How is a mooring made?

A mooring system is made up of a mooring line, anchor and connectors, and is used for station keeping of a ship or floating platform in all water depths. A mooring line connects an anchor on the seafloor to a floating structure. We will focus on mooring Mobile Offshore Drilling Units and Floating Production Systems.

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The mooring line can be made up of synthetic fiber rope, wire and chain or a combination of the three. Environmental factors - wind, waves and currents - determine which materials make up the mooring system.

Chain is the most common choice for permanent moorings in shallow water up o 100 m, whereas steel wire rope is lighter weight and has a higher elasticity than chain, which is a better choice in water depths greater than 300 m. However, synthetic fiber rope is the lightest weight of all three. Configurations include all chain, chain and wire rope (conventional mooring line to 2,000 m), chain and synthetic fiber rope, and chain, wire rope and synthetic fiber rope combinations are used in ultra-deepwater (greater than 2,000 m).

Anchors

The mooring system relies on the strength of the anchors. The holding capacity of anchors depends on the digging depth and the soil properties. The mooring lines run from the vessel to the anchors on the seafloor. Anchor types include: drag embedment, suction and vertical load.

A drag embedment anchor (DEA) is the most utilized anchor for mooring floating MODUs in the Gulf of Mexico. The drag anchor is dragged along the seabed until it reaches the required depth. As it penetrates the seabed, it uses soil resistance to hold the anchor in place. The drag embedment anchor is mainly used for catenary moorings, where the mooring line arrives on the seabed horizontally. It does not perform well under vertical forces.

Suction piles are the predominant mooring and foundation system used for deepwater development projects worldwide. Tubular piles are driven into the seabed and a pump sucks out the water from the top of the tubular, which pulls the pile further into the seabed. Suction piles can be used in sand, clay and mud soils, but not gravel, as water can flow through the ground during installation, making suction difficult. Once the pile is in position, the friction between the pile and the soil holds it in place. It can resist both vertical and horizontal forces.

Suction Pile
Source: sdi.ca

Vertical load anchors are similar to drag anchors as they are installed in the same way. However, the vertical load anchor can withstand both horizontal and vertical mooring forces. It is used primarily in taut leg mooring systems, where the mooring line arrives at an angle the seabed.

Mooring Systems

There are six types of mooring systems discussed below. They include catenary, taut leg, semi-taut, spread, single point and dynamic positioning.

The catenary mooring system is the most commonly used system in shallow water. It gets its name from the shape of the free hanging line as its configuration changes due to vessel motions. At the seabed, the mooring line lies horizontally; thus the mooring line has to be longer than the water depth. Increasing the length of the mooring line also increases its weight. As the water depth increases, the weight of the line lessens the working payload of the vessel. In that case, synthetic ropes are used. As water depth increases, conventional, catenary systems become less and less economical.

The tout leg system typically uses polyester rope that is pre-tensioned until taut. The rope comes in at a 30 to 45 degree angle on the seabed where it meets the anchor (suction piles or vertically loaded anchors), which is loaded vertically. When the platform drifts horizontally with wind or current, the lines stretch and this sets up an opposing force.

The semi-taut system combines taut lines and catenary lines in one system. It is ideally used in deepwater.

A spread mooring system is a group of mooring lines distributed over the bow and stern of the vessel to anchors on the seafloor. The vessel is positioned in a fixed heading, which is determined by the sea and weather conditions. The symmetrical arrangement of anchors helps to keep the ship on its fixed heading location. The spread mooring system does not allow the vessel to weathervane, which means to rotate in the horizontal plane due to wind, waves or current. Spread mooring is versatile as it can be used in any water depth, on any vessel, in an equally spread pattern or a group.

Single Point Mooring System
View Large: Lankhorst SMC diagram

A single point mooring system connects all the lines to a single point. It links subsea manifolds connections and weathervaning tankers, which are free to rotate 360 degrees. The single point system includes a buoy, mooring and anchoring elements, product transfer system and other components.

Dynamic positioning does not use mooring lines. Instead a computer controls the vessel's thrusters and propellers to maintain position. DP can be used in combination with other mooring systems to provide additional redundancy.

By Tom Burden, Last updated: 9/7/

A permanent mooring must remain secure for long periods while unattended, occasionally under adverse conditions. For peace of mind, it should be the right size for the job. The size of your mooring should depend on the conditions under which the boat is moored, such as the amount of fetch for waves to build up and whether your mooring is for light duty, such as overnight use in fair weather, or designed to ride out a hurricane. Below are the basic anchoring components for installing a mooring.

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Anchors

Several types of anchors are used when building a moooring, and we&#;ll review them in order of their holding power, from the wimpiest to the most tenacious:

