1. Decommissioning Scope and Practice Southern North Sea (SNS): Subsea
The following summarises the outputs of workshops held in Q3/Q4 2016, exploring the scope and practices of several SNS Operators with subsea aspects of decommissioning
The workshops focussed on scope and practices covered by approved Decommissioning Programmes
The scope and practices listed, and the drivers leading to their adoption, are examples of approaches taken
This is not a comprehensive list of all options
there may be other options which are superior to those listed
these do not necessarily represent the best, or good, practice
A fuller range of options will, in many cases emerge, as decommissioning experience develops and more Operators contribute to this body of knowledge
Likewise, the most competitive options will likely become clear from the statistics of the options adopted by Operators in coming years.
18 September 2018

2. What are the targets to engineer the solution to?
Execution Examples: Process description
Screening / Ranking
Situation/Issue Example Objectives Example Options Relative Cost
Outcome
Why is this activity being undertaken?
A clear & justified objectives statement will align with cost-effective option selection
What is the decommissioning starting point, and the issue ? Typically this is a fixed situation But it may also be a choice
Selected option
What are the costs of the options?
Safety
The options will screen differently when comparatively assessed, and a compliant yet cost-effective option can be selected
What are the different ways that the objectives & associated metrics can be practicably achieved?
Environmental
Medium
Lowest
Technical
Highest
Low
High
Societal
Metrics/Targets
Low, but risk of high
Economic
What are the targets to engineer the solution to?
Metrics should be challenged to test how unique/non-unique they are, & whether other acceptable metrics open-up more cost effective solutions
Options objectively compared using the Comparative Assessment framework in line with BEIS-ODU guidance if required / proportionate

3. Execution Examples: Pipeline Decom Preparation
Screening / Ranking
Situation/Issue Example Objectives Example Options Relative Cost
Outcome
Oil/gas removal to remove hazard
Oil Pipeline - Known contents
Oil/gas removal to allow intervention
Selected option
Oil Pipeline - Unknown contents
Reduce toxin entry into food chain
What are the costs of the options?
Pig + Water Flush + monitor/measure
Safety
Gas Pipeline - Known contents
Minimise environmental impact
Medium
Lowest
Environmental
Pig + Water Flush
Technical
Gas Pipeline - Unknown contents
Highest
Water Flush
Low
High
Societal
Low, but risk of high
Multiphase Pipeline
Others
Economic
Injection-water pipe
Metrics/Targets
Chemicals flowline
Hydrocarbons in water
Defined methodology
Options objectively compared using the Comparative Assessment framework in line with BEIS-ODU guidance if required / proportionate
Reasonable endeavours

4. Execution Examples: Mattress Decommissioning
Screening / Ranking
Situation/Issue Example Objectives Example Options Relative Cost
Outcome
Minimise hazards to offshore personnel
Minimise hazards to other users of the sea
Selected option
Mattress with High structural integrity
Minimise hazards to the environment
Total recovery of mattresses
What are the costs of the options?
Safety
Environmental
Mattress with Low structural integrity
Partial recovery of mattresses
Medium
Lowest
Technical
Buried Mattress
No recovery of mattresses
Highest
Low
High
Societal
Unburied Mattress
Rock dump over mattresses
Low, but risk of high
Economic
Mixed Burial Mattress
Metrics/Targets
Minimise snagging hazard
Options objectively compared using the Comparative Assessment framework in line with BEIS-ODU guidance if required / proportionate
Risks are ALARP

5. Execution Examples: Pipeline Decommissioning
Screening / Ranking
Situation/Issue Example Objectives Example Options Relative Cost
Outcome
Minimise hazards to offshore personnel
Large Diameter Pipeline ≥ 16”
Minimise hazards to other users of the sea
Small Diameter Pipeline < 16”
Selected option
Minimise hazards to the environment
Total recovery of pipelines
What are the costs of the options?
Safety
Buried Pipeline
Partial recovery of pipelines
Lowest
Environmental
Medium
Unburied Pipeline
Technical
Highest
Mixed Burial Pipeline
No recovery of pipelines
Low
High
Societal
Rock dump over pipelines
Flexible Pipeline
Low, but risk of high
Economic
Rigid Pipeline
Metrics/Targets
Bundle Pipeline
Minimise snagging hazard
Umbilical
Options objectively compared using the Comparative Assessment framework in line with BEIS-ODU guidance if required / proportionate
Risks are ALARP

6. Execution Examples: Subsea Equipment Decommissioning
Screening / Ranking
Situation/Issue Example Objectives Example Options Relative Cost
Outcome
Minimise hazards to other users of the sea
Minimise hazards to the environment
Selected option
Total recovery of subsea equipment
What are the costs of the options?
Safety
Equipment without an over-trawlable structure
Partial recovery of subsea equipment
Environmental
Medium
Lowest
Technical
No recovery of subsea equipment
Highest
Low
Equipment with an over-trawlable structure
High
Societal
Low, but risk of high
Rock dump over subsea equipment
Economic
Metrics/Targets
Snagging hazards
Options objectively compared using the Comparative Assessment framework in line with BEIS-ODU guidance if required / proportionate

7. Execution Examples: Post-Decommissioning Monitoring
Screening / Ranking
Situation/Issue Example Objectives Example Options Relative Cost
Outcome
Confirm there are no hazards to other users of the sea
Confirm there are no hazards to the environment
Selected option
High Frequency Monitoring
What are the costs of the options?
Safety
Environmental
As left situation, post completion of decommissioning
Medium Frequency Monitoring
Medium
Lowest
Technical
Highest
Metrics/Targets
Low Frequency Monitoring
Low
As left situation, post completion of previous surveys
High
Societal
Low, but risk of high
Economic
Critical spans
No Monitoring
Exposures
New snagging hazards
Pipeline and mattress stability
Options objectively compared using the Comparative Assessment framework in line with BEIS-ODU guidance if required / proportionate
Identify any leaks