Anchor and Mooring Systems - Coursework Example

Anchor and mooring systems al Affiliation) Introduction Anchoring and mooring system is used for station keeping of any floating platform like ships, floating production systems and offshore drilling units at all water depths. The system generally consists of an anchor, connectors and a mooring line. An anchor in the sea floor is connected to a floating platform or structure by the mooring line. Floating platforms and ships tend to drift it their location due to the effect of wind and current. Anchor and mooring system are used to hold the ships and the floating platforms in position and prevent movements caused by the wind and current. Given most sea vessels and ship are incapable of breaking, only anchorage systems and mooring can be used to slow down and hold the ship into position to avoid movement (Lekang, 2007). The unregulated and unwanted movements caused by wind and the current can cause grave consequences and repercussions including damage to property if the ship is not properly anchored or moored. Floating production systems and offshore drilling units located in deep sea require a high level of anchorage in order to maintain a firm position against the high tides, currents and wind present at high depths. Anchoring systems and mooring therefore perform a crucial function in maintaining the stability of the sea vessels without which there exist costly ramifications. This paper discusses the operation of anchors and mooring systems, their fabrication and classification. Operation Anchor and mooring system consist of the anchor, the mooring line which is used to transmit force from the moored vessel to the anchor and the tensioning system or the attachment point on the moored vessel or floating work platform. The mooring line can be made from either a chain or wire rope. In some cases the anchor line is made from the combination of the chain and wire rope with fiber line or rigid element included. In shallow waters of depth up to 100m, chains are usually preferred for permanent mooring. Steel ropes have a higher elasticity and is preferred at greater depth than 300m. Synthetic fiber ropes on the other hand is the lightest among the three choices and is used for anchorage in deep waters of up to 2,000m. The anchor usually provides the majority of the holding capacity or the resistance to motion even though the other components of the system are also at play in contributing to the anchorage of the vessel or the floating work platform (Dokkum, 2010).The portion of the anchor buried under the sea has a significant contribution to the whole system especially when a chain is used as an anchor line. The portion of the anchoring system excluding the attachment point on the vessel or the floating work platform is known as the ground leg. The ground leg is inclusive of the anchor, anchor line and other auxiliary devices. The anchor line (chain or wire rope) is connected to the windlass mounted on the sea vessel or the floating platform. The windlass can be either of vertically or horizontally designed. Most anchor and mooring systems use a vertically designed windlass mostly known as a capsan. The capsan is composed of a gypsy; drive wheel notched in order to fit the chain links. The chain is drawn along the deck from the capsan through a pawl and dropped down through hawsepipe in the deck which exits at the ships bow. The chain is then dropped down and a shackle is used to connect the chain to the anchor. The hardened steel pins of the shackle pass through a hole drilled in the anchor central shank. Material and methods of fabrication A number of materials can be used to make an anchor ranging from large blocks of rocks to sacks of sand. Commercial anchors are made from pig iron which are fabricated to form cast iron anchors. Steel anchors are also made through drop forging of carbon steel. Both the cast iron and forged steel anchors are used in deep sea while the bronze, brass and the stainless steel anchors are lightweight and are usually used by yachts and small fishing boats (Dokkum, 2010). There are a number of factors that should be put into consideration when designing an anchor: Soil data; the type of soil in which the anchor is to be used will determine the type of anchor suitable for that mooring process. The manner in which the anchor will be set also depend the type of soil. Therefore soil data plays a fundamental part in the design process of an anchor. The second factor to be considered is the load data; the anchors are usually designed based on the pre-calculated damaged design and maximum intact loads. The third factor crucial in the anchor design process is the mooring line configuration. Lastly the classification society of the anchors determines the appropriate safety factors (Lawson, 2005). The holding capacity of the anchor on the other hand depends on such factors as the soil type, size and type of the anchor. Anchors with large surface areas and higher weights tend to have a higher holding capacity than those with lower surface area and low weight. Sands and hard clays generate higher holding capacity as compared to soft clay. Classification Anchors exist and are produced in different sizes, design, holding capacity