Tech Tidbits, vol. 3

Evaluating bid tenders includes verification that the equipment bid will be able to complete the drilling

program specified, i.e. well depth, lengths of each casing string, specified mud delivery and

conditioning, etc. However, sometimes the capabilities as identified by manufacturer and model

number may not reflect the operating capacities as installed. The following are two case studies to

illustrate this situation.

1. Mud delivery

Specifications in the inquiry required 1,000 gpm at 4,500 psi. The

contractor equipment list showed two mud pumps which the vendor data sheets show as capable of

delivering approximately 430 gpm at 4,670 psi each. No problem, right?

After contracting the rig, during acceptance it was discovered these mud pumps were fitted with 1,000

hp motors. In order to deliver the nameplate specifications, a 1,600 hp driver is required. The

contractor replaced the installed motors with larger ones capable of delivering the mud needed for the

program. Recognizing this as part of the acceptance allowed execution of the drilling program as

designed.

There are several reasons why this "mismatch" could have occurred.

1. Some contractors choose to install lower horsepower electric motors than the maximum input

power rating for a specific mud pump if the anticipated service conditions will permit (e.g. high

circulation rate, low standpipe pressure), thus saving the incremental cost of the more

powerful motor.

2. Another reason for the lower horsepower drivers could be improved pump service life

(increased engineering factor of safety).

3. In certain cases, equipment availability may be an issue, and lower powered motors are

selected simply to more expeditiously get back to work. Once installed, they remain in service

until the next replacement cycle.

2. Hoisting and braking

Acceptance of one rig resulted in recognizing that the eddy

current brake recommended by the drawworks manufacturer was not installed, but one of a lower

capacity. Discussions with both the drawworks and eddy current brake manufacturers suggested the

possibility that the braking capacity was adequate, but only if the controls were configured and

installed to utilize the drawworks motors as regenerative brakes. The rig could not confirm the

controls configuration.

A drawworks braking capacity test was designed and conducted, confirming sufficiency of this capacity

as installed.

Both cases illustrate the importance of careful assessment and acceptance of rig equipment to ensure

performance requirements will be achieved. Procedures that do not require checking the matching of

system components could result in hiring a rig that is not suitable for the program you have designed.