Types of submarine pipeline boundary conditions and implications for pipeline walking behaviour

Abstract

The ends of deep-sea submarine pipelines are usually connected to wellheads, suspended chain line risers, pipeline end manifolds, etc. , and the boundary conditions at the end of the pipelines are complicated. In order to investigate the influence of different pipeline boundary conditions on pipeline walking behaviour, the boundary conditions at the end of the submarine pipelines were simplified into three types: type Ⅰ is horizontal force at both ends, type Ⅱ is horizontal force at one end and inclined force at the other end, and Type Ⅲ is elastic boundary at one end and horizontal force at the other end; Numerical models of submarine pipeline walking were set up with different boundary conditions, and the behaviour of submarine pipeline walking was comparatively analyzed with the different types of boundary conditions. The results indicate that: compared with type Ⅰ, with the increase of pipeline end tension inclination angle, the pipeline axial tension decreases, and the amount of pipeline walking also decreases in type Ⅱ boundary conditions; With the pipeline tension inclination angle increased from 0° to 60°, the maximum axial tension decreases by 6.73%, and the pipeline walking rate decreases by 50%. This means that the change of the riser shape of the suspension chain line leads to the direction of the tension force acting on the submarine pipeline changes, and the amount of submarine pipeline walking also corresponding change. The comparison between type Ⅲ and type Ⅰ boundary conditions shows that the submarine pipeline walking rate under type Ⅰ boundary conditions is a constant value, with the number of heating and cooling cycles increase, the submarine pipeline walking rate is approximately linear growth; the submarine pipeline walking rate under type Ⅲ boundary conditions is influenced by the pipeline end manifolds elastic stiffness, the greater the elastic stiffness, the greater the walking rate of the pipe line in the first temperature cycle; with the number of heating and cooling cycles increase, the walking rate decreases rapidly until it reaches 0. The amount of pipeline walking under type Ⅲ boundary conditions is usually smaller than that of type Ⅰ. The walking behaviour of submarine pipelines varies greatly under the three types of boundary conditions, which provides some reference for analyzing the types of boundary conditions at the end of submarine pipelines and selecting appropriate boundary condition in actual project.

Keywords

• ocean engineering; pipeline boundary conditions; pipeline end manifolds; inclined tension; walking rate