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11 Implications for man

A look at the exploration history of the important also bebenefits to the fishiilgindustr!: and thereis future oil arells of the \vorld proves conclusivel!- that oil promise for biotcchnolog!: and gas seeps gave the first clues to most l:inall!; wc look at the other side of the coin; the oil-lxoducing regions. Many great oil fields are the iinpacts that hurni~n activities have, or may have, on direct result of seepage drilling. seabed fl~lid flow and features associ~~tcd n-it11 it. This

I,inl<, 1952 includes actii-itics that may trigger 'ex-ents'. and those that nlaj- be harmful to delicate features associated

E geohazards, with the pote~ltial to aifect with seabed fluid flow: It is good that international and

offshore operations, arc associated with seabed fluid national legislation is now affording some protection to flo117. The petroleum industr~: in particular, has some seep ;und vent sites. learned froin experience that these geohazards 1 must

I ' they brokc one after another as thc turbidity current sxx-cpt across the Laurentian Fan and onto the Sohm Ab!-ssal Plain. The timing of the breaks (the furthest

11.1 I N T R O D U C T I O N break occurring 1.3 hours after the earthquake) enabled The i~nplications of scabed fluid flow for offshore oper- the speed of the current to bc estimated; according to ations fall into two categories: those that are hazardous, Piper t/ rll. (1985) this w s at least .~ 65 ltin h-' on the and those that are beneficial. In this chapter we discuss upper Ln. This crent prol~idcd dramatic evidence that the most important of these. Hazards associated with seabed slopes may become unstable. :Is \\-ell as causing seabed fluid flow have been of concern since the begin- significant disruption to intcrcontinental communica- ning of offshore engineering for hydrocarbon de~-clop- tions, the associated tsunami ltillcd 27 people in Nen- ment. Marine 'geohazards' include thoseassoci~~ted nit11 foundland (Locat, 2001). natural features and events (slope instability, gas escapes, Scahed slopcs may fiil even on very slight gradi- and mud-volcano eruptions, etc.), and those (such as ents, as little as O..iJ on some deltas according to Prior blo~~~outs) that hal>pcn as a direct consequence of man's and Hoopel. (1999). As the oil industry has extended its intervention with the natural scabed environment; Fig- offshore interests into the dccp ~vaters of the conti~lental urc 11.1. Benefits include the direct I-alue of seabed flu- rise, evidence ofn~;ljor slope failures has becn discovered ids (seep gascs and gas hydrates) or their by-products in manj- areas. Horvc~-cr, slope instabilitj- may occur at (l~pdrothermal minerals) as resources, and the assistance an! water depth. Thcrc hare becn several reports of gas pro\ ided to petroleum prospecting by- seeps. There may associ;~ted with shallo~v-water slope fiailures, and there

Seabed collapse Seabed fault scarps fissures

Tension fissures

Very soft soil zones with lower than with fissured and or expected shear strengths and slickensided soils variations in clay mineralogy

Figure 11.1'' Cartoon illustrnting the range of gcollazards S~vccne!; BP Csploration.)

cllcountered in the Caspian Sea. (Image courtes) of Aliltc

is a strong corrclation between regions of gas h!;dratcs and inajor submarine slides (Laberg and Yorrcn, 1993). Is this coincidental, or docs gas play a role in slopc instability? W% addrcss this clucstion by investigating some relevant examples.

11.2.1 Gas-related slope failures: case s tudies

Harnpton p t ill. (1906) identified file environments in which seabed slope failurcs are common:

fjords; active river deltas on the continental margin;

* open continental margin slopcs; submarine canyons;

* oceanic volcanic islands and ridges.

midnight. 'The slidc developed retrogressivel!; to~vnrds land. About 25 minutes after midnight a drivcr felt that his car and the road were shaking violently and stopped. T h e beach below the road was zone. hlinutes latcr he witnessed 250 m of the road brcaking in three parts and slumping into the sea. A car with one person also disappeared. Shortly afterwards, the nearcst house started to move, then sank into the mud and disappeared into thc sea. Tlirec people inside did not manage to escape. A411 this happened ~vithin 5 minutes or less. Several minor mass moverncnts occurred along the cdges of the slidc, but after 1 hour, no more slide acti\-it? was observed.

Long\-a et ill., 2003

Major clay slides like this arc not uncommon in and Fjorrls: rlze 1996 Pir~~reidfiord Slide around fjords in Norway; Alaska, and British Columbia; At about midnight on 20 Junc 1996 therc was a shoreline in man) cases screre damage has been causcd and lil-es slopc-failure in Finncidfjord in northern Norwa!~. The

fi~ilure started 50-70 m from the shore. T h e follo~ving have been lost (Hampton r t nl., 1996; Longva et nl., 1998). They are caused by the sudden liqucfaction of \\-as based on c!~e-witness accounts: 'quick' clays. oftcn as a result of an external trigger such

Eye ~vitnesscs sa\\- m7a~;cs, bubbles and \\-llirls as seismic activity, or an increase in porc fluid pressure. moving a13-aj7 from the shore somc time beforc T h e Finneidfjord slide followed a period of h a \ y rain;

Scahcd slope inst,~bilit> 357

detonations from nearbj- tunncl construction ~vorks may have contributed (Long\-a et ,I/., 2003).

Beneath the fjord, a 'bright' la!-er 011 seismic profiles, representing rclativel!. sandy sediments, n as found at the presumed dcpth of the failure plane; this vr-as underlain by acoustic turbidity (Best et (/I., 2003). It scems that gas from underlying gassy sediments was accumulating in the sandy layer, resulting in excess pore fluid pressure that ma!^ har-e contributed to the failure.

Active rizlnr dnltrrs: the FI .NZLJ~ Delta Shallow g ~ s is present over a large area of the Frascr Delta, British Columbia (scc Scction 3.20.2). Therc is elidencc of slopc failure in tn-o areas of the delta front: off Sand Heads, a i d the 'Roberts Banks Failure C o n - plex'. Both areas are characterised by sandy seabed sediments, and evidence of gas has been identified in both areas Uudd, 1995; Christian el a/. , 1997). It seems that gas has made these sandy sediments susceptible to liquefaction failurc (Atigh and Byrne, 2003; Grozic, 2003). Sand Heads is where the main distributary- of the Frascr River crosses the delta flats, depositing much of its load of silt!-/sandy sediments. T h e delta front failed fix times between 1970 and 1985; the 1985 event invohed at least 1 x 106 m b f sediment (McKenna el ill., 1992). It is probable that slope failures in this area are Iargel!. a result of the rapid deposition, but sandy sediments such as these tend to be liable to liquefaction failure during cyclic loading; Atigh and Byrne (2003) and Grozic (2003) suggested that gas may have increased their sus- ceptibility.

Further south on the delta forcslopc thc Robcrts Bank Failure Complex is a distinctive area characterised by a hard (sandy) seabed, represented on the profiles by a strong (high-amplitude) reflection, and sandwal-es. Therc is seismic evidence of slope failure (Hart and Oly~~yli, 1994). The sediments here contrast with those clsc\vhere on the slope because acoustic turbidity is not ubiquitous. I-1011-cvcr, small patches ofacoustic turbidity and variable-amplitude reflections suggest there is some gas here. It could be said that gas is relatively scarce in the failure complex because the sediments are coarser than elsen-here, and therefore unlikely to be able to retain gas at the pressures that might be found in a finer-grained sediment. An alternative is that gas was present, but escaped when the slope failed.

Slope failures involving gassy sediments have been describcd from othcr deltas, including the Alsek Delta

(Section 3.19.2), the Klamath Delta (Section .?.22.1), and the Mississippi Delta (Section 3.26). Some typical deltaic slope failure features were described in Section 7.4.2 (for further details see Prior and Colema11, 1982).

Ope?z co?ztineiztal nzn~*giizs - upper slope: the Hunzboldt Slide T h e Humboldt Slide, described by Gardner el nl. (1999) as ' a licrge, cconzpler slide zone', is located on the upper slope of the Eel R i x r Basin offshorc northcrn Califor- nia in watcr dcpths of 250 to 600 m. It affects an area of about 200 ltm2, and a sediment volume of about 6 kin'. Although dated as late Pleistocene to early Holocene, it seems that thc slidc ma!- still bc activc (Gardner et al., 1999). Gas, gas hydrates, and related features are ~videsprcad in the Eel River Basin (see Section 3.22.1). Yun et ill. (1999) questioned the role of gas in the slope failurc. Was it causc or effect? Then, Gardner et nl. (1999) suggested it may have at least contributed to the cause. This ~ ~ ~ o u l d explain, for example, why thcrc is 11-idcspread gas in the underlying sediments but little near the surface. Gardner etnl. thought that gas-induced increases in pore fluid pressure might 'col~~ribtl~e lo llze .sr,i.lrorl's szrsceplibilit)~ to slzeari?zg and sliiling' by rcducing the shear strength. S,, - see Equation (7.4). Howel-er, shallo\v gas and pockmarks nre present over a large part of the slope and outer shclf of this region, but the I-Ium- boldt Slide is the 0111~- major failurc. This suggests there must bc ;I f;lctor othcr than the presence of gas that made this pnrticular location susccptible to failure. However, Lee r t (11. (1999) had a totally different interpretation. They nctu;~ll!- questioned the concept that this is a slide, sa!.ing that ' ~ n i ~ n ) , /!fits i.hi~rirrterzstic suggesl i~ series o f ileposi~r olli~l betl/oims'.

Opeir coirtitren~al nzargiizs: tlze Stoi,egga Slidt! Cor17ple.u 'I'he Storegga Slide, on the Norwegian Margin (hlap 3), is probably the world's largest sediment slide. It mo\-ed a volume of 3500 km3 of sediment, and has a 290 kin long headn-all scar ~vi th slide material suppos- edly extending 750 lirn do~17nslope to cover 90 000 ltm" of the Norwegian-Greenland Sea Abyssal Plain (Urq-n el id., 2003). Six failure el-ents haw occurred in this area during the last 500 000 years. Flood deposits found in lakes and bogs on the Norlvcgian coast and in the UK of the same age as the most recent (about 8200 BP) event suggest that at least this one cause~l a tsunami

3.58 Implications for lnan

SW

Figure 11.2' X multichannel 2D-seismic section across the RSR, indicated 11y arrows, is probably apparent because ot'rhe

northcast tl.lnk of the Storcgga Slide. T h c lctter 'P' indicatcs thc enhancement oSunderl!~ing retlecrions by gas trapped belo\\- the locations ofcomplcx seabed pockmarks \\.it11 carhonatc ridges GI-ISZ (gas h!-tlrate stability zone). (Reproduced from Hovland (described bl- Hovland t.1 ill., 2005). 'l'he!. arc associated with et al., 2005 with pcrnmission horn Elsevier.) k~ults and 'pipes' (vertical gas n~igration pathways). T h c regional

(Bondevik t t a/., 1997). 13ecause a huge gas field, the Ormen Lange ficld, lies under the slide scar a great deal of research effort has gone in to understanding this slide complex. The motivation is to cnsure that the current seabed is stable, presenting no hazard to scabed installations.

The Storegga Slide lies in a dcpression betwccn two depocentres for glacial sediments: the North Sen Fan and the Slijoldryggen areas (Bryn r t a/., 2003). Although the detailcd history of the slide is debatable, it is inferrcd that it \+-as rclated to gas and gas h!-drates; clearly identificd BSRs (bottom-simulating reflectors) occur on thc flanks of thc current scar and project into thc level of thc slide-scar sole (Figure 11.2; Mienert er al., 1998; Bouriak et ill., 2000), and gas-escape struc- turcs rise from the BSR to the present seabcd upslope from the slide scar (see Section 6.3.3). 'he question 'what came first - the hydrates and/or the slide? was discussed by Berndt e ta / . (2001). Although they failed to say ~ n u c h about the likelihood of free gas and gas hydrates providing the conditions necessary for failure in thc first place, thcy found that the slide must have disturbed large volumes of buried gas hydrates causing it to dissociate and escape. They calculated that fluid must have escaped over a period of less than 250 ycars. Subsequent modclling by Sultan t~ id. (2003) dcmon- stratcd that various characteristics of the slide could not be cxplained if thc influence of dissociating gas hydrate \\-as excluded from thc model. They concluded that thc melting of gas hydratcs may have initiated the failure,

and that ' /heJi l i l l~re inle~:/icl~ is i ir i t i~~ted nl llze top o f the h,y~lr~zte Inj~czr. IEIIO' not a t llze /l>z>el cftlze BSR'.

O p e n c o n t i n e n t ~ ~ l nirrrgins - lomer slope: the C a p e F e ~ r r SLide The first side-scan sonar images of the Cape Fear Slidc, off the Carolinas, were published by Dillon e t a / . (1982) and Hutchinson et a/ . (1982). They sho~v that, although this is the largest on thc US Atlantic Margin, it is one of sevcral similar slides in the area (Map 31), anothcr being the Capc Lookout Slide (Popenoe et trl., 1993). The amphitheatre-shaped headn-all scarp is locatcd in the loner continental slopc at a depth of about 2600 m. I t is up to 120 m high and over 50 Itm long. A secondary complex of slumps and slide tracks extend 40 k ~ n upslope iron1 the headwall scarp. Do~~~ns lopc , a broad trough, over 150 m deep and more than 40 km across, has bcen scoured into the seabed. The slide deposits in this troagh extend for more than 250 km onto the Flat- teras Abyssal Plain (Embley and Jacobi, 1986; Popcnoe t,t (i/., 1993).

