BACKGROUND OF THE INVENTION Field of the Invention:

This invention relates to pipelines electrically heated by AC current flowing through the electrically conducting pipe that is electrically insulated from the sea water with a wateφroof coating and to a method of insulating offshore pipelines; more particularly a way to apply thermal insulating materials after the pipes are welded together, thus avoiding various complications that arise when laying pipelines in deep water. Related Art:

The use of the technique known as "subsea", or "offset" production to produce offshore oil and gas from offshore reservoirs is increasing. This technique employs small, submerged pipelines known as "flow lines" that are laid along relatively short routes between the submerged wellheads and a gathering point such as a nearby platform. For a variety of reasons it will often be beneficial or necessary to take measures to maintain high temperatures in the fluid flowing in subsea flow lines. One way to this is done is to add insulation to retain heat that already exists in the produced fluids.

Because these flow lines are generally small, relatively short, and often in deep water, installation from reel barges is often faster and less expensive than installation using lay barges which depend on stringing the pipe joints together on deck. Reeling, however, generally precludes the use of a concentric pipe with an insulation-filled annulus except on small flow lines, due to the stiffness of the composite pipe. Various plastic, rubber or foamed plastic coatings have been used to insulate reel laid pipes, but these are both more expensive for a given volume, and more conductive than conventional insulating materials that can be used in a concentric pipe design. Because several flow lines are often laid along the same short route, it can be less expensive to string the flow lines onshore and tow a bundle of several pipelines to the offshore site, particularly if insulation is needed. These bundles are usually assembled

inside a "carrier" pipe that serves to reduce submerged weight and to protect the flow lines during the journey. The bundle may be towed off-bottom at a "controlled- depth", in which case the weight of the bundle is accurately controlled to be very near to the buoyant force, or it may be kept just slightly heavier than the buoyant force so that it stays on the bottom during trip without too much drag. In either case, the submerged weight of the bundle must be held within narrow limits during the tow. In the controlled depth tow method, the void space between the carrier pipe and the flow line bundle is generally filled with ballast to sink the pipeline bundle when it reaches its destination. Liquids or slurries that partially solidify serve the combined purpose of insulation and ballast. For bundles that are towed along the sea bed, the additional ballast provided by filling the flow lines themselves may be adequate. In such cases, inexpensive, conventional insulating materials have been installed in the dry space between the bundled flow lines and the casing pipe. These insulating materials are more effective and less expensive than the submersible insulating materials used on reeled pipelines. This is taken to be a considerable advantage of this method of installing flow line bundles when insulation is desirable or necessary. In some cases this can be the deciding factor in whether to tow the pipelines, or lay them separately from a reel barge. One disadvantage of housing the bundled lines inside a casing pipe is that in very deep water the wall thickness required to keep the casing pipe from collapsing under the hydrostatic pressure makes it difficult to achieve the desired buoyancy. This has been overcome in some cases by pressurizing the casing pipe with nitrogen, but pressurizing also adds expense due to the cost of the nitrogen itself, the required filling apparatus and time to fill the casing pipe. The flotation pipe in this method is also larger than would be required to add the required buoyancy, because part of the volume of the casing pipe is occupied by the flow lines. This is space that does not contribute buoyant force as it would if the flow lines were outside the floatation pipe. Another disadvantage of this design is that some sort of carriage system is needed to get the flow line bundle pulled into the casing pipe. It would be quite difficult to connect one prefabricated length of such a bundle to another that is already in the water. Precisely because of this difficulty, most towed pipeline bundles have been fabricated in a single section on shore. This requires a very long section of land with a beach front.

In the past, most insulation for offshore pipelines has been applied to the pipe at a plant location remote from the site at which the pipes were strung together, and a separate process used to coat the small gap at the field joint. Recently, some insulating coatings have been extruded onto the pipe as it was reeled. Insulation has also been installed in the field on bundles of pipes that were towed. In at least one case, pre-cast pieces of foamed plastic were placed around the bundle before it was pulled into a casing pipe and then towed along the bottom. In more than one pipeline installed by the controlled-depth tow method, liquids or slurries that gel or set were pumped into the space between the pipelines and the casing pipe after the bundle was in place at the final destination. The collapse resistant jacket was essential to all of these towed bundles.

The benefit of increasing the thickness of a given insulating material diminishes with its thickness, but the benefit of decreasing the conductivity of a given thickness of insulating material increases linearly, and inversely. It is, therefore, not possible to generalize when comparing the merits of a low conductivity material applied one way to a high conductivity material applied in a different way. For long pipelines it may be impractical to add enough insulation to achieve the desired result. In other cases it may be practical to adequately insulate with a low conductivity material, but not a higher conductivity material. In either case, the practical length can be increased by adding more heat than is contained in the produced fluids. Heating can also be useful and cost effective even when it is not essential to operating the pipeline.

Many of the problems relating to cooling of the fluids in a pipeline are of a temporary or transient nature due to changing flow conditions. Some insulated pipelines carry materials that will freeze or turn to wax if flow is stopped, but which at full flow can be transported through the pipe fast enough to avoid cooling problems. In such cases, the pipeline must quickly be flushed with a different fluid after it is shut down. Insulation can be used to slow heat loss enough to allow time to flush the line in an un-planned shut down but the ability to heat the pipeline can provide an infinite "shut-down window" and eliminate the need to flush the line. Another type of transient flow condition is declining flow rates that occur in flow lines that carry subsea well production. Because temperature drop depends on the length of time the

fluid is in the pipe, the temperature loss increases as the flow rate decreases. In such cases the peak power need for heating may not occur until near the end of the life of the oil or gas well, meaning that much of the cost of heating is deferred. If a less effective insulating system can be installed at lower cost, then the combined cost of heat and insulation may be lower than the cost of a more effective insulation system. The problems associated with temperature are not entirely predictable, so with non- heated line the worst case must be paid for in advance. The heated line can be operated with just enough heat to suit the real need.

Pipelines can be heated by pumping hot water or steam through separate heating pipes that are thermally connected to the pipeline. One oil company is planning to apply submersible insulation to the outside of a casing pipe so that hot water can be pumped through the space between the casing pipe and the pipeline bundle. The insulation will be applied outside the casing pipe.

