Special Session : Advances in Flexible Riser Technology: Challenges of Flexible Riser Systems in Shallow Waters

Abstract While much focus is given to the challenges of designing and manufacturing riser systems for extreme deep waters, the challenges are certainly not less when one tries to push the limit at the other end of the spectrum: in very shallow water. Due to their advantages FPSO's are being used in increasingly shallow waters (less than 50m water depth is current practice in Vietnam now days). This puts real challenges on the mooring and riser designers together with FPSO contractors and forces them to come up with novel configuration designs in particular when the very shallow water is combined with harsh environmental conditions and significant vessel motions. An active collaboration between the mooring and the riser designers is then of upmost importance, and requires validation of the concept at the earliest stage of the project: during FEED studies. The main challenges reside in the need to accommodate the two extreme positions of the surface floater (Far/Ballasted and Near/loaded) which often result in contradicting requirements. Interference is also a main concern, be it with the hull of the FPSO, the seabed, the free water surface or other risers. Marine growth is also a critical issue as it impacts the riser configuration on the full length. To adequately address all those requirements, novel, generally complex, configurations are needed using combinations of clump weights, tethers, buoyancy elements and rigid or flexible structural members. This paper will present and discuss the different issues faced when designing flexible riser systems in these very shallow waters and present a number of novel solutions to address those. It will also present some actual field experience on two very shallow water riser systems installed over the last two years and will explain how the designer reached the specific chosen solution. Introduction This paper will concentrate on challenges of flexible riser systems design (this is mainly focusing on unbonded flexible pipes but may be fully applied to umbilical) in very shallow waters, typically around 50m water depth. Unbonded Flexible pipes are key elements for offshore oil & gas production, enabling connection of subsea wellhead to floating platforms in shallow to ultra deep water fields. Design of these risers is a complex and long iterative process; design criteria are well defined in relevant codes and standards (See Ref. [1], [2]).