A tight fit in icy depths
Far from land, in the freezing cold depths of the Gulf of Guinea, it is no easy task to lock together massive pieces of equipment that will gather oil and gas from new wells below the seabed. Adding to the challenge, engineers must allow existing oil and gas production to continue while they work.
On a remote ship off Nigeria’s coast, four pairs of eyes stare at a screen showing live footage from below the ocean’s surface. It is the moment of truth. The engineers carefully lower a 30-tonne metal box – packed with pipes and valves – more than a kilometre down to the seabed.
The production module is designed to sit on the seabed, gathering oil and gas from new wells for pumping to a processing facility the height of a 12-storey building that floats on the sea surface.
Before they can install it, the team uses a system of cables and joystick-controlled underwater robots with on-board cameras to remove pipeline caps from an existing module, operating since 2005. Then they must slot the new module on top.
“Usually we would do an assembly test of all the equipment on land first,” says Uche Okonkwo, lead engineer for undersea equipment on the Bonga North West deep-water project. “But in this case, we had to fit new equipment to an existing installed structure without this step.”
A window of opportunity
A remote survey showed that the existing module had not corroded and still matched its original measurements. Even so, they could not be certain the new module would fit exactly.
“We had no margin for error,” says Uche. “Carrying out this task in such a remote location requires a lot of investment – and incredible precision.”
Adding to the pressure, the team had only a small window of opportunity to install the new equipment. They took advantage of planned maintenance on the floating production facility, which was partly shut down. No extra production time would be lost.
After eight hours in total, the module was lowered into place and was locked tight – a perfect fit.
Warm and fluid
Installing the module was just one of their challenges. In temperatures around 65°C (150°F), the mix of oil, water and gas from the reservoirs must stay warm and fluid during production. If not, it cannot flow the 10 kilometres from the furthest wells to the production facility. But at these depths, sea water temperature is only around 4°C (40°F).
“If you don’t intervene to keep the oil warm, it will freeze,” says Uche. Such low temperatures threaten to turn the gas produced along with the oil into icy solids, causing blockages in the pipelines.
Project engineers introduced several measures to fight off the chill. First, they used a double layer of carbon steel pipes with insulation in between. This helps minimise heat loss, in a similar way to a vacuum flask.
They also installed a new system on the production facility that injects special anti-freeze chemicals into the pipelines whenever needed. The chemicals alter the temperature at which the fluids turn to solids.
Oil and gas from the Bonga North West field flows back to the existing floating production, storage and offloading facility. Moored over the Bonga main field, the facility is held in place by 500-tonne anchors linked by 20km of mooring lines. It receives crude from production wells on the seabed. The oil is processed on board, stored and then sent to a so-called single point mooring buoy anchored nearby from where it is loaded onto tankers for export.
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Shell Nigeria Exploration and Production Company Ltd (SNEPCo) is producing oil from the Bonga North West field, which lies at a depth of more than 1,000 metres (3,300 feet).
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