Patent Application
20190047665
The present invention relates to mooring lines and mooring systems, and related connectors and methods for attaching a mooring line, for mooring or tethering a floating or submerged object in a body of water. The disclosed mooring lines, mooring systems, related connectors and related methods enable load reductions and shock protection to be achieved as compared to traditional mooring systems.
Mooring lines have long been used to secure floating objects. The moored objects are often extremely expensive, or can be dangerous if free to move and collide with other objects, and thus the mooring lines are often over-engineered to ensure that they do not fail.
Many different types of mooring line are used throughout the world, with a catenary mooring line being the most common. In a catenary mooring line, a length of chain (or weighted line) is attached from an anchor to the device being moored. The loads from the ocean apply on the moored device and try to move it. The length of chain is substantially longer than the seabed depth (e.g. 3, 5 or 7 times the depth) and the weight of the chain acts as a restoring force to any motion. As the moored object is moved from its rest position away from the anchor, more of the chain length lifts off the seabed increasing the mass of chain responsible for the restoring force. This increase in force helps pull the device back to its rest position. Over-engineering this line typically means adding extra mass (e.g. using a thicker chain) so that the restoring force is greater. To counter the weight of this extra mooring line mass, the device being moored may need larger ballast resulting in an increase in its size, and hence an increase in the loads applied by the ocean.
An alternative approach is to use a mooring line which can stretch and which has a restoring force related to the stretch in the line. Typically these are rope or synthetic lines such as Polyester or Nylon. These lines have a small quantity of elasticity in their fibres and a small additional level of elasticity from their braiding design, but can usually only elongate by a few percent before reaching their breaking load (e.g. typically <<50%). In this scenario the restoring force is coming from the material properties and the compacting of the line. Over-engineering this line typically involves using bigger ropes with higher material properties and larger braids. Such ropes often have to be very long to deliver enough elongation to absorb the energy trying to move the device.
Multiple mooring lines are often used to split and share the restoring force. If the applied ocean force can come from multiple directions then multiple lines are often required in different directions to keep the device within a desired ocean ‘footprint’.
The main purpose of a mooring line is to control relative movement between the object being moored and its tether point. Such movement may be caused by wave, current and/or tidal motion. The mooring line must therefore apply a restoring force against movement of the object. It can be difficult to meet the demands on a mooring line where the object to be moored experiences relatively large displacements relative to the depth of water. In these environments it is desirable that the scope of the mooring line is not too large as this increases cost, where the “scope” is defined as the length of mooring line per unit of water depth. It is also desirable to minimise the footprint of the mooring system, where the “footprint” is the seabed area occupied by the mooring system.
As mentioned above, mooring lines are typically comprised of long metal chains that form a catenary mooring system. These catenary mooring lines utilise a metal chain which is attached to the object to be moored, the metal chain then goes down to the seabed where often up to 2 km of chain is laid across the sea bed, before ending in an anchor.
Due to the horizontal load reacting nature of the conventional drag embedded anchors which are used with catenary systems, the scope of the cable must be chosen such that the cable is never entirely picked up from the seabed for the given environmental conditions. Large waves can be up to 20 m high, i.e. the same order of magnitude as the water depth, and the length of chain required to deal with such changes then becomes very large. A scope of three suffices in many scenarios, but in shallower water a scope of more than five is frequently required, and scopes of >7 are often needed in extreme scenarios. Such a mooring system is often inefficient and takes up a lot of seabed space around the object, resulting in high costs and a large footprint. In the most extreme conditions the horizontal mooring force on a steel catenary system can be greater than 5000 kN.
A further challenge for mooring systems is fatigue, as the mooring lines tend to wear. There are many causes for such wear with the primary causes being from friction at any contact point (e.g. the seabed touch down point) and from material wear due to large variations in load within the mooring line. For a chain the wear comes from friction at the link contact points and results in the metal slowly wearing away and reducing the load holding capability of the line. For a rope the wear results from fibres rubbing against each other and breaking/fraying, reducing the load holding capability of the line. Reducing these variations reduces the wear on the line in both cases with large savings in inspection and replacement costs.
Whilst a catenary mooring line largely absorbs loads which are applied to the mooring line, the snatch loads and peak loads on the mooring line can be extremely high. Snatch loads are short, fast-acting loads that are applied to the mooring line when the tethered object experiences a sudden displacement. These loads arise due to the inelastic nature of the metal chain and the rapidly changing wave conditions. High snatch loads can be detrimental to the chain, causing fatigue which can often cause the chain to fail. Peak loads are the maximum loads experienced by a mooring line during operation. Whilst repetitive small loads can cause fatigue of the mooring line, the higher peak loads are known to cause the mooring line to fail in certain circumstances.
Accordingly there are a number of problems when it comes to implementing a catenary mooring system to tether a floating object, especially in offshore environments. In particular, very large scopes, seabed footprints and horizontal motion envelopes are required by a catenary mooring system to allow the object to ride the waves. In addition, the metal chains suffer fatigue and potential failure. Other problems include noise emissions, seabed scouring, roll and pitch, and cost.
Mooring components comprising elastomeric material are popular in near shore and dock mooring applications. Examples are seen in US 2005/0103251 and US 2009/0202306. Such proposals provide a number of advantages over traditional mooring solutions by allowing a flexible component inserted into the line to stretch with the heave and surge of the floating vessel or platform. However, these mooring systems are principally designed to prevent drift or buffer tidal currents, and are not designed to provide low scope, small footprint performance in deeper waters. In these mooring components one or more rubber strands may be combined in parallel with a so-called bypass cable (or “safety-locking loop”) formed of stiff synthetic fibre or steel wire that prevents the rubber strand(s) from over-extending. Such components are typically designed with the rubber elements designed to hold the full mooring load and the “safety locking loop” rarely if ever required. Such components are limited in the lengths to which they can be made, as the synthetic fibre or steel bypass cable can add disproportionately to the weight of the component. In practice such lines are no more than about 10 m long and therefore find most use in mooring pontoons and boats in a marina. The braided synthetic ropes in some of these components can also suffer from wear problems. Furthermore, such a “safety locking loop” would cause a significant problem in terms of very high peak forces in response to elongation beyond the working range of the rubber strands. Not designed for high wave loads, orders of magnitude above the normal load, these components suffer significant snatch loads when the “safety locking loop” is forced to engage, for which they are not designed. Accordingly these mooring components only work well where the change in wave height is small with respect to the depth of water in which the mooring is used, such as in-harbour pontoons, or in estuaries where tidal changes in water height are low. Such mooring components are not designed for offshore use where objects may be subject to storms and very large waves.
