DK201600396A1 - Vinylidene fluoride polymer outer sheath for unbonded flexible pipe - Google Patents

Abstract

This invention is made by Solvay Specialty Polymers and National Oilwell Varco in cooperation. Unbonded flexible pipes are suitable for use as riser for offshore production of petrochemical products and, in particular, for transportation of hydrocarbon containing products, such as crude oil, from a subsea installation such as a production well to a sea surface installation such as a platform or a vessel.

Description

Vinylidene fluoride polymer outer sheath for unbonded flexible pipe

This invention is made by Solvay Specialty Polymers and National Oilwell Varco in cooperation.

Unbonded flexible pipes are suitable for use as riser for offshore production of petrochemical products and, in particular, for transportation of hydrocarbon containing products, such as crude oil, from a subsea installation such as a production well to a sea surface installation such as a platform or a vessel.

Flexible unbonded pipes are for example described in the standard "Recommended Practice for Flexible Pipe", ANSI/API 17 B, fourth Edition, July 2008, and the standard "Specification for Unbonded Flexible Pipe", ANSI/API 17J, Third edition, July 2008.

Such pipes usually comprise an internal pressure sheath, often referred to as an inner sealing sheath or inner liner, which forms a barrier against the outflow of the fluid which is conveyed in the bore of the pipe, and one or usually a plurality of armor layers.

The pipe usually further comprises an outer sheath such as an outer protection layer which provides mechanical protection of the armor layers. Very often, the outer sheath is a sealing layer sealing against ingress of sea water, e.g. to protect metal armor layers.

In certain unbonded flexible pipes, one or more intermediate sealing layers are arranged between the armor layers.

Thus, the unbonded flexible pipes are constructed of a number of independent layers where at least two or more of the armor layers are not interfacially bonded to each other directly or indirectly via other layers.

The unbonded flexible pipes are usually very long, e.g. 500 meters or longer such as up to several kilometers, and typically have a relatively large diameter.

When the petroleum product is harvested, it is often under high pressure and it has a very high temperature, which means that all the layers of the pipe are subjected to high thermal stress. Simultaneously, the polymer layers and, in particular, the outer sheath are subjected to high mechanical stress, in particular where the outer protection layer is in contact with other elements such as buoyancy modules, mid water arc or other support elements, bend restrictors or bend limiters as well as connecting elements such as end-fitting or where the pipe is connected to other elements.

Further, where the outer sheath is in contact with other elements, it is no longer cooled by water and also, where the pipe reaches above water, it is not subjected to water cooling, which often results in that the outer sheath is subjected to a very high temperature which may damage the outer sheath, in particular where it simultaneously is subjected to high mechanical stress.

We describe hereinafter an unbonded flexible pipe suitable for use in offshore production of petrochemical fluids, such as crude oil, where the unbonded flexible pipe comprises an outer layer made of a vinylidene fluoride polymer layer which is an alternative polymer relative to the prior art vinylidene fluoride polymers.

It has been found that the vinylidene fluoride polymer outer sheath of the present unbonded flexible pipe is advantageuosly endowed with a high temperature resistance, a high heat stability as well as a high mechanical stability and a high flexibility, and is relatively easy to produce in long lengths compared to prior art vinylidene fluoride polymers.

The unbonded flexible pipe described herein is in particular suitable for offshore transportation of petrochemical fluids as well as other fluids. The pipe is in general suitable for offshore applications where it may be subjected to very high and/or very low temperatures and in particular where a relatively high degree of flexibility is desired.

The unbonded flexible pipe comprises an outer sheath with a high temperature resistance to both high (> 110°C) as well as very low (- 30°C) temperatures, while simultaneously having a high and lasting flexibility and toughness over its time of use which usually spans over about 20 years or longer.

It has been found that the vinylidene fluoride polymer of the outer sheath of the present unbonded flexible pipe has a high melt strength which makes it very suitable for forming a uniform sheath by extrusion.

The outer sheath made of the polymer (F) of the present unbonded flexible pipe has a very smooth outer surface.

The unbonded flexible pipe comprises an outer sheath made of a fluoropolymer [polymer (F)] comprising: - from 94% to 97% by moles, preferably from 95% to 96.5% by moles of recurring units derived from vinylidene fluoride (VDF), - from 3% to 5% by moles, preferably from 3.5% to 4.5% by moles of recurring units derived from hexafluoropropylene (HFP), and - optionally, up to 0.5% by moles of recurring units derived from one or more other fluorinated or hydrogenated monomers.

The polymer F may comprise the monomers in form of a compound of a homopolymer of VDF and a co-polymer of VDF and HFP.

The co-polymer may be present in an amount of more than 55 % by weight.

The polymer (F) advantageously has a melting point comprised between 148°C and 152°C.

