EP1409908A1

EP1409908A1 - Control cable

Info

Publication number: EP1409908A1
Application number: EP01932422A
Authority: EP
Inventors: Tom Grimseth
Original assignee: Havtroll AS
Current assignee: Havtroll AS
Priority date: 2000-05-05
Filing date: 2001-05-07
Publication date: 2004-04-21
Also published as: AU2001258949A1; US20030103811A1; WO2001086183A1; NO20002382D0

Abstract

The present invention relates to a cable for supplying chemicals to, and for the control of, a sub-sea hydrocarbon well comprising an external pressure pipe (31) enclosing a number of smaller pipes (32, 33), the cable having the special features that the external enclosing pipe (31) forms a channel for the hydration inhibiting agent and in addition forms an element that takes up mechanical tension in the cable, protects the internal pipes (32, 33) against radial forces and forms a nearly corrosion free environment protecting the smaller, internal pipes (32, 33).

Description

CONTROL CABLE

Field of the invention

The present invention relates to a control cable (umbilical cord) for sub sea field developments producing hydrocarbons, especially sub sea installations tied back to a central production platform by means of a flow line and an umbilical cord, where the umbilical cord supplies the sub- sea production installation with chemicals and control/monitoring functions.

State-of-the-art

During the 25 years of developing sub sea petroleum technology 2 types of sub sea control/service umbilical have become commonplace .

I . So called bundled design, where a relatively large tube, typically a 2" tube for hydrate inhibitor, is strapped to a cable consisting of a number of small bore tubes and electrical conductors, which are laid up in a helically twisted pattern, and protected by an outer sleeve. Armour may be added as required.

This design is characterised by relatively moderate mate- rial-and construction costs. The laying process is slowed down by the requirement for strapping of several elements on deck during deployment. Fig. I illustrates a typical design as described. The design includes a tube for hydrate inhibitor II, a saddle 12, on outer sleeve 13, typically of polyethylene, an inner sleeve 13', small bore tubes 14 for a variety of chemicals/hydraulic power, sepa- rated from each other by means of saddles 18, an electrical centre section 15 containing a quad 16, a strap 17 holding the hydrate inhibitor Tube 11 with the cable containing the tubes 14 and centre section 15 and armour 19.

2 . A so-called integrated design characterised by a requirement for lying of only one single element . This design is based on the hydrate inhibitor tube being located in the centre of the umbilical cord with small bore tubes and electrical cables twisted around the centre tube in a symmetrical and helical pattern. There is an industrial practice for using 25% Cr steel for all tubes in order to resist corrosion caused by sea water penetrating the interior of the umbilical cord. Particularly lines for supply of hy- draulic power fluid require high cleanliness standards and consequently often leads to requirement for high quality alloy steel .

A mixture of dissimilar metals may be disadvantageous in a marine environment. This has often resulted in selection of high quality expensive alloys for all tubulars in a sub sea control umbilical. Fig. 2 illustrates a typical design. This design includes centre tube 21 for hydrate inhibitor, small bore tubes 22 for chemicals/ hydraulic fluid as well as quads 24, separated by spacers 23, and an outer sleeve 25, typically made for polyethylene .

Both the referred main types of design have been exten- sively and successfully used for field development projects. For large robust field developments the costs associated with construction and installation are normally acceptable. However, in the tailing end of the life of an oil province there will be a large number of small hydrocarbon deposits to be developed by tie-back to exist- ing infra-structure. Such field developments may be viable but require more cost effective designs than the developments of larger fields. For development of marginal fields the referred techniques may be associated with undesirable costs. The following patents are referred to as representative of this technique:

NO 160389, GB 2246410 A, US 4709730, NO 304533. The referred methods are considered of little relevance to control cables/umbilical cords.

Purpose of the invention

The objective of the present invention is to achieve tubes and electrical optic cables accommodated inside of an outer carrier pipe. Such design (bundled pipe) is common for flow line systems for the purpose achieving temperature control of fluids in a flow line. Separate pipes for heating water is added to one or more flow lines accommodated inside an outer carrier pipe. Such bundles cannot be reeled on to a lay barge, but are towed to the final destination by several tugs.

It is the objective of the present invention to eliminate the disadvantages identified for the referred techniques and it is aimed at combining low construction cost with cost effective installation techniques. The invention has probably an economical limit related to the number of wells operated by means of a single cable of the proposed Design. It may typically be suitable for fields with 1 - 4 wells connected to a flow line and controlled by means of a single cable (umbilical cord) .

