Structural Fatigue: A Forward-Thinking Approach for the Joint Support Ship

Jonathon Williams, Martin Fuller, and LCdr Antony Carter
Jun 9, 2022
11 min read

Updated: Jun 10, 2022

As the saying goes, nothing lasts forever. Unfortunately, ships are no different. As vessels age, the ability of a ship’s structure to resist imposed stresses diminishes. The limited assortment of tools that are available to technical authorities, combined with a large number of uncertainties, make it difficult to accurately assess the remaining life of a ship’s structure. The result is that operational authorities assume a higher degree of risk when operating older vessels. The Joint Support Ship ( JSS) will be provided with additional tools which will enable technical authorities to make more informed assessments of the risk associated with operating these vessels as they approach the end of their design life.

Design Life

Design life refers to the period of time that a vessel is intended to be in service, and able to complete the tasks it was designed to accomplish — aided by maintenance, repairs and refits. Selection of a design life usually requires a trade-off between structural robustness and other capabilities such as speed, endurance, and armament, to name but a few. In modern warship construction, the design life typically ranges from 25 to 35 years for large seagoing ships, but in some cases can exceed 40 years. For example, the United States Navy’s Nimitz-and Gerald R. Ford-class aircraft carriers have a planned service life of 50 years (U.S Navy Office of Information, 2021).

A ship’s design life is dependent on several factors, one being the consideration of the various modes of failure possible for the ship’s structure, which is achieved through the application of what is known as “limit states design.” Simply put, a limit state is defined by the description of a state in which a certain structural member, or an entire structure, fails to perform its intended function. The four most common limit states that are considered in ship design are:

Accidental Limit State (ALS), which considers structural damage resulting from accidents such as collisions and fires;
Ultimate Limit State (ULS), which considers failure mechanisms such as buckling and yielding;
Fatigue Limit State (FLS), which considers failure resulting from fatigue damage; and
Service Limit State (SLS), which considers structural damage from normal in-service operations (Hageman, et al., 2014).

Recently, as the Royal Canadian Navy (RCN) fleet continues to age, there has been an increasing amount of attention paid to the consideration and understanding of the fatigue limit state.

What is Fatigue?

Fatigue is a failure process — a function of the stress amplitude applied to a structure, and the cycles in which the stress is repeated (Callister & Rethwisch, 2014). Imagine bending a paper clip. The clip can be bent once and remain intact, but given enough repetitions it will crack and break. The graphical representation of stress amplitude vs. number of cycles experienced until failure is known as the S-N curve, an example of which is shown in Figure 1. (Steel used in the construction of the JSS would follow a similar curve as 1045 steel.)

Fatigue is characterized by cracking that takes place in three distinct phases: crack initiation, crack growth, and structural failure. In the crack initiation phase, stress applied to a material that is large enough to induce fatigue causes a microscopic crack to form at the maximum point of stress within the material, typically at the point where there is a concentration of stress, or a defect. During the crack growth phase, the crack propagates with each repeating stress cycle until the crack is so large that the remaining intact portion of the material fails upon the next application of the stress. The result is structural failure. Millions of cycles might be required to reach this point, as the actual number of cycles depends on the magnitude of the applied stress, the type of material, and other factors (Sieve, Kihl, & Ayyub, 2000). However, if the frequency of the stress cycles is high, fatigue failure can occur relatively quickly (Callister & Rethwisch, 2014).

Stress concentrations are found throughout ship structures on both the global and local scale. Global stress concentrations are commonly associated with significant changes in geometry, or discontinuities, such as deck openings, superstructure terminations, and knuckles. Local stress concentrations occur in both base materials and weldments. For the former, areas such as sharp corners and transitions, and plate edges are common areas of stress concentrations. For the latter, it is common for stress concentrations to occur along the many kilometres of longitudinal, transverse, and vertical welds that connect the various pieces of a ship’s hull, often as a result of detail design decisions and weld defects (Sieve, Kihl, & Ayyub, 2000).

Stress concentrations can also result from localized and general corrosion resulting from exposure of unprotected structural surfaces to the marine environment. Surfaces may become unprotected if they are not adequately or properly coated, or if protective coatings have been damaged (Callister & Rethwisch, 2014).

Ships experience many forms of cyclical stress, with the dominant source being from wave-induced cyclical loads (Glen, Dinovitzer, Paterson, Luznik, & Bayley, 1999). The number of cycles in this environment can be in the order of millions of cycles per year, and can be composed of a wide range of amplitudes. Other sources of cyclical loading include vibrations from machinery, propeller-induced hull vibration, and the motion of fluid within tanks (Sieve, Kihl, & Ayyub, 2000).

Fatigue Life

Fatigue life refers to the time taken to repeat the necessary cycles for a material to fail due to fatigue. For example, if a material can sustain 20 cycles of a load until failure, and the load is repeated at one cycle per minute, then the fatigue life of the material would be 20 minutes. The same principle can be applied to assess the fatigue life of ship structures.

