Flexible MODS rig simplifies offshore drilling, workovers
Small rigs in demand
As production moves into deeper and deeper water, the number of spar and TLPs operating in the Gulf of Mexico has increased significantly. Demand has simultaneously increased for small, lightweight drilling and workover rigs that can operate on these platforms. This increasing demand led Nabors Offshore to develop the Modular Offshore Dynamic Series (MODS) rigs, which have worked on 13 of the 20 spars and TLPs operating in the Gulf.
The rigs’ modular design reflects how it is mobilized and demobilized. The rig breaks down into lightweight modules weighing no more than 55,000 lb each. Nabors designed the rig using high strength steel to minimize weight, which also simplifies handling requirements during load out and rig up. The operator’s platform crane can offload the lightweight modules in deepwater applications, or the rig crane can offload the modules during standard installation. This eliminates the need for a derrick barge or other heavy lifting equipment.
Space is at a premium on spars and TLPs. A drilling and workover rig must have a small footprint, be lightweight, and be portable. The lightweight modules allow components like mud pumps and mud tanks to be stacked without exceeding weight requirements or safety parameters. This maximizes the use of the existing space on the platform.
Nabors designed its MODS rigs to perform tiebacks, completions, or sidetracks.
MODS rigs must also conform to a variety of installation and application requirements because no two platforms are alike. Since offshore platforms have capping beam spacing from 30-69 ft, Nabors designed MODS rigs to rig up on these beam configurations using adaptor frames for narrow to wide applications less than 40 ft and greater than 50 ft.
“Rigs on spars and TLPs must also be able to withstand the dynamic forces caused by wind and wave action,” Joe Bruce, Gulf of Mexico president, Nabors Offshore Corp., says. “Especially important is how well the rig stands up to acceleration forces, which are loads caused by the rebound effects that occur after the platform is moved by wind or wave action and then recover to its original position.”
To withstand these dynamic forces, Nabors constructed MODS rigs with a stronger substructure capable of handling heavier loads. The company also added a dynamic assist frame to strengthen the mast and use special clamping mechanisms to secure the rig to the platform, which keeps the rig on location in spite of pitch and roll action.
“MODS rigs were designed primarily to do tiebacks, completions, or sidetracks,” Bruce says. “Generally a semisubmersible or drill ship is brought in to pre-drill the well. Then a spar or TLP platform is set over the well and a rig is added. Our rigs range in size from 750 hp to 2,000 hp so they can work in depths up to 25,000 ft.”
One of the reasons for the success of the MODS rig has been flexibility. Fixed platform rigs cannot be placed on spars or TLPs without extensive modifications and a significant increase in structural weight. Nabors designed the MODS rig to move between the two by adding or eliminating the mast dynamic assist frame and the skid base substructure adaptor frame.
Safety is essential in the offshore environment. Nabors added additional stairways to improve access, and has designed personnel tie off points into the structure, easily allowing the crew to use harnesses around the rig. The company also installed non-skid safety pads throughout the rig and non-skid plates on the stairways.
“We just rolled out the latest version of the MODS rigs,” Bruce said. “The MODS 200 and 201 are A/C powered, which provides greater control and less maintenance. It also allows us to drill top to bottom in water depths of 6,000 ft. If we go to 10,000 ft of water, I suspect we will build a 3,000 HP rig.”
