WO2016118006A1

WO2016118006A1 - Spread moored chain connector and floating structure comprising such a chain connector

Info

Publication number: WO2016118006A1
Application number: PCT/NL2016/050046
Authority: WO
Inventors: Wilhelmus Coenradus Johannes Jozephus Woldring; Pieter DIERX
Original assignee: Sbm Schiedam B.V.
Priority date: 2015-01-20
Filing date: 2016-01-20
Publication date: 2016-07-28
Also published as: BR112017015565A2; BR112017015565B1

Abstract

A vessel includes a chain connector for a mooring leg. The chain connector includes a chain connector arm, a chain guiding element and a universal joint. The universal joint is positioned at a distal end of the connector arm and is connected to the hull of the vessel. The guiding element is arranged on the connector arm near the connector arm distal end, and configured to guide the chain in a plane parallel to the length of the connector arm. The universal joint includes first and second rotation shafts, perpendicular to each other. The first shaft is connected with the hull of the vessel and the second shaft is connected to the distal end of the connector arm. The longitudinal direction of the second shaft is positioned under an oblique angle β with respect to the chain guiding plane of the guiding element.

Description

SPREAD MOORED CHAIN CONNECTOR AND FLOATING STRUCTURE COMPRISING

SUCH A CHAIN CONNECTOR

Field of the invention

The present invention relates to a spread moored chain connector according to the preamble of claim 1. Additionally, the invention relates to a floating structure comprising such a chain connector. Moreover, the present invention relates to a method for manufacturing such a chain connector. Background

In offshore technologies, use is made of floating structures for storage and/or production of hydro-carbon fluids, from undersea gas or oil fields. Such floating structures comprise floating storage and offloading (FSO) and floating production, storage and offloading (FPSO) vessels.

Such a floating structure may be secured at its position, by a structure of a number of mooring lines (or mooring legs), which connect to anchoring facilities at or in the seabed, geographically distributed i.e., spread around the position.

During use, a mooring leg, made-up completely or partially of chain links, between the floating structure and the anchoring facility, is exposed to forces caused by motions of the floating structure as exposed to sea and weather. Such forces give rise to some fatigue phenomena which could cause a chain link to fail:

• Tension-Tension (Traditionally the only analyzed phenomena)

• OPB (Out of Plan Bending)

· IPB (In Plane Bending).

OPB and IPB fatigue occurs on the links of a mooring chain, near the connection to the floating structure, where due to the relatively high tension, the chain links do not rotate with respect to each other because of interlink friction. Rather than rotating, the links start to bend in order to follow the continuously changing floating structure motions (roll, pitch yaw, etc.).

OPB and IPB relate to practically same fatigue phenomena as only the bending direction differs. IPB is bending over the 'stronger' axis of the chain link and OPB is bending over the 'weaker' axis of the chain link. Of the two, it is considered that OPB is the governing fatigue mechanism. The total chain fatigue damage is a combination of all three aforementioned fatigue phenomena.

The chain is connected to the floating structure by means of a chain connector. The main issue with OPB and IPB are the motions (behaviour) of the floating structure with respect to the chain connector and the mooring leg.

In addition it is observed that, especially on spread moored floating structures, the dependency of the azimuth angles of the mooring lines on the supporting structures of the chain connectors may lead to last minute design changes and unforeseen delays.

Moreover, a disadvantage of prior art supporting structures for the chain connectors (and fairleads) is either the relatively large weight or changes in azimuth angles due to different mooring design will lead to modifications to these structures.

Current Design and Solution

The basic functionality of a chain connector is to:

• Connect the mooring line (by means of chain) to the floating

structure

· Allow for (re-) tensioning of the mooring line

• Allow for alignment of the chain connector and mooring lines

(including floating structure motions) with the tension vectors

• Reduce the maximum interlink moment that can occur in order to reduce IPB and OPB fatigue

· Transfer loads from a mooring line to the FPSO and reduce fatigue to the mooring line

Chain guiding structures, like gutters and fairleads, and single articulated chain connectors have been introduced in the past to fulfil the above mentioned requirements. The single articulated chain connectors provide a single rotational axis in the 'in-plane' direction of the mooring leg ensuring alignment of the tension vector and chain links near the chain connector. The fairleads and gutters also provide a single 'rotational' axis in the 'in-plane' direction. After the identification of the OPB phenomenon an additional rotational axis has been introduced by Applicant into the chain connector design and/or fairlead design to ensure alignment in both the 'in-plane' and 'out-of-plane' direction of the chain links. OPB and IPB are best mitigated by two axis rotated 90 degrees with respect to each other but both perpendicular to the mooring line.

