Babcock: Structural Analysis and Validated Upgrades to LCVP for Lifting Operations



Reduced costly repairs

Load path and stresses now understood

Operational availability increased

SUMMARY

We were called upon to investigate cracks in the lifting lugs of the Landing Craft Vehicle Personnel (LCVP), which remained unexplained by traditional hand calculations. We surmised that the failure could have occurred through an underprediction of localised stresses or a cyclic or fatigue related failure through under-estimation of load magnitudes or frequencies. Using FEA, we determined the contributing factors. This model was then utilised as a virtual test bed for further investigations and ultimately a series of modifications, saving time and money.

SITUATION

We were called upon by Babcock and the Royal Navy to apply our expertise in Finite Element Analysis (FEA) to address a failure in service involving an aluminium landing craft – ‘Landing Craft Vehicle Personnel’ or LCVP.

The MK5 LCVP is an amphibious landing craft designed to transport troops or armoured vehicles from ship to shore, in this instance deployed from the amphibious warfare ship HMS Bulwark. HMS Bulwark plays a critical role in UK defence, through her ability to deploy Royal Marines and their equipment to shore but she has also assisted in a humanitarian role, with recent campaigns off the coast of Italy in which she recovered over 2,900 migrants.

During deployment, the LCVP is lowered from a davit and spreader beam assembly, with the hook loads passing into the LCVP hull structure via four lifting lugs within the LCVP gunwales.

It was at these critical junctions that failures in the field were observed; cracking around critical welds in the lifting load-path raised concerns with Royal Navy personnel, and an investigation was commissioned.

CHALLENGE

The assessment of the LCVP under lifting conditions had been conducted using a hand calculation approach to compare average stresses in the lifting lug welds against a 0.2% proof stress criterion, with various safety factors applied in order to determine the ‘allowable stress’.

This method assumed that the stress distribution around the lifting lugs was relatively consistent and hence it could be assumed that if the average stress did not exceed the allowable stress, then the lifting lug design would be structurally sound.

With the improvements in computer-aided engineering (CAE) and specifically finite element analysis (FEA), in which we have a specialism, it has become practicable to evaluate not simply the average stress in a welded interface, but the distribution of stress throughout all of the interface features.

Given that the original design basis had not substantially changed, i.e. the applied loads, structural design and materials were unaltered from the original specification; it was considered that failure could have occurred through an underprediction of localised stresses (a hotspot) or a cyclic or fatigue related failure through under-estimation of load magnitudes or frequencies.

In the first instance, it was determined that a static FEA assessment of the stresses in the lifting lug should be conducted, the outcomes reported and reviewed before proceeding with any fatigue assessment.

SOLUTION

We approach all analysis tasks through a rigorous process to ensure that both the expectations of our client and our internal quality management system are met. In our world, all analyses start with ‘The Analysis Plan’.

The analysis plan summarised the bounding load case and also identified all other critical parameters, such as material properties, operating temperature, the data source for the LCVP geometry (the drawing pack in this instance) factors to be applied, the assessment code, software of choice and stress limits.

Using the original production drawings, our nominated analyst developed a 3D mid-plane (surface) model of the LCVP structure, while taking advantage of the line of symmetry about the keel. In order to ensure that the stiffness of the model was accurate, it was necessary to include all deck and scantling structures, while the superstructure could be removed and later reinstated through the use of a point mass technique. The use of shell elements requires all plate thicknesses to be defined, along with the application of material properties and an adjusted mass to ensure the total mass and centre of gravity were respected.

Tension-only beam elements representing the lifting chains and the application of load (in this case gravity) could then be applied – thus completing the model.

Using ANSYS Mechanical, the model could then be solved and the stresses within the LCVP structures determined and, following formal review and approval activities, compared to the allowable stress.

Through this process, the factors contributing to the cracking issue were determined and the FEA model was then utilised as a virtual test bed for further investigations and ultimately a series of modifications.

BENEFITS
  • For the first time, the load path and stresses in the LCVP were understood.
  • A series of strategies could be identified and validated before commencing costly and time-consuming repairs.
  • By minimising repair time, the availability of the LCVP craft to support Royal Navy operations was improved.


