GB2156407A

GB2156407A - Marine risers

Info

Publication number: GB2156407A
Application number: GB08508300A
Authority: GB
Inventors: Minoo H Patel
Original assignee: University College London
Current assignee: University College London
Priority date: 1984-03-29
Filing date: 1985-03-29
Publication date: 1985-10-09
Also published as: GB8508300D0; GB2156407B; GB8408085D0

Abstract

The disclosure relates to a marine riser comprising a hollow tubular structure (1) for extending from a well head connector (3) in the sea bed to a vessel (4) on the sea surface (5). A tensioning arrangement (2) is provided on the vessel for supporting the riser at its upper end and in order to withstand peak bending moments the riser is constructed with increased wall thickness (7) at locations of high bending stress. <IMAGE>

Description

SPECIFICATION Marine risers This invention relates to marine risers and is concerned with arrangements for improving the operating efficiency of marine risers to reduce static and dynamic operating stresses in the riser structure.

A marine riser is a long slender steel pipe which connects a subsea well head to a surface vessel carrying out oil drilling or production activities. During drilling operations, the marine riser is the conduit through which the drill string (with a cutting bit at its lower end) is directed into the sea bed through the subsea well head. The bore of the drill pipe carries a lubricating 'mud' at high pressure to the drill bit with the annulus between the drill pipe and riser inner wall providing a return path for the low pressure mud and drill cuttings. For oil production operations, the marine riser is almost always made up of a number of bundled pipes to carry raw crude up to the platform for processing and to carry the treated crude oil back down to the well head and then to a subsea pipe line or tanker for export.

Since the marine riser is long and slender (up to 6000 ft long with diameters of 16 to 36 inches), it would buckle under its own weight without a system at its upper end to maintain it in tension. This tensioning system is also designed to take up the relative movement between the surface vessel heaving up and down in waves and the top end of the riser.

In certain circumstances, usually during the occurrence of a severe storm, the marine riser has to be disconnected from the bottom well head due either to the inability of the tensioner to meet a high stroke requirement or due to the fact that the vessel cannot maintain station above the well head due to high winds and/or currents. Following riser disconnection, the vessel operator has the choice of retrieving the riser assembly, one segment at a time or of riding out the storm with the riser hanging freely below the vessel without the bottom connected to the sea bed. The former may not be possible because of vessel movement in a severe storm. If the riser is being hung off, the tensioner is locked off and the riser upper end is regidly connected to the surface vessel by a mechanical gripper.

The riser assembly is loaded by several static and dynamic forces during its operation. Static forces include those due to riser self weight, the weight of contents, riser buoyancy, surface vessel offset from above the subsea well head and forces due to ocean currents. Dynamic loads on the riser are induced by surface vessel motions, ocean waves and inertia forces due to oscillations of the riser mass. The riser acts as a slender vertical vibrating tube and responds to these dynamic forces through a combination of several resonant frequencies and associated damping contributions. Riser inertia, damping and structural elastic as well as geometric stiffnesses all contribute to riser response and material stresses. Buoyancy module attachements on the riser affect the entire process through changes in the riser inertia and geometric stiffness.

This invention provides a marine riser comprising a hollow tubular structure for extending from the sea bed to a surface vessel or platform, the marine riser having a rigid connection for mounting on the sea bed and having a mass per unit length which varies between the sea bed and sea surface such that the mass per unit length is a maximum at the sea bed and sea surface and is a minimum generally midway between the sea bed and sea surface.

The invention also provides a marine riser comprising a hollow tubular structure for extending from the sea bed to a surface vessel or platform wherein the riser is a universally pivotable joint at the sea bed and wherein the riser is constructed with a varying bending stiffness by varying the mass per unit length from a minimum adjacent the sea bed to a maximum at the sea surface.

In either of the above arrangements the increased or reduced stiffness and mass per unit length are created by increasing or reducing the wall thickness of the tubular structure at said location or locations.

More specifically the internal dimensions of the riser may be substantially constant throughout its length and the external diameter of the riser is increased or reduced at the locations wherein increased or reduced bending stiffness and mass per unit length are required.

