Flanged Coupling Design for the Snorre TLP Risers

ABSTRACT The flanged couplings adopted for the "Snorre" TLP risers are of a new and optimised configuration. The weight is comparable to that of a threaded coupling. So ia also the fatigue strength. The ultimate strength is equal to that of the tubular cross section of the riser joint. Make up and break out can be achieved rapidly and highly accurately with small and compact tools. In comparison threaded couplings of large diameters in particular require Iarge and cumbersome torque tools which achieve. a far lass precise prestress. It is believed that flanged couplings may provide the more optimal solution in many similar applications. INTRODUCTION The "Snorre" Tension Leg Platform stands in 320 meter water depth and is located in the Norwegian sector of the North Sea. It is operated by Saga Petroleum as. The design provides for 44 production risers of 9 5/S" diameter. There will also be an oil export riser of 20" diameter and a gas export riser of 10 3/4" diameter. Each riser string consists of 17 joints which are 51? long. Flangad couplings have bean provided for the couplings of the production risers in the splash zone as well as for the oil and gas export risers The splash zone type couplings are provided in the most heavily loaded sections of the risers. The design loads are given in table 1. For the moat critical ultimate load combination the gross tubular cross section of the joint including the corrosion allowance is fully utilized. The flanges do as a consequence have an ultimate capacity in excess of that of the tubular cross section. The tubular cross section is also highly utilised in fatigue. As a consequence forged upset ends have been provided to accommodate the girth welds on the production risers and the gas export riser. On the oil export riser there are flush ends. The compact flange concept which was adopted has also been used on shore in process pipework. In order to adapt this design concept to accommodate the design requirements laid down for the "Snorre" TLP, extensive non linear finite element analysis was performed. The general purpose non Iinaar finite element programme "Solvia" /1 / was used for this purpose.("Solvia" was previously known as "Adina".) The analytical results obtained ware verified with a comprehensive programme of full scale prototype tests which are described in more detail below. DESCRIPTION OF FLANGES Drawings of the flange coupling for the production riser splash zone type joints are shown in figure. The flange coupling for the gas export riser is of an identical configuration. Drawings of the oil export riser coupling are shown in figure 2. To avoid special transition joints splash zone couplings and standard couplings are of an identical design. The flange faces are carefully profiled to give the desired stress distribution after prestressing. This is described in more detail in the relevant sectional below..