Marine tether anchoring device

A marine tether anchoring device comprises a tubular anchor body for eventual attachment at its upper end below a marine tether and a spigot for eventual fixing to the sea bed so as to be upstanding therefrom and for reception in the anchor body to establish the anchor. Latching arms within the anchor body establish a releaseable connection to the spigot. A flexjoint supported within the tubular body transmits tensile load in the anchor body as an inwardly and upwardly directed compressive load through the latching arms to the spigot whilst permitting relative tilting movement between the latching arms and the anchor body consequent upon sway of the tether. The anchoring device can be used with large diameter thin walled tendons and the flexjoint that forms part of the device is located in a compressive part of the load path.

FIELD OF THE INVENTION 
This invention relates to a marine tether anchoring device. 
BACKGROUND TO THE INVENTION 
Tension leg marine platforms for use in the offshore oil industry are known 
and a connector apparatus for connecting such a tension leg to a subsea 
foundation is described in U.S. Pat. No. 4,320,993 (Conoco). An anchoring 
device that was used in such a structure in the Hutton field of the North 
Sea is described in U.S. Pat. No. Re. 32,274 of U.S. Pat. No. 4,459,933. 
The tether line described in the Reissue specification and used in practice 
employed a tether of relatively small diameter and having a relatively 
thick wall. It has now been realised that it may be desirable to use 
tendons of larger diameter and with thinner walls but it is difficult to 
design and make a transition section that will couple such a relatively 
large tendon to a relatively small flexjoint such as is described in the 
Reissue specification, particularly where high loads are to be employed. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a marine tether anchoring 
device that can be used with large diameter thin walled tendons and that 
avoids the need to locate the flexjoint that forms part of such a device 
around the tendon load path. 
Accordingly, the invention provides a marine tether anchoring device 
comprising in combination; a tubular anchor body for eventual attachment 
at its upper end below a marine tether; a spigot for eventual fixing to 
the sea bed so as to be upstanding therefrom and for reception in the 
anchor body to establish the anchor; latching means within the anchor body 
for establishing a releaseable connection to the spigot; and flexjoint 
means supported within the tubular body for transmitting tensile load in 
the anchor body as an inwardly and upwardly directed compressive load 
through the latching means to the spigot whilst permitting relative 
tilting movement between the latching means and the anchor body consequent 
upon sway of the tether. 
OUTLINE OF PREFERRED FEATURES 
The spigot may have a head formed with a downwardly facing conical catch 
face on which the latching means engages. Advantageously the latching 
means includes an axially movable support that is generally circular in 
plan, a plurality of latching arms depending from the support at angularly 
spaced intervals and having obliquely inturned downwardly facing tips of 
latching engagement on the catch face of the spigot at a lower position of 
the support, means for moving the support to an upper position, and means 
operable on movement of the support to the upper position to radially 
expand the tips of the latching arms away from latching engagement with 
the spigot. The latching means may include a body connected to the 
flexjoint means to maintain its axial position relative to the anchor body 
as the support moves, said body having first cam means that cooperates 
with the tips of the latching arms as the support moves towards its lower 
position to urge said tips radially inwards and second means cooperating 
with formations on inner faces of the latching arms as the support moves 
towards its upper position to urge said tips radially outwards. 
For effective reaction of downward loads during establishment of the latch 
connection, the latching means comprises an axially fixed body as 
aforesaid supported on the flexjoint means and movable members supported 
on the fixed body for establishing the releaseable connection to the 
spigot, and the anchoring device includes means for reacting a downward 
load from the anchor body via a load path that includes only the fixed 
body of said latching means. The fixed body may be generally bell shaped 
with a head of the spigot passing to the fixed body and supporting the 
fixed body at a top part of its undersurface during said downward load and 
with a bottom rim of said fixed body supported on said flexjoint means, 
frame means upstanding from said rim cooperating with an inturned part 
spherical downwardly facing surface of the anchor body to receive downward 
loads from said body. The upper part of the fixed body may be connected to 
the rim of the body via spaced fingers and the frame means has upstanding 
fingers aligned with the fingers of the fixed body, through which 
upstanding fingers the downward load is received. 
