[Posted on July 2nd, 2014 by Bill Stewart]

 

When drilling platforms need to transfer gas or liquids, single point moorings help their offloading requirements. Properly designed and anchored, these moorings can secure tankers of any size safely.

Offloading crude oil or gas to single point moored tankers is vital to getting platform production from the ocean to land-based ports. This connection between tanker and platform at a single mooring point with proper subsea design and installation makes the critical transfer of platform production easier, faster and more effective. Understanding how  single point moorings work is important to ensuring their subsea design and connections are installed properly effectively holding weathervaning tankers securely prior to and during gas or liquid transfer.

Working Description of Effective Mooring Systems

Three primary components make up mooring systems.

• A mooring line;
• An anchor: connecting the mooring line and a floating buoy; and
• Various connectors, establishing the vital link of tanker and offshore platform.

Mooring lines often use different material, based on subsea and surface environmental conditions, such as wind strength, wave size and motion, and current power and direction. The most popular materials are —

• Synthetic fiber rope;
• Steel wire rope;
• Chain; and/or
• Combinations of the three materials used together for added strength.

Long-term and permanent moorings typically feature lines using chain, particularly in shallow water (usually defined as up to 100 meters). Deeper water mooring lines often use steel wire rope, maximizing the benefits of its lighter weight and increased elasticity as compared to chain. When light weight is the top consideration, mooring systems feature synthetic fiber rope, for its lighter weight properties.

Deeper water moorings, up to 2,000 meters, suggest using chain or chain and steel wire rope configurations. Combinations of all three popular mooring line materials are often used in deep water, over 2,000 meters, to get the best features of strength and elasticity.

All mooring system success depends on the strength and ‘holding capacity’ of their anchors. Outstanding anchors’ holding ability depends on their digging depth and the soil components of the sea floor. The most popular anchor type is the drag embedment style, used throughout moorings in the Gulf of Mexico.

Installing this anchor involves dragging it along the seabed until it achieves its optimal depth, penetrating sea floor soil and creating necessary resistance to hold it in place. Deep water conditions favor suction pile-type anchoring, since drag anchors lack high performance when heavy vertical forces dominate the environment, such as the pressure generated by deep water.

The third primary anchor type, vertical load devices, while similar to drag anchors perform well when facing both horizontal and vertical load issues. These types are very useful when mooring lines tend to adopt angular positions between anchor and buoy.

Steel catenary risers help the most common mooring system for shallow water allow the line to modify configurations as the surface vessel moves around. Since the mooring line lies on the seabed, it needs to be longer than water depth, increasing its weight, making catenary mooring systems less efficient as water depth increases.

When platform owners want expert advice and analysis of single point moorings, they turn to Stewart Technology Associates, industry leaders in offshore equipment and systems expertise. Our state-of-the-art technology and extensive experience analyzing and designing offshore systems help develop efficient solutions for offshore drilling and production platforms. Analysis of mooring methods is only one area of the many conditions we evaluate for our clients.