Page 237 - Pipelines and Risers

210 Chapter I2

12.6.2 Wall-thickness Design for Three Medium and Two Medium Pipelines
Subsea pipelines have historically been designed for three different mediums: air (during
installation), water (during pre-commissioning) and finally the product (gadoil). In shallow
water, air-filled pipelines at near atmosphere pressure do not cause particular difficulty
because the wall-thickness is sized for the internal pressure of the product or the pressure test.
In deep water, provided the same installation and operation approach is adopted, the pipeline
will be sized for external pressure (collapseAoca1 buckling) for the installation phase. This
phenomenon is clearly illustrated in Figure 12.25 and 12.26 that shows how the operation,
testing and installation phases dictate the pipeline wall-thickness requirements for increasing
water depths.

Water depth (m)-

Figare 1225 Wall thickness selection function of water depth (3 mediums).

Water depth (in)-

Figure 12.26 Wall thickness selection function of water depth (2 mediums).
Should the pipeline be designed to carry two mediums, water (during installatiodtesting) and
product (gadoil during operation) then the wall-thickness requirements can be drastically
reduced for the deepwater pipeline. This approach (Figure 12.25) is not limited by collapse
resistance until a significantly deeper water depth is reached (Figure 12.26). The densities of
both of the two mediums (water and oil/gas) then the wall-thickness requirements for hoop