A ratchet is commonly used to 'hold' the chain. These ratchets can be integrated into the chain connector or can be located somewhere else.

Most chain connector / fairlead designs are based on lever arms with bushes or bearings on multiple perpendicular shafts to allow articulation for in-plane and out-of-plane rotations. This is done by one of the following solutions (or a combination thereof):

• Fairlead combined with single articulated chain connector

• Double articulated chain connector

· Uni -joint

Especially for (but not limited to) spread moored floating structures, chain connectors with multiple rotational axes are combined with fairleads. Fairleads are not used to create a rotational axis but to deflect the chain from the top of the chain connector to a mooring leg tensioning system (wherever located) and / or provide the correct mean azimuth angle for the mooring chain.

The combination of chain connectors and fairleads provides too many rotational axes where only two perpendicular axes are required to mitigate OPB and IPB fatigue issues. Furthermore existing chain connector and fairlead solutions tend to be heavy and their support structures complicated to integrate into the floating structure.

It is an object of the present invention to provide a spread moored chain connector that overcomes or mitigates the above detrimental effects.

Summary of the invention

The above object is achieved by a chain connector for a mooring leg connectable to a floating structure comprising a chain connector arm, a chain guiding element and a universal joint, the chain connector arm being arranged at its distal end for connection to a hull of the floating structure by means of the universal joint positioned at the distal end; the chain guiding element being arranged on the chain connector arm near the distal end of the chain connector arm, and configured, in use, to guide a chain from the floating structure in a guiding plane defined as a plane set up by the longitudinal direction of the chain connector arm and an incoming direction of the chain; the universal joint comprising a first rotation shaft and second rotation shaft, perpendicular to each other; the first rotation shaft being connectable with the hull of the floating structure and the second rotation shaft being connected to the distal end of the chain connector arm, wherein a projection plane is defined as a second plane that is perpendicular to the longitudinal direction of the chain connector arm, and wherein a projection of the longitudinal direction of the second rotation shaft on the projection plane is positioned under an oblique angle β with respect to the incoming chain direction as projected on the projection plane.

The invention provides that by arranging a chain guiding element and a universal joint at the distal end, a compact construction is obtained with relatively low weight in comparison with prior art chain connectors. The projection of the second rotation shaft is oriented under the oblique angle β with respect to the projection of the incoming chain direction while the other first rotation shaft is oriented under a second oblique angle. Additionally, by having respective oblique angles between the first and second rotation shafts of the universal joint, and the rotation plane of the chain wheel (i.e., the guiding plane), it is possible to accommodate changes in azimuth angles of the mooring lines while the chain guiding element remains directed along the mooring line between the chain guiding element and the mooring line tensioning system.

Also, the present invention relates to a floating structure comprising a chain connector as described above.

The chain guiding element may a chain wheel or a chain gutter or any other suitable chain guiding element.

Moreover, the present invention relates to a method for manufacturing a chain connector for a mooring leg connectable to a floating structure.

Advantageous embodiments are further defined by the dependent claims. Brief description of drawings

The invention will be explained in more detail below with reference to drawings in which illustrative embodiments thereof are schematically shown. They are intended exclusively for illustrative purposes and not to restrict the inventive concept, which is defined by the appended claims.

Figure 1 shows a schematic perspective view of a chain connector arm in accordance with an embodiment of the invention;

Figure 2 shows a schematic view of the distal end of the chain connector arm along its longitudinal direction;

Figure 3 shows a schematic view of a support structure for mooring lines along a hull of a floating structure using chain connector arms according to an embodiment of the invention;

Figure 4 shows a schematic cross-section view of the mounting location of the universal joint of the chain connector arm in the support structure.

Detailed description of embodiments

Figure 1 shows a schematic perspective view of a chain connector arm in accordance with an embodiment of the invention.

