Offshore drilling rig (mobile offshore drilling unit)

01

Pick the unit type and water depth

US Coast Guard rules (46 CFR 107.111) define a mobile offshore drilling unit as a vessel capable of drilling for subsea resources, and distinguish a self-elevating unit, with movable legs that raise its hull above the sea surface; a column-stabilised unit, whose main deck is connected to an underwater hull or footings by columns or caissons; and a surface-type unit, a ship-shaped or barge hull operating afloat. [1]

Noble's BlackHawk is a Gusto P10000 dual-activity drillship rated for 12,000 ft water depth, held on station by six 5,000 kW azimuth thrusters with no mooring system. Noble's Great White is a Moss CS60E semisubmersible rated for 9,840 ft (3,000 m) as outfitted, with eight Rolls-Royce mooring winches rated at 450 t pull and eight 4,800 kW azimuth thrusters. [2] [3]

Editorial calculation: 12,000 ft is about 3,660 m and 40,000 ft about 12,190 m; ask the contractor how the drilling depth rating is referenced before comparing it with your well's total depth. [2]

Give the supplier or contractor the location, water depth, seabed and metocean conditions, well design and total depth, BOP pressure rating, hook load, mud system and pump pressure, deck load for the campaign, station-keeping method (legs, moorings, dynamic positioning), power, accommodation, helideck and flag or class requirements, and the period.

02

Check drilling package, deck load and power

The BlackHawk lists 40,000 ft drilling depth, hook load of 2.5 million lb (main) and 1.5 million lb (auxiliary), five NOV 14-P-220 mud pumps at 2,200 hp and 7,500 psi, two 18¾-inch 15k psi seven-ram BOPs, 70,459 t displacement, 21,150 t variable deck load, 210 beds, and main power from six 4,500 kW and two 9,000 kW generator sets. [2]

The Great White lists 35,000 ft (10,670 m) drilling depth, a 1,000 short-ton hook load, four 7,500 psi mud pumps, 68,375 t displacement, 7,500 t variable deck load when drilling (6,000 t in transit), 180 beds, eight 6,120 kW diesel engines and a helideck for EH-101 and Sikorsky S-61/92 helicopters. [3]

Editorial calculation: 2.5 million lb is about 1,130 t; compare the hook load with the heaviest casing string in your well design, not with the drilling depth alone. [2]

03

Check the subsea BOP configuration US rules require

For work on the US outer continental shelf, the Bureau of Safety and Environmental Enforcement's rule 30 CFR 250.734 requires a subsea BOP system to have at least five remote-controlled, hydraulically operated BOPs: at least one annular BOP, two BOPs with pipe rams and two with shear rams. Both shear rams must be able to shear drill pipe, workstring, tubing and their control lines under maximum anticipated surface pressure (MASP), and at least one must seal the well after shearing. The system must also have a redundant pod control system; accumulator capacity, including subsea bottles for the autoshear and deadman functions, to close each shear ram, the ram locks and one pipe ram and disconnect the lower marine riser package; ROV intervention capability within the response times of API Standard 53; and a trained ROV crew on the rig whenever the BOP is deployed. Dynamically positioned rigs need autoshear, deadman and emergency disconnect (EDS) systems; moored rigs need autoshear and deadman, and each must close at least two shear rams in sequence. [4]

Compare this with the 15,000 psi BOP and drilling packages listed for the drillship and semisubmersible above; for a US Gulf campaign the stack must meet the ram count and emergency-system rules as well as the pressure rating. [2] [3] [1]

Editorial calculation: 1 annular + 2 pipe-ram + 2 shear-ram BOPs gives the minimum of 5 BOPs the rule sets; a DP drillship needs three emergency shut-in modes (autoshear, deadman, EDS) against two for a moored semisubmersible.

For a US offshore contract, ask the drilling contractor for the BOP stack drawing with ram types, the shear capability test results under MASP, the accumulator calculation and the emergency system configuration for the rig's station-keeping mode.

04

ZPMC jack-ups and CIMC Raffles semisubmersibles

Shanghai Zhenhua Heavy Industries Co., Ltd. (ZPMC) lists jack-up rigs on its website, and Yantai CIMC Raffles Offshore Limited lists semi-submersible drilling platforms.

Ask each builder or contractor for the rig specification sheet with design, water and drilling depth, hook load, deck load, mud pumps, BOP and power figures. [2] [3]

05

Hull, station keeping, hoisting and well-control systems

The two Noble units hold station in different ways: the BlackHawk drillship has no mooring system and relies on thrusters, while the Great White semisubmersible combines mooring winches with thrusters.

06

BlackHawk and Great White figures, row by row

Where a row has two values, the first is the BlackHawk drillship and the second the Great White semisubmersible. Neither Noble sheet gives an operating temperature or a free deck area, so those rows have to come from the contractor.

07

Send the site, well and campaign data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Site: [location; water depth; seabed; metocean]
Well: [total depth; BOP rating; casing loads]
Unit: [jackup; semisubmersible; drillship]
Package: [hook load; mud pumps psi]
Deck/power: [t; kW]
Station keeping: [legs; moorings; DP]
Crew: [beds; helideck]
Period: [dates]

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