GB2492109A

GB2492109A - A riser bend stiffener

Info

Publication number: GB2492109A
Application number: GB1110524.4A
Authority: GB
Inventors: Jonathan Jury
Original assignee: RISERTEC Ltd
Current assignee: RISERTEC Ltd
Priority date: 2011-06-22
Filing date: 2011-06-22
Publication date: 2012-12-26
Also published as: GB201110524D0

Abstract

A bend stiffener, possibly for a riser, includes two identical but oppositely handed parts 12, 13. Each part 12, 13 may be of a tapered cross section and the two parts 12, 13 may be held together by straps 11a-11f disposed in recesses. Split rings 11a-11h may be used to transfer shear loads between the two parts 12, 13.

Description

PURPOSE DESIGNED TWO PIECE (SPLIT) DYNAMIC BEND

STIFFENER DESIGN

This invention relates to a riser ancillary product used to prevent overbending of the riser at the connection to a rigid body such as a floating production, storage and offtake vessel or at other locations on the riser where prevention of overbending of the riser is desired. The system is particularly, but not exclusively applicable to the flexible risers and umbilicals used in floating production systems for the transport of oil, gas and other high pressure gases and fluids as well as electrical power and control signals.

Flexible risers provide a recognised means of transporting well fluids and other high pressure gases and fluids to and from subsea wells and through the water column whilst maintaining a compliant system which can absorb vessel motions. At various positions along the riser but particularly at the entry to the production vessel bend stiffeners have to be supplied to prevent overbending of the riser. Typically a dynamic bend stiffener is provided at these locations. The dynamic bend stiffener has a designed gradual stiffness profile which maintains the riser at a curvature less than its design allowable. The bend stiffener is a precast single piece of polyurethane with a steel insert for connection and is compliant with the curvature of the riser.

The bend stiffener is necessarily a close fit with the riser and is slipped over the riser before the end termination (end filling) is applied to the riser. The design and manufacture of the single-piece bend stiffener needs to be completed in advance of the manufacture of the riser. In addition once the single-piece bend stiffener is fitted it cannot be replaced without retermination of the riser, a substantial task.

Recently there has been a move to exploiting more marginal fields by the use of a FPSO vessel. Commonly, the risers will only be used for a short period and could potentially be used on another field but this may require a different design of bend stiffener for different host or environment. In addition there has been a substantial numbers of bend stiffeners which have failed in-service. These failed stiffeners cannot be repaired without removal of the riser from the riser system and retermination of the riser end to allow removal and replacement of the failed stiffener.

The single-piece bend stiffener in some riser systems can be over 6m long and cause considerable problems in loading of the completed flexible riser onto reels either for transport or installation. In addition transfer of the single-piece bend stiffener through the lay systems prior to installation can be a time consuming and difficult process.

An object of embodiments in accordance with the present invention is to provide an improved two-piece dynamic bend stiffener which is specifically designed to behave as well if not better than existing single piece designs, which is quickly installed and removed and which can be retrofitted underwater if required. It is also an object of embodiments in accordance with the invention to provide a system which requires the minimum use of complicated bolting and connector systems.

Accordingly, the present invention provides a two-piece bend stiffener system comprising: means for effectively limiting the curvature in the riser to below the allowable values whilst allowing bending and deflection of the riser; and constructed of two separate parts connected by strapping and with moulded rings and grooves for ensuring the composite action of the combined section; integral to the moulded parts is a split steel insert for interface with the load bearing structure; characterised in that regularly spaced rings of a high modulus polyurethane or metallic material are provided which act as shear connectors for effectively transferring static and dynamic loading between the two halves of the two-piece bend stiffener under load; wherein the regularly spaced rings are removable and are inserted into specially designed recesses constructed at regular intervals along the length of the inner diameter of the two-piece bend stiffener during assembly; the rings can either be of a single or multiple piece construction; wherein regularly spaced strapping coinciding with the regularly spaced rings which holds the two halves together through the life of the system; wherein the upper section of the bend stiffener two halves are connected using bolts inserted with load spreading steel inserts to prevent the upper part of the two halves from bursting apart under load; wherein a steel ring interface plate to connect the two halves of the two-piece bend stiffener together at the root of the stiffener Preferably, the strapping joining the two halves together are encapsulated within moulded paths within the polyurethane and are coincident with the internal rings which transfer loading between the two halves.

