Fatigue analysis of subsea Jumper under external loads (steady current and earthquake)

Fatigue analysis of subsea Jumper under external loads (steady current and earthquake)

Kosanunt, P.

Metrikine, A. (mentor)
Hendricks, M.A.N. (mentor)
Qu, Y. (mentor)

Civil Engineering and Geosciences

Hydraulic Engineering

Offshore engineering

2015-07-21

In recent times, the oil and gas business has moved into unconventional reservoirs, especially in deep-water. One high-potential prospect was found in the deep-water area of Myanmar. A subsea production system pilot project is planned for implementation in this area. One essential element of a subsea system is a “subsea jumper”. The main function is to interface between the subsea tree and subsea manifold. There are various subsea jumper configurations used in the market. This thesis focuses primarily on the U-inverse shape jumper as a fundamental shape which gives subsea jumper a flexible characteristic. A subsea jumper that is used in a deep-water area is difficult to access for maintenance or repair. As such, it is essential to determine the reliability of a jumper system, especially thru fatigue failure analysis. The dynamic behaviors of a subsea jumper at various load conditions need to be taken into account during the design phase. In general, a subsea jumper system experiences many loads both internal and external, but there are only two key external loads, the steady sea current and earthquakes at designed area. These interesting factors are considered in this thesis. In order to be better understand the dynamic behaviors of a subsea jumper under load conditions, it is important to first analyze the dynamic characteristics of the jumper itself. A U-inverse shape jumper can be modelled by connecting three “pipe conveying fluid model” (or Euler Bernoulli Beam + internal flow effect). This is called a “subsea jumper model” or “Triple beam model”. This model gives the dynamic characteristics of a jumper in terms of “mode shape” and “natural frequency” in two vibration planes: inline and crossflow. The dynamic behavior of a subsea jumper under a current load situation can be solved by using a wake oscillator model coupled with a subsea jumper model. The results show that a mild sea current is able to dramatically induce jumper oscillation. This phenomenon is called Vortex induced vibration (VIV). It can occur in both crossflow VIV and inline VIV; however, for both cases of VIV, a subsea jumper system is safe to operate under the designed current velocity (maximum current velocity is 0.832 m/s, based on a 100-year return value). In an earthquake load condition, the subsea jumper model is coupled with an inertia load model (mass times acceleration). Two types of acceleration are considered in this thesis thru a sinusoidal model and simulation model. The first, sinusoidal model assumes that an earthquake is a continuous process with ground acceleration in a sinusoidal shape. It is used to analyze the dynamic behavior of a subsea jumper in terms of “seismic response spectra”. The second, a simulation model defines an earthquake in more realistic way by considering an earthquake as a shock of high magnitude in a small period. This model is more suitable for fatigue analysis. It should be emphasized that a pure earthquake load is a rare occasion, as the current of the nearby seabed is always present. Thus, it is more helpful to investigate the fatigue lifetime of a jumper under a combination of earthquake effects and steady current. The analysis results show that a subsea jumper can withstand up to 13,000 number of a high magnitude earthquake shock, over 7.5 Richter. However, during the designed lifetime of a subsea jumper there are typically only 600 shocks. Thus, one can conclude that a subsea jumper is safe against earthquakes in the designed area. The designed subsea jumper may require changes if it is relocated to operate in another area with the presence of a stronger current velocity and/or earthquake conditions. Subsea jumper lifetime can be improved by designing dimensions and configurations to give natural frequencies out of the load range. This can be achieved by reducing the length of a jumper or increasing its diameter. Another method is to reduce the flow rate of the contained fluid. However, these methods may stimulate another problem if slug is present inside the jumper. Adjustments in flow rate or jumper dimension changes the impact period of slug at each bend of a jumper system. When slug impact load frequency is close to a natural frequency, there will be a dramatic response. Thus, considerations of slug should be taken into account for subsea jumper design, especially with any changes in dimension, configuration and flow rate. Lastly, other mitigation methods include a more robust material, controlling surface conditions and welding method.

fatigue
subsea jumper

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Student theses

master thesis

(c) 2015 Kosanunt, P.