Hydrodynamics Reaction of Two Bodies in Close Proximity

Analysis of Hydrodynamics Reaction of two bodies in Close Proximity

MSc Thesis By Manish Shrivastava Faculty of Mechanical, Maritime and Materials Engineering TU Delft 2015 Preface Abstract

Table of Contents 1. Introduction............................................................................................................................................... 3 1.1 Inspiration for Topic ............................................................................................................................ 3 1.2 Problem Definition .............................................................................................................................. 5 1.3 Objective.............................................................................................................................................. 7 1.4 Structure of Report.............................................................................................................................. 7 2. Literature Research ................................................................................................................................... 3 9. Abbreviation .............................................................................................................................................. 8 10. Bibiliography ............................................................................................................................................ 8 11. Appendices .............................................................................................................................................. 8

1. Introduction 1.1 Inspiration for Topic It is very expensive to fabricate weld structures in sea and offshore, also it is time consuming and recycling is difficult. So to minimize costs, structures are being built onshore and then they are carried offshore on Vessels or towed (Not all structures can be towed). As structures are becoming heavier and bigger, lifting is not the most optimal or feasible way always. Floatover has advantage over lift in sense it doesn’t need extra crane capacity as long as it can be carried to the site. Another trend is these days’ hulls are being fabricated in China or Korea while machinery components are being fitted in Europe. Sometimes FPSO and semisubmersibles are carried on Floater to the site and floated. This reduces time as, sailing speed of Floater can be maintained high compared to towing time as resistance is less. Also dry transportation has its own advantages. Floatover vessels can be used for in-situ dry-docking of FPSOs and semis. Floatover is becoming new offshore trend of the day, slowly but steadily. During floatover operation (or Offshore discharging), Vessel is ballasted and it sinks in water, while load on deck ( FPSO, Semi etc) starts floating at surface. Alternately during dry-docking of FPSO or Semi vessel (Offshore loading) is de-ballasted and FPSO slowly lands on deck of Vessel. During both operations (Offshore loading and discharging), there are two major Hydrodynamic challenges, (1) Controlling horizontal relative movement of HTV and cargo, (2) Estimating Vertical movement when a very small gap exists between HTV and FPSO, in order to avoid collision of cargo on deck of HTV.

In past Hydrodynamic interaction between floating bodies have been extensively studied and three dimensional linear diffraction theory have been widely used. In sheltered water operability is not much affected by vertical motions, but in deepwater offshore where floatover operation has to be done, it affects operability. And vessel HTV has to wait for time based on operational experience to determine when it can floatover discharge or load FPSO. Other option is to floatover cargo in a sheltered area and bring it to field with tugs. If HTV can directly bring Cargo to field under controlled conditions, it would boost Heavy Marine Transport. Therefore there is urgent necessity to understand Hydrodynamic behavior of Cargo during offshore loading and discharge operation.

1.2. Literature Research So far fours methods have been used to analyze vertical motions

Analytical Solution by Drobyshevski Asymptotic Matching Program DELFRAC- 3D Multibody Diffraction program MDDM-Multi Domain Diffraction Method Modal tests were conducted by Dockwise to assess vertical cargo motions with different cargo shapes. These tests revealed difficulty in assessing motional behavior of cargo in close proximity of HTV deck. Test results show that a large force is needed to push the water out of gap domain and even large force is needed to suck the water back in. Studies of motion using 3D- multibody diffraction proved difficulty in estimating effect of narrow gap between cargo and HTV deck. Tuning of Hydrodynamic calculations was done by changing added mass and damping parameters, however no clear relation between gap size and parameters were found. Drobyshevski assumed two dimensional flow describes flow in the gap. He used matched asymptotic expansion to analytically solve radiation and diffraction potentials (to analyze motions of truncated cylinder in very shallow waters) instead of multi-body 3D diffraction. In this method he split the domain between outer domain and under bottom domain and used asymptotic matching technique to guarantee a continuous flow over the domain boundary. His method was tested and validated with truncated vertical Cylinder with flat bottom in extreme shallow water. His program gave increasing accuracy with increased H/T ratio. But his program is only for single bodies of cylindrical shape. So, it has limited scope. Asymptotic matching includes information of velocity potential at certain distance from the domain boundary, and is therefore more accurate than direct matching of velocity potentials for small gap heights. Jan De Jong developed Multi Domain Diffraction Method, combination of two dimensional flow for under bottom part for single body in shallow water and 3D-diffraction, both based on 3D- linear potential theory.

