Increase Life Cycle and Decrease Cost for Navigation Buoys
-Extension of Life, by Reduction of Maintenance for Mooring Products
Authors: Nils Bengtsson Viktor Ekström
Introduction
There is no mooring that can handle all situations. SEAFLEX Buoy Mooring is addressing the new opportunities for lowering the cost for navigational aids that have appeared through the use of new technology. These technologies are expanding the life and service intervals of navigational aids and traditional moorings are not always able to keep up with this
development. A traditional mooring like chain are now determining the service intervals for navigational aids and is thereby directly affecting the total life cycle cost. By improving a buoys mooring, the life cycle cost can be decreased by extending the service intervals.
The Impact of New Technology in Aids to Navigation
New technologies in navigation buoys have extended the required service intervals. The use of plastic instead of steel as buoy material have eliminated the need for abrasive blasting and painting every three years, the plastic has an expected maintenance free time of ten to twenty years. According to International Association of Marine Aids to Navigation and Lighthouse Authorities (IALA), the plastic buoys have the following advantage: light weight handling, corrosion resistance, low maintenance, and in some cases, lower cost.1 There are also buoys in plastic that is designed for all year usage which eliminates the need for switching buoys every autumn and spring due to ice in the winter. In extreme ice conditions, steel buoys might still be the only choice.
The use of LED light compared to light bulbs has made the lights more energy efficient and eliminated the need to change light bulbs. Today the alkaline batteries can be dimensioned for long service intervals due to more energy efficient lanterns. If the light range and/or duty cycle is high, fuel cells can be used in a near future which also increases the required interval for service.
Using stainless steel in the buoys shackle, thimble and mooring eliminates the need for Zink anodes. Stainless steel also has a significant longer life in marine environment compared to galvanized steel.
The Challenges with Traditional Fix Mooring Systems
There are two traditional fix mooring systems, with chains and with synthetic mooring lines. Both have its own advantage and disadvantage that the following text will give a brief overview of.
Chain mooring
Steel chain is the most common form of mooring line. A stretched chain based mooring has the advantage of a good position of the buoy on the optimal water level. One challenge arises when there is tide or variations in water levels, when this happens the position of the buoy will be affected. The change in water level also causes tension and no tension cycles on the mooring system, which leads to a shorter life time due to fatigue. Another challenge with a chain mooring solution is sand particles in the water. Small sand particles settles between the links of the chain, later when the chain is moving the particles causes wear on the chain between the links.
1
A normal service life of a chain mooring system is between 1 to 5 years2 due to corrosion in combination with mechanical wear and tear. In addition of the mechanical challenges with a chain mooring solution, there are also environmental affects with it. The sea bottom develops dead spots from the chain lying on top of it in low water levels, the chain is also corroding which affects surrounding environment.
Rope mooring
The main advantages of rope moorings are its light weight and elasticity compared to chain moorings. A modern rope have the same strength as a steel chain, and experience show a similar or better life span then of a chain if chafe can be avoided. The rope mooring is not affected by sand particles in the water which can be seen on chains.3 Chafe and cuts are the largest dangers for a mooring rope. Sharp edges of rocks, sea shells, or the capstan of the service ship can quickly cause permanent damage to the rope surface. Rope mooring is affected by UV light, which decreases the life time of a rope. Rope mooring also causes problems with shock loads on the anchor stone during storms since they have low ability to stretch.
The Solution: Elastic Moorings
An elastic mooring have a longer life time than a chain mooring. It also has the following advantages according to IALA:4
An elastic mooring has low weight
The elastic mooring is constantly under tension and therefore does not damage the seabed
An elastic mooring have a smaller dimension then a comparable chain mooring which leads to less marine growth
An elastic mooring can be used in shallower water and breaking waves since the rubber absorbs the energy in the buoy
Due to the low weight, a elastic mooring have low influence on the reserve buoyancy of the buoy
The buoy have a very high positional accuracy since the mooring line is always under tension
Forces in the mooring line is approximately half of those in a chain mooring due to the smooth absorption of energy by the rubber cord
There is little wear and low maintenance in an elastic mooring because the mooring line is always under tension. This give the elastic mooring approximately a doubled lifetime compared to a chain mooring which lifetime is limited by fatigue and corrosion
SEAFLEX
®Elastic Buoy Mooring
A SEAFLEX® buoy mooring is designed to withstand the worst imaginable weather conditions possible. It has good resistance against corrosive environments and has an unsurpassed ability to resist all wave motion. The SEAFLEX® mooring has low effect on sensitive seabed's and surrounding environment and require low regular maintenance.