The chain connector 1 according to an embodiment of the invention comprises a chain connector arm 2, a chain wheel 3, a universal joint 4, a ratchet and ratchet support 5 and optionally, a guide pipe 6.

The chain connector arm 2 which is arranged to guide a mooring line or mooring chain 8 is connected at its distal end with a universal joint 4. The distal end of the chain connector arm 2 is connectable to a hull of a floating structure (not shown) by means of the universal joint 4.

The universal joint is typically connected to the chain connector arm by means of a transition body 7.

Near the distal end the chain connector arm 2 is provided with a chain wheel 3 that is mounted on the chain connector arm 2.

The chain wheel is configured to rotate in a plane parallel to the chain connector arm in such a way that it directs the mooring chain towards the mooring line tensioning system that is typically arranged on the floating structure. At the proximal end of the chain connector arm 2, the chain connector 1 is provided with a ratchet and ratchet support 5, that are configured as a chain stopper arrangement.

The ratchet has the function when it is closed i.e., is blocking the mooring chain, to support and hold the mooring chain. The ratchet is designed to withstand the maximum survival mooring loads and all associated fatigue loads. The ratchet support is used to align the chain in the correct orientation to ensure that the chain can be correctly supported on the ratchet. Furthermore the ratchet is supported on the ratchet support.

Optionally, at the free end of the ratchet/ratchet support 5, a guide pipe 6 for the mooring chain is provided. Additionally, the guide pipe 6 may comprise cathodic protection elements 9.

The chain connector arm 2 may be provided with a hawse pipe to guide the mooring chain from the chain wheel to the ratchet/ratchet support.

The universal joint 4 is configured to couple the chain connector 1 to the hull of a floating structure. The universal joint 4 comprises two interconnected rotation shafts 12, 13 that are perpendicular to each other. A first rotation shaft 12 is configured to connect to an external body such as the hull of a floating structure. The second rotation shaft 13 is connected to the distal end of the chain connector arm. According to the invention, the projection of the longitudinal direction of the second rotation shaft on a projection plane perpendicular to the direction of the chain connector arm is under an oblique angle β, i.e., not being 0° or 90°, with the rotation plane of the chain wheel 3. The oblique angle β can be set by means of the transition body 7. The rotation plane of the chain wheel coincides with a guiding plane set up by the incoming direction of the chain from the hull towards the chain wheel and the longitudinal direction of the chain connector arm, i.e., is parallel to the longitudinal direction of the chain connector arm and to the direction of the incoming chain direction.

In an embodiment, the oblique angle β is obtained by the orientation of the transition body with respect to the orientation of the chain wheel by means of welding the transition body to the chain connector arm at a predetermined angle. The welding operation may be done at a late stage in the installation process of the chain connector. In this manner, the chain connector can be oriented to have the chain wheel axis of rotation to be perpendicular to the mooring line while the chain connector can maintain flexibility to accommodate changes in azimuth angles of the mooring lines by the orientation of the two rotation shafts of the universal joint. Moreover, this arrangement allows a better alignment of the rotation axes of the universal joint with the OPB and IPB direction of the mooring chain 8.

Figure 2 shows a schematic top view of the distal end of the chain connector arm along its longitudinal direction.

In Figure 2, the mount 10 of the chain wheel on the chain connector arm is directed in upward direction of the drawing. In Figure 2 the viewing plane coincides with the projection plane perpendicular to the longitudinal direction of the chain connector arm.

The rotation plane of the chain wheel is indicated by the dashed line 11. The transition body allows that the universal joint is rotated with respect to the chain wheel and the chain connector arm. The direction of the second rotation shaftl3 in the universal joint projected on the projection plane is positioned under an oblique angle β with respect to the projection of the incoming chain direction, and the direction of the projection of the first rotation shaft is under a second angle which depends on the rotation of the chain connector arm around the second rotation shaft with respect to the first rotation shaft. In an embodiment, the first rotation shaft will be under an oblique angle 90° - β and the second rotation shaft under oblique angle β, if the first and second rotation shafts are each parallel to the projection plane, since the longitudinal axis of the first rotation shaft is under 90° with the longitudinal axis of the second rotation shaft.

