A major offshore trial of a mid-depth BOP drilling riser system was conducted in 2009 by COSL in 470m water depth of the South China Sea. The objective of the trial was to demonstrate the feasibility of using a shallow water drilling platform to drill in depths which would otherwise be beyond its reach. A free-standing tieback string supported by a buoyancy unit presented an ‘artificial seabed’ and wellhead at an elevation which the platform’s existing drilling riser with its subsea BOP could connect to and drill through.

The trial mobilised a 3rd generation semi-submersible drilling platform Nanhai 5, its drilling riser and subsea BOP, a test tieback string, and a fully commissioned buoyancy unit. The full offshore transportation and installation sequences were executed for Nanhai 5 to successfully connect to a mock-up ‘real seabed’ wellhead via the afore-mentioned set-up.

The novel components in the system, particularly the tieback string and buoyancy unit, were instrumented and monitored to study their motion behaviour for correlation against analytical predictions.

In the present day ultra-deepwater drilling world, the interests for such a depth-extending system have revived, in the context of the fleet of modern 3000m-rated drilling platforms wanting to go beyond 3000m water depth. The lessons learnt from the COSL trial bear significant relevance for designing future systems.

This paper describes the trial, its design and planning, certain problems encountered and some recorded system behaviour.