August 4, 2010

Seakeeping performance in a Seastead

   Seakeeping performance becomes of increasing importance when designing floating structures: regulatory bodies and operators are becoming increasingly aware of the importance of specifying seakeeping requirements which the vessel or marine structure must meet. When designing a seastead, this analysis becomes of paramount importance.
   The seakeeping performance procedure is based upon the probability of exceeding specified ship motions in a sea environment particular to the vessel’s mission. Therefore, the seakeeping analysis is essentially a three part problem:
  1. Prediction of the ship response motion characteristics of the vessel. It is defined through the Response Amplitude Operators (RAO) of the vessel.
  2. Estimation of the likely sea environmental conditions to be encountered by the vessel. It is defined through the wave spectra of a chosen location.
  3. Specification of the limiting criteria used to assess the vessel’s seakeeping behaviour. This also defines the way in which the performance of different vessels is compared related to vessel’s mission.
   Point 1 and 2 should be combined to obtain the response of the vessel and then, compared with the point 3, the limiting criteria of the vessel mission. But, which is that limit in a seastead? Which is the mission of a seastead?

Mission of a seastead

   The mission in a seastead could be assumed to be the same as in a passenger vessel: to offer more comfortable conditions to people living on board. But the estimated seakeeping performance of a passenger vessel greatly depends on the level of limiting value selected as the seakeeping criteria.
   An important key to the acceptability of a passenger vessel is the ride comfort expressed as a low percentage of passengers getting seasick in rough seas. Vertical and lateral accelerations are mainly responsible for seasickness. ISO 2631 provides ‘severe discomfort boundaries’ as a function of frequency and exposure time, as explained and applied in the design of the Clubstead. The table below presents a tentative scale for vertical acceleration, which may be used for estimating the maximum acceptable magnitude for different activities on board and for the comfort of the crew and the passengers.

Vertical RMS

acceleration (g)


Passengers on a cruise liner. Older people. Close to the lower threshold below which vomiting is unlikely
Passengers on a ferry. The international ISO-2631 standard for 2 h exposure period. Causes symptoms of motion sickness in approximately 10% of unacclimated adults
Intellectual work by people reasonably well adapted to ship motions (i.e. scientific personnel on ocean research vessels). Cognitive/manual work of a more demanding nature. Tolerable in the long term for the crew. The international standard for half an hour exposure period
Heavy manual labor by people adapted to ship motions, i.e. on fishing vessels and supply ships
Light manual labor by people adapted to ship motions. Not tolerable for longer periods. Quickly causes fatigue
Simple light work. Most of the attention must be devoted to keeping balance. Tolerable only for short periods on high-speed craft

   When designing the Clubstead, the value for the limiting criteria for vertical acceleration was set to 0.020g (0,20 m/s2). This is the same comfort level as in any cruise vessel as shown in the previous table. Therefore, this value could be adequate criteria to assess the performance of any seastead.

Any floating seastead should be assessed through a seakeeping analysis with vertical acceleration 0,20 m/s2 as the limiting criteria. The seastead response gives the vertical acceleration and is obtained with two parameters:
    • RAOs of the vessel/structure.
    • Wave spectra of the ocean location.

    Ref. 1: Couser, Patrick. Seakeeping analysis for preliminary design. Fremantle, Australia: Formation Design Systems, 2009.

    Ref. 2: Sario¨z, Kadir; Narli, Ebru. Effect of criteria on seakeeping performance assessment. Faculty of Naval Architecture and Ocean Engineering, Istanbul Technical University. Ocean Engineering. February 2005.