Question

I observed differences in buoyancy between the Seastate (1422 MT) and the Flotation module (1459 MT). Can you please explain this?

Answer

There are a couple of reasons why you're seeing a difference in buoyancy values:

A. Seastate's Marine-Method Approximation

By default, Seastate uses the marine method, which treats each structural member as a submerged weight and applies that as a vertically distributed load along its length. In effect, Seastate does not directly compute the full displaced volume of plates "added weight", or non-modeled buoyant items. It only subtracts fluid weight from each member's self-weight, ignoring any plate or added buoyancy unless those items are modeled as separate elements or overridden on WEIGHT lines.

In your Seastate report (upright),

Total Buoyancy = 13951.954 kN, which converts to 13951.957 kN/9.81 = 1422 MT

B. Flotation's volume-based calculation, including plates and added buoyancy

Flotation computes the exact geometric volume of each member (and each plate) that lies below the waterplane and multiplies it by fluid density to obtain buoyancy. It then also adds any Added Buoyancy defined via WEIGHT lines (where you can specify non-modeled buoyant items and their submerged buoyancy) and the buoyancy contributed by all plates below the waterplane. As a result, Flotation's Total Buoyancy always includes:

• Member Buoyancy - fluid displaced by every member below the waterline, based on full cross-section area.
• Plate Buoyancy - fluid displaced by any plate elements.
• Added Buoyancy - any user-defined buoyant items. (example: lift bags, tanks, etc)

In the Flotation report, 

Member Buoyancy = 1456.847 MT
Plate Buoyancy = 3.196 MT
Added Buoyancy = 146.678 MT

Therefore, total Buoyancy = 1606.721 MT, which is > 1422 MT from Seastate.

Flotation's buoyancy is larger because:

• it integrates the actual submerged volume of ALL members (rather than approximating each member as a submerged weight only).
• it includes plate buoyancy (Seastate's marine method does not generate any plate buoyancy unless plates are modeled as separate structural elements receiving dead+buoyancy loading).
• it accounts for any added buoyancy you specified using WEIGHT lines.

C. Impact of FL/NF

In Flotation, by default, every plate (e.g mudmats, plates) that lies below the waterplane is automatically included in the volume-based integration.

In Seastate, if any members are left as non-flooded (NF), only their net-submerged weight is used. Flotation, however, assumes all members below the waterline flooded to the water surface (unless overridden) and so will compute full displaced volume accordingly.

Hence, if your Seastate input did not include any extra buoyant items, Seastate's marine method results (1422 MT) will undercount what Flotation (1606 MT) reports.

D. Orientation Shifts the waterplane intersection

When you rotate the jacket from upright to horizontal, some legs or members that were fully submerged in the upright orientation may now be only partially submerged (or may lie mostly above the waterplane when horizontal).

The net submerged length of each leg decreases, so the displaced volume and the buoyancy fall drastically.

In your Seastate report for the Horizontal Position, where the jacket's longitudinal axis is parallel to the waterplane,

Total Buoyancy Load = 11264.965 kN = 1148 T

Since the horizontal buoyancy (1148 T) < the jacket's weight (1360 T as per Flotation's weight report), the jacket sinks.

You need to ensure consistent input between the files for correct comparison:

• Water depth and density - both Seastate and Flotation must use the same water depth (e.g 150m) and seawater density (e.g 1025T/m3) to be directly comparable.
• Make sure the mudline is identically defined so that both programs know which members lie below "ground".
• Flooding overrides - if any members are forcibly set as NF in Seastate, ensure they match with Flotation's assumptions.