Page 438 - Pipelines and Risers
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Design Codes and Criteria for Risers 405
corresponding IS0 standard will be IS0 13628-2. MI Spec 175 is based on working stress
design.
2. Safety Against Collapse
Present standards have been based on a permissible utilization of 67% of the pipe capacity for
external pressure. In practice this means that the stresses in the carcass must be less than 67%
of the stresses required to collapse the carcass.
API Spec 175 uses the following formulae, see Table 21.6.
Table 21.6 API Spec 175.
I Water Depth (D) I Permissible Utilization I
D1300m 0.67
300m <D<900m (D-300)/600*0.18+0.67
For water depth less than 300 m the permissible utilization is as before. Due to the negligible
uncertainty related to hydrostatic pressure in deepwater, the permissible utilization is
gradually increased with water depth. The maximum value of 0.85 is reached at 900 m water
depth.
3. Unified Definition of Capacity and Utilization
During the MCS JIP it was found that the design methodology used by the manufacturers and
specified in the various specifications was not well defined, and the interpretation of the
requirement was not consistent.
An improved definition of both capacity and permissible utilization is implemented in API
Spec 17J. Generally speaking, the new specification allows somewhat higher utilization than
current design practices used by the manufacturers. It should, be stressed that all
specifications (both API Spec 17J and the preceding ones) are based on a calculated average
stress in each layer of the pipe. Effects like local stress concentrations and residual stresses
from manufacturing, which are important for the service life, are not considered in the design
calculations.
The design requirements were divided into two categories:
- Mandatory requirements that are auditable should be included in the specification (API
Spec 175).
- Recommendations with respect to how to satisfy the mandatory requirements, as well as
guidance for the design of flexible riser systems, are included in a separate Recommended
Practice (RP).
