US 4,377,894 discloses a method wherein a double pipe is bent using a suitable bending tool. In a further step, the outer surface of the outer pipe is heated and/or the inner surface of the inner pipe is cooled. Consecutively, an expanding force, in particular a hydraulic pressure, is applied to the interior of the inner tube. After removing the hydraulic force, the temperature of the outer pipe is caused to be lower than that of the inner pipe, in order to produce a tight fit between the outer pipe and the inner pipe. This technique is known as thermo- hydraulic technique.

A problem with this known technique is that it is relatively cumbersome and expensive to implement. A further problem is that upon bending the double pipe, the inner pipe loses contact from the outer pipe at the outer side of the bend (extrados). In addition, buckling occurs in the inner pipe at the inner side of the bend (intrados). These loosening and buckling problems are illustrated in Figure 2.

Buckling occurs at the inner side of the curve due to the fact that the inner pipe has a limited thickness, typically about 3 mm. The problem of loosening occurs due to the fact that the materials used for manufacturing the inner and outer pipes have a different coefficient of expansion. In particular, the outer pipe is made of carbon-steel and the

inner pipe of stainless-steel. The coefficient of expansion of stainless steel is larger than the coefficient of expansion of carbon-steel. In particular, the average coefficient of linear expansion for carbon steel is approximately 14*10-6/°C and for stainless steel approximately 18*10-6/°C. The stainless-steel inner pipe will therefore shrink more than the carbon-steel outer pipe. As a result, the inner pipe is loosened from the outer pipe.

SUMMARY OF THE INVENTION An object of the present invention is to provide an alternative method for forming a bend in a double pipe, achieving satisfactory results, which is less cumbersome and less expensive to implement, and wherein the buckling problem is reduced.

This object is achieved in a method according to the invention for forming at least one bend in a double pipe comprising an inner pipe located within an outer pipe, wherein the inner pipe has a coefficient of expansion higher than the coefficient of expansion of the outer pipe. The method is applied on an outer pipe having a yield point higher than the yield point of the inner pipe.

The method essentially comprises the following steps. In a first step, the inner pipe is filled with a compressible material. The compressible material is compressed in the inner pipe. This first step allows to reduce buckling and loosening of the inner pipe upon bending.

In a second step, the double pipe is bent using a pipe bending tool.

Then, the filling material is removed from the inner pipe. In a further step, an expanding force is applied to the inner pipe until the outer pipe has undergone a predetermined expansion.

The compressible material is in particular a granular material, such as sand. Use of sand for reducing buckling upon bending

single pipes is known, as indicated for example in EP-A-0 099 714.

However, filling sand has only been applied for bending single pipes.

The invention is related to a method for bending double pipes. It has been discovered that the use of the compressible material, in particular sand, reduces buckling of the inner pipe at the inner side of the bend, but also reduces loosening of the inner pipe from the outer pipe at the outer side of the bend. Furthermore, the subsequent steps of applying a heat source to the outer pipe and/or a cooling source to the inner pipe is no longer required. It is sufficient to apply an expanding force for achieving satisfactory results. Consequently, the method according to the invention is less expensive to apply than the known thermo-hydraulic method.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a section view of a portion of a double pipe before bending.

Figure 2 is a section view of a portion of the double pipe of Figure 1 after bending using conventional techniques without filling with a compressible material.

Figure 3 illustrates the double pipe of Figure 1 after bending using the method according to the invention, but before the step of applying an expanding force.

Figure 4 is a diagram illustrating the direction of movement and the speed of the bending tool and the double pipe in function of the time during the bending operation.

Figure 5 is a section view of the pipe of Figure 3 showing the extension piece wherein an opening is provided.

Figure 6 is a section view of the double pipe of Figure 1 after the expanding force has been applied.

DETAILED DESCRIPTION OF THE INVENTION The method for forming at least one bend according to the invention is applicable on double pipes or so called bimetallic pipes. In particular, a corrosion resisting inner pipe 10 is telescopically aligned inside a carbon outer pipe 12 and expanded by an expansion and calibrating process known as such. An example of such a pipe is manufactured by Butting under the trade name BUBI pipe.

The outer pipe is provided for ensuring strength, while the inner pipe is provided for ensuring protection against corrosion.

Typically, the inner pipe is made of stainless-steel and the outer pipe of carbon-steel.

Experiments have been performed with the following pipes : a carbon steel outer tube (available at Butting, API 5L-X65) having a yield strength of minimum 450 MPa (Megapascal), a coefficient of expansion of approximately 14*10-6/°C, a diameter of 219.1 mm and a thickness of 8 mm; a stainless steel inner pipe (available at Butting, ASTM A312 type 316L having a yield strength of minimum 170 MPa, a coefficient of expansion of approximately 18*10-6/°C, a diameter of 203 mm and a thickness of 3 mm.

Before bending such a double pipe, the inner pipe 10 is filled with a compressible material 11, which could for example be a granular material, in particular sand. The compressible material should be a non metal and resisting to relatively high temperatures. The compressible material is compressed in the inner tube using a technique known as such for single pipes.

