Hutton, Tension Leg Platform
Location Icon Invergordon
  • Client
    Nerida Limited
  • Date
    2021-2023
  • Role
    Demolition Consultant and CDM Advisor
  • Team
    Stephen McCann – Yazan Osaily
The Site

We worked on the decommissioning of the Hutton, the world’s first production tension leg platform (TLP), which had been a landmark in the Cromarty Firth where it had been moored for approximately 12 years before the project commenced.

The process of unmooring the TLP from the seabed - lifting the 500 tonnes of chains and anchors and towing the platform five miles to Invergordon - took approximately 36 hours and involved six tugs accompanied by a rescue boat all of which were managed by Nerida, an offshore oil and gas transport specialist who acquired the Hutton from its previous owners in 2021.

The 25,000-tonne 48 metres high platform was then towed five miles across the Cromarty Firth to Queen’s Dock in Invergordon, where it was moored for the end-of-life decommissioning package.

Once moored at the Port of Cromarty Firth, the first step was to carry out a HazDem (Hazardous Demolition Assessment) to help with the identification of site-wide and structure-specific hazards. This will then be added to the Pre-Construction Information (PCI), which is then further developed into the Designer Key Hazards. These were then communicated to the decommissioning contractor allowing the project team to adopt the principles of prevention in designing the demolition process. This included the identification or the elimination of Naturally Occurring Radioactive Materials (NORMs), asbestos, lead paint, isocyanate paint, structural damage and dilapidation, among other things.

With this completed the next step was to have a thorough 3D modelling sequence produced to determine the load paths of the structure and the weights of each section. This along with the structural stability analysis - as the structure was to remain on the water until the legs were reduced - was carried out by a specialist ship’s architects and structural engineers. With this in place the cut diagrams, cut sequence, lifting plans, and task-specific risk-assessed method statement were produced so the work could commence.

The deconstruction phase was an intricate mixture of precision cuts carried out manually using oxy-propane hot cutting equipment due to the thickness of the steel, and heavy crane lifts to lift and lower each of the designed sections to the ground for processing with demolition rigs and heavy steel cutting shears. Each of the columns was reduced in turn to maintain the stability of the water-based platform. Once the six legs had been reduced, the pontoon was removed from the water to land in sections where it was cut up manually using oxy-propane hot-cutting equipment

Our Scope

We were appointed as Principal Designer for the project where we worked with the client team to ensure the HazDem was completed so the PCI and Designer Key Hazards could be produced.

As we were dealing with offshore companies and European-based companies, it was important to ensure that all roles set out within the Construction (Design & Management) Regulations 2015 were clearly defined so each role could be discharged as they were vital to the complex contract.

Once the Construction Phase Plan (CPP) was completed, we reviewed the plan and the Risk Assessment and Method Statement (RAMS) for the project and continually commented on the revisions and updates of both sets of documents for the duration of the contract.

PERSES then did a full document review to ensure all processes and procedures were correctly established and in place to monitor works underway and control the hazards and the dynamic changes on site.

We also carried out a skills gaps analysis (SGA) for the staff based on the newly implemented processes and the methods required to reduce the complex structure and process the arising materials.

Throughout the works, we carried out monthly audits that were planned to coincide with the specific phases of the project, which included the initial CDM compliance audit and then monthly specific demolition process audits.

What we did
  • Produce the Pre-Construction Information.
  • Produce the Designer Key Hazards.
  • Review and comment on the Construction Phase Plan and issue a report.
  • Review and comment on the Risk Assessments and Method Statement and issue a report.
  • Liaise with the Port of Cromarty Firth Authority, Health & Safety Executive, and other stakeholders on the demolition sequence.
  • Carry out the pre-start site audit and issue a report.
  • Carry out an initial skill gaps analysis for the workforce.
  • Create and deliver bespoke training based on the SGA.
  • Carry out monthly site audits and issue a report on the progress.
  • Carry out continual meetings on progress with the client and contractor.
  • Ensure the completion of the Health & Safety File.

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