Structural Aspects Of Snorre Mating

ABSTRACT The deck and the hull of the Snorre TLP were successfully connected South of Stord on the Norwegian westcoast in September 1991. The 28.000 tonnes deck was lifted off its supports by a large launch barge, transported to the ballasted hull and accurately positioned over the columns. The main challenge of the operation was to control deflections of the hull, deck and barge to obtain the desired load distribution between the 16 hull support stools, 4 on each column top. On each stool the load was taken by an array of deformation tubes. The arrangement of the tubes was decided on the basis of both dynamic and static analysis. Step by step computer simulations were performed during the operations,, along with measurements of the deflection of the deck, barge and deformation tubes. This enabled full control of the operation and large flexibility when modifications to the original procedures were required. The mating took place in the autumn with risk of swell. Dynamic analysis of the hull and barge in the mating configuration was performed to decide on the requirement for additional dampers between the column tops and the deck. It was found that friction on the prearranged guides and restraints gave the necessary damping to prevent unacceptable impacts. The deck was positioned well inside the agreed tolerances of +/- 100 mm. This was achieved due to good preparations, both on the engineering and the construction sides. INTRODUCTION Snorre is a four column TLP built for the Snorre Field in the North Sea west of Nofway. The connection of the hull and deck structures took place in Digernessundet south of Stord in September 1991. The total operation was divided in two, the lift-off of the deck from the fabrication site by a barge, and the connection, or mating, of the deck and the hull. The column diameter of Snorre is 25 meters, while the centere to centre distance of the columns are 76 meters. The column height is 63 meters. The pontoon cross section is 11,5 times 11,5 meters. The deck fabricator was responsible for the lift-off and mating, while the Main Engineering contractor was responsible for the design of the hull and deck connection points and the structural integrity of the TLP. This involved a detailed arrangements of the deformation tubes, guides and ballast distribution. Key personnel from the detail engineering phase where engaged for following up of the operation. The follow-up team prepared tools and measurement schedules to enable an accurate follow-up and an early identification of possible deviations. This paper will not describe the marine operations more than what is required to give an understanding of the procedure. The main aspects will be the engineering preparations, and main parameters to control during the operations. LIFT-OFF OF DECK The deck was fabricated on 16 fabrication supports, 8 on each side of the dry dock. The supports had the same location relative to the deck as the mating stools on the hull. The barge with grillage was located under the deck at a low water level in the dock.