Gerhard Ersdal - Underwater Inspection and Repair for Offshore Structures

In all these areas, the book aims to provide an overview of previous studies relevant to each topic along with what is considered good practice.

An earlier book by Ersdal, Sharp and Stacey (Ersdal et al. 2019) addressed the topic of ageing and life extension of offshore structures, which provides a useful addition to this book. The main types of changes identified in this previous book are:

physical changes (ageing, degradation, loading, etc.);

technological changes (including compatibility and obsolescence);

changes to knowledge and safety requirements; and

structural information changes (e.g., accumulation of inspection data, loss of design data).

The focus of this book is on physical changes. However, the additional changes will also influence the safety of a structural system.

As already mentioned, there has been a significant amount of previous work, including research on each of these key elements related to inspection, evaluation and repair for offshore structures, which are in some cases unavailable to the public. These key reports have been reviewed by the authors and presented in this book with the aim of providing the reader with an awareness of this background, which is important in the context of structural integrity management.

It is important to recognize that choices made in the design will significantly influence inspection and repair of an offshore structure (the need for inspection and how it can be performed). Such choices include:

the existence of any preinstalled monitoring systems;

access for inspection and repair;

cathodic protection system design and coatings;

material selection;

design margins, design fatigue life, robustness, redundancy; and

operational restrictions given by the design.

In addition, the book will discuss the competency requirements for inspectors, structural engineers involved in integrity management and the organisational requirement for integrity management.

Section 1.3is partly based on Ersdal, Sharp and Stacey (2019).

1 Bhattacharya, S. (2019), Design of Foundations for Offshore Wind Turbines, Wiley, 2019.

2 Ersdal, G., Sharp, J.V. and Stacey, A. (2019), Ageing and Life Extension of Offshore Structures: The Challenge of Managing Structural Integrity, Wiley, 2019.

3 HSE (2006). “Plant Ageing—Management of Equipment Containing Hazardous Fluids or Pressure”, HSE RR 509.

4 SSC (1992) “Marine Structural Integrity Programs (MSIP)”, Ship Structure Committee report no 365, 1992.

5 SSC (2000). “SSC‐416 Risk‐Based Life Cycle Management of Ship Structures”, Ship Structure Committee report no 416, 2000.

6 Sundar, V. (2015), “Ocean Wave Mechanics: Application in Marine Structures”, John Wiley & Sons, Ltd.

1 1Source: Robert S. Arrighi, “Pursuit of Power: NASA's Propulsion Systems Laboratory No. 1 and 2”, 2020.

2 2Source: Sidney M. Johnson, “Deterioration, Maintenance, and Repair of Structures Modern Structure Series”, 1965, McGraw‐Hill.

3 3Source: Alexander Graham Bell.

4 4According to the 2020 web‐version of the Guinness book of records, the earliest offshore platform was the Neft Daslari in the Caspian Sea 55 km off the coast of Azerbaijan. Construction began in 1949 and it began oil production in 1951. However, other sources report that production in the Gulf of Mexico began in 1946 by the Magnolia Petroleum (now ExxonMobil) platform 18 miles off the Louisiana coast.

5 5Anomalies are in this book used for any deviation in condition (degradation, deformations, defects, damage and deterioration), configuration (change in layout, geometry and weight), design regime (new or updated requirements or practices) and design actions (changes to, e.g., metocean data leading to, e.g., insufficient strength and fatigue life) that may affect the integrity of the structure.

It takes less time to do things right than to explain why you did it wrong 1 .

—Henry Wadsworth Longfellow

Integrity—the state of being whole and undivided, the condition of being unified or sound in construction 2 .

—Oxford dictionary

A number of different regulatory regimes exist for offshore installations for different parts of the world. The regulatory regimes in USA, UK and Norway in many ways took the lead in the development of these, although well‐developed regulations can now be found in, for example Brazil, Canada and Australia. ln the North Sea, the major change has been the development of risk‐based regulations, initially in Norway and since the mid‐1990s in the UK. These regulations specify high‐level safety requirements (goals) to be achieved through the design, fabrication and operational stages. These regulations are further supported by recognised standards. Some regulatory regimes are more prescriptive with respect to the standards to be used.

Inspection and repair of offshore structures are often regulated as a part of the structural integrity management (SIM) requirements. The purpose of SIM is to identify all types of changes relevant to the safety of a structure in operation, to evaluate the impact of these changes and mitigate (e.g. repair) the impact of these if found necessary. Such changes include anomalies detected during inspections or condition monitoring, changes in loads and configuration and changes to requirements from improved standards. Mitigation will typically be various forms of repair, load reduction or operational restrictions.

A standard or recommended practice for integrity management should guide the responsible party to manage all aspects of safety and functionality for the structure. This will typically be achieved by a major hazard approach based on an established risk picture, describing what can go wrong and how one can protect the structure if such failures should occur. Hence, the structural integrity management should be based on an understanding of the possible hazards to the structure and marine systems (for floating structures) and the severity of the consequence of these types of damage. A standard should further include good methods for planning the necessary inspections and surveys to establish an understanding of any change or anomaly influencing the safety of the structure.

Ersdal, Sharp and Stacey (2019) stressed that changes to structures and marine systems may be physical, technological, knowledge based, related to safety requirements in regulations or standards and changes to information about the structure. Although all these changes can be important, physical changes will often dominate the SIM work. The detection of these physical changes results from rather costly offshore structural inspections, structural monitoring, weight monitoring and metocean observations. The other types of change should not be overlooked and these will often include document review, updated engineering methods and standards and maintaining a proper database containing all necessary information about the structure (see Section 6.1.5).

The general principles for the management of physical assets were introduced in PAS 55 (BSi 2008). Although this standard is not sufficiently detailed for structural integrity management, its principles include keeping the asset (in this context structure and marine system) unimpaired and in sound condition throughout the life cycle, whilst protecting health, safety and the environment. This type of integrity management approach has been incorporated in some of the latest structural integrity management standards and recommended practices such as API RP‐2FSIM (API 2019a), API RP2‐MIM (ISO 2019b), ISO 19901‐9 (ISO 2019a) and NORSOK N‐005 (Standard Norge 2017b), as discussed later in this chapter.