Sticking point – adhesive bonding for marine environments
Dr Jan Weitzenböck of Norwegian risk management company Det Norske Veritas discusses adhesive bonding in the maritime and offshore industry.
Global shipping is facing increasing pressure to become more environmentally friendly and efficient. New ways of production and structural weight savings are becoming necessary, leading to demand for lightweight multimaterial solutions. Adhesive bonding plays an important role in these types of structures.
In shipbuilding most current codes or class rules for maritime and offshore structures are largely experience-based. Furthermore, they are developed using a design lifetime of 20 years or more. This is difficult to document for new joining methods, such as adhesive bonding, which have a limited track record. In response to radical new ship designs, classification rules allow risk-based assessment to show compliance by documenting equivalent levels of safety. This approach is based on the guidelines for Formal Safety Assessment published by the International Maritime Organization.
Oilfields are being operated beyond their design life so require lifetime extension and retrofitting of equipment. New oilfields are being developed in more challenging environments such as arctic conditions or ultra-deep waters. One of the key success factors is the use of lightweight structures and components and the ability to join multimaterial solutions. Adhesive bonding is suitable because it does not require hot work and can join different materials.
Offshore installations are regulated by national authorities and requirements differ from country to country. As joining can limit the durability and reliability of offshore structures whether welding or adhesive bonding, the performance of an adhesively bonded joint must be documented for each case.
There are a number of actual and potential applications for adhesive bonding in shipping such as in window panes (direct glazing), propeller shafts and fibre-reinforced polymer pleasure boats. Further applications include the patch repair of cracked steel or aluminium superstructures, designing and connecting composite superstructures to a steel deck, and bonded aluminium superstructures. Most liquefied natural gas (LNG) carriers use a membrane type containment system. This insulation system makes extensive use of adhesive bonding for joining the secondary membrane. Of the applications mentioned above, only direct glazing and LNG insulation are well established and used routinely.
Direct glazing is almost standard practice on passenger and cruise ships, see image p23, above. A typical window is shown in image p23, below. It is designed in such a way that if the window sustains external water pressure the glass will be pressed onto the frame, transferring the load directly to the ship’s side shell. Only internal pressure will directly act on the bondline.
Despite the widespread use of direct glazing, it is not yet possible to predict the lifetime of the joints. However, they have a good track record as Det Norske Veritas (DNV) inspection records confirm. A preliminary analysis of passenger and cruise ship windows records for DNV classed ships gave some surprising insights. There are no reports of adhesive failures or leakage, indeed the only failures or comments are –
• Use of non-certified glass – when fire rating was required.
• Defective window wipers on lifeboats or the bridge.
• Broken glass due to heavy weather damage.
• Crack in the side shell of the hull that started at the corner of a big window.
Direct glazing has been used for about 10-15 years in ship applications. Despite the positive experience so far, practically all windows on passenger ships are still fitted with additional mechanical fasteners in case the adhesive fails.
There are fewer documented applications of structural bonding in offshore structures. One example is the use of fusion bonded epoxy to attach insulation materials to underwater pipelines and flowlines. These are demanding applications as the design lifetime can be up to 20 years without any maintenance. During installation, the pipes are reeled off which puts added strain onto the pipeline insulation.
Another important application is the use of bonded composite patches to repair damaged steel structures. Patch repairs were developed for use in floating production, storage and offloading vessels (FPSO). A FPSO (see p22) is a type of floating tank system used by the offshore oil and gas industry to take all of the oil produced from a nearby platform or templates, and process and store it until the oil can be offloaded onto waiting tankers, or sent through a pipeline. Most of these vessels are converted oil tankers.
However, oil tankers were not designed to be continuously stationed at sea. As a result, FPSOs tend to develop fatigue cracks in their steel structure. One of the main attractions of composite patch repair is that no hot work is involved, minimising the impact on oil production processes.
Guidelines have been developed to design and apply bonded composite patch repair. Three field repairs have been carried out, which confirmed the approach is successful. Bonded joints are assessed and classified according to their criticality and need for documentation. Most repairs are for cracks where growth is non-critical and the structure would survive even without any kind of patch repair.
Offshore and shipbuilding follow regulatory regimes that make it difficult to establish adhesive bonding as a new joining method for load carrying connections. The main challenge is documenting long-term performance of bonded joints. However, risk-based approaches open up new applications that could pave the way for further use of adhesive bonding on marine structures. This could be achieved by using ‘fail-safe’ designs, typically hybrid joints, to avoid having to document long-term performance.
Further information: Dr Jan Weitzenböck