The most intriguing fact about the Cape Fear and Cape Lookout slides is not that they start at great water depths, but that their head\vall scarps are locatcd close to salt diapirs. Thc Cape Fear headvvall scarp encircles five diapirs; thc largest is 8 km in diameter and its top protrudes above thc seabed (Schmucli and Paull, 1993). These form part of the line of salt diapirs extending along the seaward side of the deep Carolina Trough; thc Blakc Ridge Diapir (Scction 3.27.2) is also in this line.

S e ~ b e d slope instability 359

An extensive BSR in this area is taken to indicate gas trapped below gas hydratcs (Schmuck and Paull, 1993). Anomalous temperature and fluid-flo\\- conditions associated with the diapirs cause the BSR to rise over them, indicating a thinning of the hydrate-stable layer (Paull et nl., 2000). The BSR also riscs towards the slide scar's edges, and is less prominent near its ccntre; this may indicate gas cscape. Gas venting that is thought to occur at the hcad of the slide (Schn~uck t t ( / I . , 1992), and upslope of the slide scar (Dillon et al., 1982) may be associated with the numerous normal faults seen on seismic protiles (Paull et nl., 2000). The combination of diapirs, gas, gas hydrates, and a major slide in this are;) seems to bemore than a coincidence. Pcrhaps the slope failure and removal of sediment caused a pressure reduction and breakd0~1.n of the hydrates. Alternatively, in accordance with the hypothesis of salt-stock formation suggested by Hovland et nl., 2006 (outlined in Section 9.4.3), we suggest that warm, methane-charged fluids flom- ing out of the salt stocks might have been responsiblc for the dissociation of at least some of the gas hydrate. l i e think this weakened thc sediments, triggering the slides.

11.2.2 Associated t sunamis

Figurc 11.3 l'orces acting on a submarine slope. cr = slopc; / I =

thickness of thc sediment slice; o, = vcrtical stress; .S,, =

undrained shear srl.engt11 of the sedimcnt; rr , sin cr =

gravitational shear srrcss acting in the direction of potential

movement; 0 , and S,, m r e defined in Equations (7.1) and (7.4)

respectively. (Adaptcd from Hampton e l a/ . , 1006.)

ious stages of movement, and guides to assessing risk. Our purpose here is only to discuss the rolcs of gas and gas hydrates.

Slope failure occurs when gravitational forces (ver Tsunamis are thought to havc been associated with tical stress; a, sin a) , which tend to pull a sediment mass seabed slope failure on at least two occasions. Nurneri- downslope, exceed thc resisting forces (shear strength; cal simulations and 3D (three-dimensional)-animations S , cos a); see Figure 11.3. This happens in the weakest of large tsunami waves generated by the Storegga Slide clearly demonstrate the hazardous impact of large underwater slides. Another example, the Sissano tsunami, struck the north shore of Papua New Guinea in July 1998. Detailed seabed investigations reportcd by Tappin et nl. (2001) identified a 5-10 km3 slump. On the seabed in the slump area there were fissures, brecciatcd angular blocks, vertical slopes, and talus deposits. Also, Tappin et nl. reported 'nctive,fiz~id expulsion that main- ti~ins a cheinosjtnthetic z~eizt./&uizn'; evidencc that seabed fluid flow was implicated in the failure event.

11.2.3 Why d o submar ine slopes fail?

The study of submarine slope failures is a major topic addressed in many specialist publications: Hampton et nl. (1996), Mulder and Cochonat (1996), Locat (2001), Locat and Mienert (2003), to name but a few. These provide descriptions, classifications, analyses of the var-

- -

laycr of sediment, allowing the slice of sediment above this 'failure plane' to move downslope. Table 11.1 lists factors that makc a slope predisposed to failure, and others that trigger failure. Factors of interest here (shown in bold) are those that reduce sediment strcngth by gen- erating excess pore fluid pressure - see Equation (7.6).

It is important to acknowledge that slope failures arc not necessarily associated with gas or gas hydrates. McAdoo et nl. (2000), \vho reviewed 83 deep-water slope failurcs offshore Oregon, California, Texas, and New Jersey, identified seismicity, active sedimentation and erosion, and salt tectonics as major factors. T h q did not mention gas hydrates, and mentioned gas only in connection with four slides, all near the pockmark field in Califbrnia's Point Arena Basin. We are not trying to suggest that gas is a 'fbrgotten' fjctor (although this may bc true in some cases). Rather, we u-ish to point out that where gas or gas hydratcs are present they are likely to interact with other fjctors (rapid sedimentation,


Table 11 1 Causes of\r/bnznrzne slopefizlz~ve (~nclucli~zg ma /~ , t ~nljrorn Hamnpton et al., 1996, Pl,zor and Hoopel; 1999, Locat, 2001; and Lerouc~rl et a1 , 2003)

Predisposition factors Triggering F actors Causes o f fai lure

Changes in current Erosion

strength/direction

The prescncc of salt or Diapir movement LI1 LI1

Slope ovcr-steepcning mud diapirs at depth k ,d V)

Sediment density . F - Rapid sediment deposition, c.g. inversions r~

dcposition on deltas by river flood; L

Potcntial for rapid U

deposition following sediment - deposition or upslope mass movcmcnt Increased vcrtical stress mass movcmcnt

Large tidal risc/fall Unusually low tides Reduction in porc fluid

I I 1 pressure leading to sediment 1 I 1 failure I I ~

I Volcanically actlr e zones Volcanic activity

Seismically x t i \ c zones Earthquakes Cq clic loading

Susceptibi1it~- to storm Storms wa\-es

T h c presence of aquifers Hcavy rain in aquifcr catchment areas

1 T h e presence of 1 FIuid migra t ion 1 Exccss porc fluid pressure generation within porous

1 thermogenic gas sources 1 1 laycrs capped by 1 1 1 a t dep th 1 1 impervious layers 1 I 1 T h e presence o f 1 G a s generation 1 I I

organic-rich sed iments

T h e presence of gas G a s hydra te decompositiotl Scdimcnt liqucfaction hydrates

cyclic loading by waves, tides, earthquakes, etc.), and may facilitate slopc failures which would not occur in their absencc. Indccd, we have been struck by the fre- quency with which authors h a ~ e suggested that morc

a1 ure. than one factor has influcnccd submarine slope f '1 This seems to apply to failurcs at any water depth.

Srubrd porr fluid prrsstrr~ build z ~ p In Chapter 5 ~ v c discussed pore fluid overpressures, and explained that overpressures may result from under- compaction - Scction 7.5.2 - and the accumulation of buoyant fluids, particularly gas - Section 7.5.3;

Equation (7.12). An incrcase in pore fluid pressure results in a reduction in shear strcngth, and resistance to 1 shear failurc. Orange ct ~ 1 . (2003) suggested that seeps arc ek~idence of the existence of fluid overpressure, and that overpressure prol-ides an internal driving tt~ech- anism for failures characterised b! a flat base and a stccp, amphitheatre-shaped headscarp. They thought that persistent overpressure might lead to recurring failurc within the original feature, leading to headward migration and linear failure morphology.

I-Iainpton el nl. (1996) noted that some major deltas are apparently unaffected by slope instability.

Seabed slope instability

One example they mentioned, the Changjiang (Yangtze in thc host sediment. The increascd water content ma)- River), China, is the fourth largest contributor of sus- seriouslj affect the sediment's strength, possiblj- reduc- pcnded sediment in the ~~rorld, delivering about 0.5 x 10" ing it to mush, and making slope failure more likely. t of sediment annually. Yet, 'tzlthoz~gh .vigi1Zfi~[j111 deltaic Hydrate may dissociatc from thc seabed down~vards as i1epusit.s oa.~ir there, czs me11 izs pockmarks resulting /i.onz a result of seawater ~varming. H o n c ~ e r , when the dis- the t~ .~ ja l s io~z o f Oiogenic [microbial] gas, the s~irj/uuar is sociating hydrate is overlain by unaffected sediment, or uthermisejl.atz~re1ess'. In this case we think the escape of when dissociation occurs at the base of the GHSZ (as gas has prevented the build-up of sufficient cxcess porc explained in Scction 10.8.1), it creates a ~ ~ e a k layer; this pressure, contributing to thc stability of the delta. is where failure might occur.

It is intercsting to compare the role of gas in marine slope failures to the role of water in onshore slope

11.2.4 Predict ing slope stability failures. Onshorc, a typical failure plane occurs 11-herc vater percolating downwards through a permeablc for- Offshore geotechnical slope stability investigations havc mation encounters a fine-grained, impermeable layer. grown from the long tradition of onshore i~ivcstigations. Offshore, failurc occurs when gas migrating upwards is A 'factor of safety' (FoS), defined as the ratio b e t ~ ~ ~ e e n trapped beneath fine-graincd, impermeable sediments. cj-clic strength and induced dynamic stress in each ele- The roles of gadair and water are reversed and the pro- ment ofthe slope, is used to assess the stability ofmarine cess is inverted. slopes. Chanev (1984) pointed out that, in the absence

of a uni~rersalll- accepted FoS, it is necessary to employ T h e roles ofgns hyd~pnte 'apfiI~~ii7gJrr~Igei1ze1zt or an i~z'~~.agiizgfimr.ess to the res t~ l~s ' . 4 relationship between gas hydrate and slope failure is Of course, there ma) be uncertainties, indeed we sense suggested, if nothing else, by the freque~icy with which a 'factor of uncertai~lty' when gcotechnical engineers slope scarps coincide with thc gas hydrate stability zone have to use 'judgement and averaging' in order to assess (GHSZ). McI\-er (1981) suggested the!- mere not coinci- underwater slope stability -a situation not unlike uncer- dences, and numerous studies have subsequently inves- tainties known within geophysical interpretation. This tigated the relationship. A particularly good examplc is is, however, in no way a rcassuri~lg situation for the gen- the Atlantic Continental Slope of the US.% where there eral public, ~vho may think that wc are dealing with exact are sufticient slide scars for an analysis to be meaning- science. Locat (20lJI) addressed this situation and came ful (Paull et a/ . , 2000). There is a BSR right along the to conclusions we certainly agree with: slope, and 'data c lenr !~~ sho~il tlzal the slides are iz~itlies

Ultimatel!; the goal is to be able to carry out proper r~zndonzbl tiistributed nor are tlze)! str/~ng.(ji ~zssociated rl~ith

risk assessment analysis pertaining to submarine sleep slufies'. Paull el a / . found that the majority of the

mass movement. This could be achieved by slide headwall scarps occur at the updip limit of the

integrating thc geotechnical characterization of GHSZ, about 500-700 m water depth. This led thcm

mass movements into a risk assessment to conclude: ' the ohserved distribution ofsliilr scars is ion-

methodology, which can then be applied on a ststelzt milh the distriDution that is predicled to occus zfgrrs

regional basis. 11jidr~1te ilecumfiositiun has played a s i~~ iz jca?z l role in ctz~ls-

Locat, 2001 zng tlrese sediment Jzilures'. Similar 'coincidences' have been reported from other continental slope areas. Paull Dugan and Flemings (2002) used such an assessment et (11. (2000) remarked that ' a grerrt deal ofcircuinsiun- when considering the stability of the U S Conti~len- tin1 ecidence stror~gly sujports the concept tlzal grrs hjldsate tal Slopc offshore New Jersey. They suggested that hreakdomn is o f i m inst~unzeiztrzlin ~riggeringsedirnent mass the Neb\- Jersey continental slope was u~istable because ~riovenzeizt on the sea,floor'. of high sedimentation rates approximately 0.5 million

The role of gas hydrate varies according to its sta- years ago, but that the modelled FoS is now approx- bility. When stable it increases the mechanical strength imately 1.5 (upper slope) to 3 (lower slopc) - where and rigidity of the sediment, but dissociation rcleases FoS > 1 represents stability, and FoS 5 1 represents both water and large volumes of gas. It is assumed that instability. The approach uscd by Lee et 01. (1999), gas escapes through the seabed, but water may remain n-ho studied the Eel Margin, offshore California, was to


use a GIS (Geographical Information System) to map key parametcrs. Specifically, they determined the critical horizontal earthquake acceleration required to cause failure, k c , and plotted the ratio of kc to the peak seismic acceleration with a 1O1?o probabilit!- of exceedance in 50 pears. The resulting map effectively differentiates between areas that are, and arc not susceptibie to slope failure.

We are not qualified to comment on the reliability of these approaches to evaluating the risk of slope failure. However, we note that both Lee ' t ill. and Dugan and Flemings made various assumptions about the subseabed conditions in their respective areas, and suggest that caution should be used when methods such as these are applied to arcas in which there is fluid flow: As fluid flow tends to be focussed (Section 7.5.3), gen- eralisations about fluid pressure conditions ma!- not be valid, so locations of focusscd fluid flonr may be more susceptible to failurc than predictions might suggest.

