The installation of an eight point permanent mooring system in the Piper permanent mooring system in the Piper field

Abstract

The need for support vessels alongside North Sea platforms during offshore construction programmes has platforms during offshore construction programmes has focused attention on the problems of mooring and maintaining station in this service. In the past mooring systems subject to storm situations have been primarily designed for open ocean applications and primarily designed for open ocean applications and have not catered for the requirements of operating and surviving alongside a fixed platform. In addition, anchor handling and dragging of anchors had been seen to be the primary causes of damage to submarine pipelines.

The Piper field, located in the 58 degrees north sector of the North Sea, experienced these mooring difficulties compounded by poor top soil conditions, which led to the design and installation of a fixed permanent anchor pile mooring system around the platform. permanent anchor pile mooring system around the platform.

System Description

Eight anchor piles 60 in. diameter × 2 1/2 in. wall thickness × 94 ft long were driven with an HBM 1500 underwater hammer in a symmetric pattern around Piper 'A' providing suitable anchorage for construction vessels with operating draft displacements in excess of 40,000 L.T. Each pile was positioned 4,000 ft from the platform and has 1,010 ft of 3 1/2 and 4 in. chain attached for mooring. The free end of each chain is retrievable by pendant wire and buoy during connect/disconnect pendant wire and buoy during connect/disconnect operations.

Installation vessel choice

Pile and hammer handling capabilities Pile and hammer handling capabilities Early in the design stage, the use of stabilized lowering device such as the drill string of a drilling vessel was recognised as the optimum method of handling the hammer/follower/pile assembly. Pile/hammer stability calculations indicated the need for a sophisticated stabbing template to be pre-installed on the sea bottom if conventional unstabilized lowering devices were used. Since the template method would add considerable time and cost to the installation programme, our vessel search concentrated mobile drilling rigs.

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