Gap Holding & Weighing
Technical challenges particular to the growing decommissioning and abandonment sector include the need to accurately weigh individual modules being removed from the structure being decommissioned, before disposal. The removal and recycling of topside and substructure equipment and facilities are two significant costs in the global platform decommissioning sector and so are key targets for time and cost reductions.
Traditionally, cutting through concrete platform legs or module bases uses wedges, levers and crane supports to prevent the diamond cutting wire from becoming trapped once a significant portion of the item has been cut through. Time wasted in trying to free stuck or broken diamond wires can impact negatively on operational productivity and some structural analysis effort may be spent on avoiding this, during the engineering phase of the project.
There is a correlation between the weight of offshore platform topsides and the overall economics of a decommissioning project. Previously, determining the overall weight and centre of gravity of modules weighing multiple tonnes prior to a heavy lift, skidding or trailering operation has involved methods including making calculations from original 2D/3D construction drawings or deploying a series of large-capacity load cells underneath the object being weighed.
Why do we need a new solution?
Until recently, structures were not designed for removal so each installation demands a unique method for decommissioning. Now, innovative technology can be used to both minimise man-hours in the engineering phase and cost-effectiveness during the execution of the project.
Building on the success of the unique and field-proven ThinJack® technology, research and development has resulted in the first hydraulically inflatable, millimetric-thin steel envelope for supporting and maintaining the height of the gap during a cutting operation. The PowerPad system with its custom-designed software reads and displays in real-time individual PowerPad inflation pressures in bar, the individual load readings and the total load.
The steel envelopes are positioning in the cut gap then connected via a manifold with individual control needle valves to a hydraulic pump and the system is pressurised with water. Pressure is pumped into the envelopes which expand and, using the established formula “Force = Pressure x Area”, maintains the gap opening. These can be adjusted in real-time to compensate for any adverse forces resulting from downward pressure by the concrete mass onto the gap created by the cutting wire.
This patented application can also, with specialist software, be used for accurate weighing of any large structure under which it is placed.
The availability of a millimetric thin inflatable pad to prevent objects being cut from collapsing in on the cut gap and tapping and breaking the diamond wire thereby causing delays meets a need to deliver productivity in engineering safe asset reduction techniques in the decommissioning sector. The inflatable pads permit faster and safer operations cutting through concrete assets. The same inflatable pads, with the addition of sensors and software, can also be used in the same decommissioning project, either simultaneously or consecutively to determine the weight, load requirements and centre of gravity of heavy items being removed.
When cutting through concrete, PowerPad is more effective than a solid and passive shim. Being initially thinner than the gap, they are pushed into the gap and inflated to fill it and the pressure is then locked off. Using PowerPads will equalise the force from above and keep the gap unchanged, minimising the likelihood of tension on the cutting wire.
Conversely, a solid passive shim can be made to slide into the gap but it will not equalise the force from above completely as it cannot “pack” the gap completely. If it did it would be impossible to slide in the solid passive shim. By not supporting the force from above the weight pushes down resulting in a higher tension on the cutting wire and there will be a higher risk of “crazing” in the concrete.
When the cut is complete, the upper part of the asset will be “floating” on the PowerPads. The mass of the upper part of the asset can be calculated, as can the centre of gravity if needed. Furthermore, if PowerPads are bearing the expected total weight of the upper part of the asset, then this is an indication that the cut is completely through. With the capability to calculate the centre of gravity before the lift of the upper, cut off part of the asset, a wider range of vessels may be used for topside disconnection and thereby can contribute towards decreasing costs, with a range of structure sizes.