2
IALA Guideline 1066 - The Design of Floating Aid to Navigation Moorings May 2009 3
IALA Guidelines on Synthetic Mooring Lines
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The lower weight of a SEAFLEX® buoy mooring compared to a traditional chain mooring, makes a perfect pair with plastic buoys. The buoys can have lower buoyancy since they don't need to be able to lift a heavy chain in the water. The plastic buoy mooring eye is made of stainless steel which decreases the problem with corrosion on the buoy.
SEAFLEX® is one of the safest mooring systems on the market, it keeps buoys in place, regardless of tides and wave movements. The unique construction allows SEAFLEX® to slowly stretch and then return in a smooth and calm movement. The construction in
combination with initial pretensioning provides progressive resistance to both horizontal and vertical wave motion. A buoy anchored with SEAFLEX® is never passive to the sea, instead it offers firm resistance that substantially reduces wave movements. Navigation buoys anchored with SEAFLEX® do not change position as much as those anchored with chains, a fact that considerably improves marine safety in harbors and narrow channels.
A strong advantage with SEAFLEX® buoy mooring compared to other elastic mooring is that it can be equipped with a By-Pass system. The By-Pass prevents the SEAFLEX® hawsers from extending pass it foreseen elongation point due to extreme conditions. The largest SEAFLEX® equipped with the By-Pass has a brake load of 150 kN. This enables the SEAFLEX® buoy mooring to withstand extreme forces and loads.
Strong application areas for SEAFLEX
®buoy mooring
The SEAFLEX® buoy mooring is designed to handle all mooring of buoys. Based on the application, Navigationsteknik will help dimensioning the SEAFLEX® hawsers according to the requirements.The parameters of the SEAFLEX® mooring is calculated based on buoy type, active forces, variations in water level, wind situation, wave sizes, currents and depth. SEAFLEX® is suitable for mooring in large tidal variations, there are documented successful projects with 7 meter (23 feet) tidal fluctuation as well as 25 meter (82 feet) artificial
variation. The elastic mooring system slowly elongates and retracts in a smooth, even movement.
A SEAFLEX® mooring system works well in both shallower and deeper water. With over 1000 documented installations over the world, there are references with water levels between 6 to 200 meter (20 to 656 feet) for docks, pontoons, swing buoys, and navigation buoys. The good position of buoys with SEAFLEX® buoy mooring makes it suitable for narrow fairways.
SEAFLEX® mooring meets international standards for environmental protections. This makes the system suitable for mooring in sensible areas, such as those near coral reefs, areas of marine flora and historic sites.
SEAFLEX® buoy mooring is designed for year-around installations and can handle icy water without any problems
Environmental aspects
Mooring buoys with chain creates dead spots around the anchor, damages eelgrass and ruins underwater environments. The SEAFLEX® mooring system never touches the sensitive seabed, which leads to a minimal impact on grass and coral growth. The foot print of the
anchor itself can also be minimized if the SEAFLEX® mooring is equipped with an embedment anchor. The moorings durable components meet international standards for environmental protection and release no pollutants into the marine ecosystem.
The first SEAFLEX® was installed in 1981, that original product is still in use today which shows the potential life time. A SEAFLEX® mooring withstands the forces of nature without leaving any marks.
Technical data
The important part of a SEAFLEX® mooring solution is the reinforced homogeneous rubber hawser. The hawser is built around a homogeneous rubber core. A specially braided cord is wrapped around the core, and the outer layer consists of a durable rubber cover which forms the outer shell of the hawser.
A SEAFLEX® buoy mooring have a SEAFLEX® hawser with a stainless steel or optional titanium fitting, which is attached to the buoy with a integrated shackle. In the other end is the SEAFLEX® hawsers fitting attached to a rope with a shackle and thimble. The rope is then attached to the anchor stone with a thimble and shackle, afloat is attached to the rope to keep it stretched in quick water variations. SEAFLEX® buoy mooring is illustrated in Figure 1.
