With a maximum lifting capacity of 1,200 tonnes the BOS 45000 is the biggest crane of the board offshore crane series. The structure of the slewing bearing crane is based on an A-frame design to achieve optimal stability and reliability. The lattice boom construction enables higher outreaches. One of these large sized rope luffing type cranes is operating on board of the Seafox 5. The self-propelled DP2 jack-up vessel, owned by Seafox BV has been modified recently.  

The modification contains a boom extension of almost 20 metres, thus the crane has an optimised outreach of 96 metres in excess. 800 tonnes can be handled at the maximum outreach. With this change the crane is able to install higher wind turbines.  

“The market for wind turbines made the decision to enlarge the boom. It became necessary because of the height of the towers from wind turbines which is continuously increasing. Without this extension we could not execute our current and future projects in the North Sea”, says Alexander Eijgenraam, Rigmanager of Seafox 5. “Depending on the future market we also might look into more possibilities for our crane”, he continues. 

Back in 2012 when the crane has been assembled the first time in Singapore the 18 metres long intermediate boom section was also part of the contract. However, due to the order situation of Seafox at that time, the crane was delivered with a shortened boom.  
 

The current modification was not only a reaction on the changing market where wind turbines are getting taller. It also underlines the vessel owner’s strategy to be flexible in the offshore business and qualifies the 151 metres long and 50 metres wide vessel for a recent customer inquiry. The contract, which started in February 2018 and is now halfway completed, includes the installation of 66 six-megawatt wind turbines at Merkur, a wind farm in the German North Sea. With the modified boom length the vessel is ready to install the next generation of larger wind turbines with mast heights of up to 78 metres.  

However the idea behind this additional intermediate section was to adapt the boom length rapidly and efficiently with a minimum effort. This allows Seafox to either shorten or extend the boom depending on specific project requirements. The adjustment for the upcoming project has been solved by providing a temporary solution.  
 
A benchmark for maritime services 

The planning of this project took a lot of time. Liebherr and Seafox held dialogues for nine months before the realisation started. Both companies jointly developed a cost effective solution. The centrepiece of the approach is a hoisting gear that was especially invented to meet the project related needs of the customer. 

After dismantling the intermediate and head section in November 2017 the boom length had been adapted at DAMEN Verolme Rotterdam shipyard. As part of the work preparation for installing the extended boom the pivot piece was hoisted via bolt flanges. To estimate the maximum forces a study had been launched by Liebherr. According to the results of this study static analysis and simulations were made. Afterwards a basic concept of the hoisting gear was developed.  

Because of the need to be precise, the interaction between the mounting team, including 
Liebherr service engineers and the used support floating crane was very sophisticating.  
 “Fitting in the lengthening of the boom next to the maintenance area of the pivot section of the boom, the exchange of all outside hoses, the weather conditions and the coordination of all parties working on the crane were major challenges during this project. But due to the outstanding effort of Liebherr, EuroRope, de Gier, Damen Verolme, FluiConnecto as well as Seafox and their cooperation, this project has become a success,” summarizes Mr Evert Kistemaker, Seafox project engineer and coordinator. “The execution of the service and the support made by Liebherr has been very  satisfactory and valuable,” he adds. After 12 hours, the assembly of the boom had been done and the crane was ready for its upcoming tasks.