Subsea umbilical

Abstract

An umbilical for subsea applications has at least one longitudinal internal element and a sheath, the sheath is formed by extrusion. The internal element is suitable for communicating fluids, electrical power or signals, or for carrying loads. The sheath is made of a polymer composite having a high density filler, the polymer composite having a density in the range 3 to 11 g/cm3.

Claims

1. An umbilical power cable for subsea applications comprising: at least one longitudinal internal element; and a sheath, the sheath is formed by extrusion, wherein said internal element is suitable for communicating fluids, electrical power or signals, or for carrying loads, said internal element being selected from optical fibers and electrical quads, and wherein the sheath is made of a polymer composite having a high density filler, the polymer composite having a density in the range 6 to 11 g/cm.sup.3, and wherein said sheath is the outermost layer of the cable.

2. The umbilical power cable according to claim 1, wherein the amount of high density filler is in the range of 20 to 90 w/w % based on the total weight of the polymer composite.

3. The umbilical power cable according to claim 1, wherein the high density filler is metal based, the metal selected from the group consisting of chromium, nickel, copper, copper oxide, steel, iron, iron oxide, barium sulfate, tungsten, molybdenum and mixtures thereof, and having a density of more than 4 g/cm.sup.3.

4. The umbilical power cable according to claim 1, wherein the polymer in the polymer composite has at least one of high density polyethylene (HDPE), polyethylene (PE), polypropylene (PP), polyurethane (PU), polyamide (PA) and polybutylene terephthalate (PBT).

5. A method of manufacturing an umbilical power cable according to claim 1, said umbilical having a minimum submerged weight to length (kg/m), said method comprising the step of: providing at least one longitudinal element suitable for communicating fluids, electrical power or signals, or for carrying loads, said internal element being selected from optical fibers and electrical quads; determining the density and thickness of a sheath required to obtain the minimum submerged weight to length (kg/m); and extruding a sheath around the longitudinal element, the sheath made of a polymer composite comprising a high density filler and having a density in the range of 6 to 11 g/cm.sup.3 such that the minimum submerged weight to length (kg/m) is obtained.

6. A method according to claim 5, wherein the amount of high density filler is in the range of 20 to 90 w/w % based on the total weight of the polymer composite.

7. A method according to claim 5, wherein the high density filler is metal based, the metal is selected from the group consisting of chromium, nickel, copper, copper oxide, steel, iron, iron oxide, barium sulfate, tungsten, molybdenum and mixture thereof.

8. A method according to claim 5, wherein the polymer in the polymer composite has at least one of HDPE (high density polyethylene), PE (polyethylene), PP (polypropylene), PU (polyurethane), PA (polyamide) and PBT (polybutylene terephthalate).

9. The umbilical power cable according to claim 1, wherein the polymer composite comprises polyamide (PA) and/or polyurethane (PU) as the polymer(s), and tungsten as the high density filler in an amount of 10-60 wt % based on the total weight of the polymer composite.

10. The umbilical power cable according to claim 1, wherein the specific gravity SG of the umbilical is 1.5-3.0.

11. The umbilical power cable according to claim 1, wherein the polymer composite comprises polyamide (PA) as the polymer, and 10-30 wt % of chromium, and/or 10-30 wt % of nickel, and/or 1-5 wt % of molybdenum as the high density filler(s).

12. The umbilical power cable according to claim 1, wherein the polymer composite comprises polyamide (PA) and/or polypropylene (PP) as the polymer(s), and barium sulfate as the high density filler in an amount of 60 wt % or more, based on the total weight of the polymer composite.

13. The umbilical power cable according to claim 1, wherein the umbilical power cable has several longitudinal internal elements, and said longitudinal internal elements are multiple hydraulic lines comprising a steel tube, multiple electrical quads and multiple optical fibers.

Description

SHORT DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a cross sectional view of a prior art umbilical.

(2) FIG. 2 is a cross sectional view of an umbilical according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

(3) A subsea umbilical comprising a prior art solution for obtaining a specific w/d ratio, minimum submerged weight per length (kg/m), or specific gravity, is shown in FIG. 1. The cross sectional view is of a 127 km long umbilical which the applicant delivered to Total for the Laggan Tormore field. This specific umbilical comprises multiple hydraulic lines comprising a steel tube and a surrounding high density polyethylene (HDPE) sheath, multiple electrical quads, fibre optic elements, PP filler, profiled PE filler, PP yarn and an outer HDPE sheath 1. To achieve a required specific gravity of 1.82 in seawater (corresponding to a submerged weight to diameter ratio of 81.8 kg/m), 4 layers of steel tape 2 were added to the umbilical. Both the specific gravity and the weight to diameter ratio are calculated based on the tubes and interstices of the umbilical being flooded with seawater. The minimum submerged weight per length (kg/m) is similarly calculated based on the tubes and interstices of the umbilical being flooded with seawater.

