Finite element modeling and jacket lauch analysis using a barge

This paper presents method numerically simulate the barge jacket system for calculating the jacket launch process using a barge by finite elements software, desire to affirm the ability of our engineers in the calculation of the problems mentioned above.

FINITE ELEMENT MODELING AND JACKET LAUCH ANALYSIS USING A BARGE

Dinh Quang Cuong (1); Ngo Tuan Dung (2)

(1) Institute of Construction for Offshore Engineering (ICOFFSHORE)

University of Civil Engineering - 55 Giai Phong Street - Hanoi (2) PetroVietnam Marine Shipyard J/S Company (PVshipyard),

No 65A2, 30/4 Street, Vung Tau; Email: dqc@hn.vnn.vn

Abstract:

There jacket - barge system models can be using Software system, which have

currently on the world and in Vietnam to calculating: StruCAD*3D;

StabCAD; NEPTUNE Developed by Zentech USA; MOSES (Multi

Operational Structural Engineering Simulation) and SACS (Structural

Analysis Computer System), ProgramManual- Engineering Dynamic, Inc

USA, but their use often by foreigners The most important problem is the

system simulation includes barge and jacket This paper presents method

numerically simulate the barge - jacket system for calculating the jacket launch

process using a barge by finite elements software, desire to affirm the ability of our

engineers in the calculation of the problems mentioned above

PHƯƠNG PHÁP PHẦN TỬ HỮU HẠN GIẢI BÀI TOÁN VẬN CHUYỂN, ĐÁNH CHÌM KHỐI CHÂN ĐẾ CÔNG TRÌNH BIỂN TỪ SÀ LAN

Tóm tắt:

Có thể dùng các chương trình phần mềm theo phương pháp phần tử hữu hạn

để giải bài toán vận chuyển, đánh chìm khối chân đế từ xà lan Các chương

trình phần mềm nói trên là: StruCAD*3D; StrabCAD; SASC,… đã được trang

bị tại Việt Nam, tuy nhiên hầu như vẫn chỉ do người nước ngoài sử dụng Vấn

đề quan trọng nhất khi thực hiện bài toán là sự thống nhất về mặt phương pháp

mô phỏng hệ thống kết cấu khối chân đế và sà lan trong quá trình vận chuyển,

đánh chìm Báo cáo này trình bầy việc mô phỏng hệ thống kết cấu theo

phương pháp phần tử hữu hạn và một số kết quả ban đầu khi tính toán vận

chuyển, đánh chìm khối chân đế công trình biển bằng thép từ sà lan bằng

chương trình phần mềm theo phương pháp phần tử hữu hạn, với mong muốn

khẳng định khả năng của các kỹ sư của chúng ta có thể giải các bài toán nêu

trên đây

1 Introduction

The launch process is broadly divided into four dynamically distinct phases:

Phase 1: Jacket sliding over the launch-way of the barge towards the rocker arm

Phase 2: Jacket sliding on the rocker arm and rotating with respect to the rocker pin Phase 3: Jacket tipping on one side of the barge

Phase 4: Separation of the jacket from the barge

During each phase, the equations of motion are developed and solved using a powerful variable time step algorithm [1] [2] In the launch formulation, the barge-jacket interaction effect is incorporated and barge and jacket motions (including displacement, velocity, and acceleration) are computed for each time step, and the reaction forces and hydrodynamic forces are summarized Bottom clearance for the jacket can also be checked during the launch process

Currently, the program assumes the lateral symmetry of the barge-jacket system, and thus only Phase 1, Phase 2, and Phase 4 are simulated by the program Although the first two phases of jacket motion are constrained to the vertical plane of the barge, the hydrodynamic forces of the barge and jacket are considered in three dimensions

2 Simulate the system

2.1 Coordinate Systems: There are five major coordinate systems:

2.1.1.The input coordinate system

The input coordinate system is the coordinate system used to generate barge and jacket models and to enter launch data In general, the input coordinate system is also known as the structural global system when generating the jacket structural model The x-axis of the input coordinate system should be parallel to water surface and run along the center of the barge toward the rocker arm, i.e., the launch direction It is recommended that the x-axis of the input coordinate system be chosen along the keel of the launch barge

