Limit state design differs from conventional design methods in that each failure mode is specifically addressed (e.g. burst, collapse, local buckling, fracture due to insufficient strain capacity of the pipe wall, fatigue). Based on an extensive theoretical and experimental research programme, guidelines for limit state design of buried pipelines were presented in The Netherlands in 1986 (TGSL-1986). Later research programmes were focused on offshore pipelines as well. In the Dutch pipeline code NEN 3650 (1992). limit state design is incorporated. In the paper, a summary is given of the above mentioned research programmes, the rules in NEN 3650 and recent developments in international standards (CEN). In the general purpose finite element computer programme DIANA (1996), a pipeline module is available which enables an easy and cost effective application. Examples of practical applications are discussed. The advantages, especially in fitness for purpose evaluations, are highlighted.
Pipelines, onshore and offshore, are subjected to combinations of various loadings, such is internal pressure, external pressure, surrounding soil, bending, normal force, shear force, local loads (e.g. in supports and in spanning situations in offshore pipelines) and sometimes also torsion.In many cases, the deformation capacity is especially important. Examples are:
For offshore pipelines: during laying, pulling through J-tubes, laying on uneven sea-bottoms (e.g. sand waves).
For onshore pipelines: due to differences in the settlement of the soil along the pipeline.
The latter situation is very important in The Netherlands, where many rivers and canals have to be crossed by pipelines. The highest water level occurring in such water courses is often above the level of the adjacent land, so that dykes are needed to prevent flooding. As a rule, for pipelines laid till about 1990, the pipeline profile corresponds to the existing cross-section of the dyke and, after laying, is given a covering of clay and top soil (figure 1). In recent years, this type of construction was replaced by directional drilling techniques, whereby the pipeline crosses the water course underneath.
Figure 1 : Pipeline crossing over a dyke. (Available in full paper)
Application of traditional design methods based on the theory of elasticity for the analysis of the crossings as indicated in figure 1 has proven to be inadequate for a good insight into the actual strength and deformation properties and thus into the actual structural safety. When in the early 1970s these design methods were applied in a re-analysis of some pipeline crossings already in existence, several of these crossings were found not to meet the requirements.
Upon this, research was carried out resulting in a new method for the design and analysis of buried pipelines, which is based on limit state design applying the theory of plasticity (Gresnigt, 1986). With the theory of plasticity the real behaviour up till failure could be described much better than with the existing calculation methods. Also, a better definition of limit states and limit values could be derived.
