Scope
Jee were tasked with supporting the decommissioning planning for a North Sea gas export pipeline system connected to an offshore platform installed in the mid-1980s. The system included a 20” carbon steel export line, a piggyback line, and an associated subsea power cable. Following cessation of production, the pipelines were cleaned and purged with nitrogen, while the power cable was de-energised.
The client required a pre-engineering study to define a decommissioning approach that balanced safety, environmental impact, technical feasibility, stakeholder concerns, and cost. Jee were engaged to complete the pipeline-related elements of this work.
This included:
- A comparative assessment of decommissioning options for the export pipeline, piggyback line, power cable, and onshore sections.
- Landfall assessment of decommissioning approaches.
- Development of functional requirements for future ITT documentation.
- Definition of post-decommissioning monitoring requirements.
Solution
Jee broke down the problem into the following objectives:
Comparative Assessment of Options
- Identify and evaluate decommissioning scenarios for offshore, nearshore, and onshore assets.
- Score options against safety, environmental, technical, stakeholder, and cost sub-criteria.
- Recommend a preferred option based on overall weighted scoring.
Onshore and Offshore Interfaces
- Assess the landfall region, including the directionally drilled harbour crossing, to confirm technical feasibility of pipeline filling and reinstatement.
- Consider the riser base and offshore burial conditions, including reinstatement requirements after asset severance.
Development of Functional Requirements
- Define clear requirements to be included in future ITT packages for decommissioning contractors.
- Specify criteria for safe removal, in-situ abandonment, and reinstatement of disturbed areas.
Monitoring and Environmental Considerations
- Establish requirements for seabed surveys to confirm burial status and identify any contaminants.
- Define an appropriate ongoing monitoring regime to satisfy regulatory expectations.
- Ensure works are scheduled to minimise disruption to marine life during sensitive ecological periods.
Results and conclusions
The comparative assessment concluded that the most appropriate decommissioning approach was to sever the system at the base of the riser, leaving the offshore pipelines, piggyback line, subsurface onshore sections and anode beds in situ while removing only the onshore above-ground assets. The pipelines would then be filled with grout from the harbour entry point past the subsidence-sensitive Pollendam region and up to the onshore facility, with reinstatement of disturbed offshore and onshore areas carried out after works.
To support safe and responsible delivery of this option, Jee recommended confirming burial and contamination status through a pre-decommissioning survey, agreeing a monitoring regime with regulators and scheduling works to avoid ecologically sensitive periods. Coordination with local port authorities was identified as essential to minimise operational disruption, while the suitability of the harbour drill for grout filling needed to be confirmed, including backpressure capability and access rights. If unsuitable, the fill would be completed from the onshore facility.
The comparative assessment also considered new and emerging decommissioning technologies, including the use of the Frequensea system, a novel variation on reverse laying. This approach replaces the traditional excavation with Vibro Units, which introduce vibration into the pipeline or cable to fluidise the surrounding seabed. This enables removal without full exposure and with significantly reduced forces compared to conventional excavation methods. Finally, Jee highlighted the need to appoint contractors with experience in shallow-water, tidal environments to manage the challenging local conditions.
To contact our Technical Authority, Graham Wilson, email graham.wilson@jee.co.uk, or call +44 (0)1732 371 371.
Download our Decommissioning Capability Statement here.
Jee Ltd
