DNV GL’s Jan Rüde, Ship Type Expert MPV, discusses the challenges that vessels face when ports, either with high tidal range or ports in river estuaries, fall dry at low tide, which results to costly ship bottom deformation if the ground is uneven.
Specifically, during the construction of a vessel, if it is known that it will call such ports – as stated above – bottom damage is avoidable by designing the structure of the vessel ideally. Mr Rüde explains that from an economic perspective is it preferable to reduce the payload to prevent the possibility of falling dry or risking costly repairs.
Yet, another issue is whether the bottom of the port is level enough to enable the ship rest on it evenly.
BIMCO’s charter clause “Not always afloat but safe aground” (NAABSA) states that
a ship is allowed to moor at locations where the bottom of the harbour basin is safe (flat, homogenous and free of hard points) enough for the vessel to fall dry without suffering damage.
Although, except BIMCO’s charter party contract, the requirements for NAABSA operations are stated in several building specifications; However, Mr Rüde addresses that many operators are interested in seeing more specific provisions.
Similar to BIMCO, DNV GL issued a class notation in July 2019, providing information on the technical requirements concerning the reinforcement and structural integrity of ship bottoms, in particular the shell bottom, stiffeners, floors and longitudinal girders. Even ships not built according to the DNV GL rules can be checked to determine their fitness for the new NAABSA notation.
Moreover, the pressure a ship’s bottom structure should withstand depends on many factors; For instance, the vessel falls completely dry while touching the ground only with the flat portion of its bottom.
The distance between longitudinal girders on the ship bottom must not exceed 4.5 metres, and ship length is limited to 90 metres.
Yet, the calculations can be adapted to the individual ship as required.
In addition, Huib van der Pas, construction specialist at Groot Ship Design stated that implementing the new NAABSA rules in a newbuilding project requires knowledge but is generally not too complicated. He added that you set the customary rules, but substitute the pressure you normally use under the standard rules with the pressure to be taken into account for NAABSA.
This pressure value is easy to calculate, with only the displacement at full draft and the area of the flat bottom of the vessel needed as input quantities. Divide one by the other, apply a safety factor, and you are done.
The new rules specify structural integrity requirements concerning a ship’s hull, allowing owners to obtain the NAABSA class notation without having to meet a costly and impractical docking requirement after the first NAABSA operation. However, falling dry impacts a vessel’s ability to fight fires and cool engines, whereas equipment such as thrusters, propellers and rudder can be affected as well.
