Responses of a Tanker-Based FPSO to Hurricanes in the Gulf of Mexico

Summary

Objective

Conduct model tests to demonstrate the capability of a moored, passive tanker-based FPSO to weathervane during the passage of a severe Gulf of Mexico hurricane, and measure FPSO responses.

Approach

Model tests will be conducted in the OTRC wave basin to measure the responses of a moored, passive tanker-based FPSO during the passage of a Gulf of Mexico hurricane. Three non-parallel wind, wave, and current events will be used to simulate conditions during the passage of a severe hurricane.

Measurement will focus on vessel motions and mooring loads, and include key response such as the weathervaned heading and yaw, motions at the turret which force the riser top motions, and turret loads. These data will be available to characterize the responses in design studies and validate or improve design analysis tools.

Project Plan

Scope of Work:
The prototype FPSO will be a 200,000 DWT tanker moored in a water depth of 6,000 ft. with risers and a taut polyester mooring system.

A 1:60 model will be fabricated and instrumented to measure motions (6 DOF), turret forces, and any wave overtopping. The tests will be conducted at a single draft.

The modeled mooring system will have to be truncated due to model scale and basin depth considerations, and will be designed and fabricated to provide both static and dynamic equivalence for full prototype depth. Research results from other OTRC projects will be used in the design of the model mooring system. The modeled mooring will be a four-leg system that will be instrumented to measure mooring line tensions. Risers will not be explicitly modeled, but their impact on overall system damping will be included in the design of the modeled mooring.

The environment will be modeled by non-parallel waves, wind, and currents to simulate the confused sea conditions that can occur during the passage of a severe hurricane. The three wave, wind, and current conditions will be selected as those which cause maxima in three key responses, i.e. vertical turret motions, roll, and mooring loads. The selections will be based on analytical simulations of FPSO responses in hurricanes. Waves, currents, and winds will be measured during the tests.

In addition to the instrumentation described above, all model tests will be videotaped from surface cameras.

The experimental program will be conducted to demonstrate the FPSO overall behavior, to provide data on key FPSO responses to the modeled hurricane environment, and to provide data on key FPSO responses to waves with simulated wind and current loads. The data on FPSO responses in the modeled hurricane environment will be useful in validating design computational models. The data on FPSO responses to waves with simulated wind and current loads will be valuable for improving the analytical models.

The experimental plan will include:

1. Characterization of the modeled environments,
2. Calibration of the model vessel and mooring,
3. Static surge & sway offset tests,
4. Surge, heave, roll, & pitch decay tests,
5. Response of the moored FPSO to the separate wind, wave, and current conditions,
6. Response of the moored FPSO to simulated wind and current forces,
7. Response of the moored FPSO to the waves and simulated winds and current forces,
8. Response of the moored FPSO to the 3 non-parallel wind, wave, and current conditions.

Anticipated Results
Anticipated results include

1. Video records of tests,
2. A report documenting the tests, observations, and analyzed responses,
3. A copy of the reduced data for all tests,
4. A model that can be used for additional testing

Results from these tests will be used in related OTRC research projects seeking to develop and validate a computational analysis tool for FPSO responses (M. Kim) and mooring analyses (Zhang).

Related Publications

Ward, E. G., Irani, M., Johnson, R. P., (2001), “Responses of a Tanker-Based FPSO to Hurricanes”, Offshore Technology Conference, (OTC 13214), Houston, April.