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Petroleum Engineering 406Floating Drilling

Lesson 9c

Wellheads and Casing

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Lesson 9c - Wellheads and Casing

Temporary and Permanent Guide Bases Subsea Cementing Casing Seals

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Wellheads and Casing

A subsea wellhead, just like a land wellhead: Must support the BOP’s while drilling Must support the suspended casing

while cementing, and Must seal off between casing strings

during drilling and production operations.

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Wellheads and Casing, cont.

In floating drilling, the casing hangers, casing seals and cementing heads differ from land and platform operations in the following manner:

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Wellheads and Casing

1. The first and second casing strings are cemented with returns to the

seabed.

2. Casing is run with the last joint made-up on a casing hanger and permanently suspended prior to cementing. Mud returns flow through fluting in the hanger.

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Wellheads and Casing

3. Usually, cementing plugs are located at the wellhead and released remotely. The cementing string from the vessel to the wellhead is drill pipe.

4. Casing seals are run and set remotely.

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Wellheads and Casing

5. Special test tools are required for remotely testing the casing seals.

6. Wear bushings are essential for protecting the wellhead.

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Depth BML

240 ft

1,100 ft

4,100 ft

8,600 ft

10,100 ft

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Permanent Guide

Structure.

Temporary Guide Base

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Hole Opener

TemporaryGuide Base

UtilityGuide Frame

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Fracture gradient at

100 ft. below seabed (Santa Barbara

Channel).

Why drill with returns to the seafloor?

With this low fracture gradient it is difficult to drill with returns to the surface.

No shallow gas would be expected at this depth below the mudline.

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Shallow Gas

Blowout

Gas reduces

buoyancy!

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Typical specific gravity variations in a blowout boil have increasing effect nearer the water’s surface.

Fortunately for a semi-submersible, the rig’s primary flotation members are situated below the zones where specific gravity has been reduced the most.

Gas in the Water Column

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If there is sufficient length to the mooring system cables/ chains, the rig will be pushed off location and out of harm’s way.

However, the plume can also cause the rig to list, which reduces its freeboard and makes it more susceptible to capsizing.

Gas in the Water Column

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Fig. 4-9. Sub-sea

cementing system.

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Fig.44-10. Typical sealing

arrangement for subsea

wells.

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Metal-to-Metal Casing Annulus Seal

Assures maximum seal over extended periods, even in high-pressure holes

Eliminates dependence on seal materials that deteriorate or “cold flow”.

Available on systems up to 15,000 psi pressure integrity.

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Upper Metal Seal Lips

Resilient Compression

Element

Lower Metal Seal Lips

1. Actuating force is transferred to the2. Resilient compression element which expands, forcing the 3. Metal seal lips into contact with the surface of the4. Wellhead housing and the5. Casing hanger

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Casing Hanger and Pack-off Assembly

Single trip installation

The pack-off seal assembly is run simultaneously with the casing hanger body. All operations - installing the casing hanger, cementing the casing string and actuating and testing the pack-off seal are performed in a single trip of the running string.

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Large Flow-By Areas

Large flow-by areas can handle most drilling fluid applications with a minimal drop in pressure.

Deep 2" wide flow-by slots in the casing hanger body, and ample porting through the pack-off nut assembly, provide clear passage for cuttings and mudcake without plugging.

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Shallow Gas Plan

After the rig is properly anchored the following steps will be followed as there is a potential for shallow gas in this area:

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Shallow Gas Plan

1. Leave mooring line pawls or stoppels unset until the 20” casing has been set and cemented.

2. Mooring winches will be manned while the 8 1/2” pilot holes for the 30” and 20” casings are being drilled.

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Shallow Gas Plan

3. Mooring winches will be manned while the 8 1/2” pilot holes for the 30” and 20” casings are being opened up or under reamed.

4. The moonpool and seafloor will be observed for gas bubbles until the 20” casing is set and cemented.