Objective
The aim of the project is to have commercially available a safe, reliable and proven explosive underwater cutting system to be used in the removal of fixed steel offshore production structures.
The proposed underwater cutting method uses explosives as an engineering, as opposed to a demolition tool. The innovate charge designs minimise the amount of explosive required to effect the cut by (i) exploiting the brittle fracture characteristics of steel; (ii) improving detonation procedure and jet formation; (iii) reducing sensitivity to hydrostatic pressure and stand off.
During Phase 1 of the project, it was possible to validate the design of the charges by conducting numerous tests on small and large scale BS 4360 steel components in the form of plates and tubes of up to 65 mm thickness. Prototype charges were manufactured after modifications made to original designs. In order to make these precision tools, (i) a new rolling machine had to be designed and manufactured for making the liners of the Radial Hollow Charge (RHC) and (ii) a set of moulds for making the flexible composite waveguides of the Shockwave Focusing Charge (SWF).
The production methodology used in the manufacture of these charges is now established. The optimisation of key parameters of the process resulted in reliable cuts in different batches of BS4360 steel. The optimisation included computer modelling as well as experimental tests. The underwater cutting tools have been successfully engineered to cope with the wide variation in batches of such steels.
The reliability and repeatability of the prototypes tested so far give undoubted confidence in the ability of these charges to cut the tubular targets on land and in shallow waters (10 m). With additional testing work carried out during phase 2, it has been possible to confirm the effect of key parameters on the performance of the RHC charge (for example how the presence of sand, water, or silt between the charge and its target affects the efficiency of the cut). The RHC device however, proved to be better than the SWF device for fulfilling the programme objectives. The SWF device requires re-examination with particular emphasis on the initiation system underwater to induce a cut immediately below the initiation zone. This charge, which requires further development for underwater applications has not been included in the documentation suite developed during Phase 2. The RHC has been progressed further by design improvements, further underwater validation tests and final manufacture and documentation.
Anextensive structured suite of documents has been generated to record the design decisions taken, the design work carried out, and formal operating procedures (including safety issues).
The project is in three phases : Phase 1 deals with the problem of optimisation of charge parameters and validation of the devices on land and nearshore. Phase 2 involves the design amendments and full documentation of the production methodology and the manufacture of a set of tools. Phase 3 concerns large scale offshore field trials, final specification and approval.
The explosive underwater cutting system to be used in the removal of fixed steel offshore production structures will have to ultimately demonstrate an ability to cut steel to BS4360 Grade 50D or equivalent to 65 mm thickness or greater in tubulars of approximately 3m diameter at 75 m depth of water.
Based on many years of research and field experience, the promoters have selected two innovating explosive cutting systems for development within the project. The shock wave focussing charge (SWF) uses a convex shaped wave-guide to generate focussed tensile stress waves in the material. The charge can be made in long strips joined together and wrapped around tubulars internally or externally. The performance of this charge is independent of the hydrostatic head and is achieved using substantially less explosive than conventional linear shaped charges. The Radial Hollow Shaped charge (RHC) is configured as a pie segment with the explosive initiated at the apex causing the curved liner to collapse instantaneously around the periphery thereby generating a radially flowing jet which fans out as it travels towards the target. Fewer charges are required per unit of length of cut.
The project will produce a detailed operational methodology and appropriate design information for the underwater cutting operation as a simple hammock activity within total platform decommissioning and removal.
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DEM - Demonstration contractsCoordinator
M13 9PL Manchester
United Kingdom