NO20201051A1

NO20201051A1 - A weak link

Info

Publication number: NO20201051A1
Application number: NO20201051A
Authority: NO
Inventors: Bjørn Pettersen
Original assignee: Subseadesign As
Priority date: 2020-09-24
Filing date: 2020-09-24
Publication date: 2022-03-25
Also published as: NO346846B1

Description

Field of Invention

The present invention regards a safety arrangement for protecting subsea installations or the like.

Background

During well intervention (WI) operations, such as well stimulation, a well stimulation vessel, a mobile offshore unit (MOU), is directly connected to a subsea installation with a work string, a tubing string. Such work strings has high tension capacity and in a situation where an MOU unintentionally drift away from its WI operations area, the work string will act as a tether between the subsea installation and MOU, which could inflict damaging pulling force on the subsea installation, resulting in irreversible damage on the later.

Repairs on such subsea installations are not only costly and time consuming, the downtime of a damaged subsea well subject oil companies ongoing operational expenditures and lost revenue.

A novel technical solution is needed wherein irreversible damage on a subsea installation is avoided during WI operations, should an MOU unintentionally drift away from its WI operations area.

Summary of the Invention

The object of the present invention is to provide a safety arrangement where an MOU during WI operations drifts away from its operational area, the MOUs work string appropriately break off its connection to the subsea installation without causing significant damage to equipment within the subsea installation.

Said object is achieved in a safety arrangment as defined in the appended claims.

Brief Description of Drawings

Certain features and functions of the present invention will be described with reference to the following appended figures in which:

Fig. 1 is an overview illustration of a well operation,

Fig. 2 is a cross section through the safety arrangement’s assembly,

Fig. 3 is an illustration detailing the end section of the work string and the break rings,

Fig. 4 is an illustration detailing a subsea installation connected to a WI work string with annular shaped stiffeners.

Detailed Description of the Invention

Fig. 1 illustrates a typical offshore well intervention operation intended to improve the yield from a subsea hydrocarbon reservoir, where a work string 1 from an MOU 2, fi. a well stimulation vessel, is connected to the mentioned reservoir in the form of a subsea installation 3. A subsea installation 3 comprise of the following subsea equipment, but not limited to, a X-mas tree, a manifold, connected jumper spools, a template 4, which is a subsea superstructure designed to protect the mentioned subsea equipment from bottom trawling and dropped objects, and a attachment device 5 installed on the template 4, acting as a connecting interface between the work string 1 from the MOU 2 and the subsea installation 3.

However, the interconnected equipment in the subsea installation 3, by spools and work strings 4, is exposed to potential damage from the MOU 2 should it go adrift uncontrollably and without electrical power. The displacement of an adrift MOU 2 connected to a subsea installation 3 is more than enough force to cause significant damage to the latter, especially that which connects the subsea installation 3 and the MOU 2 is a high tension capacity work string 1 designed to work in harsh offshore conditions. The work string 1 is usually a coiled tubing aboard an MOU 2, not necessarily arranged to break off the work string 1 on the vessel side, which would require unconventional solutions to retrieve abandoned work strings 1 connected to a closed subsea installation 3, further resulting in a longer shutdown period for a subsea well.

Conversely, a work string 1 connection termination occuring on side of the subsea installation 3 is a preferable option with regards to the aboved mentioned alternative. In a situation where the crew cannot control the MOU movement, nor control WI operations, it would be suitable to have a mechanical safety arrangement where the end section 1b of a work string 1, connected to a subsea installation 3, that would break off from the subsea installation 3 when a specified level of tension stress on the work string 1 is exceeded.

Fig. 2 illustrates a cross section of the safety arrangement according to the present invention connecting an end section 1a of a work string 1 and a subsea installation 3. The end section 1a has a tapered section 1b, wherein the outer diameter is tapered to an arranged wall thickness which ruptures across the tapered section 1b when a maximum bending stress of the tapered section 1b is exceeded due to increased tension stress on the work string 1 from unintended movements from the mobile offshore unit 2. Additional characteristics of the end section 1a is an upper break ring 6a surrounding the end section 1a on the upper side of the tapered section 1b and lower break ring 6b surrounding the lower side of the tapered section 1b. Each of the upper and lower break rings 6a, b comprise contact surfaces, wherein one of the contact surfaces is flat and the other is rounded, and wherein the contact surfaces are faced towards each other and are spaced apart at a defined distance. Furthermore, each of the upper and lower break rings 6a, b are attached to the end section 1a on integrated upper and lower flanges of the end section 1a.

Fig. 3 illustrates the end section 1a and the attached break rings 6a, b, showcasing the respective contact surfaces, wherein one is flat and the other is rounded, facing each other at a defined distance arranged to allow the rounded surface to roll on the flat surface. This arrangement facilitates to increase the bending stress on the tapered section 1b deriving from an increase in tension stress on the work string 1, fi. when the work string 1 is being pulled on the other end.

Once the deformation of the tapered section 1b is such that it is in contact with the inside part of the break rings, the compression part of the tapered section will change to tension meaning that the bending movement will concentrate onto the contact point between the tapered section and the break rings. This will create a clean and predictable break.

Fig. 4 illustrates the subsea installation 3 and its attachment device 5 connected to the end section 1a to the work string 1, wherein the work string 1 have attached a set of annular shaped stiffeners 7 surrounding the work string 1. These annular shaped stiffeners 7 will act as a lever on the end section 1a, when the work string 1 is being pulled the MOU 2, further amplifying the increase of bending stress on the tapered section 1b together with the mentioned break rings 6a, b. The stiffeners 7 are shaped as spaced washers connected to the work string. When the work string is bent, the outer parts of adjacent stiffeners will come in contact and resist further bending of the work string.

The end section may be an integrated part of the work string, or optionally a separate entity that is connected between the subsea installation 3 and the work string.

Claims

1. Safety arrangement comprising an end section (1a) and a work string (1) from an mobile offshore unit (2), wherein the end section (1a) is connected to a subsea installation (3), wherein the subsea installation (3) comprise of:

a template (4), an attachment device (5) on the template (4), wherein the end section (1a) is fixed to the mentioned attachment device (5),

characterized in that

the end section (1a) to the work string (1) has a section (1b), wherein the outer diameter is tapered, wherein the tapered section (1b) is arranged to rupture across the tapered section (1b) when a maximum bending stress of the tapered section (1b) is exceeded due to increased tension stress on the work string (1) from unintended movements from the mobile offshore unit (2).

2. Safety arrangement according to claim 1, further comprise an upper break ring (6a) surrounding the end section (1a) on the upper side of the tapered section (1b) and a lower break ring (6b) surrounding the lower side of the tapered section (1b), wherein each of the upper and lower break rings (6a, 6b) comprise contact surfaces, wherein one of the contact surfaces is flat and the other is rounded, and wherein the contact surfaces are faced towards each other and are spaced apart at a defined distance.

3. Safety arrangement according to claim 1 or 2, further comprise annular shaped stiffeners (7) surrounding the work string (1) adjacent the end section (1a).

4. Safety arrangement according to any of the claims 1, 2 or 3, wherein the work string (1) and the end section (1a) is one entity.

5. Safety arrangement according to any of the claims 1, 2 or 3, wherein the end section (1a) is a seperate entity that is mechanically connected to the work string (1).