According to tests monitored by DNV (Det Norske Veritas), one single SEAFLEX® rubber hawser can withstand a force of drag greater than 10 kN and more than a 100% elongation. A SEAFLEX® buoy mooring can have up to 10 hawsers combined with a By-Pass system. The elongation enables SEAFLEX® to take care of all natural, and almost all artificial, water level fluctuations. Another special function with the SEAFLEX® hawser is that the restraining forces increases during the elongation, following a progressive curve giving an optimal dampening effect in the water which can be seen in Figure 2 and Figure 3.
Figure 2: Forces on SEAFLEX® hawser
Figure 3: The progressive load curve of SEAFLEX® hawser
Each end of the rubber hawser is attached to a high quality stainless steel or optional titanium fitting. It is important not to mix material since it can result in a galvanic corrosion which could cause severe damages to the material. It is always recommend to use at least stainless steel parts, both in SEAFLEX® hawser fittings, shackles and thimbles, in order to ensure increased durability and lifespan.
The rubber hawser with its end fittings are used in single or multiple hawser configurations depending on the force that will be handled by the SEAFLEX® buoy mooring. The rubber hawsers are then attached to stainless steel plates. The plates can be equipped with shackles and thimbles in different configurations, i.e. shackle/thimble, shackle/shackle or
thimble/thimble depending on the given conditions. The SEAFLEX® thimble is designed to be used with rope diameters from 18mm up to 32mm depending on the SEAFLEX® model. The rope is attached to the thimble by a quick splice. The quick splice has a low affect on the break strength of the rope. The synthetic rope recommended is well tested and has a break load, at least, of double the break load of the biggest SEAFLEX® model (with 10 hawsers and By-Pass). This is to ensure that the maintenance over the years will be kept at the lowest possible level.
Experiments have been performed, which illustrates the importance of elasticity in order to decrease the impact load in a mooring application. During the experiments a 50 kg weight was dropped and the loads were measured in the mooring line. The experimental setup can be seen in Figure 4 and the results can be seen in Figure 5 and Table 1.
Figure 4: Experimental design
Figure 5: Illustrates the differences of impact load in different materials. © Copyright Seaflex AB 2005 Table 1: Illustrates the differences of impact load in different materials shown in Figure 3
Polyester rope SEAFLEX® Chain Dyneema Average load 4,4 ± 0,2 kN 2,0 ± 0,4 kN 16,3 ± 0,8 kN 11,9 ± 0,7 kN Load peak 10,1 ± 0,3 kN 5,8 ± 0,5 kN 28, 4 ±1,4 kN 25,6 ± 0,7 kN Pulse duration 76 ± 3 ms 178 ± 18 ms 19 ± 1 ms 26 ± 2 ms Impulse load 334 ±28 Ns 357 ± 107 Ns 310 ± 32 Ns 309 ± 42 Ns As seen in the results a SEAFLEX® hawser can dramatically reduce the impact loads in a buoy mooring line.
Life cycle cost for SEAFLEX® mooring
A SEAFLEX® mooring system requires less maintenance than traditional mooring systems thanks to durable components.
A chain based mooring system requires a lot of maintenance. After a few years chain links may be heavily corroded, worn, fatigued and needs to be replaced. The purchase cost is only one part of the life time cost for a buoy mooring, the maintenance cost can grow larger than purchase cost over the buoy life time.
A rope based system is sensitive to the UV radiation at the surface. The radiation can dramatically decrease the life time of a rope mooring system. This is not the case with a SEAFLEX® mooring which includes a rope. In this configuration the rope is attached between the anchor stone and the SEAFLEX® and is not visible at the surface which minimizes the UV radiation on the rope.
Other cost savings with a SEAFLEX® mooring system during its life time are low initial outlay, minimal transportation weight, quick and easy installation, and a low cost per meter due to the integrated rope.
An example of an expected lifecycle cost with a SEAFLEX® mooring system compared to a chain mooring system could look like this. The lifetime of the mooring system is 10 years and the cost are approximated:
Installation cost
Inspection interval and cost for inspection
o Few areas to inspect, only shackles and thimbles
Maintenance interval and cost for maintenance o Cost of workboat per hour
o Cost of labor per hour
o Service interval on mooring compared to buoy, synchronized?
