von Bröckel, Klaus

OceanRep

ARTICLELESSONS	FROM	EXPERIENCEEcho	Sounders	versus	AirBubbles	in	Research	VesselsWithin	echo	sounding	circles	it	is	wellknown	that	air	bubbles	may	have	anegative	effect	on	echo	soundingsystems.	This	article	briefly	presentsthe	German	way	of	dealing	with	airbubbles	from	sweep-down	andcavitation	processes	as	well	as	ourexperiences	with	the	last	three	largeresearch	vessels	(RVs	Polarstern,Meteor	and	Maria	S.	Merian).	Thepractical	experience	resulted	in	thedevelopment	of	an	optimised	new	hullform	for	the	replacement	of	RVSonne.	The	new	ship	is	now	underconstruction	at	a	German	shipyardand	will	be	delivered	in	autumn	2014.The	basis	of	all	echosounding	systems	is	the	directedtransmittance	of	sound	into	the	waterat	different	frequencies	and	therecording	of	its	reflection	throughsuitable	receivers.	The	reflection(echo)	occurs	at	boundary	layers	e.g.within	the	water	column,	at	sedimentsurfaces	or	fishes.	Even	boundarylayers	within	the	sediment	can	berecorded	in	this	way.	Naturally,	any	airor	other	gas	bubble	within	the	waterpossesses	a	rather	sharp	sound-reflecting	boundary	layer	too.	So,	as	long	as	there	is	only	water	through	which	thesound	is	travelling,	everything	is	fine	and	records	are	worth	reading.	But	as	soonas	air	or	any	other	gas	(e.g.	methane)	in	the	form	of	small	and	large	bubbles	(sizerange	might	be	between	less	than	1mm	to	more	than	several	cm)	gets	in	the	wayof	the	sound,	it	is	stopped	and/or	redirected.	No	or	only	bad	echo	soundingrecords	are	the	result.	Hereinafter,	all	‘gas	bubbles’	are	referred	to	as	‘airbubbles’	for	easier	reading.Two	quite	different	sources	of	air	bubbles	exist:	(1)	Obvious	are	the	bubbles	fromthe	water	surface.	With	increasing	wind	and	wave	action	more	and	more	air	ismixed	into	the	uppermost	water	layers.	By	observing	the	water	surface,	whitishwater	with	enclosed	bubbles	can	be	seen.	Also,	as	ship	movements	accelerate,	these	bubbles	are	transported	along	the	sidesand	finally	underneath	the	hull	with	the	installed	echo	sounding	transducers.	This	process	is	called	‘bubble	sweep-down’.	(2)More	hidden	are	the	bubbles	from	cavitation	processes	hereinafter	called	‘cavitation	bubbles’.	Actually,	as	the	science	behindthis	phenomenon	is	obscure	and	not	fully	understood,	the	following	description	is	more	a	theory,	than	a	proven	fact.‘Cavitation	bubbles’	come	into	existence	when	small	and	even	very	small	vortices	exist,	e.g.	at	sharp	edges	or	weld	joints.	Thewater	may	show	different	pressures	on	a	small	scale	within	these	vortices.	The	capability	of	water	to	dissolve	air	depends	onits	actual	pressure	(this	phenomenon	is	well	known	from	pressure	cookers).	So,	if	the	pressure	changes	abruptly	at	theseedges,	air	may	be	released	in	the	form	of	bubbles.	Especially,	when	seawater	is	air	saturated	or	even	air	supersaturated,bubbles	‘spring’	into	existence.	This	is	shown	schematically	in	Figure	1.	This	phenomenon	is	well-known	from	ship	propellerswhere	it	is	normally	avoided	by	a	‘cavitation-free’	design.In	general,	any	given	problem	has	several	and	sometimes	quite	different	solutions.	These	differences	may	reach	into	the	fieldof	philosophy.	Why	should	it	be	different	when	dealing	with	the	problem	of	‘echo	sounders	versus	air	bubbles’?	Thephilosophy	in	Germany	concerning	the	installation	of	echo	sounding	transducers	is	to	keep	them	as	close	to	the	bottom	aspossible.	Thus,	the	optimal	solution	would	be	a	flat	hull	and	a	flush	installation.	Actually,	in	most	German	research	vesselsecho	sounding	transducers	are	installed	as	far	as	possible	to	the	front	and	within	a	box	keel	(height	between	50	and	70cm)enlargement.Lessons	Learnt	from	RV	PolarsternPolarstern	is	an	ice-breaking	research	and	supply	vessel	(length:	118m,	width:	max.	25m,	draught:	max.	11.2m).	Launched	in	1982,	she	mainlyworks	within	the	Antarctic	and	the	Arctic	and	also	supplies	the	German	Antarctic	station.	