More recently, it has been proposed for mooring components made of elastomeric elements to be used in traditional mooring lines to reduce the loads on the line, as can be seen in the Applicant's earlier applications WO 2011/033114 and WO 2012/127015. These publications describe elastomeric mooring solutions where an elastomeric component is inserted part way along a conventional mooring line. In these examples the elastomeric component completely replaces a section of the mooring line, i.e. connected between sections of steel chain. This means that the lines in the mooring system can be much shorter because the elastomeric components allow for a large degree of elongation and reduce the loads significantly. The elastomeric components may even reduce the vertical forces in the mooring system to a level that means anchors can be connected directly to the ocean floor instead of laying chains along the seabed.
Inserting elastomeric components into mooring lines can significantly reduce the loads on the mooring system, often by up to 90%, and thus the chances of the mooring line failing are relatively small. However, in mooring systems where an elastomeric component is physically inserted into the conventional mooring line and completely replaces a section of the steel chain, the chain must be physically broken and this may be perceived as potentially introducing a point of weakness into the mooring line. There could be concerns that if the elastomeric component, or its attachment points, were to fail or be damaged by some unknown process (e.g. more susceptible to shark bites or bio-organisms) before the steel chain then this would cause the mooring line to completely fail early.
Ideally, a deep sea mooring system needs to be adaptable to the sea states at the location at which it is placed and so it must be able to adjust its response to the applied forces from the waves over very short time periods. Ideally, shock protection from snatch loads should be combined with overall performance in handling high peak loads. Ideally, such a mooring system is self-adjusting so that risk of failure in harsh environments is reduced. Ideally, the mooring system has a failsafe component such that the mooring line is unlikely to fail. Furthermore, any mooring system should also be cost-effective.
The present invention seeks to provide improved mooring lines and systems that can withstand relatively large changes in wave height and/or tidal motion while having a low scope, small footprint and reduced risk of failure (whether that risk is actual or perceived).
Thus in accordance with a first aspect, the present invention provides a mooring line comprising a metal chain and at least one elastomeric element having an initial length; wherein the at least one elastomeric element is attached between two points of attachment on the metal chain thereby defining a bypass section of the metal chain between the two points of attachment with a predetermined length that is greater than the initial length of the at least one elastomeric element; wherein one or more further sections of the metal chain are directly linked to the bypass section by chain links so as to transmit a tensile load directly from one section of the metal chain to the next; and wherein the at least one elastomeric element is arranged such that: when an initial tensile load, below a threshold tensile load, is applied to the mooring line in use, the at least one elastomeric element stretches from the initial length, to a second, longer, length wherein the bypass section of the metal chain between the two points of attachment is not taut and thus the initial tensile load is transmitted from and to the further sections of the metal chain through the at least one elastomeric element; and when a further tensile load equal to or above the threshold tensile load is applied to the mooring line in use, the at least one elastomeric element stretches further from the second length to the predetermined length wherein the bypass section of the metal chain between the two points of attachment becomes taut, and thus the further tensile load applied to the mooring line is transmitted directly from and to the further sections of the metal chain through the bypass section of the metal chain.
It will be appreciated by those skilled in the art that such a mooring line provides one or more elastomeric element(s) capable of absorbing tensile loads applied to the object being moored, thereby reducing the peak loads on the metal chain. There are two primary ways in which this benefit can be used. In embodiments where the at least one elastomeric element is attached between two points of attachment on the metal chain of an existing mooring line, the reduction in peak loads may dramatically reduce wear and tear. This means that the lifetime of the mooring line may be extended and maintenance costs may be reduced. In embodiments where a mooring line is installed with the at least one elastomeric element pre-attached, the metal chain can be chosen to be smaller in size than in a conventional mooring line, e.g. the length of the chain and/or size/weight of the chain links chosen for the lower expected peak loads. Taking advantage in this second way means that both capital and operational cost benefits are delivered. The elastomeric element(s) also provide excellent shock protection and can eliminate snatch loads as compared to a traditional catenary mooring line.
It will be appreciated that the presence of the further sections of metal chain means that at least one, and preferably both, of the two attachment points is not at the end of the metal chain in the mooring line. In other words, the predetermined length of the bypass section of the metal chain is shorter than the total length of the metal chain in the mooring line.
In embodiments of the present invention, such a mooring line advantageously retains a continuous, unbroken, metal chain along its entire length as a failsafe in the unlikely event of failure of the elastomeric element(s). It is therefore preferable that the bypass and further sections of the metal chain are directly linked by chain links so as to form a continuous metal chain. When a relatively high tensile load is applied to the mooring line, equal to or above the threshold tensile load, the elastomeric element(s) stretches to such an extent that the bypass section of chain becomes taut and thus any further tensile load is transferred directly along the linked sections of the metal chain. This arrangement ensures that larger tensile loads are transferred along the metal chain, which may be perceived to be considerably stronger than the elastomeric element. This arrangement provides the security that, even in the unlikely event that the elastomeric element(s) were to break, the metal chain would remain to provide an unbroken mooring line. The fact that the metal chain itself forms the failsafe for the mooring line means that the mooring line is not considered to be weakened by inserting the elastomeric element(s) and this provides confidence in performance. In at least some embodiments the bypass and further sections of the metal chain consist of chain links of the same grade. This means that the various sections of the metal chain all consist of the same size/thickness and weight i.e. grade per link. Thus it can be ensured that all sections of the metal chain are designed to withstand the peak loads expected for the mooring line. All of these embodiments are advantageous over the elastomeric mooring components seen in the prior art where a separate metal cable may be provided as a “safety locking loop” but there is no direct link to any further sections of metal chain outside the mooring component. In addition, the separate “safety locking loop” provided in prior art components may be formed from braided wire and is often weaker than the typical metal chains used in mooring lines.
