Transient dynamics of drilling riser during disconnect

Abstract

Drilling of petroleum wells have traditionally be done by adjusting the mud weight to balance pressure of the well. A new method of controlling well pressure is called controlled mud cap drilling. The method uses high mud weight and constantly changing the mud height to adjust the bottom hole pressure. This allows for drilling wells which previously were considered un-drillable.

Drilling of wells might be done by a floating drilling vessel, which might drift off. In such cases an emergency disconnect of riser may be unavoidable. Hence the riser must be disconnected. The top tension is set high enough to lift the riser and LMRP clear of the BOP with a set mud height. This top tension creates recoil in the riser. During drilling the mud height changes constantly while the top tension remains constant. In essence the top tension is relatively large if disconnecting with low mud level, hence recoil would be larger.

In order to look into this issue of riser response with different mud heights a model has been established in Riflex. This has been modeled with weight on an accurate modeling of the riser and heave compensation system while the LMRP has been simplified. The mud in the riser has been modeled with three different mud heights, 80m, 150m and 200m above the seabed. During disconnect the mud can either stay in the riser (annular preventer closed) or it can fall out (open annular preventer), both of these cases have been modeled. The drilling vessel is usually not positioned directly above the BOP when an emergency disconnect is implemented. Therefore a horizontal offset of 5% of water depth was set. The study case is the Åsgard field on Norwegian Continental Shelf which has a water depth of 350 meters, hence the offset is set to be 17.5 meters.

There will be an unbalance in forces at the moment the riser disconnects. A tension wave and a mud pressure wave will travel up the riser. These waves travel up the riser induces an upward motion on the riser and a fall in axial stress. The magnitude of the fall in axial stress is highly dependent on the mud height at the moment of release. The drop in axial is greater for low mud heights and decreasing with increasing mud height. The fall might be so large that compression of the riser can occur. The risk of compression is greatest at the bottom of the riser.

Maximum bending stresses as a response to disconnect may not be at the moment of release. There might be a delay in the maximum response. These bending stresses are not large enough in themselves to impose any structural issues on the riser. There will be no issues unless a peak value in bending stresses and a fall in axial stresses coincide. This might occur at the bottom of the riser. For low mud levels the riser might experience axial compression combined with relatively high bending stresses. This combination may result in local buckling, however, this is subject for further work.

The responses are in general slightly larger when the drilling vessel has no offset.

In order to avoid extreme response of the riser it is advisable to increase the amount of mud. It may be useful to determine an optimal mud level where the responses to release are low. A part of the emergency disconnect sequence might be to fill the riser either with water or mud in order to come close to the optimal release mud height before release. It is very important that the mud level is low enough too able the riser to lift clear the BOP. It is more expectable to have damage to the riser or drilling vessel compared to the BOP.