Where waste heat is not available, electric heating usually more cost effective than fluid heat tracing, but electric heating has rarely been used offshore due to the complications of using high power heating underwater. One of the oldest ways to electrically heat pipelines is to use the pipe itself as an impedance heating element. For pipes made of magnetic material, alternating current tends to flow near the outside surface of the pipe due to self-induced eddy currents in the pipe wall. If the pipe wall is thick, this can effectively insulate the current from the fluid inside the pipe, thus allowing this method to be used even when the fluid flowing in the pipeline is somewhat conductive. While conceptually very simple, this method has several disadvantages, depending on the case. Because the resistance of steel pipelines is low, high currents are needed to generate enough heat to significantly effect the fluid temperature. Considerable power is lost through the return path unless resistance is quite low. This requires large return cables, and generally precludes earth as the return path. Furthermore, insulating materials typically used to insulate land pipelines are not designed as electrical insulators, and are not suitable for insulating high voltage. Building codes, therefore, limit the voltages to levels that are not hazardous to humans. This effectively limits the use of this heating method to pipelines shorter than those typically used offshore.

One method that has been used to overcome these problems is to cause induction heating in one or more small pipes that serves as a conduit an electrical conductor that carries high voltage alternating current. The conduit is attached to the line pipe and serves as the return current path. Heat is induced in the conduit, but the same "skin effect" phenomenon that causes current to flow on the outside of a single conductor causes the current to flow on the inside of the magnetic outer conductor of coaxial conductors carrying current in opposite directions. This method, therefore, insulates both the outside and the inside of the pipeline, allowing high voltage heating of conductive fluids without risk of electrical shock. This is described in US patent numbers 3,617,699, 3,777,1 17 and 3,975,617 and is known as skin effect heat tracing. It is common on land, and it has been used on at least one offshore pipeline. In that pipeline, the heating tubes were pre-installed on individual pipe joints, over which larger, concentric pipes were installed from the end to house the foam insulation. Each pipe joint contained a junction box that allowed connection of the heating tubes and wires offshore, after two pipes were joined. Split sleeves were installed over the joint, and welded. Because the pipes were welded offshore during the lay process, this time-consuming process was very expensive.

US. patent number 5,241,147 addresses these installation problems by proposing to induce heat in the pipe with a magnetic field created by passing an AC current through wires placed outside the insulation. This, however, may leave the wires vulnerable.

US patent 3,975,617 discloses that conductors placed near the pipe can be used for the combined purpose of inducing heat in the pipe and power transmission to production equipment. US. patent number 5,241,147 discloses that the steel windings in flexible pipe of the type used offshore can be used as a direct current, resistive heating element with earth as the return current path. This depends on plastic or rubber layers in hose or flexible pipe to electrically insulating the steel windings from sea water, and from conductive fluids inside the pipeline. The disadvantage of using DC current is that the sea water will act as an electrolyte, and the steel windings or ground electrodes could be rapidly consumed by electrolysis.

SUMMARY OF THE INVENTION

In this invention a pipehne is electrically insulated from the surrounding sea water with a wateφroof coating and the pipeline conducts alternating current that generates impedance heating in the pipeline. The power source is grounded to sea water. In the simplest embodiment the return current path is directly through sea water, but current can also be returned through an insulated cable or through another pipeline, using sea water for grounding in each case. In the preferred embodiment the electrical insulation will also serve as a thermal insulating coating. These differ from AC impedance heating applications as used on land pipelines in the prior art in the following ways: First the electrical insulating material is continuous, wateφroof and capable of insulating the pipeline from the sea water under sustained hydrostatic pressure. Second, by grounding the power supply to sea water, the entire submerged pipeline is inherently safe from shock and explosion because the insulation can only fail to ground, there is no free oxygen in the path to ground and there is no potential for a spark. This combination makes it feasible to apply the high voltage needed to use impedance heating for offshore pipelines of typical length. Because the salt water itself is a far better conductor than earth, it can be used as the primary return current path without the excessive power loss than would occur if an earth ground were used as the primary return current path. If one type and size pipe is used throughout the line, then the heat generated per unit length of the pipeline is uniform along the length. In other embodiments of the invention, the heat can be distributed differently by changing the pipe material or wall thickness, or by adding a different heating means in series with all or some of the length of pipe to be heated. Later sections of this specification disclose means for electrically insulating the pipeline over its entire length, for connecting power to the pipeline, and for connecting the pipeline to the platform while maintaining electrical continuity through the pipeline.

Another aspect of the invention is a manufacturing method whereby the thermal insulating material is cast over an assembled length of a pipeline in molds that are continuos, or directly adjacent to one another along said assembled length before it has been placed at its final destination. The insulation can be cast around several pipelines that are parallel or entwined in a helix. Typically the pipeline or pipelines will have been "assembled" by welding multiple prefabricated pipe joints together, but

it is also possible to cast the insulation onto a long, continuous pipe or pipes that have been delivered to the casting site on reels. Typically this would be done as the pipeline or several pipelines are being transferred past a work station from a reel situated on a ship or near a waterfront to the sea bed, or a lake bed or to another reel. In such case, the words "assembled length" is used here to describe a continuous length that has been wound onto a reel. More generally as used here, the term "assembled length of pipeline" is used to describe a length of a pipeline or a bundle of several pipelines that is too long to be transported over a long distance except on a reel or by towing. The mold, or molds are formed, bent or assembled around the pipelines at the stringing site, and filled with an insulating material that has some structural strength and resistance to impact and deformation when cured, so it protects and preserves the pipeline. The insulating matieral also consolidates multiple pipes into a bundle. The mold or molds, therefore, need not resist external pressure, or be capable of protecting or integrating the pipelines. The mold need not resist much internal pressure, as would be required if extruding through a die.

When compared to putting the pipes inside a pressure-resistant casing pipe, this method reduces the amount of steel and the amount of welding needed, and also eliminates the need for a carriage system to pull the flow lines inside the casing pipe. If the flotation pipe is not continuous it is easier to connect sub-sections of the pipeline bundle after part of the entire length has been towed offshore. A continuous flotation pipe can be used to carry heated fluid after the bundle has reached its destination. The economic viability of this method depends on the availability of a cost-effective, insulating material that can be cast, poured or molded and will adequately protect the bundle. As more cost-effective insulating materials are developed to withstand the hydrostatic pressure of deep ocean service, and as depth of offshore pipelines increases, the advantages of this arrangement may increase.