In such an arrangement the fingers and the latching arms may oocur in 
alternate positions about the axis of the anchor body.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
In the drawings, a tubular tendon 10 of a marine tether depends from a 
marine platform, is of diameter about 1300 cms and weight about 12,500 
kg/m. Its lower end is welded or otherwise attached to a tubular anchor 
body 12 of outside diameter about 1750 cms, slightly larger than that of 
tendon 10 and which is either formed in one piece or formed in three 
generally annular sections that are welded together as shown. The upper 
part of the body 12 is internally flanged at 13 and a bulkhead 15 is 
bolted to the underside of the flange 13 to exclude sea water from the 
bore of tendon 10. Alternatively the bulkhead 15 may be attached to the 
underside of flange 13 by welding. A flexjoint assembly 14 fits within the 
lower part of the body 12 and comprises a lower rigid annulus 16 and an 
upper rigid annulus 18 of smaller diameter and inwardly offset from the 
annulus 16, the annuli 16, 18 being interconnected by a flexible rubber 
and steel portion 20. The annuli 16, 18 are of generally triangular 
section and face oppositely as shown. The annulus 16 is a close fit in a 
cylindrical lower portion of the body 12 to which it is sealed by means of 
an annular seal 22. The annulus 16 rests on a segmented ring 24 that fits 
into a recess in the inner surface of the housing 12. A retaining ring 26 
is attached, e.g. by means of bolts 28, to the underside of the annulus 16 
and has a stepped top outside edge 30 to allow it to fit into the ring 24 
which is captive between annulus 16 and ring 26. The ring 26 is also a 
close fit in the lower portion of the body 12 and is sealed thereto by 
means of an annular seal 32. A further annular seal 34 fits between the 
annulus 16 and the ring 26, the seals 22, 32 and 34 serving to exclude sea 
water from the segmented ring 24. The inner surface of the ring 26 is 
tapered at 36 to provide an entry cone that guides a sea bed attached 
central spigot 38 into the anchor body 12 as the assembly is lowered to 
its intended final position. 
A generally bell-shaped member 40 has a solid upper part 42 connected to an 
annular lower part 44 by a plurality of angularly spaced fingers 46. A 
spigot 48 depending from the lower part 44 is a push fit in the upper 
annulus 18 of the flexjoint assembly 14. A frame structure 50 consisting 
of a multiplicity of upstanding axially spread fingers 52 (FIG. 3) united 
at their lower ends rests on the annulus 18, surrounds the upper part 42 
and fingers 46, and is located by upstanding rim flange 54 thereof, the 
angular position of the frame structure 50 being such that the fingers 52 
coincide with the fingers 46 of the member 40. The upper part 42 of the 
bell-shaped member 40 has a rod 56 connected thereto by means of bolts 58 
passing through its flanged lower end 60. A piston 62 is connected by 
bolts 64 to the upper end of the rod 56 and slides in a cylinder assembly 
66 having upper and lower outwardly directed flanges 68, 70, the lower 
flange 70 having upturned positions defining hooks as shown. A 
multiplicity of latch arms 72 depend from the cylinder assembly 66 with 
hooked upper ends 74 thereof located beneath the flange 68 on the upturned 
portions of the flange 70. The arms 72 fit between the fingers 46, 52 with 
inturned tips 76 thereof depending beneath the frame structure 50 and 
being directed parallel to an inclined seat face 77 of the lower part 44 
of the member 40. The latch arms 72 are formed on the inner surfaces with 
cam regions 79 that cooperate with an angulation 80 (FIG. 2) on the outer 
surface of bell-shaped member upper part 42 to urge the arms 72 outwardly 
as shown in FIG. 2 as the cylinder assembly 66 and arms 72 are moved 
upwardly relative to the member 40. The anchor body 12 is formed with a 
multiplicity of sight holes 86 which give access to portions of the arms 
72 to enable their angular position to be judged, e.g. by coincidence or 
otherwise of outwardly projecting lugs 88 with the outer surface of the 
body 12 to provide for visual confirmation of the presence or absence of a 
latching state. It will be noted that the frame 50 and arms 72 coincide 
with a portion of the housing 12 of enlarged internal diameter defining a 
cavity 90 providing for sway of the frame 50 and bell-shaped member 40 
within the housing 12 within an angular travel permitted by the flexjoint 
14. 
