NO345972B1

NO345972B1 - A wellhead system

Info

Publication number: NO345972B1
Application number: NO20191105A
Authority: NO
Inventors: Bjørn Pettersen
Original assignee: Subseadesign As
Priority date: 2019-09-13
Filing date: 2019-09-13
Publication date: 2021-11-29
Also published as: NO20191105A1

Description

Field of the Invention

The present invention relates to a wellhead system for subsea hydrocarbon wells.

Background

During offshore drilling operations, the subsea wellhead, and other critical parts of the well system, is subject to static and cyclic bending moments from riser loads acting in the flex joint at the top of the BOP/LRMP.

Existing WLR technology uses the principle of reducing the wellhead bending moment by various support arrangements, such as tether systems, frames connected to the BOP, wellhead load arrestor with template/seabed supports etc.

WO 2011/008834 A2 describes a method for drilling a well wherein the riser is disconnected from the well to prevent moments in the riser when the drilling operation is completed and during production. The riser is secured with a flexible coupling to an anchor. Then the riser is connected to the well via a flexible tube. During drilling there is only a common mechanical connection between the riser and well.

Other examples of prior art technique in this field are known from US 2016/0186517 A1, US 4662785 A, NO 20150446 A1 and NO 331285 B1.

Summary of the invention

The object of the present invention is to provide a wellhead system eliminating bending moments to prevent fatigue damage to the wellhead system for satellite or template subsea wells during drilling and intervention to the wells. In particular it is an object to avoid that wellhead bending moments are created from the shear in the flex joint. These objects are achieved in an invention as disclosed in the appended claims.

Brief Description of the Drawings

Further features and advantages of the invention will appear from the following detailed description with reference to the drawings, in which:

Fig. 1 is a schematic drawing of a wellhead system according to the present invention,

Fig. 2 is a diagram showing the typical critical bending moment range that cause fatigue damage,

Fig. 3 is a schematic diagram showing the bending moments affecting the joint between wellhead and casing in a conventional wellhead system, and

Fig. 4 is a corresponding diagram showing the bending moments affecting the joint between wellhead and casing in a wellhead system according to the invention.

Detailed Description

Fig. 1 is showing a wellhead system which includes a wellhead connector 1 at the bottom. The wellhead connector is normally welded to the well casing and acting as a connector to the Blowout Preventer stack 3 above. Above the BOP there is a Lower Marine Riser Package 4, a flex joint 5 and a riser stress joint 6 connecting the wellhead system to the riser. According to the invention a Bending Moment Avoidance System 2 is introduced between the wellhead connector 1 and the BOP 3. The BMAU is in principle an angular slipping device with metal sealing feature. The device will allow for the necessary rotation to avoid critical bending moments to be created and transferred into the well system. The device will be tuned to slip and allow rotation in the bending moment ranges that typically cause the fatigue accumulation, see Fig. 2 for typical range of such bending moments. The BMAU may be based on the SeAlign misalignment connector marketed by the present applicant, and which is described in Norwegian patent no. 331285, even though other joints or connectors may be applicable.

Fig. 2 shows the typical critical bending moment range that cause fatigue damage. This is an example providing typical values for an installation with a BOP weighting 200 metric tons supporting a 400 meter riser thereon.

Fig. 3 shows a conventional wellhead system in which the tension on the riser is the origin of shear forces acting on the wellhead. Due to the long moment arm from the top of the wellhead system to the joint between the casing and wellhead, large moments as illustrated in the right hand figure will occur on this joint. These moments, with the typical values illustrated in Fig. 2, may cause fatigue damage to the joint.

Fig. 4 shows how the effect of the invention in limiting the moments acting on the joint between wellhead and riser. As shown on the left hand part of the figure, the introduction of the BMAU 2 allows the upper part of the wellhead system with BOP to lean over and avoid bending forces to be transferred from the riser to the joint in question. Here, a support ring 7 prevents the BOP from leaning too far. As shown in the right hand part of the figure, the introduction of the BMAU 2 results in a substantial reduction in the moments acting on the joint. The reduction in moment is due to the shortened moment arm.

It may also be incorporated a support system that prevent the BOP to turn over due to the shear, weight and bending moments above the critical range. This support system will also be tuned to start the support after the highest critical bending moment. It will also support the BOP in the higher wellhead bending moment ranges and during high accidental riser loads. It will also incorporate a support system that prevents the BOP to turn over due to the shear, weight and bending moments above the critical range. This support system will also be tuned to start the support after the highest critical bending moment. It will also support the BOP in the higher wellhead bending moment ranges and during high accidental riser loads (these additional components are not shown in Fig. 1). The support system will transfer loads into the well support structure and/or back into the BOP structure depending on level of support needed.

The BM Avoidance System can be used for wellhead load relief systems on any well and BOP.

It should be noted that this invention is based on avoiding bending moments into the well system, while other systems are based on limiting bending moments into the well system.

Claims

1. A wellhead system including a riser, Blow Out Preventer (3) and wellhead connector (1) which is fastened to a casing,

characterized in that the system further includes a Bending Moment Avoidance Unit (2) which is placed close to or integrated into the wellhead connector (1) and allow for controlled rotation/slippage between the wellhead connector (1) and the BOP (3).

2. A wellhead system according to claim 1, further including a support ring (7) mounted on the wellhead connector (1) for limiting the range of rotation of the BOP (3).

3. A wellhead system according to claim 1, wherein the BMAU (2) is an angular slipping device with metal sealing feature.