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The Magtymguly Collector Riser (MCR-A) gravity based structure (GBS) is a 7,000 tonne fixed offshore steel gravity based structure located in Magtymguly field. The platform is located in approximately 67m water depth and 60km offshore Turkmenbashi in the Caspian Sea.
Arup’s services included detailed engineering design, structural engineering, naval architecture, installation design, geotechnical engineering and seismic engineering for the substructure. Arup also provided construction support during fabrication and installation support.
Novel Design
The configuration of the MCR-A steel gravity based platform presents a first of its type. The platform was designed to be self installed without the need for large and expensive offshore crane vessels.
The GBS is configured of large steel plated panels and more traditional tubular jacket type structures. The steel panels are formed to create large hollow pontoon sections that enable the structure to float during installation. The pontoon sections were then ballasted with water to lower the structure to the seabed. The GBS uses shallow skirted foundations that are installed via suction, eliminating the need for piles and piling equipment, and simplifies removal at the end of the field life
Complex modelling
Our multidisciplinary team overcame a number of challenges including advanced dynamic time history seismic modelling, designing the unique suction skirted gravity based foundation in stiff clays and predicting global wave loads for combined slender and larger volume members.
Arup performed complex buckling and collapse analysis of the thin shell plated structures for all permanent and temporary load cases, including load-out, transportation, installation, environmental in-place and seismic load conditions to predict the structural response during an earthquake event.
Decommissioning
The MCR-A platform is a permanent structure. However, the platform can be easily decommissioned and removed by reversing the installation methodology. The pontoon type substructure has the buoyancy and marine stability to be re-floated and towed back to shore for dismantling without heavy lifting or cutting equipment to encourage the efficient removal and ultimate resuse of the substructure or its materials at the end of its design life.
Technical excellence
The MCR-A platform has demonstrated a significant amount of technical excellence in seismic, installation, structural and geotechnical engineering. The following is a list of the advanced technical solutions Arup provided to this project:
Seismic
Due to the highly seismic nature of the site, a performance-based design approach was adopted for the platform under earthquake loading. This method predicted the non-linear deformations within both the structure (where they were interpreted as expected damage levels) and the foundation (checking overall stability). If conventional response spectrum analysis method had been used in the final assessment, a much larger foundation would have been specified significantly increasing the cost.
To accurately capture the response of the soil-structure interaction during the seismic events and accurately predict the structural response, an FE model of the platform was built in LS-DYNA including a large representation of the soil block beneath the platform. Non linear properties of the jacket, topsides structure and soils were assigned and the adjacent well-head platform was also modelled. This approach enabled the structural response, capacity foundation stability and relative motions between the adjacent platforms to be accurately predicted within the same analysis model.
Installation
For the design of the substructure installation, Arup’s float-off stability design allowed for compressed trapped air within the skirt compartments, an option that is commonly unavailable in most industry-standard stability software. The ability to include compressed trapped air allowed accurate predictions of float-off draft and hence the ability to accurately specify the requirements of the transportation vessel and float-off site. Physical wave tank testing was performed to validate the results of the installation hydrodynamic modelling (GBS and 4 tugs).
Structural
The unusual structural configuration meant that wave loads were determined using a combination of hydrodynamic linear diffraction analysis for the pontoon sections and morrisons equation using non-linear wave theory for the tubular sections.