The oblique angle β allows that rotations on the chain connector arm occur on different axes than the direction of the mooring chain 8 from the chain wheel 3 to the mooring line tension system on the floating structure (not shown).

Figure 3 shows a schematic view of a support structure 21 for mooring lines along a hull 20 of a floating structure using chain connector arms 2 according to an embodiment of the invention.

Along the hull of a floating structure, such as an FPSO or FSO vessel, support structures that are configured for spread mooring, are mounted typically near bow and stern. Each of the support structures is equipped with a plurality of chain connectors that are arranged to receive mooring lines from various locations on the seabed.

The support structures are box shaped structures on which the chain connectors are connected by the means of the first rotation shaft 12 of the respective universal joint 4.

The chain connector arm 2, with the chain wheel 3, for each chain connector is positioned at a different angle with respect to the universal joint to ensure that with a varying azimuth angle (between each mooring leg) good alignment is provided with the mooring tensioning system located at deck level (not shown).

The chain connector arm is used to create a lever arm (for both pitch and roll direction) between the ratchet and the floating structure. In case the chain connector arm comprises a hawse pipe, the hawse pipe is constructed as a box shaped structure (built-up with plates) with a guidance structure inside to guide the chain running through the hawse pipe from the ratchet to the chain wheel.

The first rotation shaft 12 that is connected to the floating structure has a longitudinal direction positioned at a non-zero tilt angle a with respect to its projected direction on the substantially vertical plane of the hull.

By orientating the shaft in such a direction, it is close to being

perpendicular to the mooring line while it allows to maintain flexibility to accommodate changes in azimuth angles of the mooring lines.

The invention may have the following advantages over the existing systems:

• The invention provides a chain connector with a reduced weight compared to existing systems due to the 'removal' of not-needed shafts / rotational axes and simplification of casted parts of the chain connector.

• The shaft that is connected to the floating structure is positioned at a non-horizontal or non-vertical angle allowing for maximization of the OPB lever arm while maintaining the flexibility to change the azimuth angles of the mooring system if needed. The chain connector functionality is provided in a single mechanical component and no separate chain wheel mounting structure is needed.

There is only one support structure required to connect the chain connector to the floating structure.

Azimuth angles of the mooring system can be changed at a late stage without the need to modify the chain connector design or the support structure. Only the hawse pipe or chain connector arm needs to be (re-)connected to the universal joint at a different angle by setting the transition body differently, i.e., in a different orientation.

Other alternatives and equivalent embodiments of the present invention are conceivable within the idea of the invention, as will be clear to the person skilled in the art. The scope of the invention is limited only by the appended claims.

Claims

1. Chain connector for a mooring leg connectable to a floating structure

comprising a chain connector arm, a chain guiding element and a universal joint,

the chain connector arm being arranged at its distal end for connection to a hull of the floating structure by means of the universal joint positioned at the distal end;

the chain guiding element being arranged on the chain connector arm near the distal end of the chain connector arm, and configured, in use, to guide a chain from the floating structure in a guiding plane defined as plane set up by the longitudinal direction of the chain connector arm and the incoming direction of the chain;

the universal joint comprising a first rotation shaft and second rotation shaft, perpendicular to each other;

the first rotation shaft being connectable with the hull of the floating structure and the second rotation shaft being connected to the distal end of the chain connector arm;

wherein a projection plane is defined as a second plane that is perpendicular to the longitudinal direction of the chain connector arm, and

wherein a projection of the longitudinal direction of the second rotation shaft on the projection plane is positioned under an oblique angle (β) with respect to the incoming chain direction as projected on the projection plane.

Chain connector arm according to claim 1, wherein the chain guiding element comprises a chain wheel, with a wheel axis of the chain wheel being perpendicular to the guiding plane, such that the incoming chain direction as projected on the projection plane is perpendicular to the longitudinal direction of the chain connector arm and is perpendicular to the longitudinal direction of the wheel axis.

Chain connector according to claim 1 or claim 2, wherein the universal joint is connected to the chain connector arm by means of a transition body. Chain connector according to claim 3, wherein the transition body is configured to set the oblique angle (β).

Chain connector according to claim 4, wherein the oblique angle (β) is a non-zero and non-perpendicular angle.

