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 enclosing a number of smaller pipes, the cable having the special features that the external enclosing pipe 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 against radial forces and forms a nearly corrosion free environment protecting the smaller, internal pipes.

Description:
FIELD OF THE INVENTION  
         [0001]    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  
         [0002]    During the 25 years of developing sub sea petroleum technology 2 types of sub sea control/service umbilical have become commonplace.  
           [0003]    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.  
           [0004]    This design is characterised by relatively moderate material-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 Il, 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, separated 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 .  
           [0005]    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 hydraulic power fluid require high cleanliness standards and consequently often leads to requirement for high quality alloy steel.  
           [0006]    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.  
           [0007]    Both the referred main types of design have been extensively 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 existing 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:  
           [0008]    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  
         [0009]    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.  
           [0010]    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  
           [0011]    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).  
           [0012]    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 DESCRIPTIONS OF THE INVENTION  
         [0013]    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  
       [0014]    In the following there is a detailed description of the invention with references to the enclosed drawings, where  
         [0015]    [0015]FIG. 1 shows a service umbilical cord according to state-of-the-art  
         [0016]    [0016]FIG. 2 shows another service umbilical cord according to state-of-the-art.  
         [0017]    [0017]FIG. 3 shows a design of an integrated service umbilical cord according to the present invention  
         [0018]    [0018]FIG. 4 shows the principle for conventional hydraulic control of a down hole safety valve, and  
         [0019]    [0019]FIG. 5 shows a schematic of the hydraulic control circuits of a control module according to the invention 
     
    
     DETAILED DESCRIPTION  
       [0020]    The concept is based on use of a regular 2-4″ pipe in carbon steel as carrier pipe and conduit for hydrate inhibitor  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 accommodated 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 .  
         [0021]    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.  
         [0022]    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.  
         [0023]    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.  
         [0024]    A key feature in this methodology is the choice of electric conductors  33  and shielding against hydrate inhibitors, especially 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 electrical 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.  
         [0025]    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 kV, i.e. voltages above the normal choice for control systems in sub-sea wells.  
         [0026]    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).  
         [0027]    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 material is comparatively cheap.  
         [0028]    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 mechanic 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).  
         [0029]    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.  
         [0030]    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:  
         [0031]    1. Pulling the center element trough an already welded external pipeline, or  
         [0032]    2. Fabrication of an external pipeline around the center element.  
         [0033]    1. Pulling the Center Element Trough an Already Welded External Pipeline  
         [0034]    Several kinds of pipes suited as an external pipeline according 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.  
         [0035]    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 element 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.  
         [0036]    The pulling wire  34  may be shot hydraulically through the external pipeline  31  with the aid of a plug, while the external pipeline  31  is filled with for example inhibited water. When the central element  36  is pulled though the external pipeline  31 , a production length is completed. The assembly of the external pipeline  31  and the central element  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 element  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, alternatively the entire cable is coiled up directly on an installation reel, whereupon the latter is transferred to a laying vessel.  
         [0037]    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.  
         [0038]    The joining of small pipes and metal clad, electric conductors is a standard practice in the industry.  
         [0039]    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.  
         [0040]    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 pipelines. 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 compressed and tensioned without imposing unnecessary stress and strain.  
         [0041]    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 (?).  
         [0042]    The central element  36  may be arranged with a number of parallel pipelines or as a spiral-configured bundle.  
         [0043]    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.  
         [0044]    Hydrate inhibiting pipes of carbon steel without the need for expensive alloys are used. This is shown in FIG. 2.  
         [0045]    Only one cable has to be laid. No strapping operations requiring stops are necessary. This is shown in FIG. 1.  
         [0046]    Only a few small pipes and electric cables are necessary. This is described above in regard with the rationalization of the control system.  
         [0047]    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).  
         [0048]    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.  
         [0049]    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.  
         [0050]    Control Module  
         [0051]    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 systems for hydrocarbon installations, because they are considered to be unreliable.  
         [0052]    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, probably because it requires a unusually large accumulator  42  on the high pressure side  40  if it is to provide a satisfactory 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.  
         [0053]    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.  
         [0054]    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 pipeline 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 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.  
         [0055]    [0055]FIG. 4 shows the principle of a conventional hydraulic control 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.  
         [0056]    [0056]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.  
         [0057]    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.  
         [0058]    A circuit like this will not be accepted for larger installations, 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 sufficiently reduced.  
         [0059]    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 properties (even if these features primarily are meant for protection 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.