Traditionally any re-entry of a subsea completion for downhole maintenance or data gathering has involved the use of a semi-submersible drilling rig or Through Flow Line (TFL) techniques. The drilling rig is slow to mobilise and is generally over equipped for the task, reducing its cost effectiveness. The TFL option increases the complexity of the well, requires a high initial capital outlay and is unsuitable for production logging.
This paper describes a subsea wireline system which can be deployed from a dynamically positioned monohull vessel providing a low cost, flexible alternative. This system has been used in an extensive wireline and production logging operation in the UK northern North Sea, demonstrating its versatility and its potential for providing a fast response with major cost savings.
There are currently some 400 subsea completions worldwide and with the trend towards subsea production systems, particularly for marginal field developments, this number is predicted to increase significantly over the next decade.
The use of a drilling rig with a conventional BOP stack and a tensioned tie-back riser has a number of significant drawbacks when used for well servicing. Transit speeds are slow, anchor handling operations are time consuming and support vessels are required. Hence the overall cost is high and has precluded all but the essential work required to maintain production. Consequently the procurement of reservoir data has generally not been attempted unless it could be combined with infrequent mechanical work on the well. Permanent downhole gauges have been used, however their long term reliability is unproven and the resultant data has been limited to pressure and temperature measurements at a single depth.
The expansion in subsea developments has created a need for a cost effective, self contained, well service system which can be rapidly mobilised to carry out a wide range of well intervention tasks.
The subsea wireline lubricator system was developed to meet this need and allows a complete range of slick line and electric line downhole operations to be carried out on subsea wells from relatively smaller dynamically positioned vessels. In fact the development of a subsea wireline system has been ongoing for nearly 20 years. In 1970 joint trials were carried out by BP, ADMA and CFP (Totan, using a Flopetrol designed Subsea Wireline Lubricator (SWL), which was deployed on a subsea well in the Zakum field, offshore Abu Dhabi, in 65 ft of water.
It was not until 1984 that a SWL was deployed commercially in the much harsher environment of the North Sea. This SWL was developed by BP, using some of the Flopetrol components, to service the subsea wells on the template beneath the Buchan Alpha moored semi-submersible production platform. The Buchan field is located in a water depth of 375ft, in the UK central North Sea. Following successful wireline operations on the template wells in the field in 1985, a project was initiated with a wireline company (Camco) and a vessel operator (Stena) to develop a handling system to enable the Buchan SWL to be deployed on satellite wells from a dynamically positioned Diving Support Vessel (DSV).
