Page 136 - Pipelines and Risers

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Chapter 8

On-bottom Stability

8.1 General
On-bottom stability calculations are performed to establish requirements for pipeline
submerged mass. The required pipeline submerged mass will have a direct impact on the
required pipelay tensions, installation stresses and the pipe configuration on the seabottom.
From the installation viewpoint, especially where spans are not a concern, the priority is to
minimize the required pipeline submerged mass.

On-bottom stability calculations shall be performed for the operational phase and for the
installation phase. For the operational phase, a combination of 100 year wave loading + IO
year current loading is to be checked, as well as 10 year wave loading + 100 year current
loading. The pipeline is filled with content at the expected lowest density when considering
the operational phase.

For the installation phase (temporary phase) the recurrence period may be taken as follows:

- Duration less than 3 days:
(i) The environmental parameters for determination of environmental loads may be
established based on reliable weather forecasts.

- Duration in excess of 3 daw:

(i) No danger of loss of human lives. A return period of 1 year for the relevant season
may be applied.
(ii) Danger for loss of human lives: The parameters may be defined with a 100-year
seasonal return period.

However, the relevant scason may not be taken less than 2 months.

If the empty pipeline is left unprotected on the seabed over the winter season, a combinations
of 10 year current + 1 year waves, and 1 year cument + 10 year wave loading will be checked.