The subsea gas field, Lille-Frigg, located in the Norwegian sector of the North Sea, was in production from 1994 to 1999. Due to high wellhead pressure and temperature of about 500 bar and 105 °C, and a CO2 content of 2.4% the field was highly corrosive.

The production fluid was evacuated in a carbon steel pipeline to the Frigg Field for further processing. In order to protect the pipeline against internal corrosion, injection of a pH stabilizer mixed with glycol was implemented.

Formation water arrived early in the production lifetime and a switch from pH stabilizer to a classical, film forming corrosion inhibitor was deemed necessary as pH stabilization is not compatible with formation water.

A subsea carbon steel/duplex stainless steel weld in the pipeline system was inspected by radiography during operation. In addition to this the carbon steel pipeline was inspected by means of an ultrasonic inspection pig. No corrosion was detected during any of the inspection campaigns. However, an investigation of the retrieved pipe after production shut-in showed preferential corrosion of the girth weld between carbon and duplex steel.

The paper describes the qualifications made prior to production start-up, the operational experience and inspection results of removed parts after decommissioning.

Subject
Test methods, Water, Piping, Corrosion rate, Pipelines, Inspection, Ph stabilization, Partial pressures, Film formation, Corrosion inhibitors, Glycol, Carbon steel, Corrosion protection
Keywords:
pH stabilization, corrosion inhibitor, dissimilar weld, sweet service, pipeline, high pressure, inspection
© 2004 Association for Materials Protection and Performance (AMPP). All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise) without the prior written permission of AMPP. Positions and opinions advanced in this work are those of the author(s) and not necessarily those of AMPP. Responsibility for the content of the work lies solely with the author(s).
2004
Association for Materials Protection and Performance (AMPP)
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