The drill system of the Norskald consists of a standard American Petroleum Institute (API) drill string for normal boring driven by a hollow spindle power swivel. For hard rock coring, a "piggyback" system is used, consisting of a wireline diamond coring rig on the top of the platform of the power swivel for the API system (Fig. F1). The piggybacked wireline is run through the main API pipe and is advanced ahead of the API pipe during drilling into hard rock. In this mode of operation, the API drill string is first lowered through the moonpool to the seabed along with the guide frame template. The API system is used to bore to rockhead, after which the drill pipe is locked in place by the seabed template. This pipe serves as casing for the wireline drill string that is run to the bottom of the hole, where diamond coring commences. Core passing into the inner barrel is retained by the core catcher and retrieved by wireline. This double-barrel arrangement reduces disturbances of the core during drilling and avoids having to pull up the drill string to retrieve the core. Despite some technical difficulties, the system was very successful in drilling basalt, even in cases of highly fractured bedrock as shown in Figure F2.
Dynamic positioning of the Norskald was accomplished by two pairs of thrusters located forward and aft with input signals from one of three motion detection systems: (1) an acoustic beacon mounted to the seabed template, (2) tension sensors on guide wires of the seabed template, and/or (3) deflection sensors on a tautwire positioned outboard of the seabed template (Fig. F1). Any two of these systems provided redundancy, and commonly the tautwire was not used.
Under normal operating conditions the vessel can be moved short distances (<400 m) in areas of smooth ground without retrieving the drill string. This is accomplished by first withdrawing the wireline system well above the seabed and then lifting the API drill string and seabed template slightly off the seabed by elevating the heave compensator to the top of the derrick. For more remote repositioning, the wireline core barrel and seabed template can be raised completely and the total length of the API drill string reduced sufficiently to avoid potential obstacles. More often, when moving the vessel >400 m, the entire API pipe has to be retrieved. These differences in operation are important because they form the basis for the site and hole designations. We designate a new site if it is necessary to completely raise the wireline system and seabed template to reposition the vessel. If relocation can be accomplished using the heave compensator to lift the API drill string and seabed template off the seabed, as explained above, we designate a new hole but retain the site number.