The at least one elastomeric element may be designed to provide a tensile response to the bulk of normal expected wave states for a given mooring line. Accordingly the elastomeric element(s) may stretch to elongations (i.e. a second length) that are less than the length of the bypass section of metal chain in response to the tensile loads applied to the mooring line most of the time. Even absorption of relatively small loads can provide a significant benefit as the elastomeric element(s) absorbs energy before the metal chain is engaged. This is what provides for an overall load reduction so that the mooring line can be shorter than a conventional catenary system using metal chains alone, or the same length but benefiting from reduced wear. If a relatively large tensile load is applied, for example in a storm, then the
elastomeric element(s) may be caused to stretch to the predetermined length at which the bypass section of metal chain becomes taut and the full tensile load is then transferred along the sections of the metal chain.
Thus in many embodiments it is preferable that the initial length is less than the second length and the second length less than the predetermined length. Further preferably there may be a wide range of lengths available for the second length, so that the at least one elastomeric element provides for a relatively high degree of elongation before reaching the predetermined length at which the bypass section of the metal chain, between the two points of attachment, becomes taut. However the bypass section of chain may frequently become taut in higher sea states. The Applicant has found that even a relatively small volume of elastomeric material can provide huge benefits in lowering the overall loads experienced by the metal chain in the mooring line.
The length of the bypass section of metal chain between the points of attachment effectively determines the predetermined length to which the at least one elastomeric element can stretch. Limiting the maximum elongation of the elastomeric element(s) reduces the risk of the at least one elastomeric element failing. Often mooring components in the prior art have been over-engineered to account for the fact that an elastomeric element might fail. For example, a plurality of elastomeric elements may be provided in case a single elastomeric element fails. The combined response provided by the plurality of elastomeric elements may not actually be necessary to absorb normal loads but is provided as a failsafe to prevent damage. According to the present invention, on the other hand, the bypass section of chain prevents the at least one elastomeric element from stretching too far, this means that additional elastomeric elements do not need to be provided to prevent failure of the mooring line. As a result, a single elastomeric element can be provided safe in the knowledge that the elastomeric element will be unlikely to fail. Thus in preferred embodiments the mooring line may comprise a single elastomeric element attached between the two points of attachment on the metal chain. This is particularly advantageous as the elastomeric elements used within mooring lines are often extremely expensive and thus reducing the total number of elastomeric elements required reduces the cost.
It will be appreciated that the mooring line has a failsafe which is provided by the directly linked sections of metal chain, however in some less preferred embodiments it is envisaged that the mooring line may be provided with additional failsafe lines if desired. For example, a non-elastomeric cable (e.g. made of stiff synthetic material or metal) may be attached in parallel with the at least one elastomeric element between the two points on the metal chain, thereby providing another failsafe line which prevents the elastomeric element(s) from extending too far.
In such embodiments it will be appreciated that the continuous metal chain then acts as a last resort in the event that the other failsafe cable also fails.
As is mentioned above, the at least one elastomeric element is arranged so that it stretches in order to absorb at least a fraction of the normal tensile loads on the mooring line and this helps to reduce the peak loads on the mooring line. The peak loads may, for example, be reduced by more than 50%. The reduction in peak loads that is achieved may be dependent on the number of elastomeric elements, the type of elastomeric elements and/or the lengths to which they are allowed to stretch before reaching the predetermined length. The skilled person will understand, for example, how to choose an initial length and thickness of an elastomeric element based on the device being moored, material properties and the expected sea states. The reduction of peak loads on the mooring line may help to reduce fatigue on the mooring line thus potentially increasing the lifetime of the mooring line. This is particularly advantageous with mooring lines which are used to moor objects in remote environments such as those at sea. Mooring lines used in offshore environments are often expensive to produce and also expensive to maintain or replace.
The predetermined length of the bypass section of metal chain and the tensile response of the elastomeric element(s) ultimately determine the loads that the elastomeric element(s) is able to absorb. For instance, one example of an elastomeric element has an initial length of 4.5 m, the bypass section of metal chain has a length of approximately 10.25 m, and thus the elastomeric element is capable of stretching by around 125% before the metal chain transmits any tensile load. Typical elastomeric elements may be capable of extensions of up to ˜400-500% without risk of failing, but may age when repeatedly extended to this limit by frequent cycles of use, for example due to the formation of micro-cracks in the elastomeric material. In a mooring line located in the ocean, for example, the elastomeric element(s) may be stretched by 3-4 million cycles a year and hence it is important to keep the maximum extension low enough to avoid long term damage. In a set of embodiments the initial length of the at least one elastomeric element, or the distance between the two attachment points on the metal chain, is chosen such that the elastomeric element is capable of extensions of at least 80%, 90%, 100%, 110%, 120%, 130%, 140%, 150%, 160%, 170%, 180%, 190% or 200% before reaching the predetermined length. On the one hand, relatively large extensions e.g. >100% ensure that the elastomeric element(s) is able to absorb a significant range of tensile loads before transferring any further tensile load, greater than the threshold tensile load, to the metal chain. By absorbing a large range of tensile loads, the at least one elastomeric element helps to significantly reduce the peak loads in the mooring line. On the other hand, it is beneficial to limit the extension range of the elastomeric element(s) so as to ensure that lifetime is not compromised. The Applicant has found that a good balance may be achieved by choosing the initial length of the at least one elastomeric element, or the distance between the two attachment points on the metal chain, such that the elastomeric element is limited to extensions of around 150%.
A further consideration is the way in which the at least one elastomeric element is attached to the metal chain. The at least one elastomeric element may be moulded around metal inserts at both ends so as to form a component of the attachment points. Such a moulded design allows for a much higher level of elongation and much higher loads than could typically be held by a crimped, clamped, or knotted elastomeric element. It also allows for ease of connection with a standard shackle to the rest of the mooring line. Preferably the at least one elastomeric element is moulded around a metal connector at each end. The surface area of the metal to elastomer surface is an important factor in maximum load handling capability and in lifetime consideration. The metal to elastomer bond at this surface can be affected by high elongations, as the volume of elastomeric material is stretched. By limiting the elongation experienced when the elastomeric element(s) stretch to the predetermined length of the bypass section of metal chain, as outlined above, a balance between material volume and performance can be achieved.
