DECOMMISSIONING Ӏ CASE STUDIES
tie down the modular crane. This was implemented safely and without any issues with one shift of welders within three days. Due to limited space on the platform deck the supply vessel was on standby within reach of the platform crane during this time allowing the rig-up and rig-down of the TC90 crane on the fly.
Another challenge was that some of the larger modules at the limit of the TC90 reach radius did not have verified weight information. A decision had to be
Autokrane
Schares removing a bridge
made on whether to reposition the crane for those lifts or whether to disassemble and breakdown down those modules into smaller components. It was concluded that the best way forward was to break down some of the modules. In addition, Thunder Cranes deployed its most experienced lifting superintendent on site to supervise the most critical lifts. By removing the production modules the overall platform weight was reduced to be within the capacity of the heavy lift vessel
which was able to remove the topside and structure in one lift. Approximate costs savings from the chosen option with the TC90 crane was estimated at $USD 1.5-2.0 million from savings in preparation time of one month, and in heavy lift vessel time of five days.
MAST REMOVAL A similar maritime uncertainty concerning load weights, though on a smaller scale and at a quayside rather than at sea, was encountered by Gemmak Engineering, who this summer removed two masts from a historic vessel docked at Swansea Marina, Wales.
Gemmak is the engineering division within Rope and Sling Specialists (RSS). The work was aboard Helwick Light Ship LV91, a historic vessel belonging to Swansea Museum. Designed and built as a floating lighthouse her hull shape and form were specifically developed to remain anchored to mark a navigation hazard in all weathers and sea states. Following a site survey by RSS to inspect the mast tensioners it was discovered that both the 87-year- old masts were in poor condition, so replacing the mast tensioners might have caused undue stresses and strains. Due to the proximity of the boat to a pedestrianised area, it was noted that the likelihood of failure and potential collapse onto that area would greatly increase with new wire tensioners acting on the masts. Matthew Edwards, engineer at Gemmak Engineering, said: “We were then contracted to undertake thickness testing on the masts, to estimate the material loss due to corrosion and the effect that would have on the stability of the masts. "Sections of the masts were tested and found to be in some locations to be between 40% and 60% of the assumed wall thickness. The recommendation based on
38 CRANES TODAY
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