Flow visualisation at the stern of a twin-screw vessel

VCT - Virtual Captive Tests

Manoeuvring performance generally features too late in the design process in SSPA’s experience. Sometimes shipyards have to face costly and time-consuming last-minute changes to the design (when the IMO standards for ship manoeuvrability are not met). And what if the manoeuvring performance is “too good”? Is the rudder too big? (Resulting in resistance that is too high). The use of a new method of CFD calculations enables SSPA to offer early-stage manoeuvring predictions with much higher accuracy than before. By sorting out the manoeuvring issues before too much time has been spent optimising the hull lines, the newbuild project will save time and money in the design stage. SSPA’s experts also believe that this approach will lead to safer, better and more sustainable ship design. Virtual Captive Tests (VCT) is one of SSPA’s “manoeuvring tools”.

The VCT method is briefly explained in this article and discussed in the context of the rest of the tools available for manoeuvring investigation. VCT can be incorporated in various stages of the design process, offering a very cost-efficient method of ensuring good manoeuvring performance with the aim of meeting the International Maritime Organization’s (IMO) standards for ship manoeuvrability.

What is VCT?

Virtual Captive Tests (VCT) are a cost and time-efficient method of investigating manoeuvring performance at an early stage of the design of a vessel. Captive model tests have been used for many years to measure hydrodynamic forces for input into a manoeuvring simulation model.

The VCT manoeuvring tool is a combination of Computational Fluid Dynamics (CFD) calculations (similar to Captive Model Tests) and manoeuvring simulations based on forces from the CFD calculations. You may wonder: why not just use CFD? The answer to this question is that this option is considered too advanced, too time-consuming and too expensive.

However, CFD calculations similar to model tests with a free model would probably give the most accurate prediction, since manoeuvring tests with a free model are the most accurate way of investigating manoeuvring performance. In the meantime, while we are all waiting for faster computers, the VCT method represents a more feasible option.

What does VCT add?

The advantage of CFD compared to physical model testing is a better understanding of the flow around the vessel because the calculations can be paused and the pressures, velocities, etc. can be studied thoroughly. This provides the naval architect with a unique opportunity to improve details based on information that cannot be obtained through physical model testing.

VCT in the design spiral

A general design spiral for manoeuvring was presented in previous issue of Highlights (63/2017). The VCT can be placed in this spiral either to make better Concept simulations or to make better Test-supported simulations.

Manoeuvring performance at speed

Overall manoeuvring performance at speed can be categorised by dynamic stability, turning ability and yaw-checking ability. A ship is dynamically stable on a straight course if it after a small disturbance soon settles on a new straight course without any corrective rudder. Turning ability is the ability to initialise and complete a turning manoeuvre. Yaw checking is the ability to counteract a turn, which is usually characterised by zig-zag tests. Poor
manoeuvring performance is of course a safety risk.

Furthermore, poor manoeuvring characteristics might also have economic implications. Low course stability may lead to excessive use of the rudder to maintain a straight course, which in turn will decrease ship speed or increase fuel consumption.

Accuracy

CFD calculations for manoeuvring seems to be a rapidly evolving field at the moment. SSPA believes that the calculations still need to be benchmarked to a database of already tested vessels and validated using model tests to ensure accuracy. SSPA’s facilities, especially the Maritime Dynamics Laboratory (MDL),
have very good capabilities for this validation. The capability to run all possible captive tests, both rotational and sinus tests, and to run tests using a free model, offers more or less all pieces of the puzzle.

The continuous improvement of naval architectural methods is vital for the development of more efficient and sustainable vessel designs. The VCT method is taking us forward, with the hope that ships will be safer and greener in the future.

Simulations

By simulations we mean simulations in SSPA software SEAMAN, which is an implementation of Nils Norrbin’s slowmotion derivatives model. This approach does not attempt to model the physics of the complicated flow over the manoeuvring hull. It is simply assumed that the forces are dependent upon the motion variables, their time derivatives, hull geometry and the rudder angle. The Taylor series expansion of forces is used.

Captive model tests

The ship model is held captive, which means that it is not free to move. Instead the model is forced to move in simple, predefined motions and resulting hydrodynamic forces are measured by a captive balance. The results from captive tests can be used to derive a mathematical model that can be used in simulations. These types of tests are also known as Planar Motion Mechanism (PMM) tests.

Model tests with a free model

Model tests with a free model are the complete opposite of captive model tests. Here the model is free to move in all directions. Forces and moments are applied to the model using propellers, and rudders and the resulting motions are measured. Model tests with a free model in the Maritime Dynamics Laboratory (MDL) are the most accurate manoeuvring test that SSPA can offer.

SSPA manoeuvring database

Manoeuvring model tests with a free model conducted over the years in the Maritime Dynamics Laboratory (MDL) have been stored in a database. Regression on this database can be used to develop generic simulation models in ShipGen.

Illustrations

Flow visualisation at the stern of a twin-screw vessel with operating propellers and large rudder angles. Pressure distribution on the hull and appendages streamlines passing through the propeller disc, and axial velocities around the rudders can be observed.

General design spiral for manoeuvring. The idea is to start the design process in the outer parts of this spiral and work our way into the centre, where various design decisions have converged into the final design. We have divided our spiral into four quadrants: manoeuvring, seakeeping, calm water and cavitation. MDL, Maritime Dynamics Laboratory, SSPA’s basin for manoeuvring and seakeeping. TT, Towing Tank, SSPA´s towing tank.