When a ship enters service it has an implied or calculated design fatigue life. Factors including construction quality, loads, material performance, and operational profile are assumed. However, these factors vary through life. Failure of structural items due to fatigue cracking can lead to increased cost of ownership and reduced capability. Therefore, to allow informed decision-making, it is necessary to monitor the variables that influence a ship's fatigue life. This paper presents a new method whereby in-service data, fleet maintenance reports, and Finite Element Analysis are incorporated to predict the fatigue life and hence support the management of the remaining service life of a ship. The method is applied to a naval High Speed Light Craft to demonstrate its merit. The fatigue lives of different welded details are predicted using a practical implementation of the S-N curve approach, employed with strain and operational data acquired from an on-board hull monitoring system.

Modern warships are often constructed from aluminium alloys or high tensile steel, and their increasing range of operational roles indicates exposure to harsh seaway loads including slamming. These factors can lead to fatigue cracking, which can reduce operational availability. The objective of the present study is to improve understanding of the influence of variables in the fatigue analysis of a weight-optimised warship. The objective is met by analysing hull monitoring data acquired from a 56 m naval aluminium patrol boat, to determine the long-term importance of slamming and the correlation between the hourly number of slams, ship speed, and fatigue damage at two structural details. It was found that the effect of the ship’s speed on the fatigue damage is not statistically significant. In addition, a sizable proportion of the fatigue damage accumulated at low to moderate ship speed, when the patrol boat experienced slamming, rather than at higher speeds. This may be due to voluntary and/or involuntary speed reduction, which is not typically taken into account in numerical fatigue analysis. That is, the use of long-term distributions of the wave environment and ship speed may mask the effects of voluntary and/or involuntary speed reduction on slamming occurrence and the fatigue damage. This finding can lead to improved requirements setting and through-life structural management of weight-optimised warships.

This paper describes a research programme to construct a Model-Based Systems Engineering (MBSE) methodology that supports acquiring organisations in the early stages of Off-the-Shelf (OTS) naval vessel acquisitions. A structured approach to design and requirements definition activities has been incorporated into the methodology to provide an easily implemented, reusable approach that supports defensible acquisition of OTS naval vessels through traceability of decisions. The methodology comprises two main parts. Firstly, a design space is developed from the capability needs using Set-Based Design principles, Model-Based Conceptual Design, and Design Patterns. A key idea is to employ Concept and Requirements Exploration to trim the design space to the region of OTS designs most likely to meet the needs. This region can be used to specify Request for Tender (RFT) requirements. Secondly, the methodology supports trades-off between the OTS design options proposed in the RFT responses using a multi-criteria decision making method. The paper includes an example implementation of the methodology for an indicative Offshore Patrol Vessel capability.

Ship structures are prone to fatigue cracking due to fluctuating loads caused by the seaway. This is especially the case for high-speed craft (HSC), as the slamming loads and associated response are known to significantly impact the stress magnitudes experienced by the structure. Therefore, slamming may have a considerable influence on the fatigue life of HSC when compared to accounting for the global wave induced stresses alone. This paper presents an investigation into various methods for identifying slams for structural response analysis. Measurements of hull girder stresses of an aluminium high-speed patrol boat are utilised to explore the characteristics of slam events. The approach to analyse full-scale time records, decomposition of the wave-induced and impact components of stress, and definition and detection of slam events are discussed. With respect to fatigue life, identification of slam events enables the evaluation of the influence of slamming and the associated whipping response in a range of speeds, headings and sea states. Such knowledge supports informed decision-making in regards to the sustainability and maintainability of the vessel.

