Deepwater floater designs consider new Gulf of Mexico metocean criteria
John Murray, FloaTEC, LLC
In the aftermath of recent hurricanes that passed through the Gulf of Mexico, the America Petroleum Institute (API) is making recommendations for assessing the behavior of existing deepwater floater installations. The API has issued interim guidelines including new environmental criteria and procedures to ensure continued safety of these installations. In accordance with the API interim recommendations, a set of metocean conditions has been defined for different regions in the GoM. In general, environmental design conditions have increased and will have an effect on the response of the floaters currently installed.
Three of the most common types of deepwater floaters in the GoM are the semisubmersible, the TLP, and the spar. Responses of these designs, including motions, air gaps, and riser and mooring tensions, will have to be reexamined for these new criteria. The original designs are based on environmental criteria now are superseded by new criteria based on observed conditions during Hurricane Katrina in August 2005.
The following compares results of predicted responses based on pre-Katrina data, which presumably were used to design the existing structures criteria, to those of the post-Katrina environments. The results compare the responses of typical, but fictitious, designs of an extended tension leg platform (ETLP), deep draft semi (DDS), and a spar hull.
The post-Katrina environment
A comparison of 100-year waves, current, and wind conditions pre- and post-Katrina showed the largest increases in waves and wind. The increase in the associated current is comparatively marginal for the 100-year hurricane.
Comparison of pre-Katrina and post-Katrina environments 100-year
Many techniques to numerical model hydrodynamic analysis of floating structures use a linear analysis which is the linear summation of randomly selected sinusoidal components. This can have limitations in achieving a prescribed maximum wave crest height or a maximum wave height in a single simulation of waves in a storm. Simulation of these environments for performance analysis of floaters generally is based on numerical computer models and scaled physical model tests. Wind forces are simulated using drag coefficients normally determined from wind tunnel tests. These coefficients relate wind velocity defined by an API wind velocity spectrum to a wind force spectrum. A similar method is used for current forces. Variations in the current profile are modeled by stratifying the velocity into sections along the structure.
A comparison of waves generated in a model test basin to a linear numerical simulation of a 100-year post-Katrina spectrum showed the crest heights from the numerical simulation are approximately one-half of the wave heights. However, the waves measured in the model test basin showed crests higher than one-half the wave height. The crests are plotted as a function of wave height. Analysis of wave data acquired during Hurricane Katrina showed steep crests similar to those generated in the test basin.
The fundamental difference is that full-scale prototype waves can have highly nonlinear components that appear in the wave train as waves with steep crests and asymmetric profiles. Simulating a wave sequence from the linear summation technique may not achieve these conditions by simply summing harmonic components. In some cases, a trial and error approach can achieve the required profiles by changing the random seed number. Particular attention should be paid to this, especially when analyzing air gap and wave run-up on the hulls.
Hull form dimensions
Preliminary investigations into the reaction of existing hull designs to the new criteria illustrate the sensitivities of these hull forms to these new environments.
The global dimensions of three typical hull forms presently deployed in the GoM, including some of the particulars such as the assumed water depth and the number of mooring/tendon lines, are shown above. The DDS has a chain/polyester/chain taut system, and the spar has chain/wire/chain semi-taut system. The DDS mooring system comprises four groups of three lines. The spar has a mooring system of three groups of three lines. The ETLP is attached to three tendons at each of the four corner porches. These designs illustrate trends on the effects of the new environmental criteria on their respective responses.
Analysis results
The results presented here are not intended to evaluate designs currently installed in the GoM. The effect that the new environment might have on an existing installation would have to be addressed specifically for that installation. In any case, there are a number of mitigating options. For example, some adjustments can be made to the moorings, ballast management, and local reinforcement of the structure, if needed. Other considerations depend on the level of conservatism used in the original designs such as multi-directional versus long crested wave conditions.
Broad observations from the analysis are summarized as follows:
null
ETLP
In general, an ETLP exhibits the largest offset motion increases. This is consistent with an increase in set-down and associated loss of air gap, which shows a reduction of about 19 ft (58 m). This air gap loss is close to the differences in wave crest heights between the pre-Katrina and post-Katrina conditions. There is also a component to air gap loss due to the increase in set-down. Increases in the rotational degrees of freedom, roll, pitch, and yaw are negligible compared to the DDS and spar because of the tendons’ constraints. The largest contribution to the increase in heave on the ETLP is the offset set-down. This set-down effect translates into an increase in the downward stroke range of approximately 3 ft (0.9 m) between pre- Katrina and post-Katrina conditions. Increases in riser stroke will also be in the range of 3 ft for the ETLP because the TTRs and the hull experience similar set-down responses as the ETLP offsets.
Spar
The spar’s increase in surge is close to the same range as the DDS. The sway increase is much less than the DDS. Thus, the total increase of offset combining both surge and sway is smaller than the DDS because of the highly nonlinear behavior of mooring restoring force on the spar at the extreme offset. Rotational motions are smaller for the spar than for the DDS because the fairleaders are higher above the center of rotation and constrain the pitch and roll motions. This constraint is evident in the higher increase in mooring line tensions on the spar in comparison to the DDS. The air gap loss of 11 ft (3.4 m) indicates that the maximum crest height could overtop the hard tank by approximately 3 ft. This could cause wave impact on the support structure for the deck.
null
DDS
The DDS has an increase in surge and sway of only 1% of the water depth. The increase in heave range is the highest of the three hull forms at approximately 11 ft. Also, due to its compliance to the incident environmental loads, the DDS has the largest increase in roll, pitch, and yaw. These increased motions induce increased stresses in the steel catenary risers. Because of the position of the fairleaders on the DDS hull, the increase in mooring line tension is small compared to the increased tension in the spar moorings.
