ABSTRACT

The costs, safety and complexity of performing downhole service operations on subsea completed wells are major concerns of offshore operators. Service costs in particular often become the deciding factor when determining the feasibility of developing small subsea fields. Of particular interest are slickline and conductor line operations. In the past, these operations were usually performed on subsea wells using a semi-submersible rig and a rigid riser.

This paper describes a subsea wireline service system in detail. The system includes a subsea lubricator stack that attaches directly to the subsea tree. The wireline unit, control systems and deployment equipment remain on the deck of the vessel. The system is designed to be deployed from a relatively small vessel such as a diver support vessel (DSV). However, operations can also be conducted from a semi-submersible rig, workboat or moored barge. A riser is not required. System components, deployment and retrieval procedures, scope of work and emergency disconnect system are described in this paper.

INTRODUCTION

In many areas of the world, especially in the North Sea, the trend in offshore field development is towards subsea completions. The number of subsea wells is expected to double by 1994, with as many as 1000 by the year 2000. Several factors have influenced this trend.

  • Cost - subsea wells are generally more economical than platform wells.

  • Marginal production from undeveloped fields - most of the remaining undeveloped fields are small with limited potential which cannot justify the expense of platforms.

  • Harsh environmental conditions - high Tides, extreme currents and ice make platform construction impractical.

  • Deep waters - some undeveloped fields are in areas where extreme water depths exceeding 1300' make platform construction cost prohibitive.

Subsea wells are completed as single well satellites or in multiwell templates. Production can be into a floating production system, platform mounted system or a shore-based facility.

Subsea well completion designs do not differ significantly from platform-based wells. The corollary to this is that well servicing requirements will, in general, be the same for subsea wells as for any other offshore well.

Conventional riser-based methods of reentering subsea wells are time-consuming and costly, and therefore not: compatible with the operating cost levels dictated by the majority of subsea developments.

The system discussed in this paper was developed over a number of years, and has been successfully used to carry out a comprehensive range of wireline operations from floating vessels in the North Sea. This system was originally designed to be deployed from a diving support vessel (DSV).

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