Axial Creeping of HP/HT Short Flowlines due to Soil Ratcheting

Abstract

The objective of the thesis was to investigate the phenomena of pipeline walking in short high pressure and high temperature (HP/HT) flowlines due to soil ratcheting. It is caused by transient temperature gradient along the pipeline during start-up of operation, but also due to unbalanced tension from risers and seabed slope. The accumulated axial creeping due to frequent start-upand shut-down of operation over a period of time could compromise the integrity of tie-ins and risers if proper mitigation measure is not devised.

The paper focuses on numerical simulation using the computer code SIMLA. A brief description of nonlinear finite element solution methods with relevance to the SIMLA computer code and the SIMLA model used in the analysis is presented. In addition the mechanisms of pipeline walking and major parameters of pipeline creeping have been discussed with special emphasis in development of analytical expressions and sensitivity studies of the major parameters.

Analytical expressions that predict the rate of walking per cycle have been proposed from first principles for all the three loading cases and validated by the computer codes SIMLA and ABAQUS. The equations are most effective in use during early screening stage of the design sothat appropriate cost effective mitigation measures could be planned and included at early stage of the design. In addition, they give a better understanding and influence of the each parameter for a specific flowline so that efficient finite element analysis approach can be planned during the final design. However, they should not be used as replacement to finite element analysis due tothe limitations they have in including such as nonlinearity behavior of the problem.

Moreover, sensitivity studies are performed for various parameters such coefficient of friction, mobilization distance, unbalanced tension and seabed slope. The results of the analysis showed that the axial creeping of flowlines highly depend on the friction coefficient selected for the specific condition. The axial walking general decreases with increase in coefficient of friction except for the temperature gradient case where the axial creeping decreases for very low friction coefficient since the soil will mobilize fast after which time there will not be any creeping.

Finally, it is concluded that proper mitigation measure should be devices during the design stage to get an optimum balance between economics and technical limitations. End support is proposed as the most effective method since it requires less force and can be in cooperated into the pipeline system easily and inexpensively.