NEWSLETTER

The Southern North Sea Business Unit (SNS BU) comprises fifty nine individual jacket & associated topsides structures, and is located up to 200 miles offshore in water depths ranging from 12m to 50m. SNS BU spans the complete evolutionary process in the design & fabrication of North Sea structures, from West Sole installed in 1965 to the latest minimal structures being installed today. This article outlines the monitoring activities, assurance processes, and documentation which delivers the structural integrity of these structures.

Importantly, during the last 35 years of service all SNS jacket & topsides structures have demonstrated excellent overall reliability & robustness. The methodologies & processes for assuring integrity have taken advantage of this strong & sizeable experience base, enabling a strongly risk based approach to structural integrity management, while achieving cost efficiency, a consistent approach, and effective management of information.

SNS structural integrity is assured on the basis of four primary processes:

• Subsea & Topsides Structural Inspection
• Structural Analysis & Engineering Assessment
• Monitoring & Reviewing Environmental Design Criteria
• Management of Weight & Topsides Loads.

For each structure these component processes are applied to address the incumbent risks associated with the parameters of age, history & design basis. These processes are the key delivery points and together allow an assessment of structural integrity to be carried out.

The purpose of structural inspection is to identify structural degradation in sufficient time to enable remedial works to be undertaken before there is a significant threat to structural integrity.

In recognition that all SNS jacket structures have demonstrable residual capacity in the event of local degradation, the risk to each individual jacket structure is monitored at a system level. The primary tool for monitoring integrity of the jacket system is Flooded Member Detection applied to 100% of the jacket structure; deployed by diver-less ROV. In parallel with this, Cathodic Potential, Scour, Visual, and Marine Growth inspections are carried out.

Topsides structural inspection is predominately visual inspection, backed up by detailed NDT to monitor significant known defects, or investigate new defects. This is carried out either from the deck elevations, or utilising rope access technicians.

In response to inspection results and any other changes, structural analysis & engineering assessments are carried out. Representative structural models of each structure are maintained for the purposes of demonstrating ongoing operational integrity, and carrying for out emergency response assessment.

Loading generated by the 'metocean' environment, topsides dead weight, and topsides live load are also systematically reviewed, monitored & where necessary updated to reflect the current status. Additional processes to address changes concerned with modifications & repairs, fabric maintenance, emergency response, and decommissioning are also applied.

The role of the Structural Engineer is to deliver structural integrity by pulling together the strands of the assurance process and provide an assessment of the overall integrity of the structure.

For SNS this is summarised in the form of a single page Annual Summary Report for each structure, with subheadings; Overall Status, Current Issues, Structural Analysis Status, Weight Control, Subsea Inspection, Topsides Inspection. This is a key document which in addition to serving as a record of integrity status, also strongly facilitates demonstrating compliance with both internal processes & current legislation.