State-of-the-art During the 25 years of developing sub sea petroleum tech- nology 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 elec- trical 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 11, 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 re- quirement 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 sym- metrical and helical pattern. There is an industrial prac- tice for using 25% Cr steel for all tubes in order to re- sist 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 poly- ethylene.

Both the referred main types of design have been exten- sively and successfully used for field development proj- ects. 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 unde- sirable costs. The following patents are referred to as representative of this technique: NO 160389, GB 2246410 A, US 4709730, NO 304533. The re- ferred methods are considered of little relevance to con- trol 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 heat- ing 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 ca- pacity. 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 in- vention 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 con- trol 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 sup- ply 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 illus- trated in fig. 3, where the carrier pipe 31 (hydrate in- hibitor 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 pur- sued 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 re- sulted 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 aggres- sively on some isolators. It is therefore a requirement on control cables that all materials are compatible with methanol. The production of electric cable terminator pene- trations in order to keep methanol away from other electri- cal parts is also problematic. According to the present in- vention, 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 tem- peratures up to 150°C, and are commonly available. Typical voltages are 2-3 kV, 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 mainte- nance 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 inhibi- tors 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 of- ten 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 ad- dition to the increased requirements for the small, inter- nal 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 in- ternal 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 calcu- lations, 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 num- ber 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 fab- rication 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 Several kinds of pipes suited as an external pipeline ac- cording to the present invention are offered on the marked.

Industrial pipes, suited for a large number of umbilical operations and high degrees of deformation without becoming oval, are offered for umbilical applications. Such pipes are suitable for external pipeline applications. 12 meter long pipes, typically seamless pipes, are also delivered for butt welding from most of the larger steelworks.

For pulling the center element 36 through the external pipe one can envisage a pipe section of limited length being rolled out on a flat surface, i. e. on shallow waters from a barge or from a vehicle on land, e. g. a shut down railway or airport, such that the pipe 31 rests perfectly straight and level. It will then be possible to pull a center ele- ment trough an external pipeline length 31. Typically, the external pipeline 31 is filled with inhibited water, while the internal pipelines 32,33 are filled with air in order to reduce frictional forces.

The pulling wire 34 may be shot hydraulically through the external pipeline 31 with the aid of a plug, while the ex- ternal pipeline 31 is filled with for example inhibited wa- ter. When the central element 36 is pulled though the ex- ternal pipeline 31, a production length is completed. The assembly of the external pipeline 31 and the central ele- ment 36 can be stored in a straight configuration or coiled on a carousel or reel. The internal friction in the coiled state will prevent large displacements of the central ele- ment 36 with respect to the external pipeline 31. Finally, the cable sections are joined together and the entire cable is coiled up on a carousel or reel for later unrolling to an installation carousel or reel on a laying vessel, alter- natively the entire cable is coiled up directly on an in- stallation reel, whereupon the latter is transferred to a laying vessel.

It is also possible to guide the bundle of internal pipes 32,33 through a vertically positioned-external pipeline

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 ele- ment 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 un- dulating-or spiral configuration in the lengthwise direc- tion as compared to the middle point of the external pipe- line 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 automati- cally 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 mate- rials 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 re- quiring 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 ef- fects, 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 pro- jects 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 de- scribed in the following.

Control module In a conventional control system it is usual to have two supply lines (see fig. 4 which shows one of them), one for

high pressure to the down-hole safety valve 47 and one for the 207 bar pressure supply to the actuators 55 on the wellhead Christmas tree. The high-pressure system 40 is characterized by a near zero fluid flow. In principle, a high pressure may be generated by the low-pressure system by means of a booster, and a low pressure may be generated by the high-pressure system by means of a reduction valve 50. Boosters are known to have been installed in new fields, but are not very common. Pressure reduction valves are usually avoided in the design of sub-sea control sys- tems for hydrocarbon installations, because they are con- sidered to be unreliable.

However, a special valve 51 of the on/off-kind only used to control a flow between two accumulators 42,52 has been build and tested. The valve 51 is developed by others and does not constitute a part of the present invention. It comprises of a shear seal, which is common in this kind of system and has proven to be reliable, and two pilot steps for turning on and off the hydraulic flow in the main unit.

This kind of valve 51 has not been implemented yet, proba- bly because it requires a unusually large accumulator 42 on the high pressure side 40 if it is to provide a satisfac- tory control of the pressures in a larger installation, in addition to that a series of error modes makes it unsuited for larger installations. However, for marginal fields with small or no interaction between the pressure supplies of the various wells, this will not present large problems.

A marginal field will probably be allowed to spill smaller amounts of water-based hydraulic fluids. This entails that the entire well may be controlled by only one hydraulic pipeline.

It will therefore be possible to provide a supply system for a marginal field that comprises one pipeline 32 for chemicals (there may be need for more than one), one pipe- line as a backup for the two first, and a quad 33. A quad

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 fi- bers withstanding high temperatures may be installed in the quad-unit 33 if real-time broadband instruments are re- quired. Optical fibers installed between the electric con- ductors 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 re- duces 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 con- trolled by the pressure in the accumulator 42 and the accu- mulator 52 in the off or on mode. A control valve 53 con- trols 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 dump- ing 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 instal- lations, one of the reasons being that the high pressure

may propagate into the low-pressure system and harm its components in case of accidents. This may be avoided by means of security measures, e. g. the pressure relief system 58, but in larger installations this measure is probably not sufficient in total (i. e. production regularity). For 1-3 wells the risks may be accepted by the operators under certain production conditions if the capital cost is suffi- ciently reduced.

Conventional control cables often has a copper content that is significantly higher than required in regard to the electric losses in the system. This is due to the concerns like mechanic strength and line control, the latter often being connected to complex supply systems of both effect and control signals to wells that are geographically spread out. The electric conductors 33 according to the present invention are metal clad and exhibit extreme mechanic prop- erties (even if these features primarily are meant for pro- tection against aggressive chemicals). If the actual copper requirement (for an electric system) is to be the deciding factor for the dimensioning of the supply system for 1-3 satellites, a considerable reduction of the copper content in, and thereby also the cross-section of, the conductors will be achieved.