The Twin Marine Lifter concept uses hinged lifting arms for the single-lift removal and installation of topsides.
A single-lift concept for platform removal and installation was launched to provide a more cost-effective heavy-lift method that is also environmentally friendly. The method was developed by Stavanger-based SeaMetric International. The Twin Marine Lifter (TML) concept is being examined by North Sea operators with upcoming decommissioning programs, according to the developers.
By mid-year, the firm was to complete a joint industry project sponsored by four members of the Decommissioning Technology Forum: Phillips and TotalFinaElf in Norway, and BP and Shell in the UK. The scope of the project applied the concept to the removal of Phillips' Albuskjell 2/4-F platform.
Development of the TML concept received financial assistance from SND, the Norwegian Industrial and Regional Development Fund.
The concept is based on simple principles and proven technologies, combined in a unique new arrangement, says SeaMetric's marketing manager Rolf Olavesen. The lifting operation is carried out by two customized barges, each with dimensions of 120 meters by 46 meters by 9 meters, and outfitted with four to six lifting arms. The lifting arms are hinged to a skid structure mounted along the center line of each barge. The lifting end of each arm is equipped with a buoyancy tank and the other end with a ballast tank.
The two barges are positioned and moored on opposite sides of the topsides to be lifted, and the end of each lifting arm is mated to the topsides by a pin which engages a receptacle pre-welded on the underside of the deck.
The jacket legs are then cut, transferring the weight of the topsides to the lifting arms by simultaneously loading the ballast tanks and deballasting the buoyancy tanks. During this operation, the angle of the lifting arms rises from approximately 12 degrees below horizontal to about 4 degrees above horizontal.
Once the topsides are lifted clear of the jacket, the barges and their load are towed away and the topsides transferred to a conventional barge for transport to shore, or to a new installation location.
The whole operation is estimated to take less than 24 hours, Olavesen says. An important feature, however, is that once 90-95% of the weight transfer has taken place, the crucial final stage of lift-off is accomplished in a matter of seconds. This significantly reduces the exposure of the operation to the effect of wave motions, and makes an important contribution to safe performance.
Another significant safety feature is that, although personnel are required on the barges during preparatory work and the initial phase of lifting, no one is present during lift-off. The whole operation is controlled from a command vessel located at a safe distance. A state-of-the-art control and monitoring system provides full control over all stages of the operation, says Olavesen.
Weight and CG
As part of the jacket leg cutting, membrane devices are attached to the legs that make it possible to measure the weight of the topsides and calculate its center of gravity (CG). The lifting arms can be individually loaded to cater for differences between the CG and the geometrical center.
One benefit TML offers is the heave compensation system, which minimizes the dynamic forces transferred from the movement of the barges to the topsides, Olavesen stresses. This means lift-off is relatively smooth, opening the way to operation of the system in a wide range of sea states. The design criterion for TML is Hs 3.5 meters significant wave height and Hs 2.5 meters operationally, meaning it can operate in wave heights up to five meters.
Each lifting arm is capable of bearing a weight of up to 1,500 tons. A total of 12 lifting arms could therefore lift a single object of up to 18,000 tons, more than 50% greater than the lift which can be achieved by existing heavy-lift crane-barges.
Various benefits stem from the concept's greater capacity, Olavesen says. To remove a large topsides, a crane-barge would perform a number of lifts over an extended period, and involve a relatively large work-force for cutting and preparing the topsides before the various parts could be lifted off.
A comparative study of the two methods done for SeaMetric by Den Norske Veritas concluded that the energy consumed in a TML removal operation would be only 10% of that required by conventional heavy-lift technology. As the TML operations would involve fewer people working for a shorter time, they would also be safer, developers claim. For the same reasons, a TML lift operation offers costs 35-40% less than heavy-lift barges could achieve, Olavesen says.
Although initial development of the concept has focused on lifting topsides, the TML will also take on both jacket removal and platform installation. Work is now proceeding on developing jacket removal techniques, for which winches will be required on the barges.
Once lifted, a jacket could either be transported in vertical position between the barges, or rotated to the horizontal and placed on a transport barge. A proposal for a joint industry project (JIP) on jacket removal has been forwarded to the Decommissioning Technology Forum. Platform installation essentially involves the reverse of the removal methods.
SeaMetric expects to operate the TML world-wide. The technology is capable of lifting the vast majority of platforms installed in the North Sea, the Gulf of Mexico, and other offshore provinces.
By providing local content, the company sees a way both of achieving low costs and facilitating access into foreign markets. It will hire support vessels, as required, close to the offshore location, and involve local contractors in the preparations and support activities, such as the dismantling and disposal phases.
Detailed engineering was due to begin late this year. In parallel, the company has been in contact with yards where the barges and lifting system might be constructed. This will likely be in the Far East.
The control and monitoring system will be developed and installed in Europe. SeaMetric was scheduled to raise the capital for further development this past summer and have the barges built by the end of next year. Allowing for a period of trials, the system could be ready for operations by early 2003.