A technological adventure 

 

One of four subsea templates on Ormen Lange 

(Foto: StatoilHydro)

The subsea development was in itself a technological adventure. This part of the project was the most demanding in technological terms. Huge templates were to be placed on the seabed over the field, almost 900 meters under the waves, 120 kilometers from shore.

When production commences in October 2007, the gas will be transported in pipelines up a slide escarpment that is as steep as the landing slope of the Holmenkollen ski jump, with a gradient of some 35 degrees. The terrain itself is an undulating landscape of peaks of between 40 and 60 meters and deep valleys. Sub-zero temperatures in the seawater and strong currents made the development extremely complicated.
 
Hydro mapped large parts of the seabed with a level of detail of up to 10 centimeters. An autonomous, unmanned submarine (AUV) was used together with remote-controlled robots (ROV) with advanced sensors to provide a precise picture of the obstacles on the seabed that had to be overcome.
 
The seabed had to be prepared with a huge degree of precision to ensure that the pipes rested comfortably deep down. For this work the team used a remote-controlled subsea dredger, Nexans Spider, that did not yet exist when the project was planned.
 
A large part of the task was to develop brand new technology for the project. The Spider has the undercarriage of a forest machine used in the Alps. The shovel has powerful jet nozzles attached to it that flush water under high pressure into the clay of the seabed.

It was remote-controlled from a support vessel that was positioned between 500 and 800 meters above the deep sea dredging. The muddy water that was hosed away created near-zero visibility and made it difficult to monitor the excavation via cameras secured to the template – and move the machine with minute precision between peaks and valleys in the sheer escarpment. The project utilized two Spiders and succeeded in digging the trenches required for the 30 inch pipelines.
 
However, other parts of the pipe trench also presented challenges. The first 33 kilo-meters from the land-based facility at Aukra and southwards to Easington pass through Bjørnsundet, a strait consisting of narrow and winding ravines. Originally there was not even enough room to lay two 30 and one 42 inch gas pipes next to each other through the narrow passages down there. Therefore wider routes had to be built for the pipes.

 Large amounts of stone were transported from the Bergen region and dumped along the planned route through Bjørnsundet. The stones landed exactly where they were supposed to, with an accuracy of within a few centimeters.
 
Another demanding challenge was the cold water. The temperature of water at this depth can be as low as -1.2 degrees Celsius, which means that the well stream, which consists of gas, condensate and water, cannot simply be transported through the pipes without the danger of hydrates and ice plugs forming, which can block the pipes and halt production.
 
So Kilde’s underwater team set about designing and building the world’s largest anti-freeze facility to prevent ice from forming. The system consists of two pipelines that transport a glycol-based liquid out to the field, where it is injected into the gas stream. On land the anti-freeze is separated out from the well stream and sent back into the system. This system minimizes the danger of ice forming.