22.2.5 Impac ts of slope failures on offshore operations

\Ve introduced this section with a comment about the cutting ofcables bj- a slope failure. Cables arc not the onlj- I ulnerable installations. 411~- seabed structure, including pipelines, platforms, etc., would be ;~ffectcd b!- the fililurc of the scabcd on which it was sitting. Site investigations must take into account not onl! the site of the i~lstallatioil being planned, hut also the surrounding area. Seabed slope-fi~ilures, particulnrl!- thosc in deep water, can affcct such enormous areas that ;In entire site may represent just n small fraction ofa single slide, so it is important t h ~ t the potential for the site to be affected b!- failures in thc surrounding area should be considered. Prior and Hooper ( 1 909) described an cxainplc from a deep-water (SSO n ~ ) site in the Gulf of Mexico where gas hj-drates n-ere implicated in slope-fi~ilures. The site investigation for a Tension Leg Platform (TLP) identified cvidcncc that slope-failure events had occurred close to the site since about 30 000 !-cars ago; the most recent within the last 1000 years. Gcoph!-sical mapping sho\vcd that debris flo\~ls started from fluid-expulsion mounds and craters from which large volumes of ol~er- pressured gas had been cxpclled. 'l'he flo~vs began on slopes of 10'-15" and cxtended up to 11 km donrns- lope. I-Io\vcvcr, more recent (< 12 000 !ear old) flon-s had stopped at least 1800 In upslope from thc installation

site, and were < 5 m thick. It was considered that the risk was acceptable, and the T L P was successfull! installecl.

1 1 . 3 D R I L L I N G H A Z A R D S

Drilling for petroleum exploration and exploitation, site investigations, and scientific research are all hazardous undertakings. Some of the most potent hazards arc associated with natural pore fluids: ovcrpressured gas and water, and gas hydrates.

11.3.1 Blowouts

Ovcrpressured gas is one of the main hazards in offshore drilling. As apollutant, oil is more damaging to the environment than gas. Ho\ve~!el; as a hazard to drilling, personnel, and infrastructure, gas is more dangerous because of its mobility, flammabilit~, negative effect on n-ater buoyant!; and difficulties in control. According to Prince (1990) shallow gas is a responsible for 'opp~.ori- ~ ~ z a t e l j ~ oil(, t1zii.d of (111 ~ I O ~ T ~ U I I I ~ ' , and has heen responsible for the loss of both lives and drilling rigs.

.%lthougll geochemists and explorationists distin- guish 1)ctneen microbial and thermogcnic methane, when it comes to hazards, there should be no such dis- tinction - both are flammable and difficult to control in large qua~ltities. In any drilling operation, formation fluids (water, oil, or gas) will flo\vinto the well bore n-hen the formation fluid pressure exceeds the pressure in the hole. If the fluid entering the n-ell bore is less dense than the drilling fluid, it nrill move upn-ards in response to buo!~anc!: Initially this is described as a 'kick'. The expansion of gas as it rises within the hole d r ixs the expulsion of drilling fluid. This reduces the n-eight and prcssure of the fluid column, encouraging more gas to enter the hole; a chain reaction results. If not controlled the result may bc a blo\vout, a "'ni~lil': ~rniustrni~~erlflom cf ,?(IS o r ~ ~ ~ s - c h a ~ e r l J ; ~ r ~ ~ z r r t r o i z Jlz~id U L the szri;j;rr.t,' (Graber, 2002). Gas blo\vouts put the drilling vessel or platform and the crew in peril in sexral waj-s: b!. releasing toxic gases, triggering fires, causing mechanical damage, and buoyancj loss. Situations may also occur 11-hcrc fluids from deep, overpressured zones flow up the borehole and encounter shallo\\ er. Ion--pressure zones. Lnder such conditions, the deep fluid (oil, gas, or water) may enter faults and frac~ures and permeable beds resulting in an uncontrollable leak - an 'undcrground blo~vout'. Such recharge and com~liunication ~vith orerpressurcd

Drilling hazards 363

COPING WITI-I PROBLEMS zones can even make future shall015 drilling in an area hazardous.

Sl~nllow-gas hlowouts 'Shallow gas' is defined differently by different people. Some regard 'shallo~v' as being within 1000 m (or some other arbitrary depth) of the seabed. A more pragmatic definition is that 'shallo~v' means above the first casing point, where a petroleum (exploration or production) ~vell is lined ~ ~ ' i t h steel casing. Until this casing is set the well cannot hc protected by a 'blo\vout preventer' (BOP), so the drilling operation is vulnerable to the influx of pore fluids (gas, oil, or \%ater). The depth of the first casing differs according to various operational parameters. but is usually within the top few hundred metres of the ~vell. Site investigation boreholes, which are not cased, ma! penetrate to 100 m or more, so they also arc vulnerable to fluid ovcrpressures in the topmost sediments (the case described in Section 7.5.3 being an example).

U\UERGROUND BLOWOUTS

Kicks can occur after the setting of casing whilst drilling deeper formations. In such cases it is normal for the well to bc shut in using the BOP, and subsecluently to 'kill' the kick, for example using overweighted drilling mud. IIo~~-ever, this opens the possibilit?~ of an 'undcrground blo~~~out'. Once it has been shut in, if the excess gas pressure fractures the formations at or heneath the casing shoe, gas will find a route to the seabed outside the casing. This is most likely to occur at the first (shallov- esc) casing point as this is probably in the least corn- paeted and, therefore, weakest sediments. The consequence of exceeding the fracture pressure - defined in Equation (7.14) - can bc that overpressured fluid forces a route to the scahed outside the casing, rendering the hlo~vout 'tu~controlled w1d t~izcoiztrnl/~~Dle' (Grace, 1994). The route taken to the surface is not necessarily close to the casing, indeed sometimes gas has been recorded Tenting some distance from the rig. The seabed can he eroded to form pockmark-like craters, some of which have been largc enough to coilsumc jack-up rigs and platforms. Grace said that 'h is /oric(~IZ~~ t l ~ e iilost i ~ ~ f n i ~ z o t ~ s rriril espei~siz't~ blonjouts in ian'zls/~:y h i s to l :~~ mt,re crssoc~aled ~ui~lz,/?nctui.in~ lo the S L I ~ ~ L L . L > , ~ ? O ~ I Z t~ i td t r /he .SLII$LCC C I I S ~ I I ~ ' .

There are two strategies for tacliling shallow-gas kicks. The excess gas pressure may be passed through a 'divertcr' at the scahed, to vent into the water column away from the rig. Alternativelj~, a diverter on the rig inay handle the gas (Prince, 1990; Grace, 1994). By d i~~er t - ing the gas, the excess pressure is allo\ved to leak an-ap Howevel; both of these strategies have their problems. In particular, the erosion of pipe work by sands produced \\-ith gas has been known to cause diverters to 'iil, increasing the risk of explosion or fire. When prob- lcms occur, the shallomncss of the source of the problem means reaction times tend to be short, making control difficult. Adams and Kuhlman (1990) urged operators and drillers to place a high priority on ensuring cquip- mcnt and proccdures suitable for dealing Tvith shallon-- gas hlomouts are in place. They noted that 'rerords ssholn ~ h i l k j r i l ~ l r e to (10 so cna resnlt ia loss of lives and iilajur pruper l ,~~ ifi~nzage'.

SHALLOW-GAS BLOWOUT EXAMPLES

During the development of the Gullfaks field it Mias known that pockmarks and shallow gas were present (see Section 2.3.6). Shallo\v gas mas encountered in 5790 of the exploration wells, and in one case resulted in an uncontrolled blo~vout. This gas is held in interlinked bodies (> -5 m thick) of unconsolidated silty sand at three levels between 130 and 230 m below seabed. Detailed investigations were undertaken after it was found that shallow gas extended heneath the site of the Gullfaks 'A' concrete gravity platform, and several wells nerc drilled solely to test or drain off this gas (Lukkien, 1985; Hovland, 1987). Pressure build-up tests undertaken in the topmost gas-bearing sand produced the follo~ving results.

Formation fluid pressure was 3320-3370 kPa (i.c. 60-1 10 liPa overpressure) indicating a gas column of 6-10 m. The gas nas 99% methane. The gas-bearing sands were 'highly permeable' (200 to 350 mD). During the test 400000 s m 3 ( m b t STP) was produced. -1 tentative estimate suggested that the volumc of gas in place was 100 x 10' Sm3.

3 64 Implications for man

The estimated porosity of sands in this zone n7as 30- 40°/o, and the mean gas saturation was 50-60%. A total gas volume of 2.5 x 10"Srnhn.a~ produced over a 35-da!- period with a stable production rate of 64 000 Sm' dp' , and a maximum of 115 000 SmQpl. Continued production proved difficult, probably because the sand is likely to collapse, destroying the permeability close to thc ~irell.

It was thought that spontaneous kicks wcre unlikely during normal drilling because pore fluid pressure was old!. slightl!- above hydrostatic, but there was a danger of a loss of control either because of a loss of circulation (when drilling fluid invadcs the permeable formation), or during swabbing (\vhcn a pressure fluctua- tion is causcd by equipment bcing moved up thc wcll). LuLkien (1985) envisaged that, if a gas escape occurrcd, an underground blowout was likely

The following examples furthcr deluonstrate thc seriousness of thc shallow-gas hazard.

1. During exploration drilling in thc German 13ight of the North Sea in 1963, the rig WIr Louie experienced a blon-out resulting in thc formation of a 400 in wide crater known now as the 'Figge Maar'. Apparcntly the crater was originallq- 31 m deepcr than the surrounding seabed (34111 deep), but it has been progrcs- sively filling up with sediment so that its depth had reduced to 22 m by 1981, and 14 n~ by 1995 (Thatjc e t a / . , 1999).

2. In 1969 a blowout in the Dos Cuadras ficld, offshore California, permitted high-pressure fluids from deepcr regions of the reservoir to migratc to the shal- lower, low-pressure zone. After\vards, several suh- stantial oil and gas seeps occurred within 300 nl of the drilling platform. They were of sufficient scver- ity to cause repeated suspension of work and evacuation of the platform. By the tiine thc well was under control, some 10 days after the blowout bcgan, the sccps had gradually increased to an arca of about 200000 m2. Observation of scep sites from a submersible showed that craters up to 'mal<l, jeer' in length and 'senercrl fief' deep were surrounded by angular rock debris which had apparently been blown from thc seabed by the force of the flow (McCulloh, 1969).

3. In Deccinber 1972 the mat-supported jack-up rig 'J. Storm 11' tiltcd. The rig was cvacuated, and sank about 20 minutcs later. A seabed survey conduc~ed the following year reiealed a flat-bottomed crater

nearly 500 nI across and approximatel!- 12 m deep. Abundant gas plumes in the water column were thought to be coming from both the broken drill stem and the floor of the crater (Urorzel and watkins. 1974).

4. A jacli-up platform drilled through a fault zone extending from the seabcd to a high-pressurc gas pocket at depth. A vigorous strcam of gas bubblcs was seen escaping at the sea surface some 300 n~ from the rig, bi-here the fault reached the seabed. These bubbles caused the sea surface to rise by betwen 12 and 22 m. A crater formed at this point and gasification caused the failure of a wedge of sediment. The rig was first set on fire by ignited gas that had escaped from the well casing; it foundered when the seabed scdiments failed (Sieck, 1975).

5. In the South Pass Area, off thc Mississippi Delta, a blo~vout occurred when shallow gas m s encountered at a depth of about 210 m. The gas ignited, and the rig collapscd and sank. A seabed crater formed by the blo\vout was surveyed five days later \\~hen it measured about 600 m across and about 30 m deep. Gas was still escaping, the m t e r column on a shallow scisluic profile being 'li~ernlly covered' with gas. The shallo~v sediments on the sides of the crater were apparently saturated with gas, and it was suggested that sediments between the drilling rig and the gas zone were blown into suspension when pressure increased beyond somc critical point (Bryant and Roemer, 1983). 1

6. A blowout in the High Island area, Gulf of NIex- ico, resulted in the loss of an entire platform into a seabed crater when a ivcll penetrated gas-charged sands at a depth of 1220 m. Subsequent surveys sho\ved that thc crater was 450 m wide and nearly 100 in deep; an estimated 4.4 million m3 of sediment had been ejccted to form the crater (Bryant and Roemer, 1983).

7. In 1985 the scmi-submersible rig 'West Vanguard' was drilling a wildcat well at the A4ikkel field on the tlaltcnbanken area off mid Norway A blo\~~out occurred whcn gas-charged channel sand was penetrated only 300 m below the seabed (Section 6.2.3). Attempts to divert the gas away from thc rig floor and into the water failed because of sand in escaping fluids. The bloivout caused one fatality, the evacuation of the rig and the following fire caused scvcre damage to the rig. An indication of the volume of gas released

Drilling hazards 365

Figure 11.4% A hlo\\.out in the I-Ialtenbanken area of thc reservoir; nntcr tlcpth 240 In. (Photographctl h!. Lcii'Berge;

Normegian Sea in October 1985. Large volumes of gas (mainly from Hovland and Judd, 1988.)

methane) are seen escaping to the atmosphere from a shallow-gas

is given in Figure 11.4. Gas bubbled continuously to the sea surface over a period of about two months.

