On the structural mechanics of multi-layered subsea pipelines

During the 1990s the world-wide offshore industry has been increasingly developing oil and gas fields in deep water - classified here as generally above 300 metres (984 feet) water depth - often combined with production from reservoirs at higher temperatures. Subsea pipelines form an essential element of these developments and one of the limitations on deep water development has been the inability to provide large diameter conventional steel pipelines and risers capable of withstanding large external hydrostatic pressures. The work presented in this thesis investigates the performance of multi-layered pipe cross-sections for the required increase in hydrostatic pressure capacity and thermal insulation for such subsea pipelines. A fundamental investigation into the structural mechanics of such multi-layered pipes is presented with an emphasis on three principal issues - The mechanics of multi-layered pipe loading due to internal pressure, its collapse due to external pressure, and the behaviour of such pipe geometry when in a free submerged catenary configuration. Initially, the stresses induced by internal pressure have been investigated based on the Lame's equations. The results were compared with a finite element analysis and demonstrated good agreement. The stress distribution due to internal pressure was then investigated for a wide range of multi-layered pipe geometries and Young's Moduli of the core material. Comparisons are also presented with the stresses within equivalent single walled pipes. The much more complex external pressure problem was then addressed. The stability of a cylindrical multi-layered shell is a complex problem and in response to this, the investigation presented in this thesis followed a staged approach. Based on the previous work of Raville (1955), an elastic classic model was developed. Following this, using the concept of an elastic foundation, a new formulation was developed to derive critical external pressure loads. This work has been compared to that of Montague (1975) for critical external pressure based on two dimensional elastic plastic deformations up to maximum shear stress or Tresca failure theory. In addition, another approach for the elastic plastic model has been developed based on three dimensional Mises failure theory. A finite element analysis was then used to compare results from these different approaches for obtaining the critical external pressure. These four methods are used for a comparative investigation of collapse pressure predictions for a wide range of pipe geometries and Young's Moduli of the annular material. These comparisons give an indication of the applications of these methods and also give some insight into possible collapse mechanisms for multilayered pipes. This thesis also examines the performance of a multi-layered pipeline in an underwater catenary configuration and compares this to the performance of a single walled equivalent pipe. This was done by the development of an analytical catenary model aimed at optimising the catenary geometry around the two critical stress points of the catenary (the top connection at far position and touch down point at near position). The results demonstrated the significant improvement that multi-layered pipes could deliver for reductions in top tension and steel wall thickness when used in a catenary configuration. In overall terms, this work demonstrated that multi-layered subsea pipelines can provide a wide range of structural performance benefits both locally and globally. Locally, appropriate design and material selection can yield combinations of reduced steel volume and greater internal and external pressure capacity. In global terms, the buoyancy contribution from the thicker walls of multi-layered pipe will yield significant reductions in top tension when in a catenary configuration. This investigation has only examined a relatively narrow range of structural benefits of multi-layered pipes. Much further work needs to be done on local structural behaviour, internal layer bonds, on the internal damping of such pipes, and on the mechanics of the pipe segment connections