Cost for replacement of parts (new chain after 5 years)
There will be a form on the webpage for SEAFLEX® buoy mooring to help calculate life cycle cost for SEAFLEX® mooring system
Deployment of SEAFLEX
®mooring
SEAFLEX® mooring have over 1000 implementations all over the world. Over the years a standardized deployment and retrieval methodology for pontoons and free floating docks have been developed.
Seaflex is currently evaluating existing deployment and retrieval methodologies for
SEAFLEX® buoy mooring. This work is conducted through test implementations with buoys together with interested partners. This work will result in updated documentation for
SEAFLEX® buoy mooring. When this work is finished, training courses will be available for customers and partners.
The SEAFLEX® mooring system increases the safety off the staff compared to a chain
mooring solution. This is the results of the rubber cover of the hawsers that eliminates the risk of crushing injuries between chain links during the handling together with the lover weight of
the SEAFLEX® hawser and rope. SEAFLEX® buoy mooring will be lift classified during 2010 in able to formalize deployment and retrieval recommendations.
Parameters for Dimensioning
NavigationsTeknik and Seaflex will provide the service to calculate the proper SEAFLEX® buoy mooring for partners and customers. The calculations are based on buoy characteristics and customer environment. The required parameters will be at least:
Dept at low water level
Water level variation
Wind Speed
Wind direction and current
Wave height (and wave length)
Buoy size
Buoy manufacturer
Type of seabed
The software will take these parameters and gives the length and number of SEAFLEX® hawsers as a result.
SEAFLEX
®buoy mooring in winter conditions
SEAFLEX® buoy mooring is designed to handle icy conditions during the winter. In
extremely icy conditions SEAFLEX® can be reinforced by using a short stainless chain in the upper part of the mooring. The length of the chain will be determined on the expected
thickness of ice.
SEAFLEX
®products
SEAFLEX® buoy mooring have a wide range of products in several dimensions to fit the various needs for mooring from customers. The following section will give a picture and brief overview of existing products and their application areas.
The SEAFLEX® By-Pass prevents a SEAFLEX® buoy mooring from being extended past its foreseen elongation point in extreme conditions. This model can also withstand extreme forces and loads and has a break load of 150 kN with 10 rubber hawsers. A SEAFLEX® By-Pass model can be equipped with 2, 4, 6, 8, or 10 rubber hawsers.
Figure 6: SEAFLEX® with 4 rubber hawsers and a By-Pass
The recommended SEAFLEX® Standard model for large off-shore buoys in steel and plastic has 8 to 10 rubber together with a By-Pass. An example can be seen in Figure 7.
Summarize
There are several advantages with using SEAFLEX® buoy mooring in both handling and financial terms. The SEAFLEX® buoy mooring is developed to withstand the worst
imaginable conditions. Each SEAFLEX® hawser can withstand a force of drag at 10 kN and a 100% elongation. A SEAFLEX® buoy mooring can consist of 10 SEAFLEX® hawser in combination with a By-Pass system which is more than enough resistance against force of drag for most buoy applications.
A lover lifecycle cost is expected for SEAFLEX® buoy mooring compared to traditional chain mooring. Most of the savings is achieved due to lower maintenance with SEAFLEX® buoy mooring. The reason for this is durable material in the components, together with minimal marine growth on the smaller surface of a SEAFLEX® hawser compared to a comparable chain mooring.
SEAFLEX® buoy mooring is designed to handle all mooring of buoys. Strong areas of use are in areas with large tidal variation in both shallower and deeper water. Its good positioning abilities makes it suitable to use in narrow fairways and harbors. SEAFLEX® buoy mooring meets international standards for environmental protection, which makes it suitable to use in sensible areas. These include areas near coral reefs, areas of marine flora and historic sites. Methodologies for deployment and retrieval of buoys equipped with SEAFLEX® mooring are currently being evaluated. Updated documentation together with training courses will be offered to partners and customers as soon this work is finished by.
A joint venture between Seaflex AB and Navigationsteknik AB is creating a larger focus on SEAFLEX® navigationalbuoy mooring products and aims to take a large part of the buoy mooring market.
