From OBR Construction’s project deliveries at Trawsfynydd Nuclear Decom Site and URENCO UK’s enrichment facility at Capenhurst, we came into contact and developed a relationship with KDC Contractors Ltd and during late 2021 we were approached by Veolia/KDC to ascertain our interest in assisting them with a rather unique opportunity to decommission one of the North Sea’s oil platforms. The support ‘jacket’ structure as it is referred to was to be brought to land in two sections at Dales Voe, Shetland Isles and it then required placing the half Ninian North Jacket onto its side on temporary supports in preparation to finally setting the jacket on the ground for final controlled dismantling / demolition. OBR’s works scope was to construct the temporary support structures upon which the structure would sit and which were discussed during Oct-Dec 2021 and the design construction finalised between all parties. 

OBR Construction’s scope was to construct eight temporary support structures set out in a grid type pattern in line with strategic load points of the jacket structure as it is placed on its side.

 Support structures ref. 1, 2, 3 and 4 were constructed where currently a concrete slab existed and structures ref. 5, 6, 7, and 8 were to be constructed on compacted hardcore areas. The ground levels across the site and particularly across the 8 points as one would expect were not consistent and therefore required regulating and the datums brought uniform so that the formation level for each support structure were the same. This task fell to Veolia who agreed to excavate down at each point until bedrock was reached and then OBR set the required datum level and a C40 concrete foundation poured into each point to form a uniform datum level for each support to be constructed from.

It was decided that each of the eight support structures would be constructed using precast concrete chamber rings, 2700DN outside diameter and approximately 4 metres in total height, infilled with C40 grade concrete. These structures were only meant for short term support, after which the intention was to carry out a controlled detonation of each support to render the jacket structure on the ground for final dismantling and demolition. Again, for clarity, OBR Construction’s involvement and physical scope ended when the support structures were constructed.

Our scope was estimated and was indeed to take some three weeks to complete and it was recommended that the completed structures were then given 56 days to fully cure before any weight was offered onto them.

 With the aid of a primed 24/366 metre concrete pump and a MEWP we imported C40/50 pump grade concrete and carefully infilled each of the support structures bringing the concrete, level with the top of the highest ring. Each support structure was completely infilled before moving on to the next to repeat the process. A 50mm 110v vibrating concrete poker was lightly used to compact the concrete which was finished by a steel trowel.

When all eight of the support structures were infilled with concrete, they were left for a further day for an initial 24hr set cure. To control the curing process in this exposed environment and time of year, each structure was wrapped using a single layer of 50mm thick insulation jacket, supplied in 10m x 1.2m rolls, which were wrapped horizontally around each ring, thus each structure required a minimum of 4 rolls to totally encase the structure. This jacket was fixed in situ by means of Sarnabar predrilled vertical securing strips, fixed into the underlying concrete ring using 70mm concrete fixings. The Sarnabar strips were then further secured with ratchet straps. The structures then remained in this state for the suggested 56 day curing period.

 The ultimate aim was to provide the half Ninian jacket an intermediate temporary support base to rest upon until such time that Veolia decided to bring the entire structure to ground by means of controlled detonation for final dismantling and recycling / disposal.

 The project was duly completed to plan and budget and we are proud to report, without incident. The Client complimented OBR Construction’s professionalism and collabortaive approach to this unique and innovative approach and following the suggested concrete curing period, the jacket was duly brought onto land and placed upon the support structure by Allseas by firstly transporting on the collosal ships Pioneering Spirit  and then transferring onto Iron Lady for load-in to the dismantling yard (illustrated below)