Master's thesis in Offshore technology : subsea technologyNowadays, oil and gas sources are found in deeper water depths and in more hostile
environments. This results in the need for more advance technologies. Riser system is
a key element in providing safety. Riser failure results in spillage or pollution and
could endanger lives. Hence, it is important to establish a high degree of reliability for
riser design.
Steel catenary risers (SCRs) have been a preferred riser solution for deep-water field
developments due to its simple engineering concept, cost effective, flexibility in using
different host platform and flexibility in geographical and environmental conditions.
Flexible riser, on the other hand, is limited by technical and economical reasons when
it comes to deep water field. Larger diameter is required in deep water to increase
collapse resistance due to high hydrostatic pressure. Consequently, increase in cost
and limit the option of host platform. Alternatively, Hybrid riser is a robust design for
deepwater and harsh environments. It is insensitive to motion induced fatigue.
However, hybrid riser is considered to be an expensive solution because it comprises
a number of complex components (buoyancy can, riser bundle, flex joint, etc).
A number of SCRs have been installed worldwide over the past years and more to
come in the future oil and gas explorations. However, there is no SCR that has been
installed in deepwater with harsh environments to date. It is mainly because SCRs in
harsh environments experience a great challenge due to large motions from host
platform such as semi-submersibles and FPSOs. Therefore, significant design effort is
required to prove that the SCRs could safely withstand environmental loads in harsh
environments and the effects of deep water.
The study investigates the feasibility of 10 inch production SCR for Offshore Norway
in a 1000m water depth with SCR attached to a semi-submersible vessel.
Conventional SCR was analyzed and found difficulty in meeting strength design
criteria at the touch down point (TDP) and at the riser hang off location. From
previous industry work, the weight variation along the riser length has demonstrated a
remarkable improvement to SCR response, particularly at TDP.
This study concentrates on fundamental aspects related to improvement from
conventional SCR to weight distributed SCR. A number of insightful sensitivity
analyses were performed in order to understand the correlation between the peak
response and some fundamental parameters such as displacement, velocity and
acceleration. Feasibility enhancement of present weight distributed SCR concept was
also studied to provide more applicable SCR configuration solution. The study
addresses global design considerations including analysis of strength and fatigue.
Deepwater SCR Installation scheme was also discussed.
The study concludes that there is significant improvement in SCR response from
conventional SCR to weight distributed SCR concept. It also proves that even though
the design of SCR in harsh environments and deep water is technically challenging,
innovative solutions can be developed
