Fatigue Assessment of a Davit Using Finite Element Analysis

Caley Ocean Systems

Caley Ocean Systems Logo. Armech Solutions client Logo.

Background

Caley Ocean Systems is a leading marine offshore handling system supplier, with clients in many industries such as defence, energy and oceanographic science. They provide heavily relied upon systems, many of which are still operational after 30 years of daily service. Due to the extensive lifetime of these structures, monitoring and evaluation are of paramount importance to ensure safe continued operation, particularly towards end of life.

As a longstanding client of Armech Solutions, Caley Ocean Systems continue to rely upon our experience and capabilities in structural integrity. We were sub-contracted a crucial, time intensive operational structural review that required ongoing Finite Element Analysis (FEA) and fatigue assessment.

Through the collaboration of Armech’s extensive experience in these areas and the clients marine and offshore structures experience, projects are undertaken to the highest possible standard. The ability to flexibly mould into their engineering team suits this particular client well and allows Armech to effectively collaborate and act as an extension of their staff, bringing additional knowledge and capabilities. Through understanding the client’s strengths, and the markets in which they operate, Armech can quickly deploy and adapt the services offered to meet their evolving needs.

The marine environment is particularly arduous for a working structure to operate over its service life, this is in part due to the multiple varying load cases that the offshore environment provides. Therefore, structures that operate under these conditions must be stringently analysed at the design, maintenance, and end of life stages to ensure continued safe operation. Armech Solutions is in a prime position to offer this analysis service and support our marine and offshore clients.

Davit connected to Rigid Inflatable Boat
Figure 1 Workboat Davit and Rigid Inflatable Boat

Problem

A Davit fleet, supplied by Caley Ocean Systems, is approaching the end of the stipulated service life. For the Davits to continue safe operation, a review must be undertaken to determine their structural condition. The Naval operators of the Davits have returned to the OEM to conduct the operational life review with a tight time scale for the work to be carried out. This will require time and labour intensive resources, creating a personnel challenge for Caley Ocean Systems.

Labelled Davit model of loads and their directions. This includes 'Motor Torqueand line load'
Figure 2-1 Loads for Operating Cycle acting on the Davit

To further support their client, Caley was eager to ensure that their product has been utilised to its full ability and potential before its removal from service. The aim of this is to help the client avoid an unnecessary onset significant capital investment, in the form of a new fleet of Davits, and the premature retirement of the current working systems.

The Davits used by the Naval operator are for fast deployment of personnel, including emergency scenarios, and therefore it is paramount that the structure can perform reliably and safely. In the event of failure in the primary loading path, a davit does not tend to have a secondary path that is able to support the load. Therefore, it is imperative that the structure be regularly maintained, and the structural integrity monitored. Structural systems subject to frequent and repeated loading can result in an area of high stress and localised damage and this is where the structure is likely to eventually fail. This fatigue behaviour can be detected through examination of the structure and techniques such as Non-Destructive Testing (NDT) however, a fatigue assessment is also required and important to highlight the areas of weakness.

During the initial design of the Davits and specification of the structure’s lifetime, a conservative approach according to the relevant standard to safeguard early failure will have been employed. It is possible the true fatigue life at the end of service is less than the designed operational loading. This is a common scenario and can arise through reduced operation, compared to the designed loading, over the service life. Therefore, through a fatigue analysis of the Davits loading history it is possible to ascertain if the service life of the structure can be extended.

A further consideration that must be taken into account is the changes in certification standards since the time that the structure was designed and manufactured. Expertise in the updated standards will be required in order to conduct the analysis.

Directions of loads applied to Davit model with two positions shown, Boomed-In and Boomed-Out
Figure 2-2 Freebody Diagram of loads exerted on the Davit

Solution

Caley Ocean Systems identified the need for external engineering assistance to manage this problem, which is where Armech Solutions are called upon for their knowledge and expertise.

Following receipt of the project specification Armech quickly began to follow the process for assessing the immediate and long-term needs of the client, and project.

Step one is to frame the problem. This involved the process of selecting the standards to be used for the fatigue assessment by performing a codes and standards review of those most applicable to the marine Davit.

Armech Solutions strives to be up to date with the relevant standards in our industries. Through a variety of different clients, Armech have been exposed to multiple codes and standards with a diverse range of applications, and therefore are well suited to make informed decisions on those most relevant.

Following the selection of the family of standards to be utilised in the analysis, the comparison was presented to the client to demonstrate the reasoning behind the decision. The standards selected were the hierarchy of DNVGL Offshore Lifting Appliances including DNVGL-ST-0498 and DNVGL-RP-C203. These were chosen as opposed to alternative methodologies, such as Eurocode and Lloyds, due to their specific focus on Davits and offshore structures including the use of a Design Fatigue Factor (DFF). The DFF is to be applied to the design life of the structure as a margin of safety to reduce the likelihood of failure. The value is dependent on the inspection intervals, inspection environment and the relevant structural element. Caley Ocean Systems, as the OEM of the structure and with their expertise in this area, made the decision on the relevant category for the Davit.

