ABSTRACT

Several platform concepts may be feasible for application in marginal oil fields located in shallow waters. One of these is the monotower platform, which presents significant advantages with respect to the costs involved in fabrication and installation. When exposed to the North Sea environment such a platform becomes susceptible to dynamic motions from waves- and from wave induced vortex shedding. This paper presents a study conducted to investigate these dynamic effects for the monotower concept. The vibration amplitudes and dynamic amplification of the in-line forces determined in an experimental test series are presented, especially for a platform with a riser caisson attached to the main caisson. A computer code has been applied to extrapolate the test results to full scale, to calculate the fatigue damage and to determine the wall thickness for various water depths. Upper water depth limits for the feasibility of the concept in the North Sea have been determined.

INTRODUCTION

Many of the oil fields discovered in the North Sea are marginal fields. Furthermore, the layers in which the reservoirs are located may have very small permeability. Consequently, it is very difficult to predict the production rate for these fields. To develop this type of oil fields it is necessary to minimize the initial investments in the field in order to limit the consequences of a production smaller than predicted. This can be accomplished by means of a small remotely operated satellite installation with provision for only one or a few wells.

For shallow water depths four types of satellite installations can compete:

  • The subsea completion

  • Jackets, either 4 or 3 legged

  • The free standing caisson (monotower)

  • Gravity platforms

A comparison between these alternatives for a field located on 35 m water depth shows that both the initial and the annual costs of the free standing caisson are smaller or equal to the other alternatives. A detailed conceptual study of the caisson has therefore been performed.

The free standing caisson or the monotower is a single pile with varying diameter driven into the sea bed. The conductors are placed inside while the risers are placed outside the structure. This type of structure is well known and have been installed at numerous locations in the Gulf of Mexico. However, it has never been used as offshore platform in the harsh environment of the North Sea. In this environment the mono tower will be governed by its dynamic behaviour and thus prone to fatigue. The only structure of this type installed in the Northern Europe so far is the free standing tower "Euro-O" located outside the coast at Rotterdam, Ref. /1/. The purpose of this tower is to collect environmental data on the behalf of the harbour authorities of Rotterdam.

Keywords:
coefficient, dynamic response, vibration, natural period, north sea, wall thickness, installation, cylinder, platform, water depth
Subjects:
Information Management and Systems
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1985. Offshore Technology Conference
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