Model tests in our towing tank ensured a safe launch of the most complex AHTS vessels ever launched at Kleven Verft shipyard.

29 September 2016

The 95 m long and 7500 tons heavy Maersk Starfish anchor handling tug supply vessels (AHTS) will provide customers with a reliable and efficient service and will also be able to carry goods to and from offshore facilities and perform oil recovery operations.

The AHTS vessels were the most complex vessels ever launched from the Kleven Verft slipway in terms of forces and safety against tipping. Kleven Verft were concerned about the vessels being damaged in the midship area due to large forces on the hull and tipping over at the end of the slipway.

”We asked FORCE Technology to perform model tests because we wanted to verify our assumptions. Our calculations indicated that a launch without additional buoyancy in the aft part of the vessel wouldn’t be responsible, but it was important for us to see how the dynamic contribution such as speed and inertia would affect the launch. Building and temporarily mounting of additional buoyancy is costly and time consuming so we wanted to avoid it if model tests could assure us that they weren’t needed. The tests actually proved that the tanks would be necessary” explains Chris Remøy, Senior Project Engineer at Kleven Verft.

Kleven Verft contracted us to perform launching tests of the vessels. The purpose was to determine the force exerted on the hull from the end of the slipway as well as the force on the forward poppet under the bulbous bow during the launch of the vessel.

The details are important

We already had a model of the Starfish in a 1:19 scale available from the seakeeping and still-water tests we had performed.

 We built a model of the slipway to match Kleven Verft’s full-scale version using their exact dimensions. The most important features to copy exactly were the slope and the water depth at the end of the slipway.

In order to support the model of the Starfish on the slipway, the model of the slipway was fitted with three longitudinal wooden rails, one in the centre and one on either side of the vessel. To reduce the friction, 10 mm POM plates were mounted under the support rails to be in contact with the slipway. The slipway tracks were mounted with 4 mm thick PEHD plate strips in order to facilitate a good surface with a minimum of friction between the vessel and the slipway.

When launching the model, we were able to measure the vertical forces at the end of the slipway as well as beneath the bulbous bow.

 Two versions of the vessel were tested; one with the original hull and one with blisters. The blisters were mounted in the stern adding buoyancy to the aft part of the ship in order to eliminate the tipping moment and ensure a positive buoyancy until the vessel was fully launched. The blisters turned out to be very efficient.

 “We obtained a lot of information by trying different positions and speeds. But most importantly, we verified how the dynamic forces compared to the static forces we had calculated in advance,” says Chris Remøy.

A successful launch

On June 10, Maersk Supply Service successfully launched the first of the six Starfish vessels from Kleven Verft shipyard in Ulsteinvik, Norway: “The model tests showed us that the forces would be up to 40% larger than at previous launches, but the first launch went exactly as expected after having performed the model tests at FORCE Technology,” concludes Chris.
 
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Model test launch of the Starfish in our towing tank
Successful launching with buoyancy tanks shown. Photo: Kleven Verft.
The first of the six AHTS for Maersk Supply Service launched from slipway at Kleven, Norway summer 2016. Photo: Christoffer Furnes