Influence of Lay Barge Motions on a Deepwater Pipeline Laid Under Tension

Tensioning is a prime requirement for laying deepwater pipelines. Of the several new or improved pipe laying methods which have been proposed, or even utilized, all require tensioning of the pipeline to minimize stress in the critical area. In these methods the use of a stinger is optional. Dixon and Rutledge produced charts by which the minimum tension and angle of inclination could be determined for laying pipelines in intermediate and deep waters. The pipeline is freely suspended from the sea floor up to the stinger, or to the lay barge if no stinger is used. It takes a shape over its unsupported length which differs from a natural catenary at its ends due to the pipeline stiffness. In the present work the authors have employed the same mathematical technique to study the sensitivity of a tensioned pipeline to lay barge motions, i.e., surge, heave, and pitch. Two general trends emerge from the results which are presented both in the form of graphs for several sizes of the pipeline and for water depths up to 1000 ft, and in the form of dimensionless charts. Surge, and to a lesser degree heave, is influential in shallow or intermediate water depths. Pitch becomes a critical parameter in deep waters where the tension is large. The influence of a sloping rather than horizontal sea floor is also studied in this work. It is shown that for a practical range of slopes the prior analysis gives a satisfactory description of the configuration. However, additional curves which have been provided are required to make application of the previous results.