MARIN (Maritime Research Institute Netherlands) lashing@sea project.
A monitoring campaign of five ships in operation, model tests of secured cargo and an extensive survey asking crew for their input was conducted in the container, ro-ro and heavy lift sectors. A consortium of 24 participants representing flag states, classification societies, shipowners and lashing equipment manufacturers, as well as crew from nearly 160 vessels, took part in the project that was sponsored by the Dutch government.
The Dutch government, together with British and Swedish maritime administrations, is set to make several recommendations to the International Maritime Organisation (IMO) and the International Association of Classification Societies (IACS) to improve safety levels and operational efficiency. The industry-wide project was led by the Maritime Research Institute of the Netherlands (MARIN) and the project was named lashing@sea project.
Although the report is from 2010 most conditions and data are still valid, except for the present day mega container ships.
Monitoring Campaign on Five Vessels
To examine cargo-lashing security in more depth, the group instrumented five vessels; two containerships, two ro-ro vessels and a heavylift ship. Results from the ro-ro and heavylift ships correlated with the design assumptions, but the containership data revealed effects that called for attention.
Container Vessels
MARIN measured accelerations along the entire hull of two container vessels, one operating in the North Pacific and one between North Europe and the Far East, for 18 months and three years respectively. Containers in the bottom and higher in the stack were also instrumented. In addition, a model test campaign was performed examining containers stacked in three rows next to each other.
Findings Monitoring Campaign
The project found that accelerations on a ship can be amplified by 50% because of the ship’s hull flexing. Both the encountered weather and the measured values were still inside design limits, however, the impact of unexpected, impulsive loads by wave slamming was clear.
Container stack dynamics were also examined by model testing with multiple rows of containers. Initially, all of the rows moved from port to starboard within expectations. However, there was a severe change when one or two of the rows were destabilised by adding weight and loosening lashing. The effect on the properly secured row was dramatic, with loads increasing up to 200%.
Although the tested configuration was “out of design”, this illustrates how loads in properly secured rows are sensitive to factors such as weight and lashing integrity and the consequential impact this has on neighbouring rows. Extra loads, in combination with other unfavourable factors, could potentially trigger a collapse of containers or securing. The probability of such events could not be evaluated within the scope of the project.
Feedback from crew:
Interviews with crew found that some 50% of those that responded said it was difficult to judge the force of developing wave and cargo loads on the very large container vessels from the bridge. This makes it impossible to evaluate whether loads remain in safe limits and when preventative action to avoid damage needs to take place.
Onboard guidance:
Onboard guidance should be provided to assist crew to identify potential problems such as recognising developing hazardous situations, how to avoid extreme rolling and slamming and how to handle extreme GM in partial load condition.
Page 36 of the report:
The effect of a wave slam is experienced in the aft ship as well. The impact passes through the hull as a travelling wave and when reaching the light and flexible aft structure results in a peak acceleration of similar amplitude as the original slam in the bow.