2.1.2 The barge body coordinate system

The barge body coordinate systems are fixed in the body with the origin located at the barge Center of Gravity (C.G.), respectively (Figure 2) The barge body coordinate system

is also used to describe relative motions between jacket and barge during the first two phases of the launch process

2.1.3 The jacket body coordinate system

The jacket body coordinate systems are fixed in the body with the origin located at the jacket Center of Gravity respectively (Figure 2)

2.1.4 The rocker arm coordinate system

The rocker arm coordinate system is fixed in the rocker arm, with its origin at the rocker pin (Figure 3) The rocker arm coordinate system is mainly used to describe phase 2 motion and jacket-barge interaction forces

2.1.5 The global (water surface) coordinate system

The global coordinate system, which is an inertial system fixed in space, that the origin

is at the water surface directly above the barge center of gravity before the launch process begins The global coordinate system has the same positive directions (X, Y, and Z) as the

input coordinate system (Figure 1) The barge and jacket motions and hydrodynamic forces are described in the global coordinate system

Originally, all the x-axes are in the direction of the barge bow to stern (i.e., in the launch direction), and all z-axes are vertical upward The y-axis is determined by the right-hand rule By specifying the jacket leading points, trailing points, and trailing edge distance, the program automatically puts the jacket on the top of the launch runner based the the assumption that the launch runner is parallel to the barge keel

2.2 Mathematical Formulation

The forces acting on the jacket-barge system due to inertial, gravitational, frictional, and hydrostatic and hydrodynamic forces are evaluated, and the equations of motion in the form

)

(t

F KX X C X

M: Total mass matrix of the global coordinate system; C: Damping matrix;

K: Structural stiffness matrix; F(t): Force vector; X;X;X : Vector of accelerations,

velocities and displacements

Thus the equations of motion are non-linear and the time domain method of analysis is inevitable

2.3 Model Generation

Other typical input which includes the barge C.G., simulation time and time step, jacket initial position relative to launch runner, and rocker arm data

The barge geometry is modeled by 'PANEL' cards Each panel is a flat area described

by connecting lines of up to 8 points All the joints in the panel must be in the same plane The order of the connecting nodes, whether clockwise or counter-clockwise, will determine the direction of the panel in accordance with the right hand rule All the panels must have inward normal (i.e., the direction of the panel) to form a complete enclosed body, i.e., the hull of the launch barge

The jacket model can be generated in any orientation in the input coordinate system (Figure 4) By defining the contacting surface of the jacket on the barge, the program will re-orient the jacket The initial position of the jacket can be determined by further specifying the initial trailing edge position and the height of rocker pin and rocker arm in the input coordinate system Since it is assumed that the barge-jacket system is laterally symmetric, it is not necessary to specify the relative position in the y-direction

The contacting surface of the jacket on the barge is defined by specifying four typical points, i.e., the leading starboard point, the leading port point, the trailing starboard point, and the trailing port point (Figure 1) These points are used to determine the coordinate system associated with the contacting surface, and the contacting length of jacket structure members on the launch runner

Therefore, the trailing points should be the aft-most points of the jacket structure members that contact the barge launch runner

2.4 Initial Equilibrium Position

The barge-jacket system is assumed to be in static equilibrium under the effect of gravity and hydrostatic forces when the launch simulation begins The initial equilibrium position can be obtained by the program based upon mass matrices, geometry, and the relative

position of the jacket and barge However, you can also choose to enter the initial draft and trim of the barge to define the initial equilibrium position In this case, the program will not check the unbalanced forces and moments, if any Therefore, you need to make sure to enter the correct initial equilibrium position that corresponds with other input data

2.5 Mass Properties

In addition to structural geometry, the mass properties of the barge and the jacket are also required for launch analysis While the mass matrix of the jacket structure is calculated internally by the program, you must enter the mass matrix of the barge yourself

