Structural Engineering Cases

Derrick

General information

The derrick is supported on the main deck of a semi-submersible platform. The derrick is 64 meter (210 feet) high. The base dimensions are 14 x 15.90 meters. The derrick is cladded around the fingerboard area and fully equipped with associated equipment, which is installed on a semisubmersible drilling unit designed for high-efficiency deepwater drilling operations. A drill pipe heave compensator is located on top of the derrick.

Global analysis

Conclusion

The derrick was checked according to NS 3472, critical members has also been checked on the basis of AISC. The environmental loads (wind and rig) has been reduced to 75%, when calculating according to AISC, this has significantly reduced the utilization of the members compared to NS 3472.
Governing load for most of the members is survival condition (horizontal rig load is the main component), but some members in the top region have max. utilization due to hook load and operational loads.

Codes

1.	Wind shape factor for Derrick is calculated on the basis of – DNV – Environmental Conditions and Environmental Loads, March 2000.
2.	Global analysis, staad model code check is based on - NS3472 Prosjektering av stalkonstruksjoner. Beregning og dimensjonering
3.	Members have been checked manually based on – NS3472, and critical members also checked according to AISC (American Institute of Steel Construction)

 

Loads
The selfweight of the derrick, cabling and equipments were applied in most realistic way, these were: 1. main structure, 2. piping and cabling, 3. wind walls, 4. platform and ladder, 11. CMC, 12. Bridge Crane, 13. Standbuilder Arm, 14 . access basket, 15. Drill Floor Manipulator Arm, 16. Finger and Bellyboard.
Variable Funciontal Loads (Live Loads) of each euipment have been applied in a separate basic loadcase: 19. min. hook load, 20. static hook load, 21. Bridge Crane, 23. Standbuilder Arm, 24. basket extended position, 25. DFMA, 26. DDM Torque, 27. Finger and Bellyboard Lean.

DDM load is applied at every level to get valid local load on DDM Guiderail.
The forces in opposite direction are applied at every corner – to get only 1x DDM load for global derrick analysis (at top position).

Environmental loads:
Windload:
3sec windload is applied on all members for realistic local results. The difference between 3sec and 15sec windload(kN/m2) is calculated according to the exact formula below the difference between 3sec and 15sec windload(kN/m2) is applied on derrick corners to reduce global result to 15sec level.
Rig motion load:
Total 1G horizontal acceleration is applied on derrick, while structure was supported in every node with pinned support. The resulting reactions are scaled according to height: multiplied with 0 at drillfloor level and 1 at top, linear interpolation in between.
The scaled forces are applied as loads on the relevant nodes.This way an angular rig load 0-1G is modelled. In the combination phase - 1G uniform load is multiplied with drillfloor level rig value, while angular rig is multiplied with the acceleration difference of drillfloor and top.Thus the acceleration is realistic at every height.

Buckling length
A realistic buckling length was necessary for all beams to achieve reliable results. Horizontal beams and primer truss members had 0.8 buckling coefficient. Secondary truss members and columns had 1.0 buckling coefficient.

 

Global analysis

Derrick structure was also analized locally at possible critical areas.
Structure was not sensitive to vortex shedding, therefore no calculation was made on this subject

Derrick without rails
Guiderails are not main structural members, derrick must remain stable without these. In order to validate this, we have removed the SBA & DDM Guiderails from the global model. The load of the Guiderails have been applied on nodes of the derrick structure. We have compared the utilization of the original and this model. The utilizations did not change significantly, therefore the structure is satisfactory without Guiderails too.

Joint Design Bolts
Beam model output data was inserted into spreadsheets to find maximum for each connection type. We have checked 26 highly stressed connections in AISC. These had utilizations ranging between 1.05-1.96 according to EuroCode (and NS 4372). We have used load combinations without load factor and environmental loads (wind+rig) divided by 1.33 for AISC calculations. The connections were acceptable due to this reduced loading according to AISC.

Welds
Beam model output data was inserted into spreadsheets to find maximum for each connection type to be checked similarly as bolts above.

Some unique and cirtical joint details were also modeled to find correct results. These were analized according to DNV rules.

Max Von Mis stresses: The "red areas" are not governing, as these are the load application areas.

BellyBoard Support Baseplate

Bellyboard supporting half of the setback load was extremely important part of the derrick. The Bellyboard – Derrick connection was checked in detail. Derrick and bellyboard was modelled by beam members, and the connection detail by plate elements. This way both the stiffness and buckling parameters of the members were included in the calculation of the joint.