We were called upon to investigate cracks in the lifting lugs of the Landing Craft Vehicle Personnel (LCVP), which remained unexplained by traditional hand calculations. We surmised that the failure could have occurred through an underprediction of localised stresses or a cyclic or fatigue related failure through under-estimation of load magnitudes or frequencies. Using FEA, we determined the contributing factors. This model was then utilised as a virtual test bed for further investigations and ultimately a series of modifications, saving time and money.



We were called upon by Babcock and the Royal Navy to apply our expertise in Finite Element Analysis (FEA) to address a failure in service involving an aluminium landing craft – ‘Landing Craft Vehicle Personnel’ or LCVP.

The MK5 LCVP is an amphibious landing craft designed to transport troops or armoured vehicles from ship to shore, in this instance deployed from the amphibious warfare ship HMS Bulwark. HMS Bulwark plays a critical role in UK defence, through her ability to deploy Royal Marines and their equipment to shore but she has also assisted in a humanitarian role, with recent campaigns off the coast of Italy in which she recovered over 2,900 migrants.

During deployment, the LCVP is lowered from a davit and spreader beam assembly, with the hook loads passing into the LCVP hull structure via four lifting lugs within the LCVP gunwales.

It was at these critical junctions that failures in the field were observed; cracking around critical welds in the lifting load-path raised concerns with Royal Navy personnel, and an investigation was commissioned.



The assessment of the LCVP under lifting conditions had been conducted using a hand calculation approach to compare average stresses in the lifting lug welds against a 0.2% proof stress criterion, with various safety factors applied in order to determine the ‘allowable stress’.

This method assumed that the stress distribution around the lifting lugs was relatively consistent and hence it could be assumed that if the average stress did not exceed the allowable stress, then the lifting lug design would be structurally sound.

With the improvements in computer-aided engineering (CAE) and specifically finite element analysis (FEA), in which we have a specialism, it has become practicable to evaluate not simply the average stress in a welded interface, but the distribution of stress throughout all of the interface features.

Given that the original design basis had not substantially changed, i.e. the applied loads, structural design and materials were unaltered from the original specification; it was considered that failure could have occurred through an underprediction of localised stresses (a hotspot) or a cyclic or fatigue related failure through under-estimation of load magnitudes or frequencies.

In the first instance, it was determined that a static FEA assessment of the stresses in the lifting lug should be conducted, the outcomes reported and reviewed before proceeding with any fatigue assessment.



We approach all analysis tasks through a rigorous process to ensure that both the expectations of our client and our internal quality management system are met. In our world, all analyses start with ‘The Analysis Plan’.

The analysis plan summarised the bounding load case and also identified all other critical parameters, such as material properties, operating temperature, the data source for the LCVP geometry (the drawing pack in this instance) factors to be applied, the assessment code, software of choice and stress limits.

Using the original production drawings, our nominated analyst developed a 3D mid-plane (surface) model of the LCVP structure, while taking advantage of the line of symmetry about the keel. In order to ensure that the stiffness of the model was accurate, it was necessary to include all deck and scantling structures, while the superstructure could be removed and later reinstated through the use of a point mass technique. The use of shell elements requires all plate thicknesses to be defined, along with the application of material properties and an adjusted mass to ensure the total mass and centre of gravity were respected.

Tension-only beam elements representing the lifting chains and the application of load (in this case gravity) could then be applied – thus completing the model.

Using ANSYS Mechanical, the model could then be solved and the stresses within the LCVP structures determined and, following formal review and approval activities, compared to the allowable stress.

Through this process, the factors contributing to the cracking issue were determined and the FEA model was then utilised as a virtual test bed for further investigations and ultimately a series of modifications.



  • For the first time, the load path and stresses in the LCVP were understood.
  • A series of strategies could be identified and validated before commencing costly and time-consuming repairs.
  • By minimising repair time, the availability of the LCVP craft to support Royal Navy operations was improved.
LCVP Lift case pre-processing
LCVP Scantling stress plot
Landing Craft Vehicle Personnel (LCVP) Docking

SECTOR

PROJECT ATTRIBUTES

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For the first time, the load path and stresses in the LCVP were understood.

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