DECLARE @timetable TABLE(_id int IDENTITY, timeval nvarchar(10))
DECLARE @hours int
,	@minutes int
,	@seconds int

INSERT INTO @timetable
SELECT '00:05:11'
UNION SELECT '00:04:35'
UNION SELECT '01:57:45'
UNION SELECT '02:52:28'

/* Use SUBSTRING and CHARINDEX to seperate hours, minutes,
 and seconds, then calculate the total time */

SELECT	@hours = hours  + FLOOR((minutes + FLOOR(seconds/60))/60)
,	@minutes = minutes + FLOOR(seconds/60) - (FLOOR((minutes + FLOOR(seconds/60))/60) *60)
, 	@seconds = seconds - (FLOOR(seconds/60) * 60)
FROM (
	SELECT SUM(CAST(SUBSTRING(timeval,1,CHARINDEX(':',timeval)-1) AS int)) AS hours
	,	SUM(CAST(SUBSTRING(timeval,CHARINDEX(':',timeval)+1,2) AS int)) AS minutes
	,	SUM(CAST(SUBSTRING(timeval,CHARINDEX(':',timeval)+4,2) AS int)) AS seconds
	FROM @timetable
	) t

SELECT CASE WHEN LEN(@hours) <= 2 THEN RIGHT('00' + CAST(@hours as varchar(5)), 2)
		ELSE CAST(@hours as varchar(5)) END + ':'
+	RIGHT('00' + CAST(@minutes as varchar(5)), 2) + ':'
+	RIGHT('00' + CAST(@seconds as varchar(5)), 2) AS [Total Time]

/* Convert to dates, sum the DATEDIFFs, then convert back to hh:mm:ss */

SELECT	@hours =  FLOOR(FLOOR(seconds/60)/60)
,	@minutes = FLOOR(seconds/60) - (FLOOR(FLOOR(seconds/60)/60) * 60)
, 	@seconds = seconds - (FLOOR(seconds/60) * 60)

FROM (
	SELECT SUM(-1 * DATEDIFF(ss,CAST(timeval as datetime),CAST('00:00:00' as datetime))) AS seconds
	FROM @timetable
	) t

SELECT CASE WHEN LEN(@hours) <= 2 THEN RIGHT('00' + CAST(@hours as varchar(5)), 2)
		ELSE CAST(@hours as varchar(5)) END + ':'
+	RIGHT('00' + CAST(@minutes as varchar(5)), 2) + ':'
+	RIGHT('00' + CAST(@seconds as varchar(5)), 2) AS [Total Time]