05.07.2009

Semisubmersible rigs are the most common type of offshore drilling rigs, combining the advantages of submersible rigs with the ability to drill in deep water. Semisubmersible rigs work on the same principle as submersible rigs; through the ‘inflating’ and ‘deflating’ of its lower hull. The main difference with a semisubmersible rig, however, is that when the air is let out of the lower hull, the rig does not submerge to the sea floor. Instead, the rig is partially submerged, but still floats above the drill site. When drilling, the lower hull, filled with water, provides stability to the rig. Semisubmersible rigs are held in place by huge anchors, each weighing upwards of ten tons. These anchors, combined with the submerged portion of the rig, ensure that the platform is stable and safe enough to be used in turbulent offshore waters. Semisubmersible rigs can be used to drill in much deeper water than the rigs mentioned above.

This is a complex, high technology activity that has its roots in the seismic characteristics of the Earth. Seismology, as practiced commercially in the search for hydrocarbons, is a large and in places very complex subject. The field activity is conducted by advanced survey vessels with typical size of approximately 300ft (5-6000 gross tons), equipped with an energy source (compressed air) to create sound waves and listening cables to register the “echoes” or returned sound waves from the subsurface of the sea bottom. The typical crew onboard the vessel consist of approximately 28 people, of which 12 people are the maritime crew and the rest represents the experts in various disciplines needed to acquire seismic data. The result of the data acquisition is partly analyzed onboard and stored on high capacitive data storage media (Mass Storage System) for further processing and analysis after being received in a data processing centre. The amount of data stored is enormous, for each survey line with approximate length of 23 kilometers typically 7-8 data cartridges are recorded and each cartridge is capable of holding 1 Gigabytes of data. In seismic surveying, energy is generated onboard a survey vessel to produce waves and send them into the earth. This energy wave doesn’t just vanish into the earth’s crust. Some of it is reflected back to the surface and we detect the returning waves with sensitive measuring devices that accurately record the strength of the wave and the time it has taken to travel into the earth’s crust and back to the sur­face. We can then take these recordings and after various adjustments, done mostly by computers, we can make them into visual pictures that give us a good idea of what the subsurface of the earth is like beneath the seismic survey area. So in summary, we cannot see directly what the rock layers are doing beneath the surface but we can use seismic surveying to get the picture indirectly.

This is the cable that is towed directly behind a survey vessel (2D, 3D). The cable is the listening device and registered signals are fed through the cables and routed to the instrument room of the vessel, where the signals are converted and stored on data tapes for further analyses.

A cable towed behind the vessel conducting the seismic survey and containing hydrophones that record the pressure changes. 2D surveys use a single cable and 3D surveys use up to 12 cables.

In parallel with the registration and data storage, an onboard quality control and preliminary data view is carried out locally onboard by the onboard Geophysicist.

Seismic cables have a slight positive buoyancy and will float to the surface if they are left with no tension in water. The cable has to be balanced to operate at a constant certain depth of 7m below the sea surface; this is achieved by pre-survey adjustments (adding/removing ballast or streamer fluid) and maintained during operations by means of cable control device (cable levelers or birds) positioned typically every 300m along the cables. Ideally, the cables should be towed straight behind the vessel. If this is not possible due to currents and influence by the environment, the streamer may be expected to ’feather’ up to 10 degrees.

Continuous information about how the shape of the cables changes and where they are located are achieved through cable mounted compasses, and GPS receivers on the tail buoys of the cables. A total picture of positions and behavior of the cables are at all times presented on various displays onboard the vessel. Should there be an emergency situation, the crew have the possibility to quickly (10 minutes) dive the cables to safe depth > 20m using the depth controlling birds, This usually has to be done only if there are ships traveling on a collision course with the cables and the crew and the assistant vessel are not able to get in contact with this ship to ask for a change of course.

Natural releases of material from the sediment to the water column, often in discrete locations. Provides evidence of hydrocarbons. Seeps occur along fractures in reservoirs or at places where the earth’s surface cuts the formation

A sill is rock that has intruded between older layers of sedimentary rock. The sill does not cut across pre-existing rocks in contrast to dikes, which do cut across older rocks. Sills are always parallel to layers of the surrounding rock. Usually they are in a horizontal orientation, although tectonic processes can cause rotation of sills into near vertical orientations.

Often accompanies the signing of a contract for an exploration license and is a sum paid by the exploration company to the licensing authority. It is often one of several elements in a competitive license application

A measure of signal strength relative to background noise. The ratio is usually measured in decibels (dB). >

A core taken from the side of the borehole usually by a wireline tool. Sidewall cores may be taken using percussion or mechanical drilling