Objectif
In-situ demolition is both significantly safer and significantly more economical than any other option for abandoning deepwater fixed North Sea installations. The project aims to enable this innovative abandonment option to be adopted and engineered with confidence.
To do this the project will identify and demonstrate design methods and generate design data required to clarify areas of uncertainty in the engineering of operations required to abandon offshore oil and gas installations by in-situ demolition. The work will incorporate the "best available techniques" and "best environmental practice" as required by recent dumping regulations with the aim of ensuring regulatory approval.
The project has completed all of the activities in Phase 1. The results of these first activities are as follows :
* INTEGRITY & RESISTANCE OF STRUCTURES TECHNICAL BRIEF: has been produced. This details the theory and experimental results and methods that are applicable to predicting the behaviour of tubular hinges. A finite element analysis procedure using ABAQUS has been identified for investigating the shell post buckling resistance of hinging tubulars subject to complex loading.
* STRUCTURAL HINGE ASSESSMENT: ABAQUS analysis of hinging tubulars is complete. Results for tubulars of varying diameter to thickness ratio and various direct loading and distortion histories indicate good correlation between the finite element results and the proposed engineers solution.
* STRUCTURAL HINGE RUPTURE PREDICTION: a fracture mechanics based methodology has been proposed for assessing the onset of rupture in hinges when strains are in excess of 5%. This methodology has been applied to material data relevant to steel jackets built in the 1970s. Results demonstrate that hinge rotations up to 45 deg. C are feasible without rupture.
* STRUCTURAL LABORATORY TESTING : a programme of tests of tubular hinges, subject to gross rotation in combination with direct axial and shear loads, has been completed. These demonstrate good correlation with the predictions of the analytical 'engineers' solution and finite element simulations.
* USE OF EXPLOSIVES TECHNICAL BRIEF: has been produced. This reviews the large number of issues associated with the safe and effective use of explosives for underwater cutting. Material issues and possible methods for assessment have been identified for use in subsequent activities.
* EFFECTS OF EXPLOSIONS ON ADJACENT STRUCTURES : this activity has developed software for predicting shock and bubble pulse loading on tubular structures due to underwater explosions. Other issues relevant to the engineering of explosive toppling are also being addressed.
* OPERATIONAL REVIEW : among other issues, this activity has developed design guidance on the selection of explosive initiation systems.
The project has completed all of the activities in Phase 2. Results of these tasks are as follows :
HINGE ASSESSMENT : ABAQUS finite element analysis followed by physical testing has resulted in a clear understanding of the effects of intervention options on hinge strains.
* HINGE RUPTURE : methodology defined in phase 1 has been demonstrated and confirmed in a hinge test incorporating defects.
* EXPLOSIVES EFFECTS : simple methodology for assessing the effects of underwater explosions on adjacent structures, developed in phase 1, has been verified by comparison with numerical simulations.
* HYDRODYNAMIC LOADING : methodologies have been developed to predict hydrodynamic loading on topsides carcasses during toppling.
* SEABED DEBRIS STABILITY : a methodology for predicting the stability of generic debris items on the seabed has been defined.
1) INNOVATIVE TECHNOLOGY
Although the analytical and design methods considered in this proposal will be mainly based on existing technology, this will be state-of-the-art technology applied to a novel operation.
The problem of engineering a predictable outcome of a structural collapse scenario will involve the use of methods in the fields of large deformation, elasto-plastic structural dynamics, hydrodynamics and response to dynamic loading which have not been applied to offshore structures in this context or combination before.
The technological investigations have to be split into the following areas :
* Structural integrity and resistance; with emphasis on understanding and demonstrating analytical methods applicable to the behaviour of structural tubulars forming hinges and undergoing gross angular distortion.
* The use of explosives; with emphasis on predicting the structural effects and identifying the development requirements of underwater explosive cutting tools.
* Hydrodynamic loading and motions; with emphasis on hydrodynamic loading on topsides carcasses during submergence and subsequent movement through the water column.
* Seabed stability of platform remains; with emphasis on the prediction of disintegration rates, debris movement and historical case data.
2) BACKGROUND
Abandonment of North Sea installations will be controlled by the regulatory environment set by national and international bodies. Currently relevant legislation includes the following :
* International Maritime Organisation (IMO) guidelines and standards allow remains to be left on the seabed in water depths over 100m subject to surface clearance restrictions.
* The New OSPAR Convention 1992 defines a list of substances which are subject to measures to prevent their release into the environment.
In-situ demolition of platforms appears to be acceptable under this regime. However, the first abandonments of large structures have yet to be approved. Early elimination of some of thetechnical uncertainties associated with toppling will assist the industry in the current debate surrounding policy formation associated abandonment plan approvals.
3) ECONOMIC BENEFITS
A number of abandonment cost studies have demonstrated savings of the order of 50% on abandonment costs, of the order of UKL 100m, for large installations by adopting toppling of intact installations rather than partial removal and remote dumping. This has been calculated to correspond to a saving equal to 1-2% of the total oil revenue for a typical installation.
Savings could exceed UKL 750 million if adopted on the 16 deepwater installations which will be abandoned on the UKCS, in the next 15 years.
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Coordinateur
GU21 5BW Woking
Royaume-Uni
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