DECOMMISSIONING is one of the fastest-growing activities in the oil and gas sector. Many offshore assets are coming to the end of their design lives or are being sacrificed to the prolonged industry downturn. Dismantling and disposing of them in a cost-effective and environmentally-friendly manner is a pressing concern for companies and governments. The North Sea is one of the new global hotspots for oil and gas decommissioning. By the mid-2050s, 470 platforms and 5,000 wells - but also 10,000 kilometres of pipe and 40,000 concrete blocks - will need to have been sealed or moved from its waters. Estimates have put the costs at anywhere from £30billion to £100billion. There are many challenges involved including in safely dismantling and transporting large structures in changeable weather and waters, plugging and abandoning wells, and safely recovering concrete "mattresses". In some lights, decommissioning emphasises the current malaise in the sector. However, it also offers lucrative commercial opportunities and spurs technical innovation. It will prompt improvements to a wide range of technologies, software and processes. For example, adapting Remote Operated Vehicles (ROVs) for decommissioning and monitoring might entail improving lighting and high-resolution cameras or developing Printed Circuit Boards (PCBs) with reduced heat output. Current regulations also require the removal of "mattresses" - the concrete structures protecting pipelines and other subsea assets. In the North and Irish seas, these include between 35,000 and 40,000 units – up to 200,000 tonnes of concrete. Few mattress decommissioning projects have been carried out in the North Sea so far, so companies will need to develop strategies for recovering different sizes and styles while minimising risk to personnel and environment. This could involve increasing the use of ROVs, developing new equipment and strategies for mattresses removal and disposal, and finding alternatives for recycling and reuse. Decommissioning will also require improved means of handling, treating, recycling and disposing of hazardous materials. These include amongst others lead paint, asbestos, and Naturally Occurring Radioactive Material (NORM). It remains to be seen how soon the oil and gas sector as a whole will bounce back from the current downturn, but it can at least offer new opportunities for innovation and commercial gain in decommissioning existing infrastructures. The UK Government has established a range of tax relief and funding opportunities for oil and gas companies, in an attempt to sustain an industry which has been vital to the national economy.