Section 1 General
Clasification Society 2024 - Version 9.40
Clasifications Register Guidance Information - Guidance Notes for Life Extension of Fixed Jacket Structures, March 2022 - Chapter 1 Guidance for Life Extension Requirements - Section 1 General

Section 1 General

Parent topic: Chapter 1 Guidance for Life Extension Requirements

1.1 Introduction

1.1.1 The life extension process ensures that the integrity of an existing structure is acceptable in a degraded state when the original design life has been exceeded. This Guidance Note provides an overview of current methods and criteria of Life Extension (LTE) of fixed jacket structures with reference to ISO, API and NORSOK standards. The phases of LTE and the applicable requirements with required documentation are also presented in this brief. Additionally, common issues, assessment methods, and strengthening and repair procedures are also presented.

1.2 Life extension overview

1.2.1 Continued use of installations beyond their design life is a well-recognised issue in the UK continental shelf and worldwide. As offshore structures are subjected to dynamic and cyclic loading, most of the focus is on the fatigue and corrosion issues that several installations face throughout their design life. However, less well recognised is the fact that many of these installations are also loaded in excess of their original design parameters, as a result of modifications and additions.

1.2.2 Lifetime extension developments are a continuing trend resulting to the use of increasingly complex analytical techniques to demonstrate suitability of these structures. The use of advanced analysis methods places an increasing burden on the technical competence and scope of work of the design consultants, duty holders and verifiers. The ageing issues relating to the offshore installations can be broadly categorised into the following two sorts:
  • Physical issues – where significant degradation leads to a failure to meet the performance standards and insufficient information on the current condition results in lack of the necessary confidence.
  • Non-physical issues.

1.2.3 The life extension related issues can be broadly categorised into the following scenarios:

  1. Physical ageing scenarios:
    1. Time-dependent degradation, e.g. widespread fatigue, accumulated corrosion, cracks, etc.
    2. Damage or deterioration, e.g. accidental damage such as ship impact or dropped object incidents, environmental overload, etc.
  2. Non-physical ageing scenarios:
    1. Changes in technology and codes and standards, e.g. improvements in technology, introduction of new design and assessment methods, more stringent design requirements, etc.
    2. Changes in external factors, e.g. failure to adapt to change or modifications, redundant equipment, etc.
    3. Loss of knowledge, e.g. corporate/workforce or original design data and operating/inspection history, etc.

1.3 Abbreviations

1.3.1 WSD – Working Stress Design

1.3.2 LRFD – Load and Resistance Factor Design

1.3.3 DFF – Design Fatigue Factor

1.3.4 ECA – Engineering Critical Assessment

1.3.5 ULS – Ultimate Limit State

1.3.6 ALS – Accidental Limit State

1.3.7 SLS – Serviceability Limit State

1.3.8 FLS – Fatigue Limit State

1.3.9 SMYS – Specified Minimum Yield Strength

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