The following is a description of a specific embodiment of the invention, reference being made to the accompanying drawings, in which: Figure 1A is a diagrammatic view of a conventional marine riser in the latched condition.

Figure 1B shows the riser of Figure 1 unlatched.

Figure 2 shows graphically typical riser displacements and bending stresses.

Figure 3A shows a modified riser in accordance with the invention; Figure 3B shows a detail of the modified view; and Figures 4 and 5 show graphically the weight distribution for risers for deep and shallow water respectively.

Figure 1A illustrates a riser, 1, with a tensioner, 2, at the upper end on a surface vessel and a subsea well head, 3 at the lower end. 4 denotes the surface vessel deck and 5 is the water surface. The tensioner, 2, takes up the relative vertical movement between the riser, 1, and the surface vessel 4. For operations in deep water, the tensioning force provided by the surface vessel is supplemented by buoyancy modules, 7, mounted along all or part of the riser length. The riser may be up to 7000 feet along with a diameter of 16 to 36 inches and would buckle under its own weight without the tensioning system.

In Figure 1 B the riser is shown disconnected from the well head (because the surface vessel has not been able to maintain station or the riser is being retrieved and in that condition the tensioner is locked off and the upper end of the riser is regidly connected to the vessel by a mechanical gripper.

The marine riser responds to static and dynamic environmental and surface vessel induced loadings by undergoing displacements in a direction perpendicular to its undisturbed length. The magnitude of these displacements is dependent on riser tension distribution, its self weight and the tube bending stiffness.

Of these, the tension distribution is also dependent on riser self weight, the top tension and the presence of boyancy modules. Lateral forces on the riser are proportional to its projected area and its immersed volume. The presence of buoyancy modules increases lateral forces due to the increased diameter, whilst having a beneficial effect on the tension distribution. The mass per unit length of the riser and its associated added mass due to acceleration of the surrounding fluid governs resonant frequencies and the dynamic response through the mechanism of inertia forces.

Current practice in riser design and operation is to use constant cross-section circular steel tubulars down the length of the riser with hydraulic or mechanical connectors at intervals. These connectors are used to split the riser into segments before installation and after retrieval of the assembly. The steel tubulars may have a concentric outer tube of buoyancy material in cases where this is required. Figures 1A and 1B illustrate a typical riser assembly although the intermediate connector details are not shown.

Risers of constant cross-section, as described above, have typical displacement and bending stress behaviour as shown in Figures 2A and 2B respectively. These curves present the mean displacement and bending stress of the riser under static loads as well as the upper and lower limit of oscillating displacements and bending stresses induced by surface vessel surge and wave action. For the bending stress curves, note the peaks in stress at two points on the riser near the subsea well head and still water level.

These stress peaks can cause riser failure due to overstressing. Furthermore, the use of risers as parts of a long term floating oil production system can lead to fatigue failures at these areas of high stress.

As described earlier, dynamic bending moments and stresses in a marine riser can be related back to the physical mass, stiffness and volume properties of the tubulars making up the riser. Figures 2 show how the displacement and stress behaviour of the riser varies substantially along its length. For constant cross-sections, the designer does not have much latitude to optimise the structure for fulfilling its function with minimum stresses.

However, for a known set of environmental conditions, a riser configuration of variable cross-section can be designed such that stresses within the riser are reduced by amounts that significantly enhance operability. Figure 3A shows the general configuration of a riser of variable cross-section where properties of the cross-section such as tubular wall thickness, outside diameter, buoyancy module diameter, buoyancy material density and so on vary with length along the riser. Practical constraints make it worthwhile to alter the cross-section at the intermediate connectors such that the riser can be installed from the surface vessel by stringing segments of known properties in a known order. Figure 3A shows a typical design. Items 1 to 5 are as defined by Figure 1A.Riser sections 8 and 9 are large diameter tubular segments disposed along the riser in such a distribution as to reduce the bending moment peaks as shown in Figure 2B. Riser design methods show that an optimum variation of riser cross-section is always available to reduce dynamic stresses and thereby improve the design.