(4) An umbilical according to the invention is shown in FIG. 2. The umbilical comprises the same internal elements as described in relation to FIG. 1. However, to obtain an umbilical having the same specific gravity as the one shown in FIG. 1, both the outer HDPE sheath 1 and the layers of steel tape 2 are replaced by an outer sheathing of a polymer composite 3 comprising a high density filler. The density of the polymer composite is such that the umbilical obtains the required specific gravity without use of any excess layers of steel tape. The thickness of the polymer composite layer 3 depends on its density and other properties such as abrasion resistance.

(5) The polymer used in the polymer composite may comprise any suitable synthetic polymer base suitable for continuous extrusion, such as, but not limited to, HDPE (high density polyethylene), PE (polyethylene), PP (polypropylene) PU (polyurethane), PA (polyamide) and PBT (polybutylene terephthalate).

(6) The polymer(s) of the polymer composite may preferably be selected from the group of thermoplastic elastomers (TPE).

(7) Further, the polymer composite may be applied to the umbilical using a conventional extrusion process, for instance as used when applying a standard HDPE sheathing.

(8) The filler used in the polymer composite is a high density filler having a density of more than 4 gi cm.sup.3, more than 5 g/cm.sup.3, or more than 6 g/cm.sup.3. The high density filler is advantageously a metal-based filler such as chromium, nickel, copper, copper oxide, steel, iron, iron oxide, barium sulfate, tungsten and molybdenum, or similar. The high density filler may be in any form suitable for an extrudable polymer composite, e.g. particles and fibres.

(9) A number of high density fillers and polymer composites comprising such fillers are commercially available, for instance those used in the Gravi-Tech compounds available from PolyOne Corporation. Further, various polymer composites suitable for extrusion, comprising high density fillers such as tungsten, are disclosed in U.S. Pat. No. 6,916,354 B2.

(10) The polymer composite may advantageously have a density in the range of 3 to 11 g/cm.sup.3, 4 to 11 g/cm.sup.3, 5 to 11 g/cm.sup.3 or 6 to 11 g/cm.sup.3.

(11) A preferred polymer composite comprises PA (polyamide) and/or PU (polyurethane) as the polymer(s), and tungsten as the high density filler in an amount of 10-60 wt % based on the total weight of the polymer composite.

(12) Another suitable polymer composite can comprise PA as the polymer, and 10-30 wt % of chromium, and/or 10-30 wt % of nickel, and/or 1-5 wt % of molybdenum as the high density filler(s).

(13) Polymer composites in the lower density range may comprise PA (polyamide) and/or PP (polypropylene) as the polymer(s), and barium sulfate as the high density filler in an amount of 60 wt % or more, based on the total weight of the polymer composite.

(14) The present invention provides a number of advantages such as a more cost effective production since less outer steel armouring is required. This both saves raw material cost and reduced manufacturing time in the armouring machine. It also reduces the need for intermittent storage of semi-finished product on turn tables. Further, for certain design requirements the outer steel armouring can be completely omitted.

(15) In one embodiment, the umbilical of the invention further has armour elements such as armour wires layer (traditional armouring process) or outer steel armouring, specifically for additional mechanical protection and for tensile strength. Thus, the armour elements are used in combination with the high density composite sheath of the invention.

(16) In another embodiment, the umbilical of the invention does not comprise any armour elements such as armour wires layer (traditional armouring process), outer steel armouring, excess steel armour comprising polyethylene (PE)-sheathed steel wires incorporated in the umbilical during the lay-up process, or steel armouring wound around the element bundle of the umbilical after lay-up (traditional armouring process) or other composite armour elements or several layers of metallic (e.g. steel) tape.

(17) Another possible advantage is that the electrical properties of the sheath can be affected by the type of high density filler, in the form of a metal, which is added to the polymer composite. In a possible embodiment such a sheath can be made semi conductive for applications where this is needed, e.g. an inner sheath of power umbilicals.

(18) A further advantage is that a polymer composite comprising for instance a metal based high density filler is harder than a HDPE sheath and will in many instances provide a better mechanical protection than the HDPE sheath used in current designs.

(19) In a prior art umbilical the required diameter necessary to obtain a required minimum submerged weight per length (kg/m) will be larger than the one necessary with the proposed polymer metal composite sheath. This means that longer delivery lengths can be achieved for a given reel/basket length capacity since the outer diameter is smaller than a traditionally armoured umbilical.