The Design of Mooring Dolphin Layout and Mooring Line Tension Analysis

Some marine industries choose to set up their activities in the coastal area northern part of Java Island, Indonesia, by considering the calmness of the Java Sea environment. A special terminal for liquid bulk cargo to transport fuel oil is being planned for construction in Sedayu Lawas village, Lamongan district, East Java Province. The ship carrying cargo oil require a pier and jetty in sufficient depth for loading and unloading. The jetty need to be enhanced to support more equipment for ship to port, which include breasting dolphin, mooring dolphin, trestle, and causeway. In this respect a study is carried out to review the stress analysis of the mooring lines (ropes) attached to jetty for various mooring dolphin layout and arrangements. The metocean data such as wind and tides in the areas were processed and analyzed. The layout of the ship and mooring dolphin rope angle were designed in accordance with OCIMF code. The stress analysis of mooring rope for various layout mooring dolphin has been analyzed to determine whether the design layout is safe during operation. The rope stress were analyzed using time domain dynamic analysis for 10,800 seconds. The safety of ship during loading for various rope condition were presented. It is suggested that all ropes should be attached to mooring dolphin for ship and port safety.


INTRODUCTION
Due to the need to maintain a berthing ship stay in position while transferring cargo oil at the port, then a well arranged mooring system with sufficient strength is needed.The mooring system consists of breasting dolphin and mooring dolphin, as shown in Fig. 1.Breasting dolphin serve to withstand the impact of the ship, while the mooring dolphin functions to hold the pull force of the ship when it is docked.
Due to the large variation in the ship sizes expected to be served by this dolphin, i.e. ranging from 1,000 DWT up to 50,000 DWT, it is necessary to design the appropriate mooring dolphin for ships that will be berthed.In addition it is necessary to analyze the mooring stress to assure the designed rope for mooring dolphin will have adequate strength to withstand largest tension load and hence safe during the entire operation in long period.

MATERIALS AND METHODS
Stages in the analysis are carried out as described in the following.

Literature Study
Literature study is performed by exploring books, theses, journals, codes/standards, legislation/regulations obtained from both national and international resources.Further, the activity include also learning the softwares to be used, covering the general floating structure design, motion in frequency domain, dynamic analysis in the time domain.

Data Collection
The data to be prepared for the analysis may be found from literature study or field study.Field study is conducted by direct observation of the object to be examined, exploration on some of the data required in accordance with the issues to be examined and the implementation of this study.The primary data required are as follows: • Main dimension data of the ships

Wind Data Processing
Wind data processing is intended to obtain the value of wind speed and direction of winds that blow on the location of mooring dolphin.This can then be used to determine the effective fetch.Wind data that has been processed and may further be used to calculate wave parameters, namely wave heights and return periods.

Tidal Forecasting
The tidal data forecasting is established on the basis of the field observation especially on the periodical high and low tides.It aims at obtaining the components of tides and tidal elevation.The least squares method is applied in this research.

Design Elevation of Mooring Dolphin
Elevation is calculated from the pier on the predictions of tidal water level, reference tidal datum and wave heights as well as the predetermined clearance of 1.5 m [2,3].As for determining the minimum depth of the pier, a factor which became the main parameter is the high laden water on ships in operation and a minimum height of receding waters.The factors are used in determining the depth of the location of the pier so that ships to be docked at the pier will not touch the seabed due to the depth of less than it is needed.

Design Configuration of Mooring Dolphin
Design configuration of mooring dolphin made based on the existing provisions in the applicable standard or code, such as OCIMF [2], fixed mooring design manuals, etc.

Ship Modeling
The initial ship structure modeling is performed using a ship design software.Modeling is aimed at finding the ship hydrostatic data to be validated by the data stability booklet.Further, modeling is carried by hydrodynamic software to obtain RAO, wave drift, added mass and damping force that will be used as input for modeling further by using analysis software based dynamic time domain, with different directions heading the environmental load (0°, 180°, and 270°).

Analysis of Tension of Mooring Rope
Mooring lines stress analysis is conducted to obtain value of tension or maximum tensile stress.For determining the safety of mooring lines during operation it is necessary to find the ratio of MBL (Minimum Breaking Load) of the rope against the maximum operational tension value of the analysis results.The MBL mooring rope, made of nylon, must be greater than the maximum operating tension by the factor of 2.2.

Analysis of the Tension when a Mooring Line Disconnected
This analysis is conducted to observe the safety of mooring dolphin configuration when one or a number of mooring ropes are disconnected.

Wind Rose and Fetch
To facilitate in determining the speed and direction of the wind, the wind data was processed in the form of wind rose.
The result of such an analysis is presented in Fig. 2. It is shown that the prevailing wind came from Southeast and North West with the maximum speed in the range of 7.5-10.0m/s.However, the wind direction considered for fetch analysis was Northwest since the Southeast area was mainly land.Further, the wind data was also processed to generate the fetch length information, as exhibited in Fig. 3

Wave Data and Return Period
Wind data has been processed and applied to predict the wave parameters following CERC [5,6].The results as listed in Table 1 comprise of wave return periods from 5 up to 100 years and the corresponding significant wave heights.

Tide Forecasting
The least square method has been employed in the tide forecasting.In this respect results of the forecasting in term of the tide components are given in Table 2. Figure 4 shows the water fluctuation level during tides observation.It is seen from Fig. 4 the types of tides was mixed tide prevailing diurnal tide.

Elevation of Mooring Dolphin
In this study the LLWS water level regarded as determining the elevation datum [7].Therefore, all the tides parameters were added by 0.1 m.The maximum elevation of mooring pile for 4.5 m significant wave height and 2.6m HHWS and 1.5m freeboard according to OCIMF was calculated as follows: HHWS + ½ Hs + 1.

