Submerged ingenuity

Designing ballast procedure – to float 255 tons SPM buoy onto barge

Sometimes an unexpected call can bring exciting adventures along – this was more or less what happened when SAL Engineering was contracted to prepare a complex loading of an SPM Buoy in Nigeria, West Africa.

A call on a Saturday morning from one of SAL’s long established clients in Nigeria, Wellman Group, urgently stating their need for SAL Engineering’s support to calculate and design a complex ballasting procedure to load a SPM buoy onto a barge, resulted in swift process, from contract signing, to design develop and on-site execution in only ten days. In contrast to what the SAL engineers normally deal with, this was not a lifting procedure, but a floating on / floating off scope that had to be dealt with.

The project centres around one 255 tons SPM buoy normally used for FPSO/FSO mooring at sea that was subject for refurbishment after years in service and to be exchanged with a reconditioned one. The buoy had been towed to Port Harcourt, a remote port up a river delta in the south-eastern part of the country.

To get the buoy to shore for refurbishment and the refurbished buoy to water required it to be floated onto an 82 metre self-ballastable barge and thereafter skidded from barge to shore and vice versa. The scope for SAL Engineering was to calculate, design and supervise the ballasting procedure for the barge, which had to be in part submerged to allow the buoy to be floated onto it.

Chris Lyovwaye, Managing Director WELLMANN Group states: “We made our own initial engineering planning, but I was not convinced that we would be able to execute the operations safely. I contacted SAL Engineering and asked for an initial assessment, and after assigning SAL to conduct the detailed engineering design a new light was brought to the project.”

hydrostatic stability calculations – and more

Essentially the aft section of the barge was submerged some 5.5 metres below surface by having 6 800 tons of water taken into its ballast tanks. It required complex computing to specify the sequence of the intake of ballast water into the barge’s 15 ballast tanks. Not only hydrostatic stability calculations had to be taken into consideration, but also attention to structural forces as there was risk of the barge bending under the massive forces of water and the buoy once submerged and upon re-floating. Lastly a special concern to avoid having the barge to capsize was also part of the complex ballast design.

SAL Engineering delivered a 132-page ballasting procedure document and had special engineer Philipp Ludewigs on site in Port Harcourt for ten days for supervision and on-site support. Philipp Ludewigs concludes: “It was a special operation with a very short lead time, but also an exciting project which was different to what we normally deal with, but states the bandwidth of the services that we can offer on an independent basis.”