Core to the capability to develop large diameter pipelines in ultra-deepwater is the wall thickness design in combination with the manufacturability of the steel plates and line pipes to the required specification. The deepwater pipelines are antagonized by the tremendous external pressure which it has to resist at high water depth for safe operations. The wall thickness design for ultra-deepwater pipeline is mainly focused upon pipe wall buckling criteria. Pipe wall buckling criteria for external overpressure includes system collapse and propagation buckling.
The thick walled large diameter pipes fabrication is feasible by two pipe manufacturing processes i.e.:
However, DNV imposes 15% reduction in compressive strength (i.e. fabrication factor, αfab
= 0.85, refer Sec.5, Table 5-5 of DNV-OS-F101: 2013
) for UOE manufacturing process under the inference that the UOE manufacturing process introduces cold deformations giving different strength in tension and compression.
For the development of one of the deepwater pipeline project, DNV was engaged to find out ways for improving the collapse strength
of the UOE pipes. Critical findings of the study are:
- Modest heating during external corrosion protection coating application on UOE pipes can enhance the compressive strength of the pipes and thus higher fabrication factor i.e. αfab = 1.0 may be utilized. However, the same shall be documented during the pipe manufacturing by performing compressive tests and/or ring collapse tests.
- Enhance production testing and tighter manufacturing tolerances for ovality and wall thickness measurement may be employed.
These findings of DNV are now being widely applied in wall thickness design of ultra-deepwater pipelines, such as Middle East to India Deepwater Pipeline (MEIDP)
, South Stream
Achieving the desired material parameters for the wall thickness required for deepwater pipelines using standard calculation methods
is on the edge of what can be produced. A small reduction in wall thickness can result in a major improvement in manufacturability, and thereby drive the actual feasibility of the project for a specific throughput and OD combination.
During the mechanical design of Middle East to India Deepwater Pipeline (MEIDP)
, which is planned to be laid at a water depth of 3450 meter (approx.), wall thickness of the pipe required for meeting the system collapse criteria was 44.6 mm considering the fabrication factor, αfab
= 0.85, as specified by DNV for UOE pipes, for DNV SAWL485 FDU (API 5L Gr. X-70 equivalent) line pipes, which is at the limit of manufacturing capabilities of most of the pipe mills with current configurations worldwide. After applying the recommendations from DNV regarding the enhancement of fabrication factor from 0.85 to 1.0, the wall thickness of the pipes selected for the ultra-deep section of MEIDP was reduced to 40.5mm.
For deepwater pipelines in water depths greater than 1000m, it is not economical to protect the pipeline from propagation buckling by further increasing the pipe wall thickness. Therefore, provision of buckle arrestor at fixed intervals to meet propagation buckling criteria shall be contemplated.
The supplementary requirements which are used to further reduce the wall thickness of the pipes selected for ultra-deepwater installation is “high utilization” (U) (refer I 500, Sec. 7 of DNV-OS-F101 : 2013
) and “dimensions” (D) (refer I 400, Sec. 7 of DNV-OS-F101 : 2013
). The supplementary requirements, high utilization (U) is applied to raise the material strength factor, αU from 0.96 to 1.00 (refer Table 5-4, Sec. 5 of DNV-OS-F101 : 2013
) and supplementary requirement, dimensions (D) is applied to reduce the fabrication tolerance to ± 1.0 mm (refer Table 7-26, Sec. 5 of DNV-OS-F101 : 2013
Therefore, the pipes suitable for ultra-deepwater application require a full scale testing program for building the confidence of the designer, manufacturer as well as the owner. These aspects will be dealt further in the line pipe manufacturing section of this article.