Armech Solutions progressed by forming the basis of analysis. Referencing the selected standards and working together with the client, Armech defined a conservative, but realistic duty cycle and set of load cases representing key operating scenarios that best represents the Davits function and safe working load. This included operational loads and parked loads that comprise the weight of the Davit and the RIB (Rigid Inflatable Boat) as well as the complex loading due to the acceleration induced on the whole ship by the waves action. The operational scenarios analysed four static models including boomed-in and boomed-out positions; and loaded and unloaded conditions. The figure illustrates the sequence of the operating cycle of the Davit.

Graph illustrating the six operational steps and the nominal stress at each stage
Figure 3-1 Operating Cycle

The original Davit CAD model, provided by the client, was modified and prepped to suit the FEA analysis using Spaceclaim. The process involved creating a mid-surface shell model to optimise computational efficiency as well as removing or simplifying geometrical aspects of the structure that did not add to the structural capability. The analysis was undertaken in two parts, a detailed structural analysis and a fatigue life analysis.

A preliminary analysis was undertaken to identify the regions of high-stress magnitude, the majority of these regions were at welded interfaces. Identifying the regions of interest allows the mesh to be refined only where high stress is expected, maximising computational efficiency.

Global mesh of Davit with detailed mesh of spring connection
Figure 3-2 Detailed Mesh for FEA

These areas were then analysed in detail and the maximum and minimum principal stress were extrapolated using the hotspot method, to mitigate FEA singularities. Due to the age of the structure, it was important to note that the FEA tools available have significantly improved since the Davits were originally designed. Armech were able to undertake an impartial assessment of the structure with the latest FEA techniques to identify areas of interest in the structural system.

The evaluated membrane and bending stress found from the structural analysis were applied as the input to the fatigue assessment through S-N curve theory, as stated in the standards methodology. This identified the number of cycles to failure for a particular load case.

A line of communication with the Naval client allowed for the supply of statistical data on each Davits loading over its operational lifetime from which Armech were able to calculate the number of cycles undertaken by the structure.

Equipped with the number of cycles to failure and cycles undertaken, the Palmgren-Miner methodology, specified by DNVGL, could be applied to obtain the cumulative fatigue damage results. The weld stress was assessed using a hot-spot stress classification with the extrapolated results shown in the figure.

Graph of extrapolated hot-spot stress and max stress at weld
Figure 3-3 Total Stress Displayed at Weld and Extrapolated Stress

A numerical method was then established to assess each Davit in the fleet on an individual basis through their different load histories.

Distribution analysis showing the allowable number of cycles for each vessel considered in the fatigue analysis
Figure 3-4 Distribution Analysis of Operational Data and Selection of Analysis Frequency

Results

The fatigue analysis undertaken by Armech Solutions found that the Davits life can be extended subject to the DNVGL standards requirements for regular maintenance and inspection.

The results allowed for the Naval client to appreciate the condition of the Davit fleet with respect to the supplied loading history of each individual structure.

The Hot Spot stresses and regions of interest taken forward to the fatigue analysis are shown below:

Maximum Principal Stress Global Contour with three hot-spot areas shown with a detailed view
Figure 4-1 Maximum Principal Stress Contour: Boomed Out-Full Load
Area of high stress at the corner of the Davit arm during boomed-out operation
Figure 4-2 Von-Mises Stress: Boomed-Out

The loading conditions applied to the Davit model are shown in the following schematic:

Maximum Principal Stress Global Contour for each operational stage including Boomed-in, loaded and unloaded, and Boomed out, loaded and unloaded
Figure 4-3 Steps for Operational Cycle
Full Load Boomed-out Total Deformation
Figure 4-4 Total Deformation: Boomed-Out

The fatigue results indicated that with a worst-case scenario of a full loading across the operational life of the structure, 90% of the fatigue life will have been utilised. However, as Armech were able to analyse the true state and load history of the Davit fleet it was found approximately 70% of the fatigue life has been performed by the system. This therefore allowed the Naval client to continue to use their Davit systems with confidence. This was very beneficial to Caley Ocean Systems who were able to add further value to their product and strengthen their client relationship. The results of the project saved the Naval operators approximately one million pounds worth of new Davit systems prematurely. Armech Solutions and Caley Oceans Systems, with their Davit experience, were able to work together to present an effective maintenance and testing plan to the Naval client to ensure the Davits are monitored as they serve the remainder of their fatigue life.

What Next

Due to the success of this and many other projects, Armech Solutions and Caley Ocean Systems have a continued long-standing continued relationship. At present, we are assisting on multiple projects that requires in depth structural calculations for a variety of systems and complex load cases.

The success in this partnership comes from the very foundations of what we offer here at Armech. It is due to our foundations that we can adapt to suit a variety of clients and industries, whilst taking a personalised and bespoke approach to each project.

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