8. In November 1990 Mobil experienced a blowout in the UK North Sea when a base-eaternary shallo\v gas source was penetrated. Although the rate ofgas release slowed after the first few days, gas was still escaping four years after the accident when it was predicted that, because of the size of the reservoir, it would continue for several years unless the vent collapsed and became blocked (Rehder e ta / . , 1998).

9. Although most shallow-gas blowouts occur during exploration drilling, production platforms are not exempt. A report from the U S Minerals Manage- ment Service (MMS, 2003) described an incident in which a sudden gas influx caught fire, even though a diverter system was used. It was thought that gas had been sucked out of a shallow sand as the drill string was being removed from the well. T h e blowout lasted only about ten minutes, but the platform had to be abandoned and damage was estimated to be two million dollars.

L E S S O N S LEARNED

There are obvious safety lessons to be learned from these case studies about the danger of fire, loss of rig buoyancy, and foundation failure, and it is clear that penetrating s+;rl'l'urc- gds mrrouirs ;fnd dri21'irtg into ~ ' & T X I ' ~ R psfif- ways can induce blowouts. There are also several lessons relevant to natural seabed gas escapes to be learned from these man-made incidents.

Gas escaping through the seabed is clearly capable of eroding the seabed to produce large craters (pockmarks) in a very short space of time. Craters may be infilled with sediment over very short periods of (geological) time. Gas escaping during catastrophic events passes through the water column to escape to the atmosphere-gas escaping after the West Vanguard blowout (Figure 11.4) passed through 240 m of water. After a catastrophic gas escape event, gas leakage may continue for a considerable period of time.

366 Implications for mail

Gas introduced into seabed sediments may cause scdiment hilure and mass movement.

Guarding ng~ci~?.s/ blomouts In order to avoid blo~vouts, oil industry- (and ODP) sites arc carefully selected and surveyetl beforc spud- ding in. (Although many oil companies have cxperi- cnced b l o ~ ~ ~ u t s , ODP has not - imainly because of a pre\-ious policy to stay an7a!. from hydrocarbon-bearing regions.) Regulations diffcr fromcountry to country, but in many cases (including U K and Norway) prc-drilling 'hazard' surveys are mandatory. In the UK, for example, detailed guidelines drawn up by the United Kingdom Offshore Operators Association (UKOOA, 1997) clarifj- relevant regulations and describe ',.mod in~/~isti:l~practici.' for the conduct of rig site surveys, including aspccts related to the seismic identification of shallow gas. These guidelines provide details about suitable survey line density, equipment specifications, data-processing require- ments, and gas indicators; we discussed the seismic indicators of gas in Section 6.2.2.

To~vnscnd and Armstrong (1990) warned against interpreting ' r r r q i lziglz a~nzplztu~le seisi?zii. rejector ns m

potential "hzglzt spot" a ~ i l to infer t l~t , pl.~'sence cfslzirllon) gzrs c ~ c c ~ ~ ~ r i ~ ~ l ~ ~ t i o l z ~ 071 i k i ~ basis'. They advised that unless

and volume, providing a fe\\~ important assumptions arc made.

Pressu7,e: In practice, gas prcssure is most easily quoted as an overpressure (i.e. the pressure above hydrostatic), rather than an absolute value. Gas pressure can be assunled to equal hydrostatic pressure - defined in Equation (7.2) -at the bottom of the gas accumulatioi1. 7 . 1 he gas overpressure (Pg,) at the top of the reservoir can then be calculated from the density contrast between gas and pore water and the height of the gas column (see Equation 7.12). Gas-column h c i ~ h t may be estimated from seismic data if there is a 'flat spot' (see Section 6.2.2) to indicate the base of the gas column (the gas- water contact) and a 'bright spot' (scc Section 6.2.2) indicating the top of the gas.

Voluine: Gas reservoir volumc can be estimated from gas col~lmn height and the areal extent of the reservoir if sediment porosity is known or can be estimated.

This approach may provide approximate values (as dcmonstrated by Salisbur!; 1990), but uncertainties about values of the various parameters mean there are shortcomings (Salisbur!; 1990). Another assump- tion made bj- this method is that the gas accumulation is not hydraulically connected to a dceper gas source, for example by a fault.

predictions of gas were reliable the interpreters ~vould lose their credibility n-ith drillers. Other papers (c.g. Games, 1990; \;\;alker, 1990) in thc same book (Ardus 11.3.2 Hydrogen sulphide and Green, 1990) cinphasised the need to choose seismic data acquisition and processing paramctcrs with care, as the nccd to identify shallow gas correctly may be frustrated if these are not appropriate. It is now common practice to use the top section of three-dimensional (31)) exploration seislnic data sets (specially repro- cessed) to provide initial indications of gas accumulations. Although it has been common to follo\r this with specialist high-resolution seislnic survcys, particularly n~here it is thought that gas accumulations may be present, Sharp and Samuel (2004) concluded that this may not be necessary. It is clear that correct interpretation is still a matter of skill and experience.

In concentrations abol-e 200 pprn, hydrogen sulphide is lethal. The gas is highly reactive and mill render vic- tiins hopelessly suffocated within minutes of exposure. This colourless, flammable, and dense (heavier than air) gas often occurs in sediments at or near methane seep sites. In non-lethal concentrations just above 10 ppm, thc gas smells like 'rotten egg.sl, but the ability to sinell it is lost after only a fen. minutes of exposure when the concentration approaches the dangcr level of 100 ppm. Extreme care must therefore bc taken when smelly cores are handled inside confined laboratories. During drilling and sampling on ODP Leg 146 (Chsca- dia Accretionar) Prism, Hydrate Ridge), the hydrogen sulphide alarm went offand no corcs were allo~ved inside

E S T I M 4 T I N G G A S P R E S S U R K S

4 N D V O L U M E S

before they had degassed outside. New safety equipment (air dilution fans, hose-fed air packs, and gas eracua- tion fans) and procedures have made handling hj-dro-

If there is evidence of a shallow gas ;~ccumulation on gen sulphide cores safcr during ODP work (Grabcr, seismic data, it may be possible to estimate gas pressure 2002).

to quantirq- nyurocaroon r ~ u x rrom me seaoeu ~ I I L ~ LIie So fa]; \-cry fc\\- incidents due to iir sit11 gas hydrates have

ocean, and because of the dynamic nature of hydrates, bccn rcnortcd b\- thc 11\-drclcarhon industr~r. Indications long-tcrm moni ' " ' of 11-l~ut ma! happen when gas 11)-drate-hearing sedi- hydrates could bc Lc,clul J L ments are disturbed have, however, been documented

b! scieiltific drilling. The ODP has drilled and cored tion sites. McGee and lioolsey ( IYYY) reported I

through BSRs on at least five scientific legs and has performing such studies in the Gulf " " '

Edmonds et 01. (2001) consiaereu mar, nrnen sampled gas hydrates associated with BSRs and sub- . . . . . drilling througb ' ' marine mud volcanoes on several of its legs (Hovland prel-ent dissociation. 1 hey recommended doing thc

~:t irl . , 1999b). Gas hydrates in sediment could dissoci- - .. . folio\\-ing: ate releasing gas, or the opposite may occur; free gas

rcleased by drilling may form gas hydratcs elsemherc in scdimcnts, ncxt to the drill-hole. - ,..:IT .- . L 11-,...-..

\Vhe11 drilli~lg through natural hydrates the greatest conccrn is associated with thc production of warm hydrocarbons, heating surrou~lding formations and causing hj~drates to decompose. For example, 'S~LIJJ I In)l- t.1.s' were repeatedly cncountered when drilling through nlari~le sediments abow a prominent BSR during ODP 1.eg 146 (Westbrook el nl., 1993). These layers mere tirstly intcrprctcd as having bccn scvcrclj disturbed b!- drilling operations, but latcr turncd out to havc hccn gas hydrate bearing. On depressurisation, dissociation led to the rclcasc of watcl; ~vhich affcctcd thc scdimcnts and turncd them into a totally structurclcss 'soup'. Potcn- tial conscqucnccs of this rapid reduction in scdimcnt strcngth and porc fluid prcssurc incrcasc (if c ~ o l ~ c d gas cannot escape) include damage to drilling equip- mcnt and seabed installations; examples of casing collapse due to excess local prcssurc causcd b! dissociating gas h!-drates are k n o ~ ~ - n to haw occurred. C;onsequentl>-, an!- offshore operation in gas hydratc-pronc arcns must be sensiti~~e to the potential for gas h ~ d r a t e dissocin- tion, and thc gossiblc conscqucnccs (Bouriah rt ( { I . , 2000; Ho~~land and Gudinestad, 2001). The assessment of a potential drilling site, deep-water construction site, or pipeline route must iilclude an evaluation of the likelihood that gas hj-drates ma!- bc prcsent, or might form. It is neccssary to use and intcrprct all available indications, including indirect (seismic, sonar, and topographic features), and direct (~~isua l observation and seabed sain- pling) means (hIax and Miles, 1999). An assessment of gas hydratc potcntial should also includc theoretical considerations. Because the regional and local diffusivc and focussed flux of light h j d r o ~ ~ r b o n s through sedi-

increase mud circulation to ensure turbulent flo\\l and

high heat transfer and to remove any gas; use a chemical additive (e.g. lecithin) to stabilise the hydrate zone; run high-strength casing in thc hydratc zonc bcforc drilling dccper.

In their opinion it is unwise to encourage 'co~zt~nllerlilis- socialion', removing evolved gas. They considered this approach 'pot~ntiolly nzol-e h/lzil~/ous'.

Gas l~yr/r.rr/rs irtside cased wells Barlier ancl Gclmez (1989) reported deep-water (350 in) d r i l l i ~ ~ g ~ r o h l c r ~ ~ s caused by gas hydrates offthe US \Irest coast. G:ls entercd the ~vc11, and thc ltill opcration, which took sewn d;lys, n-as seriously hampered bj- 11)-dratc icc forming on the BOP, choke line, kill line, and the riser. Iluring nothe her incident, in the Gulf of Mexico, in 950 In of\\-atel; the BOP failed to operate properll- due to gas hydrates, cnusing a prolonged well control operation. If h!-dratcs form in the drilling fluid the! cause a change in mud properties, ~vhich can lead to harytcs scttling out.

Barlicr and Gomcz (1989) summarised the ad\ erse effects of hydrate formation during \\-cll control ogcrations:

cholte and ltill-linc plugging, which prewnts their use in well circulation; plug formation at or helon- the BOP, preventing \I-cll- pressurc monitoring below the BOP; plug tbrmation around the drill string in the riser, BOP, or casing, prcvcnting drill-string movement;

plug formation between the drill string and the BOP, ~ilell. In extreme events there are long-lasting uncon- preventing full BOP closure; trolled flows of overpressured water and sand nrhich plug formation in the rain cavity of a closed BOP, have caused n-ell damage, casing damage, bent drill pipe, preventing the BOP froin fully opening. fhundation failurc, and complete loss of the hole. In some

cases eruptions from overpressured sands have resulted

Gns hyd~nte.f irvzazion i n drilling rrlltd in scabed cratcring, mounds, and cracks (Schultz and

T h e likelihood of hydrate formation in watcr-based mud Pickerin& 2002).

is higher than in oil-bascd mud, but water is alwa~-s As well as 'wet blowouts', geopressured sediments

present in the lnud system, and hpdratc formation is p(,s- 'nay also leak to the surface up the annulus betmcen

sible with any mud formulation. Shut-in situations, and casing and formation if the casing is Po1

cooling, particularly in choke and kill-lines (which are Other causes, brougllt about by errors in drilling, I

in mud include: sured sediments are charged and overpressured by exccss drilling mud pressure, but dischargc when cir-

keeping the temperature above or thc pressure below culation stops), parallels Mrith shalloJv-gas problems hydrate for~natioil conditions; include the fact that S W F is most ccmnlon when drilling using chemicals to depress thc hydrate forma- into overpressured zones; these include isolated sand

hydrate crl-stals; 2000). Regional studies have shown certain formations

adding chemicals that modify the growth of hydratc crystals to prevent agglomeration, so that solid plugs do not form.

Shnllotn-matev,floms Many petroleum wells have becn 'lost' because ofuncon- trollcd 'shallo~v-water flow' (SWF). Shallow-water-flow problems werc first encountered in 1985 during drilling in the Gul i of hiexico. Since then they have cost several hundred million dollars, and have occurred in many deep-water petroleum provinces including: the Caspian Sea, Norwegian Sea, North Sea, offshore West Africa, Caribbean (Alberty, 1998; Ostermeier et al., 2000). This is mainly a deep (> 500 m) water condition found when drilling at least 400 m below the seabed, but sometimes problems have been encountered both in shal-

Gulf of Mexico is on the continental slope associated with rapid late-Pleistocene sedimentation from the Mis- sissippi River.

