Background

A client of ours was responsible for project management of an intervention on a sea sump caisson. Located on a platform in the North Sea, this caisson was open ended and receiving discharge from sand wash activities, deck drains, and seawater overflow from diesel tanks.

The caisson had become blocked between -46m and -84m and needed to be cleared. For this the client needed an intervention plan.

The plan involved cutting several holes in the caisson to allow access for hoses and dredging equipment to remove the blockage. However, these holes had the potential to weaken the caisson so an assessment was required to ensure that the integrity after modification would be acceptable for the remainder of the caisson’s field life.

The client commissioned us to analyse and assess the current structural integrity of the caisson and predict its integrity following the planned intervention.

Analysis

The Jee team used the FS 2000 design software to analyse the structural integrity of the caisson and create a global model. The team also constructed two local 3D models of the caisson at the hole locations using Abaqus FEA software.

Simulations of various loads were applied to the local models. The range of loads could then be compared to determine a worst-case stress concentration factor (SCF). The calculated SCF was then applied to the global model and the results checked against the standards governing offshore steel structures such as this (BS EN ISO 19902).

Outcome

Our analysis showed that the caisson’s integrity would not be compromised by cutting the proposed maintenance holes, provided the caisson was not subject to degradation or corrosion exceeding expected levels.

To monitor this, clear data for the actual corrosion of the caisson would be required prior to drilling the maintenance holes. Since limited data was available, our team advised carrying out an ultrasonic scan to check for defects and determine wall thickness. We further recommended a visual inspection for any defects or cracks on the caisson, and physical measurements of corrosion.

We advised for a team of subsea divers, equipped with the requisite tools and cameras, to undertake the inspection and measurements of the interior and exterior of the caisson prior to drilling the holes.

This information could be used to check the results of the preceding ultrasonic scan and determine the correlation between the scan indications and actual observations.

Benefits

We were able to provide our client with clear procedures for checking the integrity of the caisson at each stage of the cleaning process thus enabling them to undertake the work safely and with confidence.

We also provided the client with contingency guidelines for drilling additional holes should the planned access holes not be sufficient to permit cleaning.

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