business or trade of the employer. The test is whether the former is usually necessary or desirable in the usual business or trade of the employer.18[18] Nex old The connection can be determined by considering the nature of the work performed and its relation to the scheme of the particular business or trade in its entirety. Also, if the employee has been performing the job for at least one year, even if the performance is not continuous or merely intermittent, the law deems the repeated and continuing need for its performance as sufficient evidence of the necessity if not indispensability of that activity to the business. Hence, the employment is also considered regular, but only with respect to such activity and while such activity exists.19[19] In the case at bar, it is undisputed that petitioners were employees of private respondents until their services were terminated on September 1, 1989. They served in their capacity as Chief Engineers, performing activities which were necessary and desirable in the business of private respondents Esso International, a shipping company; and Trans-Global, its local manning agency which supplies the manpower and crew requirements of Esso Internationals vessels. It is, likewise, clear that petitioners had been in the employ of private respondents for 20 years. The records reveal that petitioners were repeatedly re-hired by private respondents even after the expiration of their respective eight-month contracts. Such repeated re-hiring which continued for 20 years, cannot but be appreciated as sufficient evidence of the necessity and indispensability of petitioners service to the private respondents business or trade. Verily, as petitioners had rendered 20 years of service, performing activities which were necessary and desirable in the business or trade of private respondents, they are, by express provision of Article 280 of the Labor Code, considered regular employees. Being regular employees, petitioners may not be dismissed except for a valid or just cause under Article 282 of the Labor Code.20[20] In the instant case, clearly ,there was no valid cause for the termination of petitioners. It will be recalled, that petitioner 18[18] De Leon vs. National Labor Relations Commission, 176 SCRA 615 (1989) 19[19] Id., at 621. Art. 282. Termination by employer. - An employer may terminate an employment for any of the following causes: (a) Serious misconduct or willful disobedience by the employee of the lawful orders of his employer or representative in connection with his work; (b) Gross and habitual neglect by the employee of his duties; (c) Fraud or willful breach by the employee of the trust reposed in him by his employer or duly authorized representative; (d) Commission of a crime or offense by the employee against the person of his employer or any immediate member of his family or his duly authorized representative; and (e) Other causes analogous to the foregoing. 20[20]