Changes on the surface.....laser based modification of polypropylene pipe coatings for oil and gas applications

TWI Bulletin, May - June 2009

A proposal with a difference is about to be launched by TWI following its work on improving surface adhesion of polyurethane to polypropylene pipe coatings...using lasers

Richard Shepherd joined TWI in the Spring of 2007. For the 25 years prior to this he has worked for various R&D and engineering organisations, working with engineering applications for polymers; many in the Oil & Gas sector. He has published a number of papers specifically in the use of polymers in oilfield applications and is joint inventor on a patent for high pressure and temperature dynamic seals. Richard has qualifications in Mechanical and Production Engineering.

Currently he is working in a number of areas including polymers in sweet and sour environments, permeation of polymers, pipe coatings, flexible joint technologies and non-destructive test methods for high value polymeric components.

The work could revolutionise the way polypropylene pipe coating field joints are applied. Early work has shown great promise, achieving better than industry target adhesion strength between fully representative materials. As Richard Shepherd explains this work has demonstrated the benefit of scanning (pulsed) laser head technology in the surface preparation of field joints, using appropriate power inputs, the same laser could be used to prepare the steel pipe surface by replacing conventional grit blasting.

Half a dozen of the oil giants within TWI's Industrial Membership have already responded positively and there is a current initiative to develop further funding from industry to refine the technology and verify the durability and robustness of the developed joints.

Effective pipe insulation and anti-corrosion coatings are critical to the cost effective and reliable production of oil field fluids. Increasing well depths have required corresponding increases in the lengths of flow lines, which again demand efficient and long term effectiveness of the insulating system in order to prevent cooling of the fluids. In addition any improved functionality of the life of the system will extend the productivity of marginal wells while reducing the requirement for expensive workover treatments, necessary to prevent the formation of hydrate plugs and wax deposits and eliminating longer term issues of corrosion.

A typical insulation system consists of a multilayer polypropylene (PP) composite with Fusion Bonded Epoxy (FBE) as the adhesive layer to the steel (Figure 1). Specific requirements for protection or thermal insulation are taken care of by specially designed systems; resistance to deep water and high temperature can be catered for by adjusting the density of the layers, however, an integral part of the insulation system is the field joint which forms a uniform thermal insulation coating over the total length of the pipeline.

A field joint is required at each welded joint as a flowline is fabricated. Oil field flowline design is showing an increasing trend to longer distances adding increasing dependence on the overall integrity of the anti-corrosion/insulation coatings.

Offshore flowline fabrication and installation is typically performed by one of two methods; reel ship or lay barge.

For reel ship installation field joints are fabricated onshore when the individual pre-coated pipe lengths, typically of the order of 12m, are welded to form continuous sections up to a kilometre in length, for installation. For this process typically the steel is welded, there then follows a grit-blast cleaning process for the steel and then the field joint layer is injected beneath two clamped half shells that are placed over the joint region. The field joint material used here can either be from a compatible polypropylene (PP) or a polyurethane material.

When installed by lay barge the pre-coated pipelines are loaded onto the lay barge for fabrication at sea, field joints being produced as part of this process. Generally PU based systems are used here for speed and simplicity. Figures 2 and 3 illustrate reel ship and lay barge pipeline installation systems.

The grand plan

In response to industry requests the work here has been to explore the viability of laser surface treatments to enhance the adhesion between field joint materials (PU based) and the base PP insulation systems. The objective, to investigate the basic principle/viability of using lasers to enhance surface activation and hence adhesion between two otherwise dissimilar materials, is to generate initial data for the basis of a future GSP project proposal.

Currently industry field joint providers aim to achieve a minimum bond strength of 2-3MPa (believed to be in a 90° peel test geometry). This can be achieved in the laboratory under ideal conditions with some consistency, however, in the field this is not always the case. Failure of a field joint at the reeling or pipe lay stages requires costly repair and if undetected can compromise the life of the pipeline.

The story so far...

Sample materials for the work have been provided in the form of, nominally 1.6mm thick and 75mm wide solid PP strip and a fully representative five layer PP foam system (as described in Figure 1) applied to a 3m length, 12inch diameter pipe, providing a total insulation thickness in the order of 30mm.

Laser surface modification trial at TWI - Trial 1

Early work established the ability to apply laser energy in a controlled fashion to significant areas of material for bonding trials using the TWI Spectron Q-Switch Quad YAG laser with the wavelength set to 1064nm. However it was quickly apparent that the process while effective would not offer anything like the productivity required for a practical industrial solution (see Fig.4).

Scanning laser systems

From the work above it became apparent that the method of sample production would have to be significantly more efficient to allow even a small range of parameters to be explored. TWI approached an equipment supplier which provides a unique beam deflection technology to allow the efficient, systematic distribution of single laser pulses. The pulses can be aimed with some precision allowing surfaces to be rapidly scanned. For this work a CL 150 (diode pumped Nd:YAG) was used with scanning optical head, the use of fibre optic and direct coupling was also explored.

