The North Sea has already entered the arena of platform removal. In the time to come, several and large platforms will be removed, and so there is a large business market for removal services.

The Norwegian company Dr.techn. Olav Olsen started several years ago with concept development of a novel heavy-lift solution called MPU Heavy Lifter using the company’s experience from designing concrete platforms (Condeep). The MPU Heavy Lifter is now being built in Rotterdam for the company MPU Offshore Lift ASA. Sesam has been used in all phases as the engineering analysis tool. The MPU Heavy Lifter is unique in the way that it is a u-shaped concrete semisubmersible for handling large offshore installations. It is designed to remove and install jacket structures as well as topsides for both fixed and floating installations. The MPU Heavy Lifter is actively using the ‘excess buoyancy’ effect during these operations by removing water from the concrete columns. The vessel is 40m high with a footprint of 87 x 104m and can handle topside lifting up to 15,000 tonnes in one go. The maximum lifting capability for a jacket is 35,000 tonnes. The MPU Heavy Lifter is thus capable of removing large structures and move them to onshore for dismantling as compared to
traditional methods where dismantling is done offshore. During the various design stages, a number of analyses using Sesam have been carried out.

The vessel consists of a U-shaped concrete hull with two large movable steel lifting frames attached.

Static and frequency domain analyses
Similar to other floating structures, a typical setup of global and local analyses has been carried out, using the Sesam software products. They serve the purpose of establishing the sea-keeping behaviour, finding design waves governing for the structure and subsequently the structural response to static and quasi-static (wave)loading. The superelement technique was used and the structure was split into manageable parts such as the concrete hull, the lifting frame connection, steel frame and the relevant object to be lifted or installed (typically a jacket or topside structure). All in all, the size of the analysis was just above 400,000 degrees of freedom with roughly 600 loadcases from 44 design waves.

Time-domain analyses
Special for this structure is the transient load-transfer phase, when large amounts of water are released from a large mass floating structure, and the subsequent contact and load transfer from a stiff, earth-fixed structure. This causes special challenges from analysis points of view, and the features within Sesam for marine operations was used for the time-domain simulations executed. As such instant events is more difficult to predict statistically, a large number of simulations must be executed, for a number of typical seastates, topside configurations and ballast procedures. This induces several hundred realisations (about 400) and the worst-case load scenarios are then transferred to the steel-frame for a separate frame analysis to perform code checking of the beams. After another filtering of results, the governing loads (about 500) for the concrete hull are applied to the global structural analysis as static loads. The correspondence between tank-test and the linear hydrodynamic analyses of sea-keeping behaviour and impact loading is very good. In addition, the features within Sesam for hydrodynamic non-linear time domain analysis have also been employed, in order to verify local wave enhancement and air gap around the structure.

Post-processing of concrete structures
Dr.techn. Olav Olsen has previously developed Sesam's post-processor (Concode) for large-scale post-processing of concrete structures. This is based on results from a linear FEM analysis. As a reinforced concrete section is a highly non-linear, anisotropic material, the stiffness is dependent on the loads applied. Therefore, a potential for improving the correspondence between local capacity and overall stiffness has been identified. Over the past years, this software has been further developed under the name ShellDesign with the purpose to account for non-linearities without performing a non-linear analysis. The key principles are:

1. Run a linear analysis of the governing load combinations
2. Initial post-processing in ShellDesign, calculating capacity and updated stiffness
3. Transfer updated stiffness back into the model for a new linear analysis
4. Repeat step 1–3 above 3–5 times to converge at a non-linear capacity.
   

The results from a typical plate field from such a process will be increased structural capacity, similar to the increase from linear to plastic capacity (yield-line theory).

Local analyses
Additionally, local 3D analyses of highly stressed and complex areas have been conducted, such as cast items (towing brackets and lifting frame connection to column), as well as critical parts of the mechanical systems that is locking the movable lifting frame in its various positions.

Summing up
In total, we believe that all the engineering executed, with the analysis tools involved (primarily Sesam and ShellDesign), give a good overview, and a level of confidence for the designers and the customer that the concept is well-proven. The classification process has been smooth, considering that no such vessel and no such operation have been seen before. A lot of work and a lot of fun, too!

Author: Erik B. Holm, Dr.techn. Olav Olsen

The Norwegian company Dr.techn. Olav Olsen's MPU Heavy Lifter is now being built in Rotterdam.