The Ærfugl MEG (Monoethylene Glycol) injection skid, designed and produced by Hitec Products AS for installation on the FPSO (Floating Production Storage and Offloading) vessel in the Skarv field, is a mission-critical component in the field’s flow assurance strategy. To ensure mechanical integrity and operational reliability, a comprehensive static pipe stress analysis was commissioned and executed by Dynaflow Research Group. This case study presents the methodology, assessment criteria, results, and key engineering insights derived from this analysis.
Project Scope and Objectives
The primary objective was to evaluate the static stresses in the on-skid piping system under a wide range of operational, environmental, and accidental loads. The assessment also included verification of flange and pump nozzle loads to ensure compliance with international design codes and equipment specifications. The scope covered all on-skid piping, including suction, discharge, and pressure safety lines, as well as consideration of their interaction with attached piping segments during operation and transport.
Analytical Methodology
The analysis was performed using CAESAR II, with assessment criteria derived primarily from the ASME B31.3 piping code. The focus was on static load cases, with dynamic effects—such as pump pulsation—mitigated through support arrangement but not explicitly modeled in this phase. Key loads considered included:
- Dead weight (empty and water-filled conditions)
- Internal design and operating pressures
- Maximum and minimum design temperatures
- Environmental loads (snow, ice, and wave-induced accelerations)
- Transport and installation loads (including wind)
- Hydrotest conditions
The analysis incorporated actual valve weights, realistic support locations and types, and used a detailed IGES model to ensure fidelity with the as-built configuration. Stress intensification factors for welding tees were applied in accordance with ASME B31.3.
Material Properties
Piping was specified in ASTM A815 (UNS S31803), a duplex stainless steel, with properties at 20°C as follows: Young’s modulus 200 GPa, Poisson’s ratio 0.31, density 7750 kg/m³, allowable stress 207 MPa, and a thermal expansion coefficient of 12×10⁻⁶ m/m/°C.
Load Cases
A comprehensive matrix of load cases was developed, covering both operational scenarios (including design, sustained, expansion, and occasional loads) and transport scenarios. Of note were the inclusion of 10-year and 100-year return period snow, ice, and wave loads, per the project’s functional specification and field location data. For transport, both vertical and horizontal accelerations, as well as wind loads, were explicitly modeled.
Assessment of Flanges and Pump Nozzles
Flange loads were evaluated using the ASME B16.5 equivalent pressure method (Kelloggs method). Where necessary, more detailed checks would be performed per ASME BPVC VIII Div.1 Appendix 2. Pump nozzle loads were assessed against a conservative criterion of twice the API 674 allowable loads.
Results and Discussion
Stress Evaluation
Across all analyzed cases, the maximum piping stress observed was 57.4% of the ASME B31.3 allowable limit, with most cases well below this threshold. For the most demanding operational and transport cases, stress levels remained within the 35–57% range, demonstrating a robust design margin. Hydrotest and expansion cases further confirmed the adequacy of the system, with stresses of 32.4% and 19.4% (maximum expansion), respectively.
The stress distribution was generally favorable, with higher stresses localized at long, unsupported small-bore branches on the discharge lines. However, even these peak values did not exceed 54% of the allowable under the most severe 10-year return load scenario. The implemented support arrangement effectively limited both stress and displacement across the skid.
Flange and Pump Nozzle Loads
All flange loads were confirmed to be within allowable limits as per ASME B16.5 criteria. For pump nozzles, the design successfully maintained loads below twice the API 674 allowable, with a maximum observed utilization of 99%. Achieving this required the inclusion of a dummy leg support at the two discharge lines and an additional guide on one discharge line, as detailed in the support arrangement.
Support Arrangement and Vibration Risk
The support strategy was crucial in maintaining low stress levels and limiting loads at critical interfaces. The arrangement was developed to anticipate dynamic forces from pump pulsation and to control transport-induced displacements. However, it was observed that small-bore branches on the discharge lines were nearly unsupported in the received layout. While static stresses were within allowable limits, these branches are known to be susceptible to vibration-induced fatigue. The analysis thus recommended bracing these branches back to the main headers to mitigate potential vibration risks during operation.
Sensitivity to Environmental and Accidental Loads
The piping design demonstrated resilience to combined snow, ice, and wave loads, as well as to 100-year return wave events. The calculated forces, derived from field-specific environmental parameters, were translated into equivalent vertical and horizontal loads in the stress model. The approach ensured that worst-case combinations of environmental and accidental scenarios were addressed without exceeding code-defined stress limits.
Conclusions
The static pipe stress analysis for the Ærfugl MEG injection skid confirmed the robustness and code compliance of the design under a full spectrum of operational and accidental load cases. Specifically:
- All piping stresses remain well below the ASME B31.3 allowable limits, with a maximum utilization of 57%.
- Flange loads for all connections are within acceptable limits as per ASME B16.5.
- Pump nozzle loads are controlled to within twice the API 674 allowable, validating the effectiveness of the implemented support scheme.
- The support arrangement is adequate for both operational and transport conditions, with specific enhancements identified for small-bore discharge branches to guard against vibration risks.
Recommendations
To maintain the integrity and reliability of the system throughout its lifecycle, the following actions are recommended:
- Implement the support arrangement in accordance with the detailed layout provided in the analysis.
- Install appropriate bracing on all small-bore branches of the discharge lines to prevent vibration-induced failures.
- Adhere to the specified material grades and component ratings, particularly in high-pressure sections, to ensure continued compliance with ASME B31.3 and API 674 standards