John Waggoner - Technology Editor, Drilling & Production
Cylinder hoisting system technology has become a lot safer with the advent of the Cylinder Rig from National Oilwell Varco (NOV), which adds additional hoisting ropes to overcome the hazards of previous, single-rope designs.
“The safety of the cylinder hoisting rig drilling package comes from the utilization of four or six hoisting wire ropes, a hazard that has led to serious incidents on traditional drilling units,” says Øyvind Vaagland Reiten, NOV chief engineer for motion compensation equipment.
The system, designed for sixth generational ultra deepwater drilling vessels, is based on earlier CHR technology already deployed on theStena Tay and Stena Don drillships, with innovations that reduce failure risk virtually to zero.
The hydraulic powered system achieves a 2-to-1 mechanical hoisting advantage by means of long stroke cylinders that push in the center of a sheave cluster.
The system is designed so four or six off wire ropes are fixed to the deadline compensators’ equalizer yoke and positioned around the movable sheave assemblies in the sheave cluster at the rod end of the cylinder. This means that 24 m is transferred to 48 m of hoisting/hook travel. The cylinders are supported with flanges bolted on top of an A-frame mounted on the drill floor.
“In a conventional derrick there is only a single lifting cable, with the associated danger of single point failure, a hazard that has contributed to serious incidents on traditional drilling units,” says Reiten.
Besides the derrick, the drilling package upgrades the previous system to help reduce critical path rig time. One of the innovations is an off-line stand building system that permits quadruple stands of the drill string and casing to be made up and racked for future use while drilling operations are under way.
The rig system also has the advantage of reducing the weight and center of gravity by transferring the hoisting load through the hoisting cylinders directly to the drill floor without the need of a heavy lifting derrick also using the rig main deck for pipe set back, and with the fingerboard at drill floor level.
The main improvements in safety include the following:
- Unmanned derrick and open working/drillfloor areas
- Remote operation of all machines involved in routine operations
- Design prevents the top drive from colliding with the drill floor or crown block
- Lower risk of dropped objects
- Over-pull is prevented by an adjustable “limit switch”
- No cut and slip of drill line and tensioner wires.
Another element of the design is accommodation for a knuckle boom crane, which can bring equipment and tools from the deck to the drill floor in a controlled manner and help eliminate crane hazards.
Because the amount of equipment is reduced in the derrick (which also helps limit the risk of dropped objects) potentially hazardous man riding is minimized. Additionally, the use of working baskets, or “cherry pickers”, in the substructure improves the safety of personnel on the rig. Personnel on board will also appreciate a significant reduction in noise levels achieved by the design.
Key design innovations
The core of the Cylinder Hoisting Rig has been developed with state-of-the-art hoisting cylinder design. An improved stuffing box design ensures high sealing integrity and leak monitoring. Each cylinder is fitted with a back up stuffing box fixed at the rod end. The back up stuffing box has three separate back up seal arrangements to be engaged in the main stuffing box in case of sealing failure.
All parts in the cylinders are DNV approved.
The cylinders themselves have been optimized for weight, size, and rod position censoring. New rod end stabbing devices and connection buckets in the sheave cluster use innovative design and smart materials, including locking arrangements.
The design of the cylinder control valve block is optimized with respect to size, weight, selection, and quantity of components. Position measurement of cartridge valve elements in the hoisting control valve block allows for increased safety and system condition monitoring. The sheave cluster has also been simplified and structurally optimized based on experience from operation and evaluation of design fromStena Tay and Stena Don and use of 3D modeling.
Additional refinements of the system include using a combined boost accumulator for the main and auxiliary well. The boost accumulator has an improved piston positioning system which eliminates external piping and adds redundant sensors. The boost accumulator also improves safety margins with respect to sizing and system monitoring by eliminating pressurized accumulators for the piston positioning system. The accumulator is replaced with atmospheric lubrication and leak indication tanks utilizing state-of-the-art filtering technology to prevent condensation problems and overpressure, and the risk of an explosion.
The deadline compensator has an integrated bypass and relief function in the active heave manifold which provides fewer components for easier operation and maintenance. The HPU was improved with a unique compact valve manifold for main pump feeding and pressure relief. The main and auxiliary rig distribution manifold was combined into one single unit, designed with improvements in system verification and maintenance in mind, the manufacturer says.
Benefits over conventional systems
The hoisting cylinder technology provides a number of other benefits besides improved safety. Most notably is the 2.5 m/s hoisting speed of the Cylinder Rig, which is noticeably swifter than its conventional predecessors, and its improved mtr.
An effort was made to improve the ease of use, NOV says. An open drill floor frees space between well centers to facilitate material handling. Each of the hoisting cylinders and guide structure are behind the well center for the same reason.
The system is also mechanically stable as the cylinders push at the center of the sheave cluster, which helps contain sideways forces on the cylinder and guide structure. If a cylinder does fail, it is possible to operate the rig with a reduced load.
The system also permits hoisting even in a power failure with energy storage in the boost accumulator and deadline compensator. The system is able to boost and compensate for the same hoisting and lowering cycle, while energy is regenerated from lowering to lifting though storage of hydraulic pressure in the boost accumulator.
One of the factors that makes the system easier to use is the design of separate systems for hoisting and compensation, according to Reiten. Drill string compensation is achieved by means of conventional passive and active compensation principles, with a separate deadline compensator (DLC) unit located at the drill floor level to avoid complicated regulation of the HPU and boost accumulator.
Drill string compensation acts directly on the dead end of the hoisting wire ropes and the fully passive stand alone DLC system provides safety.
Another main advantage of the technology is the redundancy of the hoisting system. This makes it possible to operate with from one to three cylinders out of order. In other words, if a cylinder were to fail only the hoisting capacity of that cylinder or cylinders out of service would be lost.
However, one of the driving concepts behind the redundant wire rope arrangement is to reduce the potential of failure to a minimum. There are six and four off wire ropes in the main and auxiliary rig, respectively.
