Abstract

Axial walking is the cumulative axial displacement of a complete flowline length occurring over a number of start up and shut in cycles, which may lead to excessive end movement and ultimately the failure of tie-in jumper/spool connection. The phenomenon shall be evaluated for the relatively short deep water flowlines which are operated under the following conditions:

  • very soft clay soil

  • high slope seabed profile

  • steep temperature gradient during start up and shut down

This paper addresses a study of deep water flowline walking through detailed finite element simulation with ABAQUS. The nonlinearity of pipe-soil friction and pipe material, the seabed profile, the pressure and temperature profile, and the connected PLET resistance are considered. The effects of different parameters on the pipeline axial walking are studied, especially for the pipe-soil friction models which are critical and heavily dependent on the embedment for the deep water flowline. A range of friction curves (LB, BE and UB) and mobilization distances are considered due to the uncertainty and complexities involved in the deep water soil properties.

Possible mitigation methods for axial walking are further discussed. The advantage and disadvantages of each option are compared with respect to cost, possibility of success and conditions. The suitability of each mitigation method for the case study is analyzed.

The work presented in this paper is intended to help raise awareness of axial walking for the design of deep water flowlines and shows the importance of the decision making process of the mitigation method to achieve an optimum balance between economical and technical constraints.

Introduction

In recent years, much attention has been focused on pipeline axial walking which is the cumulative axial displacement of a relatively short pipeline occurring over a number of start up and shut in cycles. The mechanism leading to axial walking has been studied in detail (Knut Tørnes et. al, 2000). An analytical equation predicting the rate of axial walking has been presented by SAFEBUCK JIP (M. Carr et. al, 2006). The mitigation measures for deep water pipeline instability including axial walking induced by pressure and temperature variations have been discussed (D. Perinet and I. Frazer, 2006).

Axial walking itself, will not cause the pipeline failure if the pipeline is not susceptible to buckling. However, as a result of the accumulated global displacement over a number of cycles, axial walking may cause the failure of tie-in jumpers/spools. It may also increase the loading within a lateral buckle causing localized failure. Therefore, it is necessary to address this problem at a conceptual design stage since its occurrence may have a major influence on the field layout, which can have a huge impact on the project cost and development.

Pipe walking is a complex aspect in regard to the pipe/soil interaction especially in high plasticity clay deposits so often found in deep fields. The analysis that is typically used especially for cohesive soils, is a very simplified constant residual friction and elastic soil, mobilisation and recovery axial resistance model. In reality, there are a lot of parameter non-linearities, most crucial perhaps, being the suction force between embedded pipe and surficial liquid soil which is probably re-set in very short period following pipe breakout and soil disturbance. Actual axial restraint is probably very much a function of the time between heat-up and cool down cycles, pipe coating type, soil contact area and adhesion force with variable embedment and associated varying force mobilisation distance.

This content is only available via PDF.
You can access this article if you purchase or spend a download.