and performance. Anchor and mooring systems can be broadly classified into five major categories; Drag embedded anchors, deadweight anchors, grappling devices, pile anchors and direct embedded anchors. Drag Embedded Anchors Drag embedded anchors are suitable for temporary mooring, polling systems, as anchor points for the beach gear and parbuckling rigs. This type of anchor is usually lowered to the bottom of the sea and pulled until it rotates in the angle that the flukes penetrate the sea floor. The anchor is pulled after it has been lowered until the flukes penetrate to the required depth in the sea floor in order for it to generate its maximum holding capacity. There are several factors that determine a successful deployment of drag embedded anchors: the anchor geometry, soil condition and the length of the anchor line. For a deep penetration that will result into a higher holding capacity, the fluke angle of the anchor should be designed in the range of 30-35 degrees in granular soil, 50 degrees for soft soils including silk, soft clay and mud while 20-25 degrees for stiff clay. For proper and successful tripping, the anchors should be designed with heavy crowns. The fluke angle of an anchor has a major role in determining the level and depth of penetration. It is the angle formed by the flukes and the shank. There is a critical fluke angle for any combination of soil type and anchor geometry of which penetration is possible (Lawson, 2005). Penetration is possible for all flukes angle less than the critical angle. In cases where the fluke angle is almost same as the critical angle, anchor penetration is achieved via simple soil shearing. In cases where the fluke angle is greater than the critical angle, penetration becomes a problem since there is a shape rotation on the anchor. The rear of the anchor tends to rise above the sea floor causing it to overturn and drag on the side. The stability of the anchor also plays a crucial role in the successful deployment of the anchor and mooring system. Stable anchors can be dragged for long distances without breaking, overturning and lying on its side. Successful penetration and burial causes a constant traction force which in turn causes instability of the anchor. There are two major instability categories namely lateral instability and vertical instability. Lateral instability occurs majorly during dragging and penetration when the anchor rotates about the axis of the shank line.Vertical instability on the other hand occurs when the anchor moves vertically along the shank line through rotation or translation of the anchor about shanks attachment points. Lateral instability is majorly determined by the geometry of the anchor. Anchors with long and narrow flukes tend to be highly unstable laterally. Fitting long stabilizers to such unstable anchors will ensure lateral stability. Wide spacing of the flukes in most anchors ensure stability in homogeneous soils while closed spaced flukes are highly stable in heterogeneous soils (Dokkum, 2010). Drag embedded anchors can further be grouped into seven major categories including deeply penetrating anchors, anchors with elbowed shanks, anchors with large hollow flukes, high performance stockless anchors, improved stockless anchors and stocked anchors with small fluke area. Advantages Drag embedded anchors have a high capacity of more than 100,000 lbs . The continuous resistance provided by drag embedded anchor can be availed even in cases where the maximum capacity is exceeded. The drag embedded anchors are available in a wide range of shapes and sizes and can be easily recovered. Disadvantage The drag embedded anchors lack the ability to function and operate in hard sea floors. Their resistance to uplift is so minimal and an extra line scopes is needed to aid in horizontal loading in the sea floor. The drag embedded anchors function by penetrating the sea floor which can easily damage cables and pipelines under the sea. Deadweight anchors Deadweight anchors can be made of any object that can be placed on the sea floor such as concrete, steel or ferro-cement clumps. The selection and installation of deadweight anchors are determined. The force required to drag or to lift the deadweight anchor over the sea floor constitutes its holding capacity. The submerged weight of the deadweight anchor provides the resistance to vertical force also known as uplift. The suction effect also provides additional resistance to drag which is as a result of the friction between the dead weight and the sea floor. The lateral drag is determined by several factors including the tension in the mooring line, current drag, storm wave and the gravitational down the slope on a sloping sea floor (Lawson, 2005). The dead weight anchors are fitted with shear keys at the bottom that increases the lateral load capacity by enforcing the surface by which the anchor slides deep into the sea bottom. The shear keys are mostly applied to the base of anchors that are used in soils with higher lateral load resistance. The shear keys are fixed closely enough in order to force the sliding failure to occur at the shear key bases. When designing the dead weight anchors, there is a minimum number of shear keys required in each direction