Subsea protective housing

Abstract: A wellhead protector comprises a metal framework for placement about a wellhead, which has four legs, each of which is anchored by a pile driven through the lower end of the leg into the seabed, the structure being characterised in that it has no perimeter tube.

Description: This invention relates to subsea protective housings of which a wellhead protector is an example. It is common practice, in the oil industry, to protect subsea wellheads and manifolds which may be damaged by fishing tackle from boats working in the area. This may be achieved by erecting a metal framework about the wellhead. This subsea protection structure is stabilized in two ways: firstly by the use of a perimeter tube which, as its name suggests, runs around the perimeter of the structure in its position on the sea bed to prevent trawl lines snagging under the structure, and secondly, by the use of piles which are driven down through the metal framework of the structure to anchor it in place. The use of this method has certain disadvantages. Because piles are often positioned through the centre of the structure they constrict the space available for manoeuvring flowlines, umbilicals and the like from the wellhead. In addition, the perimeter tube provides further obstruction to setting up the flow lines, as a passage must be formed through the perimeter tube to allow the lines to follow the prescribed course. This passage must be created before the perimeter tube is set out on the sea bed, and may be wrongly placed for the actual position of the flowlines. Substantial time may be spent in obtaining the correct alignment. If no passages are provided the perimeter tube may be placed directly on top of the flow lines which may crush them and could be dangerous. By driving piles outside the corners of the structure so that the extremities of the structure are firmly keyed into the sea bed, the perimeter tube is no longer required. This may be achieved by using a new type of piling structure - Anchortech's Crooklatch System - which is at present the subject of a co-pending Application No 88307661.4. The Crooklatch system has its connector (which joins the pile to the structure leg) on the inside of the pile instead of the outside, as in conventional systems, and so there is no snagging surface. In accordance with the invention there is provided a subsea protective structure for placement about a subsea installation, said structure comprising a framework including a plurality of support members each having a lower end adapted, in use, to engage the seabed at a plurality of locations spaced around the installation, there being no perimeter tube interconnecting the lower ends of the support members (as is known, for example, from US-A-3063500); characterized in that the lower ends of said support members are adapted to be engaged by piling means whereby the structure may be secured to the seabed, and in that each of said support members has an upper end connected to an upper frame assembly supported above the installation by said support members, said support members extending outwardly and downwardly from said upper frame assembly. EP-A-0169653 discloses a wellhead protector comprising a rectangular perimeter tube having four protective tubes extending upwardly and inwardly from the corners of the perimeter tube to meet at a central apex, the conductor pipe being enclosed in a central tube extending vertically through said apex, and the perimeter tube being piled at its corners to secure the structure to the sea bed. Preferably, said upper frame assembly is substantially rectangular in plan view, and wherein the upper ends of said support members are connected to the corners thereof. Preferably also, each of said support members has associated therewith first and second diagonal bracing struts extending between the members and respective adjacent sides of said frame assembly. Preferably also, the piling means are piles which are triangular in plan. Preferably also, the piles include connecting means for connecting the piles to the structure in the form of a connector on the inside of the pile relative to the structure. Preferably also, a penetration of pile into the seabed of greater than 2 metres in depth and 1 metre in width is effected. Most preferably, a penetration of pile into the seabed of 4 metres in depth and 2 metres in width is effected. An embodiment of the invention will now be described by way of example with reference to the drawings in which: Fig. 1 is a side view of a subsea protective structure according to the present invention; and Fig. 2 is a plan view of the subsea anchoring system of Fig. 1. A wellhead protector 1, embodying the subsea protective structure in accordance with the invention, comprises a metal framework 2 which is fitted over a wellhead 3. The framework 2 has four legs each of which is anchored by an Anchortech Crooklatch pile 4 driven through the lower end of the leg into the seabed. In previously proposed arrangements, piles were driven through the crashrail at 5. Dotted lines 6 show the position of a perimeter tube which was present in comparable conventional arrangements but which is omitted from the present embodiment of the invention. The corner position of the Anchortech piles 4 gives a clearance above the wellhead 3 not available in previously-proposed systems. The piles 4 themselves are designed so as to avoid having projections which may snag fishing tackle. Since the structure now rests on individual legs at sea bed level, rather than the perimeter tube, damage to the sea bed flowlines during structure placement is virtually eliminated. Thus the presence of a perimeter tube is unnecessary and its absence confers the advantages of easier installation and a reduction in weight of the structure.

Claims: A subsea protective structure (1) for placement about a subsea installation, said structure (1) comprising a framework (2) including a plurality of support members each having a lower end adapted, in use, to engage the seabed at a plurality of locations spaced around the installation, there being no perimeter tube interconnecting the lower ends of the support members; characterized in that the lower ends of said support members are adapted to be engaged by piling means (4) whereby the structure (1) may be secured to the seabed and in that each of said support members has an upper end connected to an upper frame assembly supported above the installation by said support members, said support members extending outwardly and downwardly from said upper frame assembly. A subsea protective structure as claimed in Claim 1, wherein said upper frame assembly is substantially rectangular in plan view, and wherein the upper ends of said support members are connected to the corners thereof. A subsea protective structure as claimed in Claim 2, wherein each of said support members has associated therewith first and second diagonal bracing struts extending between the members and respective adjacent sides of said frame assembly. A subsea protective structure as claimed in Claim 1, Claim 2 or Claim 3, wherein the piling means (4) are piles which are triangular in plan. A subsea protective structure as claimed in Claim 4, wherein the piles (4) include connecting means for connecting the piles to the structure (1) in the form of a connector on the inside of the pile relative to the structure (1). A subsea protective structure as claimed in Claim 4 or Claim 5, wherein a penetration of pile into the seabed of greater than 2 metres in depth and 1 metre in width is effected. A subsea protective structure as claimed in Claim 6, wherein a penetration of pile into the seabed of 4 metres in depth and 2 metres in width is effected.

Inventors: MARTIN ROBERT ESQ, MARTIN, ROBERT, ESQ.
Applicants: ANCHORTECH LTD, ANCHORTECH LIMITED
Application year: 1990
Main classification: E21B33
IPC codes: E21B33, E02D27