Читать книгу Underwater Inspection and Repair for Offshore Structures - Gerhard Ersdal - Страница 14

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 anomalies⁵, 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 1 shows 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.

Offshore structures in the Gulf of Mexico have also failed during hurricanes. For example, Hurricane Andrew in 1992 caused significant damage to twenty‐two of the offshore regions, with older structures sustaining significant damage. Inspection was needed to determine the extent of the damage and in many cases this information led to the need for repair in order to resume operation. Several examples of hurricane damage in the GoM are reviewed later in this book.

Other offshore accidents have also occurred. Not all of these failures were a result of an anomaly that could be identified by inspection. Some were the results of under‐design, underprediction of loading, accidental damage and gross errors. Such failures typically initiate significant subsequent research work providing a better understanding of the cause of failure and appropriate inspection requirements. An example of such is the intensive work that was initiated on fatigue and crack inspection after the Alexander L. Kielland accident.

The reasons for inspection and repair can change over the life of a structure. Ersdal et al. (2019) review the statistics of failure for older offshore structures and show that these structures have a significant failure rate, particularly for floating structures. Figure 2 shows the types of damage to critical hull members; this includes cracking of the hull, corrosion, vibration and other types. This figure also shows the trend for increasing damage with age. This is to be expected knowing that these structures will degrade and accumulate damage, which requires regular inspection and often subsequent repair and mitigation.

Figure 2 Damage to hull structural members by different causes and ship age for all ship types. Sources: Based on SSC (1992) “Marine Structural Integrity Programs (MSIP)”, Ship Structure Committee report no 365, 1992; SSC (2000). SSC‐416 Risk‐Based Life Cycle Management of Ship Structures, Ship Structure Committee report no 416, 2000.

As shown in Figure 2, damage rate increases with age, which is also indicated in a traditional bathtub curve as shown in Figure 3. In addition, structures are also known to experience some so‐called burn in failures at an early age. These two increases in failure rate are often reflected in a typical bathtub curve as shown in Figure 3. The phases in the life of a structure related to the bathtub curve can be described as:

 an initial phase where anomalies arise from the design, fabrication and installation;

 the maturity phase representing the useful operating life; and

 the ageing and terminal phases representing the first and second part of the end of life.

Figure 3 Typical bathtub curve. It should be noted that ageing may set in earlier if the structure is not managed properly.

Source: Based on HSE (2006). Plant ageing—Management of equipment containing hazardous fluids or pressure, HSE RR 509.

It is important to recognise that frequency, purpose and method of inspection depend on the phase in the bathtub curve (HSE 2006). In the initial phase, anomalies arising from gross errors in the design, fabrication and installation should be detected by early inspections (baseline inspection). The purpose of these early inspections is to determine the existence of gross errors and to provide confidence about the state of the structure. Any anomalies detected are expected to be rectified and the structure should at the end of this period “enter” the maturity phase.

In the maturity phase a lower failure rate is expected and the purpose of the inspection is to confirm that changes to the structure’s condition, configuration and loading are in line with expectation.

In addition, identification of unexpected and unforeseen anomalies and damage is needed, such as unexpected early fatigue cracks and corrosion damage and unforeseen accidental damage and load changes. Inspection in the maturity phase will often be periodic and can be calendar‐based, condition‐based or risk‐based. These strategies for inspection are discussed further in later chapters.

In the ageing and terminal phases the failure rate is expected to increase, although this is seldom recognised by structural integrity engineers. The purpose of inspection in these phases is to provide the basis for evaluation and the need for and prioritisation of repairs to ensure that the structure is still fit for purpose. Standards in some cases also require special inspections to be performed in these phases. The frequency of inspection is expected to increase compared to that in the maturity phase and even more so in the terminal phase.