ANALYSIS OF WIND FORCE

AND WATER WAVES ON

OFFSHORE STRUCTURES By Harish Kumar Mulchandani

Under guidance of Dr. Shibani K. Jha Assistant professor Civil Engineering Department BITS Pilani, Pilani campus

1. Introduction

With most of the easy gas and oil reserves discovered and prices rebounding,

companies are now drilling far offshore in extreme weather condition

environments. As deep water wells are drilled to greater depths, engineers and

designers are confronted with new problems such as water depth, weather

conditions, ocean currents, equipment reliability, and well accessibility. So

design of offshore structures has been one of the toughest design problems

due to extreme conditions and mainly governed by the dynamic forces.

So at present we are simulate the wind and water forces experienced by

structures using the CFD simulation tool(COMSOL) and performing the

parametric study of the governing parameters.

2. Motivation

The oceans present a unique set of environmental conditions that dominate

the methods, equipment, support, and procedures to be employed in offshore

structural designing. Many literature articles have addressed the extreme

environmental events and adverse exposures as they affect design, still there

is need to evaluate the impact of the combination of wind and water waves.

In recent future the need of offshore wind farms and oil platform has increased

and it is assumed that in 2030 worlds 10% energy would be generated

through offshore wind farms.

At present we have highly sophisticated CFD tools like COMSOL, Ansys

Fluent etc. which have capability of simulating the toughest of design

problems. These tools have different caterings the different needs, being

graduate student in Structural Engineering I have tried to perform analysis of

an offshore structure (a typical monopile) using COMSOL. In the present

study Fluid Structure Interaction (FSI) module have been used to model. It

consist of two sub module Fluid flow module and Solid Mechanics.

Velocity field and Pressure are calculated using the fluid flow module

whereas the displacement and von mesis surface stress are calculated using

the solid mechanics module.

3. Conceptual Model

Above image shows the conceptual model, Offshore Structure is considered

having fixed base and have the wind forces and water forces from the same

direction. It has been designed taking a lot of assumptions in order to simplify

the design.

Assumptions:

1. Not accounted for volume forces.

2. Turbulence in water and wind flow is not accounted.

3. Linear elastic material of the solid structure.

4. In few conditions water is only accounted in the upstream.

5. Direction of Wind and water waves are same.

6. Velocity profile linearly varying with height.

4. Physics

Fluid Structure Interaction

Fluid-Structure Interaction, phenomena where a fluid and a deformable solid

affect each other. Due to the viscous and pressure forces exerted by the fluid,

the Structure is bending. With the structure undergoing a large deformation,

the fluid flow domain is also changing considerably.

It consist of Fluid flow and Structural mechanics as its sub modules.

Figure shows real offshore structure

being surrounded by water from all

sides and heavy mass at top of

structure.

First solves for flow field, giving corresponding velocity fields and pressure

then calculating the surface stresses and displacement of the structure.

5. Arbitrary Lagrangian-Eulerian (ALE) technique

he arbitrary Lagrangian-Eulerian (ALE) is a finite element formulation in

which the computational system is not a prior fixed in space (e.g. Eulerian-

based finite element formulations) or attached to material (e.g. Lagrangian-

based finite element formulations). ALE-based finite element simulations

can alleviate many of the drawbacks that the traditional Lagrangian-based

and Eulerian-based finite element simulations have.

When using the ALE technique in engineering simulations, the

computational mesh inside the domains can move arbitrarily to optimize the

shapes of elements, while the mesh on the boundaries and interfaces of the

domains can move along with materials to precisely track the boundaries and

interfaces of a multi-material system.

ALE-based finite element formulations can reduce to either Lagrangian-

based finite element formulations by equating mesh motion to material

motion or Eulerian-based finite element formulations by fixing mesh in

space. Therefore, one finite element code can be used to perform

comprehensive engineering simulations, including heat transfer, fluid flow,

fluid-structure interactions and metal-manufacturing.

Lagrangian specification of the field is a way of looking at fluid motion where the observer follows an individual fluid parcels as it moves through

space and time.

