Mean Drift Force

I have run the hydrodynamic for ship-shaped model and extracted the mean drift force. I found the mean drift force response for example,

translational "surge" is changing over the period of time. 

as you can see below, from

period 30 sec - 18 sec. the sign convention is positive

period 17.5 sec - 10 sec. the sign convention is negative

Note:

I tried rerun with Moses old version i.e. 7.03 and get the consistent sign convention for a period of 10sec - 30 sec.

+++ M E A N D R I F T F O R C E S F O R A B C +++
===================================================================

Drift Name = OPERATIO Heading = 0.00 Computed

Force Factor = 1.0000 Radiation Factor = 1.0000 Coriolis Factor = 1.00

Process is DEFAULT: Units Are Degrees, Meters, and M-Tons Unless Specified

Mean Drift Force ( With No Force Factor ) / (Wave Amplitude)**2

Reported in the Body System

E N C O U N T E R T R A N S L A T I O N R O T A T I O N
-------------------- /-----------------------------/ /-----------------------------/
Frequency Period Surge Sway Heave Roll Pitch Yaw

0.2094 30.000 5.3 0.1 1.5 -0.9 -1582.2 8.7
0.2167 29.000 5.4 -15.0 0.3 181.1 -1500.0 -1618.0
0.2244 28.000 6.2 -18.4 -0.4 201.3 -1451.6 -1980.7
0.2327 27.000 7.1 -22.8 -0.9 224.9 -1409.6 -2456.1
0.2417 26.000 8.1 -27.7 -1.6 246.8 -1360.5 -2977.1
0.2513 25.000 9.3 -35.0 -2.2 276.8 -1323.3 -3759.0
0.2618 24.000 10.6 -44.0 -2.8 305.5 -1271.6 -4700.5
0.2732 23.000 11.8 -62.5 -3.4 379.4 -1244.0 -6652.0
0.2856 22.000 12.9 -46.4 -5.7 199.4 -963.3 -4957.2
0.2992 21.000 13.4 -31.9 -7.2 10.3 -745.9 -3550.4
0.3142 20.000 12.6 -5.5 -7.5 -250.7 -580.0 -1054.4
0.3222 19.500 11.5 8.2 -6.8 -369.9 -556.7 184.6
0.3307 19.000 9.8 21.2 -5.8 -470.3 -559.5 1303.7
0.3396 18.500 7.3 25.3 -3.8 -502.1 -678.7 1590.7
0.3491 18.000 3.8 26.0 -1.7 -504.3 -814.9 1550.1
0.3590 17.500 -0.7 22.3 0.4 -474.9 -979.8 1120.8
0.3696 17.000 -6.2 16.9 2.5 -428.7 -1158.4 565.2
0.3808 16.500 -12.8 11.5 4.4 -370.7 -1354.5 45.4
0.3927 16.000 -20.3 6.6 6.1 -301.8 -1583.0 -378.6
0.4054 15.500 -28.3 3.2 7.5 -225.0 -1845.0 -597.7
0.4189 15.000 -35.9 1.2 8.7 -144.3 -2153.6 -629.4
0.4333 14.500 -42.2 0.8 9.5 -68.0 -2500.0 -474.2
0.4488 14.000 -45.8 1.6 9.8 -7.9 -2866.6 -198.4
0.4654 13.500 -45.5 3.2 9.6 22.5 -3215.0 90.2
0.4833 13.000 -40.5 4.6 9.0 11.4 -3482.0 258.8
0.5027 12.500 -31.8 5.0 8.0 -47.4 -3587.2 204.2
0.5236 12.000 -22.6 4.0 7.0 -152.2 -3461.7 -48.9
0.5464 11.500 -18.0 2.4 5.8 -287.0 -3101.6 -253.1
0.5712 11.000 -20.8 2.5 4.3 -407.7 -2612.0 -16.1
0.5984 10.500 -25.2 5.7 2.0 -432.7 -2155.3 763.4
0.6283 10.000 -19.1 9.0 -0.5 -284.4 -1830.4 1324.5