• Concrete Blocks: Many boats use 50-gallon drums filled with cement, concrete blocks, auto engine blocks and other types of dead weight. This type provides the least holding power, working on the principle of sheer weight, but is reliable if pulled out of the bottom. If they drag, they will resist motion with a constant amount of force. Note that in seawater, concrete loses about 42% of its weight when fully submerged, so a mooring anchor designed to weigh 500 pounds on the seabed will actually require about 872 pounds of dry weight concrete.
• Mushroom Anchor: the most common type of mooring anchor is the mushroom, which, under ideal conditions, with the right kind of bottom, can dig in, create suction and develop good holding power. Mushroom anchors work best in a silt or mud bottom, and are not as effective in rocks or coarse sand. If a mushroom gets pulled out of the bottom, it is less likely to reset itself completely, and will merely skip along across the bottom. A weight of 5&#;10 times boat length is a good rule of thumb, as a bare minimum. The heavier the better, as long as you don&#;t have to move it.
• Pyramid Anchor: The cast-iron Dor-Mor pyramid mooring anchor is a superior alternative to the mushroom. Its smaller size, concentrated weight and pyramid shape allows it to embed itself more rapidly, and its holding power (at a scope of 3:1) is up to about ten times its weight. Recommended by Practical Sailor/Powerboat Reports in .
• Helical Screw: while the above types rely for holding power on sheer weight or a combination of weight and embedding themselves in the bottom, the helical anchor is screwed into the seabed, usually by a barge-mounted hydraulic device. Helical screws have long, high-tensile steel shafts (8' length is common) with large screw threads (10" to 14" diameter) on the bottom and an attachment eye at the top. These professionally-installed anchors, originating in the offshore oil industry, have gained popularity with recreational boaters since the s, and have the most extreme holding power in relation to their weight.

Mooring Chain

Chapman&#;s recommends two sections of galvanized chain: a heavier, primary chain and a lighter, secondary chain. The primary (ground) chain lies on the bottom. Its length should be 1 1/2 times maximum water depth. The secondary (riding) chain, is connected to the ground chain with a galvanized shackle or swivel. It's usually half the diameter of the ground chain and equal in length to maximum water depth. The heavier chain is not used for the entire run so that the mooring buoy does not have to support an excessive amount of weight. The chain should be as large as practical; make the riding chain at least double the size of the chain on your anchor rode.

Mooring Buoy

Two styles of Taylor mooring buoys are shown on typical systems with primary and secondary chain.

Your mooring buoy supports the mooring chain and is an essential part of your mooring gear. The two preferred designs for mooring buoys are a traditional buoy with hardware or a buoy with a tube through the center. Both offer reliable flotation and will last for several seasons, depending upon the salinity of the water. Obviously, freshwater applications will extend the useful life of any mooring system.

The buoy must have about twice as much flotation as the suspended chain has weight in order for it to ride high enough in the water to be visible. The Taylor Sur-Moor&#; T3C&#; Mooring Buoys allow you to pass the chain through the center of the buoy, and attach the pendant on top. Secure the mooring chain at the top using a 4" galvanized O-ring, such as , and add the T3C&#; Mooring Collar to protect the buoy from wear by the anchor chain and extend its lifespan.

Mooring Pendant

This powerboat is moored through both bow chocks with two of New England Ropes' Cyclone Mooring Pendants . The bottom white section of the pendant is traditional three-strand nylon.

The pendant (pronounced &#;pennant&#;) attaches the chain to the boat. Large-diameter three-strand nylon line is used because its inherent elasticity (stretching about 10 percent under a load equaling 20 percent of its tensile strength) allows it to act as a shock absorber. Polyester line, Dyneema line or stainless steel wire is preferred by some for better chafe resistance. Length should be about 2 1/2 times the boat's freeboard. Diameter should be as large as is practical&#;but it must be able to fit through bow chocks and around a bow cleat.

Effective chafe protection is recommended for the point where the pendant passes through a chock. This is critical, as failure caused by chafe at this location is one of the main reasons why boats end up on the beach. A light pick-up buoy at the boat end makes it easy to grab the pendant.

Cyclone Mooring Pendants, made from Endura-12 Dyneema line, were developed by Nantucket Moorings in conjunction with MIT. Traditional pendants are made from three-strand nylon, to absorb shocks by their elastic nature. This stretch, while allowing your boat a comfortable, cushioned ride, allows the line to move across the bow chocks, creating friction and causing chafe.

Cyclone Pendants are attached (using a lunch pail hitch, also called a &#;cow hitch&#;) to your standard nylon pendant with its floating pickup buoy, and allow use of a low-stretch upper section. Their high tensile strength allows smaller mooring lines to be used, so they fit more easily on boats with smaller cleats and chocks. Very low elongation results in a dramatic reduction in friction, heat and chafing. Also, because the top and bottom sections are just looped together through spliced eyes, you can replace a damaged section without replacing the entire pendant.

Surviving a Storm

Hurricanes making landfall in locations along the Eastern Seaboard have wrecked hundreds of boats over the past twenty years, many of which were driven ashore with their complete and intact mooring systems still attached. Weather forecasters predict that we are now in a period where we can expect more storms of greater destructive force, storms like Harvey, Irma and Maria that caused tremendous damage in . Just how large does a mooring system need to be if your boat is caught in extreme weather? The following chart gives an estimate of the wind loads (based on the windage of the vessel) and the required size of a pyramid anchor.

Minimum Dor-Mor Mooring Anchor Size Requirements

Boat Length Beam Sail/Power Wind Load 64 Knots Dor-Mor Size Wind Load 100 Knots Dor-Mor Size 20' 8'/9' 1,600lb. 200lb. 3,600lb. 400lb. 25' 8'/9' 2,200lb. 300lb. 5,000lb. 500lb. 30' 9'/11' 3.200lb. 400lb. 7,000lb. 700lb. 40' 11'/14' 5,400lb. 700lb. 12,000lb. 2,000lb. 50' 13'/16' 7,300lb. 1,000lb. 16,000lb. 2,000lb. 60' 15'/18' 9,100lb. 1,000lb. 20,000lb. 2,000lb. 80' 19'/22' 13,000lb. 2,000lb. 31,000lb. 4,000lb.

Based on wind loading data at min. 3:1 scope, with suitable bottom conditions for anchor to embed. Wind loads based on data from ABYC.

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