Dunlin Alpha CGF Removal Engineering

Project Overview

Elgenio provided detailed decommissioning engineering support to Maersk Supply Service for the planned removal of subsea conductor guide frames, vertical support guides and selected conductors from the Dunlin Alpha platform.

The engineering scope developed from early feasibility and condition assessment through structural lift analysis, subsea cutting and lifting methodology, rigging design and dynamic recovery analysis.

The work was developed around an ROV-led removal philosophy intended to minimise diver intervention and support recovery of the removed infrastructure to the offshore vessel.

The Challenge

The removal scope involved ageing subsea structures that had been in service for several decades, with uncertainty around corrosion, marine growth, flooded members and the condition of the original steelwork.

Existing repair clamps and other appurtenances on selected conductors prevented them from being recovered using the conventional removal methods, requiring an alternative removal methodology to be developed.

The conductor guide frames also had to remain structurally suitable after isolation from the platform and during recovery through the splash zone, where hydrodynamic loading and vessel motion could govern the lifting operation.

The engineering therefore had to consider the complete removal sequence, including condition assessment, subsea cutting, structural integrity, rigging, crane capacity, vessel interfaces and recovery environmental limits.

The Solution

Elgenio developed the removal engineering around a coordinated subsea cutting, lifting and recovery methodology rather than treating the individual structural, rigging and marine operations as separate activities.

Preliminary structural analysis was used to identify highly utilised areas of the conductor guide frames and support development of the inspection and UT requirements needed to establish confidence in the condition of the ageing structures.

Detailed weight assessments were developed accounting for primary and secondary steelwork, clamps, flooded members and marine growth. Structural lift analysis was then undertaken for the proposed recovery configurations and subsea cut arrangements.

Rigging and vessel interfaces were developed around the selected recovery arrangement, including crane radius, structure clearance and the loads generated during lifting.

OrcaFlex dynamic analysis was used to assess recovery of the structures through the splash zone at submerged, waterline and in-air stages. Both single-piece recovery and cut-down contingency arrangements were considered to define the governing dynamic loads and allowable operating conditions.