Corrosion of caisson steelwork appears to be a major problem on many North Sea platforms, according to research by integrity, inspection and corrosion specialist iicorr of Aberdeen.
The company warns that the fact that there have been eight cases of complete replacement required in the past five years across a sample of platforms typically still within their design life appears to show that design of caissons has, in many cases, “been inadequate”.
The aim of the survey was to capture data on the likely true extent of the problem, which caisson types are particularly affected and how the UK offshore industry is working to detect and prevent failures.
iicorr says the responses to its survey demonstrated the “significant consequences” of caisson integrity problems, with incidents reported ranging from caissons being out of service through to complete replacements being required, with the resulting cost of each integrity problem estimated at £100,000.
While each may not, in itself, seem a big issue, cumulatively, caisson problems are costing operators millions of pounds a year to remedy, not counting interruptions to production of oil&gas in some instances.
Indeed, integrity problems on this vital element of offshore platforms are, according to iicorr, “common and can lead to significant consequences in terms of repair costs, production interruption and subsequent damage to primary structure”.
The survey covered three main types of caissons on offshore platforms:
Pump caissons – stilling tubes to pump seawater up to the production areas for injection, cooling or firewater uses.
Discharge caissons – for storing or draining fluids into the sea, such as water produced during production processes.
Carrier caissons – structural conduits for risers, umbilicals, cables, and so on, to provide protection.
Questionnaires returned to iicorr contained information on more than 200 caissons on 80 platforms.
A total of 59 recorded integrity problems were identified. There were also 29 incidents reported over the last five years, ranging from caissons being out of service to some requiring complete replacement. The total estimated cost of those 29 incidents is in excess of £7million.
Although the number of survey responses was small compared with the number of questionnaires issued (25%), iicorr insists that the survey is representative because of the large number of platforms and caissons the responses covered (80 and 200-plus, respectively).
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