Long-term economics of jacket member grouting supported

Ekofisk project example for other older structures

Dr. Eigil V. S rensen
Lars Andersen, Densit

Continued subsidence of the seafloor at the Ekofisk Field in the North Sea is increasing the risk of waves reaching the platform deck structure, for which reason mitigating measures are being taken. Previously structures have been jacked up, and this option was also considered in 1995 when action had to be taken on the Ekofisk Alpha and Bravo platforms, which otherwise would have to be shut down in 1998/99. The jacking option was however abandoned due to the high costs involved.

The license group instead decided to reinforce the existing structures by grouting critical tubular structural members with a high performance mortar, following structural analysis performed by Offshore Design of Norway. The reinforcement project was carried out during the summer of 1995.

Composite columns consisting of steel pipes filled (grouted) with concrete are known to have a significantly higher load bearing capacity than corresponding hollow (empty) steel pipes, partly because ovalization and local buckling are hindered. Furthermore, the load bearing capacity of composite elements have been found to increase considerably by using high strength concrete rather than ordinary concrete for the grouting, particularly in the typical case of axial compressive load combined with small bending moments.

In order to utilize the properties of ultra-high strength concrete in the structural design of the composite columns, the concrete must have a sufficiently high ductility or toughness, and does not possess the brittleness which normally accompanies high strength, if no extra measures are taken.

The ductility requirement was fulfilled by specifying the modulus of elasticity, the uniaxial tensile strength, and the so-called specific fracture energy of the concrete. In this case, a grout trade named Ducorit was used. The product is frequently used as wear resistant lining for steel surfaces, the manufacture of safes, vaults and burglary resistant panels, and hard wearing and chemically resistant industrial floor toppings.

Material composition

The product is a purely mineral Portland cement based material with a composition which allows for mixing the products at extremely low water dosages. The hardened materials are characterized by having very high strength and durability. Two products were chosen for the Ekofisk project: a high strength grout containing quartz sand aggregate and an ultra-high strength grout containing calcined bauxite aggregate. To help induce ductility into the materials, steel fibers (diameter 0.4 mm, length 12.5 mm) were added at a dosage of 2% by volume during mixing.

In the fresh state - immediately after mixing - the materials are very workable, almost self-leveling in consistency. They are easily pumpable, and they possess an inherent cohesion which ensures they do not wash out during underwater casting.

The degree of ductility is directly proportional to the modulus of elasticity and to the specific fracture energy, and inversely proportional to the square of the uniaxial tensile strength. Therefore, it can be calculated that the grout used is about five times more ductile than ordinary concrete.

Project execution

The work was carried out by the Norwegian offshore contracting company Stolt Comex Seaway. Prior to grouting, a hole was drilled in each end of the member to be grouted, and a hose connector piece with valve was mounted in each hole. Like all other underwater operations, this was done without divers, using a remotely controlled vehicle (ROV).

The grout was delivered as dry pre-mixed materials in large bags corresponding to mixing batches. A pan mixer on the platform mixed the concrete and fed it into a double-piston concrete pump. The hose from the pump was picked up by the ROV and connected to the lowest positioned hole of the member, and the freshly mixed grout was pumped into the water filled tube.

As injection progressed, the grout displaced seawater in the tube, which exited through the outlet hole. Video surveillance of the grouting showed the exhaust water was virtually clear with no mortar dissolved in it. When pure mortar flowed out the exhaust end of the tube, the ROV closed both valves and disconnected the hose.

Grouting of the vertical legs above water was done by drilling a hole and mounting a hose connector piece immediately above the grouting target level, and then the grout was pumped into the empty leg tube where it initially dropped about six meters.

A total of 1,500 tons of grouting concrete was injected into critical structural members of the jackets during a three-month period in the summer of 1995, without any materials processing or handling problems.

Verification

Prior to the offshore grouting operation a pilot test was carried out in the harbor basin at Haugesund, Norway, using an ROV for grouting tubular members with grout underwater. These members were later tested by Det norske Veritas to determine the strengthening effect of the grouting.

Two tests used pure bending by four point loading of grouted tubular members, with dimensions of 325 mm outer diameter and 13 mm wall thickness. In both tests, it was found that the grouting increased the load-bearing capacity in bending by about 35%.

Another test used axial compressive loading, again on a 325 mm tube which had this time a wall thickness of 17 mm. The load bearing capacity in this situation was found to be 2.2 times that of an empty steel tube (120% increase).

Later, a series of tests were carried out at Chalmers University of Technology in Sweden to assess the load bearing capacity of empty tubular steel columns, composite columns grouted with an expansive cement mortar, and composite columns grouted with Ducorit grout. The composite columns used in this study were 2.5 meters long, 168 mm in diameter, with a wall thickness of 4.5 mm. The column had a steel cross-sectional area about 12% of the concrete area, which corresponds closer to the real structure than the DnV columns.

The composite columns were loaded both in pure axial compression and in eccentric compressive loading (simultaneous axial compression and bending). The 28-day cylinder compressive strength of the cement mortar and Ducorit-D4 mortar were 75 MPa and 178 Mpa, respectively. At testing time, the strengths were 85 MPa and 202 MPa, respectively.

In both loading tests, the expansive cement mortar doubled the load bearing capacity of the tube, while the Ducorit D4 mortar makes it as much as four times higher. Also, even at very large deflections, the residual load bearing capacity of the Ducorit D4 grouted tube is more than twice the capacity of the original unbuckled steel tube.

The jacking option for subsidence mitigation of the Ekofisk Alpha and Bravo platforms was estimated at about NOK 1,800 million, which is why this option was abandoned. The grouting reinforcement, together with other modifications, totaled only NOK 20 million. Detailed structural analysis and design calculations have shown that this solution will allow operation of the two platforms well into the next century based on today's forecasts.

REFERENCES

Grauers, M.: "Composite columns of hollow steel sections filled with high strength concrete". Publication 93:2, Chalmers University of Technology, Department of Structural Engineering, S-412 96 Goteborg, June 1993.
Modeer, M.: "Modern design in practice," Proceedings of the "Nordic Symposium on Modem Design of Concrete Structures", Aalborg University, Denmark, 1995.
"Ekofisk project/Ducorit S5 and D4/Mechanical properties and properties of freshly mixed material," Test report, Aalborg Portland Cement and Concrete Laboratory, October 1995.
Det Norske Veritas AS, Technical Reports Nos. DV-5-R-024, 026, 032, and 033.
Grauers, M.: "Composite columns - an experimental study of the influence of the concrete strength". Report 96:5, Chalmers University of Technology, Department of Structural Engineering, S-412 96 Goteborg, September 1996.