The typical size and operational roles of high-speed naval vessels have increased over the past decades, which has led to operation in more extreme environments. This increasing operational tempo, and susceptibility to slamming, has driven interest in improving structural assessment methodologies for high-speed craft (HSC). To ensure that a new acquisition will meet its intended life, or to check that in-service modifications to the structure or operational profile do not substantially reduce the design life, a fatigue assessment is required. In many of the classification society rules the allowable stress approach is assumed to implicitly account for fatigue, by applying safety factors due to material, weld filler, and the criticality of the structural item. However, examples of recent fatigue failures of lightweight naval craft demonstrate that a more direct approach to assessing fatigue is needed. Further, the need to evaluate the random nature of fatigue life estimates is increasingly being recognised. For high-speed craft the stochastic nature of slamming phenomena, and the ability to reliably identify the number and severity of slam events, warrants the use of probabilistic fatigue analysis. This paper presents a review of available approaches to fatigue assessment, and their associated merits and limitations when applied to high-speed craft. Mechanisms to improve the understanding of the structural performance of a high-speed craft are examined by integrating maintenance reports, numerical predictions, full-scale measurements predictions, observed environmental conditions, and the operational profile of the vessel. It is recommended that a hybrid approach to fatigue assessment of HSC is valid, incorporating available data and procedures to balance the required accuracy and computational cost.

The intact stability of maritime surface vessels (ships, boats, landing craft, etc.) should be as-sessed for the most extreme environment that they are designed for or limited to operate in: namely the nominal and gust wind speeds and associated wave height and wave frequency profile.The IMO and naval weather criteria apply to ocean going vessels but each use different wind speeds. The IMO criterion uses a single nominal wind speed (26 ms(-1)) and a small gust factor (root 1.5 = 1.225) for all assessed vessels, irrespective of operational environment or expectations. The naval weather criteria uses different gust wind speeds for different operational expectations, with most significantly higher than the IMO gust wind speed. Yet these criteria are intended to assess the suitability of vessels for essentially similar operational expectations.This paper revisits the basis of the wind speeds used for stability analysis. A range of standard-ized wind speeds for different types of operational service is proposed.

Landing craft have been operated by various navies around the world, including the Royal Australian Navy (RAN), for many decades to transport vehicles and personnel, often from ship to shore. The RAN's current practise assesses the stability of landing craft using contemporary naval stability criteria, which, in turn, were developed from limited parameter ship databases. Over the years, multiple landing craft stability issues have arisen, mainly due to the unique shapes and uses of these vessel types in contrast to those of the stability criteria database. The stability criteria that need to be applied to this type of craft are currently being investigated through research programs sponsored by the Australian Department of Defence. One of these research programs that recently concluded involved extensive semi-captive model tests in beam waves, together with ship motion simulation studies. The research program was based on a series of three models having different freeboards and a removable bulwark. The various hull/bulwark configurations were tested at three KGs in regular waves. In a substantial number of cases there was significant water on deck or into the well. The data obtained have been analysed and used to develop quasi-static stability criteria specific to landing craft. These criteria are primarily adaptations of contemporary naval criteria, augmented by criteria designed to account for the peculiarities of landing craft. They meet one of the main aims of the research, which was to develop stability criteria that are suitable for application by the mainstream naval architectural community.

The views expressed in this paper are those of the authors and not necessarily endorsed by the Department of Defence, Australia. Landing craft have been operated by various navies around the world, including the Royal Australian Navy (RAN), for many decades to transport vehicles and personnel, often from ship to shore. The RAN assesses the stability of landing craft using contemporary naval stability criteria, perhaps unnecessarily penalising their design and operation. Over the years, multiple landing craft stability issues have arisen, with varying degrees of comfort felt for the solutions developed. The stability criteria that need to be applied to this type of craft are currently being investigated through various research programs sponsored by the Defence Science and Technology Organisation and the Directorate of Navy Platform Systems. One of these research programs involves extensive captive model tests in beam waves together with a parallel ship motions and stability simulation analysis. Other factors, such as wind heeling and water ingress over the bulwark, are also being examined. The ultimate aim of this research program is to derive intact stability criteria that are based on physical principles while being suitable for application by the mainstream naval architectural community.

