Rigless well intervention and trees on wire from a DPII vessel: a case study*

The Vincent field development is typical subsea development with 11 subsea wells required to be tied back to a central production facility. In 350–400 m of water all subsea operations would be diver-less and the well costs associated with using a semi-submersible rig would be a major element of the total field cost. Sequencing construction vessel and drill rig activities, and aligning this sequence with availability of suitable vessels is often a major challenge on such projects. This was accentuated on this development by the shortage of suitable vessels. In addition to this, multiple mobilisations—potentially incorporating lengthy, expensive transits from Singapore—drove a desire to minimise the use of specialist vessels. The challenges in reducing well costs by using rigless operations are both technical and commercial. Technically, the operations and proposed equipment must be matched to the capabilities and functionality of the vessel. Commercially, sufficient work needs to be identified to justify the one-off costs of preparing and mobilising the vessel and equipment, preparing the operations program, and training and familiarising the personnel. In this instance, the operator identified sufficient work to make the vessel mobilisation and equipment preparation worthwhile while not committing to a long-term deployment. Recognising the multi-role capability of the vessel permitted additional work on nearby assets. These removed the need to mobilise additional vessels to the field and improved vessel’s economy in well intervention mode. Other factors that ensured value was gained from the rigless approach included: the rig schedule being de-coupled from product delivery;the drilling program with limited drilling assets was accelerated, protecting first oil; andthe need to re-configure or modify a rig for Xmas tree (XT) handling was removed. Initial preparations for the planned operations began in the latter part of 2006. The main activities were to: confirm the specification of the subsea lubricator system and its deployment system;undertake a detailed hazard and risk assessment process on all operations;undertake an operability study for crane and deployment system;complete the modifications to the vessel for its intervention role; andprepare detailed well programs and procedures for equipment operation. The design and fabrication of project-specific interface equipment between the subsea lubricator and the subsea Xmas trees, and specification of the required well services equipment such as wireline, pumping and fluid handling services were also required. As this represented the first time such operations had been undertaken in the region by the contractor or by the operator, a large number of supporting systems were required to be developed, adopted and managed, most notably: well control and barrier philosophies;emergency shutdown and disconnection (ESD) philosophy;project management processes; andpersonnel training and familiarisation programs.

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