Structural integrity of offshore floating installations
There are a number of different designations for types of floating installations in the UKCS, but they are generally of two distinct types;
- semi-submersibles, sometimes with modifications to original design.
- floating production storage and offtake vessels (FPSOs) which are usually ship shaped and may be converted from older vessels.
The historical and current way of dealing with structural integrity is by the use of the Classification system to some extent. This is acceptable and may go some way to meeting the requirements of the Offshore Installations and Wells (Design and Construction, etc) Regulations 1996 (DCR) but the compliance using class may differ from installation to installation, or duty holder to duty holder. It is thus important to address structural integrity management matters with the duty holder, as well as ongoing integrity and damage and deterioration due to ageing.
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- To gain assurance that operators of floating installations have a structural integrity management framework appropriate to the number of defects currently occurring, and that all reasonably practicable measures to detect any future defects have been adopted.
- To ensure that all operators are aware of the good practice for these types of installations, particularly in matters of life extension and ageing.
Scope and plan
Inspections on floating installations are performed both onshore and offshore. The installations are chosen by the date of the last inspection (some may not still be in UK waters). The overall aim is to probe the management and inspection processes, especially seeking information on the control of the loss of integrity major accident hazard. A unified template is under discussion.
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Inspection report by duty holder (mainly management of integrity) and by installation covering specific integrity matters for that installation.
Considerations for the integrity of floating installations
- Role of Class in ensuring integrity and match with requirements of DCR.
- Physical agreement between the installation and as-built documentation and that:
- later modifications are accounted for in analyses and calculations;
- local weld-ons and scallops do not reduce the fatigue life of the structure;
- local modifications do not impair weather and watertight integrity.
- Additional considerations and requirements with respect to inspection and maintenance due to extended life for the structure with respect to:
- load-carrying structures, particularly fatigue, corrosion and thickness measurement
- critical areas in addition to those required for class certification
- leak detection systems
- Use of information on past performance to inform the current arrangements and measures, including results from similar installations. This may require co-operation with other duty holders and classification societies.
- Plans for replacement or repair due to ageing of load-carrying structures, including dry docking considerations and frequency.
- Review of the effectiveness and reliability of the existing barriers preventing a catastrophic failure, and consideration of further measures that may be reasonably practicable.
- Review of the effect of changes in knowledge concerning technology and environmental conditions that could influence existing barriers, or make further measures reasonably practicable.
- Inspection and repairs data to be gathered and documented as part of inspection report.