Preferably the internal rings are also formed of a polyurethane material of higher strength than the body but could also be of a non-corrosive material Preferably, the two halves have a simple tongue and groove type interlock between the two halves to prevent the two halves from bursting apart under radial loads Optionally, the banding for maintaining a composite action between the two halves could be external to the split dynamic bend stiffener held in position in moulded grooves.

Preferably, the system is capable of operating in all water depths, and have a design life in excess of the normal design life for the riser.

Optionally, the two piece dynamic bend stiffener can be installed underwater by divers where for example a repair is being made to an existing failed dynamic bend stiffener without the need to disconnect the riser from the host.

Typically the two halves of the split bend stiffener are formed from a polyurethane material with a modulus of around 7OMPa and can be from 2m to 9m long Typically the strapping is made from either polyurethane or a non corrosive material and is designed for the life of the bend stiffener Typically the bolts are also made from a non-corrosive material and use inserts in the polyurethane to spread the required pretension and shear An embodiment of the invention will now be described! by way of example only, with reference to the drawings, in which: Fig. I is a schematic three dimensional view of an embodiment of a two piece dynamic bend stiffener in accordance with the present invention, in use with a flexible riser; Fig. 2 is an exploded view corresponding to Fig. 1; Fig. 3 is a schematic three dimensional view of the ring shear connector at a particular section of Fig. 1; Fig. 4 is a detail of the interlock between the two halves which prevents bursting between the two halves corresponding to the embodiment of Fig. 1; Fig. 5 is a detail of the strap and its moulded path Referring particularly to Fig. 1, a two piece dynamic bend stiffener is attached to a dynamic riser. The two halves 12 and 13 each have a steel insert 5, 6 included in the moulded part for attachment to interface plates 7, 8 which are the primary connection to the support structure. Referring particularly to Fig. 2 for assembly the first half of the moulding is held in contact with the riser and the shear rings lOa, lOb, lOc, lOd, lOe, lOf, lOg, lOh are inserted as shown in the detail in Figure 3. The second half of the moulding is then placed in position to be connected with the interlock as shown in Figure 4. Initially the two halves are held in position by means of bolts 1 a,i b, 2a, 2b, 3a, 3b, 4a, 4b in the upper thicker section of the moulding.

The straps Ha, lib, lic, lid, lie, hf, hg are nowinserted into the pre-moulded paths as shown in Figure 5. Once the straps are tightened to the required tension the two parts of the bend stiffener behave as a composite body.

As seen in Fig.i and Fig. 2, the shear rings are placed at hO approximately 300mm spacing along the bend stiffener. The shear rings can be constructed of a whole or part of a circular disc.

The invention provides a method for maintaining curvature control for risers and umbilicals through the composite action of the two halves by shear transfer through the upper bolts (1, 2, 3, 4) but particularly the shear rings (I Oa to I Oh). The strapping prevents any loss of contact between the rings (ba to lOh) and the two halves of the moulding. The split nature of the system allows for attachment at any stage post manufacture of the riser thus giving projects the benefits mentioned.

Claims

Claims 1. A bend stiffener comprising of two identical but oppositely handed parts.
2. A bend stiffener as claimed in claim 1, wherein each part is of a tapered cross section.
3. A bend stiffener as claimed in claim I or claim 2, with a through bore of near constant diameter.
4. A bend stiffener as claimed in claim 3, wherein regularly spaced grooves are provided recessed in from the bore regularly along the length of the split bend stiffener.
5. A bend stiffener as claimed in claim 4, wherein split rings of a near square cross section fit into the provided grooves and span on both sides of the split of the bend stiffener.
6. A bend stiffener as claimed in claim 5, wherein separate split ring components located in grooves are provided to transfer shear loads between the two halves.
7. A bend stiffener as claimed in claim 6, wherein straps moulded into recesses in the two halves are coincident with the ring positions and act to prevent separation of the two halves and load the rings.
8. A bend stiffener as claimed in any preceding claim, wherein the two separate components are tapered equally.
9. A bend stiffener as claimed in any preceding claim wherein the two components can additionally be bolted through recessed holes in the body.
10. A bend stiffener as claimed in any preceding claim, wherein the two separate stiffener components have the steel inserts joined through a steel interface plate.