Recent industry trend to increase the production from current offshore assets without significant green field investment require more intervention operations. Riserless light well intervention system is gaining more popularity due to quick turn-around and efficiencies. A typical riserless light well intervention system is composed by coiled tubing downline system, umbilical and wireline system, and well control package. The existing coiled tubing and riserless light well intervention system unfortunately is not designed for open water intervention operations, especially in challenging deepwater environment.

For offshore interventions on a floating vessel, the coiled tubing is deployed from a reel through an injector, and is subject to significant dynamic movements due to wave loadings, ocean current and vessel movement. Integration of umbilical, wireline and other pressure control equipment causes additional constraints on the system's already limited operability windows. Additionally, the crowded subsea infrastructure and dropped object risks create extra challenges for safe and efficient operations.

This paper presents the key challenges and solutions faced by operators in design and operation of the offshore riserless light well intervention system. The challenges are presented from several perspectives: from the equipment capacity and integrity, system operability limitations, risk awareness, procedure controls, to industry standards. The methods and processes to tackle each challenge are presented.

The equipment capacity and weak point are identified and improvement options of various components are evaluated. The equipment improvement opportunities include LARS hoisting capacity assessment, UTA and mudmat tipping over prevention, wireline lazy wave and buoyancy configuration, injector head and guide optimization, subsea jumper and ROV pulling load specification. The operability limitations are increased by engineering analysis optimization and detailed 3D finite element modelling of critical components. Risk awareness and procedure controls are improved by operation guidance and fatigue monitoring mitigations. The novel approaches presented in this paper can be considered for improvement of other riserless light well intervention systems and development of a common industry standard.

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