The invention is aimed at saving material cost, as only lower and less costly steel quality is required due to the novel design. For some scenarios significant costs may be saved associated with installation of the cable. A further objective is to achieve a well control module, which requires fewer control lines (hydraulic) of lower capacity. This may be achieved by means of a modification of the hydraulic circuitry of the well control module.

Brief description of the invention The objectives referred above are achieved by means of an arrangement characterised by the features included in the characterising part of claim 1. Further features are described in the dependent claims .

Brief description of the drawings

In the following there is a detailed description of the invention with references to the enclosed drawings, where

fig. 1 shows a service umbilical cord according to state- of-the-art

fig. 2 shows another service umbilical cord according to state-of-the-art .

fig. 3 shows a design of an integrated service umbilical cord according to the present invention fig. 4 shows the principle for conventional hydraulic control of a down hole safety valve, and

fig. 5 shows a schematic of the hydraulic control circuits of a control module according to the invention

Detailed description

The concept is based on use of a regular 2 - 4 " pipe in carbon steel as carrier pipe and conduit for hydrate in- hibitor 31. This pipe also provides mechanical protection, stress relief and corrosion protection for all the internal components. All small bore tubing such as conduits for supply of chemicals 32, conduits for hydraulic power fluid (control system) and electrical conductors 33 are accommo- dated inside the outer carrier 31. The concept is illustrated in fig. 3, where the carrier pipe 31 (hydrate inhibitor conduit) contains a conduit for low capacity supply of chemicals, e.g. scale inhibitor or wax inhibitor, and an electrical cable 33, e.g. a quad, metal clad. Additionally there is provided a wire or a fibre rope 34 as well as a clamp/strap or other form of bundling mechanism 35.

The reason for this design is that the carrier pipe may be constructed from low cost carbon steel and protected against sea water based corrosion by means of coating and anodes, typically from zinc or aluminium as per standard sub sea design. The carrier and small bore tubes require no protection against common hydrate preventing chemicals, which are typically methanol or glycol . Neither of the chemicals are good electrical insulators nor function as electrolytes thus facilitating corrosion. In such a medium a mixture of different metals may be pursued without risk of corrosion thus facilitating selection of material for each small bore tube according to service condition. Control lines, for instance, are subject to extreme cleanliness requirements, this has often resulted in selection of 22% Cr or 25% Cr alloys for such service .

It may happen occasionally that there is a requirement for bleeding back live hydrocarbons from the flow line through the hydrate inhibitor line, such that the inner surface of the carrier 31 is exposed to corrosive well fluids. Such occurrences are rare and mostly of short duration and does not involve corrosion of any significance. Many operators do not include this facility at all.

A key feature in this methodology is the choice of electric conductors 33 and shielding against hydrate inhibitors, es- pecially methanol . It is known that methanol acts aggressively on some isolators. It is therefore a requirement on control cables that all materials are compatible with methanol. The production of electric cable terminator penetrations in order to keep methanol away from other electri- cal parts is also problematic. According to the present invention, the electric conductors 33, preferably arranged as quads, are laid down in welded diffusion resistant pipes with welded connections at both ends . This entails that the methanol only encounters extruded, rolled or welded metal surfaces along the entire length of the service pipeline, including the termination.

Such metal clad cables 33 are common in oil wells. They are in particular used for down-hole instrumentation and are designed for durable use in wells encountering high temperatures up to 150°C, and are commonly available. Typical voltages are 2-3 kN, i.e. voltages above the normal choice for control systems in sub-sea wells.

In the design practice followed in the North Sea, where the multiphase transport usually is protected against hydrates in the steady state by means of isolation and the maintenance of high temperatures through the entire transport distance, the diameter of the service pipeline only varies marginally for large or small fields, since only one well is injected with hydrate inhibitors at the time (Other oil fields may have may have other requirements and needs for methanol) .

However, the dimensions of all the other pipes and electric conductors are dependent of the number of wells, e.g. the flow capacity requirement for a small number of wells is smaller than for a field with many wells being served by the same control cable. Small fields only require pipes with small cross-sections as compared to the length of the control cable . Since these pipes and electric conductors occupy space in the external pipeline for hydrate inhibitors and contribute to the reduction of the flow capacity, the arrangement according to the present invention will be especially suited for smaller fields. A larger number of larger pipes placed in the external pipeline will lead to a disproportionately large diameter of the external pipeline, driving up the price of this pipe even if the applied mate- rial is comparatively cheap.

