Petroleum Engineering 406 Floating Drilling Lesson 9c Wellheads and Casing

Lesson 9c - Wellheads and Casing Temporary and Permanent Guide Bases Subsea Cementing Casing Seals

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.

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:

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.

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.

Wellheads and Casing 5. Special test tools are required for remotely testing the casing seals. 6. Wear bushings are essential for protecting the wellhead.

Depth BML 240 ft 1,100 ft 4,100 ft 8,600 ft 10,100 ft

Permanent Guide Structure. Temporary Guide Base

Utility Guide Frame Hole Opener Temporary Guide Base

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.

Gas reduces buoyancy! Shallow Gas Blowout

Gas in the Water Column 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 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.

Fig. 4-9. Sub-sea cementing system.

Fig.44-10. Typical sealing arrangement for subsea wells.

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.

Resilient Compression Element 1. Actuating force is transferred to the 2. Resilient compression element which expands, forcing the 3. Metal seal lips into contact with the surface of the 4. Wellhead housing and the 5. Casing hanger Upper Metal Seal Lips Resilient Compression Element Lower Metal Seal Lips

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.

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.

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:

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.

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.