Comparative risk assessment of North West Hutton removal options
Facts about the North West Hutton platform
- Installed 1983 North East of the Shetland Isles
- Production ceased in 2003
- Water depth 144 m
- Steel jacket of approx. 17.300 tons
- 30.000 tons of drill cuttings
At the end of a platform’s productive life, decisions regarding its decommissioning shall be taken. The owner of the platform faces a difficult choice between various removal options influenced by uncertain technical, environmental and safety aspects.
To facilitate BR’s choice of a removal option for the North West Hutton steel jacket and cuttings pile, COWI A/S carried out a quantitative comparative risk assessment of various removal options. These included complete removal of jacket, footing, template, and drill cuttings as well as several partial removal options.
The methodology of the study comprised of breaking down the method statement of each removal option into 20-25 sequential activities. For each activity the main hazards were identified, and their probability of occurrence was assessed form detailed technical background documentation and in expert workshop sessions. On this basis a risk model was established using Object Oriented Bayesian Networks, which is a class of probabilistic models well suited for handling interdependent variables. Using this model, the removal options were compared with respect to the technical and personnel risk as well as environmental impact.
The technical feasibility of the removal activities included quantification of major accident probabilities and schedule/ cost problems. These problems could be caused by tool failure or structural condition uncertainty. Personnel safety was evaluated in terms of fatality risk during e.g. diving, cutting or lifting operations.
The main output of the project was a well-documented quantitative basis for selection of the optimal removal option. The study provides high-level decision support based on detailed background technical documentation prepared for the decommissioning project. It included sensitivity analyses and identification of main risk contributors. The study is expected to form an important part of the basis for the public consultation process.
- Objective evaluation of risks associated with removal options
- Create focus on key questions to be dealt with as part of the cessation process
- Substantiate and form basis for firm decision making
- Decision support for selection of optimal removal option with respect to
- Technical project feasibility
- Personnel safety
- Environmental impact
- Sensitivity assessments evaluating the impact of changes in the removal method
- Documentation to support the public consultation process