ABSTRACT

Tensioning is a prime requirement for laying deep-water 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 pipeline and for water depths up to 1,000 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.

INTRODUCTION

As offshore production moves to everincreasing water depths there arises a growing demand for means of laying deeper and larger pipelines. Many new techniques have been proposed for meeting this demand; some have become practice while others are still in the development stages. Most of these methods have one important feature in common -- the use of tensioning in order to increase the unsupported length of pipeline and shorten (or eliminate) the conventional stinger. As operations move to deeper water it seems reasonable to expect the unsupported segment of the pipe to become much greater.

Recently, the Lavan pipeline, one of the world's deepest, longest and largest underwater pipelines,1 was laid in 290 ft of water using an abbreviated stinger and constant tensioning, Fig. lA. The same organization which laid this pipeline is currently building the world's largest pipe laying barge, which will be equipped with a means for applying tension, and will be capable of laying 48-in. pipe.2Another contractor, known for laying pipe from reels, found it advantageous to develop a torque converter by which a constant amount of tension could be maintained.3 It is important to point out that constant tensioning helps to reduce the influence of lay barge motions.

Still others have proposed total elimination of stingers. One such suggested technique was to lay the pipeline from the vertical derrick of a drilling vessel, similar to running drill pipe.4

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