2. Problem Definition Author wants to get simple description of vertical interaction forces and motions between two bodies. Since it would be too complex at this stage If both bodies are moving vertically with effect of waves, to simplify the problem Author has considered bottom body HTV to be not affected by water and hence in constant linear motion, unaffected by waves. This assumption will be reasonable, because as depth gets higher effect of waves and pressure variation reduces. A simple two body diagram will look like following during floatover process.

The problem has similarities with that of motions of barge in shallow water, But there are differences (a) There is no beach. (b) Wave shoaling Phenomena doesn’t occur (c) Phenomena resembles more to squeeze out and suck in of water between two blocks, but wave condition outside is deep-sea wave. (d) Wave elevation is not affected during the process. We chose to ignore small variation in wave field nearby. (e) Wave drift forces, Current force and wind forces, we chose to ignore at this stage. Further simplification we will ignore the motion of HTV while calculating the force. To simplify the problem, we would first like to analyze effect of small gap in first we try to analyze single body effect in shallow water. We will consider flow to be two dimension. There can be several approaches to analyze load during floatover using CFD, using different boundary conditions, domain and mesh. General objectives while choosing the boundary can be to (1) Reflect/retain the physics of close proximity problem. (2) Minimize domain size in order to reduce calculation time (3) Feasibility of modelling When a motion signal is given to cargo object in test, its force signal gives peaks for sinusoidal motion signal.

When cargo floats in water it moves due to hydrodynamic forces of waves. This movement has component in vertical direction. In test it can be moved in vertical with sinusoidal amplitude. In real it is sinusoidal only for perfect linear conditions as we expect in deep sea for single body. In test by Dockwise it was found that while motion of body remains sinusoidal, force has peaks for all kinds of cargo for small gap.

The reason for peakedness can be understood by explaining flow phenomena. Let us start with flow into the gap, As a first step when the Cargo moves upwards, there is suction created, but area of suction is very large compared to size of the gap. Fluid viscosity and its nearness to boundary makes flow slower and there is huge resistance. Second when cargo moves downwards, It tries to push water outside (2D, so only one direction it comes out) but water was flowing in before so to push out flow direction has to be reversed. Viscosity again gives resistance. Phenomena will be more pronounced when time period is small. Another analogy of this phenomena would be piston movement and force experienced. As piston moves up pressure on top of it increases as there is no place for the fluid to flow except leaking past the rings or leaking valves. Same during suction. Set up: Investigation has to be done for different time periods, initially starting with high time period and then narrowing to small time period. For very high time period Force curve should also match motion curve and should be sinusoidal like linear potential theory will do or CFD also. Similarly for the gap size, initially starting from very high gap size, it should match the

Consideration for domain size If we consider infinite size then correct boundary If we consider finite size then ratio of gap size to width Another consideration may be test size which can be used for verification. Boundary conditions Limitation of Comflow Fine Marine can be used if Comflow fails. After getting force plot it can be used to relate it with time period and gap size. This will give estimation of forces expected in deepsea during floatover. Next step would be to prediction motions in deepsea. It will be equilibrium of forces and motions which will decide. Motions and forces can be solved for a particular gap size and wave period, assuming cargo motions will follow wave period linearly.

1.3 Objective 1.4 Structure of Report

9. Abbreviation HTV (Heavy Transport Vessel)

10. Bibiliography 1. Offshore Dry-docking of FPSOs, a response to industry needs: By T. Terpstraw, Dockwise

11. Appendices