Alternatively, or in addition, in a set of embodiments the predetermined length of the section of metal chain between the two points of attachment may be chosen such that it is at least 1.5 times greater than the initial length of the at least one elastomeric element, preferably at least double the initial length of the at least one elastomeric element, and further preferably at least 3, 4 or 5 times longer than the initial length of the at least one elastomeric element. This means that the at least one elastomeric element will undergo significant elongation before reaching the predetermined length at which the section of the metal chain between the two points of attachment becomes taut. This increases the range of tensile loads that are absorbed by the elastomeric element(s) and thus the overall load reduction that can be achieved as compared to a traditional catenary mooring line consisting of metal chain alone.
As is mentioned above, the at least one elastomeric element provided by the mooring line allows for a range of the typical tensile loads applied to the mooring line to be absorbed by elongation of the elastomeric element(s) to a second length below the predetermined length so that the section of the chain between the two points of attachment remains slack. The initial length of the at least one elastomeric element is preferably substantially less than the predetermined length so that the elastomeric element(s) must stretch before the bypass section of chain becomes taut, so as to optimise the load reduction achieved. In some embodiments the initial length of the at least one elastomeric element may not be significantly less than the predetermined length if, for example, the elastomeric element(s) is not taut when it is attached between the two points on the metal chain. However, this would mean that initial tensile loads are absorbed by the two attachment points moving apart without a restoring force from the elastomeric element(s) being transmitted along the mooring line, reducing the benefit provided by the elastomeric element(s). It is therefore preferable that the initial length of the at least one elastomeric element is chosen so that the at least one elastomeric element is taut when attached between the two points on the metal chain. The initial length may be chosen, for example, based on the starting loads on the mooring line (e.g. due to the weight of the metal chain) and the elastic response of the elastomeric element(s). Shock protection is ensured by keeping the elastomeric element(s) taut at the initial length.
As follows from the discussion above, it is preferable that the mooring line is designed such that the at least one elastomeric element never reaches an elongation close to its elastic limit before the threshold tensile load is applied. Beyond the elastic limit of a material, permanent deformation will occur. The elastic limit, also known as the yield strength, is therefore the lowest stress at which permanent deformation is measured. When choosing the at least one elastomeric element to be attached to the metal chain it is preferable to ensure that the elastic limit is significantly greater than the threshold tensile load. This ensures that the elastomeric element will be able to repeatedly stretch to the predetermined length with minimal micro crack formation. Otherwise micro cracks can form which, over repeat elongations, can merge and grow, thereby limiting the lifetime of the elastomeric element. Choosing an elastic limit which is significantly greater than the threshold tensile load is also important as, over time, the elastomeric element(s) may deteriorate due to damages caused by the harsh environments that mooring lines are often used in (e.g. attack by ocean bio-organisms, shark bites, etc.). Deterioration of the elastomeric element(s) may cause the elastic limit to be reduced with age. Selecting an elastomeric element(s) with a significantly larger elastic limit means that even if the elastomeric element(s) deteriorates its yield strength is still unlikely to drop below the threshold tensile load.
However the elastic limit of the at least one elastomeric element does not need to be very high, in absolute terms, as the bypass section of metal chain takes over transmission of loads once the predetermined length is reached. Preferably the elastic limit (or breaking load) of the at least one elastomeric element is substantially less than the yield strength (or breaking load) of the metal chain. For example, the elastic limit (or breaking load) of the at least one elastomeric element may be about 50% or less than the yield strength (or breaking load) of the metal chain.
Ensuring that the elastic limit is greater than the threshold tensile load can be achieved in various ways. For example, if the distance between the two attachment points on the chain is already predetermined, the choice of the at least one elastomeric element can be made depending on the predetermined length of the bypass section of the metal chain relative to the initial length of the elastomeric element(s). The distance between the two attachment points may be predefined if the chain has certain links which make attachment easier, for example the shackles between shot lengths (discussed further below). Alternatively, if the initial length of the at least one elastomeric element and its rate of elongation is already predetermined, the distance between the points of attachment on the metal chain can be adjusted such that the threshold tensile load at which the chain takes the rest of the load is below the elastic limit. This may, for example, be achieved by bringing the attachment points closer together and thus reducing the predetermined length to which the elastomeric element(s) stretches before the metal chain becomes taut.
In order to provide a stress-strain response that is tailored to respond to the most frequent tensile loads which are applied to a given mooring line, e.g. taking into consideration its expected marine environment, the at least one elastomeric element may comprise at least one tensile element and/or at least one compressive element. What is meant by a tensile elastomeric element is an elastomeric element that stretches and elongates in response to an increase in tensile load. What is meant by a compressive elastomeric element is an elastomeric element that compresses and shortens in response to an increase in tensile load. An example of a compressive elastomeric element is an elastomeric element mounted between end connectors that are arranged to move closer together in response to a tensile load pulling the end connectors in opposite directions. Some examples of tensile and compressive elastomeric element are seen in the applicant's earlier publication WO 2012/127015, the content of which is hereby incorporated by reference.
In a first set of embodiments the at least one elastomeric element comprises one or more tensile elastomeric elements. In such embodiments the tensile elastomeric element(s) may be formed of a relatively soft material. For example, the tensile elastomeric element(s) may have a relatively low stiffness, such as an elastic modulus of 10 MPa or less. In these examples the tensile elastomeric element(s) may have an elastic modulus of at least 1 MPa, 2 MPa, 3 MPa, 4 MPa, 5 MPa, or 6 MPa (measured, for example, at 75% elongation). Such tensile elastomeric elements are appropriate for mooring lines which experience repetitive cyclic low loads. Tensile elastomeric elements may be chosen as they are capable of delivering a smooth response and typically have a long lifetime.
In a second set of embodiments the at least one elastomeric element comprises one or more compressive elastomeric elements, preferably in combination with at least one tensile elastomeric element. In such embodiments the compressive elastomeric elements may be formed of a relatively stiff material, such as an elastic modulus of at least 10 MPa (measured, for example, at 60% elongation). In these examples the compressive elastomeric element(s) may have an elastic modulus of at least 15 MPa, 20 MPa, 25 MPa, 30 MPa, or greater, especially towards the end of its compression range. Preferably such compressive elastomeric element(s) may provide a non-linear increase in elastic modulus with compression, meaning that an element becomes much stiffer towards the end of its compression range. This response can provide a bridge that matches the stiffness of the metal chain so that the mooring line does not experience a sharp change in load response that can result in a snatch load. As is described in WO 2012/127015, a suitable compressive elastomeric element may comprise a cylindrical corrugated or bellowed member formed of elastomeric material. The typical structure of compressive elements means that a relatively small amount of material is required to provide high protection and this contributes to reducing the overall weight of the mooring line.
