New field joint Coating system for trenchless laid steel pipes

Both application procedures require a certain period of time and are accompanied by drawbacks, which are typical for the application of reactive resins. This report describes a new procedure for the coating of welded joint areas of steel pipes with polyurethane reactiveresins. By use of a special casing system the complications of applying the coating material by spatula are avoided. Consequently one can take complete advantage of using polyurethane coatings for HDD laid pipes without suffering from typical handling drawbacks.

Corrosion protective coatings of steel pipes laid by horizontal directional drilling are exposed to high mechanical stresses. Particularly the peel- and shear stresses and also the possible abrasion and indentation stresses go far beyond the conditions of open trench pipe laying. Rocks or gravel narrowing the drilling hole represent a typical risk for coating damages

As far as factory coatings are concerned, the above mentioned risks are taken into consideration and compensated by using reinforced HDPE- or PP-threelayer coatings or in special cases by additional protection of the whole pipeline conduit by glass reinforced plastics or fibre concrete coatings. [1]. Among field joint coatings, especially thermosetting plastics (Polyurethane, Epoxy) due to their high lap shear strength and hardness are capable of withstanding the typical HDD stresses.

Furthermore such coatings can be applied in alignment with the existing mill coating. From the application point of view, Polyurethane coatings are the preferred material, because they have shorter curing times and the typical application by spatula is easier and faster than lamination procedures to produce GrP epoxy coatings. A new Polyurethane coating, which fulfils the general requirements for a joint coating on trenchless laid pipes and which meets the requirements of the current material standard for PUR pipe coatings is described in [2].

Following state of the art technology it should be required that every kind of thermosetting pipe coating is in accordance with either EN 10290 (Polyurethane) [3] or EN 10289 (Epoxy) [4]. DENSOLID HDD, which has already been used in practice [5] successfully, is a class B - type 3 coating according to EN 10290 (operating temperatures -20 to +80 °C), which indicates the suitability of DENSOLID HDD for even high corrosive and mechanical stresses.

Although the application of two component resins by spatula is easier compared to lamination procedures, the method nevertheless offers various possibilities for improvement. The main target of the subsequently described application method was the avoidance of typical drawbacks or complications of manual reactive resin application by spatula, which are:

• the possible generation of mixing errors due to incomplete mixing.
• the possibility of creating air entrapments in the blended material due to manual stirring.
• the necessity of a multi step application to obtain coatings of some mm thickness. The necessary overcoat waiting periods extend the time for application and require to maintain a proper weathering protection of the working area.
  
  Generally it would be possible to produce a viscosity, which allows to apply the two component compound in one step without sagging, but this would even increase the complications regarding proper mixing and air.
   
• the difficulty to produce an even coating thickness by manual application.This might require extra efforts to ensure the minimum coating thickness.

Furthermore it has to be considered, that all two component reactive resins have to be protected against weathering and ambient influence unless cured. It seams obvious, that a casing system could avoid a lot of the mentioned complications.

Casing systems are already used in pipeline construction applications, such as cement grouting of field joints for completion of the additional concrete pipe coating.

However, this principle can not directly be transferred to the corrosion protective coating of field joints, because the narrow annular gap would make it impossible to fill the casing without cavities.

Even grouting of low viscous reactive resins into a premounted casing would not completely avoid the formation of cavities, whichobviously can not be accepted in corrosion protective coatings.

The subsequently described application procedure (patent pending) therefore makes use of a casing, which is closed only during the filling process. By use of tensioning strips and bracing bars it is ensured, that the casing sheet can easily be tracked and that it remains safely attached to the pipe surface where already closed. The mounting of the casing system components prior to start of the filling procedure is shown in Figure 1a and Figure 1b, while Figure 2a to Figure 2f demonstrate the corresponding filling principle.

Depending on the thickness of the pipe factory coating and the height of the weld bead it could be necessary to add a spacer to the casing system. According to the relevant PUR standard (EN 10290, class B), the minimum coating thickness is 1.5 mm. Considering possible abrasive stresses during pipe laying it seams reasonable to add a safety margin by requiring a minimum coating thickness of 2.5 mm.

The coating thickness, which can be obtained on top of the weld bead, directly results from the difference of mill coating thickness and height of the weld bead.

If this difference is smaller than approx. 3 mm (see Figure 3), a minimum coating thickness of 2.5 mm can only be obtained reliably by additional use of a spacer, which could be produced by wrapping one or more layers of a corrosion protective tape on either side of the joint area (see Figure 1b).

Provided that the typical height of the weld bead is 2 mm, the use of a spacer can be avoided, if the factory coating thickness is equal to or higher than 5 mm. Such reinforced or increased thickness coatings take into account the possible stresses during trenchless pipeline construction and by this are common particularly for big diameter pipes laid by horizontal directional drilling.

If a spacer is used, the step in the transition to the factory coating can easily be bevelled in an angle equal to or lower than 30° by grinding, which eliminates this possible point of attack (see Figure 3b). As the direction of the longitudinal edge formed by the overlap of the casing ends (see Figure 1f) is equivalent to the direction of pipe pulling, bevelling of this edge is not necessary. Furthermore it has to be mentioned, that due to the very high adhesive and cohesive strength of DENSOLD HDD, mechanical stresses affecting the longitudinal edge during pipe laying would only result in abrasive chamfering instead of chip off effects.