A pipe-based design with the same pipe specifications as for all other small pipes is usually applied for a control cable, in order to maintain symmetry in design and of me- chanic forces. This often results in the guiding of low- dose chemicals, such as deposit and wax inhibitors, through over-sized and thereby unnecessarily expensive pipes (the dimension usually is decided by the largest user, most often the low pressure supply to the control system) .

For high-pressure fields, the requirements for a relatively large wall thickness of the external pipe increases, in addition to the increased requirements for the small, internal pipes for withstanding larger collapsing pressures. However, the requirement for the internal pressure capacity will increase correspondingly for all internal pipes (This also applies for the control system described in this specification. Note that for a traditional supply system with a separate pipes for control of the XMT-actuators, also known as the 207 bar system, the requirements for internal pressure will not increase . The system described here does not utilize a separate 207 bar pipe) . Therefore, it may be concluded, as also may be shown by simple calculations, that the positioning of small pipes in the hydrate inhibitor does not increase the requirements for their wall thickness. The internal pressure is the deciding factor. Small pipes usually have a higher capacity for external pressures than for internal pressures . It is only the number and the cross-sections of the internal pipelines that drive up the dimension of the external pipeline in regard to a conventional concept as shown in figs. 1 and 2.

The most important consideration in regard to a practical implementation of a control cable of this kind is the fabrication of the pipeline. Two methods may be used:

1. Pulling the center element trough an already welded external pipeline, or 2. Fabrication of an external pipeline around the center element .

1. Pulling the center element trough an already welded external pipeline

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31, e.g. on a barge. In this case, the pulling wire 34 probably may be omitted entirely.

The joining of small pipes and metal clad, electric conduc- tors is a standard practice in the industry.

In an alternate embodiment, it would be advantageous to fabricate an external pipeline 31 around the central element 36. This will cut down the number of operation steps, but will also require substantial capital investment for suitable equipment for the production of suitable pipes and their welding. The welding of pipes is known per se and will not be further elaborated.

The described concept implies a risk for tensioning the central element 36 when the external pipeline is reeled, as it is impossible to control that the that all the smaller pipes are centered in the middle, even if the purpose of the shown radial spacers is to center the smaller pipe- lines. This may be solved by giving the central element 36, i.e. the small pipes 32 and electric conductors 33, an undulating- or spiral configuration in the lengthwise direction as compared to the middle point of the external pipeline 31. Thereby the pipelines 32, 33 may be both com- pressed and tensioned without imposing unnecessary stress and strain.

A horizontally oriented pulling operation will automatically result in some slack of the internal pipes 32 as a result of the catinary suspension between the spacers 35(?) .

The central element 36 may be arranged with a number of parallel pipelines or as a spiral-configured bundle.

The described configuration is cost saving in regard materials as compared to the established alternatives, shown in figs. 1 and 2, in the following respects. -Hydrate inhibiting pipes of carbon steel without the need for expensive alloys are used. This is shown in fig. 2. -Only one cable has to be laid. No strapping operations requiring stops are necessary. This is shown in fig. 1. -Only a few small pipes and electric cables are necessary. This is described above in regard with the rationalization of the control system.

-Robust outer surfaces on the cable are provided, typically 3.5-4 mm steel, that are well suited for the pulling in J- pipes. This feature, together with electric conductors that are designed for high temperatures, e.g. 150°C, makes it possible to lay down the cable in the same trench as the flow line, thereby saving costs in regard to trenching, surveying and inspection, which are the same activities as for the flow line (The laying time does however increase somewhat as a result of the strapping operation) .

It is noted that it for conventional operations has not been possible to install flow lines and control cables in the same trench because the control cable has been prone to be damaged by the flow line as a result of temperature effects, e.g. buckling, and the fact that the outer surface of a conventional control cable has a far weaker structure than the presently proposed embodiment. Also, the most usual material used for electric isolation (polyethylene) does not withstand high temperatures. However, several projects have installed umbilicals that are strapped to the flow line with good results.

As described above, a main feature of the present invention is to keep the number of internal pipelines and their cross-sections down. This is achieved by means of a minor modification in the control module of each well, and is described in the following.

