<I>Teredo</I> Worm and Subsea Umbilicals

doi:10.3723/ut.33.239 Underwater Technology, Vol. 33, No. 4, pp. 239–243, 2016 www.sut.org The Teredo worm and subsea umbilicals: risks and recommendations Larry Parkes* and Alan Keeble Technical Briefing Technical Received 19 March 2015; Accepted 23 March 2016 Abstract Xylophagainae family are widely dispersed, being The Teredo worm is a bivalve mollusc that is well known for present in every saline ocean basin so far studied. its destructive activity caused by boring into woods under (The Xylophagainae form the deep-sea and high- water. It can also damage subsea cables and many cable latitude ecological replacements of teredinids designs incorporate anti-Teredo protection. The Teredo worm (Voight, 2007).) Currently, the only seas thought is an alien species to many marine environments, but its dis- to be free of wood-borers are the Baltic and Black tribution is increasing. It is, therefore, possible that subsea Seas, where well-preserved shipwrecks have been umbilical failures, specifically in relation to power and/or com- found (Glover et al., 2013). However, it is probable munication transmission in the oil and gas industry, that have that their distribution is increasing; both families previously been fully or partially attributed to Teredo activity, are highly successful alien species with life histo- may increase in the future. This paper reviews some methods ries and responses to waterborne cues that support of detecting impending Teredo-related umbilical failure and movements into new areas (Borges et al., 2014; makes recommendations for future subsea umbilical design. Toth et al., 2015). Protection from the effects of Teredo attacks has Keywords: Teredo worm, subsea umbilicals, design recommendations historically involved copper, initially as sheathing over the wooden hulls of ships, where its physical barrier to biomechanical activity was also associated 1. Introduction with anti-fouling properties (Anderson et al., 2003). The bivalve Teredo, commonly known as the ship- However, the Teredo worm penetrated the copper worm although it is a mollusc and not a true worm, sheathing at the holes made for the iron fastenings is perhaps the most well-known wood boring organ- that would corrode over time (Staniforth, 2005). ism in marine systems (Bianchi, 2011; Katsanevakis Submarine cables began with the first trans- et al., 2014; Dorgan, 2015). The damage caused by national cable between Dover, England, and Calais, Teredo species can generate significant detrimental France, which was laid in 1850 with a total length of impact to any marine wooden structure, and there ~25 nm (Glover, 2015). As early as 1864, when only are numerous accounts of their historical signifi- a few dozen submarine cables had been laid, Henry cance. For example, their wood-boring activity may Clifford, an engineer to the Telegraph Construc- have contributed to sinking a considerable number tion & Maintenance Company, was concerned with of the Spanish armada (Dorgan, 2015) and caused a common problem encountered when laying the frequent failure of many early Dutch sea cables in certain conditions (Burns, 2012). These defences (Glover et al., 2013). were blamed on a variety of marine borers, but the Their activity continues to threaten wooden con- most damage caused was later attributed to the structions of archaeological importance (Katsanevakis Teredo worm. The Teredo would bore through the cable et al., 2014). Mainly because the scale and history armouring and eat the jute and gutta-percha insu- of these impacts, there has been well over three lation, exposing the conductor and causing earth centuries of research into the Teredinidae, which faults. Clifford devised a number of solutions, and is the family that includes the Teredo (Glover et al., his application of a spiral-wound Muntz metal tape 2013). The Teredinidae and the closely related came into general use in the late 1870s (Burns, 2012). Muntz metal – a compound of copper, zinc * Contact author. Email address: [email protected] and tin – contained more tin and zinc in it than 239 Parkes and Keeble. The Teredo worm and subsea umbilicals: risks and recommendations ordinary brass, and correspondingly less copper. A variation of Clifford’s tape was the lead outer sheathing that was first used in 1895 to protect the India-rubber core of the Cuba-Submarine Tele- graph Company’s Cienfuegos-Batabano cable (Burns, 2012). So, since the mid to late 19th century, Teredo worm has been known to damage not only wood, but also submarine cables. There are presently large numbers of subsea power cables, subsea communications cables, and subsea power and communications cables covering large distances across nearly all of the world’s seas and oceans (KDDI Global Network Map, 2016); much of this subsea infrastructure may be vulnera- Fig 1: Typical submarine power cable ble to damage caused by Teredo activity. There are (Source: Meißner et al., 