The polymer (F) has a melt flow index comprised between 1 and 5 g/10 min, preferably between 2 and 4 g/10 min, as measured at 230°C under a load of 10 kg (ASTM D 1238/ISO 1133).

The polymer (F) has a melt viscosity comprised between 10000 and 20000 Pa . s, as measured at 230°C and at a shear rate of 1 s-1.

The polymer (F) has an elastic modulus (E) comprised between 600 and 900 MPa, as measured at 23°C.

The polymer (F) has an elastic modulus (E) comprised between 140 and 200 MPa, as measured at 110°C.

The polymer (F) is typically obtained by suspension polymerization or emulsion polymerization.

The polymer (F) is preferably obtained by suspension polymerization through radical polymerization using a suitable initiator such as t-amyl-per-pivalate.

The polymer (F) is preferably marketed by Solvay under the trademark name SOLEF® 11112.

In another aspect the unbonded flexible pipe comprises an outer sheath made of a fluoropolymer referred to as polymer (F-l) consisting essentially of: - about 91.2% by weight of recurring units derived from vinylidene fluoride (VDF), and - about 8.8% by weight of recurring units derived from hexafluoropropylene (HFP).

The composition of the polymer (F) is determined using NMR, such as 19F-NMR, e.g. as described in "Composition and sequence distribution of vinylidene fluoride copolymer and terpolymer fluoroelastomers. Determination by 19F-NMR spectroscopy and correlation with some properties", M. Pianca et al., Polymer, 1987, Vol. 28, 224-230.

The polymer F may comprise the monomers in form of a compound of a homopolymer of VDF and a co-polymer of VDF and HFP.

The co-polymer may be present in an amount of more than 55 % by weight.

The polymer (F-l) has a melting point of about 151°C.

The polymer (F-l) has a melt flow index of about 3 g/10 min, as measured at 230°C under a load of 10 kg (ASTM D 1238/ISO 1133).

The polymer (F-l) has an elastic modulus (E) of about 741 MPa, as measured at 23°C.

The polymer (F-l) has an elastic modulus (E) of about 171 MPa, as measured at 110°C.

Unbonded flexible pipes are described in the standard "Recommended Practice for Flexible Pipe", ANSI/API17 B, fourth Edition, July 2008, and the standard "Specification for Unbonded Flexible Pipe", ANSI/API 17J, Third edition, July 2008.

The unbonded flexible pipe advantageously comprises, from the inside to the outside of the pipe, an internal armor, often referred to as a carcass, a pressure sheath defining the bore of the pipe in which a fluid to be transported may flow, a pressure armor for example comprising helically wound armor elements wound with a relatively high angle to the longitudinal axis of the pipe, a tensile armor, such as a pair of layers of cross wound armor elements, preferably wound with a relatively low angle to the longitudinal axis of the pipe, e.g. 55 degrees or less, preferably 45 degrees or less and an outer sheath protecting the underlying layer(s).

The unbonded flexible pipe may comprise more or fewer armor layers and the armor layers may be arranged in a different order. The unbonded flexible pipe may further comprise antifriction layer or layers arranged above and/or below the armor layer(s) for reducing friction between the layers.

The unbonded flexible pipe may further comprise additional layers or elements which are well known in the art, e.g. for insulating, for outgassing, for monitoring and/or for reducing risk of vortex generation.

Advantageously, the outer sheath made of the polymer (F) of the present unbonded flexible pipe is the outermost liquid impermeable sheath.

An outer sheath of the unbonded flexible pipe is herein defined as the outermost polymer sheath of the unbonded flexible pipe which fully encircles an underlying layer and extends in a substantial length of the unbonded flexible pipe, such as at least about 5 m, such as at least about 10 m, such as about 50 m or more.

Preferably, the outer sheath is sealing the pipe against ingress of sea water.

The outer sheath advantageously has a thickness comprised between about 1 mm and about 10 mm, preferably between 2 mm and 5 mm.

The outer sheath made of the polymer (F) of the present unbonded flexible pipe is a wound layer, preferably in the form of an extruded and wound foil. The wound foil advantageously forms a holding layer for a tensile armor layer to prevent buckling of the tensile armor element which is also called bird cage effect.

Where the unbonded flexible pipe comprises a pair of cross wound tensile armor layers including an outermost tensile armor layer, the wound foil forms the outer sheath and advantageously the foil is wound directly outside the outermost tensile armor layer with a winding direction opposite to a winding direction of the outermost tensile armor layer. The holding foil is preferably wound with a relative high angle to the axis of the pipe.

The outer sheath made of the polymer (F) of the present unbonded flexible pipe, when forming the holding layer, is advantageously reinforced by fibers and/or a row of fibers are wound outside the outer sheath to increase the holding strength.