Numerical analysis of offshore pipe-lay subjected to environment-induced non-uniformly distributed follower loads

摘要

The global pipeline configuration in installation is mainly governed by static lay effects, which, besides pipeline material properties and geometry, primarily depend on its submerged gravity and horizontal tension provided by the laying vessel. These loads are insensitive to the pipeline deformation and preserve both their values and their directions as the pipeline deforms throughout the whole course of the installation. However, these static lay effects may be significantly amplified by the environmental factors, primarily caused by astronomical ocean tides, such as water level variations and hydrodynamic currents. These factors may substantially affect pipeline configuration and significantly increase the internal forces. Moreover, the loads caused by the ocean currents are of a fundamentally different nature comparing to the gravity and the lay tension. They are nonuniformly distributed along the pipeline axis and change both their values and their directions following the nonlinear deformation of the pipeline throughout the course of the laying process. This paper presents a feasible numerical method for a structural analysis of a pipeline static configuration in installation, subjected to non-uniformly distributed, position- and orientation-dependent loading and water level variations. The method considers the whole pipeline, which is partially suspended and partially laid-on a seabed, as a single continuous segment, and is valid for both S-lay and J-lay techniques. The numerical solution adopts finite difference discretization of the pipeline, and proceeds sequentially in an incremental way, following the actual pipe-lay process. At each length increment a new consequent equilibrium configuration is being assessed by consistent minimization of the updated total potential energy, which allows for considering follower loads, which change both their values and their directions following the nonlinear deformation of the pipeline. Representative parametric study is conducted to demonstrate the feasibility of the method. Considered the effects of a power-law current velocity depth-varying profile and water level variations. The method compared to Abaqus/AQUA and its convergence is validated through finite difference grid refinement. The proposed technique presents a less time-consuming alternative to the available special-purpose commercial software that imposes the use of cumbersome Graphical User Interface for a model definition and a result processing.