Although ship structures contain numerous types of structural details, to predict a ship’s fatigue life it is common to base the prediction on a single detail, or on a series of structural details that are prevalent in the ship’s structure. This detail will determine the limit on fatigue life, as more robust parts of the ship’s structure will have a longer fatigue life. In general, the following procedure is required to predict fatigue life:

Determine the loads that are expected to act on the structure, and the resultant response of the ship;
Determine the internal stresses in the ship’s structure based on the expected loads;
Select a suitable S-N curve for the structural detail(s) of interest; then
Compare the applied stress cycles against the stress cycles to cause failure (Sieve, Kihl, & Ayyub, 2000).

The loads that will act on the ship’s structure depend on the operational and environmental profile of the ship. The operational profile is defined by the number of days the ship spends at sea, the ship’s speed and heading when at sea, and the time that the ship will operate in different combinations of speed and heading in different wave conditions. The environmental profile is defined by the height and period of waves that will be encountered during the vessel’s service life, and the probabilities of encountering different wave conditions. Figure 2 shows the daily position of HMCS Iroquois (DDH/DDG-280) from 1972 to 2009 based on log books; this data can be used to estimate the environmental profile of the ship.

A ship will respond differently to waves with different characteristics. The response of most importance when determining fatigue life is vertical bending moment. Using the operational and environmental profile of the ship, the lifetime vertical bending moment histogram can be assessed. With this histogram, the internal stresses in the ship’s structure can be theoretically estimated through “hand” calculations, or by using tools such as finite element analysis for more complex structures. A stress range histogram can be determined at the location of the detail of interest.

S-N curves for structural details can be obtained from a number of sources such as building codes and design guides. In some cases, it may be necessary to conduct experiments in order to determine the S-N curve for the detail of interest.

The prediction of fatigue life is not an exact science. Several assumptions can lead to uncertainty, such as the assumption that the fatigue behaviour of a structural detail in a laboratory environment will exhibit the same behaviour when integrated into the ship’s structure. The quality of construction of a ship also has a significant impact on the resultant stresses in the structure, and therefore the fatigue life (Sieve, Kihl, & Ayyub, 2000). In addition, the methods used in the welding and fitting of ship structures could result in elevated residual stresses, which when superimposed with the applied cyclic stresses could accelerate fatigue in the affected ship structures.

Current Approach for Monitoring Fatigue

Historically, structural fatigue in RCN vessels has been monitored through hull surveys, which are intended to inspect areas where fatigue can occur, and identify where repairs are required. Hull surveys, while necessary to understand the material state of a ship, represent a more reactive approach when it comes to monitoring fatigue, given that when cracks are identified during surveys, fatigue has already occurred. When cracks are discovered, they are already typically several centimetres long, and extend through the thickness of plating as shown in Figure 3.

Recent efforts within the RCN to better understand fatigue have included an assessment of the remaining fatigue life of the former HMCS Iroquois. Defence Research and Development Canada (DRDC) conducted an analysis of connection details removed from the ship during disposal (Huang, 2021).

Joint Support Ship

The lead ship of the future Protecteur class is currently being constructed at Vancouver Shipyards Co. Ltd., in North Vancouver, BC. Figure 4 shows the progress of the construction of the ship, which will have a design life of 30 years. Once delivered to the RCN, the Protecteur class will be maintained to American Bureau of Shipping (ABS) classification standards. However, to augment the hull survey regime required by ABS, the Protecteur class will be delivered with several additional tools for actively assessing fatigue life. Light Structures AS, a global supplier of ship structural monitoring technology headquartered in Oslo, Norway, will install a hull monitoring system (HMS) in each ship. Furthermore, material and weld samples from the fabrication of key areas of the ship’s structure are being collected during the build process.

Hull Monitoring System

An HMS can be used to support fatigue life assessments by providing stress range histograms for areas of particular interest in a ship’s structure. The technology is not new. Hull monitoring systems have been installed in the United States Coast Guard’s Legend-class national security cutters (Hageman, et al., 2014), and one was recently installed aboard HMCS Montreal (FFH-336). However, the future Protecteur class will be the first class of RCN vessel to be fitted with an HMS at delivery. This will enable an accurate understanding of the stresses imposed on the structure from the very beginning of service life, something that can otherwise only be estimated.

The HMS for the Joint Support Ship will function by transmitting light through fibre-optic cables to fibre-optic strain sensors located in select positions on the ship’s structure. The sensors reflect a spectrum of light corresponding to the distortion of the structure, data from which can be analyzed and used to determine the stresses on the hull (Figure 5). The JSS HMS will also include two sets of three accelerometers oriented along the longitudinal, transverse, and vertical axes of the hull, as well as a six-degrees-of-freedom motion sensor. The output from these devices will enable an understanding of the ship’s motions due to wave conditions (Light Structures AS, 2019). The locations of the strain sensors, accelerometers and motion sensor on the JSS are displayed in Figure 6.

