I define input wave data as usual (Stock 5th) without current load. Direction cosine Z=1 for wave. Expect to see restraint load Fz > 0 only.

But output is showing horizontal restraint loads Fx > 0 and Fz > 0

My question: why Fx is not equal to zero?

There is a transverse force generated also. Please see the write up on Hydrodynamic Loading in the User's Guide.

I can understand the transverse (lift) force now

Considering phase 0 at all elements, the line has two nodes, guide + lim at one node and guide only at the other. Wave direction cosine is in Z.

Load case review: Wav1 only

Case #1:
The line is in vertical direction, lift force is now in X direction. Total lift force Fx from output can be checked by manual calculation = (0.5*D*rho*Cl*avgV^2)*element_length. But how to verify the sign +/- of Fx? I see the output is showing positive values.

Case #2:
The line is in horizontal X direction, submerged somewhere in the middle of water depth, the lift force now is in Y direction. I'm trying to verify Fy. My questions: Total Fy = total lift force + total inertia force? And how to verify the sign +/- of Fy?

Depending on the particular phase of the wave, the velocity vector can be positive or negative (actually as the wave passes the velocity vector varies from maximum positive to maximum negative and back). This means the transverse force (which some people unfortunately term "lift" and assume it is always up), can also be positive or negative.

The User Guide is saying "The lift force is defined as the force acting normal to the plane formed by the velocity vector and the axis of the element". This makes me think transverse (lift) force vector always perpendicular to velocity vector. While I understand your reply that transverse (lift) force vector in same direction with velocity vector

Can you explain your reply in graphic for the case considering phase 0, horizontal line in X direction, wave direction in Z direction (0,0,1)?

I believe I said the same thing as the User's Guide. The transverse force is indeed perpendicular to the plane formed by the velocity vector and the axis of the element.

For your example, if the pipe is in X, and the wave is in Z, the transverse force will be in Y. The force may be +Y or -Y, depending on the wave phase (which defines the plus/minus direction of the velocity in Z and how you defined the element's plus/minus direction.

As per your reply, I understand the rule from CII as follow:

Local +x = wave direction
Local +z = element From-To direction
Local +y = right hand rule = transverse force direction

Is this rule mentioned in any text book?

In real world, 2D wave, at phase 0 (wave crest) I image the transverse foce should always be up even if the wave direction forward or backward. This conflict with CII rule. Do I have any misunderstanding?

Right hand rule yes. Transverse force always up - No. Take your example above, but with the velocity in -x (which happens for half the wave), now the transverse force is down.

Above discussion relate to the change in direction of velocity vector without changing element direction vector.

Now I test 3 lines in same run with input order as below (keeping fluid direction no change in +Z, and changing element direction vector, but all 3 lines have the same configuration and boundary conditions):

Line 1: 10-20 X=+6000mm, 20-30 X=+6000mm
Line 2: 120-130 X=+6000mm, 120-110 X=-6000mm
Line 3: 230-220 X=-6000mm, 220-210 X=-6000mm

Node 20 coordinate: 0,0,0
Node 120 coordinate: 6000,0,3000
Node 220 coordinate: 6000,0,6000

Load case is WAV1 only

Typical output Fy is showing:
- Node 20 : Fy < 0
- Node 120: Fy = 0
- Node 220: Fy > 0
---------------------------
- Node 10: Fy < 0
- Node 110: Fy > 0
- Node 210: Fy > 0
---------------------------
- Node 30: Fy < 0
- Node 130: Fy < 0
- Node 230: Fy > 0

The restraint load at one specific location is changing because of changing of element axis direction (From-To) during input activity even the 3 lines are same in configuration, boundary condition and fluid direction

My question: which line match the real world?

I would go with either 1 or 3. Be consistent n your modeling.

You are approximating a dynamic event with a static approximation. Depending on the direction of the velocity vector, the transverse force changes direction - for a given element orientation. Numerically this is the vector cross product between the element direction and the velocity direction. The sign of the load could be plus or minus.

Its OK if Transverse Force is standing alone. Support design can use absolute value.

But in the test, phase is 0, wave solution give acceleration vector downward. This means Inertia Foce is downward.

Fy = Inertia Force + Transverse Force.

The sign of Transverse Force will impact to Fy because of change in element direction. Its effect to support design and loading information issue to Structural Discipline.

Which line (1,2,3) should be chosen? Since I don't want to be restricted by element input sequence. By logic, user can build the model whatever he want but still satisfy boundary conditions.

The question cannot be answered definitively because, as you say, the user can build the model whatever way he wants.

What you could do is setup two load cases, where the hydrodynamic loading is +X in one case and -X in the other. Use the range of loads to design the restraints.

Thank Richard, after made the question, I'm also thinking like that now