Residual Lay Tension

During pipelay, tension is applied to the pipeline by tensioner located at the firing line on the lay vessel. The tension applied will determine the curvature of the pipeline between the stinger and the touchdown point. Please note that the higher the tension applied, the touchdown point of the pipeline will be further away and vice versa.

Prior to pipeline installation operation, the installation analysis will be performed. The purpose of the installation analysis is to determine the tension required during the pipelay to ensure the strain/stress of the pipeline overbend and sagbend are within the allowable limit in accordance to the codes and standards.

The tension discussed above is also known as the top tension. Due to the tension applied, there will be residual lay tension acting on the pipeline upon completion of pipeline installation. The residual lay tension is the stress that remains in the pipeline after the tension applied to the pipeline has been removed.

There are several ways of calculating the residual lay tension which depends on the stage of engineering design:
1) Assume as 10% of the lay tension. (This is a common assumption used in calculating the fully restrained pipeline effective axial force and some other analyses if no sufficient information is available. Note that this assumption may not be suitable for some analyses as the residual lay tension may be more than 50% of the lay tension in certain scenarios)

2) Simple residual lay tension - the residual lay tension calculation will be added in the future, if there is any request!

3) Detailed FE analysis - Model the progressive pipe lay using the FEA software (i.e. Abaqus, Ansys, Orcaflex etc.) and check for the effective axial force of the pipeline upon completion of the pipe laying.