2.6 Winch Effect

A system of “winches” may be used to slide the jacket along the launch runner toward the rocker arm In this case, it is assumed that a constant winch speed is applied and the winch process is slow enough that its dynamic effect can be neglected Winching proceeds until the jacket sliding velocity is greater than the winch speed This makes the jacket slide along the launch runner by its own weight

2.7 Hydrodynamic Forces and Coefficients

While Morison’s equation is applied to calculate the hydrodynamic forces on the jacket, the hydrodynamic forces acting on the barge are described by added mass and damping coefficients You can choose to enter these hydrodynamic coefficients to override the default values assigned by the program (Currently not available) Both added mass and damping coefficients must be entered in a non-dimensional form

Non-dimensional added mass coefficients are defined as follows:

• For surge, sway, and heave: Aii/ Mass, i = 1, 2, 3

• For roll, pitch, and yaw: Aii/Iii, i = 4, 5, 6

Non-dimensional damping coefficients are defined as follows:

• For surge, sway, and heave: Bii / Mass * (L/g)1/2 i = 1, 2, 3

• For roll, pitch, and yaw: Bii/ Mass * (L/g) 1/2 * (1/L)2, i = 4, 5, 6 Where lii is mass moment of inertial, L is the length of the barge, and g is the

acceleration due to gravity

2.8 Load Generation

Each program-generated load case, which corresponds to each time point specified may include parts or all of the following loads: Buoyancy, hydrodynamic forces, barge-jacket interaction forces, and inertial forces Member buoyancy and hydrodynamic forces are generally transformed to member distributed loads, except that when only avery small number of the members is submerged, and member concentrated loads are therefore used

To generate barge-jacket interaction loads, the jacket structure joints lying on the launch runner that is to receive the launch runner reaction is defined Based upon these structural joints, the program will search for the structural members contacting the launch runner, and subsequently map the interaction forces onto these members The program assumes that the rocker arm is relatively rigid with respect to the jacket, and therefore interaction

forces are distributed uniformly along the structural members contacting the rocker arm and barge launch runner

3 Program Output

There are two levels of output In the Summary Output, Launch Parameters, Initial

Position, Weight and Buoyancy, Phase-wise Motion Summary, and the Height to Water

Surface reports are included The Detail Report includes the Rigid Body Position Result 6

Degrees of Freedom (DOF), the Velocity of Jacket and Barge (6 DOF), the Acceleration of Jacket and Barge (6 DOF), the Rocker Arm Forces, and Forces and Moments acting on the Launch System

You also have an option to choose output time steps for each phase, which can be different from the simulation time steps

To help you better understand the Launch program, sample launch outputs are explained report-byreport on the following pages

3.1 Jacket Mass Matrix Report

The Jacket Mass Matrix and Jacket Radius of Gyration are reported in the Input Coordinate System with the jacket at its original position, i.e., before the jacket is reoriented and put on the launch way (Figrue 4)

3.2 Launch Parameters and Barge Data Report

The barge-jacket system is assumed to be in static equilibrium under the effect of gravity and hydrostatic forces when the launch simulation begins The initial equilibrium position (with the jacket already on the launch way) is defined by the draft at the barge center of gravity, and barge trim and heel (Figure 2)

The Barge Radius of Gyration is relative to the body-fixed Barge Coordinate System The Rocker Pin location is reported in the Input Coordinate System with the barge launch way still parallel to the X axis of the Input Coordinate System, i.e., before the barge-jacket system achieves its initial equilibrium position (Figure 4)

3.3 Weights and Buoyancy and Initial Position Data Report

In the Input Coordinate System, the jacket weight and buoyancy are reported with the jacket at its original position, i.e., before the jacket is re-oriented and put on the launch way (Figure 4) However, in the Global Coordinate System, the jacket weight and buoyancy are reported with the barge-jacket system in its initial equilibrium position (Figure 2)

The initial position data of barge - jacket system are reported in the Global Coordinate System (Figure 2,4)

3.4 Rigid Body Position Results

The barge rigid body position (six degrees of freedom) is reported in the Global Coordinate System, which is fixed in the space with its X-Y plane coinciding with the water plane The jacket rigid body position (six degrees of freedom) is reported both in the Global Coordinate System and in the Barge Coordinate System, which is fixed in the barge with its origin at the barge C.G

