```ts
export function layConveyor(floor: string[], row: number, col: number, length: number): string[] {
  if (row < 0 || row >= floor.length) {
    throw new Error("Row out of bounds");
  }
  if (col < 0 || col + length > floor[0].length) {
    throw new Error("Run would leave the plan");
  }

  const result = [...floor];
  for (let i = 0; i < length; i++) {
    if (result[row][col + i] !== ".") {
      throw new Error("Cell is not open");
    }
    result[row] = result[row].substring(0, col + i) + "=" + result[row].substring(col + i + 1);
  }

  return result;
}
```