The Construction Of The Ninian Central Platform For The British North Sea

ABSTRACT At tow out from the construction base on the West coast of Scotland with 600,000 tonnes displacement, the Ninian Central Platform became the biggest floating structure ever built. To complete a concrete platform of this scale, construction methods were established to handle high volume and continuous concreting operations. Slip-forming of the relatively small outer wall sections was phased for programme advantage with the large, full circumference slipforms of the main walls. By using precast concrete techniques further advantages were gained from the offsite manufacture of the numerous dome roofing units for the platform floatation chamber and also from the on site casting of quadrant, sections for the deck supporting upper breakwater wall. Deep water close to the dry dock in which the base section was built afforded suitable wet dock moorings for completing the structure and for deck emplacement. Module installation and Hook Up works were accomplished at the latter anchorage in considerably calmer and more suitable sea conditions than those which prevail in the North Sea where platform decks have more traditionally been lifted into position. INTRODUCTION The Ninian Central Platform now in position in the North Sea, U.K. block 3/3 provides the main base for drilling and oil production from the Ninian Field. Designed and built by Howard Doris Limited this steel reinforced prestressed concrete gravity structure was towed out and placed on location for the Client, Chevron Petroleum (UK) Limited in May, 1978. The Platform has provision for drilling 42 wells and will support an operational 35,700 tonnes deck load. Of this, 23,800 tonnes inclusive of deck was installed and hooked up before leaving Loch Kishorn construction base on the North West Scottish coast. Water depth at the field is 136 metres. From the 140 metre diameter base slab at the sea bed, the structure rises to a height of 156 metres to the frame which supports the deck 31 metres above sea level. DESCRIPTION OF THE STRUCTURE As shown in figs 1, 2 and 3, the platform is a concrete structure formed of seven concentric walls and eight radial diaphragm walls rising from a circular base slab supported on 4 metre deep steel skirts. The outer two low level walls 7 and 6 carry the loads from the peripheral area of the base slab with perforations in the outermost wall acting to prevent sea bed scour. Further inwards, walls 5, 4 and 3 stiffen the inner base slab and are roofed over to form an upwards tapering floatation chamber which meets the tower wall 2 at the 75 metre level. This wall and the centre shaft support the deck frame directly onto the top of the shaft at 156m and through eight wall 2 columns which accept the greater part of the deck loads. Conical holes in wall 2 throughout the splash zone ensure that the wall functions also as a breakwater to reduce wave forces.