Application of Finite Element Modelling in the Qualification of Large Diameter Unbonded Flexible Risers

Unbonded flexible pipe has a proven track record in the offshore oil and gas industry for more than 20 years. The product is synonymous with the use of floating production systems spanning the water column and connecting subsea structures to facilitate the retrieval of hydrocarbons, provision of water injection systems and the export of processed or semi-processed fluids to main trunk pipelines or onshore. Unbonded Flexible pipe is a technically complex multi-layer structure of helically wound metallic wires and tapes and extruded thermoplastics. In 1996 Wellstream was awarded a major contract for the supply of flexible risers and flowlines as part of the Norsk Hydro Troll Olje Gas Province Development located in 350m water depth 80km west of Bergen. The development consists of two main fields, Troll East (31/3 and 31/6) and Troll West (31/2) which together have an estimated production life in excess of 50 years, making it one of the worlds largest offshore developments. Norsk Hydro is responsible for the development and operation of the production facilities. The scope of supply included 15-inch internal diameter, 213 barg design pressure, dynamic risers for the export of oil and gas from the platform to shore. At contract award, Wellstream was finalising the location of their European Manufacturing site, a facility which would have the capability of manufacturing unbonded flexible pipe with external diameters up to 24-inches. The design, manufacture and qualification of a large diameter oil and gas export riser for service in the Norwegian sector of the North Sea, considered to be one of the most severe environments in the offshore industry, provided unique challenges and attributes. These risers have now been in service for over two year, following an extensive qualification programme. This paper provides an insight into the integrated approach adopted during qualification with the successful application of finite element technology to aid full-scale testing. During a full-scale test program a finite element simulation of a 15 metre long prototype pipe was performed with special emphasis on the evaluation of contact forces between the flexible pipe and a bend limiting structure. The finite element analysis program package ANSYS is chosen for this simulation due to its special feature of contact/target elements. The paper illustrates that the use of Finite Element Modelling is indeed capable of predicting the observed behaviour of prototype risers, which are subjected to a series of dynamic load cases, in a Dynamic Test Rig (DTR). Finally, the paper concludes that focus should now be given to the advantages of using finite element tools that are verified by full scale testing to reduce development costs and schedules.