Figure 3B shows a typical detailed side view close to a riser connector, 10. The cross-section of one part of the riser is denoted by 11. In varying the cross-sectional geometry of the adjacent segment, 12, note that the internal riser diameter is kept constant with the wall thickness (denoted by 13) being varied in association with the outer diameter (as shown by 14) of the primary steel tubular. Additional variability in the cross-section can be introduced through a buoyancy module denoted by 15 with its external diameter and material density being variable. Note that the material density may be set at above that of water to introduce additional mass per unit length to the system.

The inner riser diameter is kept constant so as not to affect riser functionallity for running drill pipe or having unobstructed internal flow. In certain circumstances, the intermediate riser segments could be of smaller outer diameter than the rest of the riser to reduce bending stiffness and mass per unit length in these locations, For a variable cross-section riser configuration as shown in Figure 3A, typical improvements in bending stress levels are shown by the envelope of oscillating stresses in Figure 2B.

Potential stress reductions due to using risers of variable cross-section have been researched at University College London using both computer simulations and confirmatory model tests. These show that substantial stress reductions are possible to an extent that is commercially significant. The stress reductions are such as to reduce the likelihood of riser failure due to overstressing. They also have the additional effect of increasing riser fatigue lives.

However, the precise definition of the optimum variable cross-section is dependent on environmental conditions, water depth and surface vessel motion characteristics.

Figure 4 of the drawings illustrates the mass distribution over the length of a riser for deep water. (that is more than about 500ft) installation, the riser having a rigid connection to a sea-bed mounting. The riser is constructed to that its mass per unit length is a maximum at the sea bed and sea surface and a minimum half-way between.

The mass per unit length at any particular point may be calculated in accordance with the following formula:

where m1 - mass per unit length at extremeties m2 - mass per unit length at centre L - riser length Figure 5 of the drawings illustrates the corresponding mass distribution for a shallow water riser (that is up to about 500ft) having a lower ball joint providing free pivotting on the sea bed. The mass per unit length is a minimum at the sea bed and a maximum at the sea surface and conforms to the following formula: Mass distribution

where m3 - mass per unit length at upper end m4 - mass per unit length at lower end L - ruser length

Claims

1. A marine riser comprising of hollow tubular structure for extending from the sea bed to a surface vessel or platform, the marine riser having a rigid connection for mounting on the sea bed and having a mass per unit length with varies between the sea bed and sea surface such that the mass per unit length is a maximum at the seabed and sea surface and is a minimum generally mid way between the sea bed and sea surface.

2. A marine riser as claimed in claim 1 wherein the mass distribution of the riser varies in accordance with the following formula:

wnere m, is mass per unit length at the riser extremities m2 is mass per unit length at the riser centre L is riser length.

3. A marine riser comprising a hollow tubular structure for extending from the sea bed to a surface vessel or platform wherein the riser a universally pivotable joint at the sea bed and wherein the riser is constructed with a varying bending stiffness by varying the mass per unit length from a minimum adjacent the sea bed to a maximum at the sea surface.

4. A marine riser claimed in claim 3 wherein the mass per unit length of the riser varies in accordance with the following formulae:

where m3 is mass per unit length at upper end of the riser m4 is mass per unit length at lower end of the riser L is riser length.

5. A marine riser as claimed in any of the preceding claims wherein the increased or reduced stiffness and mass per unit length are created by increasing or reducing the wall thickness of the tubular structure at said location or locations.

6. A marine riser as claimed in claim 5 wherein the internal dimension of the riser is substantially constant throughout its length and the external diameter of the riser is increase or reduced at the locations wherein increased or reduced bending stiffness and mass per unit lengths are required.

7. A marine riser as claimed in any of the preceding claims wherein the tubular structure comprises a number of segments connected together end to end to extend from the sea bed to the surface vessel or platform, the elements at the locations where increased or reduced bending stiffness and mass per unit length are required having an increased or reduced external dimension compared with the remainder of the segments.

8. A marine riser substantially as described with reference to and as illustrated in the accompanying drawings.