One	of	the	most	impressive	features	of	the	ship	is	theratio	of	underwater	length	and	width.	In	a	dry	dock	she	looks	like	a	huge	duck.	From	ice-tank	model	tests	it	was	predicted	that	no	icewhatsoever	will	reach	underneath	the	ship.	Practice	killed	theory	and	after	just	the	first	cruise	into	Antarctica	the	ship’s	bottom	looked	like	anold	washboard	with	huge	dents	in	the	steel	plates	between	the	rips.	Although	rather	large	pieces	of	ice	made	their	way	down	underneath	theship,	during	normal	cruising	no	air	bubbles	interfered	with	the	echo	sounding	systems.	The	systems	were	installed	within	a	box	keel	of	about70cm	in	height.So,	lessons	learnt:	(1)	do	not	trust	all	aspects	of	tank	model	tests	and	(2)	if	a	ship	is	deep	enough	or	even	unusually	deep,	nobubbles	will	normally	reach	underneath	the	bottom.Lessons	Learnt	from	RV	MeteorMeteor	(length:	97.5m,	width:	max.	16.5m,	draught:	max.	5.6m)	is	a	globally	operating	RV.	She	came	into	service	in	1986	asone	of	the	most	modern	RVs	at	the	time.	She	has	been	working	mainly	within	the	Atlantic	Ocean	and	its	marginal	seas,	theEast	Pacific	and	the	Indian	Ocean.	During	the	planning	phase,	attention	was	paid:	(1)	to	developing	a	hull	form	that	wouldprevent	any	air	bubbles	reaching	underneath	the	hull	and	disturbing	the	function	of	the	echo	sounding	systems,	(2)	towardsan	optimal	hull	form	regarding	energy	consumption	at	maximum	as	well	as	normal	travel	speeds,	(3)	to	limiting	draught	toabout	5	metres,	so	the	ship	would	be	able	to	carry	the	German	flag	even	into	small	and	shallow	harbours.	Numerous	tanktests	were	carried	out	to	make	sure	all	these	different	tasks	were	fulfilled.	The	bubble	sweep-down	tank	tests	gave	satisfyingresults.	The	resulting	hull	form	was	characterised	by	a	huge	bulbous	bow.	The	big	surprise	during	the	first	sea	trials:	echosounding	systems	worked	poorly	or	not	at	all.	It	became	clear,	that	lots	of	surface	air	was	trapped	within	large	eddies	on	bothsides	of	the	bulbous	bow	and	swept	down	the	ship’s	sides.	Figure	2	shows	this	phenomenon	for	a	‘normal’	container	vesselon	its	way	through	Kiel	Canal.	Installation	of	cameras	below	the	ship	clearly	revealed	carpets	of	air	bubbles	being	sweptunderneath	the	ship.	Naturally,	the	bulbous	bow	had	to	be	cut	off	and	replaced	by	a	different	one.	Figure	3	shows	the	bowsbefore	and	after	conversion.	Furthermore,	echo	sounding	transducers	were	placed	within	a	lateral	extension	of	the	box	keel(height	about	50cm)	as	seen	in	Figure	4.	The	new	bow	and	the	new	installation	resulted	in	a	slightly	reduced	speed	but	also	inecho	sounding	devices	that	worked.So,	lessons	learnt:	(1)	do	not	trust	all	aspects	of	tank	model	tests,	(2)	bulbous	bows	do	not	fit	with	echo	sounding	systemsand	(3)	integrating	echo	sounding	transducers	into	an	enlarged	box	keel	away	from	the	hull	bottom	is	a	solution	that	works.Lessons	Learnt	from	RV	Maria	S.	MerianMaria	S.	Merian	(length:	94.8m,	width:	max.	19.2m,	draught:	max.	6.5m)	began	service	in	2006.	She	was	designed	according	to	theexperiences	of	Meteor.	From	the	very	beginning	there	were	no	questions	about	a	bulbous	bow	as	the	ship	was	intended	to	be	an	ice-marginvessel.	She	had	to	be	able	to	manoeuvre	between	ice-flows	and	to	break	ice.	So,	an	ice-bow	was	required.	Again	all	tank	tests	promised	no	airbubbles	would	ever	reach	the	bottom.	Surprisingly,	the	first	sea	trials	revealed	that	all	echo	sounding	systems	refused	to	give	decent	results.As	camera	systems	were	installed	underneath	the	bottom,	again	they	presented	huge	carpets	of	bubbles	passing	by.	As	there	was	nobulbous	bow,	the	source	of	the	bubbles	had	to	be	within	the	bow	itself,	namely	as	‘cavitation	bubbles’.	The	typical	icebreaker	bow	had	a	largerectangular	iron	rod	fixed	in	the	front	to	make	the	first	ice-contact.	This	rod	featured	sharp	edges	on	both	sides.	Other	edges	andunevenness,	but	less	sharp,	existed	at	the	depth	numbers	and	other	markings.	