The challenges of offshore decommissioning

Brian Nixon looks at the inherent problems of removing mature offshore

Perenco UK safely executed the heavy lift removal of the Welland gas production platform in the southern North Sea.
Perenco UK safely executed the heavy lift removal of the Welland gas production platform in the southern North Sea.

There are more than 7,000 offshore oil and gas structures around the world. As these platforms mature, operators aim to extend their economic and productive lives either through the introduction of new technologies, expertise with asset integrity and production optimisation, or the development of marginal and remote satellite reservoirs.

Despite these advances, it is now recognised that a growing number of offshore assets have either reached, or are approaching, the end of their economic lives.

In accordance with a number of regional and international regulations, these structures will have to be decommissioned and removed. Over the next 10-20 years, an average of 15-25 installations are expected to be abandoned annually.

In addition, several thousands of kilometres of pipelines will have to be removed, trenched or covered. This presents critical challenges for the owners and operators of mature assets in terms of engineering capability, safety and, significantly, costs.

Offshore decommissioning is a complex series of activities, each with its own level of skill and expertise. From operations to communications, engineering to legislation, each strand is vital. Understanding how a platform was installed, how it was operated, and the expectations for when it reaches the end of its life is a collaborative process which requires a transparent sharing of knowledge.

Decom North Sea (DNS) is the industry body that facilitates this in the North Sea. Since its inception in 2010, DNS has grown to have more than 230 members drawn from operators, major contractors, service specialists and technology developers. The aim of the group is to bring people from all over the industry together in an open environment to discuss opportunities and, above all, to learn from one another.

“In the North Sea many assets are over 30 years old. Minimal consideration was given during the design phase as to how these structures would be removed and disposed of in the future.

As a result, operators now face a number of challenges, including whether to attempt to remove vast concrete structures from the sea bed, not to mention hidden dangers, such as asbestos, as the approaches required to meet these challenges have yet to evolve.

The majority of operators gearing up to undertake their first decommissioning programme are taking time to develop their own approach and strategy.

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There is currently no time constraint for offshore decommissioning to be undertaken, which gives project teams time to commission various forms of engineering studies, surveys, inspections and analyses from the very beginning of the process,” said Brian Nixon, chief executive of DNS.

A variety of asset sizes can be found across the industry and each has its own engineering challenges and opportunities. In shallow waters, platforms typically have topsides of fewer than 1,500 tonnes and are able to be removed in a single lift using a shear-leg or single hull lifting vessel.

The supporting structures, or jackets, are in a similar weight range and can also normally be lifted from the seabed in one piece. The topsides and jackets can then be transported by barge or on the lift vessel to a shore-based facility for re-use, final cleaning and waste treatment, dismantling, disposal and recycling.

In deeper waters, assets are designed to support complex oil, gas and condensate production (including high pressure and temperature) which throughout their productive lives can experience a build-up of hazardous waste streams. Larger facilities can weigh up to 40,000 tonnes and the large majority are supported on steel jackets, which themselves weigh up to 25,000 tonnes.

These are held in place on the seabed with multiple piles, often several metres in diameter and tens of metres long. The options for removing these facilities are very different to those in shallower waters.

“The North Sea is a mature region, and in some respects, we are pioneers of the decommissioning industry. The challenges we face with these large facilities located in deeper waters are reflective of those in the rest of the world and I hope that the lessons we learn and the knowledge we gather can be of benefit to other regions as their assets mature,” said Decom North Sea’s Nixon.

“Although the large majority of North Sea installations are supported on steel structures, there are a smaller number of concrete gravity-based structures. With similar topsides as the steel ones, these installations are among the oldest in the basin, having been built at a time when design concepts were still evolving.

These platforms are acknowledged as probably the largest man-made structures ever transported, ranging from 300,000 to more than 500,000 tonnes each, and present significant engineering challenges to the industry.’’

As decommissioning gains prominence, how a platform will be removed and disposed of is considered early in the design phase. New approaches include the use of floating production and storage vessels.

It is hoped that these vessels can be relocated to alternative developments as the site matures, although it is unlikely that the associated subsea components such as flowlines, umbilicals, risers, wellhead protection covers, mooring systems, mattresses or inter-field pipelines will be able to be re-used.

It is expected that these pieces of infrastructure will also need to be cleaned, decommissioned, removed and recycled. The plugging and abandonment of several thousands of platform and subsea wells is one of the highest cost areas of any decommissioning programme, and is widely regarded as the most challenging part of any decommissioning programme with the greatest need for innovation and technology development.

Within the decommissioning sector, there are encouraging signs of cooperation, sharing of ideas, joint industry projects and open dialogue among operators, contractors and supply chain specialists – as is evident with DNS.

Sharing knowledge and working together is critical to success, so laying down these firm foundations within the industry is vital to overcoming its inherent challenges.

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