In addition, or alternatively, in a set of embodiments the at least one elastomeric element may be arranged to provide a non-linear response to the tensile loads applied to the mooring line. It will be appreciated that there are various ways that a non-linear stress-strain response can be achieved. In preferred embodiments where the mooring line comprises a single elastomeric element, whether a tensile or compressive element, the elastomeric element may include a plurality of different sections along its length comprising different elastomeric materials and/or thicknesses of elastomeric material. In less preferred examples where the mooring line comprises a plurality of elastomeric elements, whether tensile and/or compressive elements, there may be provided a plurality of different elastomeric elements. The plurality of elastomeric elements may comprise different elastomeric materials and/or thicknesses of elastomeric material and/or lengths of elastomeric material. However increasing the number of elastomeric elements may unduly increase the complexity and cost of the mooring line. In a preferred set of embodiments the at least one elastomeric element comprises a single tensile elastomeric element, optionally in combination with a single compressive elastomeric element. Preferably the tensile and compressive elastomeric elements are arranged to respond in parallel to tensile loads when combined. However, it is further preferable that the compressive elastomeric element has a higher elastic modulus than the tensile elastomeric element. This means that the two elements contribute to substantially different parts of the overall stress-strain response of the mooring line. By selecting the combination of tensile elastomeric element(s) and compressive elastomeric element(s) it is possible to provide a non-linear stress-strain response that is tuned to respond to a wide range of movements of an object being moored. However a single elastomeric element has been found to deliver the lowest cost solution with least chance of entanglement.
In various embodiments the at least one elastomeric element may be formed of any suitable elastomeric material. An elastomer is any polymer material with viscoelasticity (having both viscosity and elasticity) provided by very weak inter-molecular forces, therefore generally having high failure strain compared with other elastic materials. The elastomeric material may be a thermoset (requiring vulcanization e.g. natural or synthetic rubber) or a thermoplastic (e.g. thermoplastic polyurethanes (TPU), thermoplastic copolyesters, thermoplastic polyamides, thermoplastic olefins). In those embodiments where the at least one elastomeric element comprises a tensile elastomeric element, such a tensile elastomeric element preferably consists of a natural rubber material. Natural rubber may be chosen for its excellent lifetime performance. This results from the self-healing properties of natural rubber, as opposed to synthetic rubber materials, which allow for micro-cracks to heal rather than proliferate. In those embodiments where the at least one elastomeric element comprises a compressive elastomeric element, such a compressive elastomeric element may consist of a thermoplastic material. An example of a suitable thermoplastic elastomeric material is Hytrel from Du Pont.
A mooring line according to the present invention generally allows an object which is being moored to move spatially. It will be appreciated by those skilled in the art that mooring lines in accordance with the present invention may form part of a variety of different mooring systems. In some examples, the mooring line may be part of a taut mooring system, i.e. a mooring system in which the metal chain outside the two points of attachment is held taut. In such examples the movement of the moored object may be facilitated solely by the at least one elastomeric element stretching until the bypass section of metal chain between the two points of attachment becomes taut. Once the at least one elastomeric element is stretched to the predetermined length and the bypass section of chain becomes taut, the object is no longer able to move any further. Such mooring lines may be suitable for near shore or tidal environments where a relatively small and well-defined range of movement is required.
Alternatively the mooring line may form part of a catenary mooring system. A catenary mooring system typically consists of a first section of metal chain resting along the floor along with a second section of metal chain connecting the first section to the object being moored. A mooring line according to the present invention may be inserted into the second section of metal chain so as to provide a load reduction. In particular, the most frequent and generally low tensile loads may be absorbed by the at least one elastomeric element rather than by repeatedly lifting the first section of metal chain off the floor. This effectively converts a traditional catenary mooring line into a semi-taut mooring line and eliminates some of slack within the chain. Reducing the amount of slack chain helps to reduce wear on the chain thus improving the operational life of the mooring line. Improving the operational life is a significant benefit as mooring lines are often used in remote marine environments where replacing the mooring lines is extremely expensive not only due to the cost of the mooring lines themselves but also due to the resources required to replace the mooring lines.
As the elastomeric element reduces the loads in a catenary mooring system, this allows the total length or grade (e.g. thickness or weight per link) of metal chain to be reduced. The weight of chain along the seabed floor available to be lifted is proportional to the expected maximum loads (taking into account the holding power of the anchor). Reducing the maximum peak loads dramatically reduces the length and weight of chain required on the seabed floor, as well as the anchor requirement. This also means that as less chain rests on the seabed floor, the footprint of the mooring system can be reduced. Typically the quantity of metal chains used in catenary mooring systems is extremely expensive; reducing their overall length potentially provides a significant cost reduction. This is augmented by the fact that a relatively small length of elastomeric element can enable a relatively large length of metal chain to be removed.
In embodiments where the mooring line is part of a catenary mooring system it will be appreciated by those skilled in the art that the mooring line is capable of absorbing the tensile loads, both by stretching the at least one elastomeric element and/or lifting the metal chain off the floor. It will be appreciated that whether the metal chain is caused to lift off the floor will depend on the magnitude of the tensile load applied and the rate at which it is applied. A benefit of the elastomeric element(s) is its absorption of shock loads.
Thus according to a second aspect of the present invention there is provided a catenary mooring system comprising a floating body and a mooring line comprising a first section of metal chain resting on the floor and a second section of metal chain connecting the first section of metal chain to the floating body, and further comprising at least one elastomeric element having an initial length that is attached between two points of attachment on the second section of metal chain, thereby defining a third section of metal chain between the two points of attachment of the at least one elastomeric element with a predetermined length that is greater than the initial length of the at least one elastomeric element;
wherein the third section is directly linked to the second section by chain links so as to transmit a tensile load directly from the second section of the metal chain to the third section;
wherein the at least one elastomeric element is arranged such that:
when an initial tensile load, below a threshold tensile load, is applied to the mooring line in use, the at least one elastomeric element stretches from the initial length, to a second, longer, length wherein the third section of metal chain between the two points of attachment is not taut and thus the initial tensile load is transmitted from and to the second section of metal chain through the at least one elastomeric element; and
when a further tensile load equal to or above the threshold tensile load is applied to the mooring line in use, the at least one elastomeric element stretches further from the second length to the predetermined length wherein the third section of metal chain between the two points of attachment becomes taut, and thus the further tensile load applied to the mooring line is transmitted directly from and to the second section of metal chain through the third section of metal chain.
