The recent emphasis in performing electric line and coiled tubing operations on subsea trees from ship-shaped vessels smaller than conventional drilling units has introduced a new set of challenges in keeping people safe during major accident events. The most significant of these is preventing the impairment of escape, evacuation and rescue facilities during uncontrolled hydrocarbon accident events, such as, releases originating from the well or, in the case of coiled tubing operations, from the return of pumped and well fluids to surface.

Open water wireline systems by their nature do not involve such risks, however coiled tubing operations involve a conduit between the well and the vessel and, depending on the nature of the preventative measures incorporated in the design of the intervention system, exceeding the allowable impairment frequencies is possible. Adopting the design principles of an open water wireline intervention package to coiled tubing operations where the wellbore is restricted subsea during operations provides distinct advantages as the probability of an accident event is significantly reduced.

Reducing the probability of hydrocarbon related events is critical in the design of the well intervention package and the consequent safety of the system and its operations. This is because well intervention vessels, unlike typical offshore vessels such as semi-submersibles and drillships, do not provide the same inherent protection as traditional offshore vessels against these types of accident events.

This paper presents how to manage the safety of personnel during hydrocarbon related accident events during well intervention operations on an intervention vessel as a function of the design of the invention system in consideration of the consequences for the identified accident events. The paper also assesses the safety associated with each type of intervention system and the vessel of operation relative to each another in terms of impairment frequencies.

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