ABSTRACT

Fields currently being discovered in the North Sea are not as large as those of the 1970's and 1980's; they cannot therefore, economically sustain the implementation of a fully functioning platform The approach currently employed to extract reserves from such ‘marginal’ fields is to Install a subsea wellhead, controlling this via a control umbilical tied back to an existing platform Although this is less costly than building a platform the cost is still significant. In addition, there are also technical risks involved in laying and trenching the umbilical, which may limit the distance between the remote wellhead and the host platform.

Most, if not all subsea installations require hydraulic or electrical controls Communications to subsea trees, manifolds or templates involve the purchase and installation of umbilicals With the average cost of umbilical also ne running at around £120 per metre to purchase and installation costs at least double this figure, depending on lengths and site conditions, it is not difficult to realise costs, when Including sophisticated control modules and terminations, of between 5 to 7 million pounds for a multiplexed controls and umbilical system some 20km from a controls source.

This paper highlights the fact that for certain situations, savings in the order of 50% can be made by the acquisition and installation of an Integrated Control Buoy (ICB), together with a more reliable system.

INTRODUCTION

Control of the wellhead via long hydraulic umbilical can be problematical due to the delay associated with the pressurization of the umbilical. These problems can be reduced, to a degree, by the use of subsea electronics, but the repair of these systems, should they fall, is costly due to the difficulty in accessing the equipment on the seabed

A solution to this problem is to install all necessary control and monitoring equipment In a relatively small buoy (6 metres diameter and 6 metres high) moored adjacent to the subsea wellhead This is connected to the christmas tree via a short, vertical, hydraulic and signal riser The control systems on the buoy are then supervised, via a radio link, from a nearby master control station situated on board a Platform or Floating production facility.

The following technologies are employed in the system.

  • Buoy technology (including moorings) is well understood through experience gamed with the Oceanographic and Meteorological Data Gathering Buoys which were installed and managed under contract to UKOOA

  • Control and Monitoring Equipment will be standard, off the shelf equipment as used offshore for many years.

  • Dynamic Riser technology IS well developed in the industry. Recognised suppliers will be used.

  • Radio links have been supplied for use in many harsh environments, both offshore and onshore, for control and monitoring applications.

BACKGROUND

John Brown presented a paper on the possible use of control buoys In September 1989 at the *SPC offshore Europe conference but the concept was not developed due to lack of historical data on buoy reliability at that time

Recessionary times and present day economics have revitalized the need to pursue and investigate new and historical innovative economical but reliable ideas.

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