ABSTRACT

Recently a development has taken place extending the spans of drill and well conductors beyond previously used lengths Such long spans of well conductors carrying the weight of the xmas tree and par t of the Inner casings at the top result In rather low eigen frequencies which make them vulnerable to vortex shedding locking-on during the passage of large waves or vulnerable to resonance with the shorter waves In the sea Both phenomena are governed by the structural damping and other types of damping, Inherent in conductors. These other damping contributions are the modulation of the eigen frequencies when the high waves change the inundation of the conductors and the out-of-phase vibrations of the Inner casings and tubing The paper presents the results of analysis of these contributions and the Impact on design.

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INTRODUCTION

The analysis and design of jack-up drilling conductors and well conductors on steel jacket platform s have used established techniques for many years New technology and developments, however, have made it necessary to review the current practice. An example of new technology is the horizontal drilling from jack - ups which has increased drilling time from 1 month to 3 exposing the outer conductor to higher fatigue damage during the drilling Further jack-up operations have been extended to deeper and deeper water bringing the conductors Into a low frequency domain

Another example of new developments is the addition of well conductors to existing old platforms using their reserve structural strength to the utmost. Due to the complications of mounting conductor supports the spans f the conductors are made as long as possible, which bring the well conductors into a low frequency domain and make them vulnerableto vibrations. In both the above examples the role of vortex shedding locking-on and resonance with wave motion becomes Important The development of such vibrations are dependent on the damping contributions This paper describes some additional damping effects present in drilling and well conductors.

RESPONSE TO VORTEX SHEDDING

The paper is not meant t o be a desertion concerning vortex shedding locking - on However, a few basic concepts will be described in the following in order to appreciate the problems. For long spans exposed to steady currents, methods of analysis are well established For long spans exposed to wave action or to combined wave and current action, however, very few methods are available (Ottesen Hansen, 1982). The basic mechanisms to consider for the wave induced vortex shedding problem are

  1. Presence of vortex shedding by the wave motion around the conductors

  2. Onset for locking-onto vortex shedding.

  3. Development of vibrations during the passage of waves.

These mechanisms will be briefly discussed in the following. The presence of vortex shedding on a conductor depends on the wave kinematics. Criteria are listed for instance by Ottesen Hansen (1982).

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