Ship operability analysis - ensuring passenger safety and comfort
Passenger and crew safety are primary concerns in modern shipbuilding. In addition to continued strong cruise industry growth, shipowners pay special attention to passenger and crew comfort, enjoyment on-board and smooth operation. This places limits on ship behaviour at sea and on permissible vibration and acceleration levels in the most critical parts of the ship. Sophisticated operability studies for specific routes and navigational parameters can provide a good picture of the expected level of passenger/crew comfort in specific operational conditions.
Ship operability analysis should be performed to estimate seakeeping performance as an essential criterion for overall passenger comfort and/or crew operability in specific weather conditions.
Ship operability factors and limiting criteria
Ship operability is estimated by using known wave statistics, routes and ship performance. It is of great interest to operators and designers alike to know the percentage of time a vessel’s motions or accelerations remain within permissible levels in a given geographical area.
In practice this means calculating the probability of not exceeding given criteria at defined positions on-board for each leg of the ship route, and combining wind and wave conditions (based on statistical sea scatter diagrams) with the ship course, hull form, cruising speed and behavioural inputs. The impact of stabilizing devices, such as fin stabilizers, is also taken into account.
As indicated above, operability shows the probability of not exceeding limiting criteria, in other words, the probability of seakeeping not inducing seasickness, passenger discomfort or insecurity, or negatively affecting normal crew operation under specific conditions. For operators, operability is an indication of future cash flow and can assists them in making investment decisions.
Limiting operability criteria can be based on acceleration, motion and the probability of extreme events: slamming, deck wetness, helicopter operations, environmental forces on the structure, propeller emergence, etc.
Computing the operability index
In order to compute the operability index as a percentage of time a vessel can carry-out normal working conditions, knowledge of the vessel’s motion characteristics, a set of operability criteria and a set of environmental conditions are essential. Thus, the index is computed by evaluating vessel motions against a set of criteria for all sea states in a scatter diagram.
Floating structure behaviour in different wave conditions
There are multiple methods available to analyse the behaviour of floating structures in different wave conditions. These methods include strip theory, Computational Fluid Dynamics (CFD) and basin model testing.
In strip theory the ship is divided into 20 to 30 cross sections for which the two-dimensional hydromechanics coefficients and existing wave loads are calculated. In order to obtain three-dimensional values, these values are integrated numerically over the ship length. Finally, the differential equations are solved to obtain motion values. These calculations are performed in the frequency domain.
CFD is modern tool for ship motion calculations. It enables complicated and detailed simulations if sufficient computational resources are available. The simulations can include, e.g., the estimation of accelerations or forces experienced by the ship in ensuring the comfort of passengers, if the mass and inertia moments of the ship are known. In addition, the wave motion and the ship’s rolling motion can be simulated.
With the help of CFD analysis it can also be ascertained how much the ship’s wing stabilizer reduces the ship’s motion. For smaller vessels, for which the effect of the waves is more significant, the trim angle can be simulated. This allows better prevention of vertical swinging. It is similarly possible to analyse green water on decks or structures.
Sea condition data (also for individual routes) is specified according to custom spectra taken from wave height measurements, or by Pierson-Moskowitz, JONSWAP, Bret-Schneider or ITTC spectra for all headings.
Developing a solution for operability calculation
To calculate precise operability estimations and conduct accurate analyses a large number of parameters and factors that influence ship navigation need to be taken into account.
At Elomatic we set our sights on developing a solution for operability calculation for particular ship parts and locations on board. The main input of the analysis is RMS accelerations in three directions (x, y, z), which can be calculated using one of the aforementioned methods. Ship speed and heading angles have a great impact on these values, and so do stabilizing devices; they can be activated to improve overall operability.
It is important to consider and analyse the heading which causes the highest accelerations. For greater precision, ship routes (from port to port) are divided into small segments (legs) where the ship course and speed are constant. The calculations must be made for each leg of operability to get the final average value for the complete route.
Depending on the purpose of the operability analysis, various criteria for permissible acceleration may be used: seasickness, MSI – motion sickness incidence, MII – motion induced interruption, etc.
The availability of wave measurements has introduced so-called scatter diagrams, which reflect joint statistics about significant wave heights and average zero-upcrossing periods. Global wave statistics by BMT provide a practical basis for the design of ships. Calculations based on scatter diagrams for every season can also be included in the final results. This provides the shipowner with the option of analysing seasonal impacts and differences during ship operation all year round.
Elomatic engineers have developed a software program (ELOPER) that takes all the above-mentioned parameters into consideration and calculates operability (in %) and down-time periods. The software inputs can be varied: limiting criteria, speed, ship course, route length, scatter diagrams.
Ensuring comfort on RCCL “North Star” gondola ride
A recent project where these techniques were applied is the Royal Caribbean Cruise Lines’ Quantum and An-them of the Seas. The cruise vessels are equipped with the “North Star”, a “gondola” or “platform” suspended from a “crane” on the top deck, which is designed to offer a spectacular view of the ship and her surroundings. The ships will be built by Meyer Werft shipyard in Germany.
Apart from accelerations at the centre of gravity of the ship, local (longitudinal, transversal, vertical) accelerations were calculated for specific gondola positions. Irregular wave conditions and those of the JONSWAP wave spectrum were used in the calculations. Scatter diagrams for certain global areas were also employed.
Elomatic was tasked with reviewing current research and results and, in particular, with analysing the limiting acceleration criteria and proposing limits for maximum passenger comfort during the gondola ride.
The seasonal operability was calculated for each segment of the ship route and presented in easy-to-read graphics. Special attention was paid to gondola down-time and analysis of the system when in use.
The ELOPER software was a useful tool on the project, especially to vary and check input parameters with the customer. It was also used to emphasize the most critical ship headings and positions with the maximum generated acceleration.
The final results were included in a forecast booklet for all year round cruising and comfortable gondola rides.
Authors: Ivan Maksić, Patrik Rautaheimo
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