at the base of the anchors. The number of shear keys is computed by comparison of the design load which is parallel to the sea floor and the passive resistance developed per key. The net downward force of the which is present to drive the keys determine the depth of the shear keys. Vent holes installed at the base of the anchors assists in penetration by allowing soil trapped by the keys and water to escape. Penetration is also enhanced by sharpening the edges of the keys. (Dokkum, 2010) The overturning resistance of the dead weight anchors should also be placed into consideration while designing the anchors. When the anchors are exposed to excessive uplift loading and lateral force, the deadweight may rotate near a point on its leading edge. The forces can be resolved to parallel components which are normal to the sea floor. When the resisting moment is more than the overturning moment, stability of the dead weight anchor can be achieved (Gaythwaite, 2004). The anchor should be designed in a way that ensures that there is full contact between the supporting soil and the anchor base. To achieve this design, the resultant normal force should be made to act within the midpoint of a third of the base. The point of anchor rotation is therefore assumed to be the point at which the normal soil reaction crosses the shear key line. The overturning potential can be minimized by keeping the lateral load component as small as possible. This is usually achieved by minimizing the height of the deadweight anchor. Directed Embedded Anchors The installation of these types of anchors is different from the other anchors in that the anchors are buried before the anchor line is loaded as opposed to drag embedded anchors which bury themselves during loading. Clumps and deadmen are drilled or placed in excavated pits then buried. This type of mooring is crude but effective. The anchors are usually plate-type and are placed vertically inserted into the sea floor. They are then re-oriented in order to increase their pullout resistance. The direct embedded anchors are further divided into five major categories; vibratory driven, propeller driven, impact driven, jetted-in and augured in. The propellant embedded anchors are fired into the sea floor through the use of a gun barrel in order to achieve a high holding capacity. They have a major advantage of near instantaneous impediment on the floor. They can be used in both shallow and deep waters (Gaythwaite, 2004). The holding capacity of direct embedded anchors depends on the type of soil present on the sea floor. In coral floor, the holding capacity of the directed embedded anchor remains constant as the coral strength increases and penetration decreases. Generally, the holding capacity of these types of anchors depends on the mode of soil failure which in turn depends on the relative embedment depth. A major disadvantage of these types of anchors and mooring system is that there is a strength loss in the soil in which the anchor is drilled or inserted. The holding strength of the anchor is bound to reduce after a certain cyclic loading given that the soil losses it strength to hold the drilled anchor on the sea floor. The amount of strength lost from the soil varies considerably by such factors as soil state, soil type and the nature of cyclic loading. The soil susceptibility to strength loss can be reduced by anchoring in a denser soil, lowering the amount of cyclic loading and increasing the gap and the duration interval of cyclic loading. Pile Foundation and anchors These types of anchor and mooring system are achieved by deeply embedded anchoring devices installed by grouting and drilling. As compared to drag embedded, dead weight and direct embedded anchors, pile foundation and anchors are expensive to install. Pile anchors are suitable for short scope mooring (Gaythwaite, 2004). In conclusion, anchor and mooring systems are vital components in most sea vessels and offshore work platforms in ensuring stability against strong currents and winds. There are several types and designs of anchors as discussed in the paper. The choice of anchor design to be used depends on various factors such as the seabed soil type, the loading force, the size of the vessel to be anchored and several other factors. Each and every anchor design has its distinct advantages and disadvantages over the other designs. References Dokkum, K. (2010). Ship knowledge: a modern encyclopedia. The Netherlands: DOKMAR. Dokkum, K. v. (2008). Ship knowledge: ship design, construction and operation (5th Ed.). The Netherlands: DOKMAR. Gaythwaite, J. (2004). Design of marine facilities for the berthing, mooring, and repair of Vessels (2nd Ed.). Reston, VA.: ASCE Press. Lawson, T. B. (2005). Fundamentals of aquacultural engineering. New York: Chapman & Hall. Lekang, O. (2007). Aquaculture engineering. Oxford: Blackwell Pub.. Scott, W. (2011, June 13). Anchor Designs and Types. Bright Hub Engineering: Articles, News & Industry Information Written for Engineers. Retrieved April 12, 2013, from http://www.brighthubengineering.com/naval-architecture/119304-anchor-systems-for-ships/#imgn_1 Stan, J. (2002). Mooring systems for recreational craft. Brussels, Belgium: PIANC General Secretariat. Read More