Monetising Congolese gas

The exploitation of the Marine XII oil and gas field in Congo faced limitations due to excessive natural gas production alongside the oil. While the oil was valuable, the natural gas was less so, primarily because the country’s consumption levels were too low to deal with all the associated gas. Consequently, the strategy of Eni to liquefy this associated gas enabled not only the sale of LNG, but also an increase in oil production output.

To implement an LNG export solution in a short timeline, a combined system was developed with two existing units. The TANGO floating LNG (FLNG), a liquefaction barge with liquefaction capacity of approximately 1 billion m3/y of gas originally designed for benign conditions with limited storage capacity, and the EXCALIBUR, an LNG carrier to serve as permanent storage unit (FSU) for the terminal.

In early 2022, the EXMAR teams were tasked to find a solution to moor TANGO FLNG offshore Congo and position FSU EXCALIBUR alongside the FLNG, based on the environmental conditions of the area and considering the short timeframe to implement the new design.

EXMAR, with its expertise across the whole LNG value chain, including FLNG, and serving as the EPC contractor for this project for the implementation of the FLNG and FSU EXCALIBUR, designed the mooring system and performed the refurbishments on both vessels at Dry Docks World yard in Dubai.

Eni’s Congo LNG project leveraged the Marine XII block’s gas resources and existing production infrastructure, adopting a phased approach to achieve the goal of zero routine gas flaring. This strategy allowed for increased oil production in the Marine XII block, with previously flared gas now being monetised through TANGO FLNG’s LNG production, thereby minimising flaring to the lowest possible levels. All this was targeted to be completed in less than a year till first gas, underwriting.

A fully-integrated project

The development of the Congo LNG project was a good example of how an opportunity can become reality in a short timeframe when several companies work together to one common goal. Various divisions within the EXMAR Group worked jointly with the Eni teams to come up with an innovative solution creating value using existing assets.

To moor an FLNG barge close to an FSU in offshore conditions with a substantial swell, a new mooring system was invented. The ‘split mooring’ system is a combination of a reinforced side-by-side mooring with a conventional spread mooring system. The design from Exmar Offshore Company in Houston is made to keep both floating units close enough together to easily facilitate LNG and crew transfer while keeping a safe distance between both units in a 100-year storm design condition. In spring 2022, the concept was proven to Eni via wave basin tests. Since split mooring is a completely new concept, it means that the EXMAR teams had to prove via a technology qualification assessment (TQA) by the classification society Bureau Veritas that it would work.

In parallel with the further maturing of the split mooring, several other upgrades were engineered.

Since no helicopter platform was available, boat landing towers were added to both TANGO FLNG and the FSU EXCALIBUR to cater for personnel transfer. In this way, classical surfer boats (which were already available in Congo) could be used for the personnel transfer.

A normal LNG ship-to-ship (STS) system to transfer LNG from TANGO FLNG to FSU EXCALIBUR could not be used because of the motions of both units, and because the distance between both units is too large. A new liquid transfer system (LTS) was designed to overcome these operational challenges. On both FSU EXCALIBUR and TANGO FLNG, a tower was built to increase the height of the LNG manifold well above the mooring lines in between both units. On top of the towers, LNG swivel arms of a dedicated design were installed to always ensure that the LNG transfer hoses are free of stress and torsion. The entire system uses one 10 in. hose for LNG transfer from the FLNG to the FSU and a second 10 in. hose for vapour return from the FSU back to the FLNG. All excess boil-off gas generated by the FSU can be reliquefied on the FLNG increasing the efficiency of the terminal and avoiding unnecessary venting or flaring of natural gas.

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