In the unfortunate event that a naval vessel is damaged, decisions must be made regarding the reduction in the vessel's capability and what damage control procedures can be initiated to both restore this capability and minimise further damage. In order to make an informed decision, it is vital that Commanding Officers (COs) have access to reliable information regarding the vessel's structural integrity, stability and seakeeping performance. A reduction in any of these parameters may have a significant influence on the operability of the vessel. A preliminary program of work has been conducted to investigate the change in ship motions due to an angle of list induced on the vessel after sustaining damage. This study incorporates both experimental and numerical simulations of a listed vessel at rest in a seaway. During the experimental program several parameters were investigated to ascertain their influence on the vessel's motions. These parameters include initial list angle, direction of list and wave conditions. The results from this study show that for a vessel in beam seas there is no significant effect on vessel motions due to an angle of list. However, in head seas the introduction of an angle of list has a marked effect on the amplitude of roll motions. The example operator guidance generated shows that numerical modelling can be applied to the analysis of the operability of a vessel against set limiting criteria. This information and methodology could be incorporated into a rapid damage assessment tool along with sea-load predictions to assist COs in their decision making process in the event of an emergency.

The unfortunate event that a naval vessel is damaged, decisions must be made regarding the reduction in the vessel’s capability and what damage control procedures can be initiated to both restore this capability and minimise further damage. In order to make an informed decision, it is vital that Commanding Officers (COs) have access to reliable information regarding the vessel’s structural integrity, stability and seakeeping performance. A reduction in any of these parameters may have a significant influence on the operability of the vessel. A preliminary program of work has been conducted to investigate the change in ship motions due to an angle of list induced on the vessel after sustaining damage. This study incorporates both experimental and numerical simulations of a listed vessel at rest in a seaway. During the experimental program several parameters were investigated to ascertain their influence on the vessel's motions. These parameters include initial list angle, direction of list and wave conditions. The results from this study show that for a vessel in beam seas there is no significant effect on vessel motions due to an angle of list. However, in head seas the introduction of an angle of list has a marked effect on the amplitude of roll motions. The example operator guidance generated shows that numerical modelling can be applied to the analysis of the operability of a vessel against set limiting criteria. This information and methodology could be incorporated into a rapid damage assessment tool along with sea-load predictions to assist COs in their decision making process in the event of an emergency.

This paper describes recent research undertaken by the Defence Science and Technology Organisation (DSTO) to provide the Royal Australian Navy (RAN) with an ability to incorporate conditional probabilities within operation profiles of surface platforms. An example calculation of extreme and fatigue load analysis demonstrates the importance of realistic operational profile definitions.

Short-term damage mechanisms in naval ship structures are caused by incidents such as vessel grounding, collision, severe sea states and weapons effects. These incidents can cause damage to the ship structure, increase the risk of further failure and lead to the loss of life and reduced mission capability. In these cases it is therefore important that quick assessments be available to determine the post-damage strength of the ship and aid in further decision-making. The primary method used to ensure the global strength capability of a ship structure is by assessment of the ultimate longitudinal bending strength. Assessment methods used by DSTO Australia are described and it is shown how the ultimate longitudinal bending strength of intact and damaged structures is determined using progressive collapse analysis methods. The ultimate strength can then be compared to the expected extreme wave induced loading in the region of operation. Depending on the extent of damage, various ship management decisions can be made to either discontinue the mission or to reduce the loads on the vessel by choosing a more conservative operational profile. In this paper an example of a ship structural damage scenario is investigated to show the damage assessment method undertaken and the information required from the Navy for the analysis. A probabilistic reliability analysis is undertaken by accounting for the uncertainty in both the structural strength and wave-induced loads. Other considerations are also discussed such as the ability of a flooded vessel to remain operational, local considerations involving the residual strength of buckled plating and the effects of flooding on internal bulkheads.