According to Alberty et ul. (1999) the most common causes of overpressuring are differential compaction and compaction disequilibrium, but we suggest other processes may also be responsible. These might include tcctonic pressure, h~rdraulic connection with underlying overprcssured formations, etc., described in Chapter 7 as being responsible for gas ovcrpressures and sand intrusions. There is no seismic signature of overpressured water, so identifying potential shallow- water-flow zones is difficult. If overpressured water is associated with gas, then seismic gas detection procedures are useful, but gas may not be present. Two main approaches to S W F prediction have emerged (see Ostcr-

I ing open-hole drilling (i.e. before a marine riser and blowout prevcnter havc been installed) into oserpres- Detailed 2D and 3D high-resolution seisrnics to iden- sured sands. Isolated sand bodies enclosed within finer tify and map sediment facies and feature types known sediments may be uncompacted as pore water has becn to bc associated with S W F (McConnell, 2000; Wood unable to escape during burial (see Section 7.5.1). When ~t NI., 2000); shear wave analyses are also applicable such sands are penetrated, a sand slurry escapes up thc (Schultz and Picliering, 2002).

Hazards to seabed installations 369

Site-specific investigations of the pore fluid prcssure 2. Activc pockmarks formed in the .\rabian Gulfwithin environment, including studies of geotechnical wells a one-year period may have been triggered by the drilled before the petroleum well, and real-timc mea- construction of a platform. Gas was not actually surcments using h1WD (measurement while drilling released by the construction but triggering was using gamma ray and multi-sensor resistikity tools causcd by the disruption of the pore pressure envi- to predict lithology), SM'D (seismic while drilling ronment. Although Ellis and McGuinness (1986), in ~ h i c h either the source or receiver is deployed in xvho reported thesc pockmarks, did not divulge their the water column, and the other is downhole; Dutta sizes, their existence demonstrates that pockmarks and Nutt, 1998), and PWD (pressure while drilling). can be formed within a span time that is short not Ostermeier et (11. considered that PWD was probably only on a geological time-scale but also in human the most important technique as it provides almost terms. instantaneous variations.

indications of downhole pressure

The strategy for dealing with SWF problems used to be to move off site. Experiencc, particularly from the Gulf of RiIexico, has sho\vn that with careful planning and prognosis, it is possible to stay on site and cope with the problems. For example, the Garden Banks 785, No. 1 well was successful, even though conditions were verj- difficult and would normally have given shallow;- water flow Methods employed, using carefully weighted muds, nitrogen-foamed cement, and active use of the BOP, were described b!- Corthay (1998).

In both these cases human intervention triggered gas escape, but therc may be natural ~riggers. i\lthough it seems that pockmarks are potentially hazardous, the absence of shallo\v gas or a suitable groundwater source may suggest a very low risk. Perhaps the absence of incidents, cvcn in heavily pockmark areas of the North Sea, suggcsts they pose little danger. Or have we just bcen lucky?

Even if pockmarks present no risk, they are obsta- clcs that may affect installations, particularly pipcli~les. It is inadvisable for pipelines to span across pockinarks for several reasons: if a critical span length is exceeded

11.4 H A Z A R D S T O S E A B E D I N S T A I L L . \ T I O N S

the 01-erstvessed pipe may buckle; harmonic vibrations caused by water currents may result in the shedding of the concrete coating that provides negative buoyancy; and the chance of fishing gear or anchors snagging the

some features associated with seabed fluid flow are obui- pipeline is significantly increased. Modern pipe-laying

ous obstacles to seabed installations, ~~~i~~ mud volca- techniques can position a pipe on the seabed with great

noes and llYdrothermal vents should be avoided by both accuracy (to within a metre of the predeter~nined opti-

installations, such as occupying a small arcs, mum position) men in hundreds of metres of water. It

and by pipelines and cables, H ~ ~ ~ ~ ~ ~ , there are more is possible to avoid seabed obstructions such as pock-

subtle hazards. marks, provided a suitable route is available. Ho~vever, thcre are alternative strategies, such as trenching thc pipeline, and dumping rock in pocltmarlts.

11.4.1 Pockmarks a s seabed obstacles

Since we have been interested in pockmarks, the offshore industry has wanted to know the rate at which they form in order to assess their hazard potential. Unfortunately, as is clear from Section 7.6.3, there is no simple answer. The only guidance that can be given is as follows.

1. Catastrophic gas escape can form large, deep pockmarks in very short periods of time, as demonstrated by some of the shallo~\-gas hlou~outs mentioned in Section 11.3.1. Natural gas-escape events might lead to similar results.

11.4.2 Trenching through MDAC

The widespread MDAC (methane-derived authigenic carbonate) encountered during trenching operations in the Nor~q-egian North Sea (see Section 3.5.3) seriously impeded trenching operations. Coinrie et rrl. (2002) found that weakly cemented sediments, continuous over distances of several hundreds of metres, had no significant impact on trcnching speed or depth of burial, but more competent blocks of MDAC did. Trenching speeds were rcduced from 600 or 800 to 200 m hK1, and

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Eruptiolls and natural blowouts 07 1

IHT:.i\ Y S T R U C T U R E S inhibitors is anothcr. Edmonds e t ill. (2001) suggested four basic mcthods of remo1 ing h!-drate blockages: Hea1-p structures placed on the scabed will exert addi-

tional prcssure on the sediments, thus changing ambient dcpressurisation to dissociate the h j drate; conditions. Such structures are typically linked to hot addition of chemical inhihitors such as methanol or hj-drocarbons in casings, manifolds, or pipelines. Their glycols, which chi~nge thc stability boundary and melt stability would be thrcatened if gas release caused con- the hydrate; siderable soil mol-ements or if gas pressure is allowed to cxtcrnal (electrical) heating of pipes to dissociate thc build up under compartments of the structure when h! drate; the! protrudc into the seabed. Such compartments mechi~nical (drilling). should therefore be cquipped with rentilation for gas pressurc release.

S U C T I O N I N C H O R S

Suction anchors are becoming increasingly popular in deep water for holding floating structures on station. As for thc heavy structures discussed abovc, gas could accumulate inside the anchors threatening their safe operation. If thc anchors exert a suction force, pressurc reduction could cause local gas hydrate dissociation with possihle gas build-up insidc the anchor buckets and soil moTemcnts disturbing the friction forccs along the walls of the anchor. In order to avoid hazardous situations, suction in thc anchors should be kept to a minimum fbr locations where therc is a danger of disturbing in sill/

gas hydrates. In such areas forces on the anchor should, whenever possible, be taken up horizontally rathcr than uerticall~ employing long mooring lines on the seabcd. Becausc gas hydrate dissociation may lead to mcthane release and anoxic conditions in the scdirnents, it also leads to increases in scdiment poremater sulphide content, which could represcnt an aggressi\-e corrosive environment for steel structures. This could call for extra corrosion protection (Sahling e t (TI., 1999).

Pipelines nndflomlines

The handling of gas hydrates inside pipelines, production units, etc.. is ;I scicnce, and indeed, a business in its own right, but the!- are fundamentally the samc as those used to handle hydrates in drilling mud (Section 11.3.3). T h e risk of forming blocking gas hydrate plugs in f lo~l incs and trunk pipelines increases with ]Ires- surc, low temperature, and the amount of water ~nixcd in with thc hydrocarbons. Also any loss of flow (tur- bulence and mixing) will encourage hydratc formation. Electrical heating of pipcs is one common method of controlling hydratcs in subsea flowlines, adding kinetic

M7hcrc dcpressurisation is used, the differential pressure across the plug should not be allonrcd to bccomc too large or a 'projectile' may forin in thc pipework. In a pipcline this is best achicl-ed by depressurising from both ends of the h!-drate plug. Recent research suggests that hcat tracing is a 1-iahle option for melting or preventing a h5-drate blockage. Using coilcd tubing to circulate hot m t e r from thc surfacc is another possibility. With both techniques, the lines I\-ould need to be insulated.

Flowlines transporting hot fluids are pronc to uphcaval buckling due to thermal expansion and failure of the stability dcsign (gravel or concrete mat- tress cover). Heating could cause gas hydratcs in thc sediments to dissociate, rendering thc scabed 'soupy', and causing a loss of pipeline stabilit). and initiating buckling.

1 1 . 5 E R U P T I O N S A N D N A T U R A L B L O W O U T S

Submuine ~o lcan ic eruptioils are knonn to hc ha^- ardous to shipping. Unfortunately, this was demon- stratcd very dramatically on 24 September 1953 \\hen the research vessel No.5 KAIYO-R/lARU from the Hydrographic Department of the Japancsc hlaritimc Safety Agencj \ \as sunk by an eruption lvhilst conducting a survey of AIyojin-Sho Submarine Lhlcano on the Izu-Ogasalvara (Bonin) Island Arc. All 31 people on hoard were killed (Morimoto, 1960). In this case it secms that thc ship \\.as holed by rocks blown through the water by the force of thc eruption. T o avoid a similar f'atc befalling shipping, there is a 1.5 km exclusion zone around a ~rolcano called Kick 'em Jenny, 8 km north of Grenada, in the eastern Caribbean. The warning to shipping says that eT'en hen Kick 'em Jcnn!- is quict, there is a dangcr of a loss of buoyancy duc to gas bubbles rising from thc crater (SRU, 2000).


Another interesting discovery associated with a volcano was that of the wreck or a classical Greek ship. Divers found it in about 32 m of watcr off thc tiny island of Dattilo, near Sicily. T h e wrccli is reported to be 'actz~allJ, lying on the soji bubbling mud o f a living volcano' (Yellowless, 1987). Was this ship also a victim of a volcanic cruption?

11 .5 .1 Gas-induced b~~oyancy loss

Vigorous gas rclcasc, whether from blow outs, gas 11)-drate decon~position, or natural venting could havc consequences for thc buoyancy and stability of surface vessels, and has been demonstrated bq the loss of drilling rigs during blowouts. The idea that natural gas escapes from the seabed could be hazardous to shipping was first expressed by R. D. McIver in 1982. H e suggested that large volumes of gas might 'ruslz to tRc s~/i:f;c~.r' if a gas hydrate scal was breached. He thought that a rapid and localised gas escape would have an effect 'illenticrrl ~ a ~ i t h ~ h a l q f n blon~out caused 611 nzari~zedrillinp operi~tions (i.e., t11~re mould be n pnlch o fh igh41 ugitatedjioth,y water of' - % , . , ' < I , ) Ion] relutive IZensil~~)'. He advised that ' i ~ q l vessel

acritlenlnl<~~ e~zcounteving this patch m o ~ ~ l d lose b z ~ o , ~ ~ n ~ z g ~ anilsink aer.11 i/z~iclZ(~i'. R/IcI\,er's coninieilts were spccifi- cally concerned with the 'Bermuda Triangle'. Before we think about this we must examine the feasibility of sinli- ing ships with gas. Surprisingly this topic has not been studicd by naval architects. However, Brucc Dcnardo of thc Naval Postgraduate School, Rlontere); Califor- nia, invcstigated the reduction in buoyanel- of spherical bodies in gassy water (Denardo et al. , 2001), and Ma)- and Monaghan (2003), of the School of Mathcniatics, Monash University, Australia, asked: ' C a n asingle b~ihblc sink a ship.?'

According to Archimedes' Principle, a floating body floats when buoyancy force equals the weight of the displaced fluid. But, if thc density of the water is reduced by gas bubbles, then buoyancy is reduced and the ship \\-ill sit lower in the watcr, displacing more water until equilibrium is regained or the ship sinks. IHo\verer, there are othcr consequences, as explained by Bondarcv ct (11. (2002). They reported that ashallo~7-gas blowout in the Pechora Sca (see Figure 7.9) caused the drillship's hydroacoustic positioning system to fail and stoppcd both the main engines, leaving the ship in a dangerous situation, tilting 5-7" to the stern. Fortunatel!; ' i ~ t tlrr

l u t strokes o f l h e booster, the ship succeeded in leuzing the d~~r7gerous zone '.

Many drilling rigs have bccn lost because of a loss of buoyancy during blowouts. However, buoyancy loss is not necessarily on]!- caused by drilling accidents or gas h!-dratc dissociation.

Tlzr 111~~strq8 o f the Wilcli 's H o l e An echo sounder profile recorded during one of the first site investigations in the North Sea, for RP's For- ties field, showed a pockmark with a small, near-vertical water-column target in it (Figure 2.2). In our first book wc prcscnted this as early evidence of gas escape from a pockmarli and, likc BP, we regarded this as support for thc theory that pockmarks were formed by gas escape. Subsequently the British Geological Surve! (BGS) identified an unusual pockmark in their South Fladen pockmark study area. This is probabl) the same fcature. It is unusually large (about 120 m across and 2 or 3 111 deep), but it is distinctive because of the large number of smaller pockmarks which surround it, g i ~ ~ i n g it a pcpper-pot appearance on side-scan sonar records (Figure 2.20). Also, there is acoustic turbidit). indicating very shallo~l- (about 10 m subseabed) gas beneath it (Figure 2.19). The BGS named this pockmark the Witch's Hole (see Section 2.3.1).

