Abstract

Field developments in remote areas of deep water offshore require the services of long distance pipeline systems. Design and installation of pipelines in deep waters are expensive. Seabed terrain in deep water is rough and more numbers of unsupported spans of long lengths are expected. Design against vortex induced vibration (VIV) is of concern for the reliability and cost of deepwater pipelines.

The practice of design against VIV has been to limit allowable span lengths such that onset of VIV cannot be initiated. This is a highly conservative approach resulting in expensive span corrections. Probability of occurrences of current velocities at seabed reaching critical is very low, hence, it is proposed that the designs be based on actual fatigue damage. This is also recognized by DnV Guideline 14 (June 1998).

Existing methods of VIV response calculation produce extreme response, which leads to conservative fatigue damages. Based on a non-dimensional analysis and fitting model test results an empirical relation, as a function of Reynold's number, reduced velocity, and reduced damping, is proposed. The formula determines equivalent response, which represents overall fatigue accurately.

Introduction

Offshore pipelines transport reservoir fluid from wellhead to production platform and produced oil and gas from platform to shore terminal. It is one of the most important and expensive items of an offshore field development project. It is even more expensive for developments of fields in deep waters. The sea floor terrain is rough in deep waters, consequently, unsupported spans are large and more in numbers. One of the major elements of cost in offshore pipeline system is the span correction and quite often correction of spans are required to satisfy the design against vortex induced vibration (VIV).

Classification notes (DnV) are available explaining the principles and procedures of VIV, but until recently (June 1998) there was no regulatory guidance on the design of unsupported pipe spans against VIV. Even after the introduction of the DnV Guideline No 14, normal design practice has been to limit the length of a span in such a way that the natural frequency of the pipe is away from the shedding frequency of the vortices such that no resonant vibration can take place. Reduced velocity for no onset of inplane vibration is small. This means small allowable free span and costly remedial measure for longer spans.

The major drawback of the present design practice is that it is unreasonably conservative. It does not consider the magnitude, duration, and the combined damaging effect of the VIV in relation to the available resistance to fatigue damage of the pipe line structure. Current velocities at the seabed are mostly small and negligible. The probability of seabed current velocity reaching critical is extremely low and, therefore, a realistic approach to design a pipe span would be to design it against fatigue, like other submerged structural members are designed against wave induced fatigue.

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