Parent Vessel Design Scaling Case Study

The need

The Irish Naval Service (INS) operates a fleet of eight oceangoing vessels tasked with protecting the fisheries off Ireland's coast. In the early 2000s the INS took delivery of two 80-metre LOA patrol vessels designed by Vard Marine.

While these vessels proved very useful throughout the following decade of use in Ireland’s hostile waters, when the time came to expand their fleet the INS engaged Vard Marine to design a new vessel that would retain the best qualities of the existing vessels while enhancing key seakeeping capabilities.

The Challenges

The sea conditions in the North Atlantic and Irish Sea around Ireland are some of the most dangerous and difficult in the world, especially for vessels that must operate without limitations during search and rescue or similar activities.

The combination of short steep seas with high winds makes it unusually challenging to produce a ship that’s safe to operate, comfortable to serve on and can perform multiple functions. The vessel needs to withstand high beam winds and maintain good roll characteristics for slow-speed over-the-side operations.

This requires contradictory ship motions, i.e. high metacentric height (GM) to withstand beam winds, but lower GM for a slower, more comfortable roll to reduce crew fatigue when carrying out marine rescue operations.

Vard Marine’s solution

We knew the ideal approach would be to scale up the very successful and high performing 80-metre design into a larger more capable platform.

In discussions with the INS we discovered that, overall, the original design was too small for the higher sea states and the ship motion was too quick at slow speed during boat launching activities, which resulted in rapid crew fatigue.

Simply increasing the overall length would not be enough. The Vard Marine team analyzed the sea conditions and operational profile in more detail and determined that modifications to the hull form and the addition of a flume stabilizing tank would offer the best solution. This strategy would allow at-sea adjustments to the vessel motion characteristics to accommodate specific sea conditions and associated activities.

After extensive ship motion analysis and collaboration with the stabilizer tank designer, we established the appropriate range of metacentric height values to withstand high wind and sea states with the option to adjust the GM when working at slower speeds and performing over-the-side boat launching.

The Results

Sea trials on the first vessel in the initial year of operation confirmed the success of the solution. The vessel maintained effective operations in a higher sea state than before, with an advanced level of safety and greatly diminished crew fatigue.

Further confirmation of the vessels’ capabilities came recently when a ship carrying out humanitarian duties in the Mediterranean remained stable with more than 700 migrants on board.