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PEF LOAD TRANSFER METHOD: Transfer theory Lifting gear Sea fastening Capacity Advantages Previous page
THE STATE OF THE ARTOffshore transfer and lifting of heavy cargo and modules from a vessel to an oil platform imposes quite heavy dynamic loads on the cranes due to the energy difference between the cargo and the crane hook as the vessel moves with the sea. Dynamic forcesThe dynamic energy absorption may be regarded as an additional force acting on the crane, and is often referred to as Dynamic Amplification Factor, DAF for short.
Where: C = P/d = geometric stiffness coefficient, P = load, d = total deflection, W = the working load, Vr = Vcb + Vn = the relative velocity at load pick-up, i.e. when the crane starts to pick up the load, Vn = downward velocity of load at moment of pick-up start, Vcb= lifting speed at moment of pick-up start, g = gravitation constant This extra load is the main reason for de-rating the offshore cranes lifting capacity as function of the wave height. It is easy to comprehend that if the Vr in the formula is zero at time of load transfer, the DAF=1. This means that the load transfer is performed as if it was static. Required lifting speedThe lifting speed of the crane is also a limiting factor. It is required that the lifting speed during load transfer shall be higher than the maximum vertical speed of the waves. The reason for this is obvious, since the vessel may collide with the cargo if the lifting speed is too small. The mere fact that the crane has to perform a lift during load transfer will increase the relative velocity Vr, hence the DAF. During lifting heavier equipment, such as modules and topsides, the total lifting forces imposed on the cranes will become so considerable that lifting with huge floating cranes is necessary. The floating crane problemThe major disadvantage with using floating cranes is cost, physical size, and the fact that the lifting and load-handling must take place over other equipment. This entails particular limitations with respect to access, for example in installation or removal of equipment under the platform. Furthermore, the lifting of heavier units over vital parts of the platform means that the production process must be stopped for security reasons. The cash flow consequences of this can be significant. The huge floating cranes must be mobilized and anchored at the platform. This involves special complications if the seabed is overlaid with pipeline systems, manifolds etc. The mobilization time and costs, in addition to operating cost for this equipment, is indeed very significant, even though the lifting process per se only takes a short time. From the above, you will appreciate that current methods establish clear limitations, both technically and financially, for offshore lifting operations. BASIC PEF THEORYThe limitations mentioned above is overcome by the PEF method, which permits load transfer directly from a vessel to a platform with very low dynamic forces and without using cranes. The load transfer takes place at an instant when the cargo has insignificant kinetic energy by engaging the load transfer units, also referred to as the remote controlled grips, in a controlled manner exactly when the cargo is on the top of the wave. Thereafter, a gas/hydraulic operated sea-fastening is made to retract in a controlled manner establishing a distance between the vessel and the cargo now suspended in the lifting cables. Once the module has been transferred from the vessel to the lifting gear, the vessel may leave, and the module lifted up to its final position by the lifting jacks. Moving the module sideways is also possible to perform with the lifting gear. The lifting sequence goes like this:
The rules that apply to this kind of operation is the NORSOK rules for marine operations, the DnV rules for "Engineered lifting" and ISO 9000 quality standard. In addition, one has to adhere to the rules set forth by other statutory bodies and the Company rules for safety.
THE PEF TYPE OF EQUIPMENTThe load transfer and lifting tool "MODLIFT"
What is shown here is one string of the load transfer and lifting system, in other words; one lifting point. The system consists of a remote controlled one-way grip device connected to the module. The grip and the grip control is one of the most essential parts of the load transfer system. Therefore, the grip is designed as a master / slave unit in order to ensure proper operation. Through this grip runs a wire matrix up to the platform, where it is attached to a hydraulic lifting jack. Each jack is supported on a hydraulic load equalizer that will even out the load between the lifting jacks during load transfer. In effect, this system makes it possible to transfer the module from the vessel as if it was lifted in one single point over the center of gravity. At the end of the wire bundle is fitted a combined wire ballast weight and quick release unit. The ballast weight is used to overcome the friction between the wire(s) and the grip unit(s) during vessel heave motion. The automatic quick release will free the wire(s) from the ballast weight if the vessel for any reason leaves the load transfer area before the module has been transferred. Note that the system may be inverted, i.e. that the lifting jack may substitute the grip and the wire may then be attached to the platform with a wire anchor only.
The gas / hydraulic sea fastening system "SEACO".
The SEACO consists of four hydraulic cylinders fitted on- or into the vessel. On top of the cylinders, there are so-called container fittings locking the module in position during transport. At arrival in the lifting zone, the module is raised 2 m (6 1/2 ft) by the cylinders. On the upper side of the cylinder piston, there is compressed nitrogen gas for a rapid return. As the module is lifted off the SEACO rig, sensors activate the cylinders making then retract fully within 4 seconds, thus creating a clearance between the vessel and the module. In choppy sea, a special instrument called MAXWAVE will ensure that the module is transferred on one of the highest waves.
"Unlimited" lifting capacityWorld record is 12.680 ton ! The lifting gear is composed from standard lifting machines and matrix wire systems with a wide variety in lifting capacity. The individual lifting machines range from 12 ton and up as the wire matrix number increase. In addition, the number of lifting machines applied may vary, and so may the lifting geometry. The current world record with this type of equipment is 12680 ton during lifting the RBS-8D, "Deepwater Horizon" in Ulsan, Korea by PSC HeavyLift. The lift was performed with 34 off 600 ton strand jacks, and the lifting height was 33 meters.
Want to know more ? Go to the download section. There you can download reports, descriptions, animations and video clips explaining in detail how the system works.
Advantages:
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TR Consulting, Joh. Falkb.gt. 10, 5538 Haugesund, Norway. Tel: +47 52729124, Fax: +47 52724196, C.E.O. Cell phone. +47 90 616969 e-mail: trcons@online.no or trcons@start.no |
