Going for greenfield

A greenfield platform was a more effective option over brownfield modifications on Shell’s Leman Alpha complex in the North Sea. Elaine Maslin explains how Dutch expertise made it a reality.

The Leman AC platform engineering drawing rendering. Images from Iv-Oil & Gas.

At nearly 50 years old, anyone could forgive the Leman Alpha complex for needing a little bit of late life TLC.

The facility, comprising four bridge-linked platforms, is 50km off the coast of Norfolk, England, in the UK sector of the North Sea. The complex required new compressors to improve reliability as well as safeguard the continuity of gas export to the Leman and Corvette Pipeline Users Group fields until 2028. The question was, should a brownfield project be initiated by directly replacing the compressors, or should an entirely new platform be designed?

Operator Shell UK consulted the Dutch Engineering Company Iv-Oil & Gas and Fabrication Yard HSM Offshore to study the possibilities. This resulted in the advice to build a new platform, the Leman AC platform, complete with 40MW of new compression capacity, through two compressors (with in total three compression stages) with a capacity that runs from approximately 3 to 80 bar. The new compressors replace the compressors on the existing Leman AK platform. Additionally, the new platform also includes auxiliary equipment and a 1200 kW diesel generator.

“Greenfield work for a brownfield project was not only the more profitable option, but was also safer and more efficient, because all the systems can be tested onshore instead of offshore. This allowed production to continue on Leman Alpha,” says Marcel Stevens, Project Manager at Iv-Oil & Gas. “Greenfield work is also less time consuming than carrying out brownfield modifications on a platform, which can be hindered by limited accommodation. This could result in the need for an accommodation vessel as well as helicopter transport for workers, etc. The less you interfere in the normal production of the platform the better. In a normal brownfield modification, the interference is quite high.”

The Leman AC platform engineering drawing rendering. Images from Iv-Oil & Gas.

The conceptual design of the Leman AC platform is based on a conventional integrated steel frame structure. The topside structure of the Leman AC platform provides the framing for a compression facility containing a cellar deck, a compressor deck, a mezzanine deck, a cooler deck, a crane and laydown areas. In addition, it provides support for the bridge-link to the Leman AK platform. The bridge is 37m-long, made of a tubular steel construction with a rectangular cross section. This bridge-link supports piping and offers a location for the installation of the nitrogen storage vessels, power supply, personnel access/escape routes and trolley access for lightweight goods/equipment.

Shell UK awarded Iv-Oil & Gas the contract for the FEED phase of the project in February 2012. In August of that year, Iv-Oil & Gas received a contract for detailed engineering, procurement services and construction management (EPCM) of the project. This included the detailed engineering of modifications on the existing Leman complex in order to integrate the new platform with the existing installation. HSM Offshore was awarded the fabrication contract including mechanical completion, pre-commissioning, load out and sea fastening, which started in January 2013. Sail away of the 1030-tonne jacket and 3570-tonne topside both took place in August 2014. Heerema Marine Contractors provided transport and installation using its own crane vessel Hermod.

One of the challenges imposed on Iv-Oil & Gas was the design of the facility around the two compressor trains: one low pressure compression train from GE Oil & Gas and one high pressure compression train from Solar Turbines, which had already been purchased by Shell UK.

As part of its role as manufacturer, Iv-Oil & Gas supervised all construction and pre-commissioning services at the HSM Offshore yard during the fabrication phase.

The Leman AC platform will be monitored and fully controlled from the Leman AD1 platform, the controlling platform for the entire Leman complex. This required new fiber optic cabling to be routed through the Leman complex. First gas is expected this month, September 2015.