ABSTRACT

This paper discusses some of the problems associated with the design of subsea pipelines for the hydrocarbon industry Concentrating primarily on subsea tiebacks In the North Sea, it looks at those aspects of design which reflect the tensions between operational requirements, installation requirements and costs, and highlights the potential benefits of plastic liners The paper does not address flexible flowlines, but considers alternatives both to pipelines, and umbilicals for subsea control Life-cycle management, asset integrity and the safety of other users of the sea are also addressed

INTRODUCTION

This paper is not a guideline for the design of subsea pipelines Rather, it discusses in general terms why the design and management of subsea pipelines can present a challenge to the engineer and, through this, attempts to explain why pipelines are an exciting line of business for some people It would be considered a particular success if it were to persuade one or more young people that engineering In general, and subsea engineering in particular, IS a profession worth pursuing

The North Sea is a mature province, and it can be stated with reasonable certainty that most future developments will be subsea tie-backs to existing infrastructure In other parts of the world, there will be a greater variety of pipelines installed, with challenges such as deep water, or Ice infestation However, this paper concentrates on North Sea subsea tie-backs for hydrocarbon service because, apart from anything else, to cover all types of pipeline in all regions would require a book rather than a paper (Note that flexible flowlines - composite structures in which a thermoplastic pressure sheath is strengthened with steel wires - can be used as alternatives to rigid steel pipe but this paper concentrates on the latter)

DEFINITION OF A PIPELINE

In the context of this paper, a pipeline is a pressure vessel that IS used to transport fluids from where they are found, to where they can be used Generally, these fluids would be harmful to the environment lf they escaped and, furthermore, would pose a severe safety hazard if they were to escape In the vicinity of people The alternatives to pipelines are limited as, will be discussed subsequently It is possible to consider three fundamental types of subsea pipeline

  • the infield flowline - which carries unprocessed fluids from well to process,

  • the interfield line - which carries fluids from platform to platform,

  • the trunkline - which carries processed fluids to shore

These three types are illustrated in Figure 1 Clearly there can be a number of variations around these Consider the Infield Flowline The simplest field will consist of a single well and the infield flowline will connect the single christmas tree back to the riser of a processing facility (which may be a platform, or a floating production system (FPS), the term ‘platform’ will be used generically from here on) Where more than one complete well is required (1 e ignoring multi-lateral completions), it is common to tie several trees back to a subsea manifold

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