Design Method and LRFD for HPHT Subsea Equipment for Extreme and Survival Load Conditions

Abstract

Abstract The current API standards and technical reports for Subsea Equipment which fall under API SC 17; especially relevant to Subsea Trees and Wellheads; do not particularly provide the design methods for Extreme and Survival load conditions. As the industry continues moving forward with the High Pressure High Temperature (HPHT) application it has increased the attention towards safety, and design precautions have the utmost priority. Hence, the industry is redoubling its focus on the Extreme and Survival load conditions for the Subsea Equipment, especially for HPHT conditions. The recently published API TR 1PER15K-1 and in-work API TR 17TR8 address the design methods for Subsea HPHT equipment but do not specifically provide the design methods for Extreme and Survival load conditions. API RP 17G, recommended practice for completion/workover risers addresses these loading scenarios by providing the design condition factors, but suggests testing for plastic collapse load. API RP 2RD, recommended practice for Design of Risers for Floating Production Systems and Tension-Leg Platforms, recommends the consideration of these load conditions, but does not provide specific design rules. This paper posits a design method for Subsea Equipment governed by API SC 17 particularly for API 17D, Subsea Trees and Wellhead under Extreme and Survival Load conditions. The proposed method combines the Elastic-Plastic design analysis method from ASME Boiler and Pressure Vessel Code (BPVC), Section VIII, Div. 2 and 3 and the API RP 17G Structural Resistance Methods. It combines the API RP 17G Design Condition Factors with the ASME Section VIII BPVC Div. 2 and 3, Load and Resistance Factor for Design (LRFD) to establish new LRFDs for Extreme and Survival Load conditions. Fatigue life estimation is also addressed for these conditions. An example analysis of a Subsea Tree component is reviewed, evaluating its combined load capacity for normal, extreme and survival conditions. Additionally, the review includes the fatigue life cycle estimation for this component.