JOHN MACDONALD,
CLARUS SUBSEA INTEGRITY,
RICKY THETHI, 2H OFFSHORE, &
JONATHAN DEEGAN, RISKBYTES,
PROVIDE
AN ECONOMIC AND RISK ASSESSMENT
OF SINGLE AND DUAL CASING TOP
TENSIONED RISERS.
T
he use of single or dual casing top tensioned risers (TTRs) is a key
decision in the design of a dry tree system. The decision is risk based,
between the decreased cost of a single casing top tensioned riser
and the additional barrier provided by dual casing risers. However, without
any recorded incidents attributed to either of the riser systems, and very
few incidents with TTRs in general, there is an overall lack of industry data
for direct use in a risk analysis. This article discusses a quantitative risk
assessment methodology using historical data fromblowouts, combined
with a typical planned riser configuration and assumed time onwell.
When combinedwith the cost between single and dual cased systems, the
approach can be used to develop a point of decision between the two riser
configurations.
Calculating risks
During development of a deepwater dry tree floating facility, the operator will
face a decision between single casing or dual casing top tensioned riser (TTR)
systems. Fundamentally, the decision is risk-based on the likelihood of the
riser failing and losing pressure containment capability, versus the economic
impact of additional equipment and facility weight. The lifetime costs are a
combination of upfront capital cost (Capex), operating cost (Opex), and risk
cost (Riskex).
1
Development of Capex andOpex is straightforward given access to
reference data frompast projects. However, the development of Riskex
requires an understanding of TTR operations including completion, drilling
and production, as well as access to reliability data and failure data to assess
risk events. Fortunately for the offshore production industry, there have been
very few riser failures. However, this complicates determination of Riskex for a
dry tree riser application. For instance, the key risk often cited is the potential
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