ABSTRACT

This paper describes recent developments in techniques for the reassessment, maintenance and repair of offshore steel structures. Techniques discussed include refined modelling of tubular joints to incorporate local joint flexibilities and use of test data to describe tubular joint strengths and Stress Concentration Factors. These techniques are adopted to obtain better estimates of safety margins; these safety levels are discussed with respect to those implied by design codes such as API RP2A. Different repair techniques are discussed and evaluated including 'wet' and 'dry' (habitat for example) welding, grouted sleeves, mechanical clamps and stressed grouted clamps. The use of these techniques are illustrated by examples. The scope of the background research 1lSed to justify the increasingly popular grouted repair techniques is described.

The reassessment, maintenance and repair of existing offshore installations is now an important part of offshore engineering. Design codes provide little or no guidance in this area. The techniques discussed in this paper provide a rational structural reappraisal and identify the extent of maintenance, inspection and the need to repair. This rationale is important if unnecessary and costly strengthening or repair is to be avoided. The alternative repair techniques discussed in this paper will assist in selecting the most economic solution whilst maintaining the desired safety margins.

INTRODUCTION

This paper is concerned with the reassessment and repair and strengthening of fixed tubular steel offshore structures. The paper has been divided into two parts:

  • A - Reassessment of jacket structures

  • B - Recent developments in repair methods

Part A deals with the reasons why reassessment may be required and discusses how to achieve a realistic representation of the overall frame behaviour, including the correct application of local joint flexibilities, joint eccentricities, pile-soil and pile group effects, progressive collapse mechanisms and the potential benefits of grouted leg piles. Static strength, Stress Concentration Factor and fatigue life aspects of the tubular joints are discussed in relation to recent changes in design guidance and new test data. The inherent conservatisms in some design codes are discussed.

The major development in repair methods discussed in Part B is the increasing use of mechanical and grouted repair systems as an alternative to welded repairs. The background to these systems and the basic research studies are outlined. Some examples of the forty major repairs using these techniques are given.

PART A - REASSESSMENT OF JACKET STRUCTURES
Purpose of analysis

The engineering reassessment of an offshore platform may be initiated for one or more of several reasons including:

  • Routine updating of platform operating basis (e.g. revised deck loading)

  • Changes in design codes/design premise

  • Investigation of the effects of potential damage

  • Planning of rational inspection programmes

  • Recertification.

Routine update of operating basis

During the detail design stage for a major structure it is usually only possible to consider a limited number of loading scenarios.

Keywords:
offshore structure, investigation, grouted repair, strength, local joint flexibility, tubular joint, clamp, flexibility, fatigue performance, upstream oil & gas
Subjects:
Offshore Facilities and Subsea Systems, Platform design
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