Page 243 - Pipelines and Risers

216 Chapter 12

0 Materials and fabrication 55%
Installation 29%
Commissioning 1%
0 Insurance and miscellaneous 2%

The cost impact is discussed for each main area.

Management and design: The approach will have no direct commercial impact on the
managemenvdesign process. However the design should address all the potential limitations
of a 2 medium pipeline to ensure that they are acceptable to the operations phase. One area
that must be addressed is a system of assuring that pressure in the line never drops below a
prescribed minimum. One approach is to have isolation valves at the ends of the end, which
are activated on detection of pressure drops as a HIPPS is applied to H P lines;

Materials and fabrication: The required wall-thickness is significantly reduced. Some
reduction in wall-thickness is achieved for water depths less than 1OOOm. But in water depths
of 2000rn or more, the reduction is at least 50%;

Installation: A addressed above, for pipelines in excess of 2000m there is a significant
reduction in lay tension requirements. An added benefit is that lay rates should be faster as the
wall-thickness of the pipeline is significantly reduced. It is envisaged that there will be no
adverse commercial impact to this phase. If conventional installation methods can be used,
the cost is reduced further;

commissioning: As conventional approach of de-watering and drying the pipeline will not be
possible. However pre-commissioning techniques using liquids (methanol) at the required
pressure could be implemented. The approach may have a commercial impact;

Znsurance etc: Although this is not a well-proven technique it is technically feasible. It has
been used where the pipeline is too light to be stable on the seabed, but not in ultra deep
water. Insurance would be related to repair cost rather than risk of damage.

In summary, installation of a wet pipeline in lo00 - 3000m water depths is technically
feasible and it could reduce the pipeline CAPEX (due to material savings) by up to 27%.
There is a greater emphasis on the design aspects but with modem analysis methods and tools,
the engineering can be performed reliably and efficiently.

12.7 References

1. Bai, Y. and Damsleth, P.A. (1997), “Limit State Design of Offshore Pipelines”, Proc. of
OMAE’97.