Chain according to claim 4 or 5, wherein either the oblique angle (β) is between 10° and about 80° or the oblique angle (β) is about 45°.

Chain connector according to any one of the preceding claims 2 - 6, wherein the chain wheel is arranged on the chain connector arm above a path for a chain from the distal end to a proximal end of the chain connector arm.

Floating structure comprising a chain connector for a mooring leg connectable to said floating structure in accordance with any one of the preceding claims 1 - 7.

Floating structure according to claim 8, wherein the universal joint is connected to the chain connector arm by means of a transition body, intermediate the chain connector arm and the universal joint.

Floating structure according to claim 8 or claim 9, wherein the longitudinal direction of the first rotation shaft of the universal joint that is connected to the hull of the floating structure is under a non-zero and non-perpendicular tilt angle (a) provided between the longitudinal direction of the first rotation shaft and its projection on the substantially vertical plane of the hull.

Floating structure according to claim 10, wherein the tilt angle (a) of the longitudinal direction of the first rotation shaft is determined in such a way that, in use, the longitudinal direction of the first rotation shaft is substantially perpendicular to the direction of the mooring leg within the chain connector arm.

Floating structure according to claim 10 or claim 1 1, wherein the tilt angle (a) is between about 15° and about 60°.

Floating structure according to any one of the preceding claims 8 - 12, wherein either the oblique angle (β) is between 10° and about 80° or the oblique angle (β) is about 45°.

Floating structure according to any one of the preceding claims 8 - 13, wherein the chain guiding element is arranged on the chain connector arm above a path for a chain from the distal end to a proximal end of the chain connector arm, and the chain guiding element is arranged to guide the mooring chain between the chain connector arm and a mooring line tensioning system of the floating structure.

Floating structure according to claim 14, further comprising that the second rotation shaft of the universal joint is directed under the oblique angle (β), with respect to a longitudinal direction of the mooring chain or mooring line between the chain guiding element and the mooring line tensioning system.

Method for manufacturing a chain connector for a mooring leg connectable to a floating structure comprising a chain connector arm, a chain guiding element and a universal joint,

— providing the chain connector arm at its distal end with the universal joint for connection to a hull of the floating structure; the universal joint comprising a first rotation shaft and second rotation shaft, perpendicular to each other; the first rotation shaft being connectable with the hull of the floating structure and the second rotation shaft being connected to the distal end of the chain connector arm;

— mounting the chain guiding element on the chain connector arm near the distal end of the chain connector arm, and configuring the chain guiding element for in use guiding a chain incoming from the hull of the floating structure in a guiding plane defined as plane set up by the length of the chain connector arm and the incoming direction of the chain ;

defining a projection plane as a second plane that is perpendicular to the longitudinal direction of the chain connector arm and

configuring that a projection of the longitudinal direction of the second rotation shaft on the projection plane is positioned under an oblique angle (β) with respect to the incoming chain direction as projected on the projection plane.

Method according to claim 16, wherein the universal joint is mounted at the distal end of the chain connector arm by means of a transition body, intermediate the chain connector arm and the universal joint.

Method according to claim 16 or claim 17, comprising mounting the first rotation shaft on the hull of the floating structure in such a way that the longitudinal direction of the first rotation shaft of the universal joint is under a non-zero tilt angle (a) between the longitudinal direction of the first rotation shaft and its projection on the substantially vertical plane of the hull.

Method according to any one of the preceding claims 16 - 18, further comprising:

providing a transition body between the universal joint and the chain connector arm, and orienting the transition body with respect to the orientation of the chain guiding element to obtain the oblique angle (β).

Method according to claim 19, wherein said orienting the transition body with respect to chain guiding element comprises that the transition body is welded between the universal joint and the chain connector arm at a predetermined angle to set the oblique angle (β). Method according to any one of the preceding claims 16 - 20, comprising that the first rotation shaft is attached to a hull of a floating structure, in such a manner that the chain guiding element is arranged to guide the mooring chain between the chain connector arm and a mooring line tensioning system of the floating structure,

and in such a manner that the second rotation shaft of the universal joint is directed under the oblique angle (β), with respect to a direction of the mooring chain or mooring line between the chain guiding element and the mooring line tensioning system.