Today, oil and gas deposits in the earth's crust are extracted by drilling holes down to the deposits, so as to allow the oil and gas to be carried up to the surface.

The equipment used for drilling holes in the earth's crust mainly consists of a drilling derrick formed as a steel truss structure. Hoisting machinery is arranged in the derrick, with a wire for hoisting up and lowering drill pipes. In addition, various equipment is provided for handling the drill pipes and lowering liners into the borehole.

When it is necessary to recover the oil and gas deposits from great depths, use is made of floating or fixed installations with mounted equipment. These platforms are large and especially expensive, due to the drilling equipment being large and heavy. In addition to the actual drilling equipment, piping connections (risers) between the platform and the seabed are also required in order to circulate drilling mud during the drilling operation. Floating platforms also require the installation of a heave compensator in order to accommodate wave motion on the platform relative to the seabed.

The alternative to drilling from floating drilling platforms is to place the drilling equipment directly on the seabed, thus obviating the need for large expensive platforms. The oil industry today has developed a lot of advanced equipment for remote controlled subsea operations. These are technologies that are available and may be used in a remote control drilling operation in which the drilling equipment is placed directly on the seabed.

The present invention aims to provide a remote control drilling module that may be placed directly on the seabed to carry out drilling operations. The drilling operations in question are not purely drilling operations for the purpose of extracting oil and gas deposits, but also drilling work to be carried out in connection with plugging of old boreholes, as well as pilot-drilling and observation wells. Moreover, the use of a drilling module that may be placed on the seabed will also prove relevant for maintenance work in production wells.

With the drilling module situated on the seabed, the requirement for surface support is reduced to a smaller ship with a limited lifting capacity.

Flexible cables and hoses are arranged between the ship and the drilling module for control/monitoring and circulation of drilling mud to the drilling process. Stands of drill pipe are transported between the drilling ship and the drilling module in cassettes that are put in place in the drilling module by means of remote operated vehicles (ROV). The actual drilling module is equipped with buoyancy elements and is anchored to the seabed with a suction anchor, or against a pre-installed anchor frame.

Drilling operations often have to be interrupted under extreme weather conditions, which results'in extra costs and hazardous operations. The situation is also dangerous for rig workers who operate the equipment on platforms that are exposed to weather and wave induced stresses. A disadvantageous common feature of known techniques for using floating drilling vessels is the need for large, heavy platforms and expensive structures.

These problems form the basis of the present invention, the general purpose of which has been to provide a drilling module that may be placed directly on the seabed in order to carry out remote control drilling operations without the use of large, expensive floating drilling platforms. The drilling module should also be designed to carry out maintenance operations in the well and for special drilling in connection with plugging of wells.

For said purpose, the invention is characterised by the characteristics stated in the characterising part of the independent claim 1. Advantageous but secondary characteristics that are non-critical to the intended functioning and technical effect of the invention appear from the subsequent dependent claims.

A drilling module of this type comprises a drilling derrick structure with vertical guides for a drilling machine for rotation of the drill string, and an associated hoisting mechanism, so as to allow drill pipes connected to the drilling machine to be pulled out of the borehole and also, when required, be pushed down into the borehole during drilling operations. The drilling derrick structure is attached to a quadrangular frame structure with buoyancy elements at each corner, and is also provided with hydraulically adjustable anchors against the underlying steel structure positioned on the seabed. The steel structure is anchored to the seabed by means of a suction anchor. The drilling derrick structure with associated frame structure may also be attached to a standard subsea module where appropriate.

The drill string is built up from drill pipe stands that are screwed together with threaded connections. Several pipe stands are stored in cassettes that can be transported between the surface and the drilling module, where they are placed in suitable devices to allow each drill pipe to be handled into the drill string as this is being screwed together or dismantled. Handling of drill pipe stands between the cassettes and a position in the drilling derrick is carried out by a pipe handling device with gripping arms. The

drill pipes are screwed together using gripping and rotating means (Roughneck) placed at the bottom of the drilling derrick structure.

The drilling module is driven hydraulically with hydraulic pressure from a pumping unit placed in connection with the actual drilling derrick structure. The power supply for operation of the hydraulic unit may be electrical power through a cable from the surface, or by means of a hydroturbine supplied with pressure water through a flexible hose from the surface.

Drilling mud used to circulate out drill cuttings is pumped down and back through flexible hose connections from the surface. All operations are controlled and monitored from the surface vessel via flexible cables.

Non-limiting examples of preferred embodiments of devices designed and constructed in accordance with the present invention are explained in greater detail below with reference to the accompanying drawings, in which: Figure 1 shows a schematic diagram illustrating a surface vessel with a drilling module in the lowered position immediately prior to being connected to the foundation frame positioned on the seabed; Figure 2 is a side view of the drilling module with a pipe cassette positioned for insertion into the drilling derrick structure, plus a buoyancy anchor;

Figure 3 shows an elevation of the drilling derrick structure with the drilling machine, pipe handling equipment, drill pipe and screw means for screwing and unscrewing drill pipe threads; and Figure 4 shows a basic section through a hose for supply/return of drilling mud, communication and monitoring/control cables.

Reference is made to figure 1, where 21 denotes a light surface vessel with mounted hoisting equipment 22, positioned on the surface 23 of the sea directly above the drilling module on the seabed 15. A flexible connection 5 with hoses and control cables extends between the drilling module and the vessel. The pipe cassette 4 is hoisted into place using a wire 1 and hoisting machinery 22. The drilling derrick 10 is connected to the frame 12, which in turn is equipped with buoyancy elements 13 at each corner. The frame 12 is also equipped with anchors 16 that engage fixtures 14 on the foundation frame 24.

Figure 2 shows the drilling derrick 10 with a mounted drilling machine 9 and screw means 11 for screwing together drill pipe stands, and gripping arms 7 for handling pipes. A hydraulic drive unit 20 is positioned in connection with the actual drilling derrick structure 10. The pipe cassettes 4 with frame 3 are provided with buoyancy elements 2 for adjustment of buoyancy. Guiding elements 8 hold the pipe cassettes in place relative to the drilling derrick structure 10. A buoyancy element 6 is arranged on top of the drilling derrick structure for adjustment of buoyancy.

The foundation frame 24 is held in place against the seabed by means of suction anchors 17.

Figure 3 shows the pipe handling equipment with a vertical structure 19 having mounted gripping devices 7 for transporting drill pipes between the pipe storage 4 and a position in the drilling derrick 25. The pipe handling equipment consist of a vertical structure 19 supported at top and bottom by guides 18, so as to allow the structure 19 to rotate about the vertical axis while at the same time going through a concentric motion about the vertical axis 26 of the drilling derrick.

Figure 4 shows a basic section through a supply cable 5 between a surface vessel 21 and the drilling derrick 10. The supply cable is composed of hoses 29 and 30 for supply and return of drilling mud, and also supply 31 of energy to the drilling module. In addition, there are cables 32 for monitoring and control of the drilling module and associated equipment.