Page 435 - Pipelines and Risers

402 Chapter 21

- Partial lost of station-keeping capability
- Vessel impact
The follows should be considered as appropriate:
- Tensioner failure
- Riser collision
- Explosion and fire
- Heatflux
- Operational malfunction
21.4.4 Definition of Load Cases

Load cases that shall be checked are defined by combining functional, environmental and
accidental loads. Depending on riser orientation and directional variation in wave and current
loading, extreme combinations of wave and current loading shall be selected. The selection of
load cases is also related to the design stage. Critical load cases are needed to be considered
for preliminary and conceptual design. All the load cases are needed to be included for
detailed design.
Load cases listed in Table 21.4 shall be checked against analysis and design with allowance
for the requirements of a particular riser configuration.

A range of storm wave periods shall be evaluated to determine the governing condition. This
will depend on the riser attachment location with respect to the vessel center of gravity, wave
directionality and vessel RAO.
Load cases listed in Table 21.5 should be checked as appropriate.
Analyses should be conducted at extreme slow drift offsets, zero vessel offset, and for failed
mooring conditions.
A range of floater headings shall be considered for spread moored or tethered platforms where
RAO is dependent on wave heading. For turret moored, system analysis shall be conducted
for head sea conditions with sensitivities for non-zero wave headings as appropriate.
Typically, this will address incident angles up to 30 degrees depending upon turret forelaft
and weather philosophies.
For preliminary analysis and conceptual design, seabed can be modeled with suitable friction
coefficients applied for lateral and longitudinal directions. It is allowed to use changed
coefficients depending upon the service time of the riser. Sensitivity study of these
coefficients should be performed as appropriate.