Providing the compressible material considerably reduces buckling, ovality and loosening of the inner pipe. Indeed, if the double

pipe is bent without having been filled previously with sand or the like, the obtained bend will be shaped as illustrated in Figure 2.

After bending, the compressible material is removed. The obtained pipe bend is shaped as illustrated in Figure 3. For bending, use is made of an appropriate bending tool. In particular, a hot bending technique is applied using a heating mechanism, such as an inductor.

Using sand or another compressible material offers the further advantage that the temperature of the inner tube is essentially equal to the temperature of the outer pipe. As a result, when hot bending is applied, the deformation of the inner pipe 10 is performed at a warm temperature, since the heat from the inductor will conduct through the outer pipe and the inner pipe.

This method is in contrast with the state of the art, where the inner pipe is bent in a cold state, since a cooling source is applied to the cavity of the inner pipe. Cold bending of the inner pipe increases the yield point of the inner pipe. It is important however to keep the yield point of the inner pipe lower than the yield point of the outer pipe for reasons explained hereinafter. Hot bending has less influence on the yield point of the inner pipe.

Figure 4 is a diagram illustrating the direction of movement, the speed of the bending tool vi and the speed of the double pipe vp in function of the time t upon bending the double pipe. The bending operation is performed from time span ti to t6. Within this time period, a preheat phase (from t, to t2), a start bending phase (from t2 to t3), a bending phase (from t3 to t4), an end bending phase (from t4 to ts) and a post heating phase (from t5 to t6) are distinguished.

In the preheat phase, the inductor is moved in a first direction with a speed vi, which is constant between time t, and t2. In the start bending phase, the inductor speed vi progressively decreases.

Between time t2 and t3, the double pipe is also moved in a second direction, opposite to the first direction, as illustrated in the lower part of Figure 4, with a progressively increasing speed vp.

During the bending phase between time t3 and t4, the inductor is not moved, but the double pipe is moved with a constant speed vp.

During the end phase, comprising the end bending phase and the postheating phase, the inverse operation of the start phase is performed. In other words, the double pipe speed vp is progressively decreasing between time t4 and t5, while the inductor speed vi is progressively increasing. Between t5 and t6, the inductor is moved with a constant speed v ; white the double pipe is not moved.

The start and end phases allow to form a smooth transition between the straight part of the double pipe and the bend. In addition, local buckling at these transitions is considerably reduced and the compression of the sand is improved, due to the effect of shrinking.

Preferably, the relative speed between the inductor and the double pipe is kept essentially constant during the bending operation, as illustrated in Figure 4. In other words, the sum of the speeds vi and vp is constant.

The duration of the initial phase, the intermediate phase and the end phase, as well as values for the speed will depend on a number of parameters, such as the material used for the inner and outer pipes, the diameter, the thickness. In the example of a double pipe described hereinabove, the speed is approximately 50 mm/min, time span t2-ti approximately 30 seconds, time span t3-t2 approximately 45 seconds, time span t4-t3 approximately 35 minutes, time span tus-tu approximately 45 seconds and time span te-tu approximately 30 seconds.

After bending, an expanding force is applied on the inner pipe, as indicated by the arrows in Figure 5. As expanding force, hydraulic pressure is in particular applied. In the given example the hydraulic pressure is in order of 47.5 MPa. The expanding force is applied until the outer pipe has undergone a predetermined expansion.

The inner pipe will first be subjected to an elastic deformation and then a plastic deformation. After a determined degree of expansion, the inner pipe is in contact with the outer pipe. Keeping the expanding force will lead to a further plastic expansion of the inner pipe together with an elastic expansion and subsequently a plastic expansion of the outer pipe.

Both pipes are thus plastically expanded.

This step of applying an expanding force is required, since a space is formed between the inner pipe and the outer pipe, as shown in Figures 3 and 5. The space is present due to the heating applied upon bending the pipes and due to the different expansion coefficients of the inner and outer pipes. It should be noted that the space between the inner and outer pipes is considerably smaller than a corresponding space when applying the conventional techniques, wherein the bend is formed without using a compressible material.

An opening 15 is preferably provided for allowing air, present in the space, to escape when the expanding force is applied.

The opening is preferably provided in an extension part 16 of the double pipe, which extension part will be cut when the bending operation has been completed.

The expanding force is released when a predetermined degree of expansion of one of the pipes, in particular the outer pipe, has been reached. In the given example, the expanding force has been released when the outer diameter of the outer pipe has expanded of 1 % of the outside diameter. If the yield point of the outer tube is larger than

the yield point of the inner tube, the outer pipe will tend to shrink more than the inner pipe. This ensures a tight fitting between the inner and outer pipes.

The obtained bend after release of the expanding force is illustrated in Figure 6. It has been found that the two pipes are tightly connected together with a clamping force which is larger than the camping force of the pipes before bending. In the example given, a clamping stress, determined according to the API 5LD standard, in the order of 20 MPa has been found.

In the described embodiment, only one bend has been formed in the double pipe. It is conceivable to apply consecutively a plurality of bends in a same double pipe.

PARTSLIST 10 inner pipe 11 compressible material 12 outer pipe 13 space 14 inner side bend (intrados) of inner pipe 15 opening 16 extension