The platform PETROBRAS 25 (P-25) is operating at Albacora field, located at the northeast portion of Campos Basin, offshore Brazil, about I 10 Km from the coast, in a water depth that varies from 230 to 1600 m. This unit, originally a drilling platform, was converted into a production floating system with process capacity of 100,000 bpd of oil and 3,000,000 m3/d of gas, P-25 is moored in a water depth of 520 m, receiving the production from 13 satellite wells and 2 diverless subsea production manifolds (MSPDL-I e MSPDL-2), with 8 wells each. These 2 manifolds were considered the deepest in the world at that time, installed in 500 and 620 m, and are improved with features like multiplex control pod and vertical connection. To reach this important mark, a lot of things have been done, including operational procedures to prevent hydrate formation. Nevertheless, seven months after the unit started producing, a total blockage in two wells in each manifold caused by hydrates occurred. The purpose of this paper is to describe how hydrate formation took place and the actions taken to unblock the manifold and restore the production.
Historically, the hydrate formation in petroleum multiphase flow lines occurs after a shutdown or when the well went on stream. One classic example is, after an emergency shutdown in the unit, the down-hole safety valve (DHSV) of the well closes. Then could occur some gas segregation right down the DHSV and some free water right over it. When the well is open, the contact gas-water on a high pressure and low temperature ambient occurs around the DHSV, creating conditions to hydrate formation and, as a consequence, the production line is plugged. To avoid hydrate formation in P-25, the following procedure was adopted: Before open any well, we circulated diesel on the wellhead, flowing by the gas-lift line and keeping WI, W2 and XOV valves open. We calculated the gas-lift and production lines volume and filled them with diesel. Whenever it was possible, we launched a low-density foam pig, lead by the diesel flow, cleaning completely the lines. Then, we equalized the pressure over the DHSV valve, pumping diesel and keeping the MI valve open and WI valve closed, When, there was a diesel injection right through the DHSV, we pressurize the hydraulic line to open this valve. After the confirmation of the DHSV opening, we put the well on stream.
The start-up of the wells AB-43 (MSPDL I) and AB-38 (MSPDL2) follows the procedure that was described before, with some changes, because these wells are connected to the subsea production manifolds. As we can see in the P-25 submarine system diagram (figure I), connecting each manifold to the platform, there are a main production line (10 in at MSPDL-I and 8 in at MSPDL-2), a main gas-lift injection line (4 in for both manifolds), a test production line (4 in for both manifolds) and a service line (8 in for both manifolds) used to individual measurement of the gas-lift injection.
