Offshore drilling for exploration and scientific purposes in deep waters and subjected to harsh environments due to high current or extreme waves, or a combination of both has been a challenge for decades and lacks a complete and robust solution to allow reliable and continuous drilling operations. A unique riser restraining system is presented in this paper for improving the operability windows of drilling rigs in harsh environments. The restraining system restricts transverse deflection of the risers due to high current drag forces and coupled dynamic motions due to extreme waves that cause potential clashes with the moonpool. The restraining system proposed in this paper uses a modified tensioning ring to act as a centralizer, which is connected to hydro-pneumatic tensioners in the lateral plane for providing retention resistance and attached to removable support structures fixed to the moonpool decks. The customized upper and lower restraining joints are used for interfacing the riser system with the centralizing tensioner ring along with treble slips joint, intermediate flex joints and PUP joints to ensure smooth load transfer and reduce stress concentrations on the connection interface.

A thorough review was carried out on some of the potential offshore drilling locations that are exposed to extreme environments. These assessments were used to generate wave and current combination and three different water depths were defined to establish the design basis for analyzing the system. Numerical analysis using time domain simulations were carried out to check the global and local strength integrity of the system. The results were utilized to benchmark the improvement in operability of the riser system with the restraining solution as compared to conventional systems with and without fairings. Simulations with the proposed restraining system show that the exceedances of key parameters like flex joint angles and riser clashes with moonpool deck are reduced to significant levels while maintaining the stress envelopes within allowable limits.

Generic modifications required for implementing on existing drillships and semisubmersibles are also highlighted. The overall global and local strength integrity of the system and significant improvement in operability window are presented to demonstrate the benefits of the proposed system.

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