This analysis is performed to be used for a study of Vessel into a safe mooring system on the Offshore site. This report assesses the behaviour of the Vessel mooring system under extreme weather conditions in the field of interest, along with typical operating conditions. The mooring analysis focuses on the structural adequacy of the mooring lines, anchor uplift and anchor holding power.
The whole mooring analysis is performed using Moses and Orcaflex 9.2.
2. ANALYSIS TOOLS
Orcaflex is a marine dynamics program developed by Orcina for static and dynamic analysis of a wide range of offshore systems, including all types of marine risers (rigid and flexible), global analysis, moorings, installation and towed systems.
MOSES” is an integrated ocean simulation and analysis program for analyzing the ocean performance of systems of bodies. The program accepts a description of a model (vessel) to perform either static, frequency domain or time domain simulations. “MOSES” has been utilized to generate vessel hydrostatics, ballasting and stability of a vessel and cargo and generation of standard seakeeping results in irregular Seas.
3. ANALYSIS PROCEDURE & METHODOLOGY
Using available vessel documents, the following basic input data are generated for the MOSES program:
- Hull input offsets generated using the vessel’s various Scantling Plans to capture the columns, bracing and underwater hull forms.
- Wind areas and underwater hull/columns/bracing areas are calculated using vessel’s various General Arrangement drawings and area values are used as input into “MOSES” programs to calculate the corresponding forces.
Interfacing all the input data and program commands, the following are generated by “MOSES”:
- Respond Amplitude Operators (RAO)
- Added Inertia Coefficient
- Linear Radiation Damping Coefficient
- Linearised Wave Frequency Forces
This operating case is analyzed using Orcaflex programs. The Orcaflex program provides fast and accurate analysis of catenary systems such as mooring analysis, flexible risers and umbilical cables under wave and current loads and externally imposed motions. The analysis procedures for the operating and survival are listed below:
1.Three dimensional diffraction/radiation analysis is performed using MOSES to compute the hydrodynamic coefficients, such us added mass, radiation damping, linear wave force, wave drift force and motions RAOs.
2. The mooring lines Orcaflex, and also inputing hydrodynamic properties result by MOSES software. The Vessel is treated as a rigid body with 6 degrees of freedom, 3 translational and 3 rotational. The equation of motion has the following contributions.
Regular wave time domain simulation are performed with the maximum height and the associated wave period.
The maximum values of the offsets, mooring line tensions at fairlead and achor, uplifting force are obtained
The factors of safety of the mooring lines are compares against the API-2SK rule requirement.
4. MODELING VESSEL
There are some steps before we doing mooring analysis on ORCAFLEX, the following step are listed below;
4.1 Vessel Modeling
Before we do mooring analysis on orcaflex, we have to generate model on MOSES or other software related, such as MOSES, AQWA, HYDROMAC and many others software.
For modeling vessel on MOSES we need stability booklet of the vessel or other documents that shown LOA (length over all), B (breadth), D(Depth), Displacement at each Draft of vessel, LCF each Draft of vessel, LCG each Draft of vessel, and LCB each Draft of vessel, and lines plan drawing of Vessel to easy modeling on MOSES.
In MOSES, vessels are modeled from bow to stern and only one side( portside ). MOSES will reflect the model and generate one full body of Vessel. Modeling vessel on MOSES need to careful enough, we model each panel of vessel, point by points that indicate body of the vessel. If some points are missing, there will be negative diagonal damping on the output. Negative diagonal damping indicate that vessel not good enough for motion analysis. So we need to check point by point on Vessel model
See picture below about MOSES axis references :
See below picture for more details about modeling in moses :
4.2 Hydrostatic Analysis
After we got the model, we run hydrostatic analysis to capture displacement of vessel at each draft, LCG of vessel, and LCB of the vessel. After that we have to check result of the analysis with stability booklet of the vessel, 1% differences for each analyzed items are still ok for the model.
Please see below item for moses output sample;
4.3 Hydrodynamic Analysis
After all modeling and hydrostatic steps are completed ( hydrostatic analysis corrected with stability booklet ), now we run Motion and hydrodynamic analysis of the vessel to capture following items:
1. Displacements at targeted draft.
2. COG of vessel
3. Mean wave drift force
4. Respond Amplitude Operators (RAO)
5. Linearzed Wave Frequency Forces
6. Added Inertia Coefficient
7. Linear Radiation Damping Coefficient
8. Added mass and damping Coefficient
All mentioned items are used for ORCAFLEX input data.
See below items for MOSES’s motion and hydrodynamic analysis output samples;
Bouyancy and Weight
Mean Wave Drift Force
Response Amplitude Operator (RAO)