Decommissioning Guidelines for North Sea Subsea

Wednesday, November 09, 2011
Anthony Onukwu, senior technology analyst from ITF

A new study which aims to inform UK government guidelines on environmentally responsible methods for decommissioning offshore pipeline bundles will be launched in Aberdeen this month. The study was secured with assistance from ITF, the technology facilitator for the global oil and gas industry, following membership support for the project. It will be led by project management and engineering consultancy PDi, with assistance from eight operating companies.
Currently, there is no guidance for the industry on decommissioning pipeline bundles and the study aims to provide a basis for the Department of Energy and Climate Chance (DECC) and stakeholders to assess decommissioning proposals. There are a growing number of North Sea offshore installations due to be decommissioned in the next few years and the topic has been the focus of extensive discussion within the oil and gas industry. More than 470 offshore installations, 10,000 kilometers of pipelines, 15 onshore terminals and around 5,000 wells constitute part of the North Sea infrastructure which will eventually need to be decommissioned in the UK  Continental Shelf (UKCS) at an estimated cost of £20-25 billion. Pipeline bundles allow all of the flow lines, water injection, gas lift, chemical injection and control systems required for a subsea development to be incorporated into one carrier pipe, and offer a number of benefits including protection against external hazards, a reduction in the pipeline corridor and cost savings.
Anthony Onukwu, senior technology analyst from ITF said: “Pipeline bundles may only represent a small percentage of the total length of installed pipeline in the North Sea, but it is essential they are decommissioned in asafe and environmentally responsible manner. “The Department of Energy and Climate Change’s guidelines for the decommissioning of offshore oil and gas installations and pipelines indicate that any new pipeline bundles should be designed for future removal. However, more than 60 pipeline bundles have already been installed in the North Sea, with an estimated combined length of 250km. “Some of these are now approaching decommissioning and currently there are no guidelines on how to do this. This study should be of value to asset owners developing bundle decommissioning programmes and will hopefully provide guidance for DECC and stakeholders when assessing decommissioning proposals.”
A variety of factors, including the type of pipeline and individual circumstances, have to be considered when assessing the options for decommissioning a subsea pipeline. In some cases pipelines may be decommissioned in-situ although complete removal may be the preferred option depending on the circumstances.  Pipeline bundles, however, may present particular issues when it comes to decommissioning which require specific consideration. Graeme MacDougall, operations manager at PDi said: “When it comes to decommissioning, pipeline bundles are substantially different from other pipelines and although they may appear similar to trunk lines, as they have a large diameter and sit on the seabed, unlike trunk lines pipeline bundles are all less than 7.5km long.
“Taking into account their relatively short length, they may seem to have more decommissioning synergies with in-field lines than with trunk lines, but their large diameter generally rules out burial, refloating bundles for recovery poses significant engineering and operational challenges, and cutting it into sections for recovery will involve an extensive subsea intervention campaign and multi-handling of abnormal loads.”  

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