Since risers and field pipelines can comprise 30 percent of the cost of developing a field in deep water, reducing this cost will save hundreds of millions of kroner. Aker Riser Systems saw the potential for developing the technology in this area already in 1998, and since then has amassed expertise and developed a new concept for risers.
"It was our goal early on to develop a hybrid riser, with a more efficient tower design and simpler installation. From experience we knew that we had to reduce the weight and find alternative means of providing buoyancy and insulation," says Arne Sele of Aker Kværner.
At moderate depths, distributed buoyancy of risers has not been a dominating cost. However, at depths in excess of 1000 metres, buoyancy made of foam can drive up costs. The solution Aker has arrived at is to install an air-filled tank 50-100 metres beneath the surface to hold the riser rigid from the sea floor. There it is held in place with a gravity anchor, which functions as a base for connecting flowlines. A standard flexible riser goes from the tank and up to the surface.
Convincing the market
The hybrid riser has several conduit pipes of aluminium that function as tension legs and that also provide buoyancy. The conduit pipe brings the steel riser up to the tank, which is deformed to a helix because the pipe is held firmly in place, preventing pressure and temperature expansions in a lateral direction. Bending stresses arising due to the deformation are in theory modest.
The contact between steel and aluminium is not supposed to present corrosion problems, primarily because the steel on the inside of the conduit pipe is insulated so that no galvaniccontact arises between the metals during manufacturing or in the construction phase. In addition, the conduit pipe will be filled with a petroleum gel that will keep oxygen away and thus, corrosion as well. On the outside, cathodic protection will ensure that galvanic corrosion does not arise in the seawater. "We shall do a full-scale test of the concept to convince the market that our material theories are correct," Sele says.
One of the objections to using aluminium is its unsuitability for traditional welding methods. This concept could not have been realised if Aker had not begun its collaboration with Hydro Aluminium. They own one of the world's first and largest friction welding machines, which will do longitudinal and circular welding of the conduit pipes. The method is well known and has been used for several years, and the result is a weld that retains 98 percent of the strength of the mother material.
Entire towers towed out
The costs will be reduced especially because the anchoring no longer has to be preinstalled at depths of several thousand metres and because assembly offshore is eliminated. In this concept it is probable that up to 30 percent of the costs related to installation of risers can be reduced. This is because the buoyancy tank, riser and anchoring are assembled on land and then towed out to their destination. During tow-out the riser tower needs to have uniform and distributed buoyancy along its entire length, and the gravitation points have to be supported by the tow vessels.
The conduit pipes are filled with gas to prevent implosion when the riser tower is set down on the sea floor. The buoyancy tank is gradually filled with air, which ensures a controlled operation.
When everything is in place, the gas is released and the conduit pipes are filled with petroleum gel, which is meant to serve as a means of insulation and buoyancy.
"Gel has traditionally had a bad reputation because in earlier solutions, salt solutions were used to which unstable gel formers were added," Sele explains. To disprove the sceptics regarding the use of gel, it will also be tested.
