ABSTRACT

In 1977, three welded l6-in. pipeline-to-riser connections were success fully made in dry one-atmosphere chambers at the base of the Thistle "A" platform in the North Sea.

As well as welding, the system involved pulling the pipelines into the chambers using standard winch equipment. On completion of the work, all welds were verified with radiographic and ultrasonic equipment, also in a dry subsea habitat.

In this paper, the entire pipeline connection project at Thistle "A" is reviewed, commencing with the platform operator's requirements which led to the choice of this system. The design, fabrication and installation of the connection equipment on the platform is also discussed, followed by offshore pipelaying operations and pull-in procedures.

Dive operations, in which the chambers are accessed by service technicians to complete the connections in a one-atmosphere environment, are described chronologically, together with problems involving weather, vessel proximities and underwater hazards which caused delays and procedural changes.

Detailed times taken to complete each connection, as well as a cost summary of the hardware, equipment and service system are listed.

The experience gained at Thistle "A" has led to a further development and extension of this application to cope with larger-diameter pipelines and greater water depths.

INTRODUCTION

The Thistle "A" BNOC field development complex on Blocks 211/18 and 19 in the U.K. Sector of the North Sea is unique in many ways. At floatout and placement in 530 ft. (161 m) of water August 5, 1976, the self floating, 32,000 M ton structure became the largest and most northerly steel platform installed in the North Sea. In addition to "on jacket" modules for winter construction, piling depth and technique, the number of well slots, the use of the deepest North Sea tanker-loading facility, and numerous other "beyond-prior-experience" accomplishments, this field development saw the first commercial application of subsea one-atmosphere welding. This involved a pipeline/platform riser connection method developed by Lockheed Petroleum Services (LPS) Ltd. with large diameter pipelines and deep water in mind. This unique method permitted welded connections to be made subsea in dry, one atmosphere chambers located at the base of the platform, using tools and welding techniques normally found on dry land.

Background

Development started in 1971, and equipment and method concepts were first reported by DeJong and R.J. Brown (1973). Based on field experience with similar flowline connection pull-in components (ref. Woodlock, 1977), it become possible, in 1974, to put the pipeline/welding chamber concept to practice.

In February 1975, the Thistle "A" operators (formerly Signal Oil and Gas, at that time Burmah Oil Development Ltd., later BODL Ltd., and now BNOC (Development) Ltd.), ordered fourteen subsea chambers. The chambers, designed and supplied by LPS, are of two types. Six are platform riser chambers (PRCs) for connection of pipelines, and eight are flowline riser chambers (FRCs) for hook-up of future subsea completions adjacent to the Thistle "A" platform. Of the six PRCs, four are for nominal l6-in. (405 mm) pipe and two are for nominal l2-in. (305 mm) pipe.

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