This same method can be used to insulate individual pipelines or bundles of pipelines that are installed on a reel, ether as the pipe is being installed on a reel, or as it comes off the reel during the lay process. This method can also be used to insulate pipelines that are strung on deck, and is particularly well suited for pipelines assembled and laid vertically by the technique commonly known as J-lay.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a side elevation view of a portion of an assembled, insulated bundle of pipelines inside identical, circular sleeves that serves as molds for placing insulation. Figure 2 is a cross sectional view taken at the section line A-A in figure 1.

Figure 3 is a cross sectional view taken at section line B-B in figure 1 ,

Figure 4 shows a cross section of rectangular mold being placed around a bundle of four flow lines and a flotation pipe.

Figure 5, shows a cross section of the pipeline bundle shown in figure 4 after the mold is in place,

Figure 6 shows a cross section of the same bundle of flow lines shown in figure 5, after the insulation has been placed in the mold.

Figure 7 is a side elevation of a pipeline bundled with a smaller pipe being moved axially past a station where molds are installed and filled with insulation material, past which the insulated pipeline bundle proceeds to be wound onto a large reel.

Figure 8 is a view taken at section line A-A in figure 7.

Figure 9 is an enlarged detail of a type of sheet metal lock seam that can be used to join the edges of molds such as are shown in figures 7 and 9. Figure 10 shows pipelines that are being moved downward, off of a ship, either from a reel or a tower, as stove-pipe-like mold sections, 35, are successively positioned and filled with insulation, just before the insulated pipeline enters the water.

Figure 1 1 , shows a pipeline bundled with a smaller pipe as it being moved axially past a station where a sheet material is fed from a coil and formed with rollers into a circular mold, into which insulation is being pumped before it is wound onto a reel.

Figure 12, is a view taken at section A-A in figure 11.

Figure 13 is a side elevation of the pipe bundle shown in section 1, 2 and 3 as it is being manufactured and simultaneously pulled into the sea.

Figure 14 is a cross section of a bundle of two pipelines around which an insulating material has been cast inside a mold, and a float pipe that is attached to said insulated pipelines but not surrounded by insulation.

Figues 15 and 16 are a side and end view of the spacer, 77 in figure 15 Figure 17 shows a pipeline suspended from a platform in a catenary to the sea bed, extending from there to a subsea wellhead. The pipeline is electrically insulated from the ambient sea water and insulated from the platform with an insulating above the waterline. The terminal comprises an electric terminal that connects the pipeline to one leg of the AC power supply. The other leg of the AC power supply and the end of the pipeline near the subsea wellhead are connected to sea water electrodes. The sea water completes the circuit between the sea water electrodes.

Figure 18 shows an offshore pipeline that is connected to a pipeline riser at the sea bed and extends to a subsea well head. The riser is attached to an offshore platform, and the pipeline is electrically insulated from ambient sea water and from the riser by a submerged insulating joint. The insulating joint comprises an electric terminal that connects a flexible power lead to an AC power supply on an platform. The other leg of the AC power supply, and the end of the pipeline near the subsea wellhead are connected to sea water electrodes. The sea water ground completes the circuit between the sea water electrodes. Figure 19 shows a pipeline suspended from a platform in a catenary arc to the sea bed, extending from there to a subsea wellhead. The pipeline is electrically insulated from the ambient sea water with an insulating coating and is insulated from the platform with an insulating joint on the sea bed. The insulating joint comprises an electric terminal that connects the pipeline to one leg of an AC power supply. The connection is made with a cable to that extends through the waterline inside a conduit. The other leg of the AC power supply and the end of the pipeline near the subsea wellhead are connected to a sea water electrode. The sea water completes the AC circuit.

Figure 20 is an elevation section view of the electrically insulating joint and terminal 279 in Figure 18 and 299 in Figure 19.

Figure 21 shows an elevation cross section view of a pipeline field joint that has been insulated by casting an insulating material in a mold that encloses the weld

that joins two vertical, insulated pipes, as would exist when pipes are installed offshore using the "J" lay metho ^' d.

Figure 22 shows an end cross section view taken through a pipeline field joint that has been insulated by casting an insulating material in a mold that encloses the weld that joins two horizontal pipes, as would exist when insulated pipes are installed offshore using the "S" lay method.

Figure 23 shows an elevation section view of a cast field joint taken at section BB in Figure 220.

DETAILED DESCRIPTION AND VARIOUS EMBODIMENTS OF THE INVENTION

With the manufacturing method of this invention, the insulation is placed between an assembled length of one or more pipelines in a mold that is continuos, or molds that are directly adjacent to one another along said assembled length before it has been placed in its final position. The insulation can be cast around several pipelines that are parallel or entwined in a helix. The insulating material serves to protect, integrate and preserve the pipes, so the mold need not be collapse or impact resistant. It may be installed in prefabricated segments from the end or the side, depending on the application, it may be formed in place by wrapping lengths of flat stock around the bundle, it can be made from tube sections formed with a spiral seam, or it may be continuously formed around the pipeline from coil stock, joined at the edges with a longitudinal seam in a manner similar to that used to make pipe, and shown in figures 1 1 and 12. Figures 1 through 3 show a pipeline bundle in a circular mold. If such an arrangement is fabricated on a flat surface, as might be done for a pipeline bundle that is to be towed, some or all of the spacers can be made to extend across the entire cross section of the mold as shown in figure 15 so that it is partitioned into short sections that can be filled to the top with insulation, one at a time. A spacer of this type is shown in figures 15 and 16. If the insulation can be poured while the pipeline is tilted, as is shown in figures 7, 10 and 11 , this is not necessary.

