Harold C. Simmons boosted his stake in Sea-Land.

The Dallas investor and a group of companies that he controls have increased their holdings in the shipping firm to 34.8% from about 25% over the past three weeks and now are openly seeking control of the firm. In a SEC filing, the Simmons group said it intends to acquire at least 39.9% of Sea-Land’s outstanding shares and to gain representation on the board.