Scope and Application

This Recommended Practice applies to structural design of rigid pipelines with a potential to buckle globally. This normally implies so called HP/HT pipelines but also moderately tempered pipelines may be of relevance depending on the global buckling resistance (e.g. very light pipelines). Hence, it does not include considerations with respect to temperature and pressure effects on materials, insulation, thermal conditions and flow assurance but gives procedures and criteria to global buckling only. Consequently, there are no limitations to pressure or temperature other than those caused by structural limitations from the material properties etc.

Global buckling is a load response and not a failure mode as such. Global buckling may, however, imply an ultimate failure mode such as:

— local buckling

— fracture

— fatigue.

This Recommended Practice covers global buckling ultimately limited by local buckling. The procedures provide also best estimates and upper bound strain values that may be used in a fracture assessment. For other failure modes reference is made to DNV-OS-F101.

Three global buckling scenarios of HT/HP pipelines are covered in this Recommended Practice:

1) Exposed pipelines on even seabed. Global buckling occurs in the horizontal plane. Post buckling configuration may be allowed.

2) Exposed pipelines on un-even seabed. Global deformation occurs first in the vertical plane (feed-in and uplift) and subsequently in the horizontal plane or when this is combined with scenario I, e.g. for curves on un-even seabed. Post buckling configuration may be allowed.

3) Buried/covered pipelines. Global buckling in the vertical plane, so called upheaval buckling. Only criteria for avoiding global buckling are given.

The trawl interference evaluation in this Recommended Practice is limited to lateral buckling only. Hence, trawling in free span is not covered by this Recommended Practice but by DNV-RP-F111.

With reference to trawling applicability see also Sec.3.5.3. For other design issues, reference is made to other codes, see Sec.1.5.

The design procedures and criteria in this Recommended Practice are based on vast number of analyses. In spite of this very special combinations may give un-anticipated results and the designer shall evaluate all results critically.

The provided design procedures are considered being applicable in general. The basis for the development has been pipelines in the range from 10" to 42".