Control module

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33 provides a redundant control for one well, and depending on the configuration it may also provide some redundancy for more that one well. In addition, metal clad optical fibers withstanding high temperatures may be installed in the quad-unit 33 if real-time broadband instruments are required. Optical fibers installed between the electric conductors 33 will not increase the cross-section.

Fig. 4 shows the principle of a conventional hydraulic con- trol unit for a down-hole safety valve 47. The supply line 41 (innermost pipeline) with a connected accumulator 42, leads to a control valve 45, which in turn is connected to the valve 49. The circuit (somewhat simplified) is used by most suppliers.

Fig. 5 shows a schematic of the suggested hydraulic control system of the control system. A fixed restriction 50 reduces the flow between the high-pressure accumulator 42 and the low-pressure accumulator 52 to practical values . The valve 51 is the one referred to as two pilot steps controlled by the pressure in the accumulator 42 and the accumulator 52 in the off or on mode. A control valve 53 controls the valve 56 via the actuator 55, the low-pressure accumulator 57 providing a stable pressure in the spring chamber of the actuator. A non-return valve 54 allows dumping of the used fluid to the sea. A pressure relief system 58 is similarly provided with a non-return valve 59 that dumps used fluid to the sea.

The circuit functions in the following manner: The high- pressure accumulator loads the low-pressure accumulator 52 when it has the capacity to do so, i.e. almost always. The pilots in the valve 52 are loaded when the pressure falls bellow a certain value. The loading stops when the pressure reaches its highest allowed value, typically 207 bar.

A circuit like this will not be accepted for larger installations, one of the reasons being that the high pressure to μ> H o cπ o cπ

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Ω tr CO ii ø CQ <! Φ 0 0 9⁾ H μ- ø Φ Hi Ω ø CQ CO CQ tr φ •_* rt ⁾ Φ ⁾ H H Φ 0 0 Ω 9⁾ P. 0 rt μ- Φ ⁾ μ- Hj rt μ- 0 Hj Φ Ω μ- O 9⁾ H β P. if φ H Ω μ- rt

Φ Φ 9⁾ tr ^~. ø Φ Ω CQ rt Ω CQ ø Ω β Φ ø 9⁾ β ø Φ

<! cr Φ Hi O μ- rt μ- φ Φ rt Ω Φ Ω rt Hi Hj φ ^ Ω O rt l-h 3 H5 CQ P. CQ CQ M 0 P. rt Hi CQ μ- Ω μ-

P 0 P Hi Φ μ- 0 rt < 0 . μ- μ- μ- Φ rt 9> 0

9⁾ 0 μ- 9⁾ rt rt Ω rt HJ μ- rt 0 co B Hi ^ CQ rt

H O 3 9⁾ CQ tr 9⁾ H-¹ 0 φ ø if Hi 0 rt Φ 0 co μ- Φ 0 CQ Φ H Ω CQ B μ- 0 O rt μ- 3

0 p. ø H{ CO 0 Ω CQ rt CQ H Ω 0 ø Φ 0 rt

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Φ cr ø Ω μ- φ P. Ω s3 μ« 9⁾ CQ cr μ- H H Ω H

H μ- rt <! 9⁾ rt φ Φ 0 ^ rt μ< rt cr . — . Φ Ω 0

Ω Φ ø Hi φ rt ⁾ o H X P. CQ B- φ μ- Φ μ- PJ CO μ- Ϊ3

Hi CQ μ- β 0 Hi H rt 9i CO 0 • rt - P. 1 o H{ Ω Ω Hi P. CO CQ CQ H Φ a ø 9> Φ μ- φ 13

CQ φ 0 tr φ β μ- 3 o O Ω . o φ 0 Hi

CQ P. Hi CQ φ C^Q Φ GO rt 13 B Hj Hi Ω ø . rt Φ

1 β ^ 3 X GO tr 13 Φ Φ rt μ- φ 13 CO CQ CO O

CQ Ω rt CQ μ- 13 tr 9⁾ H-¹ ι-3 |P Φ Hi 13 Hj • • CO

Φ rt tr rt Ω Hi μ- 9⁾ rt ^ Ω tr β 0 rt 0 rt β

Ω μ- Φ Φ 9⁾ μ- cr Ω 0 μ- μ- rt Φ P. if rt H Hi rt 0 3_^ 3 μ- Ω 9i CQ B CQ Hj B- tr P. β μ- tr tr Φ μ- ø CQ CQ 9⁾ rt 0 Hj • ; rt φ 9i φ Ω Q φ μ-

