Cracks in weld seams of Jack-up r i g s l e g s o u t o f X 1 0 0 Q / S 6 9 0 L H H O a n d potential to solve the issue with a new alloying concept .
In the last decade the usage of h i g h e r s t r e n g t h s t e e l e s p e c i a l l y o f X100 / S690QLHHO became usual for the construction of Jack-up rig ( JUR ) and wind turbine installation vessel ( WTIV ) legs as well as of lec encircling cranes ( LEC ). Shipyards established their welding procedure qualification tests ( WPQT ) and built plenty of JUR which showed later in service several cracks at the legs . Means rigs designed for more than 20 years lifetime need repair after the second or third inspection interval of the high strength welded nodes .
The impact of cooling time on material toughness and strength is mandatory to achieve the required mechanical weld properties . The cooling time in the interval of 800 down to 500 ° C is in which the most important microstructural changes occur in the high strength structural steel . This interval is limited by the upper and lower boarder . Exceeding the lower boarder – too fast cooling – leads to a too hard heat affected zone which will increase the risk for hardness cracks . Exceeding the upper boarder – too slow cooling – leads to a very high transition temperature which will end in lower material toughness . The suitable mechanical tests for a proper characterization are hardness imprints and charpy v-notch impact tests or CTOD ´ s . The range between the two boarders are the socalled welding parameter window . Today , weldments will be tested for a welding procedure qualification test ( WPQT ) in accordance to a fixed procedure of the codes . The mechanical tests and its location do not take the welding parameters into account - means test location are not necessarily in areas of the most stringent weld parameters .
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A visualization of a WPQT with 100mm thickness has shown , that the areas of extreme high cooling times haven ´ t been covered by the cvn test specimen and hardness imprints . It can be assumed , that at these locations the weld reliability is not given and that these areas have a reduced crack arrest resistance .
The most important preventive action to avoid such behavior is to put more attention to the applied welding parameter window and / or use a more resilient alloying system . Vallourec developed a bainitic X100 which allows a wider welding parameter range by overmatching the minimum requirements by class . This alloy with the brand name SuperOceanFIT is approved by ABS ( EQ70 ) and DNV ( NV EO690QT ) and even down to -60 ° C ( NV FO690QT ).
Jacking Solutions
The goal of our presentation was to show that the jacking system can deeply impact , in term of cost and duration , a reactivation project . The owners don ' t always realize how deeply the condition of the jacking system may impact their project .
On several recent cases , we have inspected jacking systems ( supposed to be well maintained ) and we have discovered damaged equipment ' s ( mechanical or electrical ). Such damages require urgent replacement and are postponing the availability of the rigs .
The industry is facing a shortage of spare parts due to the worldwide uncertainties . Some parts have a lead time of 5 to 10 months . In the meantime , prices are also very high .
Availability of parts and prices are directly affecting the project . This needs to be considered when starting a reactivation project .