Two new Global Marine drillships will contain a number of industry firsts: optically coupled load sharing for the drawworks and high resolution 4-quadrant drive-controlled pipe racking. The devices are part of a package of controls developed by Electronic Power Design (EPD) of Houston to handle the drilling operations and automated pipe handling for the drillships Glomar C.R. Luigs and Glomar Jack Ryan. Both will be capable of drilling in 9,000 ft water depth.
EPD recently shipped the first of two identical control systems that will be used in the two Global Marine drillships, under construction in Belfast, Northern Ireland. Drilling operations are to begin during the fourth quarter of 1999 in the Gulf of Mexico with the Glomar C. R. Luigs. Drilling operations with the Glomar Jack Ryan are scheduled also in the Gulf of Mexico during the first quarter of 2000.
Drilling controls
The drawworks uses four load-sharing shunt-wound DC motors (totaling 5,000 hp) for the vertical motion of the pipe string. The EPD-controlled drawworks must be capable of hoisting loads of 75,000 tons. The 750-ton top drive provides the actual drilling motion. The rotary table provides a backup for the top drive and is also used for building pipe stands.
The drawworks is load-shared with fiber optics since two drives are on one side of the ship and two are on the other side. This method prevents noise from disturbing the load sharing signals.
Drilling mud provides the hydrostatic pressure, prevents the entry of formation fluids into the well, and carries the crushed rock to the surface. There are four mud pumps (National 14-P-220). The mud pumps are reciprocating, positive piston displacement and must be synchronized (load-shared) for proper operation and prevention of possible internal pump damage.
The control for all the motors and pumps were built by EPD, using Siemens Simoreg-6RA24 DC drive controllers. Each of the drives has its own field supply controller. All of the drives are speed regulated, which means that they use as much armature current as necessary to achieve the set point.
Drawworks A, drawworks B, the rotary table, and the top drive are all identical. These drives are equipped with reversing contactors, to provide for turning the motor in the opposite direction when necessary.
The drives for the mud pumps, drawworks C, and drawworks D, are single quadrant and have no reversing contactors, since they do not require reversing capability. The drawworks and the mud pumps are synchronized, via the drives, in a load-sharing arrangement.
Control logic being fed to the drives, pumps, and HMI (human-machine interface) is supplied by Siemens S7-400 as master, with S7-300 programmable logic controllers (PLC's) as slaves. EPD believes it is the first to use this advanced hardware for offshore drilling with the intro duction on Global Marine's Glomar Explorer drillship and the Glomar Celtic Sea unit.
Redundancy
A redundancy scheme assures continuous operation. Where possible, the system uses ET-200 distributed I/O (input/output) units. The use of ET-200 remote units provided significant savings by reducing wiring cost and labor, as well as saving valuable ship space.
Like the PLCs, the drives use redundancy to assure continuous operation of the drilling and mud pumps. One lineup is a mirror image of the other. The HMI for the drive line-ups consists of a Siemens OP37. Switching to any of the spares or backups requires using only the rotary switches directly beneath the HMI in the same control cabinet. The HMI reports equipment control and status, provides alarm information, and aids in pinpointing problems should they occur.
The critical nature of keeping the drives and pumps continuously running necessitates a reliable and fast field bus system. The control system connects all the controls via optical links through Profibus, using Siemens communication modules. The Profibus network operates at 1.5 Mbaud. EPD believes it is the first in the industry to implement Profibus protocol with Siemens S7 PLCs and Simoreg drives.
To assure that "the hole" is always preserved, EPD also installed analog speed control pots on the all of the drilling operation drive cabinets. This allows the personnel to manually operate the system during an emergency.
Pipe racking controls
The two pipe racking systems are being built by Oil States-Skagit/Smatco in Houma, Louisiana for later shipment to Belfast. The controls, drives, and motors for the entire system are being integrated by EPD. The joints, complete with collars are initially stored in the vertical racking storage area before they are built into "stands" and transferred in the horizontal pipe racking (HPR) area. The process for building the stands and transporting them to the HPR storage area is as follows:
A crane removes the joint from the vertical rack area. This is transported via a set of motor-powered rollers (the skate) along rails upon which fits a motor-driven cart. Once the cart gets to the rotary table, the pipe is "kicked out" and the vertical lift and horizontal travel cranes work in concert to upright the pipe above the rotary table.
A stabber mechanism grabs the pipe from the side and steadies the pipe to keep it from swinging during this process. Roughnecks on the deck manually guide the pipe into the rotary table where it is engaged for adding the other sections.
This process is repeated for a second and third time completing one stand. The stand is then removed and stored in the horizontal pipe racking storage slots called "sleepers."
The building and storage of the stands takes place before the actual drilling of the hole. It can be performed while at dock or at sea. Initially, it is performed dockside, and then the process is repeated at sea when moving from one drilling site to the next.
EPD is providing the overall control for handling the pipe sections, handling the assembly of the risers and insertion into the borehole, and then controlling all the horizontal pipe racking operations.
The racking and assembly requirements use a total of 21 motors. These motors range from 10 hp to 40 hp. All the motors use drives built by EPD utilizing Siemens DC drive controllers and receive I/O logic via S7-300 programmable logic controllers with the S7-400 being the master PLC. All the drives have 4-quadrant operation capability to provide the motors with the capability to run in the reverse direction very quickly.
These motors are equipped with high resolution encoders to provide the necessary 6-axis motion control and closed-loop feedback required for the drives. Like the control sections for the drilling operation, all of the automated racking system drives have analog pots for manual speed control backup. In an emergency, the cranes can be operated manually via an operating panel. Again, communication between all the drives and PLCs is via the Profibus network.