In 1987 Total Oil hlarine plc agreed to run some sur\~ey lines across the Witch's Hole. We asked thcm to run side-scan sonar, and to keep the towfish close to the seabed to get a good look at the 'gas plume'. To their surprise, the tomfish hit thc water-column target, which proved to be not gas, but a shipwreck! A report of this wreck, based on thc Total data, was published (Judd, 1990), together with some speculation about how it camc to lic right in the middle ofthis unusual pockmark. Did it land thcrc hj-chance, or was it sunk by gas escapingfrom the Witch's Hole? The riddlc ofthe wreck in the Mlitch's Hole remained forgotten (almost!) until about 10 years later one of us (AGJ) was approached to make a television programme about methane ( T h e Nortlz S e a ' s Be~.rn~~illi Triangle- part of the Savage Pl117iet series; Granada 'TV, 2002). Fugro U D I Ltd.'s ROV support ship, the Skandi Inspcctor, was used and T V cameras recorded footage of the wreck. This was identified by Robert Prescott and Mark Lawrcnce of the Scottish Institute for Mar- itimc Studies (Universitv of St Andrens) as an early twentieth-ccntury steam trawler. Its exact identity has not been discovcrcd.

Eruptions and natural blowouts

Fipure l1 .Y T h e Witch's Holc: multibearn echo sounder image approximately 120 111 across. LX1' = lo\vcst astronomical tide, i.e.

of the Witch's Holc; the shipwreck is clearly seen standing dcpth (in m) belovv sca Icvel. (Image courtesy of Richard

uprigh~ in the centre of the pockmark. T h c pockmark is Salisbur), Fugro SLI~T-ey Ltd.)

The RO\- surveq, and subsequent multibeam echo lire now knoq that the wreck in the 11'itch's Hole sounder surveys, have shown that the wreck does lie right in the middle of the Witch's Hole (Figure 11.5). Apart from decay (rusting plates, etc.) visual inspection of the hull revealed no evidence of damage that might have caused the ship to sink; damage to the superstruc- ture was probably caused later by fishing gear. Ho~vevel; no evidence of active gas seepage was seen either. The mystery remains unsolved. Is it possible that ships coultl be sunk by escaping gas? The probability of a ship land- ing by chance on the seabed right in the middle of a pockmark seems \-ery small. In the South Fladen area only about 8% of the seabed is occupied by pockmarks, but there are no other pockmarlis like the Witch's Hole in this 57 km2 area, and < 70h of the South Fladen area is underlain by gassy sediments, so why is the n-reck in this one? The probability is much less than 890 (0.08). However, the alternative is that the ship was sailing over the IVitch's Hole precisely when it was leaking a large volu~ne of gas. The probability of this is much smaller still! An alternative (and more probable) explanation is that the 'pockmark' \vas formed by the impact when the ship landed on the soft sediments.

is not unique. Two other pockmarks, both similar in appearance to the Witch's Hole, and both with sonar targets looking like a-recks (yet to be confirmed) were identified in 2000 (Judd, 2001). Also, the Sliagerrali sccl~s studied by Dando et al. (1994a) were reported to be close to a wreck. Are these coincidences, or is there really a 'Bermuda Triangle' in the North Sea?

The Bertjrrrda T~iangle Tdes of the loss of ships and aircraft in the so-called Bermuda Triangle arc probably exaggerated. Some commentators consider that there is nothing unusual in this area or the number of losses here. However, McIver (1982) a ~ i d others have pointed out that this area at least partially ovcrlies the gas hydrate-bearing sediments of Blake Ridge (see Section 3.27.2). Consequently, if there was a sudden h!~drate dissociation event, perhaps trig- gcrcd by earthquake activity and an associated seabed slope failure, then massive amounts of methane might be released. McIver suggested that such events might affect not only ships in the area at the time, but also


aircraft (concentrations of mcthanc in the air T T - O U ~ ~ Here we can only provide a brief introduction to cause cnginc fi,~ilure). somc of thcse benefits.

T h e benefits of seabed fluid flov fall into three main groups

1. Direct benefits: fluids and associatcd materials with resource potential. This catcgorl- includcs seeping freshwatel; which has bcen uscd, at lcast on a minor scale, for centuries. Geothermal and hydrothermal fluids might be uscd to suppll- heat (as it does on land in countries such as Iceland, Ital!; and New Zealand), and thcre is potentic11 for csploiting minerals concentrated in hydrothermal fluids. Howe\-er, thc most significant direct bcnefits and potential bell- cfits come from vast accu~llulatioils of ore lnillerals associatcd with hydrothermal venting, and thc utili- sation of methane in the form of seeping gas c~ncl gas hydrates.

2. Benefits a s a n indicator: because thcy disperse abovc the seabed, venting and seeping fluids indicate thc presence of a source. Once thc technological challeilgcs of esploiting seabed hydrothermal minerals havc been overcome, the detectio~l of hydrothermal fluids may prove to be a valuable exploration tool. Whereas this is a tool for the future, natural oil and gas sceps have bccn uscd to guide petroleum exploration for centuries.

3. Indirect benefits: the principal indircct bcnefits of seabed fluid flow are biological. The cnhanccment of biological activity by natural vents and sccps is not rcstrictcd to hot and cold chemos!~nthetic communities. Riotcchnologists and biochemists recognisc that vent cornmuilitics include microbes with capabilities not possessed by other organisms. Some of these are alreadq- being utilised, for example to process indus- trial sulpllide wastes, producing biomass that is itself useable (Ihn Dover, 2000) and for production of pristine bioproteins (Hovland and IZIortenscn, 1999). T h e injection of microbcs, substrates for microbcs, and nutrients (including hitchhikers) into the ~vater column havc as yct unquantificd benefits to biological productivity Is it a coincidence that petroleum basins such as the North Sea and the Ne~~foundland Grand Banks arc (or were, hefore 01-erexploitation) prolific fishing grounds? \Ne think not.

11.6.1 Metallic ore deposits

I2,Ictalliferous ore bodics haw been exploited for mil- Icnilia. For example, Romans mined the North Penilinc Oreticld, in Northern England. By thc end of the tncn- ticth century > 4 x 10" t of lead concentrate, 2.4 x lo6 t of barium minerals, and 2.1 x 10" t of fluorspar had bccn extracted, along TI-it11 iron, copper, and zinc mill- erals (Dunham, 1990). Hannington rt a/ . (1995) estimated global ~nassive sulphidc mineral production and reserves from 'fossil' hydrothermal deposits to be at least fire billion tonncs. Volcanogenic-hostcd m:lssi\-e sulphide dcposits form in subduction-relatctl island- arc settings (Kurolio type), at mid-ocean or back-arc spreading centres (Cyprus type), and in sediment-filled sprcading centres (Besshi-type deposits); cach has a characteristic mineral assemblage. Once it was understood that such ore bodies are linked to seabed fluid flow; the existence of ore bodies beneath the seabed TT~;IS

rcalised. For example, the ore bod! in thc TAG field, 11-hich is of the Cyprus type, comprises nearly four million tonnes (Hannington et d., lC)95), and the metalliferous brines of the Atlantis I1 Deep in the Red Sea constitute a dcposit of over 90 million tonnes.

Only a few seabed ore bodies have been found to date, but it is clear that thc j are represcntatircs of a massi~~e and widespread resource. Econonlic esploita- tion of these dcposits has yet to take place, but they arc significant to the present-day metalliferous mining industry bccausc, IIOTT- that thcir modc of forlnation is understood, 'fossil' occurrences can be searched fbr with greater efficiency. Equall); field studies on land provide evidence of the pcological settings in ~vhich modern h~~drothermal deposits are likely to be found. It secms that back-arc settings are particularly fa~iourable for ancient economic ores (Scott, 1995).

Full coverapc of this important topic is clearly bcyond the scope of this book; we refer readers to specialist tests such as Scott (1 997) and the Special Issue of Ec.ono71tic Gtologl~ prefaccd by Rona and Scott (1993).

11.6.2 Exploiting gas seeps

In 1982 , 1 K O (Atlantic Richficld Company) installed two large steel pyramids to capture the seeping fluids


T h e d i s l r i b u ~ i o n o f g a s hydrutes Hovland et nl. (1997a) suggested that the most promis- ing arcas to look for gas hydrate resourccs arc active deep-water mud volcanoes. Rlilkov and Sassen (2002) suggested that 'structural accumulations' (i.e. those associated with faulting, mud volcanoes, and other geological structures) such as those in northwestern Gulf of Mexico, and Hydrate Ridge, Cascadia Margin have the greatest co~nmercial promise. They also thought some stratigraphic accumulations where the hydrate is widely disseminated in coarsc sediments may be com- mercially viable; the massive methane hydrates held in thick deposits of sandy turbidites in the Nankai Trough offJapan are an example which is receiving close attention.

Thcrc have been many attempts to map the distribution of gas hydrates, all ofwhich seem to be out ofdate by the time they are published as new sites continue to be discovered. Like others, Kvenl-olden and Lorenson (2001) distinguished between locations where the presence of gas hydrates has been 'proved' by sampling, and those that are 'inferred', for esample by the occurrcncc of a BSR; their inventory included 19 of the former and 77 of the latter. They said that most of the sampled hydrates were rcportcd to be of microbial origin. What- ever the true distribution, it is clear from available data (some of which is revie\ved in Chapter 3; sce also Map 36) that gas hydrates are widely distributed around the world, from polar regions to equatorial regions. Soloviev (2001) estimated the distribution of gas hydrates by considering the extent of conditions suitable for the formation of gas: sedimentary basins, locations with high rates of Cenozoic sedimentation, subduction zones, and accretionary wedges. He considered that gas hydrate- prone arcas must have a minimum sediment thiclincss of 2 km. He estimated that these covered a total of 35.7 x lo6 km', about 10% of thc area of the world's oceans (Figure 11.6), and he calculated that this area was distributed between the oceans as follo~vs:

Antarctic coastal regions: 19.7"o; Arctic Ocean: 12.3'h; Atlantic Ocean: 38.2Okj; Indian Ocean: 14.4%; Pacific Ocean: 15.4'/0.

The enormity of thc gas hydrate reservoir is boosted by the fact that 1 1n3 of hydratc will, on dissociation, yield

164 m3 of gas (assuming a 90% gas-filled lattice; Collett, 2002).

1 1.6.4 Technological challenge

Of course, gas hydrates will not be viable as a resource until the technology to exploit thcm has been de~e l - opcd. The techniques that have been investigated fall into two main types: those that decompose hydrates by prcssure reduction, and those that favour heat injection (Sawyer et nl., 2000). Sevcral years ago, Japanese researchers teamed up with Canadian and US scientists to explore commercial exploitation of gas hydrates by drilling through known, thick hydrate occurrences in Arctic Canada, the Mallik \\-ell (Dalli~nore e ta / . , 1999). In 2001 this project was expanded into a ~nultinational campaign, including personnel from India and Ger- many. They have investigated both warm-water circulation and pressure release as production means of free- ing up the subsurface hydrate-locked gas. Adam (2002) reported that the amount of methane produced was encouraging: 'errol/g/~ to ignite a,flnre sinlililr to those xeen hui.rri~l,c o u r oil 1.i3.c'. However, he was not sure 'w/ze/l~cr 1/2~~,~~cllo1a,/ii1t~ze is ~ j ~ ~ l l ~ o l i c or a genuine step fbrmards'.

Releasing thc gas from the hydrate is only one stage of exploitation. Many more technological challcngcs must be met before hydrate energy is produced.

F z ~ l u r e perspectives Despite strategic interest and obvious signs of progress, it mill take several more years beforc the dream of hydrate cnergy is turned into reality. The results of a survey of industry specialists led Bil(2000) to concludc that onshore hydrates ma!, be developed by 2015, but offshore hydrates will take much longer to exploit, 2060 was thought realistic. Nevertheless, the 'Big Prize', vast quantities of 'clean' energy, will surely drive research onwards. As at least 95% of hydrates are in continental- slope sediments ' ~ k e qfilrore re$lPesents t / ~ e , f i ~ n d ~ ~ n z e n t a l c/zallen~z rrndpotentzull~i the greatest remord' (Bil, 2000).

Explora t ion , f i r hydrocarbons Sceps are effective tools for determining whether or not a sedimentary basin has petroleum potential. They shon that the 'petroleum sy stem' is \\ orking, thdt source rocks are present, and that they are mature. Link (1952), Hedberg (1981), and many others have commented on

378 Iinplicatioils for mail

the importance of secps to cxploration: 'Historicr~llj~, nzost oftlze morl/l's t?z/!~rpe~rolez~~~z-benri~zg al.easa~zd azaqt

q f'ils I~~rgesl oil atzd grzs jelrls ~u~>r~,.first calle/l to attentiolz because of'ckihle oil n ~ z d gas seepages' (Hedberg, 1981). Homcvcr, thc absence of seepage does not mean that there is no petrolcum; it may indicate an abscnce of migration pathways.

