In Depth: Conwenna

KOHBEHHA

When heavy transport is to be shipped over water in Eastern Europe, Silverburn Shipping Group, operating under the name Morwenna, is one of the companies specialised in it.

The company owns a fleet of barges, tugs, accommodation vessels and tankers, all geared for the specific environment ranging from the Caspian Sea all the way to Northern Europe and Russia.

The working area is characterised by an extreme climate — ice in winter and sizzling heat in summer — shallow draught and limited waterways. There is no way to get there with a typical heavy lift vessel, due to both draught and airdraught constraints, so the company
predominantly uses its own barges and pusher tugs. Occasionally, they charter other vessels.

Design brief

Ship owner Morwenna came to Thecla Bodewes Shipyards with a concept design, based on their experience and their requirements for their three new vessels. The four characteristics that best define these vessels are power, versatility, shallow draught and operational readiness. The main task will be the transportation of barges with heavy cargoes on the Russian rivers and surrounding seas, all the way from the Baltic and the White (Barents) Sea in the North to the Caspian Sea in the South.

When transporting a barge on a river, it is done by pushing, and the barge is coupled to the tug. This is not possible on the sea, due to the strains on the connection in waves, and that’s when the tug is used to pull the cargo with a towing wire. Conwenna and her sister vessels Wenna and Trewenna excel at both.

Bollard pull

With a specified set of main engines and a minimal required bollard pull of 35 tonnes, it was up to the shipyard to make it happen, within the shallow draught of 2.3 metres light loaded. They succeeded: the vessel achieved a bollard pull of 39 tonnes.

The solution was found in a very flat and shallow aft ship – ensuring enough water can be drawn by the propellers – and fixed pitch propellers with nozzles. SIP Marine supplied the propellers and nozzles and carried out a CFD analysis of the hull to determine the optimum propellers.

The propeller nozzles are not welded onto the hull but are rotatable to an angle of 35 degrees either side. Inboard, there is no difference with a regular steering gear, driven by hydraulic cylinders. Welded onto the rear side of each nozzle is a small fixed rudder flap, which in addition to creating lift, also keeps the largest blocks of ice out of the propellers when going astern.

The solution is clean and simple. As each nozzle can be steered individually, this provides great handling characteristics to those who know how to wield the controls. A conventional tunnel bow thruster of 200 kW helps manoeuvring at low speed. The electrical system was installed by eL-Tec (Hattem), and derives its power from two Cummins QSM-11 generator sets, rated at 248 ekW each.

 “These tugs have an asset that distinguishes them from competitors in the same market. At the stern, there are two stern anchors, which are not required by regulations, but they allow the ship and coupled barge to be easily anchored in a river with current from the back, without the need to uncouple or rotate,” Project Engineer Arthur Willebrandts from TB Shipyards said.

Gap analysis

The original concept was a non-SOLAS design which had to be upgraded to a SOLAS vessel for worldwide operations. The yard wished to build under Bureau Veritas class and Dutch flag, while the owner wanted Russian Register class and the flag of Kazachstan.

A gap analysis was made to see where the rules differed and wherever there was a discrepancy, the most stringent rule was applied. Right after delivery of each of the vessel, they are reclassed and reflagged without a glitch.

Something which had a bigger impact on the design is the Ice Class rating IC. Although a light rating, meaning that the vessel can safely sail in a channel of broken ice of 0.6 metres thickness, it has an impact on engine power, gearbox certification and ice belt strengthening in the hull.

The propeller shafts are lubricated with an open-loop water system from Maprom. To ensure a constant supply of fresh water, even in ice, the vessel has an ice tank on each side. This is essentially a sea chest with heating elements in it. The cooling of the main engines and generators is entirely closed-loop with boxcoolers in the sides.

Supply

In addition to her pushing and pulling qualities, Conwenna is also a full-fledged anchor-handler and supply vessel. For her supply duties, she is equipped with a large aft deck with container fittings for nine 10foot containers, a 3,150 kg SWL deck crane, and ample capacity for fuel, fresh water and sewage.

Anchor handling is done with the towing winch and a wide stern roller at the transom. Two towing winches are mounted in a cascade arrangement, ensuring that there’s always a spare cable and winch. Each drum has the capacity for 750 metres of wires and a holding power of 40 tonnes. In the aft deck, retractable towing pins and a sharkjaw are mounted from C-Nautical. They consist of a vertical part and an eccentric top on each pin, which can be rotated to close the top.

Wheelhouse

A wheelhouse on a hydraulic pedestal is nothing new. The challenge for TB Shipyards’ designers was to build one which would be solid enough to withstand the rolling and pitching motions at sea.

It’s a bit of a theoretical discussion, as the vessel will normally be towing at sea, with the wheelhouse down, but nevertheless, they succeeded and Conwenna is one of the few vessels in the world that can sail in waves with the wheelhouse raised.

The solution was found in the use of two concentric square tubes with enough overlap to transfer the forces in case of heel. The accommodation has a HVAC system, ensuring that climate conditions are workable inside even when it is 40 degrees outside. For heating in winter, there is a central heating system with an oil-fired boiler. Conwenna has a total of 10 cabins for its crew of 12 and has a capacity for an additional 8 people.

Side launch

The launch of Wenna (the first in this series) was the first side launch since the former Peters Shipyard in Kampen was taken over by Thecla Bodewes Shipyards in 2015.

For smaller vessels, the shipyard also has a lift, but a side launch is a more spectator-worthy event than a slow and safe lift launch. Each vessel was sailed to the TB Shipyards location in Harlingen for the finishing touch and the sea trial program. Before the Volga freezes over for the winter months, TB Shipyards will have delivered the third and last vessel in this series. At the same time, the yard is finishing a fishing vessel and is starting up a new passenger ferry project.

Bruno Bouckaert


This article was originally published in the fourth edition of Maritime Holland 2018.

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