| Popis: |
Periodically on semi-submersibles, it is required to perform planned and occasionally unplanned disconnects of the marine riser from the subsea BOP stack. The reasons are diverse but always result in a change out of fluids in the riser. Typical circumstances for riser disconnects are: preparation for upcoming bad weather situations, changing from drilling to completion riser and the termination of operations on an exploration well. Should the fluid in the riser happen to be of an environmentally unfriendly nature at the time of disconnect, this fluid must first be displaced to some benign fluid type (e.g. seawater). In the typical displacement, contamination of fluid in the marine riser occurs which generates significant volumes of interface waste or "slop". This might result in a volume of emulsified fluid being created when oil-based drilling fluid or "mud" (OBM) mixes with seawater for example. Since this displacement process often leads to undesirable mixing of two fluids, several prevention methods are often attempted, e.g. the use of viscous pills or similar as a "barrier". Despite these measures, undesirable mixing issues still persist due to the large inside diameter (ID) of the riser compared to the wellbore below. This means that fluid velocities in the riser during a typical displacement are drastically reduced which in turn reduces the effects of the barrier pills. The interfaces between the different fluids have been recorded as being in the region of 30-60 m3 (188-377 bbl) when the rig to sea bed distance is ±350 m (±1,148 ft). The problem is obviously exacerbated significantly as water and hence riser depths increase. Another issue with riser disconnect is mud residue adhering to riser walls. Should these residues contain hydrocarbons or other environmentally unfriendly substances, the potential exists for discharge to the sea while the riser is open-ended in the water while the rig awaits better weather. In many cases, upon recovery, the riser needs to be cleaned on the rig deck prior to the next operation. Riser disconnect can become a costly operation when logistics, planning and slop volume treatment/disposal are all taken into account. |