This can be visualized as sitting in a boat and drifting down a river.

Eulerian specification of the flow field is a way of looking at fluid motion that focuses on specific locations in the space through which the fluid flows

as time passes.

Adaptive Mesh (User define

Meshing), triangular Meshing

Moving Mesh in the Present

Problem, its an example ALE

This can be visualized by sitting on the bank of a river and watching the

water pass the fixed location.

ALE could be converted into either Eulerian or lagrangian specification

through fixing the space frame.

6. Governing Equations

1. Fluid Flow equations

The fluid flow in the channel is described by the Navier-Stokes equations,

solving for the velocity field u = (u, v) and the pressure, p, in the spatial

(deformed) moving coordinate system:

Where I is the unit diagonal matrix, and F is the volume force affecting the

fluid. The model neglects gravitation and other volume forces affecting the

fluid, so F = 0.

2. Structural Mechanics Equations

The structural deformations are solved for using an elastic formulation

and a nonlinear geometry formulation to allow large deformations.

The Structure is fixed to the bottom of the fluid channel. All other object

boundaries experience a load from the fluid, given by where n is the

normal vector to the boundary. This load represents a sum of pressure

and viscous forces.

We could see from equations that 3rd equation in fluid flow give us force

exerted by the fluid and 1st equation considers the same force to

calculation the strain in the solid structure.

7. FVM formulation

Diffusion always occurs alongside convection in nature so here we examine

methods to predict combined convection and diffusion. The steady convectiondiffusion equation can be derived from the transport equation for a general

property by deleting the transient term.

Finally converting this equation into algebraic form using nearby nodes

values

8. Modelling

2D Model

Middle blue color

represents solid

structure

3D Model

Model has water layer up to a certain height and remaining height is covered

with air.

Modelled in both 2D and 3D

Structure height 47.5 m

Water height 0, 20, 45, 100 m

Total height 100m

Boundary Conditions

Fixed Base

At the outflow (right-hand boundary), the condition is p = 0.

At left boundary inlet velocity linearly varying with height is given

v = U* y/H where U is considered as 33.33 m/sec as max velocity

On the solid (nondeforming) walls, no-slip conditions are imposed, u = 0, v = 0 (fixed constraint)

9. Parametric Study

Height of

water

Displacement

of Structure

Max Surface

stress(KN/m2)

0 1.28 m 0.930

20 m 2.08 m 1945

45 m 11.69 m 101000

Displacement of structure

0 meter water height

20 meters water Height

45 meter water height

Surface Pressure

0 meter Water Height

20 meter water height

45 meter Water Height

10. 3D Model

11. Solver Used It uses MUMPS solver, two segregated solutions, segregated solution 1, and

segregated solution 2.

Segregated 1 solution comprises of spatial coordinates and displacement

Segregated 2 Solution comprises of Velocity field and pressure.

12. Future Prospectus

Modelling of torsional effect on offshore structures due do wind and water waves.

Change of material properties with displacement Behavior of offshore structures under earthquake conditions

References

[1] Tetsuro Tamura, Kojiro Nozawa, Koji Kondo, AIJ guide for numerical prediction of wind loads

on buildings, Journal of Wind Engineering and Industrial Aerodynamics, Elsevier, Volume 96,

Issues 1011, OctoberNovember 2008, Pages 19741984

[2] Bert Blocken, Jan Carmeliet, Ted Stathopoulos CFD evaluation of wind speed conditions in

passages between parallel buildingseffect of wall-function roughness modifications for the

atmospheric boundary layer flow, Journal of Wind Engineering and Industrial Aerodynamics,

Elsevier, Volume 95 (2007) 941962

[3] Ben C Gerwick, Construction of Marine and Offshore Structures Taylor & Francis Group 2007

[4] Domic Reeve, Andrew Chadwick, Chris Fleming, Coastal Engineering, CRC publication 2014

[5] COMSOL4.4 user manual

[6] COMSOL Examples Fluid Structure Interaction and Obstacle in a fluid