2006). lessons to be learned by the oil and gas sector from the electrical power and communications indus- tries that installed and use these subsea cables. This account reviews present-day designs of subsea cables in relation to withstanding Teredo attack, and then discusses how future oil and gas subsea umbilical designs can be built to prevent ingress of the Teredo worm. 2. Submarine power cables Teredo worms burrow into the outer surface of submarine cables and create electrical shorting paths through the body of the Teredo worm. All subsea power cables, whether alternating current (AC) or direct current (DC), incorporate a metallic barrier to minimise this risk. One or more layers of copper, brass or similar tape (e.g. phosphor bronze, lead alloy sheath) are applied over the taped bundle to protect against boring marine organisms. A typical submarine power cable is shown in Fig 1 with a lead alloy sheath (no 6) around each conductor. Another example of a typical modern submarine cable is a copper tape shield with an additional lead alloy sheath (Fig 2). This is normally because of the severe environmental demands placed on submarine cables. The lead-alloy sheath is often specified because of its compressibility, flexibility and resist- Fig 2: Typical modern subsea power cable ance to moisture and corrosion. The sheath is usu- (Source: http://www.openelectrical.org/wiki/index. ally covered by a number of outer layers, comprising php?title=File:Submarine_cable.JPG. a polyethylene jacket, metal wire armouring and outer cover of polyethylene. The optical fibres are enclosed in a metallic tube for the main reason of eradicating the problem of hydrogen gas affecting signal attenuation. 3. Subsea optical fibre cable The hydrogen gas can be generated by corrosion of Modern submarine optical fibre cables are encased the armour wires (including stainless steel wire) inside a copper or aluminium tube (Fig 3); for optical from stray DC current pick-up by the armour, as fibre inclusion into oil and gas umbilical or communi- well as from hydrogen-producing bacteria in the cation cables, items 8, 7 and 6 are mandatory, while surrounding area. An electrical current in water item 5 could be an option, and items 4 to 1 would causes the water to separate into its component depend on the build of the host cable/umbilical. parts, and as hydrogen is a very small atom it can 240 Underwater Technology Vol. 33, No. 4, 2016 that some of these ‘faults’ were located along the length of the power cables and not at the power cable end connectors. The connectors had been the assumed failure area modes, based on previous umbilical copper cored power and communication cables. Where the connectors were not the failure point, the problems were put down to ‘other’ issues, such as cable manufacturing error, diesel spillage on cable or damage while onshore. Some of these power cables that were affected along the power cable length were of a similar construction, being twisted pairs (or quads) of stranded copper cores with polyethylene (PE) insulation. The twisted pairs (or quads) were over sheathed with Fig 3: A cross-section of a modern submarine optical fibre communications cable: (1) polyethylene; (2) mylar tape; polyethylene. There was no copper screen over the (3) stranded steel wires; (4) aluminium water barrier; power cores within the power cable, or a metallic (5) polycarbonate; (6) Copper or aluminium tube; outer sheath, and so some failures of these power (7) Petroleum jelly; (8) optical Fibres cables within their umbilical have now been attrib- (Source: https://en.wikipedia.org/wiki/Submarine_ uted to Teredo worm damage. communications_cable). In the oil and gas industry, power cables are mostly within a subsea umbilical, along with tubes permeate through low density polyurethane for fluids (super duplex being the preferred mate- (LDPE), medium density polyurethane (MDPE) rial), fibre optics (where the cores are within a cop- and even high density polyurethane (HDPE). How- per or aluminium tube) and other copper cored ever this copper (or aluminium) tubing also pro- cables for communications, which is subject to the tects the optical fibres against theTeredo worm. type and manufacture of the subsea control system being used. In addition, there are some subsea umbilicals that have a flowline/pipe within them. 4. Subsea umbilicals There are several subsea umbilical designs, and In the oil and gas industry, there have been a few these reflect the manufacturer, the end user, reports of umbilicals that have had problems with whether it is static or dynamic use, water depth and their electrical power cables. These problems tend location. In addition, there are subsea workover to be the shorting of the power cables to earth – umbilicals required for intervention and mainte- where the ocean water comes into direct contact nance work to be performed on subsea equipment with the copper core(s) carrying the voltage within on the seabed.

Load more