It will be appreciated by those skilled in the art that a floating body may not necessarily be a body which is floating on the water's surface. The body may be partially submerged or fully submerged and still be considered to be floating. Any floating body that is supported above the floor by water is considered to be a floating body which may require a mooring line.
Typically the metal chain in a catenary mooring system comprises several lengths or sections of metal chain attached together. Whilst the entire mooring line may comprise a single type (e.g. thickness) of metal chain, often two different chains are connected in series. Therefore, in a set of embodiments the mooring line comprises a plurality of different metal chains, or different sections of metal chain, connected in series, preferably a plurality of different thickness metal chains or sections of metal chain. This is often the case with catenary mooring systems where the overall chain length is often very large and of the order of kilometres. In such mooring systems often a length of heavy gauge chain which rests along the floor is attached to a smaller gauge chain which connects the heavy gauge chain resting along the floor to the moored object. Accordingly the first section of metal chain may be thicker than the second section of metal chain.
As the mooring lines in catenary systems are relatively long, as is discussed above, they typically comprise a plurality of sections of chain connected in series. Typically each section of metal chain is referred to as a shot length. Each of these shot lengths is often connected to the adjacent shot length using a shackle or chain link which is welded closed. It is important that the shackle or link is formed from the same grade and strength material as the chain itself. This ensures that these points of connection do not form a weak point in the chain. In a set of embodiments the metal chain comprises a plurality of shot lengths connected in series. In such embodiments the attachment points of the at least one elastomeric element are preferably located at each end of one shot length. In other words, the predetermined length of the third section of chain may correspond to a shot length. It will be appreciated that the metal chain may be formed from a variety of different types of chain, for example, a stud-less chain and/or a stud-link chain. Also the mooring line may comprise multiple shot lengths, potentially with different shot lengths of different grades along the length of the mooring line.
The metal chain used in mooring systems is typically stud-link or stud-less. Stud-link chain has a bar across the width of the links which makes it difficult to attach a connector to it. Many heavy duty chains are stud-link as it is heavier. Metal chain comes in pre-determined lengths of 27.5 m (a shot length) which are then welded together (or shackled) and it makes sense to connect to these points, or to cut the chain and replicate such a connection. Preferably the third (“bypass”) section of metal chain has the same specification as the second section of the chain in the mooring line so that it does not form a weak point once the third section is taut. This means that, even if the at least one elastomeric element becomes disconnected, the entire mooring line forms a typical chain line with no weak points.
Whether or not the mooring line forms part of a catenary mooring system, it will be appreciated by those skilled in the art that connecting the at least one elastomeric element directly to the chain may not always be possible. Whether it is possible to directly attach the at least one elastomeric element may depend on the size, gauge and type of metal chain e.g. there may not physically be space in the internal hole of the chain links to directly attach the at least one elastomeric element, especially for a stud-link metal chain. This problem may be further increased in embodiments where a plurality of elastomeric elements is attached to the metal chain.
To form the attachment points, it may be possible to cut a stud-link chain at the desired point and attach the elastomeric element(s) with shackles or through welding in a stud-less link of similar strength. However, in a preferred set of embodiments the at least one elastomeric element is attached to the metal chain at each attachment point by a connector. Whilst it is appreciated that this connector may, for example, take the form of a simple shackle which can attach the chain and the elastomeric mooring component directly together, it is appreciated that a shackle may not always be appropriate. In some embodiments the connector may comprise a chain clamp, especially if the chain is a stud-link chain. In a set of embodiments the connectors may be shaped such the (third) section of metal chain between the two attachment points defines a bypass section of chain that is not able to come in contact with the at least one elastomeric element at least when the bypass section of chain is taut. For example, a chain clamp may comprise a shoulder portion that extends between the elastomeric element and the bypass section of chain to prevent them from touching at the attachment point (at least when the bypass section of chain is taut). Such a shoulder portion may be positioned between first and second attachment points of the chain clamp. However the connectors are preferably shaped so as to provide an arrangement of three attachment points.
This is considered to be novel and inventive in its own right, and thus when viewed from a further aspect the invention provides a connector, for attaching at least one elastomeric element to a mooring line which comprises a metal chain, wherein the connector comprises a first attachment point for the elastomeric element, a second attachment point for a first section of the metal chain and a third attachment point for a second section of the metal chain, the attachment points being arranged such that when a tensile load is applied to the first section of metal chain, the connector aligns itself so that the tensile load is transferred along the at least one elastomeric element, and the second section of metal chain is not able to come into contact with the at least one elastomeric element at least when the second section of metal chain is taut.
By providing a connector to attach the at least one elastomeric element to the metal chain it means that the connector can be designed so that the elastomeric element is not directly attached to the metal chain. Additionally, by providing a connector that it designed such that the at least one elastomeric element and the second “bypass” section of metal chain are unable to come into contact with each other at least when taut (e.g. when a tensile load equal to or greater than the threshold tensile load is applied to the mooring line) ensures that the elastomeric element and the bypass section of metal chain do not rub against each other and cause wear problems. This helps to prevent reduction in the lifespan of the mooring line. As elastomeric elements are typically formed of a weaker material than metal chain, if they were to repeatedly come in contact with one another, the metal chain may damage the elastomeric element potentially causing it to break. Once the elastomeric element has been caused to break the mooring line is no longer capable of absorbing the tensile loads as efficiently. As discussed previously, ensuring a significant lifetime for a mooring line is important as mooring lines are typically placed in remote locations where replacement is often difficult and expensive.
