Simon Frost explains the role of engineered composite repairs in late life asset management.
A tubular brace, having suffered wall loss through external corrosion, was reinstated to its original flexural strength with an engineered structural composite repair.
The North Sea’s aging assets pose a number of challenges for the industry in the UK. Several assets on the UK Continental Shelf have been in operation for 30 years and are expected in some instances to remain in service for a further 20 years. Recent fiscal changes, including a brown field tax allowance and greater certainty on decommissioning costs through the decommissioning relief deed, will likely lead to an increasing number of asset life extension projects.
Operators should recognize that the structural integrity of an asset needs to be maintained during its operational life and even after cessation of production, through any unmanned phase and preparations for removal, until the asset is finally removed. This means structural integrity, subsea, in the splash zone and on topside structures, including components such as pipes and pipework, beams, decks, struts, stairways and towers, must be closely monitored, maintained, and plans made for their continued safe operation.
While operators have the option to replace components, the evolution of many assets has rendered some components inaccessible, making replacement difficult. Replacement can also be costly and might not be deemed good value for a platform nearing decommissioning or where bed space offshore is limited, as replacement can require additional manpower over relatively long periods.
A container storage area was identified as suffering from severe corrosion or through wall defects totaling an area of 48sq m requiring repair.
Engineered composite repair solutions
Engineered composite repairs offer an alternative solution and can be completed in shorter timeframes. Composite repairs have been used offshore for a number of years, primarily on pipework with localized corrosion defects, but also in tanks and vessels. However, the engineering principles are also valid for the repair of structural components.
Composite repairs are governed by ISO/TS 24817 or ASME PCC-2 Article 4.1 standards, which define the qualification requirements, design rules, installer training requirements and installation guidance.
Composite repairs are used for a number of reasons. First, no hot work is required – an important benefit when offshore safety requirements render hot repair solutions impractical. Second, once applied, composite repair systems are corrosion resistant and can be guaranteed for up to 20 years – potentially the life of the field. Furthermore, if the corrosion on the structure is external, applying the repair will prevent further material loss. Third, in some instances it is possible to apply a repair while the pipework is in service, allowing production to continue. Finally, composite solutions can be used to repair most defect types in the offshore oil and gas environment. The only defect type where careful consideration is required is crack-like defects.
|A Walker Technical technician working infield.|
A composite repair consists of two main components: a composite laminate (either an impregnated carbon or glass cloth with a thermosetting resin, e.g. epoxy or polyurethane) and an adhesive (either an epoxy or polyurethane resin).
A critical aspect of the installation process is adhesion, irrespective of the specific application. In most cases, the performance of the composite repair is not limited by whether the repair can withstand the applied loads, but whether the loads can be transferred from the underlying pipework or structural component into the composite laminate. To transfer the loads, the adhesive layer between the component and composite repair must withstand and carry out this transfer. Therefore, it is crucial to understand the parameters that influence adhesion. There are five main factors for consideration. These are:
The most important parameter is surface preparation. To achieve the greatest adhesion, a surface prepared to Sa2.5 is required. Other surface preparation techniques, i.e. power tools or hand tools which provide an ST3 or ST2 surface preparation, respectively, can also be considered. It should be noted, however, that a surface prepared to an ST2 preparation has 25% of the adhesion value of a surface prepared to Sa2.5, i.e. the pressure retention capacity of the repair is significantly reduced when composite repairs are applied to pipework prepared only to ST2.
It is critical that the installation is implemented according to the provided installation method statement. If the surface preparation and other installation procedures are not correctly followed, the repair may leak or fail, regardless of the sophistication of the design.
A bespoke quality control/quality assurance process should also be developed for each composite repair application.
A Cunifer grey water drain line was suffering from a pin hole leak. An engineered Cunifer composite repair was applied to reinstate the integrity of the pipe to its original form.
Walker Technical performed a composite repair on a tubular brace on the cellar deck of a platform in the UK Southern North Sea. The 14in. diameter, 16mm-thick carbon steel brace, had been in service for 26 years and had suffered up to 10mm wall loss through external corrosion at the intersection with a vertical column member. An engineered composite repair that would provide the required flexural stiffness was required, as there were concerns that the brace may be unstable under compressive loading at the corroded area. Furthermore, it was expected that humidity would create limited windows for installing the repair.
The design approach adopted for the repair was split into two parts. The first involved the use of a uni-directional carbon fiber, epoxy resin reinforcement (Technowrap HP PRS), to provide the flexural stiffness to the brace, and the second consisted of a quasi-isotropic carbon fiber, epoxy resin reinforcement (Technowrap SRS) to ensure load transfer continuity between the brace and the vertical column. This ensured the repair would both withstand the applied loads and remain operational for a 20-year lifetime. The repair application took 14 days to complete.
Walker Technical was asked to undertake a deck repair within a food container storage area covering about 48sq m on a 15-year-old floating production vessel in the northern North Sea. The work scope was to provide a repair able to withstand a 800kg/sq m working load and impacts up to 1.5kJ, including design calculations, and the installation of Walker Technical’s Technowrap DRS (Deck Rehabilitation System, a rubber toughened composite repair system).
In order to remove surface scale and corrosion product the engineering team used ultra-high-pressure jetting to prepare the surface of the area to be treated. However, the jetting caused some flash rusting issues. Bristle blasters were then used to remove any remaining corrosion products. The Technowrap DRS system was applied and an anti-slip coating was then applied over the repaired deck area. The entire project took seven days.
Previously, the quality of adhesion obtained when applying composite repairs to cunifer pipe was poor as the material is soft and therefore, when prepared, the surface finish does not have sufficient roughness or irregularity. To solve this issue, Walker Technical developed a resin system (Technowrap Cunifer) that provided greater resistance to interfacial crack growth, enabling the repair to achieve improved pressure containment. This technology was used when a 4in. Cunifer grey water drain line had suffered internal erosion, which had created a pin hole through wall defect. To stop the leak, a temporary repair was applied over the defect.
Due to its role in the operation of the platform, the line could not be shut down or depressurized, implying that it was not possible to remove this temporary repair. Walker Technical had to reinstate the integrity of the grey water drain line and provide a 20-year design life. The drain line’s surface was prepared using a bristle blaster and the temporary repair profiled over using fast curing cementitious filler. The composite repair was then applied over the temporary repair and extended onto the cunifer pipework.
Engineered composite repair solutions offer a cost-effective solution for the myriad of integrity challenges facing operators as part of late life asset extension projects, ensuring that the integrity of their assets can be maintained while still in production, through to the point where they are removed as part of decommissioning process.
Simon Frost serves as technical director for Walker Technical Resources. He holds a PhD from Cambridge University. He is a visiting professor at Newcastle University, chairman of ISO/TC 67/SC 6 workgroups on GRP piping and composite repairs. He is also a member of the structural materials college for assessing grant submissions to the Engineering and Physical Sciences Research Council.