The use of StraiN Based Design (SNBD) criteria in offshore pipeline technology has been widely discussed during the last decade. In fact there have been many instances where the pipeline engineer has had to adopt SNBD in order to avoid onerous measures necessary to meet the traditional STress Based Design (STBD) criteria advocated by international codes. SNBD was adopted to make provision for the limit capacity of available laying equipment in difficult installation conditions, to minimize seabed preparation and free span correction works for pipelines crossing uneven seabeds, and to make allowance for the severest operating conditions in high temperature and pressure pipelines. Occasionally this gave rise to a dispute between laying contractors and,' pipeline operators as well as between pipeline operators and authorities. Conditions for which SNBD were applicable, as well as permissible deformations in relation to safe operation of the pipeline in the long term, were of major concern. In this paper the state of the art technology and engineering practice for the implementation of SNBD versus STBD is discussed. The applicability for pipelines under different conditions of equilibrium between external loads and internal reactions is reviewed. In addition different aspects to be considered in the definition of permissible strains are introduced
STress Based Design (STBD) is almost universally used to assess the structural integrity of pipelines. The application of the proposed formats is straightforward and simple elastic analyses are required to calculate the effects of applied loads in the specified conditions. Load effects are subsequently compared with allowable stresses and undesirable effects are controlled through a safety margin with respect to the characteristic strength of the material (Stephens and McComell, 1985). The characteristic strength is usually taken as the stress at 0.5% strain referring to a uniaxial stress-strain curve taken from standard tests (specified minimum yield stress, SMYS).
At the moment STBD criteria, in relation to allowable hoop stresses and minimum wall thickness requirements for pressure containment (which date back about 40 years; Verley et al., 1994), are under revision (Jiao et al., 1996). An effort to account for both the current status of steel and line pipe technology with respect to that existing at the time of their development and for the actual failure modes experienced during the 25 years of offshore pipeline use (Bruschi and Tura, 1995), is currently being carried out. This revision process also involves STBD in the presence of longitudinal stresses, which are usually based on the comparison between the calculated equivalent stress according to Von Mises and allowable stress expressed in terms of the characteristic strength of the material, multiplied by a usage factor. Sometimes this may lead to arbitrary and variable safety levels in relation to actual failure conditions (Sotberg and Bruschi, 1992). In particular, this can be observed when comparing STBD for land and offshore pipelines.
The advent of new materials and modem welding technology has removed the hazard of failure modes associated with brittle fracture of the material which is the basis for proposing and using STBD.