Q: We are considering to use MOSES for lowering analysis. A typical case is for instance to install a cover or a template at the seabed. We typically want to analyze 4 different phases:

1. Lifting in air before the cover meets the surface
2. Lowering through the surface
3. Cover fully submerged, typically 5m below surface
4. Cover close to seabed, a few meters above the sea bed.

I will typically model the lifting vessel and the cover as two bodies, and then connect them with sling connectors. I think that phase 1 and 3 easily can be analyzed by using the Static process approach for lifting analyses and then combine these analyses with frequency domain analyses to incorporate the vessel motions and wave forces (phase3) acting on the cover.

However; I am not sure of what the best approach would be for phase 2. I am thinking about carry out many time domain analyses of this phase, where I use different waves for each run. Then report the maximum and minimum (to check for uplift) loads in the slings for each run. After carrying out maybe 30-40 runs, I could collect the data and establish confidence intervals for the mean loads in the slings and use these results to establish limiting environmental criteria for the lifting operation. A Monte Carlo approach so to speak. Do you have any experience with these kind of analyses in Moses? Is it possible to define a "lowering velocity" for the cover when doing time domain analyses of the lifting process? As the cover is partly submerged, will Moses take buoyancy variations as the wave passes by into account? In that case, I guess I must use very small time steps in the analyses...

With respect to phase 4, I wonder whether Moses takes into account the increase of added mass as the cover gets close to the seabed.
REV 6.01

A: I normally look at the problem a bit differently. Basically, you have one body connected to another; one floating, the other being supported. The two important factors are that the length of the support line (and hence its stiffness) is being changed and the second body changes it proximity to the water and the bottom.

We recently did a study of lowering a jacket and we found that the maximum load occurred before the jacket entered the water but the maximum DAF occurred when the jacket was completely submerged. Also if the water depths are large then a resonance will occur for some length of line.

That having been said the answers to your specific questions depend on how you model the cover.

• Morrison’s equation is treated "correctly"; i.e. the current position is used in computing the added mass and the excitation. The bottom effect is ignored.
• Diffraction is "correct" at the position used to compute the pressures. As the body is lowered a true diffraction solution does not change. There use of &DEFAULT -WAVE_RUN, which will compute the Froude Kryloff pressure "correctly", but the change in added mass is still ignored.
• A frequency domain approach is ok before the cover can contact the water and perhaps after it is totally submerged. I would, however, use the time domain for locations from the cover being at +- the maximum wave amplitude to the water plane.
• You can define a line change speed and simulate the lowering as a true time domain process.
• I would probably look at three locations: cover in the air, cover just touching the still water, and cover just submerged. Now, run time domains at these positions to find which one is critical and use it to establish an environmental limit.
• Actually, the above may not be what I would do, depending on the water depth. For deep water, you need to consider the mass and weight of the line and need to look at the response for all depths since you can have a resonance for some intermediate depth. Here, I would probably use frequency domain to find the critical depth, do a time domain at that depth, and then use the critical position to find the environmental limit.
• The simple connectors do not include the effect of their mass. You must either add 1/3 of this mass to the cover (a standard vibration approximation) or use a rod element.