Page 313 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

OIL AND GAS EXPLORATION AND PRODUCTION 291

The extent of internal corrosion in a particular oil field environment is largely a
function of the amount of water produced. As a field ages and the water increases,
corrosion control will become more costly. Increased water means increased levels of
bacteria and hydrogen sulfide, and in cases where miscible gas is reinjected, increased
levels of CO .
2
It is useful to consider the case of an installation of a subsea gathering system for
a natural gas production field. The pipeline design for a new gas production facility
consisted of 20 cm diameter subsea gathering lines (flow lines) emptying into a 19 km,
50 cm diameter subsea transmission gas pipeline. The pipeline was to bring wet gas
from an offshore producing area to a dehydration facility on shore. The internal corro-
sion was estimated to be 300–400 mpy. The corrosion mitigation options considered
were: (i) carbon steel treated with a corrosion inhibitor; (ii) internally coated car-
bon steel with a supplemental corrosion inhibitor; (iii) 22% Cr duplex stainless steel;
(iv) 625 corrosion-resistant alloy (CRA). The chance for success was estimated from
known field histories of each technique, as well as the analysis of the corrosivity
of the system and the level of sophistication required for successful implementation
(Table 4.42).
On the basis of the risk factors and economics coated carbon steel with a supple-
mental corrosion inhibitor was preferred over the duplex stainless steel in spite of
higher risk of the coated steel. The material selected for flow lines and trunk lines is
shown in Table 4.43.

TABLE 4.42 Field Histories
Option Chance of Success (%)
Bare carbon steel with inhibitor 65
Coated carbon steel + supplemental inhibitor 90
Duplex stainless steel 95
625 CRA 98

TABLE 4.43 Material Selected for Flow Lines and Trunk Lines
Flow Lines Pipe and Internal Corrosion Duplex Stainless Steel Alloy
Protection (22% Cr)
Cathodic protection Duplex stainless steel alloy (22% Cr)
and external coating
Trunk Lines Pipe and internal Coated carbon steel with supplemental
corrosion protection corrosion inhibitor and corrosion
allowance
Cathodic protection Coated carbon steel with supplemental
and external coating corrosion inhibitor and corrosion
allowance