As the pace of deepwater oil and gas exploration continues to grow, so too does demand for modern drilling vessels with equipment capable of operating in water depths of 10,000ft or greater. These greater water depths place significant demands on the drilling riser and the riser tensioning system. Modern riser tensioners are complex hydro-pneumatic systems and far from applying a constant top tension, the stiffness and damping characteristics associated with the tensioner mean that the applied tension can vary substantially as the tensioner strokes in response to vessel heave. As a result it is critical that the riser tensioner system response be captured in sufficient detail when evaluating the loads on the drilling riser.

Riser tensioner systems for deepwater drilling must be capable of supplying the required tension to satisfy the minimum stability tension requirement, as per API RP 16Q; however this recommended practice does not adequately account for dynamic tensioner load variation, which can be up to 50% of the nominal tension. For deepwater drilling riser systems, where riser load limits are being approached, accurate modeling of the tensioner system load variation is required to ensure that the riser does not experience compression or excessive stresses. Furthermore, as the dynamic tension variations are largely velocity dependent, they can be relatively independent of water depth. Thus larger percentage variations in tension are experienced at low tensions when compared to higher tensions. This is an important consideration when calculating minimum top tensions for deepwater drilling rigs in shallower water depths.

This paper presents a comparison of the response of a direct-acting riser tensioner (DAT) system for a range of environments. The comparison is based on results from detailed tensioner models that include the individual hydraulic and pneumatic components of the tensioner system and that are fully integrated with a non-linear 3D structural FE analysis tool [1]. The FE model is based on a widely-validated-non-linear software tool [3]. The detailed tensioner model has been validated against manufacturer performance data for existing in-service tensioner systems. The detailed tensioner model has also been used as part of a drilling riser recoil analysis study [1] which provided a good comparison of recoil analysis results against a published recoil test case. The impact on the global riser response of accurately modeling the tensioner system behavior is demonstrated, while the implications for the calculation of minimum top tension are also discussed.

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