The submerged weight of the pipeline, or pipeline bundle can be reduced with a continuous or non-continuous ^" pipe, as shown in figure 1 , with wooden timbers or other light-weight material, including particles that are lighter than the insulating composition and are dispersed in it. If the light-weight material contains void spaces, then it must be strong enough to resist any hydrostatic sea water pressure that is transmitted to it through the insulating material. This will depend on the nature of the insulating material. The insulating material itself may include void, or gas-filled spaces if it will withstand the hydrostatic pressure for long periods. In some cases the pipelines may be sized so that the correct submerged weight can be achieved without using materials that serve no puφose other than reducing weight. The practicality of this will depend on the operating pressures of the pipelines and the ocean depth. For pipelines or pipeline bundles that are to be installed by reeling, low submerged weight may not be important. In most, but not necessarily all cases, the mold will be left in place to give additional protection to the insulating material. On bundles that will be towed on the sea bed, it can act as a hard cover that will resist the abrasion as it is dragged across the sea bed. For pipes or pipe bundles that are to be reeled, a thin metal mold can also serve as a jacket to help resist deformation under point loads due to bending reactions on the insulated pipeline or pipeline bundle. The insulation protects the pipe bundle from impact, and supports the mold, while the mold acts as a jacket that protects the insulation against gouging and abrasion.

As is illustrated in figures 4, 5 and 6, the mold need not have a circular cross section. There are several advantages to other shapes. If, for example, the bundle itself is not round, then a round mold would result in more insulation thickness in some places around the bundle than others. A different shape would be more efficient in terms of the total volume of material needed to achieve the desired thermal resistance. Furthermore, for bundles towed on the sea bed, flat contact with the sea bed could distribute contact pressure over a larger area and thereby reduce abrasion during the tow. For reeled pipes, this the benefit of better volumetric efficiency of the insulation may allow a greater amount of insulated pipe to be installed on a reel. Furthermore, a flat contact surface may better distribute bending loads, changing what would be a point load on a circular cross section, to a line load. These benefits are

not practical if the pipe or bundle of pipes is protected by a casing that depends on its circular shape to resist collapse under ocean pressure.

Towed bundles can be fabricated as they are towed offshore with a cable, as shown in figure 13. The cable can be pulled by a boat, a winch anchored offshore on a boat, a structure, or an anchor or by an onshore winch that pulls a cable through a sheave that is anchored offshore.

The insulation in this invention can be any material that can be cast, molded or injected to completely fill the space between the mold and the pipe bundle and cures to a solid or semisolid that provides adequate thermal insulation flexibility and strength. For the puφose of being specific in this disclosure, a thermal insulator will be defined as a material that exhibits thermal conductivity less than 0.2 BTU/hr-ft-°F in the conditions that will exist when the pipeline is in service. Ideally it will be a wateφroof material, but if not, the mold can be designed as a wateφroof jacket. This arrangement will be distinguished from encased pipeline bundles of the prior art by the fact that in this arrangement the insulation is cast in the mold and it structurally supports the mold or housing against impact or collapse under hydrostatic pressure. Waxes, foamed plastics, thermoplastic rubber, or thermosetting plastics can be used, but most of these submersible insulating materials that have been used to insulate pipelines in the past are either expensive, not ready adaptable to molding, water absorbing, or structurally weak. Low-cost, moldable, submersible insulating materials that exhibit considerable compressive strength that can be applied by the methods of this invention to make towed bundles which are less expensive than bundles installed in dry casing pipe because of the savings in steel, assembly and welding. Bitumen or bituminous compositions have low thermal conductivity, and can be melted to a liquid that can be cast, and are, therefore, well suited to insulate pipelines of this invention. When the object of insulating the pipeline is to keep the pipeline warm, the stratum of bitumen nearest the sea water will be cooler and harder than the stratum near the pipes, and will act as a sleeve that contains the semi-solid warm stratum. This can be effectively employed in this invention. When the stratum of bitumen that contacts the metal mold cures, it adds support to the mold, allowing the mold to be made of a thin sheet metal that need not be thick enough to resist the impact and hydrostatic pressure that will occur during installation. Thus, the sleeve can be construed as a laminate of

sheet metal and a cool stratum of bitumen. If the bitumen adjacent to the pipeline bundle will not support the bundle at the operating temperature of the pipeline, then the mechanical link used to maintain the spacing between the pipeline bundle and the "composite sleeve" can be the same spacers that are used to hold the mold in place while the insulation is being placed. Other embodiments of the invention may use bitumen combined with particulate, fibrous or cellular fillers, chemical modifiers or polymers, or any other composition comprising bitumen.

The the heater-cooler, shown as 30, in figure 7 or 50, in figure 11, represents one or more devices that can cool thermoplastic insulating materials, such as bitumen, or that heat the mold to keep the viscosity of the insulation low as it flows into the mold. This would enable thinner layers to be cast. In some cases it may be useful to have both a heater and separate cooler, that is placed slightly further along the path of the bundle. In their simplest forms, the heater can be an open flame, and the cooler can be a water spray. It may also be useful to pump steam or hot or cold water or other fluids through the pipelines as the insulation is being applied. When molten insulation such as bitumen is poured into the mold as the pipe is laid from the ship, as shown in figure 10, then sea water itself can act as the coolant.

A heater may make it possible to reel pipelines insulated with materials that are brittle at cold ambient temperatures. Heating, either by pumping steam or hot fluids through the pipelines, or by heating the mold from the outside, can soften the material enough to bend or straighten the pipe without cracking the insulation, either as it is transferred from one reel to another, or as it is spooled off during laying. This could be useful where the insulation is bitumen or modified bitumen on a pipeline that is installed at temperatures greatly different from their operating temperature. If buoyancy is added with a pipe, it can be flooded to provide dead ballast, and if it is a continuous pipe it can transport a hot fluid to add heat to the bundle. If this is the intention, then the flotation pipe should be thermally insulated from the sea water and "thermally connected" to the pipelines. This can be done by applying the mold, and the insulation around both the pipelines and the flotation pipe, and by ensuring that the flotation pipe contacts all of the pipelines in the bundle that need to be heated. As shown in figures 2 and 3, the "thermal connection" between the flotation pipe and the pipelines can be further enhanced by applying a thermally conductive material, 4,

so that it contacts a significant area of both the flotation pipe, 2 and the pipelines, 1. For the puφose of this disclosure a thermally conductive material will be defined as a material which has thermal conductivity of 1 BTU/hr-ft.oF or more. One particular benefit of this arrangement is that hot water is often a by-product of processing that is done on the platform. The platform is at the outlet, and hence the cool end of the pipeline, but the hot water is flowing the opposite direction. Therefore, the water temperature need not be as hot as wellhead temperature to be beneficial, so long as it is hotter than the minimum desirable temperature, and so long as there is an adequate volume of water to carry enough heat. Such a large heat tracing line will probably carry all available heating water with little back pressure. If the flotation pipe is not to be used for heat tracing it need not be surrounded by insulation. A cross section of such a pipeline bundle is shown in figure 14. Similarly if only some of the pipelines do not need insulation, they may be attached to the insulated pipes, but not surrounded by insulation. This can reduce the amount of insulation needed to retain the same amount of heat in the other pipes. It also allows the flotation pipe to be releasable so that it can be retrieved once the insulated pipelines are in place.

