# Question regarding underestimation of Surge response for IEA-15-240-RWT-UMaineSemi **URL:** \ **Category:** Structural Analysis\ **Created:** [December 16, 2023, 4:43am UTC](https://forums.nlr.gov/t/question-regarding-underestimation-of-surge-response-for-iea-15-240-rwt-umainesemi/6203 "2023-12-16T04:43:45Z")\ **Posts on this page:** 1\ **Showing post:** 5
**Author:** ![Yosuke.Matsumoto](https://avatars.discourse-cdn.com/v4/letter/y/779978/32.png) [@Yosuke.Matsumoto](https://forums.nlr.gov/u/Yosuke.Matsumoto)\ **Post date:** [December 18, 2023, 7:55am UTC](https://forums.nlr.gov/t/question-regarding-underestimation-of-surge-response-for-iea-15-240-rwt-umainesemi/6203/5 "2023-12-18T07:55:42Z")
Thank you for your answer … I would like to confirm the details regarding #2. Q1 Can I use WvDiffQTF and WvSumQTF only when PotMod=1? I want to consider 2ND-ORDER WAVES in strip theory only calculations (PotMod=0). The reason and situation is as follows. I am trying to find the sectional force of a floating body using SubDyn. If I set “PotMod=1”, I get very large moments at the reference point, as discussed in the forum below. > [@Sudden change in the bending moment of the Floater member](https://forums.nlr.gov/t/sudden-change-in-the-bending-moment-of-the-floater-member/5593/6): > > Dear @Yosuke.Matsumoto, OpenFAST supports the modeling of hydro-elastics and member-level loads, but to get proper member-level loads, you must properly distribute the hydrodynamic loads across the member. This is possible through the strip-theory of solution of HydroDyn, but not through the potential-flow solution. In the potential-flow solution of HydroDyn, the potential-flow body is assumed to rigid whereby hydrodynamic loads are lumped at a point. This is why in my forum post above I sug… I am trying to find the hydrodynamic loads acting on the floating body using only strip theory with “PotMod=0” for HydroDyn. If WvDiffQTF or WvSumQTF is set to True under these conditions, the following error occurs FAST\_InitializeAll:HydroDyn\_Init:HydroDynInput\_ProcessInitData:DiffQTF can only be used with PotMod==1. Turning off HydroDynInput\_ProcessInitData:SumQTF can only be used with PotMod==1. Turning off Q2 I would like to know how to calculate the cross-sectional force of a floating body while taking into account the secondary wave forces. Is the only way to do this the following method you suggested in the same forum? Create multiple WAMIT files and assign them to the multiple elements that make up the float. --- _[View the full topic](https://forums.nlr.gov/t/question-regarding-underestimation-of-surge-response-for-iea-15-240-rwt-umainesemi/6203)._