For Phase 1, the only non-zero relative motion between the jacket and barge is in the

barge Хdirection, since the jacket is sliding on the launch way

For Phase 2, the jacket has relative skid and rotational motion with respect to the barge

These relative motion components are described as motion in 'X', 'Z' and 'Pitch' in the Barge Coordinate System

For Phase 4, the jacket has been separated from the barge Thus no relative motion is

reported

Figure 8: Phase 4 Motion Figure 9: Height to Water Surface and

Bottom Clearance

3.5 Velocity and Acceleration of Jacket and Barge

The barge velocity is reported in the Global Coordinate System, which is fixed in space with its X-Y plane coinciding with the water plane

The jacket velocity is reported in both the Global Coordinate System and in the Barge Coordinate System, which s fixed in the barge with its origin at the barge C.G

3.6 Force Table

The total contacting forces between the jacket and barge are reported in the rocker arm coordinate system, which is fixed in the rocker arm with the local z direction always perpendicular to the rocker arm top and the x-direction originally in the launch direction (Figure 3) Note that the rocker arm Y-direction is determined by the right hand rule

3.7 Forces and Moments acting on the Launch System

All the forces and moments acting on the barge are reported in the coordinate system, which origin is located at the barge C.G and the X-Y-Z axes in the same direction as those

of the Global Coordinate System (Figure 5)

All the forces and moments acting on the jacket are reported in the coordinate system, which origin is located at the jacket C.G and the X-Y-Z axes in the same direction as those

of the Global Coordinate System (Figure 5)

3.8 Phase 1 Motion Report

In this report, the barge motions are reported in the Global Coordinate System Since the only relative motion between the jacket and the barge is in the x-direction of the Barge Coordinate System (fixed in the barge), the jacket motions are better described as relative motion Note that the x-direction of the Barge Coordinate System will be different from that of the Global Coordinate System if the barge has a non-zero pitch/trim angle

Only the most important barge motions, i.e., surge, heave, and pitch, are included in this summary report The other motion information can be found in the detail report

3.9 Phase 2 Motion Report

For Phase 2, both the barge and jacket motions are reported in the Global Coordinate System The relative skid velocity is reported in the Rocker Arm System (fixed in the rocker arm) The relative position (X-direction) is still reported in Barge Coordinate System to be consistent with the Phase 1 relative motion In addition, the rocker arm rotating angle and rocker arm contacting force in the rocker arm z-direction are also reported The rocker arm force in the rocker arm y-direction can be found in the detail report Note that the x-direction of the Rocker Arm Coordinate System will be different from that of the Global in Barge Coordinate System if rocker arm has a non-zero angle Only the most important barge and jacket motions, i.e., surge, heave, and pitch are included in this summary report The other motion information can be found in the detail report

3.10 Phase 4 Motion Report

For Phase 4, both the barge and jacket motions are reported in the Global Coordinate System Since the jacket has separated from the barge, no relative motion will be reported The bottom clearance is calculated based upon the reference joints you define

Only the most important barge and jacket motions, i.e., surge, heave, and pitch are included in

this summary report The other motion information can be found in the detail report

3.11 Height to Water Surface and Bottom Clearance of Jacket

The “Height to Water Surface’ is positive if the joints are above the water surface The “Deepest Point” is calculated based upon the reference joints you define

4 Conclusions

There jacket - barge system models can be using Software system, which have currently

in ICOFFSHORE to calculating: StruCAD*3D; StabCAD; NEPTUNE Developed by

Zentech USA SACS, Structural Analysis Computer System, Program Manual-

Engineering Dynamic, Inc USA

5 Reference

1 Bathe K J -Finite Element Procedures in Engineering Analysis-1992

2 Bathe K.J - Finite Element Procedures - USA-1996

3 Peter Bettes - Infinite element - London - 1993

4 Softwares: StruCAD*3D; StrabCAD; NepTune developed by Zentech – USA

5 SACS, Structural Analysis Computer System, Program Manual- Engineering Dynamic, Inc USA