In a set of embodiments the connector is shaped so as to pivot about the second attachment point. Accordingly each connector may comprise a first attachment point for the elastomeric element, a second attachment point for the metal chain and a third attachment point for the second (“bypass”) section of chain, wherein the connector is shaped so as to pivot about the second attachment point. This means that the first section of metal chain can be generally aligned with the elastomeric element(s) while the second “bypass” section of chain is swung away from the elastomeric element(s). This may be achieved, for example, by an L-shaped or V-shaped connector comprising two substantially straight arms connected by a bend. Two of the three attachment points may, for example, be arranged at either end of the arms of the connector and the pivoting attachment point at the bend between the two arms. In some such embodiments the first attachment point is provided at the end of one arm, the second attachment point is provided at the bend between the arms, and the third attachment point is provided at the end of the other arm. The at least one elastomeric element may therefore be attached at one end of the L-shape or V-shape, and the metal chain may be attached at the other end of the L-shape or V-shape and at the bend between the two arms. Attaching the at least one elastomeric element and the first and second sections of metal chain at these three spaced apart points ensures that the second “bypass” section of metal chain and the at least one elastomeric element cannot come into contact with each other at least when the second “bypass” section of metal chain is taut, e.g. when the threshold tensile load is reached. In an L-shaped connector, one arm may be longer than the other so as to maximise the distance of the third attachment point from the bend and hence increase the spacing between the “bypass” second section of chain and the elastomeric element(s). Additionally, by attaching the metal chain and the at least one elastomeric element in this way means that as the threshold tensile load is reached the connector acts as a lever which pulls the elastomeric element(s) to one side and brings the “bypass” second section of metal chain into a parallel arrangement. This means that the tensile load can be appropriately transferred along the metal chain from the first section to the “bypass” second section.
It will be appreciated that the primary purpose of the at least one elastomeric element attached to the metal chain is to assist in reducing the loads the mooring line experiences while retaining some of the traditional metal chain as a failsafe. It is therefore preferable that the connector is formed from a material which is as strong as the metal chain to which it is connected. Further preferably, the connector is made from substantially the same material as the metal chain. Forming the connector from the same material is advantageous as it ensures that there is even wear between the connector and the metal chain. If, for example, the connector was made from a stronger material it may wear on the metal chain forming a weak spot on the chain which may lead to it failing. Furthermore, forming the connector from a similar or the same metal as the chain may be important to prevent galvanic corrosion.
Whilst the connector connects the at least one elastomeric element and the metal chain together, it will be appreciated that it is still necessary to attach the connector to the metal chain and the at least one elastomeric element. This may be achieved by integrated connection means in the connector. Alternatively, in a set of embodiments the at least one elastomeric element and the metal chain are each attached to the connector by a shackle. The shackle is preferably of the same grade, thickness and size as the metal chain itself.
The present invention also extends to a method of modifying a mooring line which comprises a metal chain, the method comprising: attaching a first end of at least one elastomeric element to a first attachment point on the metal chain and attaching a second end of the at least one elastomeric element to a second attachment point on the metal chain, wherein the metal chain between the first and second attachment points has a predetermined length that is greater than an initial length of the at least one elastomeric element. This means that the predetermined length of metal chain between the first and second attachment points defines a “bypass” section of metal chain that is slack when the elastomeric element(s) is under tension at its initial length.
According to preferred embodiments of such a method, the metal chain is unbroken. However it is also envisaged that the method may further comprise: breaking the metal chain and inserting one or more chain links to allow for attachment of the at least one elastomeric element. For example, a link or section of a stud-link chain may be replaced with a link or section of a stud-less chain.
It will be appreciated that such a method can be used to modify a mooring line that is already in situ. For example, a catenary line comprising a metal chain being used to moor a buoy at sea may be modified using this method. According to embodiments of this aspect of the invention it is preferable that the metal chain is not broken during attachment of the elastomeric element. For example, the elastomeric element(s) may be attached by a connector or shackle as discussed above. In such embodiments the mooring line advantageously retains a continuous, unbroken, metal chain along its entire length as a failsafe in the unlikely event of failure of the elastomeric element(s). It is therefore preferable that the “bypass” section of the metal chain between the first and second attachment points is directly linked by chain links to the rest of the metal chain so as to form a continuous metal chain for the mooring line. Preferably at least one, and preferably both, of the two attachment points is not at the end of the metal chain in the mooring line. In other words, the predetermined length of the bypass section of the metal chain is shorter than the total length of the metal chain in the mooring line.
Such methods of modifying the mooring line require that the predetermined length of the chain between the attachment points is longer than the initial (e.g. unstretched) length of the elastomeric element(s). It will be understood that in order to attach the elastomeric element in such a manner the “bypass” section of chain between the attachment points may to be gathered such that the physical distance between the attachment points allows the elastomeric element to easily be attached. This may, for example, be achieved by physically grouping a section of the chain together so that the tension on the chain is released and then attaching the elastomeric element(s) at the two points of attachment and then allowing the tension to be re-applied and thus transferred along the elastomeric element(s). Such a method is advantageous as it means that the object that is being moored, for example at sea, is not released from the mooring line and thus reduces the risk of it becoming detaching and floating away from its ideal position. This method may also be appropriate when the mooring line comprises a catenary line—in such an example it may be possible to bunch together a section of the chain as a small section of chain can be lifted away from the sea floor to account for this bunched section.
Alternatively, the at least one elastomeric element may be stretched to a certain extension and then attached to the metal chain, with the length of the stretched element and the bypassed section of chain to which it is being attached being the same. Attaching the elastomeric element in this way effectively establishes the elastomeric element in a fully stretched state that matches the predetermined length of the bypassed section. The elastomeric element(s) may then reduce in length and slack may be taken from the rest of the metal chain to allow for this. Attaching the elastomeric element in this way may be advantageous in cases where it is particularly difficult to bunch and hold the chain together whilst the elastomeric element(s) is being attached.
However, it will be appreciated that modifying a mooring line in a way as described above may not always be suitable, particularly in situations where there may not be sufficient slack chain. Therefore, in an alternative set of embodiments the method comprises temporarily detaching the mooring line from the object to which it is attached. This allows a section of the chain to easily be gathered and the elastomeric element(s) attached. This method may be preferable when the object is moored by a plurality of mooring lines and thus detaching a single mooring line has minimal effect on the security of the moored object.
Whilst it is possible to add at least one elastomeric element into an existing mooring line it may be easier to attach one or more elastomeric elements to a new mooring line. Thus in an alternative set of embodiments the method is applied to a new mooring line which is not yet attached to an object. Applying the method to a new mooring line is likely to be simpler than modifying an existing line as it is likely not to be in a hostile environment such as being at sea. Also when the mooring line is not in use there is no requirement to remove the tension on the mooring line in order to attach the elastomeric element(s).