0 β — Hi Hj Φ CQ Hi μ- P. CO tr rt CQ CQ

B 0 13 μ- . μ- Φ P. rt O Q Ω ^ μ- 3 13 ^

Hi 13 O H X μ- CQ Φ H μ- 9i 9⁾ 0 Φ Hi 3 CQ

0 H H >< rt ø Φ 3 » P. ø 13 rt 0 9⁾ φ 9i rt

Hi rt ^ rt Hi Hi Q 0 CO β Ω μ- if O O ^ Φ if 0 9⁾ Φ i rt φ Hi o rt Φ Hi β O 3

Φ O rt Hi 3 rt HJ o tr Φ 13 9⁾ Φ Hi β cr rt ^ cr tr φ Φ O 9⁾ Hi Φ rt CQ 13 o CQ Φ H φ 9⁾ tr a O Φ φ 13 0 9⁾ Φ 13 β φ ø

Φ 0 rt 3 3 rt tr cr H Hi Hi Ω Φ H μ-

0 9⁾ rt P. 0 Hi 9⁾ CQ Hi < ^ P

13 Φ rt 9⁾ Φ Φ tr μ- S

Ω 13 3 t Ω 9⁾ Ω φ Ω rt rt if Ω CO 9i Hi Φ 0 tr

0 φ Φ rt 0 tr 9⁾ B- rt Φ rt 0 rt rt μ- 13 (-^■ μ- 9⁾

0 H Hi β rt 9> 13 H Φ o B o rt Hi μ- P Hi

P. o p. 9⁾ ø Hi M φ Hj Ω rt μ- Hi ^* o φ Φ 3 β Ω Hi Φ i-> Hi μ- φ Hi o rt Φ CQ O cr Hi p.

Ω o Ω 0 Ω CO Hi O B tr B • 9⁾ μ- rt 0 H μ- Ω Hi Φ CO φ Hi Ω Φ rt CO β cr CQ cr rt

0 rt I P. 0 13 0 13 Ω rt Φ β 0 *) H ^ ^ CQ

H Φ CO μ- 13 13 Hi rt Hj rt Φ Hj rt Hi P. 0 ^ O

W 0 ø 13 Hi 0 Φ 0 0 B- Hi Φ Hi rt rt CQ Φ 0 13 9> CQ 9⁾ μ. Hj Φ

Hj 1 P. rt 1 3

Claims

C l a i s

1. A method for supplying chemicals to a sub-sea hydrocarbon well and controlling a sub-sea hydrocarbon well, c h a r a c t e r i z e d i n that a per se known coiled tubing (31) is being used as a control cable, said coiled tubing (31) being lead through the same J-pipe and possibly the same ditch along the sea bed as a flow line.

2. A method according to claim 1, c h a r a c t e r i s e d i n that a hydrate inhibitor (32) is being lead through said coiled tubing (31) , said coiled tubing (31) also being provided with channels (32, 33) for electric cables and/or hydraulic lines in connection with a electro-hydraulic control system, said coiled tubing also being provided with channels (32, 33) for the supply of chemical injection fluids.

3. A method according to claim 1 or 2, c h a r a c t e r i s e d i n that said coiled tubing (31) is provided with a complete set of pipelines that provide channels (32, 33) for electric cables (33) and/or hydraulic pipes, as well as channels for chemical injection fluids .

4. A method according to one of the previous claims, c h a r a c t e r i s e d i n that said coiled tubing (31) forms a nearly corrosion free environment permanently protecting the smaller internal channels (32, 33) .

5. A method according to one of the previous claims, c h a r a c t e r i s e d i n that said coiled tubing (31) is provided with an external protective sheath and/or a central element (35) able to take up tensional forces, said central element (35) being provided with said channels (32, 33) .

6. Use of a per se known coiled tubing (31) as a control cable, said coiled tubing being provided with a complete set of channels (32, 33), such as channels for hydrate inhibitors, electric cables (33) , hydraulic effect, and chemical injection fluids, said coiled tubing (31) being lead through the same J-pipe and possibly the same ditch along the sea bed as a flow-line.