Model tests are a significant investment and availability of model test slots in the basin do not always match project deadlines. For many purposes, CFD simulations become a powerful complementary approach, allowing the designer to understand the hydrodynamic effects of a design change, a couple of hours only after performing the change itself.
ULSTEIN also offers services to ship owners (such as trim optimizations) and to propulsion suppliers (such as new product development, including hull-propulsor interaction studies). At the same level, we offer hydrodynamic services in waves such as forces, motions, power consumption and performance predictions.
CALM WATER RESISTANCE & HULL OPTIMIZATION
Getting reliable calm water resistance predictions is fundamental in order to understand the consequences of design choices and keep reasonable design margins. CFD allows for innovative ideas to be explored with a minimal lead time. Confidence in the numerical results has been achieved over the last couple of years thanks to extensive validations including various designs, draughts, speeds, appendages, etc. In particular, 160+ comparisons with model scale experiments are showing a good match, typically less than 5% relative difference on the calm water resistance for more than 95% of the cases. Such simulations are now part of the standard design process and can return a complete resistance curve overnight. Often, optimization is carried out for limited number of speeds. At ULSTEIN, the whole resistance curve is usually estimated for several draughts in order to have the complete performance overview available to take into account the operational profile of the vessel. In our experience, customer requirements can change and small changes can make significant differences.
Successful bow optimization leading to 5% fuel saving in the mid-speed range in calm water (while compromising high speeds that were less important according to the vessel operational profile).
Added resistance in waves is a difficult topic, in particular in CFD where both user experience and software choice are key. Three years of internal research have led to the use of ANANAS™, developed by Lemma, a talented French startup. ULSTEIN has industrialized the process using this tool which offers a unique way of solving the Navier-Stokes equations (the SWENSE method) leading to high quality predictions in a short amount of time. The combination of the industrialization and the tool opens the doors to affordable hull optimization in waves during early design. The method has been subjected to an extensive validation against model test experiments and the development team has been challenged and have delivered. Comparisons on many cases have shown less than 10% relative difference when simulating the ULSTEIN PX121 in various realistic wave conditions, including steep waves at the edge of breaking. DP (Dynamic Positioning) as well as transit in waves have been covered by the validation.
ULSTEIN PX121 transiting at 10kt in irregular waves (Hs 2.5m – Tp 9.5s). Hydrodynamic pressure distribution on the hull and isolines for the diffracted wave elevation.
These simulations can be incorporated as part of the standard design process. Five hull variants can be compared in a given sea state within 24 hours, giving an appreciable speed/cost advantage compared to model tests.
ULSTEIN PX121 in DP (Dynamic Positioning) in steep stern waves of 2.5m height and 5.5s period. Typical result of the SWENSE method, comparing the total wave elevation with the diffracted wave elevation and highlighting the sheltered zone in front of the bow.
ULSTEIN X-BOW transiting at 9kt in regular head waves of 4m height and 5.5s period.
Comparison of the ULSTEIN X-BOW with a conventional bow. Transit at 14kt in irregular waves (Hs 2.5m – Tp 6.5s).
THRUSTER & HEADBOX ALIGNMENT
Every hydrodynamic detail that might influence the fuel oil consumption or the vessel performance has its importance. Of particular importance is the alignment of the propulsion system with local flow, as this will affect the long term power consumption by up to 4%. An optimization method developed in-house can determine overnight the optimum propulsor orientation for each combination draught – speed.
Streamlines in and around the propeller zone of the ULSTEIN PX121 and their influence on the free surface.
Moonpool operability is a key aspect for offshore operations. Weather, motions or transit speed can cause unsteady effects such as pumping and sloshing inside the moonpool. By using CFD, the free surface motions can reliably be predicted as well as the added drag due to the moonpool itself.
The SWENSE method applied to the ULSTEIN SX174 in DP equipped with a work moonpool and damping grids. Simulation performed in head waves (Hs 3m – Tp 9s). Model scale verifications performed at Marintek.”
Hull-propeller interactions can cause noise and vibrations. The complexity of the CFD models used for such simulations makes the computation time longer, however, once the solution is available, it reveals a lot of information which is hard, or very expensive, to get from model testing. Such “state of the art” calculations are not part of the standard design process yet and are performed occasionally to investigate potential issues if exact propulsor models are available, in collaboration with our valued suppliers.
Accurate wind force predictions are needed to make sure that the vessel meets the DP requirements. To get the wind forces, the superstructure of the vessel is modeled and a typical wind profile is used to ensure realistic results. Several simulations are then made to get the coefficients all around the vessel. This is an area of further study and research.
Successful application of industrialized CFD has enabled early verification of innovative concepts such as ULSTEIN X-STERN™.
Trim and draught optimisation
We offer Trim/Draught Optimisation run in CFD (Computational Fluid Dynamics), that give the customer full scale calculations faster and cheaper than with regular model tests.Trim and draught optimisation