While it is discussed that the metal chain typically comes into action during the use of the mooring line, it will be appreciated that in some situations the metal chain may never come into action, i.e. the at least one elastomeric element may be designed so as to take into account the full possible range of motion of the moored object. In such an embodiment the metal chain is there in the extremely unlikely event of failure of the elastomeric element(s), however it may not act to transfer tensile loads in any other situation other than the elastomeric element(s) breaking.
The invention has various applications, particularly in marine mooring, however it is appreciated that it is not limited to marine mooring situations. The mooring line according the invention may, for example, be used to moor: aquaculture farms, buoys, wind farms, tidal farms, wave farms, fenders used in ship-to-ship transfer or storage of mothballed ships. All of the examples given above typically involve expensive objects which are preferably securely moored. The present invention helps to reduce the loads experienced by the mooring lines mooring these objects and thus reduces the chance of these lines failing thus increasing the security of the moorings.
It will be appreciated a mooring line in accordance with the present invention can be used to moor a range of different objects in a range of different environments. For example, the mooring line may be used to moor objects at sea, in rivers, in lakes or in pools.
It will be appreciated that certain preferred features of the invention, which are, for clarity, described above in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various preferred features of the invention which are, for brevity, described in the context of a single embodiment may also be provided separately or in any suitable sub-combination.
Some preferred embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings, in which:
line 8 as it is picked up from the ocean floor 6. As the water depth increases due to large waves the catenary chain 8 is lifted off the seabed and the platform 4 moves upwards and to the left. For small waves, the chain 8 is laid along the seabed 6 as the water depth decreases and the platform 4 drifts downwards and to the right. Thus very large amounts of chain and a large space envelope is required to allow horizontal movement as water depths rise and fall. The mooring forces (Fmax) are high and transmitted through the entire chain 8, experienced at all points.
Attached in parallel with a section of the 28 mm chain 24 is an elastomeric element 28 having an initial length in its unstretched state. The elastomeric element 28 is attached to the 28 mm chain 24 at a first point 30 and a second point 32 such that a “bypassed” chain section 34 of the 28 mm chain 24 between the attachment points 30, 32 is longer than the initial length of the elastomeric element 28. The “bypassed” chain section 34 is therefore slack while the elastomeric element 28 is taut. In one example, the elastomeric element 28 has an initial i.e. unstretched length of 4.5 m and the “bypassed” chain section 34 between the attachment points 30, 32 has a length of about 10.25 m. As the floating body 12 rises and lowers due to changes in the sea depth (e.g. tidal or wave motion), a tensile load is transmitted along the 28 mm chain 24. If there is a sudden change in depth of the water, this load increase is transferred into the elastomeric element 28 causing it to stretch. This allows the floating body 12 to rise with the increased water depth, without transferring shock loads to the steel chains 20, 24. Depending on the elasticity of the elastomeric element 28 and its initial length relative to the bypassed section of chain 34, after a certain extension the chain 34 will become taut and further tensile load will be transmitted along the 28 mm chain 24. Accordingly, as the floating body rises it may also cause the length of 38 mm chain 20, 22 on the seabed to be lifted away from the seabed. This provides a restoring force such that when the sea level lowers, the floating body 12 also lowers and remains securely moored. In the example mentioned above, the bypassed section of chain 34 is only taut once the elastomeric element 28 stretches by 125% or more.
The length and/or elasticity of the elastomeric component may be chosen such that the 38 mm chain 20, 22 on the seabed is only caused to lift when the elastomeric element 28 has been fully extended to just below its elastic limit. Whilst this may be the case in some embodiments, it is preferable that the length and/or elasticity is chosen such that the 38 mm chain 20, 22 lifts before the elastomeric element 28 is fully extended, i.e. well before reaching its elastic limit. This ensures that the elastomeric element 28 is safely operational, and can assist in reducing shock loads, at a range of different sea depths.
The 28 mm chain 24 is then attached to the second attachment hole 40 positioned at the bend between the two arms of the L-shaped connector 36. A slack portion 44 of 24 mm chain 24 is then kept loose and the 28 mm chain 24 is attached again at the third attachment point 42. The purpose of keeping the slack portion 44 is to allow the connector 36 to freely pivot during operation without putting excess forces on the 28 mm chain 24. The other end of the elastomeric element 28 is attached to the 28 mm chain 24 in a similar manner. The distance between the third attachment points 42 of the two connectors 36 is chosen such so as to bypass a slack chain section 34 of the 28 mm chain 24 which has a predetermined length greater than the initial e.g. unstretched length of the elastomeric element 28. This “bypassed” chain section 34 will only become taut and transmit tensile load once the elastomeric element 28 has reached an extension corresponding to its predetermined length. Of course the 28 mm chain 24 may be attached to the elastomeric element 28 via different connectors, some examples of which will be discussed in more detail below.
Operation of the mooring line 10 will now be described with reference to
Thus a mooring line comprising an elastomeric element rated at 15 kN and having an initial length of 10 m is shown to provide >50% peak load reduction, as well as shock protection, as compared to traditional catenary mooring lines.
A mooring line in accordance with embodiments of the present invention can be provided to assist in reducing peak loads in a wide variety of mooring situations. For example, many structures moored at sea have multiple different types of mooring lines. For example, aquaculture cages are large structures that are moored at sea. They typically comprise a large cage formed from polymer rope. Such aquaculture cages are typically moored by multiple bridle lines attached to each corner of the aquaculture cage. These bridle lines are subsequently attached to chain mooring lines. A “bypass” elastomeric element as described above may be added to the bridle lines and/or the mooring lines.
In each of the scenarios discussed above, the chain clamp 50 functions to attach the chain 24′ to the connector 36 and also allows the nearby sections of the chain 24′ to pivot and rotate as the chain 24′ is brought into tension under a tensile load. It can be seen from
Of course the chain clamp 50 may be used to connect the chain 24′ directly to the elastomeric element without a connector 36.
Of course the chain clamps 50, 64 discussed above are not limited to use with stud-link chain and may be in fact be used to attach various different types of chain, including those without studs, to the elastomeric element 28 or connector 36.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1603026.4 | Feb 2016 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB2017/050442 | 2/21/2017 | WO | 00 |
