Marine failures can be costly, dangerous, and environmentally damaging.
When designing ships and offshore platforms, or subsea infrastructure and mooring systems, all teams that operate in marine environments must be certain that their assets maintain the utmost structural integrity and everyone comes home alive.
Beyond the risks, operators who wish to comply with international marine safety and classifications standards such as ABS (American Bureau of Shipping), DNV (Det Norske Veritas) and Lloyd’s Register, will have to prove due diligence in design and construction to obtain certification.
Furthermore, for those working in the naval defence sectors, designing shock-resistant military vessels will need to comply with MIL-STD-901D & MIL-STD-810G standards.
These are a lot of parameters to navigate well before getting close to the water.
The Enginuity Marine Engineering team has a long-standing history of designing marine systems and components that withstand the harshest conditions on earth (well, at sea).
Of the utmost considerations are structural integrity, fatigue-life prediction, high-load handling, corrosion, wave & current effects and shock/vibration resistance.
Well before one piece of steel is welded, a thorough and diligent Finite Element Analysis (FEA) is performed to model for dynamic and static loads to predict fatigue. This exercise also accounts for high load handling shock resistance and corrosion.
TOP 6 CAUSES OF MARINE STRUCTURAL FAILURES
Using the numerical technique called the Finite Element Method (FEM) allows engineers to simulate any given physical phenomenon. The following is a list of the most common failures this method helps eliminate.
Fatigue Cracks
Most commonly found in welds, fatigue cracks are a result of constant cyclic loads. Even low-stress cycles can extend cracks, leading to fracture and compromising structural integrity over time. Material selection complicates things. Opting for lighter weight aluminium is much more challenging when calculating fatigue limits than steel.
Wave induced cyclic loads are the main contributors to fatigue failure. Irregular and ever-changing wave-induced loads make analysis very challenging. Fatigue cracks are most often found in the following:
- Bottom and side longitudinal stiffeners
- Keels
- Waterline area
- Connections between longitudinal stiffeners and web frames
Fatigue cracks with FEA
Buckling of Panels (or Plate Buckling)
Buckling is lateral deformation under compression, causing surface wrinkling. This instability risks structural failure, especially in marine environments with complex loads. Addressing buckling during design is crucial for structural safety and reliability.
Predicting the use case is the most effective prevention method for this failure type.
- Overloading or improper load distribution
- Compressive loads during hogging2 and sagging of ships
- External forces such as large waves or collisions
Corrosion
A significant long-term issue that weakens structural components manifesting in various forms:
- General corrosion, reducing plate thickness
- Pitting corrosion in specific areas
- Galvanic corrosion when dissimilar metals are in contact
Acidity and salinity are inputs of the design that inform section loss over time, material selection or both. For example: a mooring component would typically be calculated and designed to withstand a reduction of section thickness of approx. 0.5 mm per year.
Enginuity designs beyond End of Life (EOL), meaning the component will continue to withstand the service loads well beyond its service time. The corrosion rate will be determined by the environment (acidity/salinity/etc.) and the material. Between materials selection, environmental conditions, and service life and service loads, the Enginuity team designs for those factors.
Poor Design or Construction
Needless to say, inadequate design, improper stiffening, or substandard construction techniques can introduce weak spots compromising in a reduction of marine durability. It is imperative that environmental factors such as such as storms, rogue waves, and hurricanes, be thoroughly considered in design and during construction.
For example, on February 15, 1982 a semi-submersible offshore drilling platform called the Ocean Ranger met a fateful end, resulting in the deaths of all 84 crew members. Several design flaws contributed to the sinking, notably a poorly chosen location for a simple porthole window, that allowed water to enter the ballast control room. Subsequently the Canadian government made 136 recommendations regarding design, training, and safety protocols for oil rigs.
Vibrations
Constant vibrations from ship engines, machinery, or rough seas can gradually weaken marine structures.
In the world of renewable energy, offshore wind turbines are constantly exposed to powerful wind and wave forces.
Such persistent vibrations can severely impact the turbines’ longevity and operational efficiency, posing a significant challenge to their long-term performance and reliability in harsh marine conditions.
Enginuity’s Lou Manuge often uses a linear dynamics package that can investigate steady state vibration loads.
Even moorings are susceptible to vibrational fatigue. As an input for calculating the fatigue limit state, the Enginuity marine team will obtain a statistical model of tension in the mooring generated from a hydrodynamic analysis of the mooring assembly, and then simply scale the expected live stresses from a linear static finite element model using the tension data.
Thermal Stresses
In ships, temperature differences between warmer cargo and colder seawater can cause high thermal stresses, potentially leading to structural failures in the hull. Expansion and contraction in all marine components and offshore structures are particularly susceptible to cracking, fatigue, structural weakening and reduced component lifespan.
With respect to thermal loads, strains from thermal expansion/contraction using appropriate material thermo-mechanical properties such as coefficient of thermal expansion/conductivity or heat capacity, among others are easily obtained. Our engineers then drive these models with temperature boundary conditions, heat dissipation loads, simplified convection and radiation loads.
The Enginuity team can also couple Computational Fluid Dynamics (CFD) and FEA tools to generate thermo-fluid models, where the flow velocities and fluid temperatures can be incorporated to get accurate convective loads. Thus, solving convection, conduction and radiation physics simultaneously with a high degree of accuracy.
Benefits of Using FEA in Marine Engineering
These six factors often interact and compound each other. It is paramount that all factors are considered. All marine operators desire reduced risk of failure and an extended service life.
By correctly performing FEA, fatigue, corrosion, thermal stress, ensures that ships, offshore platforms, wind turbines and mooring systems will withstand cyclic loads, wave impacts and corrosion and maintain structural integrity. The results of the simulation analysis keeps costs to a minimum and ensures everyone gets home safe.
