Inspection and Repair of Fixed Platforms in the North Sea
Authors: Winkworth W.J. 
Published: October 2008

A major part of the maintenance cost of fixed offshore steel platforms concerns the underwater inspection of the jacket structure. Visual inspection alone will normally only detect serious damage since even through-thickness cracks are unlikely to be detected visually. However, because of the size of the structures non-destructive examination (NDE) of each weld on a regular basis is impracticable.

Various inspection strategies have been developed by operators to maximise the benefit and minimise the cost of inspection. Drawing on experience with 160 platforms, some of which have been installed for over 20 years, the paper describes the inspection strategies that have been adopted and reviews their effectiveness.

The problem of platform repair is also discussed. Typical damages and the repair solutions most commonly adopted are also described.

Introduction

This paper summarises Lloyd's Register's (LR) experience with underwater inspection of fixed steel platforms in the North Sea (NS) since certification began 20 years ago. There are now approximately 160 platforms in the UK sector of the NS representing many different operations.

Over this period there has been considerable development in the inspection tools and strategies employed and the paper end eavours to trace this development and to indicate possible future trends.

The comments made in this paper cannot generally be referenced to published documents as they are based on confidential records of actual inspections and reportedexperience but references are given where possible.

Inspection

Legislation and Government Guidelines

The requirements for UK Certification are given in UK Statutory Instrument 289 published in 1974. The document specifies that a major survey of the installation should be completed before it is re-certified for a further 5 year period. The surveys to be carried out on the underwaterstructure are briefly described in the UK Department of Energy's (DEn) Guidance Notes.

The first edition of the DEn Guidance Notes in 1974 (Ref. 1) required a minimum of 10% of nodes to be inspected. When the Guidance Notes were revised in 1977 thewording was changed to a more general requirement, viz:- In-service inspections of fixed installations should be planned by an experienced engineer who has examined the design characteristics, the records of severe environmental and other loads to which the structure may have been exposed and any available records of structural behaviour such as settlement, differential settlement, tilt, distortion or abnormal response. The initial inspection schedule should take account of the nature of the deterioration to which steel and concrete structures are liable in a marine environment and of the regions in which defects are most prone to occur (e.g. sudden changes in section, discontinuities, etc) and of members or regions known to have been, or likely to have been, highly stressed or subjected to severe fatigue loading. Special attention should be given to areas of suspected damage or deterioration andto areas repaired following earlier surveys. Only after the nature and probable positions of defectshave been established should the inspection schedules be prepared.