If the flotation pipe is not continuous or not insulated, additional heat tracing lines may be added to the pipe bundle. These may be pipes that transport hot fluids, electric cables, small steel tubes that contain an electric wire that carries a high frequency electric current.

The procedure for fabricating and installing a pipeline bundle in this way is as follows.

1. Fabricate the bundle of flow lines.

2. If one of the pipes in the bundle or an electric cable, is used to add heat to other pipes, attach it in such a way that heat is easily transferred between the pipes by welding, soldering or applying a heat conducting cement.

3. If a separate flotation is needed, and if it is not dispersed in the insulating composition, attach it to the bundle.

4. Install the mold around the bundle, either during or after the assembly of the bundle.

5. Fill the space between the mold and the pipeline bundle For reeled pipelines, or pipelines that are ^' strung and laid from a barge, this will be done continuously or in increments, as the mold is being installed.

Heating pipelines is only cost-effective if the thermal resistance between the heating element and the fluid in the pipeline is low compared to the thermal resistance between the heating element and the environment. If the pipeline is submerged, the available ways to achieve this are constrained by the electrical and thermal conductivity, and hydrostatic pressure of sea water. This is further complicated by the high cost of time during offshore pipeline installation. Many of the methods for heating onshore pipelines were conceived to overcome the problem of electric shock.

In this invention the electrical conductivity of sea water is used as a benefit by serving as an inherently safe ground and return current path. An electrically conductive pipeline is electrically insulated from the surrounding sea water with a wateφroof coating and the pipeline conducts alternating current that generates impedance heating in the pipe. Preferably, one coating will electrically and thermally insulate the pipeline.

The heating method of this invention has the following features:

1) The pipeline is made from an electrically conductive material. If the pipe wall is thick enough and the pipe material is magnetic, conductive fluids can be carried in the pipeline without applying an insulating coating inside the pipeline.

2) One leg of the AC power supply is connected to the pipeline and the other leg is grounded to sea water.

3) The end of the pipeline nearest the AC power supply is insulated from sea water or ground, typically by insulating it from the offshore platform that supports the power supply.

4) The heated portion of the pipeline is electrically insulated from sea water with a wateφroof coating.

The method of achieving each of these requirements is discussed in the following paragraphs. Most offshore pipelines are made of steel, which is both electrically conductive and magnetic. In most embodiments the skin effect will effectively insulate the pipe from the fluid inside the pipe the pipe wall will be thick enough that

little current reaches the fluid because the pressure in offshore pipelines usually requires that the pipe wall be thick enough for that puφose. In other cases increasing frequency will reduce the depth of the layer of pipe in which current flows, and in yet other cases the inside diameter of the pipeline will be insulated over a significant length.

Flanges and joints that insulate one section of a pipeline from another are commonly used to isolate the cathodic protection system of a pipeline from the facility it serves. Examples are shown in Advance Products and Systems Company's product brochures "Flange Insulating Gasket Kits" and "Iso- Joint. In the preferred arrangement, the pipeline will contain no fluid connectors in the section to be heated so as to avoid the complication, expense and questionable reliability of underwater connectors that are capable of carrying high currents and high fluid pressure. It may be preferable to put the power terminal and the insulating joint above the waterline. To meet both of these goals the pipeline will be installed without fluid connectors in the heated section of the pipeline. In shallow water this can be achieved by pipeline installation methods known in the prior art as "stalking", "J-Tube pulls", "habitat welding" or with a "catenary riser". In the stalking method, an elbow is welded onto the pipeline when the barge is near the platform. The pipeline is then lowered to the bottom as vertical sections are welded together. To use this method for the puφoses described here, the elbow would be thermally and electrically insulated. A J-Tube is "J" shaped pipe that is previously installed on the platform, through which the pipeline is pulled to form a riser. "Habitat welding" is used to make an under water pipeline welds inside a dry bubble that is created by pumping air or gas under a "welding habitat". In some applications the same methods used to cast insulation at the field joint on deck will also be used in a welding habitat. "Catenary risers" are used in deep water pipelines that are installed using vertical pipe lay methods (J-lay or reeling). The riser is connected after laying toward the platform by simply transferring the vertical end of the pipeline from the lay ship to connecting a hanger on the platform, leaving it hanging in a "J" shaped catenary to the sea bed. The suspended portion of the pipeline is called a "catenary riser. Figure 17 shows a pipeline connected to a platform with a catenary riser. Depending on the application, the platform can be supported on the sea bed, on a ship, a barge, or a semi-

