MEMORANDUM 

To:	 Kelly	Fortin,	EPA	Region	4	

CC:	 Eva	Land,	EPA	Region	4	and	Katie	Maness,	Anadarko	

From:	 Sara	Carlson,	Anadarko	and	Rahul	Pendse,	Trinity	

Date:	 July	18,	2014	

RE:	 Revisions	to	the	Proposed	Main	Engine	BACT	Limits	in	BlackHawk	and	BlackHornet	PSD/Title	V			

Permit	Application

This	technical	memorandum	discusses	the	need	for	a	different	approach	for	establishing	best	available	control	

technology	(BACT)	based	emission	limits	and	proposes	revisions	to	BACT	limits	included	in	the	BlackHawk	and	

the	BlackHornet	drillship	air	permit	application	submitted	to	EPA	in	September	2013.	

BACKGROUND 

The	primary	emissions	source	on	drillships	currently	permitted	under	OCS	regulations	for	operation	in	the	

eastern	Gulf	of	Mexico	(EGOM)	are	the	diesel‐fired	drillship	main	engines	(main	engines.)		The	prevention	of	

significant	deterioration	(PSD)	permits	issued	for	these	sources	contain	BACT	limits	that	limit	emissions	for	

each	pollutant	for	which	PSD	review	was	triggered.		The	BACT	limits	are	provided	in	the	form	of	grams	per	

kilowatt‐hour	(g/kW‐hr)	measured	over	an	appropriate	averaging	period	for	each	pollutant.					

Compliance	with	the	BACT	limits	in	g/kW‐hr	may	be	demonstrated	via	an	EPA‐approved	continuous	emissions	

monitoring	systems	(CEMS),	EPA‐approved	parametric	monitoring	method,	or	with	prior	written	approval	by	

the	EPA,	an	EPA	approved	source	testing	emissions	monitoring	method.		The	BlackHawk	and	the	BlackHornet	

drillships	will	be	equipped	with	the	parametric	monitoring	system.		However,	in	order	to	establish	

representative	emission	factors	and	the	BACT	limits	for	the	main	engines	on	these	drillships,	Anadarko	

conducted	the	source	tests	according	to	USEPA	methods	under	an	approved	test	protocol.		The	source	tests	were	

completed	in	June	2014	and	the	report	was	submitted	to	USEPA	on	July	12,	2014.		Consistent	with	the	previous	

air	permit	applications	submitted	to	EPA,	BACT	limits	were	proposed	in	g/kW‐hr	in	the	air	permit	applications	

for	these	drillships.	

NEED FOR REVISIONS TO BACT LIMITS 

USEPA	requires	operators	to	test	at	various	operating	loads	for	three	1‐hour	source	test	runs	at	each	load.		The	

tested	loads	are	intended	to	be	selected	to	be	most	representative	of	the	expected	range	of	operating	conditions.		

The	expected	operating	loads	for	the	BlackHawk	and	the	BlackHornet	drillships	is	expected	to	vary	between	30	

and	60	percent.		The	distribution	of	operating	loads	in	this	range	is	determined	by	the	particular	operations	

being	performed	by	the	ship	and	the	requirements	of	dynamic	positioning.	

In	general,	the	source	tests	performed	on	the	drillship	main	engines	have	indicated	actual	engine	emissions	vary	

significantly	with	operating	load.		In	general,	the	g/kW‐hr	emission	rates	increase	as	the	load	decreases.		For	

some	engines	and	pollutants,	the	tested	emissions	are	less	than	the	g/kW‐hr	limits	at	high	loads,	but	may	exceed	

the	g/kW‐hr	limits	at	lower	loads.		In	order	to	reconcile	the	permit	with	actual	engine	performance	and	

minimize	potential	non‐compliant	situations,	permit	modifications	are	often	necessary	to	build	in	more	

flexibility	for	low	load	conditions.	
Ms.	Fortin	–	Page	2	

July	18,	2014	

In	the	pre‐application	setting,	estimating	actual	engine	emissions	performance	across	the	range	of	loads	is	

difficult	due	to	factors	including	but	not	limited	to	the	following:		

Each	engine’s	performance	changes	from	the	test	bed	to	actual	operation;	

Source	test	results	have	shown	that	each	engine,	even	of	the	same	model,	performs	differently;	

Emission	control	technologies	such	as	turbocharger	do	not	function	efficiently	at	low	loads	since	they	are	

designed	for	efficient	operations	at	higher	loads;	

Operational	history	may	impact	future	performance;	and	

Main	engine	operating	loads	are	inherently	dynamic	and	may	not	be	comparable	to	the	emission	results	in	a	

controlled	test‐bed	environment.	

