Inservice Inspection And Repair Of Offshore Structures
Authors: Roland Bjorn Thore 
Published: December 2007

With reference to the present regulations (1, 2, 3) for inservice inspection of offshore structures in the British and Norwegian sector of the North Sea, a description will be given of a simple system for periodic inspection and evaluation developed to satisfy the intention of these regulations. Further a review is included of feasible methods and materials for repair of offshore structures. For inspection and repair main emphasis has been laid on operations under water.

Introduction

Limited statistical material exists on structural defects and reliability of offshore structures inservice that may serve as documentation for present day requirements for inservice inspection. However, with structures and materials introduced for new purposes involving radical increases in size, loadings and consequence of failure, the regulations reflect the realization that:even with extensive documentation and control of design materials and construction prior to the operation phase, only a systematic inspection will confirm the number of assumptions made regarding structural behavior and safety inservice.such inspection and systematic handling of data thereform will contribute significantly to the basis for improved criteria for design, construction and choice of materials in the future.in order to keep the effort of inservice inspection at a realistic level, the evaluation of data should at all times provide the documentation for the extent and frequency of such inspection.

Formal Requirements

The regulations for inservice inspection in the British and Norwegian Sectors of the North Sea are similar in principle. The main features are requirements for annual surveys and certification periods of 5 and 4 years for the British and Norwegian sector respectively as basis for judging continuous structural fitness. In the British arrangement with Certificate of Fitness (CF), Det norske Veritas as a Certifiying Authority (CA) is responsible for enforcing the requirements for inspections and recertification of structures on behalf of Department of Energy. On the Norwegian side the Norwegian Petroleum Directorate enforces the corresponding requirements, DnV being involved on consultative basis.

On both sides all the formal and practical arrangements for inspection and reporting to CA are with the owner of the platforms, and as a principle a DnV surveyor will be present during all inspection operations, witnessing these and carrying out spot check inspection as deemed necessary.

DnV Offshore Rules (3) and Appendix I (4) to the rules form the general basis for DnV's work as Certifying Authority with the necessary adjustments to relevant national regulations. With reference the Rules requirements for inspection (in principle nearly identical to British regulations) the procedure for inspection and recertification will be as follows:A long term program for inspection (minimum for one certification period) to be prepared by the owner for CA approval.From long term program and continuous experience gained from inspections, annual (periodic) inspection programs to be prepared in detail and submitted to CA for approval.