submersible vessel, but in other embodiments of this invention the pipeline will extend from shore, in which case a foundation or the ground itself replaces the platform. In applications where submerged fluid connections are used to connect the pipeline to the riser, submerged electrical connections can be avoided by using a submerged insulating joint and connecting a flexible power cable to a wateφroof terminal on the pipeline before it is submerged. The lead can be strapped to the pipeline and then floated to the surface, it can be paid out as the pipeline is lowered to the sea bed, or it can be made up under a bubble trapped under a bell or dome. A pipeline constructed by this method is illustrated in Figure 18. The optional buoy 278 on the cable 277 can reduce the risk breaking the cable without increasing the risk it will become tangled. The buoy is particularly helpful for floating platforms. Figure 20 shows an elevation section view of the submersible electric insulator/terminal 79 in figure 18. This employs an insulating joint similar to insulating joints of the prior such as the "ISO joint", but which also comprises a terminal connection. It is possible to separate the two functions, but for use on pipelines that will carry conductive fluids there can be a benefit to combining the two functions. The skin effect causes current to flow near the outer surface of the pipeline, but in the region between the insulating joint and the terminal the magnetic fields are more complicated. The inside insulating sleeve or coating 224 in Figure 20 insulates the pipeline from the fluid in this area. The additional resistance is proportional to the length of the inside coating. If the insulated area is long enough, impedance heating can limit current flow into the fluid from a thin wall pipeline. If all the parameters are known it is possible to use this to cause resistance heating in the fluid itself. There is an argument in favor of connecting the electrical terminal and the insulating joint beneath the water line, even when there are no submerged hydraulic connectors. If the terminal is well isolated from any shock hazard the potential for spark could ignite escaped gas. In areas where this is deemed to be a hazard, most safety codes require that all electrical wiring and components be enclosed in housings or conduit that have been classified as being "explosion proof. Ports or pipe connections in the enclosures are specified, in part, by the length of the path through which a flame would have to travel to get outside the housing if hazardous gas reaches the electrical connection and cause an explosion inside. For that puφose the insulated pipe proposed here would be treated

as an insulated wire. If the insulating joint and terminal were put inside an enclosure that extends through the waterlfne then very small gas leaks would accumulate. An alternative is to place the terminal and the insulating joint beneath the water line where it is inherently safe for several reasons. There is no spark path to ground and the hydrostatic pressure of sea water is higher than atmospheric pressure, so there is no pressure potential to cause gas to enter void space around the terminal. Finally, the terminal can be made to contain no void or gas-filled space into which an explosive mixture could enter. Figure 20 shows a cross section of the submersible insulating joint 279 in Figure 18 and 299 in Figure 19. Preferably, but not necessarily, one coating will electrically and thermally insulate the pipeline. One suitable way to achieve this is with bituminous compositions, which can be used to thermally insulate pipelines and pipeline field joints. These are very well suited to both the manufacturing method of this invention and the combined function as electrical and thermal insulator. Field joints cast from liquid thermoplastics including, but not limited to bitumen can be made to form a seamless fusion bond to thermoplastic coatings previously applied to the rest of the pipe, to form a continuos, wateφroof coating on the pipeline. In addition to its low thermal conductivity, bitumen has high dielectric strength and high electrical resistivity. It is now disclosed here that cast thermoplastics, and extended thermoplastics including bituminous compositions, can also be made to reliably fuse to produce a continuos, electrically insulating pipeline coating. Thermoplastics other than bituminous compositions, such as waxes, and polyolefins that can be cast or molded can also be used, but most are considerably more expensive and less effective as thermal insulators. If thermal insulation is applied separately, then the thickness required to electrically insulate the pipe will typically be less than 0.25 inches, and almost always be less than 0.5 inches. In these cases the thickness may be driven by toughness and the additional cost may be justified. While all coatings provide some thermal insulation, the cost of power is such that economics would require such coatings to be at least 0.5 inches thick to serve as the insulating coating. For the puφose of this invention , a thermally insulating coating is deemed to be one which is either : a) an electrically insulating coating that is more than 0.5 inches thick, or

b) a coating which provides more resistance to heat transfer between the contents of the pipe and the surroundings than would a 0.375 inch thick coating of a material with conductivity of 0.1 BTU/hr*ft*F° that is achieved directly to the pipe. Some embodiments of the invention may also comprise other thermally insulating coatings. Various thermosetting polymers such as polyurethane, and polyester resins can also be adapted to this puφose, but they are more expensive than bitumen or bituminous compositions. These materials can also be extended with glass bubbles to lower their density and thermal conductivity in some water depths.

Ideally the electrically insulating coating will have very high resistivity and completely isolate the pipe from the sea water, but it is not always essential that the insulation remain totally leak proof. In some embodiments some permeability or penetration of sea water to the pipe is tolerable. If, however, slight water ingress occurs, the matter of electrolytic decay becomes very important. It is impossible to generate high power that is purely sinusoidal, so in some embodiments of the invention, a small direct current bias is impressed on the system to ensure that the pipe remains the cathode. One way to achieve this is to apply sacrificial anodes. If the return path is through sea water, one or more of the electrodes may be made of, or comprise of materials that will behave as anodes.. In such case, the system will be designed, to the extent possible, place it so that it is available for inspection or replacement. Due to the very high currents, the detailed design of such a system will be affected by the power supply. In some embodiments, it may be preferable to design the power supply to to maintain an impressed DC bias on the AC current

In any case, where sea water is used as the return current path, the electrodes must be properly designed and have large enough surface area to reduce current density to low levels, and to be replacable or large enough to remain effective for long periods. In most embodiments, one or both electrodes will be made of be made of a a non-corrosive material such as stainless steel, or a non-metalic, but conductive material, such as compositions or materials comprising carbon or carbon graphite.

Notwihstanding the problems related to electrolysis, the electrical insulation is deemed to be effective even when power loss to sea water over the length of the pipeline is up to ten percent.

The integrity of the electrical insulation is of particular concern at field joints. Figure 19 shows a cross section of cast field joints made where the pipe is laid vertically from a ship in the pipeline installation method known in the prior art as the "J-lay method". Figures 22 and 23 show cross sections of cast field joints for pipes that are assembled horizontally on barges or ships and installed using the installation method known in the piror art as the "S lay method. In this method the pipes are joined as they lay on horizontal rollers on the barge deck and the barge then pulls itself from under the joined pipes as they are deployed to the sea bed. If the insulation is very thick then thermoplastics may not be cool fast enough to completely solidify before the pipe is moved to the next station relative to the barge. In that case the field joint insulation will not support the weight of the pipe as it moves over the rollers. One solution is to add stiffeners to the mold, or simply use a mold that is made thick enough to support the pipe on the roller. Figures 22 and 23 show that a pre-cast insulating insert 247 can be used to reduce the length that mold must span and to reduce the time needed for cooling. The insert 247 is shorter than the space 4a between the insulation 246 so that the liquid field joint material 248 bonds to the insulation 246 and the insert 247. The insert 247 in figures 22 and 23 is made of a flexible, somewhat elastic material that allows it to be slipped over the pipe 241 by expanding the opening at the top of the insert 247. The opening then springs back to keep the insert in place while the mold is being installed. If this "snap on" feature is not needed, the insert need only cover the section of the pipe that will support roller loads. This will be 'less than half the circumference of the pipe, so inflexible materials can, therefore, also be used The insert can be attached to, or made a part of the mold. If the mold is strong enough to cross the rollers, or if the insulation cools fast enough, no insert is needed. The preferred embodiments of the field joint and how it should best be applied are subject to wide variation within the scope of the invention. The field joint coating described above is said to be cast, but injection, spraying and wrapping are also contemplated. Other embodiments are envisioned where no field joints exist in most, or all of the pipeline, such as when the pipeline is installed by reeling.