Filling of the annular gap between pipe and casing sheet can obviously not done out of tins or buckets, because at least the bottom of the pipe is inaccessible by this procedure. Additionally, even if a low viscous material was used, grouting without forming cavities would be extremely difficult.

For that reason DENSOLID HDD is packed in two chamber cartridges, which allow both the reliable mixing via an attached static mixer and the precise filling of the annular gap between pipe and casing.

During the application process, either side of the pipe is filled one after the other. Prior to filling, the casing is mounted in a way that the casing end, which is closed first, ends in 12 o’clock position. During filling it is of most importance, that a bulge of material is present over the whole coating width, by which air enclosures are avoided.

Tracking of the casing sheet is done by means of the bracing bar on either side of the pipe. The central bracing bar always remains at the bottom of the pipe. By this system and combination of tensioning strips and bracing bars it is ensured, that the casing sheet always has tight contact to the factory coating or spacer where already closed, which prevents undesired shifting or lifting.

The casing sheet is made of rigid polypropylene, which is nevertheless flexible enough to permit bending around pipe diameters from 4“ upwards. In pipe longitudinal direction, the casing sheet’s stiffness prevents dent formation and by this ensures an even coating thickness and the necessary coverage of the circumferential weld bead.

The casing furthermore protects the coating material against weathering, rainfall and ambient influence unless cured. Tents or other protective measures for the working area only have to be maintained until the casing is closed, which significantly reduces the on site efforts, compared to other application procedures.

The removal of the casing can be carried out as soon as the coating is tack free, which in case of DENSOLID HDD takes approx. 2,5 hours at 20 °C.

Because of its very smooth surface, the casing sheet can easily be lift off from the tack free cured coating without residues. Same as the other components, the casing sheet can therefore be reused after removal. Considering an application period of approx. | hour per joint (depending on the pipe diameter), only 5 casing sets are required for a continuous progress of work.

Currently the volume of commercially available two chamber cartridges is limited to 400 ml. To obtain a coating thickness of 5 mm, between 2 cartridges for a 4“ pipe and 14 cartridges for a 40“ pipe are thus required per welded joint.

Particularly in case of bigger diameters or long pipeline conduits [6] it could be desired to make available higher contents per packing unit, which would reduce costs by faster application and less packaging waste.

For this purpose and in addition to the standard size cartridges, a newly developed, pneumatically driven discharging machine (patent pending) with a working volume of approx. 6 l is available (Figure 5).

With one filling, consisting of two 2 l hose packs of resin and one 2 l hose pack of hardener, welded joints on 40“ pipes can be coated continuously without refill interrupts.

Summary
The newly developed casing system and procedure described in this report significantly simplifies the process of coating welded joints on steel pipes with two component thermosetting compounds.

The employment of such coating materials is particularly useful on pipes laid by horizontal directional drilling. Compared with standard manual application procedures, the new coating system provides various advantages, which are

• Fast working progress by one step application, regardless of the final coating thickness.
• Mixing errors or air enclosures, resulting from standard manual mixing, are
  avoided by application from a two chamber cartridge with an attached static mixer.
• The uncured coating is protected against weathering and other ambient influences. By this, the efforts for protection of the working area are significantly reduced.
   
• The new casing system allows to produce an even coating thickness, which also and reliably ensures the required minimum coverage on top of the weld bead.

References
[1] Funk, D. "Anforderung an die Umhüllung für im Bohrpress- bzw. im Horizontal-Drilling- Verfahren verlegte Rohrleitungen", bbr, Sonderausgabe "Grabenlose Rohrleitungsbauverfahren", 2000

[2] Quast, M. "Neue Polyurethan-Schweißnahtbeschichtung für Horizontalbohrungen", 3R international (42), Heft 10- 11/2003, Seite 703 - 707

[3] DIN EN 10290 (August 2004) "Stahlrohre und -formstücke für On- und Offshoreverlegte Rohrleitungen - Umhüllung (Außenbeschichtung ) mit Polyurethan und polyurethan-modifizierten Materialien; Deutsche Fassung EN 10290:2002"

[4] DIN EN 10289 (August 2004) "Stahlrohre und -formstücke für On- und Offshoreverlegte Rohrleitungen - Umhüllung (Außenbeschichtung ) mit Epoxi- und epoxi-modifizierten Materialien; Deutsche Fassung EN 10289:2002"

[5] Fengler, E. "Umlegung einer Gashochdruckleitung durch Kreuzung des Dortmund-Ems-Kanals", bbr 1/05, Seite 35 - 37

[6] Lukas, A. "The practicality of drilling very long pipelines under hazardous terrain - 5 km, 10 km", Paper for IPLOCA Convention 2004, Berlin.

[Source: 3R INTERNATIONAL: Journal for Piping, Engineering, Practice
Special Edition 13/2005]