As	can	be	seen,	none	of	these	factors	are	related	to	improper	maintenance	or	operation	of	the	engines,	but	the	

factors	nonetheless	may	have	real	impacts	on	actual	emissions	performance	and	are	inherent	to	the	intended	

use	of	the	engines.			

USEPA	has	expressed	the	intent	to	have	BACT	limits	that	allow	for	emission	and	fuel	efficiency	comparison	

among	main	engines	on	different	drillships,	and	this	intent	has	been	the	driving	force	behind	having	the	BACT	

limits	in	g/kW‐hr	for	each	pollutant	and	each	engine.		Given	the	discrepancy	between	low	load	and	high	load	

emissions	shown	by	recent	emissions	tests	and	the	magnitude	in	the	difference	between	low	load	and	high	load	

emissions	for	some	pollutants,	the	BACT	limits	given	in	terms	of	a	single	g/kW‐hr	applicable	to	all	loads	will	be	

very	challenging,	if	not	impossible,	to	comply	with.		In	Anadarko’s	and	Trinity’s	opinion,	an	approach	to	help	

minimize	the	above	mentioned	issue	would	be	to	define	the	BACT	emission	limits,	as	proposed	in	Proposed	

BACT	Limits	section	of	this	memorandum,	to	consider	the	trend	observed	in	recent	source	test	results	without	

significantly	sacrificing	emission	and	fuel	efficiency	comparison	opportunities	provided	by	g/kW‐hr	limits.	

PROPOSED BACT LIMITS 

For	some	engines,	source	tests	have	shown	that,	although	emissions	in	terms	of	g/kW‐hr	are	higher	at	low	loads,	

the	absolute	mass	emission	rates	(lb/hr)	are	often	lower	at	low	loads	when	compared	to	high	loads	indicating	

that	g/kW‐hr	emission	factors	are	important	for	limiting	emissions	for	higher	loads	but	become	less	important	

as	loads	decrease.		Inability	of	emission	control	technologies	to	work	efficiently	at	lower	loads	is	the	key	

contributor	to	higher	g/kW‐hr	results	at	lower	loads.		Situations	where	lower	load	conditions	with	low	absolute	

emission	rates	exceed	the	specified	g/kW‐hr	limit	may	result	in	counterintuitive	scenarios	where	increasing	

absolute	emissions	could	be	a	corrective	action	to	eliminate	situations	that	could	be	viewed	as	“non‐compliant.”		

To	minimize	this	potential	and	minimize	the	need	for	future	permit	modifications,	a	hybrid	limit	that	considers	

both	lb/hr	and	g/kW‐hr	emissions	may	be	appropriate.	

The	proposed	methodology	is	to	have	a	g/kW‐hr	limit	for	higher	loads	where	the	g/kW‐hr	tend	to	be	more	

constant	and	have	a	lb/hr	limit	for	lower	loads	where	the	lb/hr	emissions	tend	to	vary	less	than	the	g/kW‐hr	

emissions.		The	key	to	this	approach	is	selecting	the	load	(referred	as	BACT	Limit	Switch	Load	hereinafter)	

below	which	a	lb/hr	emission	limit	and	above	which	g/kW‐hr	emission	limit	is	used	for	compliance	

demonstration.		For	main	engines	on	the	BlackHawk	and	BlackHornet	drillships,	50%	load	is	proposed	as	the	

BACT	Limit	Switch	Load	since	the	emission	control	technologies	are	expected	to	operate	efficiently	at	50%	and	

higher	loads.		The	proposed	BACT	limits	for	main	engines	on	the	BlackHawk	and	BlackHornet	drillships	are	

listed	in	the	table	below.	
Ms.	Fortin	–	Page	3	

July	18,	2014	

Proposed BACT Limits for 9H32 Series Main Engines

Proposed BACT Limits NOx CO VOC PM PM10 PM2.5 GHG

50% load and above

(g/kW‐hr) 10.57	 2.49	 0.92	 0.49	 0.49	 0.49	 837	

Below 50% load

(lbs/hr) 52.41	 12.35	 4.58	 2.41	 2.41	 2.41	 4,149	

Proposed BACT Limits for 18H32 Series Main Engines

Proposed BACT Limits NOx CO VOC PM PM10 PM2.5 GHG

50% load and above

(g/kW‐hr) 10.57	 2.49	 0.92	 0.49	 0.49	 0.49	 837	

Below 50% load

(lbs/hr) 104.82	 24.70	 9.16	 4.82	 4.82	 4.82	 8,299	

Having	the	g/kW‐hr	limit	will	still	allow	for	easy	emission	and	fuel	efficiency	comparison	across	different	main	

engines	and	having	the	lb/hr	limit	will	eliminate	the	counterintuitive	scenarios	described	above	and	reduce	the	

likelihood	for	permit	modifications	due	to	discrepancies	between	low	load	and	high	load	g/kW‐hr	results.