Page 97 - Well Control for Completions and Interventions

Completion Equipment 89

(and cemented) the BOP must be removed, a new casing spool fitted,
and the BOP rigged back up. This process is both time consuming and
costly. Moreover, a multispool construction means multiple flanges and
gasket seals, and therefore many potential leak paths. Conventional, mul-
tiple spool wellheads are relatively tall and need a deep cellar (land wells).
On offshore production platforms, conventional trees need more eleva-
tion between the cellar (wellhead) deck and the deck giving access to the
trees.
In 1961, Gulf Oil introduced the first compact (Unihead) spool well-
head. Originally developed for subsea wells, the technology now finds
widespread applications on land and offshore platform wells. In all
respects the compact spool is a technically superior design.
• Compact design—less space needed.
• Uninterrupted BOP cover—the BOP remains in place for the whole
drilling operation.
• Reduced wellhead related flat time—no need to remove the BOP at
each casing point.
• Improved pressure retention—fewer flanges and gaskets (Fig. 3.2).

Figure 3.2 Compact spool wellhead. Image courtesy of GE.