If the pipe material and dimensions do not change through the length of the pipeline then the distribution of heat generated through the pipeline is constant, and

cumulative along the length. The effect on the temperature near the inlet is, therefore, slight because only a small fraction of the heat is generated there. There are some applications where this uniform heat distribution is not the best use of power. An example is paraffin control. The temperature required to remove paraffin deposits is higher than the temperature at which they precipitate on the pipe wall. It may be more cost effective to periodically raise the temperature of the pipeline to above the melting temperature than to continuously heat the pipeline to above the paraffin precipitation temperature. If the temperature at the inlet of the pipeline is below the melting temperature of the paraffin deposits, but above the paraffin precipitation temperature then paraffin may accumulate near the wellhead. In this case the same amount of heat would be much more effective if it were applied near the inlet. One way to achieve this is to use pipe made of a different material or different wall thickness in different sections of the length. Another solution is to use the pipe as a conductor for current that operates a heater that exchanges heat near the end of the pipeline. One method is to connect a cable in series between the pipeline and the return current path, whether that be another pipeline or a sea water ground. The cable can then be coiled around a section of the pipeline at the end, increasing the number of ampere turns in the pipeline and causing induction heating that adds to the impedance heating near the end of the pipeline. The heating can be concentrated near one end by simply putting the ground electrode nearer the electrical terminal section, that is by simply heating a shorter length of pipeline. Referring to Figure 19, this can be done either by moving the insulating joint and terminal 99 nearer the wellhead or by putting the ground electrode 94 nearer the platform. In other embodiments of the invention the current flowing pipeline can be in series with other types electrical apparatus or equipment, so that the AC current flowing through the pipeline can also power said electrical apparatus. One such embodiment uses this power to run pumps in the susea well. In another embodiment, the equipment that is directly connected in series with the pipeline is the primary winding of a transformer coil that induces power in the secondary coil, that powers other equipment.

Detailed Description of The Drawings

Figure 1 shows a side elevation view of a portion of an assembled, insulated bundle of pipelines, 8, that are inside identical circular mold sections, 5, that overlap along the length, and are held in place by fasteners, 9 around circular rims, 3, to form a mold for casting insulation around the pipelines, 1 , and a flotation pipe, 2.

Figure 2 is a cross sectional view taken at the section line A-A in figure 1, showing circular molds, 5 held in place by fastener, 9, that keeps the edges of the mold at a fixed distance from one another, around the flow lines, 1, and flotation pipe, 2, leaving an opening, 10, through which the insulation, 7 was poured or placed. The flotation pipe is thermally connected to the flotation pipe by conductive cement, 4.

Figure 3 is a cross sectional view taken at section line B-B in figure 1, and showing that the circular molds, 5, that are formed around the bundled pipes, 1 and 2, by wrapping or forming lengths of sheet metal or other thin material around a circular rim, 3, that is attached to the flow line bundle by spacers, 6. The sleeves are held in place with fasteners, 9. Successive, identical, sections, 5, overlap to form a continuous mold along the entire length of the bundle. The space between the mold segments, 5 and the pipes, 1 and 2 is filled with insulation, 7, is cast through opening, 10, in the top of the mold. The flotation pipe is thermally connected to the flotation pipe by conductive cement, 4. Figure 4 shows a cross section of a bundle of four flow lines , 11 and flotation,

12, while a section of a rectangular mold, 5 is being installed around it. Spacers, 16 will serve to maintain the correct spacing between the mold and the bundled pipes, 11 and 12, after the mold is installed.

Figure 5, shows a cross section of the pipeline bundle shown in figure 4, taken after the mold, 15, is placed, and has been secured with fasteners, 19, leaving opening, 18, at the top, but before insulation is in place.

Figure 6 shows a cross section of the same bundle of flow lines shown in figure 5, after the insulation, 17, has been placed through opening, 18, inside the mold, 15 and around the pipes, 1 1 , and 12. Figure 7 shows a side elevation of a pipeline, 21, bundled with a smaller pipe,

22. These pipes are shown after they have been strung together at their ends with other pipes, and are being moved in increments, downward on rollers, 23, from the

upper left of the page to the lower right. Between each increment of movement, sleeves, 25, much like sections of stove pipe, are placed and held in position around the pipes, 21 and 22, with spacers, 26. Insulation is pumped or injected through pipe, 29, into the space between the sleeves, 25 and the pipes, 21 and 22, to level, 24. A water spray, from heater/cooler, 30, is used to cool the insulating composition. As the molded, insulated bundled moves, it is spooled onto a reel, the outermost layer of which, 28, is shown.

Figure 8 is a view taken at section line A-A in figure 7, showing that the mold section, 25, was installed from the side around pipes 21 and 22, and then joined at seam, 20, and held in position relative to the pipes, 21 and 22, by spacers, 26. Insulation is then pumped or injected into the space between the pipes and the mold, 25 through pipe, 29.

Figure 9 is an enlarged detail of a sheet metal lock seam, which is one type of many possible types of seams, 20, shown in figure 8, that may be used to join the edges of the mold, shown as 25 in figure 8.

Figure 10 shows pipes 31 and 32, that are being moved downward, off of a ship, 38, into the water, 36, as sheet metal stove-pipe-like mold sections, 35, are successively positioned from the to overlap each other and form a continuous mold around the pipes, 31 and 32. Insulation is transported through pipe, 39, to fill mold sections 35, that are then progressively lowered with the pipes into the water, 36.

