INTRODUCTION

Working underwater is a lot more difficult than working above water or on land, therefore the incentives to do so must be found in some aspects of project realization, such as

  • reduced cost,

  • advantageous schedule,

  • improved technology

In this chapter these aspects will be considered for a number of cases of floating systems and subsea developments which have been carried out The North Sea provides examples of floating systems, starting with Hamilton Brothers'/Argyll, and the development of the buoyant tension-legged platform of Conoco Hutton TLP in production in 1984

Subsea systems are complementary to existing fixed structures and increase the recoverability, for example, BP Magnus has seven satellite wells

Subsea production began with the Zakum Subsea Production Scheme (1969–72) and now subsea systems are used m field developments in the North Sea, for example, the Shell UMC, Texaco Highlander and North-East Frig

This chapter also discusses some prospective developments in applications, techniques and equipment. Other chapters in this volume will elaborate on many of the topics which will be raised.

Early practitioners of the art of working underwater include marine salvagers and offshore drillers, and techniques which were developed for these activities are being elaborated and unproved upon for those more complex activities of offshore oilfield development Experience, and common sense, has taught that complicated construction work should be done as completely as possible on land before taking units offshore for installation, and that only minima1 assembly of pieces be required subsea Examples of subsea construction activities include well templates, manifolds, pipelines, multiple flowlines, anchor foundations, use of divers and remotely operated vehicles Prospects for the future of subsea activities are assured by the present keen interest in floating production systems and subsea satellite developments to be tied-back to existing installations

The future North Sea development m the UK sector will be primarily

  • smaller fields,

  • satellite fields,

  • condensate/sour gas

Deepwater developments present greater challenges but, with the discovery of sufficient reserves, there is no doubt that these developments wtl1 also stimulate new requirements as well as new capabilities for underwater construction.

ARGYLL FIELD

The Argyll Field layout is represented in Fig 1, and the key information summary is presented m Table I The production riser system is of particular interest

Production riser system

The production riser system at Argyll is made up of standard drilling components assembled in what was a unique system, in 1975 The(Fig 1 is available in full paper) system consists of five basic elements from the sea bed upwards - the mass anchor, permanent base, manifold, risers and flexible connections.

The central riser is of 10 in * nominal bore and serves as the main supporting member and export (or shipping) riser. This element consists of a stab sub-assembly, universal joint and SLX 40 ft* joints of 10 in riser pipe connected with standard marie riser joints.

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