Huisman-Itrec has extended its multipur-pose tower (MPT) concept to handle dual-activity drilling. This follows delivery of the first MPT system last October for installation on Cal Dive's semisubmersible Q-4000, which is designed to drill in water depths down to 3,000 meters.
The tower can also be integrated into the deck or substructure of drillships, jackups, SPARs, tension leg platforms (TLP), or workover vessels. Through use of tensioners, it can be adapted for coiled tubing, J-lay and flex-lay operations, template installation, and subsea equipment handling.
As a drilling system, the MPT employs a vertical box structure in place of the lattice structure applied to conventional derricks. This box structure provides both the main load-carrying element and an enclosed environment for mounting all key equipment items. Huisman-Itrec claims that the plated box structure is also more efficient for carrying drilling loads and moments than lattice tower structures.
Configurations
The MPT is available in varying configurations to handle triple 30-in. drillpipe, triple 45-in. drillpipe, or quadruple 30-in. drillpipe stands. Its main components are:
- A mast structure, including integrated crown block
- Dual drawworks and controls
- Splittable traveling block
- Dual passive heave compensation
- Active heave compensation on drawworks.
Advantages claimed over conventional derrick/drilling systems include:
- Large, open drillfloor, with no V-door limitations and improved drillfloor access
- Adjustable hoisting capacities and speeds due to splittable blocks
- Storage for 28,600 ft of 5 1/2-in. drillpipe in two carousels
- Smaller installation area (4 meters by 4 meters, to 7 meters by 7 meters), with space for under-balanced drilling or well test packages on the cantilever
- Simple to install or retrofit, and easy to access for maintenance. The MPT can be assembled and tested by Huisman-Itrec in Schiedam, Netherlands, prior to delivery, either as a single unit or in components, depending on customer requirements.
Huisman-Itrec also made a detailed comparison of 30-in. quadruples MPT with 30-in. triples conventional derrick. The firm concluded that in this mode, the MPT provides a weight reduction of 126 tons, a reduction of 3 meters in its center of gravity, faster tripping times, and reduced connection time while drilling.
There are four basic configurations for the MPT: single (equivalent to a single-activity derrick), auxiliary, twin or dual (with the auxiliary tower integrated into the main MPT). The twin or dual MPT is claimed to offer the advantages of a dual activity drilling system on a far wider range of vessels than a conventional dual-drilling derrick.
System components
In more detail, the MPT's hoist tackles are equipped with splittable traveling blocks comprising a main block with two sheaves and four or six detachable single sheave blocks. Splitting is achieved through detaching pairs of sheave blocks from the traveling block and attaching them to the crown block. Maximum hoisting speed is attained with a four-fall block.
Splitting is performed with the traveling block in the top position, using hydraulic cylinders operated remotely from the driller's control panel. Unused sheave blocks are locked to the masthead mechanically via spring-loaded latches. The flexible design allows traveling speed to be raised 400% (as opposed to being fully reeved), as the hook load decreases during tripping.
The tower features a double drum drawworks. These drums are driven by AC motors with synchronized control. During operation, the two winches operate simultaneously, with the drill line kept stationary over both the central equalizer sheave in the masthead and the unused sheave blocks of the traveling block. This approach reduces the overall line speed, leading to decreased line wear. That in turn raises wire service by a factor of two during normal drilling operations.
Should one winch fail, the other can still operate the traveling block, although at half the normal speed. This eliminates the single point failure tendency of conventional rigs. Brakes are also incorporated for emergency braking and parking of the drum. There is no requirement for a deadline anchor and the conventional "cut-and-slip" procedure, because of the dual-winch drum layout. This feature alone should save the operator at least four days' non-productive time per year, Huisman-Itrec claims.
Drawworks are available in capacities ranging from 2,000 kW to 4,000 kW, and can be mounted either in a separate winch frame on the mast's rear face (single, twin, or auxiliary) or inside the mast structure (dual MPT). For the twin and dual MPTs, the capacity of the auxiliary drawworks is typically half the capacity of the main drawworks.
The MPT's active heave compensation is achieved through automated control of the drawworks' electric motors. Two real-time heave motion reference units provide a signal for the winches' AC motor drives. Redundancy in the motion sensors should minimize the risk of compensation signal failure. Passive heave compensation is achieved through hydraulic cylinders and accumulators mounted inside the mast and linked to the traveling block.
Huisman-Itrec's patented Downhole Control System is designed to improve control while drilling without full-time running of the active heave compensation system. This is a wire rope between the riser tensioning ring and the traveling block. The connection comprises a 50 mm diameter wire rope which is reeved from winches mounted in the drawworks winch frame - via sheaves connected to the compensator blocks - to another block on the forward hoisting side of the mast.
The distance between the traveling block and the riser ring remains constant, and the wire rope ensures that relative force between the vessel motion and riser tensioning ring is passed directly to the traveling block, leading to a constant weight on bit. That in turn gives the driller finer control over drilling loads.