Because of thcir valuc as an cxploration tool, con- sidcrable efforts have been expended to develop effective seep-detecting technologics. All major oil companies have made use of them. Clarke and Cleverly (1991) reported that BP had compiled ;i seep database. Kor- nacki el nl. (1994) summarised Shell's successful use of seeps in evaluating the petroleum potential of the continental slope of the Gulf of Allexico. A benchmark papcr by Thrashcr et al. (1996) not only re~~iewed strate-

gies used by 13P and Statoil for seep studies, but also explained thc nccd for understanding the geology of a basin when designing an exploration seep study and interpreting the results. Isaksen et (11. (2001) explained how ExxonMobil has uscd secp technology in evaluating the petroleum 'risk' in the Rockall Trough, west of the British Isles.

Seu-su~f ice seep studies Documentary evidcncc ofnatural oil slicks in the Gulf of Mexico has been available for hundreds ofyears (see Sec- tion 3.26; also offshorc California - see Section 3.22.4), the first detailed map of slick distribution was published in 1910 (Solej; 1910; see alsoMacDonald, 1998). Nowa- days, nlorc sophisticatcd techniques arc used to detect

Benefits 379

oil sliclts (natural and man-made) from abole the sca sur.Face, talting adv'lntage of characteristics summarised b! Bro~tn t7t (11. (199.5).

Oil produces a surface shecn on water, reflecting light ovcr a broad spectrum of n~a~c lcngt l~s (certain thick- nesscs cause the familiar 'rainbow' colours). Oil absorbs solar radiation and re-emits some of thc radiation as thermal energ!-; thick (> 150 pm) oil sliclts appcar 'hot' on infrared images, but thin (<-SO pm) slicks arc not visiblc. Even thin ( 4 0 1 pm) oil layers are highly reflectkc in ultraviolet (UV) light. Oil fluorcsces n~hen cxcited by UV lasers. Bccause oil dampens capillary waves on thc sca sur- F~ce, the normally 'cluttered' and chaotic radar image of the sea surface changes when oil is present. Syn- thctic aperturc radar ( S I R ) is particularly suited to oil dctcction. It can be used at night and through most clouds, although wave conditions can rnakc data unusable, so a suitablc ~veather \\-indow is required.

* Water and oil both emit microwa\-c radiation, but at different intensities, oil's bcing about tnricc as strong as nater's.

SEEP S P O T T I N G FROM SPACE

Oil seeps, including gas seeps ~ i t h oily bubblcs, product s&~-surface slicks visible from space (Figurc 11.7). Examination of a single photograph from thc space shuttle Atlantis enabled MacDonald et ( I / . (1993) to idcn- tify at lcast 124 sliclts within an arca of 15 001) km' in the Gulf of Mexico. In a later papcr MacDonald et al. (2002) used SAR (s!-nthctic aperture radar) and Land- sat data to identify seeps in the Gulf of Mexico, and to monitor their acti\-ity over timc. They found that some arecontinuousl!- active, and others are intermittent. Dc Beukelacr et a/. (2003) provided further esamplcs of slicks identified by SAR and linkcd to seabed fcatures (rccorded on sidc-scan sonar) bl- acoustic water-column plumcs.

Satellitc-based SAR surveys have been particularly successful in proving the presence of oil in frontier areas. 'Offshore Basin Screcnirlg', a remotc-sensing technique dcreloped by two UK companies, the NPA Group and TREICoL, provides confidence that more detailed exploration using surface-based techniques are likcly

Figurc 11.7* ('l'op) Oil drops renching thc sca surhce in a

discretc Sootprint (nrro~\ed); sun-glint is enhanccd in the are3 oS

floating oil. (Bottom) Photograph from the space shuttle s h o ~ ~ ~ s

suil-glint from slicks in four distinct places (arroued).

(Reproduced \\-it11 permission from hlacDonal~l c.1 nl., 2002.)

to bc ~vorthwhile, particularly nrhcn combined with othcr regional studics such as gravit!. surveys (derived from satellite altirnetry). At relatively low cost, even dccp-water basins can be screened, as demonstrated by Williams and Lawrence (2002) who identified seeps from dccp-water basins offshore Brazil and .Ingola. Of course, oil slicks arc not aln-ays a rcsult of natural secp- agc, but inspection of images takcn during succcssi\c passes of a satellite can eliminate pollution from ships


Figure 11.8' Typical pancakc-shaped oily bubbles s u ~ ~ l c i n g in ..\APG \\,hosc permission is ~.equired for further use;

the sc~uthern Caspian Sea. (I'~o111 IVilliams and Lawrence, 2002. AAPG02002.)

I.l/lPG S~zmdier in CL~olog, j~ No. 48. Reprinted b! permission of the

and oil rigs etc., and thinner natural surface films of of the seeping oil can be analysed by gas chromatogra- other natural substances (NPA Group, 2003). phy to pro\ ide details of composition.

ALF Srziffcr sur-2:eys Sniffcrs essentially comprise a towfish containing a

More detailed surveys of seeps can be obtained b j airborne studics. The most commonl!- used systcm in oil exploration is ALF, the airborne laser fluorosensor. Although data are not gcnerall!- relcased to the public domain, ALF surveys have been extensively used world- wide, proving to be an effccti,-e tool for scep detection. They are able to detect much smaller targets than SAR.

G R O U N D T R U T H l N G

Confirmation that slicks spotted by satellite- or airborne-systems are caused by natural seeps is done by shipborne 'ground truthing'. When oil-covercd bubbles break surface, the oil spreads out to form 'pancaltcs' (Figure I 1.8). These coalesce to form continuous seeps that drift away from the point i ~ t which thcy surface because of mind, wave, tide, and current forces. Samples

pump to lift \vater to the mother-ship, a stripper to cxtract gas from the n-ater, and a gas chromatograph to analyse for methane and the higher hydrocarbon gases. These have proved extremely useful as seep-detection tools. The data density- (illustrated in Figurc 3.41) is fir greater than that acquired from traditional water-bottle deployments (as described in Section lO..C.I).

Jones el n l . (1999) presented a case study from the High Island area of thc Gulf of hlcxico. The Sniffer surl~ey covered 385 ltm, including detailed grid survcy-s and regional lines. rlnalyscs were perforrncd at three-minute intervals on water pumped from about 9 m abovc the seabed, providing data points about 450 m apart. Anomalous concentrations of >500 nl 1 ~ ' methane, < 5 nl 1-' ethane, and0.5 to 1.Onl 1-' propane contrasted with background values of - 100, < 0.7, and < O..i 1111-', respectively Rclative concentrations

Implications for rrlan

6000 5000 4000 3000 2000 1000 CDP

t --.----

300 -

200 - 2 a Methane ~n

bottom waters 100 SAR oil slicks at

edge of seal

/--

0 NW ........ - ' . . , ~ . .

3694 SE

2694 0

1694 I -1 ...--_.-

694 SP I

- S 0 5

E .- - 2

1 .o

Figure 11.9' Seismic profilc across the I - ; l r~~pi Shclf, offshore diagenctic zonc) of highlj ~.etlective scabcd, n-ater-coluii~n-gas Australia, shoning the rclarioilship betyeen thc cdgc of an methane anomalies, ant1 satcllitc-dctcctcd sea-surface slicks. effective peti-olcum seal, an HRDZ (hydrocarbon-related (Reproduced n~ i t h permission limm O'Urien e l nl., 2002.)

of the wells drillcd during the evploratioll of this field offihore field (Paul Nadeau and _%ndre~~-Horbur); 2004, fell within the anomaly. personal communications).

The Cantarcll field, a super giant oil field with 17 bbl rcserves, was finnlly discovered in Junc 1977 thanks to a humble and persistent fisherman, Sefior Cantarell. H e noticed that there was always an oil seep at a ccrtain spot every timc he returned therc. I-Ie figured that if it had come frorn a boat it could not keep reappearing. H c took his story to the local office of Pemcx (the Mexican oil company), but he was laughcd out the door. H e went to the regional office and the sarnc thing happened. Finall!; hc took his story to thc main Pemex building in Mexico City and thcy listened, shot somc seismic to check, and the rest is history About four years ago Pemex, drilled beneath the C::~ntarell structure to disco)-er the Sihil structure, which alone is bigger than thc largest U S

In 1995 a 3D seismic survey n-as undertaken in an arca offshore Equatorial Guirlea, and a 'sz~spectetl IS

rlzi i t~t~c)~' Jvas identified. Subseq~~entlq- a seabcd gco- chemical exploration programme was undertaken, and three piston-core salnples were collccted fi-om the top of the chimney. Analyses proved thc presence of thermogenic oil and gas in two of these cores. These results encouraged further action; a site survcy was undertaken. Rathy-metric mapping showed there was a 'large crnlei.' (400 rn across and 17 m dccp) above the gas chimncx and shallow seismic data sl~o\ved the seabed sediments wcrc gassy, and that therc wcre seep plumes in the water. The prospect \\-as drilled and ' the success uf E.~~rell i~-1, I P / Z Z C / I ~ ? ? ~ O ~ ~ l l ~ l . e d about 200-4 [60 mJ g ioss of

&)~drocn~bon-beni,i~ig s ~ o l J , ~ .m~! f i~med tlzis ~Iirecl lqldrocal- 30°h of its normal le\-el by an anosic event in 1994, with boll i~zdicator' (Canales, 2002). serious economic implications for Namibia.

11.6.5 Benefits t o fishing? 11.6.6 Secps, vents, a n d biotechnology

Nrc have come across several anecdotes suggesting that ~ h , biotcchnologl- industry is clea-~-. .L .- - .... fish productivity is enhanced in seep areas. Orange ' resour ,,,, to be found among el al. (2002) described submarine dives on the off- chcmosrnthctic communjties ofvents and seens. l,,deetl shore part of the Eel River delta, northern Califor- it is as ttlle Irzos Pr c,,----,-, ,---., --- --- r --.. ' -- nia which documented active gas sceps. The!- reported bioactivc conlpounds ff;,,. :.- I.. .A,.;- I - - . , . ; - . . I , - . ,. . I

that local fishermen provided 'alze~.ilotirl r?'iiktni,c,fi~i,oils l.oIznle12tal, p~zarlllaceLllicai OzcbOli~g a t ~ l ze sen surfi~filce a t tlzis i'otir~ion 11s nlrll rrs/i~i. 2003). illtllough discove ir~i.r~~n.~eil,~s/zi~z~y,~~ields ill tlzis ~,cgio~z'. Con~~ersations nith present enormous potential, this remains l a rF r l t - North Sca fishermen re\-ealed that many could identify plorcd. N~ doubt this u.ill change r; 34D.%C (methane-derived authigenic carbonate), and About 7.5 km east of the Sula some associated it nit11 good fishing grounds; one loca- in ,id NorTvaJ: statoil has const,. L,,,, , ,* tion was exen referred to as the 'gas bubble' (Louise for bioprOteins, is the first ,,';+, L;, Tizzard, 2004, personal communication). located at the Tjeldbergodden proct

Is it just a coincidence that the North Sea has ural gas near ~ ~ ~ ~ d l ~ ~ i ~ . v g E i E tr tn-o major petroleum pro\-inccs and a high biolog- t h c ~ a l t c n p i p c trunlcline ical productivity? Maybe so, but there are other 180 km offshore, ~l~~ ailm coincidences; for example the Grand Banks of N e w annually using methane with ammollia foundland uscd to be(beforeseriousoverfishing) amajor and oxygen) as a for b--+-..:..- M,,+L.-,I-

fishing ground, and is also a significant oil province i . l l P , ~ l ~ ~ l , ~ l ~ ~ ~ , ~h~ pr c,(.eqq nt,l.tl.