While the primary intent is that the data collected from the HMS, through analysis, will enable a more accurate assessment of platform end-of-life, the exact nature of how this data will be used remains to be determined. Some investment will be required to either develop the necessary skills within DND, or contract out for them, in order to collect and analyze the data to provide meaningful results. The data could help inform a number of decisions, including whether there is a need to perform hull surveys beyond the prescribed classification society regime should the JSS encounter conditions outside the expected operational and environmental profile. The collected data could also show how quickly fatigue damage is accumulating, and thereby assist decision-making regarding the periodicity of performing hull surveys. Finally, the data could support a number of parallel initiatives within DND, including:

Supporting the validation of numerical and theoretical models used to predict fatigue life;
Supporting the development of a digital twin of the vessel which could:
1. Provide technical agencies a more holistic understanding of the material state of the JSS; and
2. Enable simulations to be performed to determine fatigue states in localized structure, and thus enable more targeted hull survey regimes.

Material Samples

Given the significance of ship-construction materials in determining the fatigue life of the ship, baseline material and weld samples are being collected during the construction of the JSS. The steel samples will include both plates and profiles for all structural steel grades used in the design, with the thickness and width determined from median figures for each steel type used. Samples of profiles and weld coupons taken from JSS, which can be seen in Figure 7, are being shipped to the Quality Engineering Test Establishment (QETE) in Ottawa. The Directorate of Naval Platform Systems (DNPS 2) and QETE have a number of experiments they are planning to conduct upon receipt of the samples. The remaining samples will be stored under temperature- and humidity-controlled laboratory conditions until they are needed. These samples could be analyzed by QETE in the future to confirm assumptions made regarding the fatigue behaviour of the materials and weldments used in the construction of JSS, and allow more accurate S-N curves to be developed.

Conclusion

Condition-based assessments have enabled the RCN to extend vessels beyond their design life in the past, but predicting fatigue failure using these methods remains difficult. This leads to decisions that carry increased risk, or result in inflated safety margins being applied to account for the uncertainties.

While the hull monitoring equipment itself is not new or innovative, the fact that the JSS will be equipped with this system at delivery is a novel approach for the RCN. Despite the work that remains to be done to determine how best to utilize the collected data, it should at a minimum provide technical authorities with greater insight into the fatigue life of the ship’s structure. The material samples will enable experimental data to be collected, which will further provide technical authorities with a greater understanding of fatigue behaviour. The end result is that better, more informed decisions can be made by reducing uncertainties, which, ultimately, enables greater confidence in the safe operation of aging vessels.

Acknowledgment

The assistance of LCdr Mark Bartek, DNPS 2-2 Ship Structures, and Dr James Huang, DNPS 2-4 Naval Material & Petroleum Engineering, in the preparation and technical review of this article was gratefully received and very much appreciated.

Works Cited

Callister, W., & Rethwisch, D. (2014). Materials Science and Engineering. Hoboken: John Wiley & Sons Inc.

Glen, I., Dinovitzer, A., Paterson, R., Luznik, L., & Bayley, C. (1999). Fatigue-Resistant Detail Design Guide for Ship Structures. Washington, D.C.: Ship Structure Committee.

Hageman, R., Drummen, I., Stambaugh, K., Dupau, T., Herel, N., Derbanne, Q., Kim, P. (2014). Structural Fatigue Loading Predictions and Comparisons with Test Data for a New Class of US Coast Guard Cutters.

Huang, J. (2021, December 20). Email: MEJ Article // Fatigue & JSS Hull Monitoring System.

Huang, J. (2021, September 15). Email: Ship Life Extension // Current Strategy. Light Structures AS. (2019). 194-160.00-025 Functional Description for Hull Monitoring System. Oslo.

Light Structures AS. (2022). The technology behind our structural monitoring systems. Retrieved from: https://www.lightstructures.com/structural-monitoring/

Seaspan Shipyards, @. (2021, October 20). From our #VancouverShipyards modernization, to designing and building world-class vessels for the @CoastGuardCAN and the @RoyalCanNavy [tweet]. Retrieved from Twitter: https://twitter.com/MoreThanShips

Sieve, M., Kihl, D., & Ayyub, B. (2000). Fatigue Design Guidance for Surface Ships. West Bethesda: Naval Surface Warfare Center.

Smith, M. (2017). HMCS Iroquois Structural Testing Program. NATO ST PSDS Meeting. Helsinki.

U.S Navy Office of Information. (2021, November 12). Aircraft Carriers - CVN. Retrieved from America's Navy: https://www.navy.mil/Resources/Fact-Files/DisplayFactFiles/Article/2169795/aircraft-carriers-cvn/

Jonathon Williams is an Engineer-in-Training who completed a rotation with the Joint Support Ship Project Management Office as a junior naval architect.

Martin Fuller is a Naval Architect for the Joint Support Ship Project Management Office in Ottawa.

LCdr Antony Carter was the Naval Architecture Manager for the Joint Support Ship Project Management Office in Ottawa.

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