Gerhard Ersdal - Underwater Inspection and Repair for Offshore Structures

The way you learn anything is that something fails, and you figure out how not to have it fail again 1 .

—Robert S. Arrighi

Repair is an exacting, technical matter involving five basic steps: (1) finding the deterioration, (2) determining the cause, (3) evaluating the strength of the existing structure, (4) evaluating the need for repair and (5) selecting and implementing a repair procedure 2 .

—Sidney M. Johnson

Before anything else, preparation is the key to success 3 .

—Alexander Graham Bell

Offshore structures for the production of oil and gas have a long history. The early offshore oil and gas exploration started in the 1940s in the Gulf of Mexico (GoM) and the Caspian Sea 4 . This was followed by the development of the North Sea and Brazil in the 1960s and later activities in the Persian Gulf, Africa, Australia, Asia and other areas. More recently offshore structures for wind energy production have been developed, initially in Denmark in the early 1990s followed by significant growth in several European countries, particularly the UK.

These offshore structures are continuously exposed to:

a sea‐water environment, which can cause corrosion and erosion;

active and environmental loads, which may cause fatigue cracking and buckling; and

incidents and accidents causing physical damage such as dents and bows.

Damage of these kinds can cause loss of integrity of the structure and decrease the margin of safety. In addition, many oil and gas structures and the earliest of the wind structures are now ageing and many have been through a life extension process. Nevertheless, there is a continuing requirement to demonstrate that these installations remain safe for the personnel that operate them.

Unfortunately, there have been a number of accidents over the years with considerable loss of life resulting from structural failures related to inadequate inspection or failure to mitigate anomalies. A typical example is the semi‐submersible Alexander L. Kielland accident in 1980 resulting in the loss of 123 lives. The cause of the accident was the loss of a brace member from fatigue and fracture leading to overturning and sinking. Fatigue failure initiated at a fabrication defect as a result of a combination of poor welding and lack of in‐service inspection, which led to a catastrophic failure.

Other accidents with serious loss of lives have occurred in which structural failure played a part. In the shipping industry the MV Erika and MV Prestige accidents are examples of structural failures in storms where anomalies in these vessels failed to be detected and mitigated. It is clear from these accidents that in‐service inspection and repair of structures are vital for the safety of structures. In addition, these are also normally required by regulators and class societies (i.e. ship classification societies, also known as ship classification organisations).

There are numerous incidences of damage and deterioration that had they not been detected by inspection and subsequently repaired or remediated could have led to serious accidents and loss of life. As later shown in this book these types of damage and deterioration include severe corrosion, fatigue cracks, dents and bows from impact loads, and severed members that could have resulted in more widespread structural failure and ultimate collapse of the structure. While these instances are well known to the companies involved and the relevant regulators, they are not necessarily well reported in the public domain. However, they show the importance and value of undertaking inspection and repair in a timely way for the prevention of escalation and maintaining safety. In addition, many offshore structures are now in an ageing phase where inspection and repair are likely to be more important. The authors believe that this is an opportune time to review this previous work on inspection, evaluation, repair and mitigation of such structures.

There are few books on underwater inspection and repair and those that exist are now significantly out of date. However, a significant amount of previous work is available from research and technology developments on the topic providing an extensive expertise accumulated in inspection and repair of structures through many years of offshore operational experience. Unfortunately, many of these reports are presently unavailable in the public domain.

This book is intended to indicate the current practice in these fields for those involved in keeping offshore structures safe, including practicing engineers involved in structural integrity management and also for students in the field.

Although we intend to design, fabricate and install structures for safe operation during their design life, the environment, cyclic loading and accidental events will cause anomalies 5 , which if not detected and repaired have the potential to cause failure of the structure. Ageing increases the likelihood of such anomalies being present.

Figure 1shows the drivers for why structures are inspected. These include factors such as the balance between minimizing the life cycle cost and ensuring safe and functional structures (safe operation) by means of inspection and repair. These two drivers will often be in conflict but will also in some cases coincide as failures that lead to major repairs, loss of functionality or in the worst case, collapse of structures will have a major impact on cost also. An optimal integrity management that ensures safe and functionality at a minimum cost is hence often an important goal in planning inspection and repair of a structure.

Figure 1 The elements of why we inspect structures.

Inspections and surveys also provide a means to determine the current condition of a structure and if necessary, timely undertake appropriate and cost‐effective mitigation and repair measures to preserve the integrity of the structure. This will be discussed further in the book, especially in Chapter 6 on long‐term inspection planning. In addition, regulatory and code requirements need to be met and these may define a minimum inspection level.

The diagram also illustrates the different changes and uncertainties in the current condition of the structure that can be detected by different types of inspection and surveys. The primary goal is to identify any changes, damage and anomalies to the structure but inspections that indicate that no anomalies are present are also important. Such information is vital in reducing uncertainty about the condition of a structure and, hence, providing the owner and the responsible engineer with confidence in that the operations remain safe and assumptions are valid when no significant or unexpected anomalies are detected.

The decision to undertake any form of repair or mitigation of an anomaly, detected by inspection, needs to be based on a thorough evaluation and often a more detailed inspection of the anomaly and its effect on the structural safety, based on established standards and knowledge. If repair or mitigation is required, the necessary decisions need to be made on how this can be achieved effectively. Failure to repair or mitigate critical anomalies could cause structural failure with the possibility of significant consequences. Thus, this emphasises the important role of inspection, evaluation, mitigation and repair in maintaining a safe structure.

Structural failures have occurred offshore with significant loss of life. The first of these was the Sea Gem incident in 1965 in UK waters with the loss of 13 lives. The resulting inquiry concluded metal fatigue in part of the suspension system linking the hull to the legs was to blame for the collapse. Fatigue cracking and lack of in‐service inspection were significant in the Alexander L. Kielland capsize in 1980 killing 123 people, as already mentioned.