substantially the winding angle a.

10. The pipe installation of any one of the preceding claims, wherein the fiber sensor is located in a groove of a helically wound elongate armor element.

11. The pipe installation of any one of the preceding claims 1-9, wherein the fiber sensor is located in a protection tube, in at least a part of its length.

12. The pipe installation of claim 11, wherein the protection tube comprises perforations located at measuring site(s) of said polymer fiber length section.

13. The pipe installation of claim 11 or claim 12, wherein the protection tube comprises a protection house arrangement in which at least one of said measuring site(s) are located.

14. The pipe installation of claim 13, wherein said at least one measuring site comprises a measuring site of said polymer fiber length section and said protection house arrangement allows the polymer fiber measuring site to come physically into contact with fluid(s) located in said annulus.

15. The pipe installation of claim 13 or claim 14, wherein said pipe installation comprises at least one additional fiber sensor located in said protection tube, said additional fiber sensor comprises a FBG measuring site located in said protection house arrangement, preferably said protection house arrangement is configured for protecting said FBG measuring site of said additional fiber sensor from coming in physical contact with fluid(s) in said annulus, more preferably said at least one additional fiber sensor is a polymer fiber length section containing additional fiber sensor and said FBG measuring site is a polymer fiber length section FBG measuring site.

16. The pipe installation of any one of the preceding claims, wherein the polymer fiber length section comprises a coating comprising a chemically sensitive element, preferably the coating with the chemically sensitive element is located at the measuring site.

17. The pipe installation of claim 16, wherein the chemically sensitive element is chemically sensitive to concentration changes of H2S, CO2 and/or reaction products thereof and/or chemically sensitive to pH changes.

18. The pipe installation of any one of the preceding claims, wherein the polymer fiber length section is sensitive to changes of moisture in the annulus, preferably at least the measuring site(s) of the polymer fiber length section is sensitive to changes of moisture in the annulus.

19. The pipe installation of any one of the preceding claims, wherein said polymer fiber length section comprises a photonic crystal fiber, comprising a core region and a surrounding cladding region, said cladding region comprises an arrangement of voids extending along at least a part of the length of the polymer fiber length section.

20. The pipe installation of any one of the preceding claims, wherein said polymer fiber length section comprises a material selected from poly(metyl methacrylate) (PMMA), polystyrene (PS), polycarbonate (PC), amorphous Perfluoropolymer and/or cyclic olefin copolymer (COC).

21. The pipe installation of any one of the preceding claims, wherein the fiber sensor is a single mode fiber sensor.

22. The pipe installation of any one of the preceding claims, wherein said silica fiber length section(s) and said polymer fiber length section are optically coupled to each other via a ferrule, the ferrule is preferably a ceramic ferrule.

23. The pipe installation of any one of the preceding claims, wherein said silica fiber length section and said polymer fiber length section are optically coupled to each other via a ferrule, wherein a fiber end of the silica fiber length section and a fiber end of the polymer fiber length section are mounted in the ferrule.

24. The pipe installation of claim 23, wherein the ferrule is a ceramic ferrule, such as a ferrule having an outer diameter of less than 2 mm, such as about 1.25 mm or less.

25. The pipe installation of claim 23 or claim 24, wherein the fiber end of the silica fiber length section and the fiber end of the polymer fiber length section are mounted in the ferrule by an adhesive, preferably the ferrule has a lumen extending from a first to a second end of the ferrule and where in the fiber end of the silica fiber length section extend into the lumen via the first end and the fiber end of the polymer fiber length section extend into the lumen via the second end.

26. The pipe installation of any one of the preceding claims, wherein said interrogator comprises a light source arrangement, said light source arrangement is configured for emitting laser light comprising a selected wavelength or range of wavelength.

27. The pipe installation of any one of the preceding claims, wherein light source arrangement is optically coupled to said fiber sensor for feeding light to said fiber sensor optionally via an optical distribution network.

28. The pipe installation of any one of the preceding claims, wherein said interrogator comprises a spectrum analysis arrangement, said spectrum analysis arrangement being optically coupled to said fiber sensor for receiving reflected light from the fiber sensor, said optically coupling is optionally via an optical distribution network.

29. The pipe installation of claim 28, wherein said interrogator is configured for analyzing said received light and determine at least one characteristic property relating to a local environment located adjacent to said at least one polymer fiber length section.

30. The pipe installation of claim 29, wherein said at least one characteristic property comprises at least one of pH value, humidity, pressure and gas content, such as the content of one or more gas selected from H2S, CO2.

31. The pipe installation of any one of the preceding claims, wherein said sensor system is configured for sensing least one characteristic property comprising humidity, pressure, gas content, strain and/or temperature.

32. The pipe installation of any one of the preceding claims, wherein said sensor system comprises at least one additional fiber sensor selected from an all silica fiber and a silica-polymer fiber sensor comprising a silica fiber length section and a polymer fiber length section.

33. The pipe installation of any one of the preceding claims, wherein said unbonded flexible pipe is a riser pipe, preferably extending partly or fully between a seabed installation and a sea surface installation.

34. The pipe installation of claim 33, wherein said riser pipe being arranged to have a local maximum in its height relative to sea surface and said at least one polymer fiber length section being located in the annulus at said local maximum in height.

35. The pipe installation of claim 33, wherein said least one polymer fiber length section being located in the annulus at its lowermost end, at a distance from the second end-fitting up to about 100 m, such as up to about 50 m, such as up to about 25 m.

36. The pipe installation of any one of the preceding claims, wherein said polymer fiber length section has a length up to about 10 m, such as from 2 cm to 5 m, such as from 3 cm to 3 m, such as from 4 cm to 1 m, such as from 5 to 50 cm.