Page 424 - Pipelines and Risers

Design OfDeepwater Risers 391

capacity. The previous analyses tools required results from full-scale test for service life
prediction.
- Analyses have shown that the use of simplified analysis tools based on average stresses in
the layer may recommend the use of hoop spirals where local stresses are very high. One
example is use of a rectangular back-up spiral as an additional hoop strength layer.

20.5 Drilling and Workover Risers
Deepwater drilling and workover is presently performed with jointed metallic risers. The
vessels and equipment have been upgraded to work in a water depth down to more than 1700
m. In deepwater and harsh environment the challenges related to operation are large due to
use of buoyancy, fairings etc.

The drilling contractors are presently building new vessels and upgrading existing vessel to
meet the deepwater requirement. Smedvig and Navion have contracted a new drillship MST
ODIN to be rented by Statoil. The vessel is fully equipped for drilling in 2500m water-depth.
Drilling in even deeper water is pIanned. The technology status is, however, presently limited
to approximately 2500 m.

Two of the critical items for deepwater drilling are riser weight and riser control. In order to
reduce the riser weight, alternative materials are considered. SeaFlex and Raufoss have
recently completed the first phase of JIP project related to composite risers. At the Heidrun
TLP a titanium drilling-riser has been installed and one composite drilling joint has been
qualification tested and is ready for offshore trial in the Gulf of Mexico. A free hanging
titanium catenary riser is being considered as the production riser for Asgard B field
development.

20.6 Riser Projects in Norway
In Table 20.1 recent on-going riser projects are summarized (Lertveit and Bjzmm, 1995).