TS-101
Seakeeping Analysis in the Design of Passenger Ships
Mario U. DOGLIANI *
ABSTRACT
In the near past there has been a growing interest for fast ships resulting in a number of large high speed craft (HSC) entering in operations. As per today, HSC having length between perpendiculars in excess of 120 m travelling at 42 kn are state of the art.
More recently, the shipping community has heavily invested in fast ro-ro passenger ferries which presents speeds up to 30 kn. Both ship types present the designer with problems from the seakeeping standpoint due to the need to keep the weight of the hull as low as reasonable practicable while granting safety and comfort on board. As a result, seaworthiness is an important design feature which calls for significant efforts,
In this paper RINA's R&D efforts in this field are summarised and the results of two practical cases are presented.
Key Words: seakeeping, slamming, High Speed Craft, Comfort, hydrodynamics
1. INTRODUCTION
In the near past there has been a growing interest for fast ships resulting in a number of high speed craft (HSC) of steadily growing dimensions entering in operations. As per today, HSC having length between perpendiculars in excess of 120 m travelling at 42 kn are state of the art.
More recently, the shipping conununity has heavily invested in fast ro-ro passenger ferries which, although not matching the HSC definition, are considerably faster than traditional ro-ro passenger ferries: speeds up to 30 kn are normally considered and 35 kn is now a short term target.
Both ship types, due to their novelty, present the designer with several problems from the seakeeping standpoint due to the need to keep the weight of the hull as low as reasonable practicable as well as to meet passengers' expectations in term of comfort. As a result, the ship seaworthiness is an important design feature which calls for significant efforts.
In this respect, the ability to quickly but accurately predict wave induced ship motions and loads are of the utmost importance, in particular in order to extrapolate the results of model tests canied out in the early design stage. For this reason, strip theory would be the most appropriate tool as it is rather well known and documented.
From a theoretical point of view, however, the use of strip theory for fast ships and severe sea states is questionable. While the latter problem can be solved by using a non linear strip theory [1, 2], the former is inherently connected with the forward speed approximation of the theory.
Nevertheless, comparisons between strip theory and full scale measurements, recently carried out [3] showed a good agreement in particular for hull girder loads and accelerations.
Aim of this paper is to illustrate the results of R&D investigations canicd out and ongoing at RlNA on this subject as well as to summarise the theoretical background and the results of two cases of practical application, namely:
- the determination of bow flare slamming design pressures for a cruise ship of the last generation;
- the definition of limiting sea states of an high speed craft of about 130 meters length.
2. BOW FLARE SLAMMING PRESSURES FOR CRUISE SHIPS
2.1 General
Due to their large bow flare, fast ships such as containerships, cruise ships and fast ro-ro ferries are subject to significant bow flare slamrning which results in considerable wave induced pressures on the forward part of the ship hull.
As a consequence, slammilng pressure is the design parameter for the plates and stiffeners of the bow region; this is the reason why slamming pressures are specified in class rules. However, due to the complexity of the physical phenomenon, rule book rules formulas may be not sufficiently accurate to cope with the structural optimisation need of cruise ships and model tests are often carried out for the purpose of detennining the extent and intensity of the slamming pressure.
Two main problems arise when using slamming model test results, namely the extrapolation to full scale and the extrapolation to the design value i.e. the highest value in the ship lifetime. These two aspects have been specially considered in the study.