Especially,	the	edges	of	the	iron	rod	at	the	bow	supposedlyproduced	cavitation	bubbles	during	cruising.	Not	everybody	involved	was	convinced.	But	there	was	no	other	logical	reason	and/or	possibilityfor	the	echo	sounding	system	failures.	So,	the	sharp	edges	of	the	iron	rod	were	cut	off	and	ground	down.	Furthermore,	the	whole	front	part	ofthe	bow	was	smoothened.	The	situation	before	and	after	can	be	seen	in	Figure	5.	The	next	sea	trials	with	similar	sea	conditions	and	shipmovements	as	before	produced	good	echo	sounding	results.So,	lessons	learnt:	when	constructing	a	research	vessel’s	bow,	avoid	any	sharp	edges	and	unevenness	where	small	vorticescould	build	up	and	produce	cavitation	bubbles.General	Conclusions	on	Air	Bubbles	(1)	bubble	sweep-down:	-	bulbous	bow	should	be	avoided	-	echo	sounding	systems	should	be	positioned	as	far	to	the	bow	as	possible	-	echo	sounding	systems	can	be	installed	in	an	enlarged	box	keel	(2)	cavitation	bubbles:	-	their	production	and	appearance	is	actually	not	yet	fully	understood	and	thus	less	predictable	(e.g.	the	installation	on	RVMeteor	of	a	retractable	bow	thruster	with	edges	directly	underneath	the	bottom	did	not	produce	cavitation	bubbles)	-	nevertheless,	any	sharp	edges	at	the	bow	part	should	be	avoided;	it	should	be	as	smooth	as	possible	-	furthermore,	creation	of	cavitation	bubbles	can	also	produce	noise	at	different	frequencies,	which	may	interfere	with	any	acoustic	devices.Hull	Design	for	RV	SonneThe	‘old’	RV	Sonne	with	a	hull	age	of	nearly	46	years	is	to	be	replaced.	The	new	Sonne	will	be	a	deep-sea	research	vessel	(length:	116m,width:	max.	20.6m,	draught:	max.	6.6m)	thought	mainly	to	operate	in	the	Pacific	Ocean	and	its	adjacent	areas.	The	largest	echo	sounding	arrayhad	to	be	considered	(0.5°	x	1°beam-opening,	transducers	length	and	width	about	16	and	8m	respectively).	Naturally,	all	the	experiences	fromformer	research	vessels	had	to	be	taken	into	consideration.	Furthermore,	the	results	of	a	discussion	that	took	place	in	2010	during	aworkshop	on	‘Optimisation	of	Research	Vessel	Design	for	Acoustic	Sensors’	at	the	23rd	International	Research	Ship	Operators	Meeting	werekept	in	mind.	At	the	time,	a	hull	form	was	emphasised	that	would	channel	surface	born	air	bubbles	from	bubble	sweep-down	far	along	thesides.	They	would	then	only	reach	the	bottom	behind	the	installed	transducers.	This	led	to	the	development	of	a	new	hull	form	(see	Figure	6).The	hull	exhibits	a	‘dent’	along	each	side	to	channel	the	surface-originated	air-water	mixture	far	behind	the	installed	transducers.	Below	thisdent,	all	transducers,	huge	and	small	ones,	are	arranged	within	a	kind	of	integrated	gondola.	The	avoidance	of	cavitation	bubbles	is	assured	bysmoothing	the	whole	front	part	of	the	bow.	This	applied	mainly	to	welding	joints.To	date,	only	‘normal’	tank	tests	and	computer	simulations	could	be	carried	out.	Both	showed	bubble	sweep-down	behind	thelast	crossbeam	of	the	multibeam	array	(see	Figure	7).	And	although	tank	test	results	should	be	regarded	with	some	cautionas	noted	above,	they	are	still	the	tool	most	used	for	predicting	hull	performances.Naturally,	the	first	sea	tests	to	be	held	early	2014	are	eagerly	awaited,	to	see	whether	practice	will	be	consistent	with	theorythis	time.AcknowledgementsThe	experiences	and	advice	of	numerous	persons	contributed	to	the	design	and	construction	of	the	RV	Sonne.	Many	thanksto	them,	in	particular	to	the	technicians	and	scientists	of	the	Scientific-Technical	Advisory	Committee,	the	Federal	WaterwaysEngineering	and	Research	Institute	(Hamburg)	and	the	Meyer	Shipyard	(Papenburg).	This	project	was	founded	through	aBMBF-project	03F0653I	(Federal	Ministry	for	Education	and	Research).https://www.hydro-international.com/content/article/echo-sounders-versus-air-bubbles-in-research-vessels

Echosounders versus Air-Bubbles in Research Vessels

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