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Light Well Intervention Vessel

In completed wells, e.g. in connection with the recovery of hydrocarbons, it may be necessary at times to perform various types of working operations downhole. The purpose of such working operations, or well interventions, may vary. Normally an intervention is carried out with the aim of disposing or removing components and equipment in the well, e.g. plugs/packers or valves, or with the aim of performing various measurements in the well, for example measuring of internal pipe diameter or measuring of production parameters in the well, possibly of perforating casing or tubing within the well. Typically, the intervention is carried out when the well is pressurized, for example by production of formation fluids.

Some wells do not produce as expected and must be replaced. "Planned workovers" are required to re-complete to new zones because high production rates of these wells result in relatively fast zone depletion. Most reservoirs are layered and faulted. Most wells water-out and/or production rates drop to uneconomical levels in a relatively short time. For each zone, initial production rates, rate of production decline and total recoverable reservoirs must be considered in a lifecycle evaluation to ensure that future operational requirements are not overlooked in the original planning.

Some well system component failures should be anticipated for all wells. Tubing string leaks and sand control failures are inevitable and stimulation operations may be required to maintain acceptable production rates. Subsea wells must also contend with subsea facilities failures in control system components, flying leads, manifold and tree valves, flowlines, etc.

Known intervention technique for a subsea well normally assumes the use of a mobile drilling device which is positioned above a well location. Especially by greater water depths it may be necessary to use a floating drilling device, e.g. a drilling rig or a drill ship, such a drilling device often being kept in the correct location by means of dynamic positioning equipment.

A connection between the drilling device and the subsea well is normally constituted by a riser connected to a blow-out preventer (BOP) below, these being lowered from the drilling device and connected to the wellhead of the well. Subsequently the intervention tool is lowered trough the riser on a flexible supply string, through the BOP and wellhead, further down into the well to perform the operation in question. The supply string that the intervention tool is connected to, is formed, for example, of a wire (slickline, wireline) or coiled tubing, the supply string being coiled on a drum. When the intervention tool is being lowered into the well, the string is supplied from the drum by means of an injector as it is being lowered from the drilling device. Conversely, when the intervention tool is lifted from the well, the supply string is coiled onto the drum. In this connection the drilling device is provided additionally with at least one pressure safety device, e.g. a lubricator/stripper, through which the supply string is passed during the intervention work, the securing device forming a pressure barrier between the pressurized well and the surface environment.

In connection with such intervention work successive and/or different intervention operations are often carried out, and it may be necessary in each intervention operation to use another and/or different intervention tool. Therefore, on the surface it will be necessary, possibly for each intervention operation, to change the intervention tool, after which the supply string is run in and out of the well for every intervention operation.

The most obvious drawback of known intervention techniques is that a mobile/floating drilling device is normally used to carry out the necessary intervention operation or operations in a subsea well. Normally hiring and using such drilling devices involves great cost.

Another drawback of mobile/floating drilling devices is that operations are often limited by weather-dependent conditions, mainly wave height and wind force, so that a large portion of the contracting time may be spent in waiting for better weather conditions, the contracting costs thereby increasing.

To a great degree such weather-dependent stops in the operations are related to the presence of a riser connecting the drilling device with the subsea well. In strong wind and/or great wave movements, and thereby great movement of the drilling device, the riser will have to be disconnected. Thereby all types of well operations carried out through the riser, including intervention operations, stop, and the use of a riser connection between the well device and the subsea well is therefore a disadvantage. Besides, the initial installation and the final removal of a riser are time-consuming and thereby cost-driving.

In connection with subsea wells the intervention tool and the supply string must be lowered through the rinser from the drilling device on the surface to the wellhead/BOP on the water floor, in order then for intervention work to be carried out. By possible replacement of the intervention tool, the intervention tool and the supply string must then be lifted all the way up to the surface. The extra time, and thereby extra cost, spent on running the intervention equipment in and out through the riser, represent a further drawback of known intervention techniques, especially by greater water depths.

The Seawell; the principal asset of UK based, Well Ops (UK) Limited, is an established leader in the mature North Sea Oil & Gas market place. The Vessel has been operating in the North Sea as a Light Well Intervention (LWI) vessel since 1987, during which time has entered over 250 Subsea wells, abandoned 75 live wells and removed in excess of 100 suspended wells. As a purpose-built vessel introduced in 1987 the Seawell has performed more than Seawel intervention projects and decommissioned more than 100 wells and 15 subsea fields. These operations have been conducted in all areas of the North Sea (the UK, Norwegian, Dutch, Danish and Irish Sectors).

The Seawell is custom designed to dynamically positioned (DP3) specification and can accommodate up to 135 people in 78 cabins. With a purpose built derrick over a 7m x 5m working moonpool and a travelling block rated to 65t MT in active compensation mode and 80t MT capacity in passive mode, it is in a class of its own; unique in both structural design and operational relevance.

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