ABSTRACT

A pipe tow method is one of the most economical ways to construct offshore pipelines. It is a method that does not require large equipment such as a tensioner or a lay barge. The long pipe strings, however, are affected by waves and currents. If a surface tow is adopted, the action of waves is severe and results in high dynamic stresses. The effect of wave force has strong nonlinear characteristics. It is therefore important to establish ways to evaluate the motions of long pipe strings in waves.

Model tests were conducted using three types of models. A computer program based on the finite difference method was also developed to analyze the dynamic response of pipe strings to waves. Test models were analyzed by the program, and the validity of the analysis method was confirmed.

This paper discusses the results of the model tests, theoretical calculations and fatigue investigations. It also describes the near-surface tow method used for offshore pipeline construction in Chengbei oil field in 1985.

INTRODUCTION

For constructing offshore pipelines, the most popular method is the lay barge method. For waxy crude .oil pipelines insulated by a coaxial system, however, a large amount of welding is required. But a tow method presents great economical advantages, compared with the lay barge method. In an area where the water is shallow and the seabed consists of soft clay, the surface tow method and the near-surface tow method are preferred over the mid-depth tow method or the bottom pull. method.

During towing, the pipe strings are affected by waves and currents. They are bent by currents and develop dynamic stresses under wave action. Wave induced cyclic stresses may cause fatigue damage in the pipe strings. The waves also create dynamic forces. in the bands which connect the pipe and the floater.

The method to predict the motions of long towed pipe strings was reported by Verner et al.(1984)! No reports have been published on the influence of wave action on pipe string motion except by Costello et al.(1983),2 though they provided no detailed information concerning wave influence.

Until now, no elaborate method to predict the pipe motion during towing has been available because of the considerably complicated analysis that results from the interaction between the wave and the pipe-floater system. A constructor has to select comparatively conservative conditions on the basis of past experience.

If we can evaluate the critical sea conditions for towing and the degree of fatigue damage during towing, the safety of surface and near-surface towing can be ensured and the tow method can be applied to a wider range of sea conditions than previously.

To predict the influence of wave action on pipe strings, model tests, theoretical calculations and fatigue investigations were conducted. The results are described in this paper.

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