ABSTRACT

Over a long period of time, remnants of fixed offshore structures may corrode and split into smaller pieces from repeated loading from waves and currents. For the safety of other users of the sea, debris must not migrate away from the site of the structure.

Hydrodynamic loads on debris may be assessed from the effect of waves and currents on the boundary layer close to the sea bed. Expressions for lift, drag and inertia forces are available from pipeline stability studies. New expressions have been derived for stability and resistance of isolated tubular and prisms on the sea bed, and compared with the result of model tests on sand and clay. The effect on stability of effective density, size, shape, soil conditions, environmental forces and height of fall to the seabed are discussed. Tubulars tends to sink into soft clay but may roll on hard clay. Data from wrecks indicate the potential for self burial due to sediment transport.

INTRODUCTION

This paper describes efforts to predict what might happen to the remains of steel structures, which may be left on the seabed, following decommissioning of particular concern are remains left in the waters deeper than 100m of the northern North Sea, from which complete fixed platform have yet to be removed. If platform remains are to be left on the seabed the Regulatory Authorities will require evidence that they will remain substantially intact, stable and not migrate across the seabed to become a hazard to fishermen and other legitimate users of the sea. Criteria are needed to determine what items need to be recovered from the seabed and what items may be left in place with confidence that they will not become a problem in the future. To achieve this, it is essential to understand both the time scales and the modes by which the debris will break up into progressively smaller items, and also understand the influences on its subsequent and resistance to migration across the seabed.

DATA ON WRECKS AND STRUCTURAL DISINTEGRATION

Records of all known wrecks around the coast of the UK are held by the Ministry of Defence Hydrographic Office, Taunton, These records point to a number of possible sources of detailed data, including :

  • MoD Hydrohraphic Office archives

  • MoD contract surveyors

  • Salvage companies (dangerous wrecks)

  • Amateur divers (normally only for shoreline wrecks)

  • European national records

Survey data has been identified for twenty wrecks in the vicinity of the North Sea(see Figure 1). Data were obtained from the MoD Hydrographic office, Taunton and from a salvage contractor for wrecks of :

  • ships, in the same region of the northern North Sea as the fixed platform in deeper water.

  • oil related installations and vessels, including jack-up rigs, semi-submersible rigs and capsized jackets

  • ships that have been surveyed or recovered by salvage contractors and as a result can provide indications of actual corrosion rates.

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