Jackup Conductor Tension System
Authors: Childers Mark A. , Sadberry Kenneth H. 
Published: December 2007

The need to place proper top tension on the drilling conductor for exploration wells drilled by jackup MODUs has become more critical as operating water depths increase, weight of BOPs and wellheads increase, and environmental loads on the jackup and conductor become more of a factor. In the past and for shallower water depths, four-wirerope "bazooka" lines tensioned by manual winches laterally helped keep the conductor from excessive horizontal movement and possible collapse. However, in some cases due to rig and conductor pipe motion, the lines often broke, especially during a hurricane, resulting in the conductor possibly collapsing with the BOP and wellhead on top of the conductor. This could result in loss of the well.

Mat and independent leg jackups, especially older designs, sway and rock with wave action enough to have an impact on the attachments to the conductor. Since the conductor is far from the rig's centriod of motion, the tension lines may slacken as the hull dips down and then over tension as the hull rises up with passing wave action. These deflections may be small, but enough to cause this effect. In addition, the conductor pipe flexes back and forth (roughly estimated at up to 3 degrees depending on top tension and soil fixity) as waves pass through the rig. The latter motion helps explain why rigid tension lines attached to the conductor often break when large waves pass through the jackup.

Motion analysis via wave action on a KFELS MOD V Enhanced B-Class jackup (which is one of the new, large, deepwater, ultra-premium design MODUs) shows that even with hull to leg locking devices and reasonably assumed leg fixity to the ocean bottom, the hull will rock back and forth. For a 42 ft. wave, 15-second period, 350 ft. water depth and with the conductor located 30 ft. aft of the aft transom, the vertical motion of the conductor platform relative to the conductor pipe is roughly estimated at plus/minus 4 to 5 ½ inches with up to 6 to 7 ½ ft. lateral movement. With the latter rig motion translated to the conductor pipe and the flexure of the conductor pipe, a motion compensation conductor tension system is required to prevent tension line failures and thus possible collapse of the conductor.

Placement of the Conductor Tension System (CTS) can be on the cantilever beams underneath the well center or off the aft transom via an extended platform. Since almost all jackups (cantilever or slot) are required to skid in for survival conditions, placement of the tension system on the cantilever beams has a significant disadvantage. However, placement of the tension system on the aft transom does not allow conductor tension from all exploratory drilling slots. This should be a minor disadvantage since most exploration wells are drilled from a single position or several specified positions.

This paper presents a 500,000 lb. motion compensated CTS that has been placed on the 400 ft. water depth rated jackup "Atwood Beacon". The issue of how the exact tension value was decided upon, the ability to move the platform to more than one well slot, the amount of motion compensation, jack stroke length, cutting and installing wellheads, use with different size conductor pipes and overall reasons for designing of the system are discussed in this paper.