Final Report Summary - DIFIS (Double Inverted Funnel for Intervention on Ship-Wrecks)
The development of the installation procedures and the design of the DIFIS components were closely linked. The specifications for the system design originated not only from the operational requirements, such as wreck size water depth, current conditions and wave conditions, but also from the developed deployment procedures. The shape and functions of the Dome Interface and the Riser Tube steel connectors, for example, were largely determined by the proposed installation method. In this section the step-by-step DIFIS deployment procedure is presented.
In the system deployment of DIFIS the following steps are made.
1. Site survey - After localising the ship wreck the site is investigated using an ROV. The local water depth, bottom geometry, soil properties and current conditions are determined.
2. Placement of the anchor blocks - The concrete anchor blocks are placed on the bottom using a work vessel with either a crane or a winch of sufficient capacity.
3. Launching of the dome - The folded dome (with dome interface) is transported on a barge. It is lowered into the water and brought alongside an installation vessel. Here it is connected to the Dyneema lines and the first section of the riser tube.
4. Construction and lowering of riser tube - The riser tube is built section by section, each time placing a steel connector on top of the previous pipe section, after which another pipe section is added. As the length of the Riser Tube increases, the folded Dome is gradually lowered until it is close to the Ship Wreck.
5. Unfolding of the Dome - The Dome is unfolded using a set of Lifting Bags, connected to the Mooring Lines. The Lifting Bags are simultaneously released, after which they pull open the folded Dome.
6. Installation of the buffer bell - After the buffer bell is connected to the riser tube it is de-ballasted to provide sufficient top tension to the system. Finally, the system is disconnected from the installation vessel.
7. Periodical offloading and inspections - Once installed the DIFIS system is completely passive and requires no human operator. Offloading operations are scheduled and periodic inspections take place to ensure the integrity of the system.
During the model tests motions, loads, accelerations and thruster RPMs were measured, using different types of instrumentation. The motions of the ship models were measured by an optical motion measurement system (Krypton), which uses a set of infra-red diodes on the model and 3 cameras in a housing on the basin carriage. With this contactless measurement system accuracies of better than +/- 0.5 mm or +/- 0.1 deg can be achieved. The motions of the buffer bell were measured using a special version of the same system, dedicated for underwater measurements. Tensions in the mooring lines and offloading hose were measured using ring-shaped strange-gauge transducers. In addition, the tension in the riser tube was measured at 3 locations, using strain-gauge transducers. Accelerations on the ship models and at the Dome Interface were measured using piezo-type accelerometers. Finally, the DP shuttle tanker thruster RPMs were measured using pulse counters.
The offloading operation is only foreseen in operational environments, since a shuttle tanker will not be able to connect to the buffer bell in survival conditions. The test programme for the offloading tests therefore included operational conditions of combined current, waves and wind. The results of the offloading tests included measured motions of the shuttle tanker and buffer bell, as well as tensions in the riser tube and mooring lines. Also the tension in the offloading hose was measured. In addition, top view contour plots (plotted every 120 s) of the shuttle tanker and buffer bell were made.
The test programme for the deployment tests included stationary tests and dynamic tests. The purpose of the stationary tests was to obtain reliable statistic data of certain specific phases in the installation process. The objective of the dynamic tests was to test the mechanical process of the actual unfolding of the dome.
The scope of work of the DIFIS model tests in MARIN's offshore basin included survival tests, operational tests, offloading tests and deployment tests. Based on the results of these model tests the following conclusions were drawn.
Survival and operational tests
1. The overall behaviour of the DIFIS system in operational and survival conditions meets the expectations; the concept appears to be feasible. No unexpected or instable behaviour was observed in the operational and survival conditions.
2. In current the dome shape remains intact. Only minor deformations of the dome can be observed.
3. The buffer bell motions are limited, both in operational and survival conditions. This confirms that the buffer bell is sufficiently far below the water surface.
4. In waves, the tension in the mooring lines of the dome showed larger variations than expected. This aspect needs to be addressed in the final design of the system.
5. For further evaluation of the DIFIS system in operational and survival conditions an extrapolation of model test results (in a water depth of 612 m) to actual water depths of up to 4,000 m is required.
6. The overall behaviour of the DIFIS system during offloading meets the expectations; the offloading operation is feasible.
7. The presence of the DP shuttle tanker does not influence the performance of the DIFIS system. The buffer bell and riser tube are not affected.
8. The overall behaviour of the DIFIS system during the deployment meets the expectations; the investigated stages of deployment are feasible. No unexpected or instable behaviour was observed during the stationary tests.
9. The dynamic deployment tests showed that the dome correctly unfolds, both in calm water conditions and in current.
10. For further evaluation of the DIFIS system deployment an extrapolation of model test results (in a water depth of 612 m) to actual water depths of up to 4 000 m is required.