Figure 11, shows a pipeline, 41, bundled with a smaller pipe, 42. These pipes are shown after they have been strung together at their ends to other pipes, and are being moved downward, from the upper left of the page to the lower right, as a sheet material is fed from a coil, 43 and formed with rollers, 46, into a circular mold, 45, around the pipes, 41, and 42. Insulation is pumped through pipe, 50, to fill the space between the mold, 45, and the pipes, 41 and 42, before the edges of the mold are joined with a tool such as a crimping or welding machine, 47. A water spray from a heater-cooler, 50, is being used to cool the insulating composition if needed. As the molded, insulated bundle moves, it is spooled onto a reel, a portion of the drum of which, 48, is shown.

Figure 12, is a view taken at section A-A in figure 1 1.

Figure 13 is a side elevation view of the pipeline bundle in figures 1, 2 and 3.

Insulation is pumped through ^" hose, 64 into molds, 5, that are wrapped around floatation pipe, 2, and flow lines, 1, that are supported on rollers, 61 to form a continuous cast bundle. As successive molds, 5 are filled, cable 65 is used to pull the entire cast bundle assembly offshore, beneath the waterline, 63,

Figure 14 is a cross sectional view of a bundle of two pipelines, 71 surrounded by insulating material, 77 that is cast inside mold, 75 by pouring the insulating material through the opening, 79 at the end of each mold section, 75. Floatation pipe 72 is attached to the insulated pipelines, 71, but not enclosed in the insulation. Figure 15 is an end elevation of a spacer that is made from an insulating material. Spacers are placed periodically along the assembled length of the pipelines before molds are placed and act as form against which the molds, such as mold, 275 in Figure 14 are positioned relative to the pipelines, such as pipeline, 71 in Figure 14. The spacer is also used to close the ends of said molds, so that they can be filled with the insulating material, shown as 77 in figure 14.

Figure 16 is side elevation of the spacer shown in Figure 14. Figure 17 shows pipeline part 201 suspended from a platform in a catenary to the sea bed 204, extending from there to a subsea wellhead 207 that is connected to the pipeline part 201 with a jumper pipe 208 with quick connecting fluid connectors 209 at each end. The pipeline part 201 is electrically insulated from the ambient sea water and insulated from the platform by insulating joint 202. The joint 202 comprises an electrical terminal that electrically connects the pipeline part 201 to an electric cable 210 that is in turn, connected to one leg of AC power supply 212. The other leg of AC power supply 212 is connected by cable 211 to a ground electrode 213 that is submerged in sea water. The end of the pipeline part 201 near the subsea wellhead 207 is grounded to sea water and the circuit is thereby completed through the sea water.

Figure 18 shows an offshore pipeline part 261 that is connected near the sea bed 264 to pipeline riser 276. The pipeline part 261 extends from there along the sea bed 264 to subsea wellhead 267 to which pipeline part 261 is connected with jumper pipe 268 and connectors 269. The pipeline part 261 is also connected to ground electrode 274 near the wellhead 267, and is electrically insulated from ambient sea

water and from riser 276 by submerged insulating joint 279. Insulating joint 279 comprises a terminal that connects pipeline section 261 through a flexible power lead 277 to AC power supply 272 on platform 265. The other leg of AC power supply 272 is connected to sea water electrode 273 through cable 271. The end of pipeline section 261 near subsea wellhead 267 is also connected to sea water electrode 274. The sea water completes the circuit.

Figure 19 shows a pipeline part 281 suspended from a platform 285 in a catenary arc to the sea bed 284. Said pipeline section 281 extends from there along the sea bed to join to subsea wellhead 287 with connectors 289 and jumper pipe 288. The pipeline part 281 is electrically insulated from the ambient sea water with an insulating coating and is insulated from the platform 285 with a submerged insulating joint 299. The insulating joint comprises an electric terminal that connects the pipeline part 281 to a cable 252 that extends through conduit 250 and connects to a leg of an AC power supply 292. The other leg of the AC power supply 292 is connected to a sea water electrode through a conduit 251. The end of the pipeline section 281 is also connected to a sea water electrode 294 near the subsea wellhead 287. Sea water thus completes the circuit between the sea water electrodes.

Figure 20 is an elevation section view of an electrically insulating joint such as joint 279 in Figure 18 and 299 in Figure 19. It comprises an insulating joint 220 similar to those in the prior art covered with an insulating coating 222 that extends over the segment 215 of a pipeline which corresponds to a segment of pipeline part 261 in Figures 18 and pipeline part 281 in Figure 19.

Figures 21, 22 and 23 show details of pipelines of this invention comprising lengths of pipe joined by field welds. Figure 21 shows an elevation cross section view of a pipeline field joint that has been insulated with an insulating material 236 that is field cast inside a sheet metal mold 238, that encloses the weld 232 that joins two vertical pipes 231 that are insulated along the bulk of their length with an insulating coating 237. The insulating coating 236 bonds to the previously applied insulation 237 to form a wateφroof, electrically insulating coating that is continuous across the pipeline. This is one embodiment of an electrically insulating field joint for pipelines of this invention that are installed vertically offshore using the "J" lay pipe installation method.

Figure 22 shows an end cross section view taken through a pipeline field joint that has been insulated by casting ^" an insulating material 248 in a sheet metal mold 244 that is held in place by bands 243 so that it encloses the field weld that joins two horizontal pipes 241. This is one embodiment of an electrically insulating field joint for pipelines of this invention that are installed offshore horizontally using the "S" lay method. The flexible insert 247 supports the mold under the weight of the pipes 241 as the mold 244 rolls over support rollers on the pipe lay barge.

Figure 23 shows an elevation section view of a cast field joint taken at section BB in figure 22. The insulating material 248 cast around the field weld 242 and the flexible insert 247, and bonds to the insert 248 and the previously applied bulk insulation 246 to form a wateφroof electrically insulating coating that is continuous across the pipeline. The insert 247 shortens the axial distance spanned by the mold 244 while the material 248 is curing. The distance is reduced from the length 4a that would exist without the insert 247 to lengths 4b. In some embodiments the mold would be able support the imposed loads without the insert, and in those embodiments the insert would not be used.