Trial 2

An initial trial was arranged where an extensive range of process parameters was explored (Fig.5). Due to the limited time available a quantitative laboratory evaluation of every possible permutation of laser parameters (including pulse duration, effective energy and scanning speed, pulse spacing) and their effect on adhesion was not possible. Therefore an objective evaluation based on appearance was made allowing a more practical number of process parameters to be explored (Table 1).

Table 1 Summary of selected trial parameters - Trial 2

Once again, using the solid PP strip, samples were prepared with the laser, however, on this occasion bonded samples were prepared by TWI in-situ using a commercially available PU based adhesive; DELO-PUR^® 9694 2K-Polyurethane cold curing system. Several lap shear geometry specimens were prepared for the various parameters described in Table 1. These were returned to TWI where they were tested. Table 2 summarises the data.

Table 2 Lap shear test data for Trial 2

* X Hatch = crosshatch pattern ie two passes, scans angled to each other.

While the P7 results for those samples that were not subject to two passes of the laser (crosshatch pattern) are clearly improved on the cross-hatched version, the direct coupled parameter set, P1, showed the greatest promise here.

Trial 3 - Time dependency

From this preliminary trial using a scanning laser system TWI requested that further samples were prepared using the P1 (direct coupled) parameter set which provided the most promising data from Trial 2. The surface of the PP sheet was prepared and the sample mailed to TWI, this served to examine any time dependency in terms of subsequent deactivation of the prepared surface. CleanLaser prepared some 300mm of the PP strip and returned them by post. When received at TWI the sample was prepared as lap shear specimens at TWI using the previously described PU adhesive system; ten days had elapsed since the laser treatment.

Table 3 shows lap shear test data for the Trial 3 test samples prepared at TWI. While the samples do not yield at the stated objective of 2-3MPa they have reached the yield point of the parent PP without failure.

Table 3 Lap shear test data for Trial 3 (ten days elapsed time)

Trial 4 - PP insulation materials

Having identified a limitation of the lap shear testpiece geometry in the previous trial (yield in the parent material), a thicker substrate was required. As a final trial, material was prepared from a fully representative five layer PP insulation system applied to a 300mm diameter section of flowline (provided free-issue by Bredero Shaw). Samples were prepared and to form plaques nominally 6mm thick and 25mm wide, taken from the foamed polymer portion of the system (Figure 6). To avoid potential issues with gripping the samples the decision was taken to use double shear testpiece geometry once the material had been processed and bonded using the selected PU system, in this way the samples could be tested in compression.

Table 4 summarises the data from Trial 4, during this trial two further parameter sets were developed (again based on the subjective assessment of the material after treatment) these were P21 and P22; Table 5 describes the specific settings.

Table 4 Double shear test data for Trial 4 (five days elapsed time)

Table 4 describes summary data from Trial 4, including data from control samples used to establish the behaviour of the PU/PP system for double shear geometry and the representative pipe insulation material.

As can be seen the laser surface modification of the foamed PP insulation material provides clearly enhanced bond strength (Figure 7) when compared to the unmodified material. While there are limited data the previously developed laser parameter set, P1, appears to have been further improved upon by P21 and perhaps to a lesser extent P22. The difference between these parameter sets is the pulse frequency, which affects the energy applied to the material. Thus, P1, with the highest frequency of 30kHz applies the lowest amount of energy to the surface (ie this is the shortest pulse duration). P21, with a frequency of 20kHz represents the middle ground for this combination of parameters and has produced the highest bond strengths here. P22 with a pulse frequency of 12kHz, ie the longest pulse of laser energy here (per spot) may be at the stage of causing excessive degradation at the surface.

Conclusions and recommendations

A brief investigation into the use of laser surface modification to enhance adhesion between PP pipe coatings and subsequently applied PU systems has shown improvements in the resultant adhesion. Preliminary lap shear type testing using a generic PP material indicated some improvements from the process; however, the test geometry limitations were soon exceeded. Using representative materials from a five layer PP insulation system allowed the use of double shear test specimens, Tests indicated that with certain parameters an industry 'target' bond strength of 2-3MPa was repeatedly achieved.

An additional benefit of a laser modification system for field joints in oil and gas pipelines is that it may also be used to clean exposed areas of the actual pipeline. Current practice is to use a grit blast process which in turn introduces the potential for contamination of the subsequent field joint application processes. The use of laser cleaning of metal surfaces in the automotive sector is increasingly common practice with high productivity and effective cleaning.

The work presented here successfully demonstrates benefits of using laser modification to enhance adhesion of PU based systems to PP. Such systems are used extensively for the insulation and corrosion protection of pipeline in the offshore oil and gas industry. A thorough parametric study would be required to optimise the process fully.