Tools for investigating manoeuvring performance
All merchant ships longer than 100 metres must fulfil the International Maritime Organization’s (IMO) standards for ship manoeuvrability. The best way to validate the criteria is model tests. Running model tests will avoid the risk of very unpleasant surprises during full-scale sea trials. The manoeuvring model tests are usually performed late in the design process. If the requirements are not fulfilled at this time, undesired design changes may be needed at the very end of a project. This article shows how SSPA’s “manoeuvring tools” can fit the design spiral for new-build merchant ships and thus avoid costly last-minute changes. The tools have been identified by looking at numerous merchant newbuild projects run at SSPA during the past few years. The tools are defined and evaluated in terms of accuracy and cost.
Manoeuvring performance at speed
Overall manoeuvring performance at speed can be categorised by dynamic stability, turning ability, yaw-checking ability and stopping ability. A ship is dynamically stable on a straight course if, after a small disturbance, it soon settles on a new, straight course without any corrective rudder. Turning ability is the ability to initiate 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 rudder to maintain a straight course, which in turn will decrease ship speed or increase fuel consumption.
The IMO (International Maritime Organization) has created the IMO standards for ship manoeuvrability (IMO Criteria), which is a set of tests, each with certain criteria, that should ensure that ships are safe in terms of manoeuvrability. All ships longer than 100 metres have to fulfil the IMO Criteria. The ships have to perform turning-circle tests, zig-zag tests and stopping tests at a certain manoeuvring speed. Some parameters from these tests, such as advance and overshoot angles, have to be within certain limits.
Some operators define criteria for berthing. The ship should typically be able to achieve a limited transverse speed in some predefined wind and wave condition.
Tools in the design spiral
We have visualised the “manoeuvring tools” by putting them in the context of a design spiral (see figure). The “manoeuvring tools” have been put in quadrant 1. There are obviously other tools in the other quadrants for seakeeping, calm water and cavitation. They are not discussed in this article but will be addressed in future issues of SSPA Highlights.
We usually try to find a programme for investigating manoeuvring performance that best fits the customer’s needs. The challenge is to find the most cost-effective solution that will generate the best ship design. Some projects may require all the tools in the spiral, while some may require only one or two of them, depending on the complexity of the project. The design process includes a number of “tollgates”, where the design has to fulfil certain requirements in order to proceed in the spiral. If a “tollgate” is failed, we can give advice on design changes. In the case of manoeuvring performance, it might be that the rudder and/or the hull have to be redesigned or a fin has to be fitted to the hull.
The idea behind the design spiral is to avoid late and expensive redesigning by starting with less expensive tools with lower accuracy and adding tools with higher accuracy for each lap we complete in the spiral. All quadrants have to be considered for each lap in the spiral. Output from the manoeuvring quadrant may for instance lead to changed rudder design, which will be input into the following calm water resistance quadrant and so on.
1. Concept simulations (IMO Criteria)
As a first step we can offer simulations of manoeuvres to investigate IMO Criteria. A simulation model can be developed at an early design stage, based on regression of model tests with similar vessels previously tested in the MDL (Maritime Dynamics Laboratory, SSPA’s basin for manoeuvring and seakeeping). These simulations will give a rough estimation; the accuracy is not good enough to validate IMO Criteria. The result will give advice on design changes or guidance on cost-effective solutions to verify needed manoeuvring requirements.
2. Berthing simulations (Berthing Criteria)
Simulations of berthing is a good idea for ships where berthing without tug assistance is a requirement. The choice of total control system, propellers, rudders and tunnel thrusters, can be studied and evaluated.
3. Test-supported simulations (IMO Criteria)
Test-supported simulations is an intermediate step between points 1 and 4. This tool is a combination of a limited captive model test programme in the SSPA Towing Tank and simulations. This is a very cost efficient solution since an existing towing tank model can be reused. The tool will assess the risk of the ship design later failing the IMO Criteria validation (point 4).
4. Model tests (IMO Criteria)
Model tests with a free model are the most accurate way of investigating manoeuvring, which is why it can be used as the final validation of IMO criteria. At SSPA, this is performed in the MDL (Maritime Dynamics Laboratory). These tests are performed with a smaller scale model than in the Towing Tank, which means that one additional model (besides the towing tank model) needs to be manufactured. These tests are therefore a bit more expensive, but can also be considered as a very cheap insurance against later failing the fullscale sea trials. Using some simpler methods to investigate manoeuvring prior to these tests (for instance, points 1 and 3) is advised, when the risk of failing the IMO criteria is high.
All merchant ships longer than 100 metres must fulfil the IMO standards for ship manoeuvrability (IMO Criteria). The most accurate way to validate the criteria is model tests with a free model. Often these tests are performed late in the design process. Hence, if the requirements are not fulfilled at this time, undesired design changes may be needed at the very end of a project. A better way to address the issue of ensuring good manoeuvring performance is to perform preliminary manoeuvring assessments early in the design process (as suggested in our spiral). By performing lower accuracy analyses at the beginning of and throughout a project, the risk of failing an important “tollgate” late in the project is significantly reduced.
Another question to consider when thinking about the design spiral: in which quadrant should the first iteration be started? Traditionally most model test projects at SSPA start off with model tests or CFD calculations in calm water (quadrant 3). Is this the best way? (There are both pros and cons to this solution.) We are happy to discuss this kind of question on receipt of your request to find the best solution to your problem.
By simulations, we mean simulations in the SSPA software SEAMAN, which is an implementation of Nils Norrbin’s slow motion derivatives model.
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.
Model tests with a free model
A model test with a free model is the complete opposite situation to a captive-model test. Here the model is free to move in all directions. Forces and moments are applied to the model by propellers and rudders, and the resulting motions are measured. Model tests with a free model in the MDL are the most accurate manoeuvring tests that SSPA can offer.
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.