The tendon 10 can be unlatched from engagement with the spigot 38 by 
relieving the axial load in the tendon 10 and supplying fluid under 
pressure to the full bore side of the piston 62, causing the cylinder 
assembly 66 to move upwardly from the position shown in FIG. 1 to the 
position shown in FIG. 2 where it is spaced above the bell-shaped member 
upper part 42 and where the arms 72 are lifted from their tapered seatings 
78 and are forced outwards clear of the internal bore by cam region 79 and 
angulation 80 so that the tendon 10 and anchor body 12 may be withdrawn 
from the spigot 38. Latching is achieved by reversal of the above 
procedure and application of hydraulic pressure to the annulus side of the 
piston 62. Subsequent downward motion of the cylinder assembly 66 drives 
the arms 72 downwardly into re-engagement with the tapered seating 78 of 
bell member lower part 44 after which arms 72 are forced inwards to the 
position shown in FIG. 1. When the arms 72 are in the latching position 
the axial load in tendon 10 is applied, engaging the tips 76 of arms 72 
with a conical underface 95 of the mushroom-headed spigot 38 via which the 
load is transmitted. In the normal position with tension in the spigot 10 
the convex top face 97 of the spigot 38 is clear of the concave 
undersurface 99 of the member 40, but when tension is relieved the face 97 
serves to react the weight of the tendon 10 and anchor body 12 via the 
surface 99. This clearance allows for heave of the tendon 10 to be taken 
up during the period while the latch is being established and before the 
tendon 10 can be tensioned. 
The above marine tether anchoring device has the advantages that: 
(a) the load line through flexjoint assembly 14, bell-shaped member lower 
part 44 and latch arms tips 76 to conical face 95 of spigot 38 is 
compressive; 
(b) the aforesaid load line is extremely short; 
(c) the load line passes from the anchor body 12 to the spigot 38 through 
the tips 76 of the latch arms only. The remainder of the latch arms 72 and 
the latching and unlatching mechanism are not subject to load during 
normal service; 
(d) the load interfaces 78, 76, 95 between components 38, 72, 40 are 
conical or inclined, thus minimising the possibility of the latch sticking 
due to corrosion; 
(e) in normal service the rod 56 is retracted into the cylinder assembly 66 
and so is protected from the effects of sea water corrosion and silt 
deposits; 
(f) the latch mechanism, consisting as it does of a series of cones nesting 
from below, is to a high degree fail safe and defect tolerant; 
(g) the latch mechanism has a minimum of moving parts; 
(h) the latch is incapable of inadvertent release. To effect release 
firstly the tensile load in the tether 10 must be relieved and secondly 
hydraulic pressure must be applied from the platform to raise the cylinder 
assembly 66 to the release position of FIG. 2; 
(i) the split ring 24 is protected from sea water corrosion (which is 
promoted by the cyclically varying loads to which the anchorng device is 
subject) so that the flexjoint 14 and latch mechanism supported thereby 
may be removed for inspection; 
(j) the diameter of the flexjoint 14 is not dependent on the diameter of 
the tendon 10; 
(k) tendons of diameter above 1 meter and of relatively thin walls (about 
3-6 cms.) can be anchored via a flexjoint to the sea bed; 
(l) upturned fingers 52 serve to prevent tensile loading being applied to 
the flexjoint 14 during latching by abutment with a concave spherical face 
23 in the anchor body 12 over the designed degree of sway. The spherical 
face 23 is provided beneath a second inturned flange 21 of the anchor body 
12. There is therefore a path for reacting downward loads from the body 12 
via face 23 to fingers 52 to the bell member lower part 44 and thence via 
bell member upper part 42 and surfaces 97, 99 to the spigot 38; and 
(m) the piston 62 and cylinder assembly 66 not only move the locations from 
which the latch arms 72 are supported but also lock in a raised or lowered 
position. This is of advantage during the operation of mooring the 
platform. Once the platform is installed, the load across the tips 76 of 
the latching arms 72 holds them in place.