(Lev!- and Lee, 1988). Similarly, fishing is important ral process suspect is ,,,,,,,,, b "'L""' L"

in numerous other shelf seas underlain by pctrolcum beneath large coral banlcs. ~h~ provinces. Another example of a 'coincidental' geo- g ro ,n , dried, and then mixed into animal (cattle, graphical link between sceps and biological productiv- and chiclien) and salr-- c-AA-.. TL,. ..-- ..,:..,. .L..-

ity comes from the Rias Baixas, northxi-cst Spain (see consist of700~o, Section 3.6.1), an area noted for nlussel farming. fat, the rest being fibres ,,,, LLL,,,,

Although the success of' mussel farms is normally ., losins, patlzoge,ls, ucycin oge.en s, o r , .. . .

attributed to upwelling along the Atlantic Continental l,, ,p,,l,f~s;f~e~efficts~ U, bl, Huslid, 1 c ) ~ ~ nrrsnn;l l Corn- Margin, it seems that mussels thrive best in the internal munication), plant starte(l I ,n ;, parts of the rias, \\-here there is extensive shallo\v gas it , n ,n~, l , t ;n, o.nalc h,. 30

Is this a coincidence? The link betwccn petroleum accumulations and

seeps is not so difficult. We discussed the jump from sccps to biological productivity in Section 8.3.4. Although further work is required to confirm (or den)-) the link between seabed fluid flow and biological pro- ducti~ ity, it sccms that dismissing it may be premature. -4t the other extreme, evidence from offshore Namibia (see Section 3.7.2) suggests that massivc rclcascs of' gas, including hydrogen sulphidc, from the seabed are responsible for killing fish. Weeks el nl. (2004) reported that the cape hake population was reduced to less than

1 1 . 7 Ih j lPACTS O F H U A I A N . I C T I V I T I E S O N S E A B E D F L U I D F L O W . I N D . I S S O C I A T E D F E A T U R E S

11.7.1 Potential triggers

Some human actil-ities, mainll; but not exclusively those of' the offshore pctrolcum industry, may affect scabcd and suhseahed conditions to the extent that fluid pressure regimes and migration are affected; the effect of pctrolcum production on seepage rates in the Santa Barbara Channel, California (Section 11.6.2) is an

384 lrnplications for man

example. The following activities, most of them identi- thc mounds, and had been exploiting them, causing fied by Kvalstad el a/. (2001), may affect stress and pore- damage to the reefs within two years of their discov- prcssure conditions, triggering, or potentially triggcring ery. Indeed, dwindling shallow-water fish stocks have the formation of new features, or 'events': driven trawlers into deeper and deeper waters, posing

drilling wclls creating blowouts to the seabed, with the possible formation of craters (examples were described in Section 11.3.1); underground hlowouts changing the pore pressure in shallow layers, with possihle implications for slope stability; oil production leading to hcat flow and tempcraturc increase around wells and well clusters, possibly leading to the dccomposition of gas hydrates (with implications for sediment strength, slope instability, etc.); depletion of reservoir pressurc resulting in increased effective stress, and reservoir subsidence (a clas- sic cxample of this being the Ekofisk field in the Norwegian North Sea), and stress changes in ouer- lying sediments; installation activities (rock dumping, and the emplacement of structures, particularl!. gravity struc- turcs) increasing vertical stress, with implications for pore-pressure conditions ~vithin the sediments; mooring installations and anchoring forccs imposing short- and long-term lateral forces.

A very real conccrn is that seabed installations on the continental slope will triggcr a slope failure. Should such an event occul; natural examples described in Section 11.2.1 indicate thc potential for catastrophe for structures locatcd on an area that fails, in the path\vay of the debris flow, or as a result of a tsunami (Kvalstad et a]., 2001). It would be prudent to take careful pre- cautions. We consider that a good understanding of thc fluid prcssure and plumbing systems is essential hcfore installations arc put in place. This, of coursc, mcans identifying features associated with activemigration and scabed fluid flow, and subseahed reservoirs, and taking into account existing natural triggers (as discussed in Section 7.5.5).

11.7.2 Ellvironnlelltal protection

hlapping of the Dar~vin ~Mounds, off the coast of north- west Scotland (see Section 3.5.4), revealed that deep-sea tr'nvling is a threat to biological communities of seeps and vents. It seclns that fishermen found the roundnose grcnadier (Coryphae71oz~le~ rr~pestns) fish l i ~ ing around

a severc threat to deep-water coral reefs and carbonate banks (Robcrts et al., 2003). This fishing practice is more-or-less equivalent to going logging in the Sahara, targctting all oasis-related date palms in the vast desert. If it is not restrictcd soon by regulation, we may be left only with rcmnants of deep-water rcefs and banks, having to try to understand their ecology by reconstruc- tion, rather than by true field observation and analq-- sis. In recent pears awareness of the need to protect the marine environmcnt, including the deep scabed, has led to the introduction of various forms of protection for designated areas of the seabed, including some features associated with seabed fluid flow. Various organisations havc jurisdiction within the marine environmcnt. T h e following cxamples show how some important sites are (or will be) receiving protection.

Internatiotznl prolection Under provisions of the U N Convention on the L,a~v of the Sca (UNCLOS), and other international law, every country has an ohligatio~l to protect the marine environmcnt: to 'protect arrrlpreser-ce rare o~fiilgile ecosys- tents (is 1ue0 rrs the k~~bi ta t c!fdeplrt~d, threatened or eudan- gered sper.its, anrl othe~.fornis c!fnzari~le I@' (UNCLOS, Article 194/5, 1982). In order to safeguard thosc parts of the marine cnvironment beyond the limits of national jurisdiction (known as the 'Area'), UNCLOS cstab- lished the International Seabed Authority (ISA), and determined that the Area and its resources wcre 'the coinuzon heritage o/rrr(o~kind'; the ISA --as made responsiblc for the Area, on behalf of mankind. At first, its principal function was concerned with the mining of polymctallic nodules, but according to the ISA web page (lSA, 2003), it now has responsibility for other rnarinc activities including polymetallic sulphides (i.e. hydrothcrmnl vents), cobalt-rich crusts, gas hydrates, and petroleum, and for the marine environment in gcneral. The ISA may designate sensitive sites as no- mining areas. The U N World Summit on Sustainable Development (WSSD; Johannesburg, 2002) called for action to maintain the productivity and biodivcrsity of important and vulnerable marine areas both within and beyond national jurisdiction (UNDESA, 2005). It urged nations to adopt thc ecosystem approach by 2010 and the

Impacts of human activities 385

establishment of representative networks of WIP.4s (Marine Protected Areas) by 2012. The General Assem- bly of the U N adopted Resolution a/57/L.48 (2002) endorsing the Plan of Implementation adopted at WSSD, and calling for 'urgerit ~ n d coorditzclled actio~z to protect cuii~erable beatlzic kabilnls' (UNCLOS, -1rti- cle 194/5, 1982). To encourage this process, the WWF (World Wildlife Fund) have proposed some 'pilot casc studies'. They proposed that thc Logatchev hydrothermal-vent field, the largest (200 000 m') vent field reported to date on the Atlid-Atlantic Ridge should be designated as ' a Showcase Eravzple for a Hi'yh Seas -Netrnork o f Marine Prolec~eil Areas' (UNCLOS, -4rticle 194/5, 1982).

WithinEurope, the two mainorganisations with respon- sibilities for the n~arine environment are OSP.1R and the European Commission. Each has played a different role.

to become a 'High Seas Marine Protected Area' (HSMPA).

We are unsure if any of the other sites (which include seamounts and deep-water coral reefs) are associated with seabed fluid flow, but they may be (Hovland and Risk, 2003).

Thc urgency of the call for protection is not just a reaction to their obvious fragility, but also to the increase in deep-water fishing, and the discovery that fishing gear was causing serious damage to important sites. .4fter a campaign by the WWF, the UK government finally secured official protection for the Darwin Mounds in August 2003. A news release (DEFRA, 2003) stated that he use c!f'hottom trnruls or similar tc1r2,ed nets oper- ating- ztz contact ini~lz the bottotrt of the sen' is prohibited within the arca bounded by the following coordinates: latitude 59"37' to 59'54' N, longitude 6'47' to 7 3 9 ' MT.

OSPAR T H E EL1liOPEXN COMhIISSION

The Convention for the Protection of the Marine Environment of the North-East L4tlantic (the OSPAR Convention; a product of agreements made in Oslo and Paris) has been signed and ratificd by: Belgium, Denmark, the European Commission, Finland, France, Germany, Iceland, Ireland, Luxembourg, the Nethcr- lands, Norwx!; Portugal, Spain, Sweden, Switzerland, and the UK. It provides protection for a vast arca, from thc Greenland coast to the European coast, and from the Strait of Gibraltar to the North Pole. The work of OSPAR includes pollution prevention, environmental assessment, and the protection and conserwation of ecosystems and biological diversity. The North-East Atlantic Programme of the WWF has contributed by reviewing selected habitats (Gubba!; 2002)- and recom- mending individual sites for protcction as MPAs. By 2003 they had identified 21 candidatcs, of ~vhich thc follo\ving arc associated with seabed fluid flow:

Sula Ridgc and Reef deep-water corals off Norway; the Lucky Strike hydrothermal vents, within the Exclusive Economic Zone (EEZ) of the Portuguese Azores Islands; the Darwin A'lounds; the Rainbow hjdrothermal field: this lies in international waters, so the W W F are pushing for this

In 1992 the European Comnlunity Council adopted 'Directive 92/43/EEC on the censer\-ation of natural habitats and of wild fauna and flora' (EEC, 1992). It is gencral1~- known as the 'Habitats Directive'. The Habi- tats Directive requires member states to introduce legislation enforcing the directive, and to propose a list of national sites worth!- of designation as 'Spccial Areas for Conservation' (SAICs). These, togcther with sites designated for the protection of wild birds, will form the 'Natura 2000 Network'. Two habitats associated with seabed fluid flow have been rccognised by the 'Interprc- tation Manual of European Union habitats (LC, 1999) Directive' (EUR 15/2).

1. Natura 2000 code 1170: Reefs

Rocky substrates and biogenic concretions, which xrisc from the seafloor in thc sublittoral zone, may extend into the littoral zone. Thesc rcef~ generally support a zonation of benthic communities of algae and animal species including concretions, encrustations and corallogenic concretions.

2. Natura 2000 code 1180: Sub-marine structures made by lcaking gases


Spectacular sub-marinc co~nplcx structures ations. Companies arc no longcr free to driw pipclines or consist of rocks, pavements and pillars up to foul. cables across the seabed at \\-ill, nor to drill or placc fixed metrcs high. These formations are due to the aggregation of sandstone by carbonate ccment resulting from microbial oxid;ition of gas emissions, mainlj- methane. T h c methanc most probably originated froin microbial decomposition of fossil plant materials. l 'he formations are interspersed mith gas vents that intermittently relcase gas. Thcse formations shclter a highl! di\-ersc ecos)~stern ~vith brightly colourcd specics.

NATIONAL P R O T E C T I O N

It is clear from preceding sections that, onc way- or another, individual nation states havc an obligation to care for thc marine cnrironment. The U K government has responded with tmo initiatives: a response to the Habitats Directivc (Johilsto~le el nl. , 2002) and a scries of 'Strategic Environmental Assessments' (SErls) which will eventually c o x r the whole UK continental shelfand dceper Ivatcrs to thc west of thc British Isles. Examples of features associated mith seabed fluid flow are identified in both.

Natura 2000 Codc 1180 clcarly refers to the 'Bub- bling Rcefs' of thc Kattegat (see Scction 3.3.4), and several Danish seeo sites with MDAC that havc been designatcd as Sites of Conservation Interest (SCIs; HELCOM,' 1996). There are no kno\vn equivalcnt structures in U K waters, but Johnstone zt ill. recognised that MDAC is a 'oarint ini~ qf 1lzi.s lzabitnr I J ~ ' ,

and named two pockmarlts wit11 'i.ar.bn~rirte strur-~zii.es ,fi~sfiied 411 leakirlgprses': the Scanner pockmark in Block UK15/25 (see Section 2.3.7), and thc Braemar pockmark inBlock UI<16/3. The sccond SEA (SEA2) rcport (DTI, 2001) includcd a section on pockmarks, and t ~ v o technical reports that discussed thcm and thcir biology (Judd, 2001; Dando, 2001); SEA6 included a report on MDAC in the Irish Sea Uudd, 2005b).

It is reassuring that such attention is being paid to features associated with scabed fluid flow

structures exactlj- where they want without considering the environinental consequences, including the implications for benthic habitats. To be fair, man!- companies developed environmental policies, and conducted envi- ronmcntal impact assessments before legal pressures lverc irnposed on them. At first thesc tended to be studies of specific sites, but in solnc areas consortia of operators undertook rcgional studies. A$n excellent example of this \\-as the w-ork of AFEN, the 'Atlantic Frontier Environmental Network', xhich undertook a regional environmental assessment of a large area to thc north- west of the UK (AFEN, 2002). Ho\vcver, a significant diffcreilce has heen made by legislation. For exanlple in the E U and the USA, it is non. the duty of the companj- to den~onstr;~te that proposcd work will not affect any specified habitats, even sites that have not becn designated for protection. The following example illustrates this new philosoph!~:

Chcmos~nthetic commu~lities are susceptible to physical impacts from structurc placement (including templates or subsea completions), anchoring, and pipcline installation. N T L 98-1 1 pre\cnts these physical impacts by requiring aloidancc of potential chemos! nthetic communities.

bli\'lS, 2001

It is no longer acceptable to say ' J v ~ iIiiln'1 ~ ~ Z O I I J i~ 113ns tlzere'; instead surxys must be undertaken to demonstrate the IL/J.TP~I(.P of sensitive sitcs before pernlission to proceed is grantcd. Statoil sct a good cxample by re- routing the Haltenbanken pipeline to avoid coral reek. This was recognised by the1i1LVF who, in 2003, awarded a 'Gift to the Earth' award to Norway for coral-reef preservation measures implemented on the Norwcgian Continental Shelf.

We n~elcomc this acknowledgemcnt that thc last frontier of our precious planet requires tendcr lo\-ing care. We hope such measures mill bc adopted interna-

Implications for c?ffshorr ofirrarions tionall!: and that thcy will be enough to ensure the well- The protectirc environment described abovc places a being of all sensitive sites - not only those associated great responsibility on thosc conducting offshorc oper- with seahcd fluid flow.

' HCLCOM is thc go\-er-ning hody of the Comention on thc Protccrion of thc hlarinc En\-ironment of thc Baltic Sen . b e a - morc

usuni1~- knon'n as thc Hclsinki Convention.

11 implications for man - velkommen til institutt for ... implications for man a look at the...

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