CASE STUDIES Ӏ DECOMMISSIONING
There are 12,000 oil and gas platforms dotted around the world and many of them are nearing the end of their useful life. They need to be decommissioned, and removed, and their materials recycled. The North Sea alone has more than 1,500 such installations; their average is 25-years-old. Another 1,500 oil and gas platforms in the Gulf of Mexico are more than 30-years- old. And in the Asia-Pacific region more than 2,500 platforms will need to be decommissioned in the next decade. So says the World Economic Forum. Decommissioning is big business. And not only in offshore energy.
Infrastructure everywhere ages and needs to be replaced. Bridges, buildings, plant machinery, even cranes, become redundant and outlive their usefulness. Removing them presents
difficulties. They are old, and possibly corroded; so their strength is not known. Their weight may not be known either; original plans, even if they can be found, might allow their original weight to be calculated but years of use, say of a steel tank in a chemical plant, may have added deposits that increase its weight, or brought corrosion that reduces it; and both of those can move the centre of gravity to an extent that can only be guessed at. And in the years that have intervened since the thing was erected other structures – buildings, roads, more industrial plant – may have been put up around them, making access tight or close to impossible. All of which makes decommissioning an art every bit as much as a science. Experience will count, even more than it does on new-build projects. Difficulties will emerge, sometimes partway through the job, that will call for ingenuity as well as expertise. And, as ever, planning is paramount.
As an example of some of the
complexities involved, consider the decommissioning of an oil platform completed in August this year by Thunder Cranes. Thunder Cranes is based in Labuan, Malaysia, and specialises in lifting solutions for the offshore oilfields of Asia. Modular rental cranes are
a speciality; its rental division employs a proprietary crane technology with a unique self- lifting system that allows its cranes to be rigged up on platforms without assistance. Decommissioning offshore oil platforms is a major area of its work. Such platforms have their own mounted cranes, of course, and it might seem logical to use these for lowering modules to strip the platform before removal. But as offshore platforms near the end of their life their fixed cranes may no longer be operational or may lack sufficient lifting capacity for
Gemmak
removes a corroded mast from the lightship ‘Helwick’
removing topside modules in the decommissioning stage. Hence the solution of rental cranes, temporarily installed on the rig to be decommissioned, for topside preparation works such as ‘weight shedding prior to topside removal by a heavy lift vessel. Global energy company Chevron Thailand engaged Thunder Cranes to assist with the decommissioning preparation work on a topside installation. The requirement was to remove equipment and associated modules and installations totalling some 300 tonnes from the main deck of the platform. This weight shedding was carried out to bring the topside weight to within the heavy lift vessel’s lifting capacity for topside removal. The ‘reverse-installation’
process consisted of isolating and disconnecting all lines connecting to equipment and disconnecting equipment skid bases from the deck plate. The equipment modules
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