Mooring Dolphin Configuration
There are several criteria that must be met in the design the configuration of mooring dolphin [3]  Due to the relatively large variation of the vessel size, then it is not possible to make all configuration in one mooring dolphin.Therefore, the overall configuration of mooring dolphin is divided into three segments, as exhibited in Fig. 6.The figure shows three different berthing combination for ships size 1,000 -5,000 DWT, 5,000-10,000 DWT and 10,000 -50,000DWT, respectively.Therefore, there are three terminal or three mooring dolphin configuration prepared for services vessel in one jetty.Due to limited space in this paper, only the result of mooring configuration for 10,000 -50,000DWT will be presented.

Result of Ship Modeling
Modeling of the ship is performed using hydrodynamic software based 3-D diffraction theory and by considering the data of loading, environmental and technical drawing [8].This modeling generates the hull form and panelization as shown in Fig. 9.

Ship RAO Analysis
RAO of ship motion behavior was computed using the hydrodynamic software for wave directions of 0°, 45°, 90°, 135° and 180°.In the case of a 50,000 DWT tanker, as can be seen in Fig. 10, the largest surge motion occurs in the wave heading 0 o and 180 o .For sway motion, the largest intensity is due to the 90 o wave, as indicated in Fig. 11.Further, as shown in Fig. 12, the largest heave motion occurs also in 90 o wave direction.Among the three translational modes, the peak of heave RAO motion eventually is the largest, that is in the order of 1.473 m/m.While the peak of surge and sway RAOs are only approaching 1.0 m/m.

Tension Analysis on the Mooring Line
Analysis of tension related to the mooring tensile strength was accomplished by the assistance of a software based on dynamic analysis in time domain.The analysis was performed on the 6 mooring lines induced by the environmental loading propagating in the directions of 0 o , 90 o , and 180 o .The analysis uses time history simulation and run for 3 hours or 10,800 secs in accordance with the provisions of OCIMF.Further, the simulations were also conducted for the cases of high and low tides.
Mooring lines used in this analysis are nylon ropes with a diameter of 50 mm.MBL of the rope amounted 348.34 kN.The mooring lines are considered safe when the maximum applied tension is less than the MBL divided by the safety factor of 2.2 as required by OCIMF.The following tables, shows the maximum tension and the ratio of MBL/ Maximum tension at each line based on simulation result for three environmental load (0 o , 90 o and 180 o ) at low and high tide for 10,000 -50,000DWT mooring configuration as mention above.From Tables 3 to 8 it can be seen that at the high tide, maximum tension occurs at line 3 on 90 o incoming environmental load, while the lowest maximum tension occured at 180 o on line 1 during low tide.Either at low or high tide, the highest maximum tension occurs on 90 o incoming environmental load.There are no significant difference of max tension for 0 o and 180 o incoming environmental load between low and high tide.In average, tension at low tide was smaller than tension at high tide.By observing the ratio of MBL/Tension, it can be concluded that the designed configuration of mooring dolphin was safe for operation as there is no value below 2.2 [3,9].

Tension Analysis if a Mooring Line Disconnected
To evaluate the safety of the mooring line configuration, a condition of broken rope was simulated.The condition of maximum tension for each line when particular single line was broken or failed was evaluated.The following tables shows the values of ratio MBL/maximum tension for various fail condition.

Figure 2 .Figure 3 .
Figure 2. Chart of wind rose year 2005 -2015 . For the current study an effective fetch with length of about 235 km is attained.The prevailing wind direction for effective fecth estimation was Northwest, following the chart of wind rose in Fig 2 above.
5 (freeboard according to OCIMF) = 2.6 + 2.25 + 1.5 = 6,35 m from LLWS (datum) As for the depth of the mooring dolphin is calculated by considering the largest loaded from ships to be berthed.If a laden ships of 50,000 DWT having a characteristic draft of 12 m, then the minimum draft of the mooring dolphin is calculated as follows: Minimum draft = 1.1 x Ship draft Minimum draft = 1.1 x 12 Minimum draft = 13.2 m of HHWS Tolerance Freedom = 1 m Final minimum draft = -14.2m from LLWS (datum) Considering the above values, the design elevation of the mooring dolphin is then visualized as in Fig. 5.

Figure 5 .
Figure 5. Elevation of the mooring dolphin namely:  Maximum horizontal angle from the bow and stern line to the line perpendicular of the bilge is of 45 o  Maximum horizontal angle between the breast line to the line perpendicular of the hull of 15 o  Maximum horizontal angle of spring line against the hull is 10 o  Maximum vertical angle of a mooring line to the ships is 25 o  The distance between the berthing dolphin is 0.3 LOA  The distance between the structures platform and the highest maximum water level is 1.5 m  The distance between the ship to the mooring dolphin ranges between 30 ~ 50 m  The maximum number of mooring dolphin in one configuration is 6  The maximum number breasting dolphin in one configuration is 4

Figure 9 .
Figure 9.The hull model of a 50,000 DWT ship

Figure 13 .
Figure 13.Roll RAO of a 50,000 DWT tanker

Table 1 .
Wave return period and heights

Table 3 .
Mooring line tensions due to 0 o environmental load at high tide

Table 4 .
Mooring line tensions due to 0 o environmental load at low tide

Table 5 .
Mooring line tensions due to 90 o environmental load at high tide

Table 6 .
Mooring line tensions due to 90 o environmental load at low tide

Table 7 .
Mooring line tensions due to 180 o environmental load at high tide

Table 9 .
Values of MBL/max tension for the case of mooring line-1 is disconnected

Table 10 .
Values of MBL/max tension for the case of mooring line-2 is disconnected Table 11.Values of MBL/max tension for the case of mooring line-3 is disconnected