April 2016

Decommissioning subsea infrastructure – geotechnical considerations

Andy Small

Andy Small

Between now and 2040, around one-sixth of approximately 470 North Sea installations will require decommissioning. Cost estimates for undertaking this mammoth scale of work is rising exponentially - the total UK offshore decommissioning expenditure is now predicted to hit £50 billion by 2055, according to Oil & Gas UK’s Activity Survey 2016. As all subsea infrastructures interact with the seabed in some manner, understanding the geotechnical aspects and managing geotechnical risk will be of significant benefit to executing decommissioning projects successfully and cost-efficiently.

Full field decommissioning in the North Sea presents an emerging technical challenge to operators and offshore service companies. Over the coming decades a wide range of subsea infrastructure will need to be decommissioned, ranging from flowlines and umbilicals to subsea structures and moorings. 

Gravity-based structures

During the engineering phase of a decommissioning project a number of soil failure mechanisms must be considered to determine the likely magnitude of the recovery loads. This is of particular importance for removal of gravity-based structures (GBS) to ensure that lifting equipment and rigging is suitability sized, and that no shock loads are imparted on the system. The recovery load, when compared to the original as-installed submerged weight of the structure, will be larger primarily due to soil-structure interaction effects. These include adhesion, soil strain rate effects and negative pore pressure development generating underbase resistance. The latter can be particularly significant for structures on soft clay seabeds, such as those located in the Witch Ground Basin in the central North Sea. In some cases there is a risk that the soil plug could be recovered during extraction and recovery, appropriate mitigations for this should be incorporated into operational planning.

North Sea fields typically feature subsea tie-backs with subsea structures.
North Sea fields typically feature subsea tie-backs with subsea structures.

Suction caissons and anchors

Obtaining and reviewing original geotechnical survey and design reports is crucial to understanding the likely behaviour of the caisson during recovery operations. For suction caissons and anchors, this will highlight the magnitude of likely overpressure, which must be overcome to allow extraction. Removal of subsea structures founded on suction caissons can theoretically be achieved by reverse installation techniques, such as the application of overpressure (in place of suction) between the underside of the mud mat plate and the top of the soil plug (seabed inside the caisson).

The accumulation of marine growth around valve work, corrosion, the ability of lifting eyes to take load and the ability of weld seams, as well as structural steelwork, to resist the required overpressure can present a number of issues for recovery. A pre-engineering general visual inspection (GVI) should be conducted to determine the condition of the suction caisson with specific focus on the condition of pipework, hatches, ROV stabs, valves and anodes. This will assist in determining if the structure can be removed by overpressure alone.

Where drag anchors have been used for moorings, the anchors and chain can normally be fully recovered using an anchor handling vessel (AHV) with suitable bollard pull and reused elsewhere following inspection and refurbishment. As such, they are frequently used for temporary floating facilities such as flotels and semi-submersible drilling rigs. Recovery is usually achieved by the application of tension in the opposite direction to the setting direction at a high angle.

Piled structures and moorings

It is generally considered impractical to recover driven piled foundations and moorings due to the magnitude of the soil resistance which must be overcome to allow extraction from the seabed. As such, typical practice is to detach the structure from the foundation piles and recover in a single or series of lifts. The remaining piles are subsequently cut at a specified depth below seabed level, depending on various factors. This cut can be performed either externally by excavating around the pile head or internally using internal dredging and cutting techniques.

A number of North Sea structures installed in the 1990s featured vibro-installed piles, at least one of these subsea structures was completely removed successfully with vibro techniques. It may be possible to extend this practice to allow complete removal of driven piles which may offer an interesting alternative to current practice.

Pipelines, pipeline bundles and umbilicals

There are diverging precedents in the North Sea regarding pipeline decommissioning. Some have been decommissioned in-situ, while others have been recovered to shore for recycling. Increasingly, subsea pipelines and umbilicals are left in-situ as recovery can be uneconomical and, or, more damaging to the environment compared to decommissioning in-situ. However, this can result in future costs incurred for the operator for periodic inspection requirements together with obligations to rectify any snag hazards which arise.

Reverse S-lay has been proposed as one method of recovering large-diameter pipelines, but the concept has not yet been proven. In the case of small diameter reeled surface laid or non-backfilled pipelines, reverse reeling of pipelines is possible and has been successfully executed in the UKCS. Piecewise recovery is also possible, but potentially time consuming.

Numerous offshore facilities are planned for decommissioning and each presents individual geotechnical challenges.
Numerous offshore facilities are planned for decommissioning and each presents individual geotechnical challenges.

Pipeline bundles are relatively unique to the North Sea and generally comprise multiple production products and associated service lines contained within a singular carrier pipe with manifold structures connected at each end. The first subsea bundle in the North Sea was installed in the Murchison field in 1980 and since then around 80 bundles have been installed. They generally comprise multiple production products and associated service lines contained within a singular carrier pipe with manifold structures (towheads) connected at each end. At present, a number of options have been proposed to decommission a pipeline bundle including, reverse submerged tow, surface tow, subsea piecewise deconstruction, or full (or partial) abandonment in-situ.

An emerging technical challenge

The bespoke nature of subsea infrastructure, the range of foundation options employed, differences between installation and reversal of this procedure and changes that have occurred since installation raise issues which need to be addressed in decommissioning engineering. In addition to these issues, an early focus on technical feasibility and constructability is required to realise opportunities for efficiency and cost reduction in conjunction with mitigating risk.

The transfer of knowledge and experiences between operators, contractors and consultants will also be crucial to enable the industry to develop decommissioning practices further. The design of new structures with a “design for decommissioning" philosophy together with knowledge gained from removal of existing structures should, in time, reduce the decommissioning burden on operators for new field developments.

Andy Small is a Principal Geotechnical Engineer for Xodus based in Aberdeen. He holds a MEng in Civil Engineering Design and Management from the University of Dundee, United Kingdom. Before assuming his current position in 2013, he worked for offshore EPIC Contractors and specialist offshore geotechnical consultancies.