Deployment of a Riserless Mud Recovery System Offshore Sakhalin Island

Abstract

Abstract: A riserless mud recovery system enables dual gradient subsea drilling operations to take place with the well open at the seabed. There are no pressure containment devices at the wellhead, but as with earlier systems developed for deep water drilling [1], mud and cuttings are returned to the rig by means of a subsea pumping system, fluid recovery hose and umbilical. The system was field tested as part of the Norwegian DEMO2000 project [2]. Subsequently, it has seen operational service in a multi-well drilling campaign in the Caspian Sea. This paper describes how the system was implemented in a remote area exploration drilling operation off Sakhalin Island. Following experience in 2004 and 2005 [3] a clear business case emerged with the underlying drivers of limited weather window, shallow gas and stringent discharge regulations. Accordingly a formal project was established and a number of critical risk reduction studies were carried out in relation to shallow gas and integrity of the return hose under high current conditions. Correct interfacing onboard the rig required careful choice of location, power supply, wellhead and remotely operated subsea vehicle. There changes were verified by means of a formal hazard and operability review. A significant part of the effort involved gaining certification of the equipment for use in the Russian Federation in parallel with the fabrication, acceptance testing, mobilisation, offshore installation and commissioning of the system. The paper concludes with a review of the operational experience from the 2006 drilling season along with the associated lessons learned and forward plan. Introduction Elvary Neftegaz, a joint venture between Rosneft and BP, drilled wells on the northeast Sakhalin shelf during the summers of 2004, 2005, and 2006. The operation encountered numerous challenges in regard to logistics, schedule, location, harsh environment, limited weather window, environmental sensitivities, and stringent regulations [3]. Discharges of drilling waste, even water-based fluids and cuttings are prohibited in the waters offshore Sakhalin once the 30″ conductor is set. In the surface hole, before the blowout preventers are run, mud and cuttings have to be returned to the rig by attaching a marine riser directly to the 30″ wellhead with a hydraulic latch connector. With this arrangement, the only protection against shallow gas blowouts is the rig's diverter system. This practice was the cause of a number of serious incidents during the 1980s [4]. As a result, the industry has moved away from using a riser when drilling surface-hole and present practice is to drill with returns to seabed whenever possible. So, in accordance with Company Policy, having to run the riser system to satisfy the discharge regulations, a risk assessment was carried out. It determined that a pilot hole should be drilled to surface casing depth to prove absence of shallow gas before the riser system was installed. Clearly, this is a time-consuming and still potentially hazardous way of operating and an alternative and safer means had to be investigated. There is a clearly defined weather window within which operations can be conducted. Although satellite images suggest that locations first become clear of ice in early May, the area is affected by drifting pack ice until late June. Severe storms and rapidly decreasing air temperatures in the autumn result in a very clear cut-off date in mid-October for an un-winterised drilling unit. Effectively, the weather window lasts for four months, from about 21st June until 21st October. Clearly, elimination of activities from the "critical path" would enable more to be achieved within a strictly limited period. The combination of being compelled to recover cuttings from the surface hole, to eliminate discharges associated with the pilot hole, to mitigate the risks posed by shallow gas and maximise productive activity within the rigidly constrained weather window drove an urgent search for a different approach. A riserless mud recovery system developed by AGR Subsea AS of Bergen, Norway [2] and actively used in the Caspian was identified as a possible candidate to achieve all the desired objectives. Accordingly, a phased project was initiated to review the feasibility of the technology and, if viable, proceed with implementation.